Effect of selective aldosterone deficiency on acidification in nephron segments of the rat inner medulla

Effect of Sodium Zirconium Cyclosilicate on Serum Potassium and Bicarbonate in Patients with Hyperkalemia and Metabolic Acidosis Associated with Chronic Kidney Disease: Rationale and Design of the NEUTRALIZE Study

INTRODUCTION

Sodium zirconium cyclosilicate (SZC) is a selective potassium (K+) binder for hyperkalemia management that provides rapid and sustained correction of hyperkalemia. The NEUTRALIZE study is investigating whether SZC, in addition to correcting hyperkalemia and maintaining normal serum K+, can provide sustained increases in serum bicarbonate (HCO3-) in patients with hyperkalemia and metabolic acidosis associated with chronic kidney disease (CKD).

METHODS

This is a prospective, randomized, double-blind, placebo-controlled phase 3b study of US adults with stage 3-5 CKD not on dialysis with hyperkalemia (K+ >5.0-≤5.9 mmol/L) and low-serum HCO3- (16-20 mmol/L). In the open-label correction phase, all eligible patients receive SZC 10 g three times daily for up to 48 h. Patients who achieve normokalemia (K+ ≥3.5-≤5.0 mmol/L) are then randomized 1:1 to once-daily SZC 10 g or placebo for a 4-week, double-blind, placebo-controlled maintenance phase. The primary endpoint is the proportion of patients with normokalemia at the end of treatment (EOT) without rescue therapy for hyperkalemia. Key secondary endpoints include mean change in serum HCO3-, the proportion of patients with an increase in serum HCO3- of ≥2 or ≥3 mmol/L without rescue therapy for metabolic acidosis, and the proportion of patients with serum HCO3- ≥22 mmol/L at EOT.

CONCLUSIONS

NEUTRALIZE will establish whether SZC can provide sustained increases in serum HCO3- while lowering serum K+ in patients with hyperkalemia and CKD-associated metabolic acidosis and may provide insights on the mechanism(s) underlying the increased serum HCO3- with SZC treatment.

Hypokalemic Distal Renal Tubular Acidosis

Distal renal tubular acidosis (DRTA) is defined as hyperchloremic, non-anion gap metabolic acidosis with impaired urinary acid excretion in the presence of a normal or moderately reduced glomerular filtration rate. Failure in urinary acid excretion results from reduced H⁺ secretion by intercalated cells in the distal nephron. This results in decreased excretion of NH₄⁺ and other acids collectively referred as titratable acids while urine pH is typically above 5.5 in the face of systemic acidosis. The clinical phenotype in patients with DRTA is characterized by stunted growth with bone abnormalities in children as well as nephrocalcinosis and nephrolithiasis that develop as the consequence of hypercalciuria, hypocitraturia, and relatively alkaline urine. Hypokalemia is a striking finding that accounts for muscle weakness and requires continued treatment together with alkali-based therapies. This review will focus on the mechanisms responsible for impaired acid excretion and urinary potassium wastage, the clinical features, and diagnostic approaches of hypokalemic DRTA, both inherited and acquired.

Hyperkalemic Forms of Renal Tubular Acidosis: Clinical and Pathophysiological Aspects

In contrast to distal type I or classic renal tubular acidosis (RTA) that is associated with hypokalemia, hyperkalemic forms of RTA also occur usually in the setting of mild-to-moderate CKD. Two pathogenic types of hyperkalemic metabolic acidosis are frequently encountered in adults with underlying CKD. One type, which corresponds to some extent to the animal model of selective aldosterone deficiency (SAD) created experimentally by adrenalectomy and glucocorticoid replacement, is manifested in humans by low plasma and urinary aldosterone levels, reduced ammonium excretion, and preserved ability to lower urine pH below 5.5. This type of hyperkalemic RTA is also referred to as type IV RTA. It should be noted that the mere deficiency of aldosterone when glomerular filtration rate is completely normal only causes a modest decline in plasma bicarbonate which emphasizes the importance of reduced glomerular filtration rate in the development of the hyperchloremic metabolic acidosis associated with SAD. Another type of hyperkalemic RTA distinctive from SAD in which plasma aldosterone is not reduced is referred to as hyperkalemic distal renal tubular acidosis because urine pH cannot be reduced despite acidemia or after provocative tests aimed at increasing sodium-dependent distal acidification such as the administration of sodium sulfate or loop diuretics with or without concurrent mineralocorticoid administration. This type of hyperkalemic RTA (also referred to as voltage-dependent distal renal tubular acidosis) has been best described in patients with obstructive uropathy and resembles the impairment in both hydrogen ion and potassium secretion that are induced experimentally by urinary tract obstruction and when sodium transport in the cortical collecting tubule is blocked by amiloride.

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.

A mathematical model of the rat nephron: glucose transport

Mathematical models of the proximal tubule (PT), loop of Henle (LOH), and distal nephron have been combined to simulate transport by rat renal tubules. The ensemble is composed of 24,000 superficial (SF) nephrons and 12,000 juxtamedullary (JM) nephrons in 5 classes (according to LOH length); all coalesce into 7,200 connecting tubules (CNT). Medullary interstitial solute concentrations are specified. The model equations require that each nephron glomerular filtration rate (GFR) satisfies a tubuloglomerular feedback (TGF) relationship, and each initial hydrostatic pressure yields a common CNT pressure; that common CNT pressure is determined from an overall distal hydraulic resistance to flow. By virtue of the greater GFR for JM nephrons, fluid delivery to SF and JM tubules is comparable. Glucose reabsorption is restricted to the PT, cotransported with one Na in the convoluted tubule (SGLT2), and two Na in the straight tubule (SGLT1). Increasing ambient glucose from 5 to 10 mM increases proximal Na reabsorption and decreases distal delivery. This is mitigated by a TGF-mediated increase in GFR, and may thus be an etiology for TGF-mediated glomerular hyperfiltration. With SGLT2 inhibition by 95%, the model predicts that under normoglycemic conditions about 60% of filtered glucose will still be reabsorbed, so that profound glycosuria is not to be expected. Compared with glucose-driven osmotic diuresis, SGLT2 inhibition provokes greater natriuresis. When hyperglycemia is superimposed on SGLT2 inhibition, the model suggests that natriuresis may be severe, reflecting synergy of a proximal diuretic and osmotic diuresis. In sum, the model captures TGF-mediated diabetic hyperfiltration and predicts glomerular protection with SGLT2 inhibition.

Regulation of luminal acidification by the V-ATPase

Specialized cells in the body express high levels of V-ATPase in their plasma membrane and respond to hormonal and nonhormonal cues to regulate extracellular acidification. Mutations in or loss of some V-ATPase subunits cause several disorders, including renal distal tubular acidosis and male infertility. This review focuses on the regulation of V-ATPase-dependent luminal acidification in renal intercalated cells and epididymal clear cells, which are key players in these physiological processes.

Maternally-inherited diabetes with deafness (MIDD) and hyporeninemic hypoaldosteronism

Maternally-inherited diabetes with deafness (MIDD) is a rare form of monogenic diabetes that results, in most cases, from an A-to-G transition at position 3243 of mitochondrial DNA (m.3243A>G) in the mitochondrial-encoded tRNA leucine (UUA/G) gene. As the name suggests, this condition is characterized by maternally-inherited diabetes and bilateral neurosensory hearing impairment. A characteristic of mitochondrial cytopathies is the progressive multisystemic involvement with the development of more symptoms during the course of the disease. We report here the case of a patient with MIDD who developed hyporeninemic hypoaldosteronism.

Regulation of transport in the connecting tubule and cortical collecting duct

The central goal of this overview article is to summarize recent findings in renal epithelial transport,focusing chiefly on the connecting tubule (CNT) and the cortical collecting duct (CCD).Mammalian CCD and CNT are involved in fine-tuning of electrolyte and fluid balance through reabsorption and secretion. Specific transporters and channels mediate vectorial movements of water and solutes in these segments. Although only a small percent of the glomerular filtrate reaches the CNT and CCD, these segments are critical for water and electrolyte homeostasis since several hormones, for example, aldosterone and arginine vasopressin, exert their main effects in these nephron sites. Importantly, hormones regulate the function of the entire nephron and kidney by affecting channels and transporters in the CNT and CCD. Knowledge about the physiological and pathophysiological regulation of transport in the CNT and CCD and particular roles of specific channels/transporters has increased tremendously over the last two decades.Recent studies shed new light on several key questions concerning the regulation of renal transport.Precise distribution patterns of transport proteins in the CCD and CNT will be reviewed, and their physiological roles and mechanisms mediating ion transport in these segments will also be covered. Special emphasis will be given to pathophysiological conditions appearing as a result of abnormalities in renal transport in the CNT and CCD.

More actors in ammonia absorption by the thick ascending limb

This review will briefly summarize current knowledge on the basolateral ammonia transport mechanisms in the thick ascending limb (TAL) of the loop of Henle. This segment transports ammonia against a concentration gradient and is responsible for the accumulation of ammonia in the medullary interstitium, which, in turn, favors ammonia secretion across the collecting duct. Experimental data indicate that the sodium/hydrogen ion exchanger isoform 4 (NHE4; Scl9a4) is a sodium/ammonia exchanger and plays a major role in this process. Disruption of murine NHE4 leads to metabolic acidosis with inappropriate urinary ammonia excretion and decreases the ability of the TAL to absorb ammonia and to build the corticopapillary ammonia gradient. However, NHE4 does not account for the entirety of ammonia absorption by the TAL, indicating that, at least, one more transporter is involved.

Aldosterone stimulates vacuolar H(+)-ATPase activity in renal acid-secretory intercalated cells mainly via a protein kinase C-dependent pathway

Urinary acidification in the collecting duct is mediated by the activity of H(+)-ATPases and is stimulated by various factors including angiotensin II and aldosterone. Classically, aldosterone effects are mediated via the mineralocorticoid receptor. Recently, we demonstrated a nongenomic stimulatory effect of aldosterone on H(+)-ATPase activity in acid-secretory intercalated cells of isolated mouse outer medullary collecting ducts (OMCD). Here we investigated the intracellular signaling cascade mediating this stimulatory effect. Aldosterone stimulated H(+)-ATPase activity in isolated mouse and human OMCDs. This effect was blocked by suramin, a general G protein inhibitor, and GP-2A, a specific G(αq) inhibitor, whereas pertussis toxin was without effect. Inhibition of phospholipase C with U-73122, chelation of intracellular Ca(2+) with BAPTA, and blockade of protein kinase C prevented the stimulation of H(+)-ATPases. Stimulation of PKC by DOG mimicked the effect of aldosterone on H(+)-ATPase activity. Similarly, aldosterone and DOG induced a rapid translocation of H(+)-ATPases to the luminal side of OMCD cells in vivo. In addition, PD098059, an inhibitor of ERK1/2 activation, blocked the aldosterone and DOG effects. Inhibition of PKA with H89 or KT2750 prevented and incubation with 8-bromoadenosine-cAMP mildly increased H(+)-ATPase activity. Thus, the nongenomic modulation of H(+)-ATPase activity in OMCD-intercalated cells by aldosterone involves several intracellular pathways and may be mediated by a G(αq) protein-coupled receptor and PKC. PKA and cAMP appear to have a modulatory effect. The rapid nongenomic action of aldosterone may participate in the regulation of H(+)-ATPase activity and contribute to final urinary acidification.

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.

The ABC transporter PGP-2 from Caenorhabditis elegans is expressed in the sensory neuron pair AWA and contributes to lysosome formation and lipid storage within the intestine

The functional role of the ABC transporter PGP-2 from the nematode Caenorhabditis elegans has been studied by combining phenotype analyses of pgp-2 deletion mutants or pgp-2 RNAi treated worms with reporter gene studies using a pgp-2::GFP construct. pgp-2 mutants showed a strong reduction of lipid stores. In addition, we found that in the case of the pgp-2 mutant or after pgp-2 RNAi the worms were unable to perform pinocytosis and to acidify intestinal lysosomes. Especially under cholesterol-restricted conditions, the viability of the mutant was reduced. Surprisingly, the chemosensory AWA neurons in the head region were identified as expression sites by reporter gene studies. These neurons are known to be involved in attraction behaviour towards odorants associated with potential food bacteria. Our results imply that PGP-2 is involved in a signalling process that connects sensory inputs to intestinal functions, possibly by influencing acidification of intestinal lysosomes, which in turn may affect pinocytosis and lipid storage.

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.

Nongenomic stimulation of vacuolar H+-ATPases in intercalated renal tubule cells by aldosterone

Renal collecting ducts play a critical role in acid-base homeostasis by establishing steep transepithelial pH gradients necessary for the almost complete reabsorption of bicarbonate and the effective secretion of ammonium into the urine. The mechanisms of urine acidification in collecting ducts involve active, electrogenic hydrogen (H+) secretion and, less importantly, potassium (K+)-H+ exchange. Deranged renal acidification and the inability to lower urine pH are hallmarks of distal tubular acidosis and often result from inborn errors of metabolism involving vacuolar H+-ATPase subunits in the collecting ducts. Three factors regulate H+-ATPase activity in intercalated cells of collecting ducts: the acid-base status, angiotensin II, and aldosterone. Most effects of aldosterone involve activation of the mineralocorticoid receptor and genomic changes in transcription and protein synthesis. Here we demonstrate a nongenomic pathway of vacuolar H+-ATPase activation in intercalated cells of isolated mouse outer medullary collecting ducts (OMCD). In vitro exposure of isolated outer medullary collecting ducts to aldosterone (10 nM) for times as short as 15 min increases vacuolar H+-ATPase activity approximately 2- to 3-fold. Neither inhibition of mineralocorticoid receptors nor of transcription and protein synthesis prevented aldosterone-induced stimulation of H+-ATPase. Incubation with colchicine, however, abolished the stimulatory effect of aldosterone, suggesting a role of the microtubular network for H+-ATPase stimulation. Immunohistochemistry in kidneys from aldosterone-injected mice showed increased apical H+-ATPase staining in OMCD-intercalated cells. The stimulatory effect of aldosterone was associated with a transient rise in intracellular Ca2+ and required intact PKC. Thus, rapid nongenomic modulation of vacuolar H+-ATPase activity in OMCD-intercalated cells by aldosterone may play an additional role in hormonal control of systemic acid-base homeostasis.

A mathematical model of rat collecting duct. I. Flow effects on transport and urinary acidification

A mathematical model of the rat collecting duct (CD) has been developed by concatenating previously published models of cortical (Weinstein AM. Am J Physiol Renal Physiol 280: F1072-F1092, 2001); outer medullary (Weinstein AM. Am J Physiol Renal Physiol 279: F24-F45, 2000); and inner medullary segments (Weinstein AM. Am J Physiol Renal Physiol 274: F841-F855, 1998). Starting with end-distal tubular flow rate and composition, plus interstitial solute profiles, the model predicts urinary solute flows, including the buffer concentrations required to assess net acid excretion. In the model CD, the interstitial corticomedullary osmotic gradient provides the basis for the flow effect on the transport of several solutes. For substances that have an interstitial accumulation and that can have diffusive secretion (e.g., urea and NH(4)(+)), enhanced luminal flow increases excretion by decreasing luminal accumulation. For substances that are reabsorbed (e.g., K+ and HCO(3)(-)), and for which luminal accumulation can enhance reabsorption, increasing luminal flow again increases excretion by decreasing luminal solute concentration. In model calculations, flow-dependent increases in HCO(3)(-) and NH(4)(+) approximately balance, so net acid excretion is little changed by flow, albeit at a higher urinary pH. The model identifies delivery flow rate to the CD as a potent determinant of urinary pH, with high flows blunting maximal acidification. At even modestly high flows (9 nl x min-1. tubule-1, with 6% of filtered Na+ entering the CD), the model cannot achieve a urinary pH <5.5 unless the delivered HCO(3)(-) concentration is extremely low (<2 mM). Nevertheless, simulation of Na2SO4 diuresis does yield both an increase in net acid excretion and a decrease in urinary HCO(3)(-) (i.e., a decrease in pH) despite the increase in urinary flow. This model should provide a tool for examining hypotheses regarding transport defects underlying distal renal tubular acidosis.

A mathematical model of the inner medullary collecting duct of the rat: acid/base transport

A mathematical model of the inner medullary collecting duct (IMCD) of the rat has been developed that is suitable for simulating luminal buffer titration and ammonia secretion by this nephron segment. Luminal proton secretion has been assigned to an H-K-ATPase, which has been represented by adapting the kinetic model of the gastric enzyme by Brzezinski et al. (P. Brzezinski, B. G. Malmstrom, P. Lorentzon, and B. Wallmark. Biochim. Biophys. Acta 942: 215-219, 1988). In shifting to a 2 H+:1 ATP stoichiometry, the model enzyme can acidify the tubule lumen approximately 3 pH units below that of the cytosol, when luminal K+ is in abundance. Peritubular base exit is a combination of ammonia recycling and HCO3- flux (either via Cl-/HCO3- exchange or via a Cl- channel). Ammonia recycling involves NH4(+) uptake on the Na-K-ATPase followed by diffusive NH3 exit [S. M. Wall. Am. J. Physiol. 270 (Renal Physiol. 39): F432-F439, 1996]; model calculations suggest that this is the principal mode of base exit. By virtue of this mechanism, the model also suggests that realistic elevations in peritubular K+ concentration will compromise IMCD acid secretion. Although ammonia recycling is insensitive to carbonic anhydrase (CA) inhibition, the base exit linked to HCO3- flux provides a CA-sensitive component to acid secretion. In model simulations, it is observed that increased luminal NaCl entry increases ammonia cycling but decreases peritubular Cl-/HCO3- exchange (due to increased cell Cl-). This parallel system of peritubular base exit stabilizes acid secretion in the face of variable Na+ reabsorption.

Chronic hyperkalemia impairs ammonium transport and accumulation in the inner medulla of the rat

Previously we demonstrated in rats that chronic hyperkalemia had no effect on ammonium secretion by the proximal tubule in vivo but that high K+ concentrations inhibited ammonium absorption by the medullary thick ascending limb in vitro. These observations suggested that chronic hyperkalemia may reduce urinary ammonium excretion through effects on medullary transport events. To examine directly the effects of chronic hyperkalemia on medullary ammonium accumulation and collecting duct ammonium secretion, micropuncture experiments were performed in the inner medulla of Munich-Wistar rats pair fed a control or high-K+ diet for 7-13 d. In situ pH and total ammonia concentrations were measured to calculate NH3 concentrations for base and tip collecting duct and vasa recta. Chronic K+ loading was associated with significant systemic metabolic acidosis and a 40% decrease in urinary ammonium excretion. In control rats, 15% of excreted ammonium was secreted between base and tip collecting duct sites. In contrast, no net transport of ammonium was detected along the collecting duct in high-K+ rats. The decrease in collecting duct ammonium secretion in hyperkalemia was associated with a decrease in the NH3 concentration difference between vasa recta and collecting duct. The fall in the NH3 concentration difference across the collecting duct in high-K+ rats was due entirely to a decrease in [NH3] in the medullary interstitial fluid, with no change in [NH3] in the collecting duct. These results indicate that impaired accumulation of ammonium in the medullary interstitium, secondary to inhibition of ammonium absorption in the medullary thick ascending limb, may play an important role in reducing collecting duct ammonium secretion and urinary ammonium excretion during chronic hyperkalemia.

Evaluation of carbonic anhydrase isozymes in disorders involving osteopetrosis and/or renal tubular acidosis

Carbonic anhydrase II (CA II) deficiency in man is an autosomal recessive disorder manifest by osteopetrosis, renal tubular acidosis, and cerebral calcification. Other features include growth failure and mental retardation. Complications of the osteopetrosis include frequent bone fractures, cranial nerve compression symptoms, and dental malocclusion. The anemia and leukopenia seen in the recessive, lethal infantile form of osteopetrosis are not seen in CA II deficient patients. The renal tubular acidosis usually includes both proximal and distal components. Symptoms of metabolic acidosis respond to therapy, but no specific treatment is available for the osteopetrosis or cerebral calcification. We review here the role of carbonic anhydrases in bone resorption and renal acidification, and discuss clinical features and laboratory findings which distinguish CA II deficiency from other disorders producing osteopetrosis, renal tubular acidosis, or brain calcification. Methods to evaluate patients with pure proximal renal tubular acidosis for deficiency of CA IV are also discussed.

Expression and distribution of renal vacuolar proton-translocating adenosine triphosphatase in response to chronic acid and alkali loads in the rat

Renal hydrogen ion excretion increases with chronic acid loads and decreases with alkali loads. We examined the mechanism of adaptation by analyzing vacuolar proton-translocating adenosine triphosphatase (H+ ATPase) 31-kD subunit protein and mRNA levels, and immunocytochemical distribution in kidneys from rats subjected to acid or alkali loads for 1, 3, 5, 7, and 14 d. Acid- and alkali-loaded rats exhibited adaptive responses in acid excretion, but showed no significant changes in H+ ATPase protein or mRNA levels in either cortex or medulla. In contrast, there were profound adaptive changes in the immunocytochemical distribution of H+ ATPase in collecting duct intercalated cells. In the medulla, H+ ATPase staining in acid-loaded rats shifted from cytoplasmic vesicles to plasma membrane, whereas in alkali-loaded rats, cytoplasmic vesicle staining was enhanced, and staining of plasma membrane disappeared. In the cortical collecting tubule, acid loading increased the number of intercalated cells showing enhanced apical H+ ATPase staining and decreased the number of cells with basolateral or poorly polarized apical staining. The results indicate that both medulla and cortex participate in the adaptive response to acid and alkali loading by changing the steady-state distribution of H+ ATPase, employing mechanisms that do not necessitate postulating interconversion of intercalated cells with opposing polarities.

Inner medullary collecting duct Na(+)-H+ exchanger

Cells from the inner medullary collecting duct (IMCD) exhibit Na(+)-H+ exchange. The present studies were performed to address certain important characteristics of this process in cultured IMCD cells. First, Na(+)-H+ exchange was found to be present both at 37 degrees C and at 25 degrees C, in contrast to Na(+)-independent H+ extrusion, which was only observed in some cultures and only at 37 degrees C. Second, with the use of image analysis techniques, virtually all cells in IMCD cultures were demonstrated to possess Na(+)-H+ exchange, whether or not the cells exhibited Na(+)-independent intracellular pH recovery from acid loads. Also, Na(+)-H+ exchange was found to be expressed on the basolateral aspect of these cells, but not on the apical membrane. These properties of IMCD Na(+)-H+ exchange are consistent with a function to regulate intracellular pH rather than mediate transepithelial acid-base transport. Na(+)-H+ exchange in IMCD cells was also compared with that in cultured renal proximal tubule cells. Despite physiologically distinct roles in vivo, Na(+)-H+ exchange in these two cell types in culture was found to be similar with respect to the Km for Na+ and the Ki for 5-(N-ethyl-N-isopropyl)amiloride. These data are consistent with functionally similar (if not identical) processes mediating Na(+)-H+ exchange in these two cell types, but with opposite polarity.

Acute metabolic acidosis enhances circulating parathyroid hormone, which contributes to the renal response against acidosis in the rat

Acute PTH administration enhances final urine acidification in the rat. HCl was infused during 3 h in rats to determine the parathyroid and renal responses to acute metabolic acidosis. Serum immunoreactive PTH (iPTH) concentration significantly increased and nephrogenous adenosine 3H,5H-cyclic monophosphate tended to increase during HCl loading in intact and adrenalectomized (ADX) rats despite significant increments in plasma ionized calcium. Strong linear relationships existed between serum iPTH concentration and arterial bicarbonate or proton concentration (P less than 0.0001). Serum iPth concentration and NcAMP remained stable in intact time-control rats and decreased in CaCl2-infused, nonacidotic animals. Urinary acidification was markedly reduced in parathyroidectomized (PTX) as compared with intact rats during both basal and acidosis states; human PTH-(1-34) infusion in PTX rats restored in a dose-dependent manner the ability of the kidney to acidify the urine and excrete net acid. Acidosis-induced increase in urinary net acid excretion was observed in intact, PTX, and ADX, but not in ADX-thyroparathyroidectomized rats. We conclude that (a) acute metabolic acidosis enhances circulating PTH activity, and (b) PTH markedly contributes to the renal response against acute metabolic acidosis by enhancing urinary acidification.

Basolateral membrane sodium-independent Cl-/HCO3- exchanger in rat inner medullary collecting duct cell

Previous studies have shown that the middle third of the rat inner medullary collecting duct (IMCD-2) secretes protons despite the absence of intercalated cells, the cell thought to secrete protons in other portions of the collecting duct. A new cell, the IMCD cell, is the predominant cell in IMCD-2. The mechanism responsible for base exit in the IMCD cell was characterized by measuring cell pH of isolated perfused tubules with 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein. Reduction of bath HCO3- caused a significant and reversible decrease in cell pH, whereas a similar change in luminal HCO3- had a significantly smaller effect, indicating that the HCO3-/H+ permeability of the basolateral membrane is much larger than the apical membrane. The rate of cell acidification induced by reduction in bath HCO3-, a measure of basolateral HCO3- transport, was significantly decreased in the absence of bath and lumen Cl. Decreases in bath Cl caused a significant and reversible increase in cell pH, which was not changed significantly by complete removal of Na from perfusate and bath, but was significantly inhibited by basolateral 4',5'-diisothiocyanostilbene-2,2'-disulfonic acid. A chemical voltage clamp did not inhibit the rate of cell alkalinization after bath Cl removal, indicating that Cl-/HCO3- exchange is not via parallel Cl and HCO3- conductances. Cell pH was measured in single cells by low-light-level imaging to show that most cells contain the chloride-dependent HCO3- pathway. We conclude that the rat IMCD cell possesses a basolateral Na-independent CL-/HCO3- exchanger which may serve as the base exit step for transepithelial proton secretion.

Renal tubular acidosis

The term renal tubular acidosis (RTA) is applied to a group of transport defects in the reabsorption of bicarbonate (HCO3-), the excretion of hydrogen ions, or both. On clinical and pathophysiological grounds, RTA can be separated into three main types: distal RTA (type 1), proximal RTA (type 2) and hyperkalaemic RTA (type 4). Some patients present combined types of proximal and distal RTA or of hyperkalaemic and distal RTA. Diagnosis of RTA should be suspected when a patient presents a normal plasma anion gap, and hyperchloraemic metabolic acidosis. A normal plasma anion gap (Na(+)-[Cl- + HCO3-] = 8-16 mEq/l) reflects loss of HCO3- from the extracellular fluid via the gastro-intestinal tract or the kidney, dilution of extracellular buffer or administration of hydrochloric acid (HCl) or its precursors. Distinction of RTA from other disorders is greatly facilitated by the study of the urine anion gap (Na+ + K+ - Cl-). This index estimates the urinary concentration of ammonium in a patient with hyperchloraemic metabolic acidosis. A negative urine anion gap (Cl- much greater than Na+ + K+) suggests the presence of gastro-intestinal or renal loss of HCO3-, while a positive urine anion gap (Cl- less than Na+ + K+) is indicative of a distal acidification defect. Determination of plasma potassium, of urine pH at low plasma HCO3- concentration, and of urine PCO2 and fractional excretion of HCO3- at normal plasma HCO3- concentration permits the differentiation between the various types of RTA.