New Insights Into the Consequences and Mechanisms of Renal Impairment in Obstructive Nephropathy

PHYSIOLOGY AND CELL BIOLOGY UPDATE

New Insights Into the Consequences and Mechanisms of Renal Impairment in Obstructive Nephropathy Saulo Klahr, MD INDEX WORDS: Glomerular filtration ratej renal plasma flowj potassiumj acidification defectj urinary tract obstruction.

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BSTRUCTIVE nephropathy refers to the renal disease resulting from impaired flow of urine or tubular fluid as a consequence of structural or functional abnormalities in the urinary tract. These abnormalities are referred to as obstructive uropathy. Obstructive uropathy is a common entity. Its exact incidence is difficult to ascertain because obstruction of the urinary tract is a manifestation of a variety of diseases that require hospitalization and surgical intervention. It has been calculated that 166 patients per 100,000 population were hospitalized in the United States in 1985 with a presumptive diagnosis of obstructive uropathy.! In 1985, slightly in excess of397,000 hospital discharge diagnoses were recorded as obstructive uropathy. In addition, there were more than 482,000 patients with a discharge diagnosis of benign prostatic hyperplasia, and it is probable that most of these individuals had been admitted to the hospital because of obstruction secondary to an enlarged prostate. The effects of obstructive uropathy in the kidney result from a variety of factors with complex interactions. z,3 Changes in renal blood flow, glomerular filtration, and tubular function occur. Most of the information about the consequences of urinary tract obstruction on kidney function and structure has been derived from functional, biochemical, and histological studies in experimental animals. Most studies have examined the effects of complete, short-term ureteral obstruction «36 hours' duration) on renal function. Both bilateral ureteral obstruction (BUO) and unilateral ureteral obstruction (UUO) have been studied, and it is apparent that the two models differ significantly. 3 The effects of partial, longstanding obstruction of the urinary tract have been less comprehensively studied. For the sake of clarity, the alterations in renal function in response to obstruction are described in the dis-

cussion below as affecting either glomerular function or tubular function. However, it is important to realize that such alterations are interdependent. GLOMERULAR FUNCTION IN OBSTRUCTIVE NEPHROPATHY

Changes in Renal Blood Flow After Acute Ureteral Obstruction Acute ureteral obstruction causes a transient increase in blood flow to the kidney, followed by progressive vasoconstriction. This initial increase in renal blood flow is due to afferent arteriolar dilatation and is mediated by intrarenal mechanisms, as indicated by its occurrence in the denervated kidney4 and in the isolated perfused kidney.5 The decrease in afferent arteriolar resistance results in an increased glomerular capillary pressure, which partially offsets the decrease in net filtration pressure resulting from the increase in proximal tubular pressure.6 Thus, in the initial phases of acute obstruction, single-nephron glomerular filtration rate (SNGFR) is maintained at approximately 80% of preobstruction values despite the marked increase in proximal tubular pressure. Most of the evidence indicates that local production of eicosanoids, mainly prostacyclin and PGEz, may account for the increased renal blood flow observed after the onset of obstruction. This increase in blood flow can be eliminated by From the Department of Medicine. Jewish Hospital ofSt Louis at Washington University Medical Center, St Louis,

Mo.

Supported by US Public Health Service National Institute of Diabetes and Digestive and Kidney Diseases Grants No. DK-09976, DK-07126. and DK-40321 . No reprints available. Address correspondence to Saulo Klahr, MD. Simon Professor ofMedicine, Director. Department ofMedicine. Jewish Hospital of St Louis. 216 S Kingshighway. St Louis. MO 63110. © 1991 by the National Kidney Foundation. Inc. 0272-6386/91/1806-0011$3.00/0

American Journal of Kidney Diseases, Vol XVIII. No 6 (December), 1991 : pp 689-699

689

690

indomethacin administration. 7,s Obstruction at the single-nephron level (wax block placed in the proximal tubule) generated an identical glomerular hemodynamic response. 9 This was attributed to disruption of tubuloglomerular feedback due to acute cessation of distal tubular fluid to the macula densa.1O However, Ichikawa II found that glomerular blood flow did not increase if proximal tubule pressure was maintained within the normal range. This suggests that the changes in glomerular hemodynamics are related to intratubular dynamics, rather than to interruption of fluid delivery to the distal tubule. Usually 3 to 5 hours after the onset of obstruction, there is an increase in intrarenal resistance. Micropuncture l2 and microspherel 3 studies indicate that this vasoconstriction is due predominantly to an increase in the resistance of afferent arterioles. The increase in resistance is mediated by several vasoconstrictors, including angiotensin 11,14 thromboxane A2,15 and antidiuretic hormone (ADH).16 In addition, decreased production of endothelium-derived relaxing factor (EDRF), a vasodilator, may also have a role in the increased vascular resistance that occurs 3 to 5 hours after the onset of obstruction (unpublished observations). Changes in Glomerular Filtration Following the Onset of Obstruction Glomerular filtration rate (GFR) declines progressively following the onset of complete ureteral obstruction. Changes in ureteral pressure appear to be instantaneously reflected in changes in proximal tubular pressure, the latter always being higher than the former. 17 The magnitude of the proximal tubular pressure increase depends on (1) the degree of hydration of the animal, the increase in pressure being greater when the animals are saline-loaded; and (2) whether one or both kidneys are obstructed. Within 1 hour of ureteral obstruction, intratubular pressure increases ls ; simultaneously, glomerular capillary hydraulic pressure increases. However, this increase is not proportional to the increase in intratubular pressure. Consequently, a decrease in the net hydraulic pressure gradient across glomerular capillaries occurs, resulting in a decline in GFR. This is the major factor responsible for the decline in GFR after the onset of obstruction.

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Within 4 to 5 hours after the onset of obstruction, proximal pressure declines. In animals with UUO of 24 hours' duration, intratubular pressures are lower than, or equal to, values before obstruction; however, this does not result in an effective filtration pressure, since intraglomerular capillary hydraulic pressure decreases at an even faster rate and to levels below preobstruction. 12 After BUO, proximal tubular pressures are about twofold higher than with unilateral ureteral obstruction. 6 Over the course of 24 hours of obstruction, the values decrease, but not to baseline levels. At the end of 24 hours, glomerular capillary pressure is not different from preobstruction values. Thus, in this setting, high intratubular pressure contributes significantly to the decrease in GFR. In both models of obstruction (unilateral or bilateral ureteral ligation), the decrease in whole kidney GFR seen after release of obstruction is due to both a decrease in SNGFR and a decrease in the number of filtering nephrons 3 (Fig 1). The decrease in SNGFR is due to a decrease in plasma flow per nephron, a decrease in net hydrostatic pressure, and a decrease in the filtration coefficient, K[, presumably due to a decrease in the surface area available for filtration. Three major vasoconstrictors ofthe renal circulation, angiotensin II, thromboxane A2, and ADH, playa role in the decrease in renal plasma flow per nephron and in the decrease in SNGFR that occurs after ureteral obstruction. 14-16 Besides being potent vasoconstrictors, both angiotensin 11 19 and thromboxane Alo have been shown to

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Fig 1. Total kidney GFR, SNGFR, and percent of filtering nephrons in rats 3 to 4 hours after release of UUO of 24 hours' duration. The decrease in total kidney GFR is due to both a decrease in single nephron GFR and a decrease in the percent of filtering nephrons.

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OBSTRUCTIVE NEPHROPATHY: NEW INSIGHTS

contract mesangial cells in culture and, therefore, can potentially reduce the glomerular capillary area available for filtration. Administration of inhibitors ofthromboxane synthesis to rats before obstruction significantly increased GFR and effective renal plasma flow after unilateral release of BUO of 24 hours' duration when compared with values obtained in untreated rats with BUO. 15 Micropuncture studies have localized the site of action of thromboxane A2 to the afferent and efferent glomerular arterioles. 21 Pretreatment of rats with enalapril, an angiotensin-converting enzyme (ACE) inhibitor, before and during obstruction, significantly increased GFR and effective renal plasma flow. 15 Moreover, simultaneous inhibition of both angiotensin II and thromboxane A2 production markedly increased GFR and renal plasma flow in the postobstructed kidney, but not to normal levels, suggesting that the increased vasoconstriction of the renal microcirculation may be related to the effect of additional vasoconstrictor substances. 15 It appears that ADH contributes to the renal vasoconstriction and the decrease in GFR observed in rats with BUO. 16 We found that rats with BUO of 24 hours duration had significantly higher plasma values of ADH than sham-operated rats (Fig 2). Pretreatment with a specific antagonist of the Vi type receptor for ADH significantly increased GFR and effective renal plasma flow and decreased mean arterial blood pressure in rats with BUO. These results suggest that high levels of circulating ADH may play an important role in the decrease in GFR and effective renal plasma flow observed in rats with BUO of 24 hours' duration. Leukotrienes, potent mediators of inflammation, are synthesized by cells through the 5-lipoxygenase pathway. We found increased synthesis of leukotriene B4 in isolated glomeruli from rats with BUO. Inhibition of the 5-lipoxygenase pathway in vivo ameliorated the decrease in GFR and in effective renal plasma flow seen after unilateral release of BUO. 22 We have recently examined the potential contribution of EDRF to the changes in glomerular filtration and effective renal plasma flow observed after unilateral release ofBUO (unpublished observations). Since the kidney is the major site of synthesis of arginine from citrulline for export to other organs and since arginine is the precursor

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Fig 2. Plasma levels of ADH (vasopressin) in normal control rats and in rats with SUO of 24 hours' duration.

in the generation of nitrous oxide (EDRF), we postulated that arginine may be rate-limiting for the synthesis of EDRF in rats with BUO. The role of EDRF in postobstructed kidneys was examined by using W-nitro-L-arginine methyl ester (W-NAME), a competitive inhibitor of the synthesis of nitrous oxide from arginine. We also examined the effects of arginine administration immediately after the release of obstruction on glomerular filtration and effective renal plasma flow. Arginine administration markedly increased GFR and effective plasma flow in the obstructed kidney. On the other hand, administration of the competitive inhibitor of arginine after release of obstruction resulted in a marked decrease in GFR and effective renal plasma flow. Rats with BUO pretreated with the competitive inhibitor of arginine were anuric after release of the obstruction. The results of these studies suggest a decreased availability of the substrate for EDRF synthesis during BUO. The results can also be interpreted to indicate that decreased EDRF activity during obstruction plays a role in the hemodynamic changes observed after release of BUO of 24 hours' duration. Recent data also suggest that platelet-activating factor (PAF) has a vasodilatory role in obstructive nephropathy.23 Ureteral obstruction increases renin secretion. 2,24 Increased renin release may result from stimulation of intrarenal mechanisms due to reduced delivery of sodium and chloride to the macula densa or to a reduction in transmural pressure at the baroreceptor as a consequence of the dilatation of the afferent arteriole described above. This increase in renin release is almost

692

completely abolished by pretreatment with the cyclooxygenase inhibitors indomethacin or meclofenamate. 8,25 Thus, vasodilatory eicosanoids, such as prostacyclin or PGE2, playa role in renin release from juxtaglomerular cells. 26 The increased renin secretion leads to increased intrarenal production of angiotensin II. At least two sources appear to account for the increased synthesis of thromboxane in the obstructed kidney, infiltrating leukocytes27 and intrinsic glomerular cells. 28 Infiltrating leukocytes consist mainly of macrophages and T cells and are described in more detail below. It has also been shown that 24 hours after the onset of obstruction, there is increased production of vasodilatory eicosanoids, particularly PGE 2 and prostacyclin. This increased production of vasodilatory eicosanoids has been found to be mediated by angiotensin II through increased activity of phosphoethanolamine-specific phospholipase A 2, and increased activity of cyclooxygenase. 29 These vasoactive eicosanoids are functionally important, since in the setting of inhibition of thromboxane A 2, administration of inhibitors of the cyclooxygenase markedly decreases effective renal plasma flow and GFR. 14

Infiltration of the Renal Parenchyma by Leukocytes in Obstructive Nephropathy A mononuclear cell infiltrate was initially described in the renal parenchyma of rabbits with chronic unilateral ureteral obstruction. 30 Subsequent studies linked the increased production of PGE 2 by the obstructed kidney of the rabbit to this leukocyte infiltrate. 31 It was suggested, but not proven, that the invasion of the renal parenchyma by mononuclear cells may be the mechanism responsible for the increased release of thromboxane A2 and PGE2 in response to the administration of bradykinin or endotoxin. 32 We have studied the infiltration of the kidney by leukocytes in a model of acute ureteral obstruction in the rat. 33 The leukocyte influx was observed shortly after the onset of ureteral obstruction. An infiltrate was already present after 4 hours of obstruction, but its peak response occurred after 24 hours. In many instances, the invading mononuclear cells formed distinctive rings around tubular cells, particularly distal tubules. The significance of this observation has not been

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clarified. It should be noted that normal kidneys have only a small number of resident macrophages in the renal cortex, located mainly in glomeruli. 34 On the other hand, the normal medulla is completely devoid of resident leukocytes. In the obstructed kidney, mononuclear cells were present in both the cortex and the medulla. At the same time, a depletion of resident glomerular macrophages was observed. The fate ofthese resident mesangial macrophages has not been established. Macrophages comprise most of the leukocyte infiltrate; the second most abundant cells are T lymphocytes of the cytotoxic suppressor cell subclass. 33 The finding that T lymphocytes of the helper type do not account for a significant portion of the infiltrate despite the fact that they predominate in the peripheral'circulation suggests a certain degree of selectivity. After 24 hours of ureteral obstruction, there was no evidence for the presence of B lymphocytes or neutrophils in rat kidney. The infiltrate disappeared slowly after release of obstruction. The levels did not return to normal until several days following the release of obstruction. The macrophage content of the cortical interstitium increased modestly 2 days after release of obstruction and then decreased to levels seen before obstruction by 6 days after release of the obstruction. On the other hand, T lymphocytes in the cortex diminished rapidly to less than 20% of their values during obstruction within 2 days of release of ureteral ligation. However, as late as 1 week after release of obstruction, a small increase in both cell populations was present when compared with the number ofleukocytes in normal kidneys.

Pathophysiological Role of Infiltrating Leukocytes The arrival of the macrophages and T lymphocytes in the kidney coincides with a decline in renal blood flow and GFR. As mentioned above, after the onset of obstruction there is a transient increase in renal blood flow that is mediated by prostaglandins. Four to 5 hours after the onset of obstruction, renal blood flow diminishes and, by 24 hours, the values for renal plasma flow are 40% to 70% of those observed before obstruction. One of the mechanisms responsible for this decrease in renal blood flow is related to increased release of thromboxane A2. The ob-

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OBSTRUCTIVE NEPHROPATHY: NEW INSIGHTS

structed kidney displays an enhanced capacity to metabolize arachidonic acid and the activity of both cyclooxygenase and thromboxane synthase is increased. The administration of thromboxane synthase inhibitors improves postobstructive renal hemodynamics and reverses partially the renal vasoconstriction of acute ureteral obstruction. 14,15,21 The role of infiltrating cells in the decrease in renal plasma flow and GFR seen with ureteral obstruction was studied by subjecting rats to total body irradiation before obstruction. 27 Irradiation abolished the leukocyte infiltration in the obstructed kidney (Fig 3). Irradiation had no effect on renal morphology or function in normal rats.27 By contrast, elimination of the infiltrate by prior irradiation in rats with BUO markedly decreased thromboxane excretion in the urine and significantly increased renal plasma flow and GFR in the postobstructed kidney, as reflected by greater inulin and PAH clearances (Fig 4). This suggests that the infiltrating leukocytes play an important role in the hemodynamic changes observed in the postobstructed kidney. It has been proposed that the leukocyte infiltrate accounts in part for the decline in GFR and renal plasma flow seen after obstruction, possibly via the production of thromboxane A2. However, elimination of the leukocyte infiltrate from the renal parenchyma does not restore the function of the postobstructed kidney to that 30 ~ (-) rads (+) rads Control

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Fig 3. Effect of irradiation on leukocyte content of rat kidneys with ureteral obstruction of 24 hours' duration. The stippled bars represent the leukocyte content per gram of tissue in rats with obstruction, but not subjected to irradiation. The black bars refer to those animals with BUO that were irradiated before the onset of obstruction. The horizontal bars indicate the level of leukocytes present in normal kidneys. Irradiation markedly decreased the levels of infiltrating leukocytes to levels comparable to or below those seen in normal animals.

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Fig 4. Effect of irradiation on inulin and p-aminohippurate clearances after unilateral release of BUO in rats. Irradiation of the rats before obstruction resulted in significantly greater values for both inulin clearances (Cln ) and p-aminohippurate clearances (C PAH) after release of obstruction of 24 hours' duration. Nonirradiated animals are represented by the black bars and irradiated animals by the stippled bars.

of kidneys of normal animals. This suggests that mechanisms other than leukocyte infiltration also have a role in postobstruction kidney function. Irradiation of animals with ureteral obstruction does not reduce thromboxane excretion in the urine to the levels seen in normal animals. This is consistent with the proposal that obstruction causes enhanced production of this vasoactive prostanoid by structures intrinsic to the kidney, such as glomerular cells. 28 It is likely that production ofthromboxane A2 by intrinsic renal cells may also modulate renal hemodynamics. Isolated glomeruli from rats with BU028 or glomeruli from the experimental kidney of rats with UUO produce greater amounts ofPGE2, 6keto PGF 1a , the stable metabolite ofprostacyclin, and thromboxane B2, the stable metabolite of thromboxane A2, than glomeruli isolated from kidneys of normal animals. Since, as mentioned above, endogenous macrophages from glomeruli are depleted in the obstructed kidney, the increased synthesis ofthromboxane B2 in these isolated glomeruli is most likely related to its production by intrinsic glomerular cells. Rats treated with an ACE inhibitor before obstruction had levels of eicosanoid production comparable to those seen in normal rats. This suggests that endogenous angiotensin II modulates the increased synthesis of eicosanoids in isolated glomeruli from rats with BUO. The activities of both phosphoethanolamine-specific phospholipase A2 and

694

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cyclooxygenase are increased in glomeruli from rats with BVO. 29 Pretreatment of the rats with an ACE inhibitor restores the activities of both enzymes to levels similar to those seen in shamoperated rats. This suggests that the breakdown of phospholipids and the increased generation of eicosanoids are related to the increased intrarenal levels of angiotensin II. The above studies indicate that the increased thromboxane excretion in the urine of animals with BVO has two origins: increased release by intrinsic renal cells28 and production by macrophages27 that invade the renal parenchyma during obstruction. The long-term effects of this mononuclear cell infiltrate on renal function and structure are not known. It has not been established that substances other than thromboxane A2 that would affect epithelial cell function are released by these infiltrating macrophages. 35 Focal glomerulosclerosis is a common pathological finding in patients with ureteropelvic obstruction. 36 This focal segmental glomerulosclerosis is usually present in areas closely associated with intense interstitial and periglomerular inflammation. Growth factors released by invading leukocytes may have a role in the development and progression of the fibrosis and sclerosis that occur in the chronically obstructed kidney.37 Thus, the cellular infiltrate may contribute to the renal damage and to the progressive decrease in renal function observed with chronic urinary tract obstruction. The specific contribution, if any, of suppressor T lymphocytes present in the infiltrate to the functional changes that occur in obstruction requires further study. The mechanisms responsible for the influx of mononuclear cells and T lymphocytes into the renal parenchyma have not been completely elucidated. Initial studies indicate that obstructed kidneys release a chemoattractant substance. This substance has been shown in vitro to be a chemoattractant for mononuclear cells, specifically macrophages. The initial evidence also indicates that this chemoattractant is a lipid. 38 TUBULAR ABNORMALITIES IN OBSTRUCTIVE NEPHROPATHY

Several abnormalities in tubular function have been described in obstructive nephropathy. These abnormalities include decreased reabsorption of

solutes and water, impaired excretion of hydrogen and potassium, and inability to concentrate the urine. Reabsorption of Solutes and Water

Despite a decrease in GFR, absolute sodium excretion is comparable in the postobstructed kidney and the contralateral untouched kidney of rats with VVO. 39 Fractional sodium excretion is greater in the postobstructed kidney than in the contralateral control kidney. In addition, fractional water excretion is increased. Micropuncture experiments have demonstrated increased reabsorption of salt and water in proximal segments of the postobstructed kidney.39 On the other hand, micropuncture studies of deep nephrons suggest a marked decrease in salt reabsorption in proximal segments of juxtamedullary nephrons. 39 Also, perfusion of isolated nephron segments in vitro has shown decreased reabsorption of salt and water in proximal tubules of juxtamedullary nephrons, but not in proximal tubules of surface nephrons. 40 The fluid reaching the bend of the loop of Henle in the postobstructed kidney is markedly increased as compared with the amount of fluid in the contralateral untouched kidney. This may be the consequence of both decreased salt and water reabsorption in proximal segments, as well as decreased abstraction of water in the thin descending limb of Henle due to a decrease in the medullary gradient. The excretion of salt and water is greater in animals with unilateral release of BVO than in rats with release OfVVO.41 Reabsorption of salt and water is decreased in the proximal tubule of surface nephrons of rats with BVO. In addition, microperfusion experiments40 have shown decreased reabsorption of sodium chloride in the thick ascending limb of postobstructed kidneys, which will result in decreased deposition of solute in the interstitium of the medulla and lessen the osmolality of the medulla as a consequence of decreased sodium chloride deposition. The differences in salt and water excretion from unilaterally and bilaterally obstructed kidneys (Table 1) may relate to urea retention, hyperkalemia, and retention of other impermeable solutes in the latter model. Also, the levels of atrial peptide in the circulation are markedly greater in animals with BVO than in those with VV0 42 (Fig 5). Hence,

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OBSTRUCTIVE NEPHROPATHY: NEW INSIGHTS

the plasma levels of urea and atrial peptide may contribute to the greater natriuresis and diuresis observed after BVO. The mechanisms underlying the decreased reabsorption of sodium are thought to be due at least in part to changes in the activity of Na,Kadenosine triphosphatase (ATPase) in the nephron. Wilson et al43 found reduced Na,K-ATPase activity in cortical and medullary homogenates of rat kidney I to 7 days after relief of either VUO or BVO and correlated this change with alterations in sodium reabsorption and postobstructed diuresis. Williams et al44 also reported a reduction in Na,K-A TPase after ureteral obstruction in the dog. Sabatini and Kurtzman,45 using tubular segments, described a decreased Na,K-ATPase activity throughout several nephron segments after 24 hours of VUO. We have recently examined the potential mechanisms underlying these reported decreases in Na,K-ATPase. 46 The Na,K-ATPase activity was markedly reduced in basolateral membrane vesicles prepared from the cortex of the obstructed kidney of rats with VVO of 24 hours' duration when compared with basolateral membrane vesicles obtained from the contralateral kidneys of the same rats or to basolateral membrane vesicles from kidneys of sham-operated animals. However, such differences disappeared 3 days after release of VVO. This restoration of Na,K-ATPase activity in basolateral membranes, 3 days after release of obstruction, differs from results published previously using whole kidney homogenates. The reason for these differences is not immediately apparent, but may relate to the methodology used. When basolateral membrane vesicles were incubated with sodium dodecyl sulfate to permeabilize the vesicles, no differences in the degree of

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Fig 5. Plasma levels of atrial peptide (pg/mL) in control (sham-operated) rats with (first open ba.) or without (second open bar) anesthesia and in rats with UUO or BUO of 24 hours' duration. No significant differences in the levels of atrial peptide in plasma were observed between sham-operated animals under anesthesia or in the awake state and animals with unilateral ureteral obstruction. By contrast, a significant increase in the levels of atrial peptide was observed in animals with bilateral ureteral obstruction. (Reprinted with permission. 42)

enzyme latency were detected between the basolateral membrane vesicles from obstructed kidneys and those from sham-operated rats. Immunoblotting techniques and analysis via antibodies to the a-subunit of Na,K-ATPase showed equal amounts of enzyme in the basolateral membrane vesicles from contralateral kidneys, obstructed kidneys, and kidneys from sham-operated rats. When incubated with liposomes under conditions conducive to fusion and lipid exchange, basolateral membrane vesicles from obstructed kidneys demonstrated Na,K-A TPase activity reconstituted almost to normal levels. Such an increase in enzyme activity did not occur in basolateral membranes from contralateral kidneys or in membranes from kidneys from

Table 1. A Comparison of Tubular Abnormalities After Release of UUO or BUO in Rats

Concentrating defect Absolute sodium and water excretion Fractional sodium and water excretion Absolute potassium excretion Fractional potassium excretion

uuo

BUO

400 mOsm/kg H20 Comparable to contralateral kidney

400 mOsm/kg H20 Increased

Increased (t) compared with contralateral kidney Decreased (~H> Decreased (~)

Increased

(ttt)

m

Decreased Increased (tt)

696

sham-operated rats incubated under similar conditions. Thus, the reduction in Na,K-ATPase activity of ureteral obstruction seems not to be related to reduced quantity of the enzyme nor to its sequestration in impermeable vesicles, but to changes in the lipid environment of the basolateral membrane. It should be noted that lipid changes occur in the kidney after 24 hours of obstruction. Tannenbaum et al47 noted an increase in triglycerides and a decrease in phospholipid content after 24 hours of ureteral occlusion in the rat. There was also increased incorporation of 14C-oleic or 14C_ arachidonic acid into triglycerides in both cortex and medulla. Morrissey etal48 also reported decreases in phospholipids and cholesterol in renal tubular membranes. Some of these lipid changes may underlie the decrease in Na,K-ATPase observed in obstruction. Impaired ability to concentrate the urine is evident after relief of obstruction in rats with either UUO or BUO. 39,41 Vasopressin administration does not reverse the defect. 40,49 A decrease in the solute content of the papillary interstitium may be a major factor. 39,41 One definite contributing factor is a relative increase in blood flow to papillary structures. Although GFR of deep nephrons can be reduced by 50% to 60% during obstruction, inner medullary plasma flow either does not change from control levels or increases slightly. The hyperosmolality of the medulla is generated by the greater reabsorption of solutes than water from the thick ascending limb of Henle's loop. Thick ascending limbs of animals with ureteral obstruction when perfused in vitro demonstrate decreased sodium chloride reabsorption. 40 A decrement in the medullary tonicity decreases water efflux from the descending limb of Henle's loop and increases the amount of fluid delivered to the bend of the loop after relief of obstruction. This, in tum, may decrease the sodium chloride gradient necessary for the passive efflux of sodium chloride from the thin ascending limb of the loop. In addition, a decrease in medullary solute content will decrease the diffusion of water out of the collecting duct. The hydroosmotic response to vasopressin is decreased in the cortical collecting duct. 4o,49 When this segment of the nephron was perfused in vitro, after dissection from kidneys with ure-

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teral obstruction, it was noted that the hydroosmotic response to both vasopressin and cyclic adenosine monophosphate (cAMP) was markedly decreased. Changes in the levels of G proteins and a decrease in cAMP generation after ADH seem to underlie part of the defective responsiveness of the cortical collecting duct to vasopressin. However, post-cAMP events also appear to have a role in the altered response to ADH. Thus, the concentrating defect in obstruction is presumably due to a decrease in the number of juxtamedullary nephrons; a decreased removal of solute from the thick ascending limb of Henle's loop in functional juxtamedullary nephrons; washout of solute from the medulla due to increased medullary flow; and a decr~ased hydroosmotic response of the cortical collecting duct to ADH.

Potassium and Hydrogen Excretion in Obstruction The fractional excretion of potassium is less in patients with obstructive uropathy than in patients with comparable degrees of renal insufficiency due to a variety of renal diseases. Hyperkalemic/hyperchloremic acidosis is observed in patients with chronic obstructive uropathy. 50 Three major mechanisms may explain how it develops: (1) a defect in renal hydrogen ion secretion, so that the pH of the urine cannot be lowered maximally in the presence of systemic acidosis and the urinary excretion of both ammonium and titratable acid is decreased (type IV distal renal tubular acidosis); (2) a defect in aldosterone secretion probably secondary to diminished production of renin by the kidney (hyporeninemic hypoaldosteronism); and (3) a combination of these two defects. Decreased secretion of potassium may be due to altered sensitivity of the distal tubule to aldosterone. Batlle et al 50 postulated that a defect in sodium reabsorption in the distal nephron results in decreased intraluminal negative potential difference. This voltage-dependent defect could account for decreases in both potassium and hydrogen secretion. Kimura and Mujais reported a progressive decrease in Na-K pump turnover in situ in intact cortical collecting ducts of rats with unilateral ureteral obstruction. 51 This was associated with an inability of the obstructed kidney to excrete

OBSTRUCTIVE NEPHROPATHY: NEW INSIGHTS

an acute potassium load. It was suggested that impaired in situ turnover of the Na,K pump contributes significantly to the abnormal potassium excretion that accompanies obstructive uropathy. An inability to acidify the urine is seen after release of BUO or UUO in both humans and experimental animals. 52-57 Moreover, a distal renal tubular acidosis with inability to lower the urine pH to normal minimum values in response to acidemia is common in patients with obstruction uropathy.53,54 Although this acidifying defect is reversible in most instances, it may persist in some patientsY The inability to acidify the urine, similar to that seen in humans, has been found after release of ureteral obstruction in both the rat 58 and the dog.59 The urine from the postobstructed kidney of rats had a pH greater than 7.0, as compared with a pH of 5.5 to 6.0 in the contralateral kidney. Micropuncture studies and bicarbonate titration studies after release of UUO found no decrease in proximal reabsorption of bicarbonate in the postobstructed kidney. 58 Urine Pe02 values in the postobstructed kidney did not increase appropriately after bicarbonate loading. The results suggested an acidification defect located in distal segments of the nephron. Laski and Kurtzman 60 found a decrease in bicarbonate transport in perfused nephron segments from rabbits with ureteral obstruction. This defect was more manifest in the medullary collecting duct segment. As obstruction was prolonged, there was a slight fall in the reabsorption of bicarbonate in cortical segments. These data suggest that the major site responsible for the acidifying defect is in medullary segments. Using an indirect method to evaluate the activity of the hydrogen ATPase, Sabatini and Kurtzman 45 found that the activity of the ATPase was markedly reduced in animals with acute ureteral obstruction. The n-ethylmaleimide (NEM)-sensitive ATPase activity was significantly decreased in cortical collecting ducts, but to a lesser degree than that seen in medullary collecting duct segments. These findings suggest, but do not establish, an abnormality in acidification located in both the cortical collecting duct and the medullary collecting duct, with the major defect in acidification at the latter segment. Purcell et aI, using monoclonal antibodies to the 3 1kd subunit of the H+-ATPase, have demonstrated, by histochemical techniques, a loss of cell

697

convexity and a decrease in the apical staining for H +-A TPase in intercalated cells of rats with ureteral obstruction of 24 hours duration 61 (and unpublished observations). The decrease in staining was interpreted as a loss or removal of H+-ATPase from the apical border of intercalated cells during the period of obstruction. Three days after release of obstruction of24 hours' duration, the cell convexity was restored, but the staining for H +-ATPase in the apical border was still abnormal. Five and 10 days after relief of obstruction, the staining for H +-ATPase in the apical border was normal and this was accompanied by urine pH values in the postobstructed kidney similar to those in the contralateral control kidney. The results indicate that a decreased number of H+-ATPase pumps in the apical surface of intercalated cells may account for the acidifying defect seen after release of ureteral obstruction. The events leading to the removal of H + pumps from the apical membrane of acid-secreting cells have not been elucidated. It is also of interest that Purcell et al noted a decrease in the number of intercalated cells with more prolonged periods of ureteral obstruction (3 to 5 days). In summary, changes in the major enzymes responsible for sodium reabsorption, potassium secretion, and hydrogen secretion occur in obstructive uropathy. Such changes may account for the altered reabsorption and secretion of the elements these enzymes mediate. Furthermore, at least for Na,K-ATPase, a change in the lipid environment of the enzyme appears to account for most of the decrease in enzyme activity observed. The mechanisms underlying the decreased activity and the removal of H+ -ATPase from apical segments of intercalated cells remain to be determined. Although considerable progress has been made in understanding the mechanisms responsible for some of the changes in renal plasma flow, GFR, and tubular function in obstruction, much remains to be learned about the biochemical and molecular events that convert a physical signal (increased intratubular pressure) into a series of complicated events that encompass marked alterations in the activity of several enzymes, decreased synthesis of certain compounds, increased synthesis of others, and an immunological response with invasion of the renal parenchyma by macrophages and T cells, pre-

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sumably mediated by a specific lipid chemoattractant. Elucidation of the intermediate steps by which increased pressure is transformed into this cascade of biochemical and cellular events would

provide a more detailed picture and a greater understanding of the mechanisms by which obstruction results in progressive renal damage and loss of renal function.

REFERENCES I. National Kidney and Urologic Diseases Advisory Board: The Scope and Impact of Kidney and Urologic Diseases, Chapter I in Long-Range Plan. Washington, DC, US Government Printing Office, 1990, pp 7-35 2. Klahr S, Harris KPG: Obstructive nephropathy, in Seldin DW, Giebisch G (eds): The Kidney (ed 2). New York, NY Raven, (in press) 3. Klahr S, Harris KPG, Purkerson ML: Effects of obstruction on renal function. Pediatr NephroI2:34-42, 1988 4. Vaughan ED Jr, Shenasky JH, Gillenwater JY: Mechanism of acute hemodynamic response to ureteral occlusion. InvestUroI9:109-118, 1971 5. Navar LG, Baer PG: Renal autoregulatory and glomerular filtration responses to graduated ureteral obstruction. Nephron 7:301-316, 1970 6. Wright FS: Effects of urinary tract obstruction on glomerular filtration rate and renal blood flow. Semin Nephrol 2:5-16, 1982 7. Allen JT, Vaughan ED Jr, Gillenwater JY: The effect of indomethacin on renal blood flow and ureteral pressure in unilateral ureteral obstruction in awake dogs. Invest Uro115: 324-327, 1978 8. Blackshear JL, Wathen RL: Effects of indomethacin on renal blood flow and renin secretory responses to ureteral occlusion in the dog. Miner Electrolyte Metab 1:271-278, 1978 9. Tanner GA: Effects of kidney tubule obstruction on glomerular function in rats. Am J Physiol 237:F379-F385, 1979 10. Bricker NS, Klahr S: Obstructive nephropathy, in Strauss MB, Welt LG (eds): Diseases of the Kidney, vol 2. Boston, MA, Little Brown, 1971 , pp 997-1037 II. Ichikawa I: Evidence of altered glomerular hemodynamics during acute nephron obstruction. Am J Physiol 242: F580-F585, 1982 12. Dal Canton A, Corradi A, Stanziale R, et al: Effects of 24-hour ureteral obstruction on glomerular hemodynamics in rat kidney. Kidney Int 15:457-462, 1979 13. Gaudio KM, Seigel NJ, Hayslett JP, et al: Renal perfusion and intratubular pressure during ureteral occlusion in the rat. Am J Physiol 238:F205-F209, 1980 14. Yarger WE, Schocken DD, Harris RH: Obstructive nephropathy in the rat: possible roles for the renin-angiotensin system, prostaglandins, and thromboxanes in postobstructive renal function. J Clin Invest 65:400-412, 1980 15. Purkerson ML, Klahr S: Prior inhibition of vasoconstrictors normalizes GFR in postobstructed kidneys. Kidney Int 35:1305-1314, 1989 16. Reyes AA, Robertson G , Klahr S: Role of vasopressin in rats with bilateral ureteral obstruction. Proc Soc Exp BioI Med 197:49-55, 1991 17. Gottschalk CW, Mylle M: Micropuncture study of pressures in proximal tubules and peritubular capillaries of rat kidney and their relation to ureteral and renal venous pressures. Am J Physiol 185:430-439, 1956

18. Andreucci VE, Herrera-Acosta J, Rector FC Jr, et al: Effective glomerular filtration pressure and single nephron filtration rate during hydropenia, elevated ureteral pressure, and acute volume expansion with isotonic saline. J Clin Invest 50:2230-2234, 1971 19. Ausiello DA, Kreisberg 11, Roy C, et aI: Contraction of cultured rat glomerular cells of apparent mesangial origin after stimulation with angiotensin II and arginine vasopressin. J Clin Invest 65:754-760, 1984 20. Mene P, Dunn MJ: Contractile effects of TxA2 and endoperoxide analogues on cultured rat glomerular mesangial cells. Am J Physiol 251 :FI029-FI035, 1986 21. Ichikawa I, Purkerson ML, Yates J, et al: Dietary protein intake conditions the degree of renal vasoconstriction in acute renal failure caused by ureteral obstruction. Am J Physiol 249:F54-F61, 1985 22. Klahr S, Lefkowith J, Reyes AA: Leukotrienes (LTs) have a role in the decrease of GFR and ERPF of obstructive nephropathy. J Am Soc Nephrol 1:444, 1990 (abstr) 23. Reyes AA, Klahr S: The role of platelet-activating factor (PAF) in obstructive nephropathy. J Am Soc Nephroll:602, 1990 (abstr) 24. Kaloyanides GJ, Bastron RD, DiBona GF: Effect of ureteral clamping and increased renal arterial pressure on renin release. Am J PhysioI225:95-99, 1973 25. Cadnapaphornchai P, Aisenbrey G , McDonald KM, et al: Prostaglandin-mediated hyperemia and renin-mediated hypertension during acute ureteral obstruction. Prostaglandins 16:965-971 , 1978 26. Gerber JG, Olson RD, Nies AS: Interrelationship between prostaglandins and renin release. Kidney Int 19:816821 , 1981 27. Harris KPG, Schreiner GF, Klahr S: Effect ofleukocyte depletion on the function of the postobstructed kidney in the rat. Kidney Int 36:210-215, 1989 28. Yanagisawa H , Morrissey J, Morrison AR, et al: Role of angiotensin II in eicosanoid production by isolated glomeruli from rats with bilateral ureteral obstruction. Am J Physiol 258:F85-F93, 1990 29. Morrissey J, Yanagisawa H , Klahr S: Mechanism of enhanced prostanoid production by glomeruli from rats with bilateral ureteral obstruction (BUO). J Am Soc Nephrol I: 446, 1990 (abstr) 30. Nagle RB, Johnson ME, Jervis HR: Proliferation of renal interstitial cells following injury induced by ureteral obstruction. Lab Invest 35:18-22, 1976 31 . Okegawa T, Jonas PE, DeSchryver K, et al: Metabolic and cellular alterations underlying the exaggerated renal prostaglandin and thromboxane synthesis in ureter obstruction in rabbits. Inflammatory response involving fibroblasts and mononuclear cells. J Clin Invest 71 :81-90, 1983 32. Lefkowith JB, Okegawa T, DeSchryver-Kecskemeti K,

OBSTRUCTIVE NEPHROPATHY: NEW INSIGHTS

et al: Macrophage-dependent arachidonate metabolism in hydronephrosis. Kidney Int 26: 10-17, 1984 33. Schreiner G, Harris KPG, Purkerson ML, et al: The immunological aspects of acute ureteral obstruction: Immune cell infiltrate in the kidney. Kidney Int 34:487-493, 1988 34. Schreiner G, Unanue ER: Origin of the rat mesangial phagocyte and its expression of the leukocyte common antigen. Lab Invest 51 :515-523, 1984 35. Schreiner GF, Kohan DE: Regulation of renal transport processes and hemodynamics by macrophages and lymphocytes. Am J Physiol 258:F761-F767, 1990 36. Steinhardt GF, Ramon G, Salinas-Madrigal L: Glomerulosclerosis in obstructive uropathy. J Urol 140:13161318, 1988 37. Nathan CR: Secretory products of macrophages. J ain Invest 79:319-326, 1987 38. Rovin BH, Harris KPG, Morrison A, et al: Renal cortical release of a specific macrophage chemoattractant in response to ureteral obstruction. Lab Invest 63:213-220, 1990 39. Buerkert J, Martin 0, Head M, et al: Deep nephron function after release of acute unilateral ureteral obstruction in the young rat. J Clin Invest 62:1228-1239, 1978 40. Hanley MJ, Davidson K: Isolated nephron segments from rabbit models of obstructive nephropathy. J Clin Invest 69:165-174,1982 41. Buerkert J, Head M, Klahr S: Effects of acute bilateral ureteral obstruction on deep nephron and terminal collecting duct function in the young rat. J Clin Invest 59:1055-1065, 1977 42. Purkerson ML, Blaine EH, Stokes TJ, et al: Role of atrial peptide in the natriuresis and diuresis that follows relief of obstruction in rats. Am J Physiol 256:F583-F589, 1989 43. Wilson DR, Hall WK, S· en AK: Renal sodium- and potassium-activated adenosine triphosphatase deficiency during post-obstructive diuresis in the rat. Can J Physiol Pharmacol 52:105-113, 1974 44. Williams RD, Fanestil DO, Blackard CE: Etiology of post-obstructive diuresis: Ouabain sensitive adenosine triphosphatase deficit and elevated solute excretion in the postobstructed dog kidney. Invest Urol 14:148-152, 1976 45. Sabatini S, Kurtzman NA: Enzyme activity in obstructive uropathy: Basis for salt wastage and the acidification defect. Kidney Int 37:79-84, 1990 46. Brunskill N, Hayes C, Morrissey J, et al: Changes in the lipid environment decrease the activity of Na,K-A TPase in obstructive nephropathy. Kidney Int 39:843-849, 1991

699 47. Tannenbaum J, Purkerson ML, Klahr S: The effect of unilateral ureteral obstruction on the metabolism of renal lipids in the rat. Am J Physiol 245:F254-F262, 1983 48. Morrissey J, Windus 0, Schwab S, et al: Ureteral occlusion decreases phospholipid and cholesterol of renal tubular membranes. Am J PhysioI250:FI36-FI43, 1986 49. Campbell HT, Bello-Reuss E, Klahr S: Hydraulic water permeability and transepithelial voltage in the isolated perfused rabbit cortical collecting tubule following unilateral ureteral obstruction. J Clin Invest 75:219-225, 1985 50. BatHe DC, Arruda JAL, Kurtzman NA: Hyperkalemic distal renal tubular acidosis associated with obstructive uropathy. N Engl J Med 304:373-380, 1981 51. Kimura H, Mujais SK: Cortical collecting duct Na-K pump in obstructive nephropathy. Am J Physiol 258:FI320F1323, 1990 52. Earley LF: Extreme polyuria in obstructive uropathy: Report of a case of "water-losing nephritis" in an infant, with a discussion of polyuria. N Engl J M ed 255:600-605, 1956 53. Gillenwater JY, Westervelt JFB, Vaughan JED, et al: Renal function after release of chronic unilateral hydronephrosis in man. Kidney Int 7:179-186, 1975 54. Better OS, Arieff AI, Massry SG, et al: Studies on renal function after relief of complete unilateral ureteral obstruction of three months' duration in man. Am J Med 54:234-240, 1973 55. Ericsson NO, Wineberg J, Zetterstrom R: Renal function in infantile obstructive uropathy. Acta Pediatr Scand 44: 444-459, 1955 56. Berlyne GM: Distal tubular function in chronic hydronephrosis. Q J Med 30:339-355, 1961 57. Massry SG, Schainuck LI, Goldsmith C, et al: Studies on the mechanism of diuresis after relief of urinary tract obstruction. Ann Intern Med 66: 149-158, 1967 58. Walls J, Buerkert JE, Purkerson ML, et al: Nature of the acidifying defect after the relief of ureteral obstruction. Kidney Int 7:304-316,1975 59. Thirakomen K, Kozlov N, Arruda JAL, et al: Renal hydrogen ion secretion after release of unilateral ureteral obstruction. Am J PhysioI231:1233-1239, 1976 60. Laski ME, Kurtzman NA: Site oftbe acidification defect in the post-obstructed collecting tubule. Min Electr Metab 15: 195-200, 1989 61. Purcell H, Harris KPG, Lim I, et al: Mechanisms of the acidifying defect after release of unilateral ureteral obstruction. Kidney Int 35:461 , 1989 (abstr)