ANG II reduces net acid secretion in rat outer medullary collecting duct

Molecular mechanisms and regulation of urinary acidification

The H(+) concentration in human blood is kept within very narrow limits, ~40 nmol/L, despite the fact that dietary metabolism generates acid and base loads that are added to the systemic circulation throughout the life of mammals. One of the primary functions of the kidney is to maintain the constancy of systemic acid-base chemistry. The kidney has evolved the capacity to regulate blood acidity by performing three key functions: (i) reabsorb HCO3(-) that is filtered through the glomeruli to prevent its excretion in the urine; (ii) generate a sufficient quantity of new HCO3(-) to compensate for the loss of HCO3(-) resulting from dietary metabolic H(+) loads and loss of HCO3(-) in the urea cycle; and (iii) excrete HCO3(-) (or metabolizable organic anions) following a systemic base load. The ability of the kidney to perform these functions requires that various cell types throughout the nephron respond to changes in acid-base chemistry by modulating specific ion transport and/or metabolic processes in a coordinated fashion such that the urine and renal vein chemistry is altered appropriately. The purpose of the article is to provide the interested reader with a broad review of a field that began historically ~60 years ago with whole animal studies, and has evolved to where we are currently addressing questions related to kidney acid-base regulation at the single protein structure/function level.

Pyk2 regulates H+-ATPase-mediated proton secretion in the outer medullary collecting duct via an ERK1/2 signaling pathway

Acid-secreting intercalated cells respond to changes in systemic pH through regulation of apical H(+) transporters. Little is known about the mechanism by which these cells sense changes in extracellular pH (pH(o)). Pyk2 is a nonreceptor tyrosine kinase activated by autophosphorylation at Tyr402 by cell-specific stimuli, including decreased pH, and is involved in the regulation of MAPK signaling pathways and transporter activity. We examined whether the Pyk2 and MAPK signaling pathway mediates the response of transport proteins to decreased pH in outer medullary collecting duct cells. Immunoblot analysis of phosphorylated Pyk2 (Tyr402), ERK1/2 (Thr202/Tyr204), and p38 (Thr180/Tyr182) was used to assay protein activation. To examine specificity of kinase activation and its effects, we used Pyk2 small interfering RNA to knockdown Pyk2 expression levels, the Src kinase inhibitor 4-amino-5-(4-methylphenyl)-7-(t-butyl)pyrazolo[3,4-d]-pyrimidine (PP 1) to inhibit Pyk2 phosphorylation, and the MEK inhibitor U0126 to inhibit ERK1/2 phosphorylation. The pH-sensitive fluorescent probe 2'-7'-bis(carboxyethyl)-5(6)-carboxyfluorescein-acetoxymethyl ester (BCECF-AM) was used to assay H(+) transporter activity. The activity of H(+) transporters was measured as the rate of intracellular pH (pH(i)) recovery after an NH(4)Cl prepulse. We show that Pyk2 is endogenously expressed and activated by acid pH in mouse-derived outer medullary collecting duct (mOMCD1) cells. Incubation of mOMCD1 cells in acid media [extracellular pH (pH(o)) 6.7] increased the phosphorylation of Pyk2, ERK1/2, and p38. Reduction in pH(i) induced by an NH(4)Cl prepulse also increased the phosphorylation of Pyk2, ERK1/2, and p38. Consistent with our previous studies, we found that mOMCD1 cells exhibit H(+)-ATPase and H(+),K(+)-ATPase activity. Pyk2 inhibition by Pyk2 siRNA and PP 1 prevented Pyk2 phosphorylation as well as H(+)-ATPase-mediated recovery in mOMCD1 cells. In addition, ERK1/2 inhibition by U0126 prevented acid-induced ERK1/2 phosphorylation and H(+)-ATPase-mediated pH(i) recovery but not phosphorylation of p38. We conclude that Pyk2 and ERK1/2 are required for increasing H(+)-ATPase, but not H(+),K(+)-ATPase, activity at decreased pH(i) in mOMCD1 cells.

cAMP stimulates apical V-ATPase accumulation, microvillar elongation, and proton extrusion in kidney collecting duct A-intercalated cells

Kidney proton-secreting A-intercalated cells (A-IC) respond to systemic acidosis by accumulating the vacuolar ATPase (V-ATPase) in their apical membrane and by increasing the length and number of apical microvilli. We show here that the cell-permeant cAMP analog CPT-cAMP, infused in vivo, results in an almost twofold increase in apical V-ATPase accumulation in AE1-positive A-IC within 15 min and that these cells develop an extensive array of apical microvilli compared with controls. In contrast, no significant change in V-ATPase distribution could be detected by immunocytochemistry in B-intercalated cells at the acute time point examined. To show a direct effect of cAMP on A-IC, we prepared cell suspensions from the medulla of transgenic mice expressing EGFP in IC (driven by the B1-subunit promoter of the V-ATPase) and exposed them to cAMP analogs in vitro. Three-dimensional reconstructions of confocal images revealed that cAMP induced a time-dependent growth of apical microvilli, starting within minutes after addition. This effect was blocked by the PKA inhibitor myristoylated PKI. These morphological changes were paralleled by increased cAMP-mediated proton extrusion (pHi recovery) by A-IC in outer medullary collecting ducts measured using the ratiometric probe BCECF. These results, and our prior data showing that the bicarbonate-stimulated soluble adenylyl cyclase (sAC) is highly expressed in kidney intercalated cells, support the idea that cAMP generated either by sAC, or by activation of other signaling pathways, is part of the signal transduction mechanism involved in acid-base sensing and V-ATPase membrane trafficking in kidney intercalated cells.

Losartan chemistry and its effects via AT1 mechanisms in the kidney

Besides the importance of the renin-angiotensin system (RAS) in the circulation and other organs, the local RAS in the kidney has attracted a great attention in research in last decades. The renal RAS plays an important role in the body fluid homeostasis and long-term cardiovascular regulation. All major components and key enzymes for the establishment of a local RAS as well as two important angiotensin II (Ang II) receptor subtypes, AT1 and AT2 receptors, have been confirmed in the kidney. In additional to renal contribution to the systemic RAS, the intrarenal RAS plays a critical role in the regulation of renal function as well as in the development of kidney disease. Notably, kidney AT1 receptors locating at different cells and compartments inside the kidney are important for normal renal physiological functions and abnormal pathophysiological processes. This mini-review focuses on: 1) the local renal RAS and its receptors, particularly the AT1 receptor and its mechanisms in physiological and pathophysiological processes; and 2) the chemistry of the selective AT1 receptor blocker, losartan, and the potential mechanisms for its actions in the renal RAS-mediated disease.

Effect of chronic inhibition of converting enzyme on proximal tubule acidification

The acute effect of angiotensin-converting enzyme inhibition (ACEi) on proximal convoluted tubule (PCT) function is well documented. However, the effect of chronic treatment is less known. The aim of this work was to evaluate the effect of chronic ACEi on PCT acidification (J(HCO(3)(-))). Rats received enalapril (10 mg.kg(-1).day(-1), added to the drinking water) during 3 mo. Micropuncture experiments were performed to measure the effect of chronic ACEi on J(HCO(3)(-)). Nitric oxide (NO.) synthesis in kidney cortex homogenates was assessed by quantifying the conversion of [(14)C]-L-arginine to [(14)C]-L-citrulline. Western blot analysis was performed to determine the abundances of V-H(+)ATPase and NHE3 isoform of the Na(+)/H(+) exchanger in proximal brush-border membrane vesicles (BBMV). Enalapril treatment induced an approximately 50% increase in J(HCO(3)(-)). Luminal perfusion with ethyl-isopropyl amiloride (EIPA) 10(-4)M or bafilomycin 10(-6)M decreased J(HCO(3)(-)) by approximately 60% and approximately 30%, respectively, in both control and enalapril-treated rats. The effect of EIPA and bafilomycin on absolute J(HCO(3)(-)) was larger in enalapril-treated than in control rats. Acute inhibition of NO. synthesis with N(G)-nitro-L-arginine methyl ester abolished the enalapril-induced increase in J(HCO(3)(-)). Cortex homogenates from enalapril-treated rats displayed a 46% increase in nitric oxide synthase (NOS) activity compared with those from untreated animals. Enalapril treatment did not affect the abundances of NHE3 and V-H(+)ATPase in BBMV. Our results suggest that PCT acidification is increased during chronic ACEi probably due to an increase in NO. synthesis, which would stimulate Na(+)/H(+) exchange and electrogenic proton transport.

Angiotensin II stimulates vacuolar H+ -ATPase activity in renal acid-secretory intercalated cells from the outer medullary collecting duct

Final urinary acidification is mediated by the action of vacuolar H(+)-ATPases expressed in acid-secretory type A intercalated cells (A-IC) in the collecting duct. Angiotensin II (AngII) has profound effects on renal acid-base transport in the proximal tubule, distal tubule, and collecting duct. This study investigated the effects on vacuolar H(+)-ATPase activity in A-IC in freshly isolated mouse outer medullary collecting ducts. AngII (10 nM) stimulated concanamycin-sensitive vacuolar H(+)-ATPase activity in A-IC in freshly isolated mouse outer medullary collecting ducts via AT(1) receptors, which were also detected immunohistochemically in A-IC. AngII increased intracellular Ca(2+) levels transiently. Chelation of intracellular Ca(2+) with BAPTA and depletion of endoplasmic reticulum Ca(2+) stores prevented the stimulatory effect on H(+)-ATPase activity. The effect of AngII on H(+)-ATPase activity was abolished by inhibitors of small G proteins and phospholipase C, by blockers of Ca(2+)-dependent and -independent isoforms of protein kinase C and extracellular signal-regulated kinase 1/2. Disruption of the microtubular network and cleavage of cellubrevin attenuated the stimulation. Finally, AngII failed to stimulate residual vacuolar H(+)-ATPase activity in A-IC from mice that were deficient for the B1 subunit of the vacuolar H(+)-ATPase. Thus, AngII presents a potent stimulus for vacuolar H(+)-ATPase activity in outer medullary collecting duct IC and requires trafficking of stimulatory proteins or vacuolar H(+)-ATPases. The B1 subunit is indispensable for the stimulation by AngII, and its importance for stimulation of vacuolar H(+)-ATPase activity may contribute to the inappropriate urinary acidification that is seen in patients who have distal renal tubular acidosis and mutations in this subunit.

Regulation of acid-base transporters by vasopressin in the kidney collecting duct of Brattleboro rat

AIM

The objective of these studies was to examine the effects of long-term vasopressin treatment on acid-base transporters in the collecting duct of rat kidney.

METHODS

Brattleboro rats were placed in metabolic cages and treated with daily injections of 1-desamino-8-D-arginine vasopressin (dDAVP), a selective V2-receptor agonist, or its vehicle (control) for up to 8 days.

RESULTS

dDAVP treatment resulted in a significant reduction in serum bicarbonate concentration, and caused the upregulation of key ammoniagenesis enzymes, along with increased urinary NH4+ excretion. Northern hybridization and immunofluorescence labeling indicated a significant increase (+80%) in mRNA expression of the apical Cl-/HCO3- exchanger pendrin (PDS), along with a sharp increase in its protein abundance in B-type intercalated cells in the cortical collecting duct in dDAVP-treated rats. In the inner medullary collecting duct, the abundance of basolateral Cl-/HCO3- exchanger (AE1) and apical H+-ATPase was significantly reduced in dDAVP-treated rats. Kidney renin mRNA increased significantly and correlated with an increase in serum aldosterone levels in dDAVP-injected rats. Serum corticosterone levels were, however, reduced and correlated with increased mRNA levels of renal 11beta-hydroxysteroid dehydrogenase-2 (11beta-HSD2) and decreased mRNA expression of 11beta-hydroxylase in the adrenal gland of dDAVP-injected rats.

CONCLUSION

Chronic administration of dDAVP to Brattleboro rats is associated with the upregulation of PDS and downregulation of H+-ATPase and AE1 in the collecting duct, along with increased ammoniagenesis. Stimulation of the renin-angiotensin-aldosterone system and/or decreased glucocorticoid levels likely plays a role in the transduction of these effects.

Recent advances in our understanding of intercalated cells

PURPOSE OF REVIEW

This review will summarize newly described novel functions of renal intercalated cells.

RECENT FINDINGS

Over the past 20 years, the importance of intercalated cells in the process of renal net acid excretion has been recognized. More recently, many of the molecular mechanisms responsible for this cellular function have been described. Functionally, type A and type B intercalated cells are largely mirror images in that type A intercalated cells are H secreting cells, whereas type B intercalated cells are OH secreting cells. Whether non-A, non-B intercalated cells represent H or OH secreting cells or whether they interconvert between these functions is unclear. Transporters such as pendrin (Slc26a4, Pds), AE1 (Slc4a1), the H-ATPase, and NBC3 (Slc4a7) contribute to the ability of intercalated cells to secrete H or OH equivalents. In addition to mediating secretion of H or OH equivalents, however, intercalated cells also regulate vascular volume, and hence blood pressure, likely by regulating renal Cl excretion.

SUMMARY

The molecular mechanisms of net H/OH secretion by intercalated cell subsets have been largely defined over the past decade. Moreover, targeted genetic disruption of these transporters has revealed novel roles, such as blood pressure regulation. Thus, some of these transporters might be the target of future antihypertensive drugs.

Angiotensin II Increases H + -ATPase B1 Subunit Expression in Medullary Collecting Ducts

Metabolic alkalosis is a common feature of hypokalemic hypertensive syndromes associated with angiotensin II excess. The alkalosis-generating effect of angiotensin II is usually ascribed to its stimulatory effect on aldosterone secretion, a hormone that upregulates collecting duct hydrogen ion secretion. We studied the effect of angiotensin II infusions on the expression of B1 and a4 protein, subunits of the renal H + -ATPase in adrenalectomized rats. Adrenalectomized rats were given either angiotensin II or vehicle for 7 days via osmotic mini-pumps. H + -ATPase B1 protein expression was evaluated by Western blot analysis in isolated medulla and cortex plasma membrane preparations from one kidney, whereas the contralateral kidney was used for immunostaining. By Western blotting, the relative abundance of B1 protein was 2-fold higher in renal medulla membranes from rats with intact adrenal glands (sham surgery) than from adrenalectomized rats (219±47%, n=12; P <0.05). In contrast to renal medulla, adrenalectomy did not significantly alter the relative abundance of B1 protein in renal cortex. Angiotensin II also did not significantly alter the relative levels of B1 protein in the cortex, but it increased it significantly in renal medullary membranes (231±56%, n=8; P <0.005). Moreover, enhanced H + -ATPase B1 subunit protein immunoreactivity was found in medullary collecting duct segments of rats infused with angiotensin II. In contrast to B1, expression of a4, another subunit of the H + -ATPase was not altered by adrenalectomy or angiotensin II. We conclude that adrenalectomy decreases whereas angiotensin II increases H + -ATPase B1 subunit expression in medullary, but not in cortical collecting ducts. By increasing the relative abundance of the B1 subunit of H + -ATPase in the collecting duct, angiotensin II excess may lead to increased hydrogen ion secretion and thus metabolic alkalosis—a common feature of hypertensive syndromes associated with angiotensin II overactivity.

Regulatory and functional interaction of vasoactive factors in the kidney and extracellular pH

A growing body of evidence suggests that vasoactive factors produced in the kidney such as nitric oxide, endothelins, angiotensin, and prostaglandins participate actively in the regulation of acid-base homeostasis under physiologic conditions. In addition, recent reports indicate that alterations in the systemic acid-base status may also influence the generation of vasoactive cytokines in the kidney, which in turn may mediate the renal effector processes that tend to restore normality under such conditions. Metabolic acidosis, which so frequently accompanies many forms of chronic renal failure (CRF), may contribute to down-regulation of intrarenal nitric oxide production that characterizes CRF. Reduced extracellular pH inhibits inducible nitric oxide production in mesangial cells by altering the reduced form of nicotinamide adenine dinucleotide phosphate (NADPH) oxidation, an important posttranslational mechanism in the inducible nitric oxide synthase (iNOS) activation. The underlying defects resulting in the uncoupling of NADPH oxidation in acidemic microenvironment are discussed. Acidosis stimulates renal production of endothelins, which mediate proximal tubular acidification by enhancing sodium-hydrogen exchanger-3 (NHE-3) activity. Renal endothelins mediate enhanced urinary acid excretion following dietary acid ingestion, an effect that is effectively blocked by endothelin receptor blockers. Reduced extracellular pH stimulates endothelin secretion from renal microvascular endothelial cells, which may promote enhanced acid excretion from the distal tubule under conditions of acidosis. These phenomena as well as the role of angiotensin and renal prostaglandins in mediating renal acidification in normal and acidotic conditions are discussed in this review, which describe the regulatory interaction between extracellular pH and renal vasoactive factors.

Renal vacuolar H+-ATPase

Vacuolar H(+)-ATPases are ubiquitous multisubunit complexes mediating the ATP-dependent transport of protons. In addition to their role in acidifying the lumen of various intracellular organelles, vacuolar H(+)-ATPases fulfill special tasks in the kidney. Vacuolar H(+)-ATPases are expressed in the plasma membrane in the kidney almost along the entire length of the nephron with apical and/or basolateral localization patterns. In the proximal tubule, a high number of vacuolar H(+)-ATPases are also found in endosomes, which are acidified by the pump. In addition, vacuolar H(+)-ATPases contribute to proximal tubular bicarbonate reabsorption. The importance in final urinary acidification along the collecting system is highlighted by monogenic defects in two subunits (ATP6V0A4, ATP6V1B1) of the vacuolar H(+)-ATPase in patients with distal renal tubular acidosis. The activity of vacuolar H(+)-ATPases is tightly regulated by a variety of factors such as the acid-base or electrolyte status. This regulation is at least in part mediated by various hormones and protein-protein interactions between regulatory proteins and multiple subunits of the pump.