Normal Renal Physiology


Bruce R. Gilbert, M.D., Ph.D.
Bruce R. Leslie, M.D.
E. Darracott Vaughan, Jr., M.D.
Cambell's Urology 6th Edition
ed. PC Walsh, AB Retik, TA Stamey and E.D. Vaughan, Jr.
W. B. Saunders Co.,1(2):70-90, 1992

Introduction:
The kidneys receive 20 per cent of the cardiac output while constituting only one-half of 1 per cent of the total body mass. The 180 liters of glomerular filtrate produced each day are finely processed to maintain the internal milieu with exquisite precision.

The nephron, the functional unit of the kidney, consists of a glomerular capillary network, proximal convoluted tubule, loop of henle, distal convoluted tubule and a collecting duct. There are approximately two to three million nephrons within two adult human kidneys. At rest only one tenth of this amount is required to maintain homeostasis. Therefore a large reserve exists. It is the precise quantification of this reserve that enables decisions regarding therapy to be made, when renal dysfunction occurs. This chapter will review normal renal physiology and examine the impact of alterations in renal function on urology. We will also discuss the limitations of various quantitative methods while presenting several "bedside techniques" for the assessment of renal function.

Historical Perspective:
The evaluation of renal function dates back to the early 1800's. As described so eloquently by Haycock (Haycock 1981), it was John Bostock (Bostock and Bright 1827) who was working as a lecturer in clinical chemistry in London, who first demonstrated an impaired concentrating ability in a group of Richard Bright's patients with advanced renal disease. It wasn't however, until almost a decade later in 1917, that Van Slyke and his co-workers introduced and defined the term "renal clearance" (Austin, Stillman et al. 1921) the central concept in the measurement of renal function . Although Rehberg in 1926 (Rehberg 1926) was the first to measure and equate the clearance of creatinine with glomerular filtration rate, it was Homer Smith (Smith 1951) who delineated the full potential of clearance measurements in the evaluation renal function.

Developmental Physiology:
During embryologic development three bilateral excretory systems develop. The earliest is the pronephros, which is composed of approximately 7 tubules, the proximal ends of which form nephrostomes which enter into the coelomic cavity while the distal ends coalesce to form the pronephric duct which empties into the cloaca. The pronephros appears to be non functional in mammals. At about the forth week of development the second system, the mesonephros develops caudal to the first. It consists of a glomerular structure, a proximal tubular segment and a distal tubular segment. This structure might produce some tubular fluid, at least transiently. In the female this structure regresses by the third month of gestation. In the male the mesonephric tubules and duct ( the former pronephric duct) develop into the efferent ductules of the epididymis, the duct of the epididymis, the ductus deferens, the seminal vesicle and the ejaculatory duct. The third system, the metanephros, originates from two different embryologic tissues at about the eighth week of fetal life. (1) The glomerulus and tubules from the mesenchyme of the nephrogenic ridge. (2) The excretory portion (collecting duct, calyces, pelvis and ureter) which arises from a specialized structure of the mesonephric duct, the ureteric bud. The nephrons in the metanephros appear to function as early as the 11th to 12th week of fetal life. It appears that morphologic and functional maturation of nephrons occurs from the deep nephrons to the superficial nephrons.

The morphologic and functional effects of renal senescence begin in the cortical regions and progresses towards the medullary portions of the kidney. This is just the opposite of nephron maturation. Between the 4th and 8th decade the kidney loses over 20% of its weight. While the greater loss occurs in the cortex the medullary portion is involved with a generalized fibrosis similar to that seen with chronic hypertension. However, even without hypertension arteriolar hyalinization and glomerular sclerosis intervenes with the resultant loss of 50% of glomeruli by the eighth decade (Lindeman, Tobin et al. 1985).

RENAL HEMODYNAMICS
Functional Organization of the Renal Circulation

The renal circulation is designed to simultaneously accomplish bulk filtration and reabsorption and precise selective regulation of the constituents of normal urine (Figure 2-1). From an enormous blood flow of about a liter per minute only about 1 ml of urine per minute is formed. The energy requirement is about 10 per cent of basal oxygen consumption (Beeuwkes, Ichikawa et al. 1981), yet the efficiency of the kidney is reflected in its low arteriovenous oxygen difference.

Originally, the renal circulation was quantified by clearance techniques measuring total renal blood flow. More recently, micropuncture and microangiographic techniques have advanced the understanding of the renal microcirculation ((Barger and Herd 1973); (Bookstein and Clark 1980)). The kidney is not composed of a single homogeneous circulation but of several distinct microvascular networks. These include the glomerular microcirculation, the cortical peritubular microcirculation, and the microcirculation that nourish and drain the inner and outer medulla (Beeuwkes, Ichikawa et al. 1981) (Figure 2-1).

The gross anatomy of the renal vasculature has previously been described. The interlobular arteries taper as they pass through almost the entire cortex, and each gives rise to about 20 afferent glomerular arterioles supplying one or more of the 1.5 million glomeruli of the human kidney. The vascular pathways in the glomerulus change under different physiologic conditions. Hence there is intermittent flow within glomeruli, which may play a role in regulation of glomerular filtration rate (GFR). The discovery that the glomerular mesangium contains contractile elements that respond to angiotensin II (AII) and other vasoactive substances supports this hypotheses ((Handler and Kreisberg 1991)). Beyond the glomerulus the efferent arterioles either form dense peritubular capillary plexuses that nourish the proximal or distal convoluted tubules situated in the cortex or pass into the medulla (especially from juxtamedullary glomeruli) and divide into bundles of vasa recta that parallel medullary rays ((Bookstein and Clark 1980)). Microdissection and injection studies recently have shown that except for the initial portion of the peritubular capillaries in the outer cortex, the efferent peritubular capillary network and the nephron arising from each glomerulus are dissociated(Beeuwkes 1979). There are distinct outer and inner medullary capillary networks. In the inner medulla, the degree of organization of vascular-tubular relations correlates with concentrating ability (Kaissling, deRouffignac et al. 1975).

The major changes in hydraulic pressure across the renal vascular bed are shown in Figure 2-2. The role of physical factors in the regulation of GFR will be discussed later.

Measurement of Renal Blood Flow

The clearance of organic iodides, Diodrast, and para-aminohippurate (PAH) is used to estimate renal blood flow (RBF) and is based upon application of the Fick principle. These substances, at low plasma concentrations, are almost totally secreted by the renal tubules and there is no extrarenal metabolism, storage, or production. Accurate utilization of the technique requires normal renal function and extraction and assumes a renal venous concentration approaching zero. Since the extraction is probably never complete, the term "effective renal plasma flow" (ERPF) has been used. In disease states, venous sampling with actual determination of PAH extraction (Epah) is required to calculate true renal plasma flow.

Accordingly, Cpah is calculated by the formula Ui x Qu = (Ai - Vi) x RPF where
Ui = concentration of indicator in urine (mg/ml)
Qu = urine flow rate (ml/min)
Ai = concentration of indicator in arterial plasma (mg/ml)
Vi = concentration of indicator in venous plasma (mg/ml)
RPF = renal plasma flow rate (ml/min)

UiQu
Rewriting this equation, RPF = Ai - Vi

Since extraction is assumed to be almost complete in the clinical setting and Ai is kept constant, the equation for Cpah becomes:
UiQu /Upah x V
RPF = Ai or Ppah

where,
Ppah = Plasma concentration of PAH

of note,
UiQu = excreted load of the indicator (mg/min)

The conversion of renal plasma flow rate to blood flow is achieved by dividing RPF by the plasma fraction of whole blood as estimated by the hematocrit.
RPF/RBF = 1 - HCT

The complexity of determination of RBF by the Cpah technique and the requirement of normal renal function have led to a search for alternative techniques. The use of a single-injection technique of a variety of radionuclides followed by measuring the rate of disappearance of the isotope tag from the blood or by noninvasive monitoring is discussed elsewhere (Chapter 9). The radionuclide monitoring techniques also allow calculation of differential RBF from each kidney, which is often the critical information desired in the clinical setting.


Distribution of Renal Blood Flow

Total RBF estimated by Cpah technique is 1200 ml/min/1.73 m2 , a value that has been confirmed by a variety of methods. In infants up to 1 year of age, RBF is about one half of the adult flow; it reaches the adult level at about 3 years of age (McCrory 1972). It should be remembered that RBF falls after age 30 and is about one half of maximum at age 90 (Davies and Shock 1950). When related to renal mass, RBF is remarkably similar in various species, being about 4 ml/gm/min.

Although it is well documented that the perfusion rate in different regions of the kidney is not uniform, there remains considerable disagreement about regional blood flow measurement obtained by different methods under differing experimental conditions. Moreover, no clear correlation exists between distribution of renal blood flow and renal function. The utilization of inert gas washout, radioactive microspheres, or nondiffusible indicators is beyond the scope of this review (see (Dworkin and Brenner 1991); (Barger and Herd 1973)). This area remains under investigation and may be relevant to our understanding of the pathophysiology of acute renal failure (Brenner and Stein 1980). The attractive hypothesis that there is a causal relationship between the distribution of RBF and sodium handling awaits confirmation.

The renal cortex receives about 90 per cent of the total renal blood flow (5 to 6 ml/min in outer cortex), while outer medullary flow is only about 1 ml/min. However, this medullary flow, considered "sluggish" relative to the cortex, is still greater per gram than flow to the liver, brain, or resting muscle.

Glomerular Filtration Rate

The elaboration of urine begins at the glomerulus with the formation of a nearly protein-free ultrafiltrate of plasma, which enters Bowman's space. As the filtrate passes through the tubules, substances may be removed (reabsorption) of added (secretion). Clearance is "a quantitative description of the rate at which the kidney excretes various substances relative to their concentration in plasma" (Smith 1951). It is calculated as follows:

Ux = concentration of x in a timed urine collection (mg/ml)
V = volume of urine per unit time (ml/min)
Px = concentration of x in plasma (mg/ml)
UxV = rate of urinary excretion of x = excreted load (mg/min)
Cx = UxV/Px = the (plasma) clearance of x (ml/min)

Cx is the volume of plasma containing x that would have to be completely cleared of x per unit time to supply an amount of x for urinary excretion at the measured rate. A summary of site and transport mechanisms for nephron handling of solutes is presented in Table 2-1. Clearance does not necessarily mean that an actual volume of plasma is, in fact, completely cleared of x. Rather, it refers to a "virtual volume" of plasma that would provide the measured amount of x.

A substance that is freely filtered and undergoes neither reabsorption nor secretion will have a clearance equal to the GFR. The clearance of inulin, a carbohydrate polymer of fructose, measured during a constant infusion, is the standard for measurement of GFR. A clearance greater than that of inulin indicates that a substance also undergoes tubular secretion; a clearance less than that of inulin implies tubular reabsorption.

rewriting the clearance equation for x = inulin, we have;

Cinulin = Uinulin V/Pinulin

Because of the difficulty in performing inulin clearance, the clearance of endogenous creatinine is used for clinical purposes as an estimate of GFR. Its plasma concentration remains stable during a 24-hour period, and its rate of excretion does not vary with urine flow. Thus, creatinine clearance (CCr) can be calculated during a 24-hour collection of urine, with a plasma sample obtained at any time during the collection period. In normal man, filtered creatinine does not undergo tubular reabsorption; some tubular secretion does occur. At the plasma creatinine concentration that prevails at normal GFR, the ratio Ccreatinine/Cinulin is close to 1, implying negligible secretion. At progressively lower GFR's, however, tubular secretion plays an increasingly important part in creatinine excretion. At GFR's below 30 ml/min, Ccreatinine may overestimate Cinulin by 50 to 80 per cent. Because this represents small absolute differences at low GFR's, Ccreatinine is satisfactory as an estimate of GFR in chronic renal insufficiency.

Kidney weight correlates better with body surface area than with either height or weight. In order to compare renal function in persons of different sizes, GFR is frequently described per standard unit of body surface area, 1.73 square meters (Smith 1951).

Often the need arises to be able to accurately predict the creatinine clearance without awaiting the results of a 24 hour urine. For example, to adjust the dose of various potentially toxic drugs when beginning therapy. Several investigators have devised formula to do this utilizing serum creatinine, body weight, age and sex as variables. The most widely used of these is that described by Crockcroft and Gault (Crockcroft and Gault 1976). Estimated GFR is given by the following equation:

(140-age)*(weight) (ml/min) /72 Scr
(multiply the equation by0.85 for women)

where:
1. Scr is serum creatinine (mg/dl)
2. Age is in years
3. weight is in kg

The major prerequisite is that renal function be at its steady state (as defined by a stable serum creatinine). Their study group consisted of 249 patients ranging in age from 18 to 92 years old with measured mean creatinine clearances from 37.4 to 114.9 cc/min. Their correlation coefficient between measured and calculated creatinine clearance was 0.84, a value not significantly different then the correlation between two consecutive creatinine clearances determined on the same individual.

Factors Affecting Glomerular Filtration

Micropuncture studies of individual nephrons in the Munich-Wistar rat and the squirrel monkey, which posses glomeruli on the renal cortical surface, have permitted direct measurement of the factors that determine single-nephron glomerular filtration rate (SNGFR) ((Brenner and Humes 1977); (Tucker and Blantz 1977); (Osgood, Reineck et al. 1982)). Assuming that all nephrons behave in a manner similar to those accessible to micropuncture, the regulation of whole-kidney GFR can be understood in terms of changes in one or more of the forces that regulate SNGFR. The symbols designating these forces, used in the following discussion, are summarized in Table 2-2.

The principal driving force for glomerular filtration is the hydrostatic pressure at the glomerular capillary (Pgc). It is a consequence of the forces that maintain systemic blood pressure - cardiac output and systemic vascular resistance. The Pgc, which favors ultrafiltration, is opposed by the hydrostatic pressure in Bowman's space of the renal tubule (Pt). The difference between these values is the transmembrane hydraulic pressure gradient (sP):

sP = Pgc - Pt

Complementing these hydrostatic forces are the osmotic pressures exerted by plasma proteins, known as colloid osmotic pressure or oncotic pressure. The oncotic pressure of glomerular capillary plasma (pgc) tends to oppose transcapillary fluid movement; the oncotic pressure of tubular fluid (pt) tends to favor it. The difference between these two forces at any point is the transmembrane oncotic pressure (sp):

sp= pgc - pt

Under normal circumstances, filtration of plasma proteins is negligible and pt is essentially zero.

At any point along the length of the capillary, the effective filtration pressure (Puf) can be calculated as follows:

Puf = sP - sp

As filtration proceeds along the length of the glomerular capillary, the concentration of protein, and hence pgc, rises. In plasma reaching the efferent arteriole, pgc has risen to a value equal to sP. This local equality of sP and pgc is known as filtration pressure equilibrium (FPE). Precisely where along the length of the capillary FPE occurs cannot be determined, but at this point SNGFR becomes zero. FPE occurs in the surface glomeruli of the Munich-Wistar rat under hydropenic conditions. Whether FPE occurs in human glomeruli remain uncertain. In addition to Puf, SNGFR is determined by both the hydraulic permeability of the glomerular capillary (kf or Lp) and the total surface area available for ultrafiltration (A). The hydraulic permeability of the glomerular capillary is much greater than that of capillaries in non-renal tissues. Because kf and A cannot at present be independently measured, they are considered together as their product, the glomerular ultrafiltration coefficient, Kf or LpA:

Kf =kf x A

The factors that determine SNGFR can thus be summarized by any of the following equations:

SNGFR = Kf(Puf)
SNGFR = Kf (sP - sp)
SNGFR = Kf ((Pgc - Pt) - ( pgc - pt))

The actions of these forces are illustrated in Figure 2-3.

Changes in any of the foregoing variables in health or disease will have predictable effects on SNGFR ((Dworkin, Ichikawa et al. 1983);(Fried and Stein 1983)).

Autoregulation of Glomerular Filtration Rate and Renal Blood Flow

Autoregulation of GFR and RBF is believed to occur mainly through variations in afferent arteriolar resistance. In response to changes in arterial pressure, parallel regulation of GFR and RBF results (Figure 2-4). Note that over a wide range of perfusion pressure, from 80 to 180 mm Hg, there is less than 10 per cent change in RBF or GFR. This phenomenon was described as early as 1947 (Forster and Maes 1947) and appears to be a critical mechanism controlling renal homeostasis. Only at very low arterial perfusion pressure does an increase in efferent arteriolar resistance contribute to the maintenance of Pgc, sustaining SNGFR at reduced RBF (Blythe 1983).

The mechanism of autoregulation remains incompletely defined. Autoregulation is not unique to the kidney but is most efficient in the renal and cerebral circulations. Since it is present in innervated, denervated, and isolated kidneys it is assumed to be mediated by events intrinsic to the kidney - hence the term " autoregulation". At present it appears that more than one proposed mechanism of autoregulation namely, myogenic, metabolic, tubuloglomerular feedback, and/or humoral systems may be etiologic. There is particular interest in juxtaglomerular apparatus playing a critical role, although experiments utilizing a variety of angiotensin and prostaglandin blockers are conflicting (Thurau and Schnermann 1982).

Evidence from micropuncture studies supports the hypothesis that changes in the rate of fluid flow in the distal tubule elicit changes in glomerular arteriolar resistance. This phenomenon is known as distal tubuloglomerular feedback. The morphologic association of the macula densa portion of the distal tubule and the afferent arteriole of the same nephron suggests that these structures are involved in the autoregulatory response. Considerable controversy persists, however, over (1) what aspect of distal tubular flow is perceived as the signal that engages autoregulation and (2) what are the mechanisms and sites of changes in arteriolar resistance. The initiating signal appears to be tubular fluid chloride and its reabsorptive transport by cells of the macula densa ((Wright and Briggs 1977), (Schnermann, Ploth et al. 1976) ). Many recent studies have indicated that chloride might not be the the only signal ((Bell, McLean et al. 1981), (Bell and Reddington 1983)). Increases in distal fluid osmolality appears to result in an increase in intracellular free calcium concentration ( {Ca2+}i ). This increase in {Ca2+}i might have two separate and opposite effects. First, the increase in {Ca2+}i may enhance contraction of both the afferent and efferent arterioles. This would result in a decreased SNGFR and prevent an acute loss of fluid and electrolytes. Secondly, however, an increased {Ca2+}i also results in a decrease in renin release from the granules of the juxtaglomerular cells of the afferent arteriole. In the face of a continued stimulus, such as volume expansion, this would reduce local angiotensin II production and increase SNGFR resulting in an increased excretion rate.

An alternative theory first proposed by Bayliss ((Bayliss 1902)) explains autoregulation as a consequence of variations in afferent arteriolar tone that occur as a direct result of changes in arterial blood pressure (myogenic theory). An increase in pressure, which stretches the arteriolar smooth muscle, elicits contraction of the muscle layer, thus increasing afferent arteriolar resistance (Fried and Stein 1983). Thereby regulating both GFR and RBF.

The metabolic theory predicts that vasodilatory metabolites accumulate with a decrease in organ
perfusion resulting in a return to baseline blood flow rates. The major objection to this theory results from the well known relationship between renal blood flow and renal metabolism (Spielman and Thompson 1982). Renal metabolism is primarily determined by the rate of sodium reabsorption which in turn is directly related to GFR. Since GFR is known to vary with RBF it would follow that an increase in metabolism would result in an increase of the putative vasodilator which would lead to an increase in RBF. This would make autoregulation of this parameter impossible.

Autoregulatory factors including vasodilatory prostaglandins (Schnermann, Briggs et al. 1984), kinins (Maier, Starlinger et al. 1981), adenosine (Spielman and Thompson 1982), and the renin angiotensin system ((Schnermann, Briggs et al. 1984),(Levens, Peach et al. 1981c), (Moore, Casellas et al. 1990), (Briggs and Schnermann 1986)) have been implicated in the autoregulation of RBF and GFR. However, much controversy persist and the role of these humoral factors needs to be better defined.

Most likely all of these systems contribute, in part, to the phenomena of renal autoregulation. Future studies will likely delineate the relative contribution of each.


Glomerular Permeability

The fluid entering Bowman's space is nearly free of albumin and larger molecules. Restriction to glomerular filtration of certain molecules is known as glomerular permselectivity. The determinants of glomerular permselectivity include effective glomerular "pore" size and the electrostatic charge on the glomerular filtration barrier. The degree of filtration of a molecule is thus determined by its size, shape, and charge. Filtration of macromolecules such as albumin may also be affected by renal hemodynamics.

The filtration of molecules larger than inulin (molecular radius = 14 A) is progressively restricted, approaching zero at molecular radii of about 40 A (Brenner and Humes 1977). The molecular radius of albumin is 36 A.

The glomerular filtration barrier is covered by sialoproteins that bear fixed negative charges. Albumin is a polyanion at physiologic pH. Hence, it is also restricted from glomerular filtration by the interaction of these similar electrostatic charges. Neutral dextran, with a molecular radius equal to that of albumin but without a net negative charge, is filtered more than 100 times as easily as albumin (Brenner, Bohrer et al. 1977). In certain forms of renal disease, diminution in the glomerular charge barrier may increase glomerular permeability to albumin, resulting in proteinuria.

The glomerular filtration of albumin may also be increased by a reduction in renal plasma flow unaccompanied by a change in glomerular filtration rate. Such an increase in "filtration fraction" (the ratio of GFR/RPF) may lead to increased in the concentration of albumin in the glomerular capillary and thereby augment the gradient favoring albumin diffusion into the filtrate.

Sodium and Water

Sodium (Na) and its associated anions (mostly chloride and bicarbonate) are confined to the extracellular fluid (ECF) compartment and are the principle determinants of ECF osmolality. Because water moves freely across cell membranes, and because the osmolality of ECF is kept constant, it follows that the volume of ECF is directly related to the total body content of Na. Renal tubular reabsorption of the Na and water preserves ECF volume despite glomerular filtration of large volumes of plasma. Changes in tubular Na reabsorption defend ECF volume against changes in the filtered Na load produced by changes in GFR. In addition, changes in Na excretion maintain Na balance at varying levels of Na intake. The handling of Na by the nephron is summarized in Figure 2-5.

The reabsorption of Na ion takes place against electrical and chemical (concentration) gradients and requires expenditure of metabolic energy. Such a process is described as active transport. The energy for the bulk of Na reabsorption derives from aerobic metabolism. There is a direct, linear relationship between the rate of Na reabsorption and oxygen consumption by the kidney (Lassen, Munck et al. 1961).

The exact mechanisms of Na transport throughout the nephron continue to be investigated. In the proximal tubule, Na in the tubular lumen travels down its concentration gradient, across the luminal (apical) membrane, into the tubular epithelial cell. Within the cell, the Na concentration is kept low by pumps in the basal and lateral membranes that extrude Na into the peritubular space, from which it can enter the peritubular capillaries. These pumps, involving the enzyme Na,K-ATPase, represent the "active" (energy-consuming) component of Na transport. For the most part, Cl reabsorption follows Na as a consequence of the negative luminal potential created by outward Na movement.

Water is reabsorbed "passively," by moving down a gradient of osmolality between tubular fluid (lower) and peritubular (higher). This gradient is established by the reabsorption of Na and its attendant anions. Because the water permeability of the proximal tubule is high, only a small osmotic gradient is required to effect water movement.

Similar Na reabsorptive processed probably operate in the distal tubule and collecting ducts. Unlike these other segments, however, the thick ascending limb of the loop of Henle has a positive luminal potential. Among the mechanisms proposed to account for this is active transport of Cl with secondary, passive Na reabsorption ((Burg and Green 1973); (Warnock and Eveloff 1982)).

Regulation of Na Excretion

Defense mechanisms ensure that the bulk of filtered Na is reabsorbed in the proximal nephron segments. Renal autoregulation keeps GFR, and hence the filtered Na load, constant. Should a hemodynamic disturbance occur that does change GFR, the filtered load of Na would change. The proximal tubule alters its absolute rate of Na reabsorption in parallel, so that the fractional Na reabsorption remains constant. The phenomenon, termed glomerular-tubular balance, prevents loss or accumulation of large amounts of Na. The mechanism by which glomerular-tubular balance is achieved remains uncertain. One hypothesis stresses the importance of physical factors ((Brenner and Troy 1971)). Because the transglomerular passage of plasma protein is restricted, a change of GFR produces a parallel change in the protein concentration in the glomerular capillary. This fluid passes into the efferent arteriole and thence to the peritubular capillaries. Thus, a rise in GFR would result in an increase in peritubular oncotic pressure. This tends to favor net Na reabsorption. Decrements in peritubular oncotic pressure would have the opposite effect. The importance of peritubular protein concentration in the regulation of proximal Na reabsorption has been challenged ((Knox, Mertz et al. 1983)). An alternative theory suggests that the glomerular filtrate itself contains a substance that stimulates its own reabsorption ((De Wardener 1978)). The identity of this substance is unknown.

Comparatively small changes in Na intake, as with chronic dietary changes, produce changes in urinary Na excretion via changes in the handling of Na by the collecting ducts. Except with extreme changes in ECF volume, such as can be produced by parenteral infusions, chronic Na loading is usually not associated with changes in proximal Na reabsorption. Na reabsorption by the collecting ducts is stimulated by aldosterone. Aldosterone secretion is regulated in part by ECF volume through the activity of the renin-angiotensin system. Other humoral factors may also regulate Na excretion. these include substances whose production in the kidney is related to the state of Na balance, including prostaglandins, angiotensin II, dopamine, and bradykinin.

The existence of one or more "natriuretic hormones" produced in the central nervous system or at other sites in response to Na loading has also been proposed ((De Wardener and Clarkson 1982); (Genest 1983)). Natriuretic hormone may act on the collecting duct to inhibit Na reabsorption, perhaps by inhibition of Na, K-ATPase. There is growing evidence that an atrial natriuretic factor (ANF) may play a physiological role in the regulation of salt and water balance in humans (Laragh 1985),(Atlas and Laragh 1988),(Goetz 1988),(Lang, Unger et al. 1987),(Richards, McDonald et al. 1988). ANP is secreted principally by the atrial myocytes in response to increased intravascular volume. It acts in concert on the vasculature, kidneys and adrenal glands to reduce systemic blood pressure and intravascular volume both chronically as well as acutely (Brenner, Ballermann et al. 1990). The reduction in systemic blood pressure is the result of a reduced peripheral vascular resistance, diminished cardiac output and decreased intravascular volume. In the kidney, ANP acts on specific receptors to induce hyperfiltration, inhibition of Na+ transport and suppression of renin release. These actions result in a natriuresis, diuresis and lowering of arterial blood pressure (Ballermann, Zeidel et al. 1991). ANP also inhibits aldosterone biosynthesis by both inhibiting renin secretion from the renal juxtaglomerular apparatus and directly by a receptor mediated action on adrenal glomerulosa cells. These actions also tend to lower arterial blood pressure and intravascular volume.

Changes in peritubular hydrostatic pressure in the renal interstitium can alter Na handling by the proximal tubule, loop of Henle and possibly by the collecting duct ((Gilbert and Maude 1977),(Gilbert, Maack et al. 1979),(Knox, Mertz et al. 1983)). An increase in interstitial pressure resulting from a rise in arterial pressure, renal vasodilation or alterations in filtration fraction (resulting from changes in filtration dynamics at the glomerular tuft) can increase renal Na excretion acutely. Whether such physical factors influence tubular function directly or through resultant changes in the levels of intrarenal hormones remain undetermined. Stimulation of the adrenergic innervation to the kidney increases tubular Na reabsorption, independent of changes in GFR or renal plasma flow ((DiBona 1977)). The site at which sympathetic stimulation acts appears to be the proximal tubule.

The rate of sodium excretion is of diagnostic importance in determining the cause of oliguria. With prerenal azotemia, sodium excretion is usually less than 15 mmole/l whereas sodium excretion is usually higher with renal causes (e.g., acute tubular necrosis). However,prerenal factors often coexist with renal disease, thus there is considerable overlap in the urine sodium concentration (UNa) in these two situations ((Espinel and Gregory 1980)). Therefore, only values that are clearly high or low are diagnostic. The fractional excretion of sodium (FENa%), obtained by dividing the clearance of sodium by the clearance of creatinine, provides a much better index by which to differentiate renal from prerenal causes. FENa% is calculated by the following formula:

FENa% = (UNa * P Cr * 100 )/ ( PNa * UCr) = (CNa / CCr) * 100

where; UNa = urine sodium concentration
P Cr = plasma creatinine concentration
UCr = urine creatinine concentration
CNa = clearance of sodium
CCr = clearance of creatinine

A value lower than 1% favors a prerenal etiology while a value above 1% favors a renal cause. Although fairly sensitive and specific, FENa% values less than 1% have been reported in a variety of causes of acute renal failure other than prerenal disease (e.g., myoglobinuria or hemoglobinuria, radiocontrast nephropathy, renal azotemia superimposed on chronic prerenal failure as in hepatic cirrhosis) ((Kamel, Ethier et al. 1990)).


Urinary Dilution and Concentration

In the proximal tubule, where some two thirds of the glomerular filtrate is reabsorbed, water follows NaCl reabsorption, and the tubular fluid remains isotonic to plasma (normally 270 to 285 mOsm/kg). Separation of NaCl from water reabsorption occurs in the loop on Henle. This mechanism generates tubular fluid that is hypotonic (dilute) compared with plasma. In the absence of vasopressin (ADH), the final urine remains dilute, permitting the excretion of a water load. The loop of Henle also helps generate a hypertonic medullary interstitium. Under condition of water deprivation (hydropenia), ADH causes fluid in the collecting ducts to equilibrate osmotically with the medullary interstitium. The results in a hypertonic (concentrated) urine, conserving water. Urine osmolality may vary normally from 50 mOsm/kg to approximately 1200 mOsm/kg.

The excretion of water relative to solute may be described quantitatively as free water clearance (CH2O). Not a true clearance in the sense of inulin or creatinine, CH2O is the difference between the measure urine flow rate and the "osmolar clearance," i.e., the rate or urine formation that would be necessary to excrete the measured urinary osmolar load at a tonicity equal to that of plasma. The calculation of CH2O is summarized below.

CH2O = V - Cosm
V = urine flow rate (ml/min)
Cosm = osmolar clearance (ml/min)
= (Uosm x V)/ Posm
Uosm = urine osmolality (mOsm/kg)
Posm = plasma osmolality (mOsm/kg)
Uosm x V = urinary osmolar load (mOsm/min)
CH2O = V - ((Uosm x V)/ Posm)
CH2O = V (1 - (Uosm/ Posm) ) (ml/min)

When dilute urine is produced, Uosm Posm and CH2O is positive, implying net free water excretion. When concentrated urine is produced, Uosm > Posm, and CH2O has a negative value, implying net free water conservation. Negative free water clearance (-CH2O) is symbolized as TcH2O.

The formation of hypotonic tubular fluid occurs in the ascending thick limb of the loop of Henle. This segment is impermeable to water. The reabsorption of Cl and Na separates solute from water and reduces the osmolality of fluid leaving this segment to approximately 100 mOsm/kg. This process occurs irrespective of external water balance. Administration of a water load reduces systemic extracellular fluid tonicity and inhibits ADH (antidiuretic hormone or vasopressin) secretion. In the absence of ADH, the cortical and medullary portions of the collecting duct remain impermeable to water. Because NaCl reabsorption can continue in these segments, urine osmolality may be further reduced to the minimum of 50 mOsm/kg. The excretion of dilute urine maintains ECF tonicity in response to a water load.

Concentrated urine is elaborated in response to hydropenia, which raises ECF osmolality and stimulates ADH secretion. In man, ADH binds to receptors on the basolateral membrane of collecting duct epithelial cells. This activates the enzyme adenylate cyclase, which catalyzes intracellular cyclic AMP (cAMP) formation. cAMP, via a cAMP-dependent protein kinase, facilitates phosphorylation of a component of the apical membrane. Through a process that also involves cellular microtubules, this increases the permeability of the collecting tubule of water (Figure 2-6). In the medullary portion, urea permeability is also increased. Most water reabsorption during hydropenia occurs in the cortical collecting tubule, where hypotonic luminal fluid leaving the loop of Henle equilibrates with the isotonic interstitium of the cortex. The isotonic fluid in the cortical collecting tubule subsequently becomes hypertonic in the medullary collecting duct, by osmotic equilibration with the hypertonic medullary interstitium.

The kidney generates medullary hypertonicity by means of the loop of Henle. According to the "countercurrent hypothesis," reabsorption of NaCl without water in the ascending limb, and its deposition in the interstitium, creates a local (horizontal) osmotic gradient. The increase in medullary tonicity causes water to leave the descending limb, raising its tonicity. The proximity of the descending and ascending limbs of the same tubules (arranged in hairpin curves) causes a constant gradient of tonicity at any given horizontal level to be multiplied along the vertical axis, creating high tonicities at the bend of the loop. This creates the progressive gradient of medullary tissue osmolality from corticomedullary junction to the papillary tip (Figure 2-7). Modification of the countercurrent hypothesis has been proposed to account for the importance of urea in augmenting urinary concentration ((Kokko and Rector 1972); (Jamison and Robertson 1979). According to the modified hypothesis, the process of urinary concentration begins with active chloride transport in the ascending thick limb. Urea and water, to which this segment is impermeable, remain behind in the hypotonic fluid. In the cortical and outer medullary collecting tubules, ADH augments water but not urea permeability. Water leaves these segments, raising the luminal concentration of urea. In the inner medullary collecting duct, ADH enhances urea permeability as well. Urea thus leaves the tubule and accumulates in the interstitium. This action raises medullary tonicity and causes water to leave the descending limb of Henle's loop, which is permeable to water but not to NaCl or urea. The raises the NaCl concentration of fluid reaching the hairpin turn. The ascending thin limb is impermeable to water but not to NaCl or urea. In this segment, NaCl diffuses down its concentration gradient, into the interstitium. Urea diffuses in, but at a slower rate, leading to progressive reduction of luminal tonicity in the segment. The osmotic gradients that are established are multiplied by the countercurrent mechanism. An additional effect of this mechanism is to cause some urea to remain in the medulla, where it recycles between the collecting duct, the interstitium, and the loop of Henle. This mechanism is summarized in Figure 2-8.


Acid-Base Balance

To maintain the acid-base balance of the extracellular fluid (ECF) the kidney must excrete net acid at a rate equal to the rate of extrarenal net acid production ( approximately 0.3 to 1.0 mEq/kg day). The kidney maintains the pH of the ECF by regulating the plasma bicarbonate (HCO3-) concentration. It does so by two processes: (1) reclamation of filtered HCO3- and (2) generation of new HCO3- by means of net acid excretion. The proximal tubule lowers the luminal pH from 7.3 to approximately 6.7 and thus reabsorbs the major portion of HCO3-. The collecting tubule provides the final urinary acidification with titration of ammonia, phosphate and other titratable buffers (Alpern, Dennis et al. 1991).

The normal filtered load of HCO3- is about 4500 mEq per day. Less than 0.1 per cent of filtered HCO3- appears in the final urine. Approximately 80 per cent of filtered HCO3- is reclaimed by the proximal tubule. Although the net result of this process is referred to as HCO3- reabsorption, the HCO3- in tubular fluid is not directly reabsorbed as such. Instead, the tubular epithelial cells add HCO3- to peritubular blood as a consequence of proton (H+) secretion. The enzyme carbonic anhydrase with then tubular epithelial cells catalyzes the hydration of carbon dioxide (CO2) to form carbonic acid (H2CO3). Dissociation of H2CO3 yields H+ and HCO3-. The H+ is secreted into the tubular lumen, where it combines with filtered HCO3- to form H2CO3. Carbonic anhydrase is also present in the brush border, where it catalyzes the dehydration of luminal H2CO3 to CO2 and water. The CO2 can diffuse back into the cell, where it may be hydrated to form additional H2CO3. The HCO3- generated within the cell diffuses into peritubular blood, possibly via specific pathways ((Warnock and Rector 1979)). The bulk of H+ secretion by the proximal tubule appears to be couple to Na reabsorption in a direct exchange mechanism (Figure 2-9) ((Warnock and Rector 1979)).

The factors that can affect proximal HCO3- reabsorption include (1) extracellular fluid volume - decrements in absolute or effective volume enhance HCO3- reabsorption, whereas increments have the opposite effect; (2) arterial Pco2 - hypercapnia stimulates HCO3- reabsorption, whereas hypocapnia inhibits; (3) body K stores- there is a slight stimulation of proximal HCO3- reabsorption by prior K depletion; (4) parathormone - inhibits reabsorption; and (5) phosphate depletion - inhibits reabsorption.

The kidney adds new HCO3- to blood by secreting H+ in excess of that which is necessary to reclaim filtered HCO3-. This process results in net acid excretion. Under normal circumstances, net acid secretion replaces the HCO3- consumed in buffering the strong acid by-products of metabolism, mainly sulfuric and phosphoric acids. Depending on diet, some 40-70 mEq of H+ derived from such acids are produced daily. Net acid excretion may also increase in response to the addition of ketoacids or lactic acid in disease states, or to compensate for hypercapnia. The secreted H+ is taken up by urinary buffers, principally monohydrogen phosphate (HPO4=) and ammonia (NH3), which are converted to H2PO4- and NH4+. Net acid excretion is equal to the sum of H2PO4- and NH4+ excretion, minus any HCO3- that escapes reabsorption. The H2PO4- is also referred to as titratable acid. This term denotes the amount of strong base required to titrate urine back to pH 7.4. A comparatively small proportion of secreted H+ is unbound to buffers, and it is this component that can result in a minimum urinary pH of about 4.4.

Most net acid excretion occurs in the collecting ducts, where H+ is secreted by pumps that are not directly coupled to Na reabsorption. This process also depends on intracellular carbonic anhydrase. There is no brush border carbonic anhydrase in the collecting ducts. The rate of net acid excretion can be modified by (1) the electrical gradient between the tubule cell and the lumen - Na+ reabsorption in the collecting ducts creates a negative intraluminal potential; this favors H+ secretion. The gradient is augmented by increased Na delivery and reabsorption, especially when Na+ is accompanied by a poorly reabsorbed anion. This enhancement of H+ secretion by Na+ reabsorption brings about an indirect coupling of these processes; (2) mineralocorticoids - aldosterone can directly stimulate the capacity of the H+ pump ((Al-Awqati, Norby et al. 1976)). In addition, by stimulating Na+ reabsorption in the collecting duct, aldosterone enhances the electrical gradient that favors H+ secretion; (3) buffer availability - NH3 is produced in the kidney from glutamine. It gains access to the tubular fluid in both the proximal and the distal nephron by nonionic diffusion. Acidosis increases renal NH3 production. Increased buffer availability, by taking up free H+, reduces the chemical gradient against which H+ is pumped and this stimulates H+ secretion. The mechanism by which a change is systemic pH affects ammoniagenesis remains undefined. Ammoniagenesis can also be stimulated by K depletion and inhibited by K loading. An adaptive increase in ammoniagenesis is the principal mechanism for the excretion of increased acid loads ((Tennen 1978)).

Defective Tubular Hydrogen Ion Secretion

Defects in renal tubular hydrogen ion secretion also termed renal tubular acidosis (RTA) may occur either in the proximal tubule (RTA type 2) or in the distal tubule (RTA type 1) (Rodriquez-Soriano and Edelman 1969).

Proximal RTA (Type 2)

Three major processes occur in the nephron to rid the body of excess acid: sodium bicarbonate reabsorption, ammonia trapping, and titratable acid formation. Unlike the distal tubule, in the proximal tubule the formation of titratable acid and NH4+ is negligible; its major function is the reabsorption of bicarbonate. One bicarbonate ion is reabsorbed for each hydrogen ion excreted. The proximal tubule has a high capacity bicarbonate transport system, it reabsorbs approximately 80 percent of filtered bicarbonate. In proximal RTA the reabsorptive threshold (Tm) for bicarbonate is thought to be lowered (Morris and Ives 1991)Figure 2-10. This results in large amounts of sodium bicarbonate delivered distally. The distal tubule has a relatively low capacity for bicarbonate reabsorption (however, it is a "high gradient system" being able to secrete hydrogen ions across a large gradient). Thus the hydrogen ions secreted distally primarily go towards absorption of the nonreabsorbed bicarbonate with little NH4+ and titratable acid secretion. This results in an alkaline, bicarbonate rich urine. Extracellular volume contraction occurs secondarily to this massive anion (HCO3-) loss. In an attempt to compensate for this extracellular volume contraction chloride reabsorption is increased resulting in a hyperchloremic metabolic acidosis. As systemic acidosis progresses, the filtered load of sodium bicarbonate diminishes allowing only small amounts of bicarbonate to be delivered distally. Thus, below this lowered bicarbonate reabsorptive threshold (Tm), distal acidification processes can compensate for defective proximal acidification. This results in more complete bicarbonate reabsorption distally with titratable acid and NH4+ production and urine ph of 5.0 or less. However, the amount of bicarbonate required to maintain a normal serum level is massive, since it must equal the amount of bicarbonate excretion. In proximal RTA, potassium and calcium excretion are increased. However, since citrate excretion is relatively normal, nephrocalcinosis and renal calculi formation are rare. Clinically, the effects on children include: osteomalacia, rickets, abnormal gut calcium absorption, decreased phosphorus, and vitamin D metabolism .

Distal RTA (Type 1)

In distal RTA the distal tubule is unable to secrete hydrogen ions against a large gradient and thus unable to produce a urine pH less than 5.4 even when challenged (Morris and Ives 1991). Minimal urine pH in distal RTA ranges from 5.4 to 6.5 depending on the severity of the transport defect. The distal tubule normally accounts for at most 15 percent of total bicarbonate reabsorption. Since proximal reabsorption is not affected in distal RTA, the urinary bicarbonate concentration is only 5 mEq/1 even at a urine ph of 6.5. Thus daily excretion of bicarbonate is not usually greater than 10 to 15 mEq/day in distal RTA. Possible causes of abnormal acidification in cells of the distal nephron are schematically shown in Figure 2-11 .

Therefore, in distal RTA, acidosis is more easily controlled as compared to the acidosis of proximal RTA. Systemic acidosis results in increased bone reabsorption and in turn increased urinary calcium. In addition, the urine is mildly alkaline (unlike proximal RTA where an almost normal urine pH is found) and nephrocalcinosis is common in distal RTA secondary to the low solubility of this excess calcium in a mildly alkaline urine with a decreased citrate content.

Two subsets of patients with RTA have been described. The first group present with complete RTA (cRTA) and comprises the majority of patients with RTA. These patients are acidotic and usually present with the known renal manifestations of the disease (e.g. nephrocalcinosis). Another group of patients present with incomplete RTA (iRTA). Patients with iRTA are nonacidotic and present with nephrocalcinosis. Like patients with cRTA theses patients are also unable to increase the urinary excretion of titratable acid to a normal maxima when presented with an acid load (Wrong and Davies 1959). These patients are considered to have a "milder" form of the disease. Functional renal mass and glomerular filtration rate is relatively well maintained and excretion of NH4 is sufficient to prevent frank acidosis (Wrong and Feest 1980).

The acidosis of RTA is non-anion gap acidosis (anion gap being defined as the difference between the major intravascular cations, sodium and potassium minus the sum of the major anions, chloride and bicarbonate; normally between 12 and 16) associated with hyperchloremia and hypokalemia in contradistinction to the acidosis associated with ATN or reduced GFR (a hypochloremic and hyperkalemic acidosis). The hyperchloremia results from increased NaCl reabsorption stimulated by volume contraction secondary to sodium bicarbonate loss in the urine. The hypokalemia results from stimulation of the renin-angiotensin-aldosterone (secondary to volume contraction) axis as well as increased distal Na-K exchange occurring with the increased distal delivery of NaHCO3.

Potassium

Over 90 per cent of plasma K undergoes glomerular filtration. Most of it is reabsorbed in the proximal tubule and the loop of Henle FIGURE 2-12. The bulk of K in the final urine is added to tubular fluid by secretion in the late distal tubule and cortical collecting duct. Tubular epithelial cells in these segments take up K from peritubular fluid by a mechanism involving Na,K-ATPase ((Wright and Giebisch 1978)). This gives rise to an intracellular transport pool of K. Potassium secretion is favored by the negative intratubular potential created by distal Na reabsorption, and by the concentration gradient between intracellular K and tubular fluid. In addition to these passive forces that influence K secretion, an active transport mechanism may exist ((Giebisch and Stanton 1979)). There is no evidence either for a coupled exchange between Na+ absorption and K+ secretion or for competition between intracellular K+ and H+ for tubular secretory pathways.

Potassium excretion is augmented by an increase in distal tubular fluid flow rate (as with saline or osmotic diuresis, diuretic drugs, or postobstructive diuresis). This promotes K secretion by maintaining a steep K concentration gradient between the cell and the tubular fluid. In addition, increased quantities of Na are presented to distal reabsorptive sites. The negative intratubular potential created by increased Na reabsorption also promotes K secretion.

Mineralocorticoids stimulate K secretion, possibly by stimulating Na,K-ATPase in the basolateral membrane ((Giebisch and Stanton 1979)). This would increase K uptake and raise intracellular K concentration, thereby enhancing K secretion.

Calcium

Only that portion of plasma calcium which is not bound to plasma proteins is filtered at the glomerulus. Ultrafilterable calcium (Ca) represents about 60 per cent of total plasma calcium. There is subsequent tubular reabsorption of 96 to 98 per cent of this filtered load. The bulk of calcium reabsorption occurs in the proximal tubule and the ascending this limb of the loop of Henle. Additional reabsorption occurs in the distal convoluted tubule and cortical collecting duct ((Sutton 1983)). Calcium movement along the nephron appears to be subject to two transepithelial transport processes. One is a paracellular and gradient-dependent (concentration) process that predominates in most segments of the nephron. The other is a transcellular, energy dependent process that characterizes calcium transport in the distal nephron (Bronner 1989). Recent work has been focused on the role of a cytosolic calcium binding protein (CaBPr) located in the distal tubule that might modulate calcium transport in this nephron segment (Huang and Christakos 1988),(Varghese, Lee et al. 1988). Regulation of calcium by several known factors (e.g. Vitamin D, PTH) might be due to their effect on production of CaBPr in the distal tubule.

Calcium reabsorption in the proximal tubule occurs in parallel with sodium reabsorption, with a component of calcium absorption being directly sodium-dependent ((Suki 1979)). Calcium reabsorption in the proximal tubule is inhibited by PTH, cyclic AMP, acetazolamide, exogenous Na loading, and phosphate depletion. The effect on urinary Ca, however, depends on the behavior of more distal nephron sites. In the loop of Henle (as well as the distal convoluted tubule and collecting duct), Ca reabsorption is stimulated by PTH. This accounts for the hypocalciuric effect of this hormone despite its inhibition of proximal Ca absorption. Ca reabsorption in the loop of Henle is inhibited by furosemide. When diuretic-induced extracellular volume depletion is prevented by replacement of salt and water losses, furosemide causes an increase in urinary Ca excretion (Costanzo 1988). This accounts for the efficacy of furosemide in the emergency treatment of hypercalcemia. Final modulation of urinary Ca excretion occurs in the distal tubule and collecting ducts. In these segments, active transport of Ca occurs, which can be dissociated from Na reabsorption. thus, chlorthiazide, which inhibits distal tubular Na transport, also directly stimulates Ca absorption in this segment ((Costanzo and Windhager 1978)). This may be the major explanation for the reduction in urinary Ca excretion with chronic administration of thiazides. An additional hypocalciuric effect of thiazide diuretics may result from extracellular fluid volume contraction, with consequent stimulation of Ca reabsorption in the proximal tubule.

Other factors that stimulate Ca reabsorption between the late proximal tubule and the early distal convoluted tubule include hypocalcemia, metabolic alkalosis (increased tubular HCO3-), vitamin D, and phosphate loading. Reabsorption is inhibited by hypercalcemia, metabolic acidosis, hypermagnesemia, and phosphate depletion ((Sutton 1983)).

Phosphate

Plasma inorganic phosphate, existing as a mixture of HPO4= and HPO4-, is 80 to 90 per cent ultrafilterable at the glomerulus. Of the filtered load, 80 to 97 per cent is reabsorbed. The tubular reabsorption of phosphate can increase to nearly 100 per cent in response to phosphorus deprivation. Most phosphate reabsorption occurs in the proximal tubule. The existence of a distal site of phosphate transport is also suspected ((Knox, Osswald et al. 1977); (Dennis, Stead et al. 1979)). PTH inhibits phosphate reabsorption in the proximal tubule and increases urinary phosphate. This effect is associated with increased urinary excretion of nephrogenous cyclic AMP.

EXCRETION OF ORGANIC SOLUTES

Urea

Urea is the major end product of protein catabolism in man. It is freely filtered at the glomerulus. Water reabsorption increases the urea concentration in tubular fluid, with subsequent urea diffusion out of the tubule. At typical urine flow rates of 1 ml/min, 30 to 40 per cent of filtered urea is reabsorbed in the proximal tubule. The medullary collecting ducts are also permeable to urea, and their permeability is enhanced by vasopressin. Reabsorption of urea in the collecting duct is enhanced by antidiuresis and inhibited by water diuresis. Urea reabsorbed in the collecting ducts contributes to the hypertonicity of medullary interstitial fluid and plays an important role in urinary concentration.

The rate of urea reabsorption is inversely related to tubular fluid flow rate. At urine flow rate of about 2 ml/min, during water diuresis, 60 to 70 per cent of filtered urea is excreted, i.e., urea clearance is 60 to 70 per cent of the glomerular filtration rate. At low urine flow rates, during antidiuresis or reductions in renal blood flow, urea clearance may fall to 10 to 20 per cent of the glomerular filtration rate. this accounts for the disproportionate increase in BUN compared with serum creatinine in states of "prerenal azotemia".

Uric Acid

Uric acid is the end product of purine catabolism in man. On a low-purine diet, uric acid production from endogenous sources is about 700 mg per day. Two thirds of the uric acid load is excreted by the kidneys. Intestinal excretion, with degradation by bacterial enzymes, accounts for the rest. Because of its low pKa (5.75), it exists in plasma almost entirely as urate. The low pH attained in urine in the distal nephron favors the formation of uric acid, which is of limited solubility in water.

Current evidence favors a four-component model for renal handling of urate ((Levinson and Sorenson 1980)): (1) Plasma urate is freely filtered at the glomerulus; (2) filtered urate undergoes nearly complete tubular reabsorption; (3) approximately 50 per cent of this reabsorbed urate is secreted into tubular fluid; and (4) postsecretory reabsorption reclaims about 80 per cent of the secreted urate. Antiuricosuric agents such as pyrazinoic acid (the metabolite of the antituberculous agent pyrazinamide) inhibit the tubular secretory mechanism. Probenecid, a uricosuric agent, acts by inhibiting postsecretory reabsorption ((Fanelli 1977)). The majority of patients with gout appear to have an impairment in renal uric acid excretion, which is incompletely characterized ((Rieselbach and Steele 1974)).

Urate secretion is accomplished by organic anion secretory mechanisms located in the proximal tubule. A variety of other substances share and mutually compete for this mechanism. They include oxalate, lactate, hippurate, penicillins, cephalosporins, thiazides, furosemide, and ethacrynic acid. A separate organic cation secretory mechanism also exists. Among the substances transported by this system are creatinine, cimetidine, and trimethoprim.

Glucose

Although glucose is freely filtered at the glomerulus, it undergoes essentially complete reabsorption in the early portion of the proximal tubule, so that urine is normally glucose-free. With progressively higher filtered loads (at higher plasma glucose concentrations), reabsorption increased until a tubular maximum glucose reabsorption rate (TmG) is attained. Filtered glucose in excess of the TmG appears in the urine ((Smith 1951)).

Glucose transport is linked to proximal Na reabsorption. When the latter is inhibited, as by ECF volume expansion, the TmG falls. These observations have given rise to the following model: Glucose in tubular fluid interacts with a carrier mechanism in the luminal membrane of the tubular epithelial cell. This carrier, which exhibits saturation kinetics, facilitates the entry of glucose into the tubular epithelial cell. Na is required for glucose-carrier interaction. Once transported into the cell, glucose may diffuse down its own concentration gradient from tubular epithelial cell to peritubular blood.

Amino Acids

Circulating amino acids readily cross the glomerular filter and undergo nearly complete reabsorption by proximal tubular cells. This occurs via mechanisms in the brush border membrane. As in the case of glucose, this process appears to be carrier-mediated, Na-dependent, and energy-requiring ((Schafer and Barfuss 1980)). Separate transport mechanisms in the basolateral membrane also mediate cellular uptake of amino acids from peritubular fluids.

Much attention has been focused on the tubular transport mechanisms for cystine and the cationic (dibasic) amino acids arginine, lysine, and ornithine. Reabsorption of these amino acids is defective in classic cystinuria. Because of the insolubility of cystine, urinary stones are formed. A model for tubular handling of theses amino acids must account for the following observations: (1) Whereas patients with classic cystinuria have excessive excretion of all four amino acids, patients have been described with either isolated cystinuria or hyperdibasic aminoaciduria (arginine, lysine, ornithine) without cystinuria. (2) In cystinuric patients, the clearance of cystine can exceed creatinine clearance, implying tubular secretion of cystine. (3) Renal cortical tissue slices from cystinuric patients may show no defect in taking up cystine from bathing medium, compared with slices from normal subjects.

According to the currently favored model, there are separate transport systems in the basolateral and brush border membranes. In the basolateral membrane are two uptake mechanisms - one for cystine and another for arginine, lysine, and ornithine together. Amino acids that accumulate within the cell may be secreted into tubular fluid. In the brush border are three separate reabsorptive mechanisms. One is shared by all four amino acids; the second is for arginine, ornithine, and lysine only; and the third is for cystine alone ((Broadus and Thier 1979)).

Citrate

Urinary citrate may help prevent calcium nephrolithiasis by chelating urinary calcium. Plasma citrate, present at concentrations of 0.05 mM to 0.3 mM, undergoes glomerular filtration and subsequent proximal tubular reabsorption. Citrate excretion in man ranges between 10 per cent and 35 per cent of the filtered load ((Simpson 1983)). In addition, some of the citrate that escapes filtration is taken up by the tubular cells from postglomerular blood. Citrate that enters the cell is metabolized via the citric acid cycle to CO2 and water.

Citrate excretion is profoundly influenced by systemic acid-base balance, through consequent changes in tubular cell pH. Metabolic alkalosis increase citrate excretion. A rise in cell pH inhibits citrate metabolism, leading to a rise in intracellular citrate concentration. The latter tends to inhibit citrate reabsorption. Metabolic acidosis has the opposite effect. Distal renal tubular acidosis and administration of acetazolamide are associated with hypocitruria and relatively alkaline urine. Both conditions predispose to calcium nephrolithiasis and nephrocalcinosis.

The Effects of Hormones on Renal Function

The kidney is an important endocrine organ, complementing and often interacting with its excretory functions. The major renal endocrine systems include renin-angiotensin, prostaglandin, kallikrein-kinin, erythropoietin, and vitamin D-metabolizing enzymes. These systems are involved in numerous aspects of renal function, including blood flow, salt and water balance, renin release, red blood cell formation, and calcium metabolism ((Felsen and Vaughan 1983)). These will be discussed in depth in a separate chapter. In addition, there are several hormones which directly affect renal function. Here we will discuss those hormones that have a direct effect on renal function (see also (Kurokawa 1989)).

There are two general types of receptors for a hormone: 1) peptide hormone and catecholamine receptors (cell membrane receptors); 2) Steroid hormone receptors (cell cytosol receptors). Receptors for peptide hormone and catecholamine receptors are generally on the cell membrane (Figure 2-13). Binding to these receptors results in activation of the membrane bound adenylate cyclase (possibly via G-protein mediated effector systems; Figure 2-13) which converts ATP (adenosine triphosphate) to cAMP (adenosine 3',5'- monophosphate). cAMP stimulates a class of proteins called cAMP dependent protein kinases which results in phosphorylation of proteins. This phosphorylation modifies the functional properties of either membrane or cytosolic proteins. Steroid hormone receptors are present in the cytosol of the cell. After crossing the cell membrane the hormone-receptor complex binds to the nuclear chromatin. This binding serves to modulate the level of a specific messenger RNA, thus leading to the synthesis of proteins translated by the messenger.

Hormones with cell membrane receptors

Antidiuretic hormone


Antidiuretic hormone is species specific. Arginine vasopressin (AVP) is the hormone in man and is central to maintenance of of the tonicity of body fluids as well as well as water homeostasis. At physiological concentrations, AVP increases the water permeability of the lumenal membrane of the collecting tubule (Figure 2-6). As for other peptide hormones. AVP binds to a membrane receptor on the collecting tubular cell and activates the membrane bound adenylate cyclase. Production of cAMP results in phosphorylation of luminal membrane proteins and an increase in water permeability. All the specifics of the cellular events are not well understood, the microtubule system appears to play some role.

Modulation of AVP- dependent adenylate cyclase activation occurs. The hormones that have been implicated include prostaglandins (in particular PGE2), a-adrenergic catecholamines and somatostatin. Of note, PGE2 not only affects the function of the collecting tubules but also the maintenance of medullary hypertonicity. This action is probably secondary to the effect of PGE2 on the thick ascending limb of Henle's loop (TAL). PGE2 (as well as high calcium concentrations) appear to decrease AVP-dependent cAMP formation. AVP also seems to stimulate PGE2 production thereby regulating its own action through a local feedback system.

In addition, in-vitro perfusion of the TAL of rabbit, rat and mouse kidneys has shown a stimulatory effect of AVP on NaCl reabsorption. Thus AVP may play a significant role in developing and maintaining the high medullary concentration gradient required for maximal urine concentration.

AVP is known to be a potent vasoconstrictor. In Munich-Wistar rats AVP has been shown to decrease the glomerular ultrafiltration coefficient (Kf). This is thought be mediated through mesangial cell contraction which might reduce the surface area available for glomerular ultrafiltration.

Morphologic analysis suggests that nephron heterogeneity might be induced by the presence of AVP. It is known that the glomerulus of juxtamedullary nephrons is larger and has a greater GFR then does the superficial nephrons. Brattleboro rats lack AVP ( and therefore have congenital diabetes insipidus). In these animals there is no nephron heterogeneity. However, in rats with diabetes insipidus chronic administration of AVP produces a selective increase in size and SNGFR of the juxtamedullary nephrons.

Parathyroid hormone

Parathyroid hormone (PTH) acts both on bone and kidney via activation of adenylate cyclase. PTH has three primary effects on kidney function: 1) it suppresses tubular reabsorption of inorganic phosphate, 2) enhances tubular calcium reabsorption and 3) stimulates production of 1,25(OH)2D3 from 25(OH)D3. Calcium plays a central role in the action of PTH (Kurokawa 1987). It not only acts a cellular messenger (together with cAMP) but also is an important mediator of PTH actions on the kidney.

In the kidney, the sites of action include the glomerulus, the proximal convoluted tubule (PCT), the proximal pars recta (PST), the cortical portion of the thick ascending limb of Henle's loop (CTAL), the distal convoluted tubule (DCT) and the connecting tubule(CT). In the glomerulus PTH like AVP decreases the available filtration area resulting in a decrease in Kf. Activation of adenylate cyclase by PTH also leads to inhibition of sodium-dependent phosphate reabsorption in both the PCT and PST. PTH also inhibits hydrogen ion secretion in the proximal tubule by an effect on the Na+ - H+ exchange mechanism in the brush border membrane of the proximal tubules. Gluconeogenesis, a process limited to the cells of the PCT and PST is augmented by PTH. This effect is mediated by intracellular calcium.


Calcitonin


Receptors for calcitonin appear to be located in both proximal and distal nephron segments. Calcitonin stimulates calcium reabsorption in the distal segments as well as increasing water permeability of the collecting tubules (Kurokawa 1987). Both of these actions being mediated through cAMP. Since a calcitonin sensitive adenylate cyclase has not been identified in proximal nephron segments the well documented inhibition of phosphate reabsorption by calcitonin is not mediated by cAMP. Recent data suggests that this proximal effect of PTH is mediated by PGE2.

Renin-Angiotensin-Aldosterone System

The renin-angiotensin-aldosterone (RAAS) system is one of the major renal hormonal mechanisms involved in the regulation of systemic blood pressure, sodium and potassium balance, and regional blood flow ((Laragh and Sealey 1981),(Ballermann, Zeidel et al. 1991),(Rosivall, Blantz et al. 1990)). Its role in the pathogenesis of renovascular hypertension is reviewed fully in Chapter 54. However, it is equally important to recognize the major role the RAAS plays day to day in normal homeostasis ((Vaughan 1983)). Renin is secreted in response to factors that reduce arterial blood pressure or renal perfusion, such as hemorrhage, sodium depletion, or heart failure ((Keeton and Campbell 1980)). The system reacts immediately to acute stimuli with the generation of the intense vasoconstrictor angiotensin II (AII) to restore blood pressure. Following chronic stimulation, such as sodium depletion, homeostasis is achieved by AII stimulation of the adrenal zona glomerulosa to secret aldosterone, which mediates distal tubular sodium retention FIGURE 2-14. Eventually, if effective fluid volume and tissue perfusion are restored the further release of renin is shut off. Hence, in normal subjects, plasma renin activity (PRA) rises in response to sodium depletion and falls with sodium loading ((Laragh and Sealey 1981)). Accordingly, if a normal subject exhibits a high PRA, then, by definition, he is sodium-depleted. Clinically, the PRA can be utilized as an index of sodium or volume status. Further implications of the role of AII in sodium depletion have been derived from the administration of AII antagonists to sodium-depleted animals and man. Angiotensin II blockade with saralasin or captopril in sodium-depleted normotensive rats results in a fall in blood pressure ((Gavras, Brunner et al. 1973); (Levens, Peach et al. 1981b)). Similar AII dependency of blood pressure has been shown in normotensive human subjects ((Streeton, Anderson et al. 1976)).

The effect of elevated PRA and AII on renal function during sodium depletion is of potentially greater clinical importance. In addition to the systemic vasoconstriction and aldosterone biosynthesis induced by AII during sodium depletion, homeostasis is ensured further by a direct effect on renal function to retard sodium and water loss. Kimbrough and coworkers first showed this effect by giving intrarenal saralasin or teprotide to sodium-depleted and repleted animals ((Kimbrough, Vaughan et al. 1977)). AII inhibition or blockade resulted in an increase in renal blood flow, glomerular filtration rate, and sodium excretion only in the sodium-depleted animals. The primary antidiuretic effect of AII is more complex. AII modulates water excretion in both sodium-depleted and sodium-repleted animals ((Levens, Peach et al. 1981a)).

Taken all together, the RAAS plays a major role in protecting against sodium depletion, at the expense, however, of systemic vasoconstriction and decreased organ perfusion. It is easy to envision an additive stress such as intraoperative hemorrhage or hypotension leading to further renal insult and the potential of acute renal failure in the setting of pre-existing sodium depletion.

In the kidney AII directly acts upon the renal vasculature, the glomerulus and possibly the proximal tubules. Systemically administered AII results in a decrease in renal plasma flow and GFR together with an increase in filtration fraction. Micropuncture and microperfusion studies have shown that these effects are due to an increase in efferent arteriolar resistance and a decrease in the Kf. The effect on Kf appears to be a function of mesangial cell contraction which is mediated by calcium, since calcium channel blockers block the decrease in Kf observed in response to AII.

Atrial natriuretic peptide

As discussed previously, atrial natriuretic peptide (ANP) is produced in the atrium of the heart and appears to regulate both sodium metabolism and systemic blood pressure (Brenner, Ballermann et al. 1990). In the kidney ANP increases GFR, RPF and decreases tubular sodium reabsorption. It appears that an increase GFR with ANP can occur in the absence of an effect on RPF, which may in part account for its natriuretic action. However, ANP when given at dose that does not cause an increase in GFR still results in a marked diuresis. Therefore, the effect of ANF on the glomerulus is not the only factor responsible for its natriuretic action. It might be that redistribution of blood to the vasa recta would result in an increase in medullary blood flow without an effect on overall renal blood flow. This could also account for the diuresis seen with ANF.

ANF also has a direct effect on vascular smooth muscle cells as well as the adrenal gland. In vascular smooth muscle cells ANF causes cell relaxation and vasodilation while in the adrenal gland aldosterone secretion from the zona glomerulosa is inhibited. Many of the effects of ANF on the kidney vasculature are similar to those of dopamine. In fact carbidopa, an inhibitor of dopamine synthesis as well as dopamine receptor blockade, can suppress the diuretic action of ANP. However, ANP does not change urinary dopamine excretion.

Glucagon

Given in pharmacologic doses glucagon appears to increase sodium and phosphate excretion. Glucagon is known to stimulate the adenylate cyclases of extrarenal tissues resulting in a marked rise of cAMP. This elevated CAMP is thought to result in an increased amount of cAMP in proximal tubular fluid might directly effect the luminal membrane solute transport systems.

Insulin

Insulin directly decreases urinary excretion of sodium and phosphate . Insulin receptors are present in the kidney as they are in other tissues. Autoradiographs suggest high concentrations in the glomerulus. This correlates with the renal characteristics of diabetes mellitus including enhanced glomerular basement membrane collagen biosynthesis and the increased GFR found in the early stages of diabetes mellitus. Whole kidney phosphate reabsorption is increased by the proximal tubular affect of insulin to suppress PTH-dependent cAMP production in the renal cortex. Although no convincing data is available on the site at which insulin stimulate sodium reabsorption, it appears to be distal to the proximal tubule.

Somatostatin

Somatostatin introduced either intravenously or into the renal artery induces diuresis in hydropenic animals. It also decreases the antidiuretic effect of AVP in animals undergoing antidiuresis without having a significant effect on renal blood flow GFR or osmolar clearance. Somatostatin appears to inhibit the AVP-dependent increase of cAMP production in the collecting tubules.

Catecholamines

Several components of renal function are affected by catecholamines. These include renal blood flow and distribution, GFR, renin secretion and tubular fluid reabsorption. Although the precise sites and mechanism of actions has not been defined the actions appear to be mediated through either a or b receptor stimulation.

Stimulation of a-adrenergic receptors results in vasoconstriction, while stimulation of b-adrenergic receptors is associated with adenylate cyclase activation. b-adrenergic receptor activation is also thought to be primarily responsible for renin release from the juxtaglomerular apparatus.

Both a and b-adrenergic receptors are present in the proximal convoluted tubule. Renal nerve stimulation increases proximal sodium reabsorption while renal denervation results in both a natriuresis and diuresis. Whole kidney clearance studies together with micropuncture and microperfusion of isolated tubules have demonstrated that b-adrenergic stimulation enhances while a-adrenergic stimulation inhibits sodium reabsorption in the proximal nephron.

The effect of catecholamines on renal water excretion are likely multifactorial. Norepinephrine results in a diuresis during hydropenia while isoproterenol appears to be antidiuretic. However, the systemic effects of these catecholamines result in systemic alteration in hemodynamics with baroreceptor mediated AVP release.


Hormones with cytosol receptors

Vitamin D sterols


The kidney is the major site of conversion of 25-hydroxy-vitamin D3 (25(OH)D3) to the highly active 1,25(OH)2D3 or 24,25(OH)2D3 by the mitochondrial enzymes 25-hydroxycholecalciferol-1a-hydroxylase or 25(OH)D3-24-hydroxylase respectively ((Lee, Brautbar et al. 1981)). There is a fine regulation of these two enzymes such that there is a reciprocal change in their activities. 25-hydroxycholecalciferol-1a-hydroxylase is primarily regulate by PTH. In addition, plasma calcium and phosphate as well as circulating vitamin D modify the 1a-hydroxylase activity.

Studies in both vitamin D replete as well as vitamin D-deficient rats suggests a direct stimulatory effect of 1,25(OH)2D3 on renal tubular reabsorption of calcium (Kawashima and Kurokawa 1986). In the kidney the site of action of both PTH and the vitamin D sterol on renal tubular calcium reabsorption appears to be the distal nephron. There seems to be at least two distinct cytosolic receptors for 1,25(OH)2D3 in the kidney: a vitamin D dependent calcium binding protein in the distal nephron and the 25(OH)D3-24-hydroxylase in the proximal nephron.


Aldosterone

Aldosterone is known to act on the distal nephron segments to enhance the reabsorption of sodium and excretion of potassium. Occupancy of the high affinity cytosol receptors appears to directly correlate with both sodium and potassium transport. Therefore, the greater the plasma aldosterone concentration the greater the reabsorption of sodium and excretion of potassium.

The major site of action of aldosterone appears to be the cortical and medullary collecting tubule. This has been worked out by both perfusion studies of isolated rabbit tubules as well as micropuncture studies on rat tubules. Aldosterone is known to induce certain proteins such as Na-K-ATPase and citrate synthetase. The increase in Na-K-ATPase appears to be secondary to stimulation of luminal sodium reabsorption since it is blocked by amiloride. In contrast, spironolactone not amiloride blocks induction of citrate synthetase.

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