Regulation of the renal Na-HCO3 cotransporter: V. mechanism of the inhibitory effect of parathyroid hormone

Hydrogen, Bicarbonate, and Their Associated Exchangers in Cell Volume Regulation

Cells lacking a stiff cell wall, e.g., mammalian cells, must actively regulate their volume to maintain proper cell function. On the time scale that protein production is negligible, water flow in and out of the cell determines the cell volume variation. Water flux follows hydraulic and osmotic gradients; the latter is generated by various ion channels, transporters, and pumps in the cell membrane. Compared to the widely studied roles of sodium, potassium, and chloride in cell volume regulation, the effects of proton and bicarbonate are less understood. In this work, we use mathematical models to analyze how proton and bicarbonate, combined with sodium, potassium, chloride, and buffer species, regulate cell volume upon inhibition of ion channels, transporters, and pumps. The model includes several common, widely expressed ion transporters and focuses on obtaining generic outcomes. Results show that the intracellular osmolarity remains almost constant before and after cell volume change. The steady-state cell volume does not depend on water permeability. In addition, to ensure the stability of cell volume and ion concentrations, cells need to develop redundant mechanisms to maintain homeostasis, i.e., multiple ion channels or transporters are involved in the flux of the same ion species. These results provide insights for molecular mechanisms of cell volume regulation with additional implications for water-driven cell migration.

The electrogenic sodium bicarbonate cotransporter and its roles in the myocardial ischemia-reperfusion induced cardiac diseases

Cardiac tissue ischemia/hypoxia increases glycolysis and lactic acid accumulation in cardiomyocytes, leading to intracellular metabolic acidosis. Sodium bicarbonate cotransporters (NBCs) play a vital role in modulating intracellular pH and maintaining sodium ion concentrations in cardiomyocytes. Cardiomyocytes mainly express electrogenic sodium bicarbonate cotransporter (NBCe1), which has been demonstrated to participate in myocardial ischemia/reperfusion (I/R) injury. This review outlines the structural and functional properties of NBCe1, summarizes the signaling pathways and factors that may regulate the activity of NBCe1, and reviews the roles of NBCe1 in the pathogenesis of I/R-induced cardiac diseases. Further studies revealing the regulatory mechanisms of NBCe1 activity should provide novel therapeutic targets for preventing I/R-induced cardiac diseases.

Phosphaturia in kidney stone formers: Still an enigma

Calcium kidney stones are common worldwide. Most are idiopathic and composed of calcium oxalate. Calcium phosphate is present in around 80% and may initiate stone formation. Stone production is multifactorial with a polygenic genetic contribution. Phosphaturia is found frequently among stone formers but until recently received scant attention. This review examines possible mechanisms for the phosphaturia and its relevance to stone formation from a wide angle. There is a striking lack of clinical data. Phosphaturia is associated, but not correlated, with hypercalciuria, increased 1,25 dihydroxy-vitamin D [1,25 (OH)₂D], and sometimes evidence of disturbances in proximal renal tubular function. Phosphate reabsorption in the proximal renal tubules requires tightly regulated interaction of many proteins. Paracellular flow through intercellular tight junctions is the major route of phosphate absorption from the intestine and can be reduced therapeutically in hyperphosphatemic patients. In monogenic defects stones develop when phosphaturia is associated with hypercalciuria, generally explained by increased 1,25 (OH)₂D production in response to hypophosphatemia. Calcification does not occur in disorders with increased FGF23 when phosphaturia occurs in isolation and 1,25 (OH)₂D is suppressed. Candidate gene studies have identified mutations in the phosphate transporters, but in few individuals. One genome-wide study identified a polymorphism of the phosphate transporter gene SLC34A4 associated with stones. Others did not find mutations obviously linked to phosphate reabsorption. Future genetic studies should have a wide trawl and should focus initially on groups of patients with clearly defined phenotypes. The global data should be pooled.

NHE3 phosphorylation at serines 552 and 605 does not directly affect NHE3 activity

Direct phosphorylation of sodium hydrogen exchanger type 3 (NHE3) is a well-established physiological phenomenon; however, the exact role of NHE3 phosphorylation in its regulation remains unclear. The objective of this study was to evaluate whether NHE3 phosphorylation at serines 552 and 605 is physiologically regulated in vivo and, if so, whether changes in phosphorylation at these sites are tightly coupled to changes in transport activity. To this end, we directly compared PKA-induced NHE3 inhibition with site-specific changes in NHE3 phosphorylation in vivo and in vitro. In vivo, PKA was activated using an intravenous infusion of parathyroid hormone in Sprague-Dawley rats. In vitro, PKA was activated directly in opossum kidney (OKP) cells using forskolin and IBMX. NHE3 activity was assayed in microvillar membrane vesicles in the rat model and by 22Na uptake in the OKP cell model. In both cases, NHE3 phosphorylation at serines 552 and 605 was determined using previously characterized monoclonal phosphospecific antibodies directed to these sites. In vivo, we found dramatic changes in NHE3 phosphorylation at serines 552 and 605 with PKA activation but no corresponding alteration in NHE3 activity. This dissociation between NHE3 phosphorylation and activity was further verified in OKP cells in which phosphorylation clearly preceded transport inhibition. We conclude that although phosphorylation of NHE3 at serines 552 and 605 is regulated by PKA both in vivo and in vitro, phosphorylation of these sites does not directly alter Na+/H+ exchange activity.

The sodium bicarbonate cotransporter: structure, function, and regulation

The role of the Na(+)-coupled HCO(3)(-) transporter (NBC) family is indispensable in acid-base homeostasis. Almost all tissues express a member of the NBC family. NBC has been studied extensively in the kidney and plays a role in proximal tubule HCO(3)(-) reabsorption. Although the exact function of this transporter family on other tissues is not very clear, the ubiquitous expression of NBC family suggests a role in cell pH regulation. Altered NBC activity caused by mutations of the gene responsible for NBC protein expression results in pathophysiologic conditions. Mutations of NBC resulting in important clinical disorders have been reported extensively on one member of the NBC family, the kidney NBC (NBC1). These mutations have led to several structural studies to understand the mechanism of the abnormal NBC1 activity.

Coordinated down-regulation of NBC-1 and NHE-3 in sodium and bicarbonate loading

BACKGROUND

Bicarbonate reabsorption in the kidney proximal tubule is predominantly mediated via the apical Na+/H+ exchanger (NHE-3) and basolateral Na+: HCO(-3) cotransporter (NBC-1). The purpose of these studies was to examine the effects of Na+ load and altered acid-base status on the expression of NHE-3 and NBC-1 in the kidney.

METHODS

Rats were placed on 280 mmol/L of NaHCO(3), NaCl, or NH(4)Cl added to their drinking water for 5 days and examined for the expression of NHE-3 and NBC-1 in the kidney.

RESULTS

Serum [HCO(-3)] was unchanged in NaHCO(-3) and NaCl-loaded animals versus control (P> 0.05). However, a significant hyperchloremic metabolic acidosis was developed in NH4Cl-loaded animals. A specific polyclonal antibody against NBC-1 recognized a 130 kD band, which was exclusively expressed in the basolateral membrane of proximal tubules. Immunoblot studies indicated that the protein abundance of NBC-1 and NHE-3 in the cortex decreased by 74% (P < 0.04) and 66% (P < 0.03), respectively, in NaHCO(3) loading and by 72% (P < 0.003) and 55% (P < 0.04), respectively, in NaCl loading. Switching from NaHCO(3) to distilled water resulted in rapid recovery of NHE-3 and NBC-1 protein expression toward normal levels. Metabolic acidosis increased the abundance of NHE-3 (P < 0.0001) but not NBC-1 (P> 0.05).

CONCLUSIONS

NaHCO(-3) or NaCl loading coordinately down-regulates the apical NHE-3 and basolateral NBC-1 in rat kidney proximal tubule, presumably due to increased Na+ load. We propose that the down-regulation of these two Na+- and HCO(3)-absorbing transporters is, to a large degree, responsible for enhanced excretion of excess of Na+ and alkaline load and prevention of metabolic alkalosis in rats subjected to NaHCO(-3) loading.

Sodium-potassium-adenosinetriphosphatase-dependent sodium transport in the kidney: hormonal control

Tubular reabsorption of filtered sodium is quantitatively the main contribution of kidneys to salt and water homeostasis. The transcellular reabsorption of sodium proceeds by a two-step mechanism: Na(+)-K(+)-ATPase-energized basolateral active extrusion of sodium permits passive apical entry through various sodium transport systems. In the past 15 years, most of the renal sodium transport systems (Na(+)-K(+)-ATPase, channels, cotransporters, and exchangers) have been characterized at a molecular level. Coupled to the methods developed during the 1965-1985 decades to circumvent kidney heterogeneity and analyze sodium transport at the level of single nephron segments, cloning of the transporters allowed us to move our understanding of hormone regulation of sodium transport from a cellular to a molecular level. The main purpose of this review is to analyze how molecular events at the transporter level account for the physiological changes in tubular handling of sodium promoted by hormones. In recent years, it also became obvious that intracellular signaling pathways interacted with each other, leading to synergisms or antagonisms. A second aim of this review is therefore to analyze the integrated network of signaling pathways underlying hormone action. Given the central role of Na(+)-K(+)-ATPase in sodium reabsorption, the first part of this review focuses on its structural and functional properties, with a special mention of the specificity of Na(+)-K(+)-ATPase expressed in renal tubule. In a second part, the general mechanisms of hormone signaling are briefly introduced before a more detailed discussion of the nephron segment-specific expression of hormone receptors and signaling pathways. The three following parts integrate the molecular and physiological aspects of the hormonal regulation of sodium transport processes in three nephron segments: the proximal tubule, the thick ascending limb of Henle's loop, and the collecting duct.

The affinity of the organic cation transporter rOCT1 is increased by protein kinase C-dependent phosphorylation

Members of the organic cation transporter (OCT) family are mainly expressed in kidney, liver, intestine, and brain. The regulation of the OCT type 1 from rat (rOCT1) stably transfected in HEK293 cells was examined using a fluorimetric technique, 1-[(3)H]methyl-4-phenylpyridinium uptake studies, and fast-whole-cell patch-clamp recordings. For the fluorescence measurements, the cation 4-(4-(dimethylamino)styryl)-N-methylpyridinium iodide (ASP(+)) was used as substrate. Uptake of ASP(+) via rOCT1 was electrogenic, and its inhibition by other organic cations was consistent with previously reported radioactive tracer flux measurements. The inhibitor quinine was not translocated by the organic cation transporter in contrast to tetraethylammonium. Stimulation of diacyl glycerol-dependent protein kinase C (PKC) by sn-1,2-dioctanoyl glycerol (1 microM) resulted in an increase in initial ASP(+) uptake rate by 216 +/- 28% (n = 29). The effect was completely antagonized by the PKC inhibitor tamoxifen (20 microM, n = 22). Forskolin (1 microM), which activates adenylate cyclase and thereby protein kinase A (PKA), stimulated the initial rate of ASP(+) accumulation by 51 +/- 6% (n = 19). This effect was inhibited by the specific PKA inhibitor KT5720 (1 microM, n = 12). Inhibition of tyrosine kinases by aminogenestein (10 microM) reduced ASP(+) uptake by 63 +/- 7% (n = 7), while genestein or tyrphostin AG1295 (each 10 microM) were without significant effects. Incubation of the cells with sn-1, 2-dioctanoyl glycerol (1 microM) increased the affinities of the transporter to tetraethylammonium, tetrapenthylammonium, and quinine by a factor of 58, 14.5, and 2.4, respectively. Western blot analysis revealed that rOCT1 protein was phosphorylated at a serine residue upon stimulation of PKC. In conclusion, it has been demonstrated that the organic cation transport by rOCT1 is stimulated by PKC, PKA, and endogenous tyrosine kinase activation. The PKC phosphorylates rOCT1 and leads to a conformational change at the substrate binding site.

Physiologic and molecular aspects of the Na+:HCO3- cotransporter in health and disease processes

Approximately 80% of the filtered load of HCO3- is reabsorbed in the proximal tubule via a process of active acid secretion by the luminal membrane. The major mechanism for the transport of HCO3- across the basolateral membrane is via the electrogenic Na+:3HCO3- cotransporter (NBC). Recent molecular cloning experiments have identified the existence of three NBC isoforms (NBC-1, NBC-2, and NBC-3).1 Functional and molecular studies indicate the presence of all three NBC isoforms in the kidney. All are presumed to mediate the cotransport of Na+ and HCO3- under normal conditions and may be functionally altered in certain pathophysiologic states. Specifically, NBC-1 may be up-regulated in metabolic acidosis and potassium depletion and in response to glucocorticoid excess and may be down-regulated in response to HCO3- loading or alkalosis. Recent studies provide molecular evidence indicating the expression of NBC-1 in pancreatic duct cells. NBC is activated by cystic fibrosis transmembrane conductance regulator (CFTR) and plays an important role in HCO3- secretion in the agonist-stimulated state in pancreatic duct cells. The purpose of this review is to summarize recent functional and molecular studies on the regulation of NBCs in physiologic and pathophysiologic states. Possible signals responsible for the regulation of NBCs in these conditions are examined. Furthermore, the possible role of this transporter in acid-base disorders (such as proximal renal tubular acidosis) is discussed.

Regulation of the renal Na-HCO(3) cotransporter. XI. Signal transduction underlying CO(2) stimulation

We have previously shown that CO(2) stimulation of the renal Na-HCO(3) cotransporter (NBC) activity is abrogated by general inhibitors of protein tyrosine kinases. The more selective inhibitor herbimycin also blocked this effect at concentrations known to preferentially inhibit Src family kinases (SFKs). We therefore examined a role for SFKs in CO(2)-stimulated NBC activity. To this end, we engineered OK cells to express the COOH-terminal Src kinase (Csk), a negative regulator of SFKs. CO(2) stimulated NBC activity normally in beta-galactosidase-expressing and untransfected control cells. In contrast, Csk-expressing cells had normal baseline NBC activity that was not stimulated by CO(2). CO(2) stimulation increased both total SFK activity and specific tyrosine phosphorylation of Src. The specific MEK1/2 inhibitor PD-98059 completely inhibited the CO(2) stimulation of NBC activity as well as the accompanying phosphorylation and activation of ERK1/2. Our data suggest the involvement of both SFKs, probably Src, and the "classic" MAPK pathway in mediating CO(2)-stimulated NBC activity in renal epithelial cells.

Regulation of the renal Na-HCO3 cotransporter: IX. Modulation by insulin, epidermal growth factor and carbachol

To examine the role of tyrosine kinase (TK) on basolateral membrane (BLM) transport, we looked for the presence of TK activity in these membranes and showed that the synthetic substrate for TK, poly [Glu80 Na, Tyr20] caused a three-fold increase in tyrosine phosphorylation. This effect was completely blocked by the TK inhibitors, 2-hydroxy-5(2,5-dihydroxybenzyl) aminobenzoic acid (HAC), 1 microM, and methyl 2,5-dihydroxycinnamate (DHC), 5 microM. We then examined the effect of agents that cause TK stimulation on tyrosine kinase immunocontent and on the Na-HCO3 cotransporter activity in BLM and in primary cultures of the proximal tubule. We utilized the cholinergic agent, carbachol (10(-4) M), epidermal growth factor (EGF 10(-8) M), and insulin (10(-8) M), well known activators of TK. Carbachol, insulin, and EGF caused a significant increase in TK immunoreactive protein content which was blocked by HAC and DHC. In BLM, carbachol significantly stimulated HCO3-dependent 22Na uptake and this effect was totally prevented by the monoclonal antibody against TK. In cultured proximal tubule cells, carbachol, EGF and insulin at physiologic concentration caused a significant stimulation of the cotransporter activity and this effect was completely blocked by the TK inhibitor, HAC. Increasing the dose of insulin 100-fold did not cause further stimulation of the cotransporter indicating that insulin plays a permissive role on the cotransporter. These results demonstrate the presence of TK in renal proximal tubule cells and show that activation of this kinase by dissimilar agents enhance the activity of the Na-HCO3 cotransporter.

Regulation of the renal Na-HCO3 cotransporter: VII. Mechanism of the cholinergic stimulation

Cholinergic agents regulate proximal tubule acidification but the mechanism responsible for this effect is unclear. We examined the effect of the cholinergic agent carbachol on the activity of the Na-HCO3 cotransporter in primary cultures of the proximal tubule of the rabbit. The activity of the cotransporter was assayed either as HCO3-dependent 22Na uptake or as the recovery of intracellular pH in cells perfused continuously with Cl-free physiologic solution containing amiloride to block the Na-H antiporter. Carbachol caused a dose-dependent stimulation of the cotransporter activity with a maximum increase of 90% above control values at 10(-5) M and half maximal stimulation at 10(-7) M. The stimulation was blocked by atropine and pirenzepine indicating an effect through the M1 muscarinic receptor. Carbachol increased intracellular calcium fourfold and the rise in cytosolic calcium was prevented by the intracellular calcium chelator, BAPTA. BAPTA also blocked the effect of carbachol on the cotransporter. Because carbachol activates phospholipase C and protein kinase C, we examined the effect of carbachol in the presence of the phospholipase C inhibitor, U73122, or the PKC inhibitor, calphostin C, or PKC depletion. The phospholipase C inhibitor prevented both the effect of carbachol on the cotransporter and on the intracellular Ca. Calphostin C and PKC depletion also prevented the stimulation of the cotransporter. Carbachol increased PKC activity and caused translocation of the PKC to the particulate fraction. We also examined the effect of the phosphatase inhibitor, calyculin A or the calmodulin kinase inhibitor, W-13 on carbachol stimulation. Calyculin A and W13 likewise prevented the carbachol-induced stimulation of the cotransporter. These results demonstrate that cholinergic stimulation modulated the activity of the cotransporter through multiple pathways including phospholipase C/PKC and phosphatase systems.