Intra- and inter-nephron heterogeneity of ammoniagenesis in rats: effects of chronic metabolic acidosis and potassium depletion

An Update on Kidney Ammonium Transport Along the Nephron

Acid-base homeostasis is critical to the maintenance of normal health. The kidneys have a central role in bicarbonate generation, which occurs through the process of net acid excretion. Renal ammonia excretion is the predominant component of renal net acid excretion under basal conditions and in response to acid-base disturbances. Ammonia produced in the kidney is selectively transported into the urine or the renal vein. The amount of ammonia produced by the kidney that is excreted in the urine varies dramatically in response to physiological stimuli. Recent studies have advanced our understanding of ammonia metabolism's molecular mechanisms and regulation. Ammonia transport has been advanced by recognizing that the specific transport of NH3 and NH₄⁺ by specific membrane proteins is critical to ammonia transport. Other studies show that proximal tubule protein, NBCe1, specifically the A variant, significantly regulates renal ammonia metabolism. This review discusses these critical aspects of the emerging features of ammonia metabolism and transport.

Emerging Features of Ammonia Metabolism and Transport in Acid-Base Balance

Ammonia metabolism has a critical role in acid-base homeostasis and in other cellular functions. Kidneys have a central role in bicarbonate generation, which occurs through the process of net acid excretion; ammonia metabolism is the quantitatively greatest component of net acid excretion, both under basal conditions and in response to acid-base disturbances. Several recent studies have advanced our understanding substantially of the molecular mechanisms and regulation of ammonia metabolism. First, the previous paradigm that ammonia transport could be explained by passive NH₃ diffusion and NH₄⁺ trapping has been advanced by the recognition that specific transport of NH₃ and of NH₄⁺ by specific membrane proteins is critical to ammonia transport. Second, significant advances have been made in the understanding of the regulation of ammonia metabolism. Novel studies have shown that hyperkalemia directly inhibits ammonia metabolism, thereby leading to the metabolic acidosis present in type IV renal tubular acidosis. Other studies have shown that the proximal tubule protein NBCe1, specifically the A variant NBCe1-A, has a major role in regulating renal ammonia metabolism. Third, there are important sex differences in ammonia metabolism that involve structural and functional differences in the kidney. This review addresses these important aspects of ammonia metabolism and transport.

NBCe1-A is required for the renal ammonia and K+ response to hypokalemia

Hypokalemia increases ammonia excretion and decreases K⁺ excretion. The present study examined the role of the proximal tubule protein NBCe1-A in these responses. We studied mice with Na⁺-bicarbonate cotransporter electrogenic, isoform 1, splice variant A (NBCe1-A) deletion [knockout (KO) mice] and their wild-type (WT) littermates were provided either K⁺ control or K⁺-free diet. We also used tissue sections to determine the effect of extracellular ammonia on NaCl cotransporter (NCC) phosphorylation. The K⁺-free diet significantly increased proximal tubule NBCe1-A and ammonia excretion in WT mice, and NBCe1-A deletion blunted the ammonia excretion response. NBCe1-A deletion inhibited the ammoniagenic/ammonia recycling enzyme response in the cortical proximal tubule (PT), where NBCe1-A is present in WT mice. In the outer medulla, where NBCe1-A is not present, the PT ammonia metabolism response was accentuated by NBCe1-A deletion. KO mice developed more severe hypokalemia and had greater urinary K⁺ excretion during the K⁺-free diet than did WT mice. This was associated with blunting of the hypokalemia-induced change in NCC phosphorylation. NBCe1-A KO mice have systemic metabolic acidosis, but experimentally induced metabolic acidosis did not alter NCC phosphorylation. Although KO mice have impaired ammonia metabolism, experiments in tissue sections showed that lack of ammonia does impair NCC phosphorylation. Finally, urinary aldosterone was greater in KO mice than in WT mice, but neither expression of epithelial Na⁺ channel α-, β-, and γ-subunits nor of H⁺-K⁺-ATPase α₁- or α₂-subunits correlated with changes in urinary K⁺. We conclude that NBCe1-A is critical for the effect of diet-induced hypokalemia to increase cortical proximal tubule ammonia generation and for the expected decrease in urinary K⁺ excretion.

Regulation of renal NaDC1 expression and citrate excretion by NBCe1-A

Citrate is critical for acid-base homeostasis and to prevent calcium nephrolithiasis. Both metabolic acidosis and hypokalemia decrease citrate excretion and increase expression of Na+-dicarboxylate cotransporter 1 (NaDC1; SLC13A2), the primary protein involved in citrate reabsorption. However, the mechanisms transducing extracellular signals and mediating these responses are incompletely understood. The purpose of the present study was to determine the role of the Na+-coupled electrogenic bicarbonate cotransporter (NBCe1) A variant (NBCe1-A) in citrate metabolism under basal conditions and in response to acid loading and hypokalemia. NBCe1-A deletion increased citrate excretion and decreased NaDC1 expression in the proximal convoluted tubules (PCT) and proximal straight tubules (PST) in the medullary ray (PST-MR) but not in the PST in the outer medulla (PST-OM). Acid loading wild-type (WT) mice decreased citrate excretion. NaDC1 expression increased only in the PCT and PST-MR and not in the PST-MR. In NBCe1-A knockout (KO) mice, the acid loading change in citrate excretion was unaffected, changes in PCT NaDC1 expression were blocked, and there was an adaptive increase in PST-MR. Hypokalemia in WT mice decreased citrate excretion; NaDC1 expression increased only in the PCT and PST-MR. NBCe1-A KO blocked both the citrate and NaDC1 changes. We conclude that 1) adaptive changes in NaDC1 expression in response to metabolic acidosis and hypokalemia occur specifically in the PCT and PST-MR, i.e., in cortical proximal tubule segments; 2) NBCe1-A is necessary for normal basal, metabolic acidosis and hypokalemia-stimulated citrate metabolism and does so by regulating NaDC1 expression in cortical proximal tubule segments; and 3) adaptive increases in PST-OM NaDC1 expression occur in NBCe1-A KO mice in response to acid loading that do not occur in WT mice.

NBCe1-A Regulates Proximal Tubule Ammonia Metabolism under Basal Conditions and in Response to Metabolic Acidosis

Renal ammonia metabolism is the primary mechanism through which the kidneys maintain acid-base homeostasis, but the molecular mechanisms regulating renal ammonia generation are unclear. In these studies, we evaluated the role of the proximal tubule basolateral plasma membrane electrogenic sodium bicarbonate cotransporter 1 variant A (NBCe1-A) in this process. Deletion of the NBCe1-A gene caused severe spontaneous metabolic acidosis in mice. Despite this metabolic acidosis, which normally causes a dramatic increase in ammonia excretion, absolute urinary ammonia concentration was unaltered. Additionally, NBCe1-A deletion almost completely blocked the ability to increase ammonia excretion after exogenous acid loading. Under basal conditions and during acid loading, urine pH was more acidic in mice with NBCe1-A deletion than in wild-type controls, indicating that the abnormal ammonia excretion was not caused by a primary failure of urine acidification. Instead, NBCe1-A deletion altered the expression levels of multiple enzymes involved in proximal tubule ammonia generation, including phosphate-dependent glutaminase, phosphoenolpyruvate carboxykinase, and glutamine synthetase, under basal conditions and after exogenous acid loading. Deletion of NBCe1-A did not impair expression of key proteins involved in collecting duct ammonia secretion. These studies demonstrate that the integral membrane protein NBCe1-A has a critical role in basal and acidosis-stimulated ammonia metabolism through the regulation of proximal tubule ammonia-metabolizing enzymes.

Ammonia Transporters and Their Role in Acid-Base Balance

Acid-base homeostasis is critical to maintenance of normal health. Renal ammonia excretion is the quantitatively predominant component of renal net acid excretion, both under basal conditions and in response to acid-base disturbances. Although titratable acid excretion also contributes to renal net acid excretion, the quantitative contribution of titratable acid excretion is less than that of ammonia under basal conditions and is only a minor component of the adaptive response to acid-base disturbances. In contrast to other urinary solutes, ammonia is produced in the kidney and then is selectively transported either into the urine or the renal vein. The proportion of ammonia that the kidney produces that is excreted in the urine varies dramatically in response to physiological stimuli, and only urinary ammonia excretion contributes to acid-base homeostasis. As a result, selective and regulated renal ammonia transport by renal epithelial cells is central to acid-base homeostasis. Both molecular forms of ammonia, NH₃ and NH₄⁺, are transported by specific proteins, and regulation of these transport processes determines the eventual fate of the ammonia produced. In this review, we discuss these issues, and then discuss in detail the specific proteins involved in renal epithelial cell ammonia transport.

Potassium restriction, high protein intake, and metabolic acidosis increase expression of the glutamine transporter SNAT3 (Slc38a3) in mouse kidney

Kidneys produce ammonium to buffer and excrete acids through metabolism of glutamine. Expression of the glutamine transporter Slc38a3 (SNAT3) increases in kidney during metabolic acidosis (MA), suggesting a role during ammoniagenesis. Potassium depletion and high dietary protein intake are known to elevate renal ammonium excretion. In this study, we examined SNAT3, phosphate-dependent glutaminase (PDG), and phosphoenolpyruvate carboxykinase (PEPCK) regulation during a control (0.36%) or low-K(+) (0.02%) diet for 7 or 14 days or a control (20%) or high-protein (50%) diet for 7 days. MA was induced in control and low-K(+) groups by addition of NH(4)Cl. Urinary ammonium excretion increased during MA, after 14-day K(+) restriction alone, and during high protein intake. SNAT3, PDG, and PEPCK mRNA abundance were elevated during MA and after 14-day K(+) restriction but not during high protein intake. SNAT3 protein abundance was enhanced during MA (both control and low K(+)), after 14-day low-K(+) treatment alone, and during high protein intake. Seven-day dietary K(+) depletion alone had no effect. Immunohistochemistry showed SNAT3 staining in earlier parts of the proximal tubule during 14-day K(+) restriction with and without NH(4)Cl treatment and during high protein intake. In summary, SNAT3, PDG, and PEPCK mRNA expression were congruent with urinary ammonium excretion during MA. Chronic dietary K(+) restriction, high protein intake, and MA enhance ammoniagenesis, an effect that may involve enhanced SNAT3 mRNA and protein expression. Our data suggest that SNAT3 plays an important role as the glutamine uptake mechanism in ammoniagenesis under these conditions.

Role of angiotensin II in the enhancement of ammonia production and secretion by the proximal tubule in metabolic acidosis

Acidosis and angiotensin II stimulate ammonia production and transport by the proximal tubule. We examined the modulatory effect of the type 1 angiotensin II receptor blocker losartan on the ability of metabolic acidosis to stimulate ammonia production and secretion by mouse S2 proximal tubule segments. Mice given NH4Cl for 7 days developed metabolic acidosis (low serum bicarbonate concentration) and increased urinary excretion of ammonia. S2 tubule segments from acidotic mice displayed higher rates of ammonia production and secretion compared with those from control mice. However, when losartan was coadministered in vivo with NH4Cl, both the acidosis-induced increase in urinary ammonia excretion and the adaptive increase in ammonia production and secretion of microperfused S2 segments were largely blocked. In renal cortical tissue, losartan blocked the acid-induced increase in brush-border membrane NHE3 expression but had no effect on the acid-induced upregulation of phosphate-dependent glutaminase or phosphoenolpyruvate carboxykinase 1 in cortical homogenates. Addition of angiotensin II to the microperfusion solution enhanced ammonia secretion and production rates in tubules from NH4Cl-treated and control mice in a losartan-inhibitable manner. These results demonstrate that a 7-day acid challenge induces an adaptive increase in ammonia production and secretion by the proximal tubule and suggest that during metabolic acidosis, angiotensin II signaling is necessary for adaptive enhancements of ammonia excretion by the kidney and ammonia production and secretion by S2 proximal tubule segments, as mediated, in part, by angiotensin receptor-dependent enhancement of NHE3 expression.

Ammonium Homeostasis and Human Rhesus Glycoproteins

The brain ammonium production is detoxified by astrocytes, the gut ammonium production is detoxified by hepatic cells, and the renal ammonium production plays a major role in renal acid excretion. As a result of ammonium handling in these organs, the ammonium and pH values are strictly regulated in plasma. Up until recently, it was accepted that mammalian cell transmembrane ammonium transport was due to NH(4)(+) transport by non-specific transporting systems, and to non-ionic NH(3) diffusion, whereas lower organisms (such as bacteria, yeasts and plants) were endowed with specific ammonium transporters (Amts). Sequence homologies between Amts and human Rhesus (Rh) glycoproteins (RhAG, from erythroid cells, and RhBG and RhCG from epithelial cells) raised the hypothesis that Rh glycoproteins act as specific ammonium transporters, further sustained by the polarized distribution of RhBG and RhCG in gut, kidney and liver. Results from functional studies agree that Rh glycoproteins are the first ammonium transporters reported in mammals. However, the nature of the transported specie(s) is much debated: in particular, it is proposed that Rh glycoproteins mediate a direct NH(3) transport, or that they mediate an indirect NH(3) transport (resulting from NH(4)(+) for H(+) exchange). Direct NH(3) transport (associated or not with NH(4)(+) transport) raises the exciting hypothesis that Rh glycoproteins may also transport other gases than NH(3) (namely, CO(2)).

Functional characterization of rat organic anion transporter 5 (Slc22a19) at the apical membrane of renal proximal tubules

A novel member of the organic anion transporter (OAT) family, Oat5 (Slc22a19), has been reported to transport a naturally occurring mycotoxin, ochratoxin A (OTA). However, neither its endogenous substrate and driving force nor physiological functions have been determined. Herein, we report the functional characterization of rat Oat5 (rOat5), as well as its intrarenal distribution and membrane localization. When expressed in Xenopus laevis oocytes, rOat5 mediated the transport of sulfate conjugates of steroids such as estrone-3-sulfate (E(1)S; K(m) = 18.9 +/- 3.9 microM) and dehydroepiandrosterone sulfate (K(m) = 2.3 +/- 0.2 microM) in a sodium-independent manner, in addition to OTA. The rOat5-mediated E(1)S transport was strongly inhibited by four-carbon (C4) dicarboxylate succinate and longer dicarboxylates (C7-C9). The uptake of [(3)H]E(1)S via rOat5 was significantly trans-stimulated by succinate, and the efflux of [(14)C]succinate was significantly trans-stimulated by E(1)S. A similar trans-stimulatory effect of preloaded succinate on E(1)S uptake was also detected in cells stably expressing rOat5 (S(2) rOat5). rOat5 interacted with chemically heterogenous anionic compounds. The rOat5-mediated E(1)S transport was inhibited by several sulfate conjugates, such as 4-methylumbelliferyl sulfate and beta-estradiol sulfate, but not by glucuronide conjugates. An immunohistochemical study showed that rOat5 was localized at the apical membrane of renal proximal tubules in the corticomedullary region. rOat5 mRNA was expressed in the late segments (S(2) and S(3)) of proximal tubules. These results indicate that rOat5 is renal organic anion/dicarboxylates exchanger and, under physiological conditions, may function as an apical reabsorptive pathway for organic anions in proximal tubules driven by an outward gradient of dicarboxylates.

Ammonia production and secretion by S3 proximal tubule segments from acidotic mice: role of ANG II

ANG II has potent effects on ammonia production and secretion rates by the proximal tubule and is found in substantial concentrations in the lumen of the proximal tubule in vivo. Because our previous studies demonstrated that acid loading enhanced the stimulatory effects of ANG II on ammonia production and secretion by S2 proximal tubule segments, we examined the effect of ANG II on ammonia production and secretion by isolated, perfused S3 segments from nonacidotic control mice and acidotic mice given NH4Cl for 7 days. In the absence of ANG II, ammonia production and secretion rates were no different in S3 segments from acidotic and control mice. By contrast, when ANG II was present in the luminal perfusion solution, ammonia production and secretion rates were stimulated, in a losartan-inhibitable manner, to a greater extent in S3 segments from acidotic mice. Ammonia secretion rates in S3 segments were largely inhibited by perfusion with a low-sodium solution containing amiloride in the presence or absence of ANG II. These results demonstrated that isolated, perfused mouse S3 proximal tubule segments produce and secrete ammonia, that NH4Cl-induced acidosis does not affect the basal rates of ammonia production and secretion, and that ANG II, added to the luminal fluid, stimulates ammonia production and secretion to a greater extent in S3 segments from acidotic mice. These findings suggest that S3 segments, in the presence of ANG II, can contribute to the enhanced renal excretion that occurs with acid loading.

Mechanisms of NH4+and NH3transport during hypokalemia

Along the collecting duct, secretion of ammonium (NH) is thought to occur through active H+ secretion in parallel with the non-ionic diffusion of ammonia (NH3). Thus NH3 is secreted into the collecting duct lumen down its concentration gradient. Moreover, the low NH permeability and high NH3 permeability observed in collecting duct epithelia minimizes back diffusion of NH. In general, an increase in the NH3 concentration gradient between the interstitium and the collecting duct lumen correlates with increased NH secretion. However, our laboratory and others have shown an important role of direct NH transport by the Na,K-ATPase. As K+ and NH compete for a common extracellular binding site on the Na,K-ATPase, reduced interstitial K+ concentration, such as during hypokalemia, augments NH uptake. Na,K-ATPase-mediated NH uptake provides an important source of H+ for net acid secretion during hypokalemia and contributes to the increase in NH excretion and metabolic alkalosis observed in this treatment model.

Desensitization of type 1 angiotensin II receptor subtypes in the rat kidney

Differences involving serine residues in the sequence of the carboxyl-terminal tail of type 1 angiotensin II (Ang II) receptor subtypes AT(1A) and AT(1B) suggest differences in desensitization ability. We examined the Ang II-induced homologous desensitization patterns of both receptor subtypes in freshly isolated renal structures: glomerulus (Glom), afferent arteriole, and cortical thick ascending limb (CTAL), whose content in each subtype mRNA is different, by measuring variations in intracellular calcium concentration. A preexposure to a maximal dose of Ang II, followed by a second application of the same concentration, induced: 1) a complete desensitization in Glom, where AT(1A) and AT(1B) mRNAs were expressed in similar proportions, and 2) no or partial desensitization in afferent arteriole and CTAL, where AT(1A) mRNA was predominant. In the absence of nephron structure containing only AT(1B) mRNA, we studied rat anterior pituitary cells that exhibit high content in this subtype and observed that desensitization was not complete. In Glom, CTAL, and pituitary cells, desensitization proceeded in a dose-dependent manner. In Glom and CTAL, desensitization occurred via a PKC-independent mechanism. These results suggest that desensitization does not depend on the nature of Ang II receptor subtype but either on the proportion of each subtype in a given cell and/or on cell specific type. This could allow adaptive biological responses to Ang II appropriate to the specific function of a given cell type.

Impact of K(+) homeostasis on net acid secretion in rat terminal inner medullary collecting duct: role of the Na,K-ATPase

For the past 50 years, the mechanism of ammonium (NH(4)(+)) transport along the collecting duct has been thought to occur through active H(+) section in parallel with the nonionic diffusion of ammonia (NH(3)). This model is supported by two basic experimental observations. First, NH(4)(+) secretion generally correlates with the NH(3) concentration gradient between the interstitium and the collecting duct lumen. This NH(3) gradient is generated through both luminal acidification, which reduces luminal NH(3) concentration, and through countercurrent multiplication, which increases interstitial NH(3) concentration. The result is secretion of NH(3) into the collecting duct lumen down its concentration gradient. Second, because NH(4)(+) permeability is low relative to that of NH(3), there is significant secretion of NH(3) into the collecting duct lumen with minimal back-diffusion of NH(4)(+). However, our laboratory, as well as others, has shown that this model is an oversimplification of the mechanism of NH(4)(+) transport along the collecting duct. NH(4)(+) is transported through a variety of K(+) transport pathways including Na,K-ATPase. K(+) and NH(4)(+) compete for a common extracellular binding site on Na, K-ATPase. During hypokalemia, interstitial K(+) concentration is reduced, which augments NH(4)(+) uptake by the Na(+) pump. In K(+) restriction, Na,K-ATPase-mediated NH(4)(+) uptake provides an important source of H(+) for net acid secretion and for the titration of luminal buffers in the terminal inner medullary collecting duct. This pathway contributes to the increase in NH(4)(+) excretion and metabolic alkalosis observed during hypokalemia.

Differential expression and acid-base regulation of glutaminase mRNAs in gluconeogenic LLC-PK(1)-FBPase(+) cells

LLC-PK(1)-FBPase(+) cells, which are a gluconeogenic substrain of porcine renal LLC-PK(1) cells, exhibit enhanced oxidative metabolism and increased levels of phosphate-dependent glutaminase (PDG) activity. On adaptation to acidic medium (pH 6.9, 9 mM HCO(-)(3)), LLC-PK(1)-FBPase(+) cells also exhibit a greater increase in ammonia production and respond with an increase in assayable PDG activity. The changes in PDG mRNA levels were examined by using confluent cells grown on plastic dishes or on permeable membrane inserts. The latter condition increased the state of differentiation of the LLC-PK(1)-FBPase(+) cells. The levels of the primary porcine PDG mRNAs were analyzed by using probes that are specific for the 5.0-kb PDG mRNA (p2400) or that react equally with both the 4.5- and 5.0-kb PDG mRNAs (p930 and r1500). In confluent dish- and filter-grown LLC-PK(1)-FBPase(+) cells, the predominant 4.5-kb PDG mRNA is increased threefold after 18 h in acidic media. However, in filter-grown epithelia, which sustain an imposed pH and HCO(-)(3) gradient, this adaptive increase is observed only when acidic medium is applied to both the apical and the basolateral sides of the epithelia. Half-life experiments established that induction of the 4. 5-kb PDG mRNA was due to its stabilization. An identical pattern of adaptive increases was observed for the cytosolic PEPCK mRNA. In contrast, no adaptive changes were observed in the levels of the 5. 0-kb PDG mRNA in either cell culture system. Furthermore, cultures were incubated in low-potassium (0.7 mM) media for 24-72 h to decrease intracellular pH while maintaining normal extracellular pH. LLC-PK(1)-FBPase(+) cells again responded with increased rates of ammonia production and increased levels of the 4.5-kb PDG and PEPCK mRNAs, suggesting that an intracellular acidosis is the initiator of this adaptive response. Because all of the observed responses closely mimic those characterized in vivo, the LLC-PK(1)-FBPase(+) cells represent a valuable tissue culture model to study the molecular mechanisms that regulate renal gene expression in response to changes in acid-base balance.

In rat tIMCD, NH4+ uptake by Na+-K+-ATPase is critical to net acid secretion during chronic hypokalemia

The purpose of this study was to determine the magnitude of Na+ pump-mediated NH4+ uptake in the terminal inner medullary collecting duct (tIMCD) at K+ and NH4+ concentrations observed in vivo in the inner medullary interstitium of normal and in K+-restricted rats. Interstitial K+ and NH4+ concentrations in the terminal half of the inner medulla were taken to be 10 and 6 mM in K+-restricted rats, but 30 and 6 mM in K+-replete rats. In tubules from K+-restricted rats, when perfused at a K+ concentration of 10 mM, addition of ouabain to the bath reduced total bicarbonate flux (JtCO2) by 40% and increased intracellular pH (pHi), indicating significant NH4+ uptake by the Na+-K+-ATPase. In tubules from K+-restricted rats, JtCO2 was reduced with increased extracellular K+. At a K+ concentration of 30 mM, ouabain addition neither reduced JtCO2 nor increased pHi in tubules from rats of either treatment group. In conclusion, in the tIMCD from hypokalemic rats, 1) acute changes in extracellular K+ concentration modulate net acid secretion, and 2) Na+ pump-mediated NH4+ uptake should be an important pathway mediating transepithelial net acid secretion in vivo.

Aminopeptidase A activity and angiotensin III effects on [Ca2+]i along the rat nephron

BACKGROUND

This study examined the specific effects of angiotensin III (Ang III) along the nephron.

METHODS

We examined the distribution of aminopeptidase A (APA) activity by using a specific APA inhibitor and by immunostaining with an antirat kidney APA antibody, the Ang III-induced variations of [Ca2+]i by using fura-2 and the characterization of the receptor subtype involved in the response to Ang III in cortical thick ascending limb (CTAL).

RESULTS

APA activity was found all along the nephron but was higher in the cortex than in the medulla. This was confirmed by immunostaining. Increases in [Ca2+]i elicited by 10(-7) mol/liter Ang III were observed all along the nephron. The characterization of the receptor subtype involved in the [Ca2+]i response to Ang III in CTAL indicated that EC50 values for Ang III and Ang II were similar (13.5 and 10.3 nmol/liter, respectively), and Ang III-induced responses were totally abolished by AT1 receptor but not by AT2 receptor antagonists. There was a cross-desensitization of [Ca2+]i responses to 10(-7) mol/liter Ang III and Ang II, and the [Ca2+]i responses to 10(-7) mol/liter Ang II and Ang III were not additive.

CONCLUSION

These results show that in CTAL, the [Ca2+]i responses to Ang II and Ang III occur through the same AT1a receptor because this subtype is predominant in this segment. Taken together, these data suggest that APA could be a key enzyme to generate Ang III from Ang II in the kidney.

Dietary K+ restriction upregulates total and Sch-28080-sensitive bicarbonate absorption in rat tIMCD

In tubules from the terminal segment of the inner medullary collecting duct (tIMCD) from rats with chronic metabolic acidosis, our laboratory has shown that bicarbonate absorption (JtCO2) is inhibited by removal of K+ from the luminal fluid or by the addition of Sch-28080 to the perfusate. The present study asked whether total and/or Sch-28080-sensitive JtCO2 is regulated by changes in systemic K+ homeostasis. Rat tIMCD tubules were perfused in vitro in symmetrical, HCO-3/CO2-buffered solutions containing 10 mM KCl + 6 mM NH4Cl. Total and Sch-28080-sensitive JtCO2 were measured in rats with varying K+ intake. In K+-replete rats, baseline JtCO2 was 2.1 +/- 0.3 pmol . mm-1 . min-1 (n = 6). In rats fed a K+-deficient diet for 3 days, JtCO2 was 5.4 +/- 0.7 pmol . mm-1 . min-1 (n = 16, P < 0. 05). To determine the mechanism for the increase in HCO-3 absorption observed with K+ restriction, the Sch-28080-sensitive component of JtCO2 was measured in each treatment group. Following the addition of Sch-28080 (10 microM) to the perfusate, a 40% reduction in JtCO2 was observed in K+-restricted rats. JtCO2 was not reduced following the addition of Sch-28080 in rats with normal K+ intake. Because Sch-28080-sensitive JtCO2 was increased in K+-restricted rats, Sch-28080-sensitive JtCO2 was studied further in tIMCD tubules from rats in this treatment group. In K+-restricted rats, JtCO2 decreased by 20% following the addition of 5 mM ouabain to the perfusate. This ouabain-induced decline in JtCO2 was observed both in the presence and in the absence of Sch-28080. We conclude that total and Sch-28080-sensitive net acid secretion is increased with dietary K+ restriction. However, since approximately 50% of JtCO2 is insensitive to both Sch-28080 and ouabain, future studies will be necessary to define other mechanisms of luminal acidification in the rat tIMCD.

Cisplatin-induced alterations in renal structure, ammoniagenesis and gluconeogenesis of rats

Cisplatin [cis-diamminedichloroplatinum (II): CDDP] is a widely used cancer chemotherapeutic agent which has been shown to cause dose-related acute renal failure. The kidney damage is histologically characterized by widespread tubular necrosis, predominantly found in the third segments (S3) of the proximal tubules. To identify the intranephron targets of CDDP more precisely, we examined alterations in ammoniagenesis (AMG) and gluconeogenesis (GLG) using rat kidney slices (for AMG and GLG), tubule suspensions (for GLG), and microdissected nephron segments (for AMG). Ultramicroassay of AMG was carried out using the enzymatic cycling method, and GLG was measured by the HK/G6PHD method. GLG obtained from kidney slices and tubule suspensions on day 3 and day 7 following CDDP treatment did not change significantly from levels in control rats. In contrast, AMG increased on day 3 in the first and third kidney slices cut from the surface inward and decreased significantly on day 7 in the third and fourth slices. Microdissected nephron segments examined on day 7 showed decreased AMG in the second segments (S2; 20.3 +/- 7.7 pmol/mm/15 min vs. 78.7 +/- 9.7 for control, P less than 0.005) and the third segments (S3; 26.3 +/- 14.4 pmol/mm/15 min vs. 79.2 +/- 7.8 for control, P less than 0.005) of the proximal tubules. Additionally, we observed morphological changes under light microscopy to examine the relationship between metabolism and morphology. On day 3 following the CDDP treatment, typical acute tubular necrosis was seen primarily localized in the outer stripe of the outer medulla, while on day 7 the lesion appeared to be recovering. Our data imply a prominent dissociation between renal metabolic and morphologic changes induced by CDDP.

Electrophysiology of ammonia transport in renal straight proximal tubules

To test for electrogenic transport of ammonium ions in straight proximal renal tubules, isolated perfused tubules have been exposed to peritubular ammonium ions during continuous recording of cell membrane potential. As a result, 20 mmol/liter NH4+ leads to a rapid, reversible depolarization of the cell membrane by 9.0 +/- 0.3 mV (N = 86). This depolarization is not significantly affected by 10 mmol/liter barium or 0.1 mmol/liter amiloride on both sides of the epithelium, but is significantly blunted by omission of extracellular bicarbonate and CO2 (3.8 +/- 0.4 mV, N = 9), by 1 mmol/liter acetazolamide (4.3 +/- 0.3 mV, N = 11), by 1 mmol/liter peritubular amiloride (4.3 +/- 1.1 mV, N = 7), by 1 mmol/liter SITS (5.7 +/- 0.4 mV, N = 6), and by replacement of extracellular sodium with choline (4.7 +/- 0.5 mV, N = 8). In the presence of both amiloride (1 mmol/liter) and acetazolamide (1 mmol/liter) in the bath, the NH4+ induced depolarization is completely abolished. Furthermore, the combined omission of bicarbonate and addition of 10 mmol/liter barium eliminates the NH4+ induced depolarization. About 50% of the depolarization can be explained by enhanced electrogenic bicarbonate exit due to the intracellular alkalosis. The other 50% is explained by amiloride and barium sensitive electrogenic entry of NH4+ into the cell.

Renal ammoniagenesis in humans with chronic potassium depletion

Renal ammonia production and distribution and ammonia precursor utilization were evaluated in eight patients with chronic potassium depletion (CPD) and aldosterone-producing adenoma and in 20 controls. In CPD, urinary ammonia excretion and ammonia added to renal venous blood were about twofold higher than in controls; thus, total ammonia production was significantly augmented (88.0 +/- 10.3 mumol/min.1.73 m2 vs. 45.0 +/- 2.6 in controls). Total ammonia production was inversely correlated with serum potassium and directly correlated with urine flow. Stepwise multiple regression analysis showed that both factors, mainly serum potassium, significantly influence ammonia production and account for 61.4% of variations in ammonia production. Renal extraction of glutamine was significantly increased (56.6 +/- 5.9 mumol/min.1.73 m2 vs. 34.6 +/- 3.1 in controls), and this could account for ammonia production. The ratio of urinary ammonia excretion to total ammonia production, an index of the intrarenal ammonia distribution, was similar in patients and controls, and was significantly correlated with urine pH and true renal blood flow (RBF). Stepwise multiple regression analysis showed that RBF, urine pH and urine flow also significantly affected ammonia distribution. However, these factors accounted for only 41.7% of variations in intrarenal ammonia partition, urine pH having a minor role. We conclude that in patients with CPD other factors besides urine pH, urine flow and RBF intervene in the ammonia partition between urine and blood.

Influence of acute metabolic acidosis on the monomer and polymer forms of renal phosphate-dependent glutaminase

Phosphate-dependent glutaminase (PDG) was measured in kidney cortex homogenates and mitochondria from control and acutely acidotic rats. The effect of plasma from acutely acidotic rats on PDG activity in homogenates from normal rats was also studied. Acidosis or incubation in acidotic plasma enhanced enzyme activity when measured at 1.0 mM but not at 20.0 mM glutamine. This effect was not due to increased mitochondrial permeability since similar results were obtained after solubilization of the enzyme with Triton X-100. Increased enzyme activity was observed with either the Tris (monomer) form or the borate (polymer) form of the enzyme, indicating that enhanced activity is not due to polymerization but probably to a conformational change in the enzyme such that the Km for glutamine is lowered.

Effects of potassium on ammonia transport by medullary thick ascending limb of the rat

Renal ammonium excretion is increased by potassium depletion and reduced by potassium loading. To determine whether changes in potassium concentration would alter ammonia transport in the medullary thick ascending limb (MAL), tubules from rats were perfused in vitro and effects of changes in K concentration within the physiological range (4-24 mM) were evaluated. Increasing K concentration from 4 to 24 mM in perfusate and bath inhibited total ammonia absorption by 50% and reduced the steady-state transepithelial NH+4 concentration gradient. The inhibition of total ammonia absorption was reversible and occurred when K replaced either Na or N-methyl-D-glucamine. Increasing K concentration in the luminal perfusate alone gave similar inhibition of total ammonia absorption. At 1-2 nl/min per mm perfusion rate, increasing K concentration in perfusion and bathing solutions had no significant effect on transepithelial voltage. With either 4 or 24 mM K in perfusate and bath, an increase in luminal perfusion rate markedly increased total ammonia absorption. Thus, both potassium concentration and luminal flow rate are important factors capable of regulating total ammonia transport by the MAL. Changes in systemic potassium balance may influence renal ammonium excretion by affecting NH+4 absorption in the MAL and altering the transfer of ammonia from loops of Henle to medullary collecting ducts.

Intra- and inter-nephron heterogeneity of gluconeogenesis in the rat: effects of chronic metabolic acidosis and potassium depletion

The intra- and inter-nephron heterogeneity of renal gluconeogenesis within rat proximal tubules and the effects of chronic metabolic acidosis and chronic potassium(K)-depletion were studied using isolated proximal tubules of rats by directly measuring glucose synthesized. The gluconeogenic activity from pyruvate and glutamine in control rats was almost limited to within the early proximal tubule (S1: 45.4 +/- 5.7 pmol/mm/60 min from pyruvate; 58.0 +/- 6.0 from glutamine). Very low, but detectable gluconeogenesis was observed in the middle portion of the proximal tubule (S2: 9.9 +/- 2.2 from pyruvate; 4.8 +/- 1.1 from glutamine). The rate of glucose production in the terminal proximal tubule (S3) was negligible. Furthermore, gluconeogenesis from glutamine of superficial (SF) nephrons was significantly higher than that of juxtamedullary (JM) ones, whereas no difference was seen in gluconeogenesis from pyruvate. In acidotic and K-depleted rats, significant increase could be seen in S1 and S2, but the increase in S3 was not significant. By the serial determination in acidosis, the glucose production from both substrates was found to be the highest at the second 1 mm segment from the glomerulus, and it decreased downward along the proximal tubule. In acidosis, glucose production from both substrates in SF nephrons and that from glutamine in JM ones were elevated significantly compared with the control, but that from pyruvate in JM nephrons did not change. These results suggest that S1 of the SF nephron plays the most important role in gluconeogenesis in the control, whereas S1 of the JM nephron and S2 contribute to gluconeogenesis in acidotic and/or possibly K-depleted rats.