Effect of hypokalemia on renal expression of the ammonia transporter family members, Rh B Glycoprotein and Rh C Glycoprotein, in the rat kidney

State of knowledge on ammonia handling by the kidney

The disposal of ammonia, the main proton buffer in the urine, is important for acid-base homeostasis. Renal ammonia excretion is the predominant contributor to renal net acid excretion, both under basal condition and in response to acidosis. New insights into the mechanisms of renal ammonia production and transport have been gained in the past decades. Ammonia is the only urinary solute known to be produced in the kidney and selectively transported through the different parts of the nephron. Both molecular forms of total ammonia, NH3 and NH4+, are transported by specific proteins. Proximal tubular ammoniagenesis and the activity of these transport processes determine the eventual fate of total ammonia produced and excreted by the kidney. In this review, we summarized the state of the art of ammonia handling by the kidney and highlighted the newest processes described in the last decade.

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.

The proximal tubule through an NBCe1-dependent mechanism regulates collecting duct phenotypic and remodeling responses to acidosis

The renal response to acid-base disturbances involves phenotypic and remodeling changes in the collecting duct. This study examines whether the proximal tubule controls these responses. We examined mice with genetic deletion of proteins present only in the proximal tubule, either the A variant or both A and B variants of isoform 1 of the electrogenic Na+-bicarbonate cotransporter (NBCe1). Both knockout (KO) mice have spontaneous metabolic acidosis. We then determined the collecting duct phenotypic responses to this acidosis and the remodeling responses to exogenous acid loading. Despite the spontaneous acidosis in NBCe1-A KO mice, type A intercalated cells in the inner stripe of the outer medullary collecting duct (OMCDis) exhibited decreased height and reduced expression of H+-ATPase, anion exchanger 1, Rhesus B glycoprotein, and Rhesus C glycoprotein. Combined kidney-specific NBCe1-A/B deletion induced similar changes. Ultrastructural imaging showed decreased apical plasma membrane and increased vesicular H+-ATPase in OMCDis type A intercalated cell in NBCe1-A KO mice. Next, we examined the collecting duct remodeling response to acidosis. In wild-type mice, acid loading increased the proportion of type A intercalated cells in the connecting tubule (CNT) and OMCDis, and it decreased the proportion of non-A, non-B intercalated cells in the connecting tubule, and type B intercalated cells in the cortical collecting duct (CCD). These changes were absent in NBCe1-A KO mice. We conclude that the collecting duct phenotypic and remodeling responses depend on proximal tubule-dependent signaling mechanisms blocked by constitutive deletion of proximal tubule NBCe1 proteins.NEW & NOTEWORTHY This study shows that the proximal tubule regulates collecting duct phenotypic and remodeling responses to acidosis.

A Deep Insight Into Regulatory T Cell Metabolism in Renal Disease: Facts and Perspectives

Kidney disease encompasses a complex set of diseases that can aggravate or start systemic pathophysiological processes through their complex metabolic mechanisms and effects on body homoeostasis. The prevalence of kidney disease has increased dramatically over the last two decades. CD4⁺CD25⁺ regulatory T (Treg) cells that express the transcription factor forkhead box protein 3 (Foxp3) are critical for maintaining immune homeostasis and preventing autoimmune disease and tissue damage caused by excessive or unnecessary immune activation, including autoimmune kidney diseases. Recent studies have highlighted the critical role of metabolic reprogramming in controlling the plasticity, stability, and function of Treg cells. They are also likely to play a vital role in limiting kidney transplant rejection and potentially promoting transplant tolerance. Metabolic pathways, such as mitochondrial function, glycolysis, lipid synthesis, glutaminolysis, and mammalian target of rapamycin (mTOR) activation, are involved in the development of renal diseases by modulating the function and proliferation of Treg cells. Targeting metabolic pathways to alter Treg cells can offer a promising method for renal disease therapy. In this review, we provide a new perspective on the role of Treg cell metabolism in renal diseases by presenting the renal microenvironment、relevant metabolites of Treg cell metabolism, and the role of Treg cell metabolism in various kidney diseases.

Regulation of Rhcg, an ammonia transporter, by aldosterone in the kidney

Rhesus C glycoprotein (Rhcg), an ammonia transporter, is a key molecule in urinary acid excretion and is expressed mainly in the intercalated cells (ICs) of the renal collecting duct. In the present study we investigated the role of aldosterone in the regulation of Rhcg expression. In in vivo experiments using C57BL/6J mice, Western blot analysis showed that continuous subcutaneous administration of aldosterone increased the expression of Rhcg in membrane fraction of the kidney. Supplementation of potassium inhibited the effect of aldosterone on the Rhcg. Next, mice were subjected to adrenalectomy with or without administration of aldosterone, and then ad libitum 0.14 M NH4Cl containing water was given. NH4Cl load increased the expression of Rhcg in membrane fraction. Adrenalectomy decreased NH4Cl-induced Rhcg expression, which was restored by administration of aldosterone. Immunohistochemical studies revealed that NH4Cl load induced the localization of Rhcg at the apical membrane of ICs in the outer medullary collecting duct. Adrenalectomy decreased NH4Cl-induced membrane localization of Rhcg, which was restored by administration of aldosterone. For in vitro experiments, IN-IC cells, an immortalized cell line stably expressing Flag-tagged Rhcg (Rhcg-Flag), were used. Western blot analysis showed that aldosterone increased the expression of Rhcg-Flag in membrane fraction, while the increase in extracellular potassium level inhibited the effect of aldosterone. Both spironolactone and Gӧ6983, a PKC inhibitor, inhibited the expression of Rhcg-Flag in the membrane fraction. These results suggest that aldosterone regulates the membrane expression of Rhcg through the mineralocorticoid receptor and PKC pathways, which is modulated by extracellular potassium level.

Potassium depletion induces cellular conversion in the outer medullary collecting duct altering Notch signaling pathway

Potassium depletion affects AQP2 expression and the cellular composition of the kidney collecting duct. This, in turn, contributes to the development of a secondary form of nephrogenic diabetes insipidus and hypokalemic nephropathy. Here we show that after 14 days of potassium depletion, the cellular fraction of A-type intercalated cells increases while the fraction of principal cells decreases along the outer medullary collecting duct in rats. The intercalated cells acquired a novel distribution pattern forming rows of cells attached to each other. These morphological changes occur progressively and reverse after 7 days of recovery on normal rat chow diet. The cellular remodeling mainly occurred in the inner stripe of outer medulla similar to the previously seen effect of lithium on the collecting duct cellular profile. The cellular remodeling is associated with the appearance of cells double labelled with both specific markers of principal and type-A intercalated cells. The appearance of this cell type was associated with the downregulation of the Notch signaling via the Hes1 pathways. These results show that the epithelium of the collecting duct has a high degree of plasticity and that Notch signaling likely plays a key role during hypokalemia.

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.

Effects of chronic hypercapnia on ammonium transport in the mouse kidney

Hypercapnia and subsequent respiratory acidosis are serious complications in many patients with respiratory disorders. The acute response to hypercapnia is buffering of H+ by hemoglobin and cellular proteins but this effect is limited. The chronic response is renal compensation that increases HCO3- reabsorption, and stimulates urinary excretion of titratable acids (TA) and NH4+ . However, the main effective pathway is the excretion of NH4+ in the collecting duct. Our hypothesis is that, the renal NH3 /NH4+ transporters, Rhbg and Rhcg, in the collecting duct mediate this response. The effect of hypercapnia on these transporters is unknown. We conducted in vivo experiments on mice subjected to chronic hypercapnia. One group breathed 8% CO2 and the other breathed normal air as control (0.04% CO2 ). After 3 days, the mice were euthanized and kidneys, blood, and urine samples were collected. We used immunohistochemistry and Western blot analysis to determine the effects of high CO2 on localization and expression of the Rh proteins, carbonic anhydrase IV, and pendrin. In hypercapnic animals, there was a significant increase in urinary NH4+ excretion but no change in TA. Western blot analysis showed a significant increase in cortical expression of Rhbg (43%) but not of Rhcg. Expression of CA-IV was increased but pendrin was reduced. These data suggest that hypercapnia leads to compensatory upregulation of Rhbg that contributes to excretion of NH3 /NH4+ in the kidney. These studies are the first to show a link among hypercapnia, NH4+ excretion, and Rh expression.

Renal potassium physiology: integration of the renal response to dietary potassium depletion

We summarize the current understanding of the physiology of the renal handling of potassium (K⁺), and present an integrative view of the renal response to K⁺ depletion caused by dietary K⁺ restriction. This renal response involves contributions from different nephron segments, and aims to diminish the rate of excretion of K⁺ as a result of: decreasing the rate of electrogenic (and increasing the rate of electroneutral) reabsorption of sodium in the aldosterone-sensitive distal nephron (ASDN), decreasing the abundance of renal outer medullary K⁺ channels in the luminal membrane of principal cells in the ASDN, decreasing the flow rate in the ASDN, and increasing the reabsorption of K⁺ in the cortical and medullary collecting ducts. The implications of this physiology for the association between K⁺ depletion and hypertension, and K⁺ depletion and formation of calcium kidney stones are discussed.

Mechanism of Hyperkalemia-Induced Metabolic Acidosis

Background Hyperkalemia in association with metabolic acidosis that are out of proportion to changes in glomerular filtration rate defines type 4 renal tubular acidosis (RTA), the most common RTA observed, but the molecular mechanisms underlying the associated metabolic acidosis are incompletely understood. We sought to determine whether hyperkalemia directly causes metabolic acidosis and, if so, the mechanisms through which this occurs.Methods We studied a genetic model of hyperkalemia that results from early distal convoluted tubule (DCT)-specific overexpression of constitutively active Ste20/SPS1-related proline-alanine-rich kinase (DCT-CA-SPAK).Results DCT-CA-SPAK mice developed hyperkalemia in association with metabolic acidosis and suppressed ammonia excretion; however, titratable acid excretion and urine pH were unchanged compared with those in wild-type mice. Abnormal ammonia excretion in DCT-CA-SPAK mice associated with decreased proximal tubule expression of the ammonia-generating enzymes phosphate-dependent glutaminase and phosphoenolpyruvate carboxykinase and overexpression of the ammonia-recycling enzyme glutamine synthetase. These mice also had decreased expression of the ammonia transporter family member Rhcg and decreased apical polarization of H⁺-ATPase in the inner stripe of the outer medullary collecting duct. Correcting the hyperkalemia by treatment with hydrochlorothiazide corrected the metabolic acidosis, increased ammonia excretion, and normalized ammoniagenic enzyme and Rhcg expression in DCT-CA-SPAK mice. In wild-type mice, induction of hyperkalemia by administration of the epithelial sodium channel blocker benzamil caused hyperkalemia and suppressed ammonia excretion.Conclusions Hyperkalemia decreases proximal tubule ammonia generation and collecting duct ammonia transport, leading to impaired ammonia excretion that causes metabolic acidosis.

Roles of renal ammonia metabolism other than in acid-base homeostasis

The importance of renal ammonia metabolism in acid-base homeostasis is well known. However, the effects of renal ammonia metabolism other than in acid-base homeostasis are not as widely recognized. First, ammonia differs from almost all other solutes in the urine in that it does not result from arterial delivery. Instead, ammonia is produced by the kidney, and only a portion of the ammonia produced is excreted in the urine, with the remainder returned to the systemic circulation through the renal veins. In normal individuals, systemic ammonia addition is metabolized efficiently by the liver, but in patients with either acute or chronic liver disease, conditions that increase the addition of ammonia of renal origin to the systemic circulation can result in precipitation and/or worsening of hyperammonemia. Second, ammonia appears to serve as an intrarenal paracrine signaling molecule. Hypokalemia increases proximal tubule ammonia production and secretion as well as reabsorption in the thick ascending limb of the loop of Henle, thereby increasing delivery to the renal interstitium and the collecting duct. In the collecting duct, ammonia decreases potassium secretion and stimulates potassium reabsorption, thereby decreasing urinary potassium excretion and enabling feedback correction of the initiating hypokalemia. Finally, the stimulation of renal ammonia metabolism by hypokalemia may contribute to the development of metabolic alkalosis, which in turn can stimulate NaCl reabsorption and contribute to the intravascular volume expansion, increased blood pressure and diuretic resistance that can develop with hypokalemia. The evidence supporting these novel non-acid-base roles of renal ammonia metabolism is discussed in this review.

Hydration status affects osteopontin expression in the rat kidney

Osteopontin (OPN) is a secretory protein that plays an important role in urinary stone formation. Hydration status is associated with the development of urolithiasis. This study was conducted to examine the effects of dehydration and hydration on OPN expression in the rat kidney. Animals were divided into three groups, control, dehydrated, and hydrated. Kidney tissues were processed for light and electron microscope immunocytochemistry, in situ hybridization, and immunoblot analysis. Dehydration induced a significant increase in OPN protein expression, whereas increased fluid intake induced a decrease in protein expression. Under control conditions, OPN protein and mRNA expression were only detected in the descending thin limb (DTL). Dehydration induced increased expression in the DTL and the development of detectable expression in the thick ascending limb (TAL). In contrast, OPN expression levels declined to less than the controls in the DTL after hydration, while no expression of either protein or mRNA was detectable in the TAL. Immunoelectron microscopy demonstrated that hydration status altered tubular ultrastructure and intracellular OPN expression in the Golgi apparatus and secretory cytoplasmic vesicles. These data confirm that changes in oral fluid intake can regulate renal tubular epithelial cell OPN expression.

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.

Mathematical Model of Ammonia Handling in the Rat Renal Medulla

The kidney is one of the main organs that produces ammonia and release it into the circulation. Under normal conditions, between 30 and 50% of the ammonia produced in the kidney is excreted in the urine, the rest being absorbed into the systemic circulation via the renal vein. In acidosis and in some pathological conditions, the proportion of urinary excretion can increase to 70% of the ammonia produced in the kidney. Mechanisms regulating the balance between urinary excretion and renal vein release are not fully understood. We developed a mathematical model that reflects current thinking about renal ammonia handling in order to investigate the role of each tubular segment and identify some of the components which might control this balance. The model treats the movements of water, sodium chloride, urea, NH₃ and NH4+, and non-reabsorbable solute in an idealized renal medulla of the rat at steady state. A parameter study was performed to identify the transport parameters and microenvironmental conditions that most affect the rate of urinary ammonia excretion. Our results suggest that urinary ammonia excretion is mainly determined by those parameters that affect ammonia recycling in the loops of Henle. In particular, our results suggest a critical role for interstitial pH in the outer medulla and for luminal pH along the inner medullary collecting ducts.

Effect of dietary protein restriction on renal ammonia metabolism

Dietary protein restriction has multiple benefits in kidney disease. Because protein intake is a major determinant of endogenous acid production, it is important that net acid excretion change in parallel during protein restriction. Ammonia is the primary component of net acid excretion, and inappropriate ammonia excretion can lead to negative nitrogen balance. Accordingly, we examined ammonia excretion in response to protein restriction and then we determined the molecular mechanism of the changes observed. Wild-type C57Bl/6 mice fed a 20% protein diet and then changed to 6% protein developed an 85% reduction in ammonia excretion within 2 days, which persisted during a 10-day study. The expression of multiple proteins involved in renal ammonia metabolism was altered, including the ammonia-generating enzymes phosphate-dependent glutaminase (PDG) and phosphoenolpyruvate carboxykinase (PEPCK) and the ammonia-metabolizing enzyme glutamine synthetase. Rhbg, an ammonia transporter, increased in expression in the inner stripe of outer medullary collecting duct intercalated cell (OMCDis-IC). However, collecting duct-specific Rhbg deletion did not alter the response to protein restriction. Rhcg deletion did not alter ammonia excretion in response to dietary protein restriction. These results indicate 1) dietary protein restriction decreases renal ammonia excretion through coordinated regulation of multiple components of ammonia metabolism; 2) increased Rhbg expression in the OMCDis-IC may indicate a biological role in addition to ammonia transport; and 3) Rhcg expression is not necessary to decrease ammonia excretion during dietary protein restriction.

Effects of chronic lithium administration on renal acid excretion in humans and rats

Lithium therapy's most common side effects affecting the kidney are nephrogenic diabetes insipidus (NDI) and chronic kidney disease. Lithium may also induce a distal renal tubular acidosis. This study investigated the effect of chronic lithium exposure on renal acid–base homeostasis, with emphasis on ammonia and citrate excretion. We compared 11 individuals on long‐term lithium therapy with six healthy individuals. Under basal conditions, lithium‐treated individuals excreted significantly more urinary ammonia than did control subjects. Following an acute acid load, urinary ammonia excretion increased approximately twofold above basal rates in both lithium‐treated and control humans. There were no significant differences between lithium‐treated and control subjects in urinary pH or urinary citrate excretion. To elucidate possible mechanisms, rats were randomized to diets containing lithium or regular diet for 6 months. Similar to humans, basal ammonia excretion was significantly higher in lithium‐treated rats; in addition, urinary citrate excretion was also significantly greater. There were no differences in urinary pH. Expression of the critical ammonia transporter, Rhesus C Glycoprotein (Rhcg), was substantially greater in lithium‐treated rats than in control rats. We conclude that chronic lithium exposure increases renal ammonia excretion through mechanisms independent of urinary pH and likely to involve increased collecting duct ammonia secretion via the ammonia transporter, Rhcg.

This study investigated the effect of chronic lithium exposure on renal acid–base homeostasis, with emphasis on ammonia and citrate excretion. Chronic lithium exposure increases renal ammonia excretion through mechanisms independent of urinary pH and likely to involve increased collecting duct ammonia secretion via the ammonia transporter, Rhcg.

Ammonia transport in the kidney by Rhesus glycoproteins

Renal ammonia metabolism is a fundamental element of acid-base homeostasis, comprising a major component of both basal and physiologically altered renal net acid excretion. Over the past several years, a fundamental change in our understanding of the mechanisms of renal epithelial cell ammonia transport has occurred, replacing the previous model which was based upon diffusion equilibrium for NH3 and trapping of NH4(+) with a new model in which specific and regulated transport of both NH3 and NH4(+) across renal epithelial cell membranes via specific membrane proteins is required for normal ammonia metabolism. A major advance has been the recognition that members of a recently recognized transporter family, the Rhesus glycoprotein family, mediate critical roles in renal and extrarenal ammonia transport. The erythroid-specific Rhesus glycoprotein, Rh A Glycoprotein (Rhag), was the first Rhesus glycoprotein recognized as an ammonia-specific transporter. Subsequently, the nonerythroid Rh glycoproteins, Rh B Glycoprotein (Rhbg) and Rh C Glycoprotein (Rhcg), were cloned and identified as ammonia transporters. They are expressed in specific cell populations and membrane domains in distal renal epithelial cells, where they facilitate ammonia secretion. In this review, we discuss the distribution of Rhbg and Rhcg in the kidney, the regulation of their expression and activity in physiological disturbances, the effects of genetic deletion on renal ammonia metabolism, and the molecular mechanisms of Rh glycoprotein-mediated ammonia transport.

Expression of glutamine synthetase in the mouse kidney: localization in multiple epithelial cell types and differential regulation by hypokalemia

Renal glutamine synthetase catalyzes the reaction of NH4+ with glutamate, forming glutamine and decreasing the ammonia available for net acid excretion. The purpose of the present study was to determine glutamine synthetase's specific cellular expression in the mouse kidney and its regulation by hypokalemia, a common cause of altered renal ammonia metabolism. Glutamine synthetase mRNA and protein were present in the renal cortex and in both the outer and inner stripes of the outer medulla. Immunohistochemistry showed glutamine synthetase expression throughout the entire proximal tubule and in nonproximal tubule cells. Double immunolabel with cell-specific markers demonstrated glutamine synthetase expression in type A intercalated cells, non-A, non-B intercalated cells, and distal convoluted tubule cells, but not in principal cells, type B intercalated cells, or connecting segment cells. Hypokalemia induced by feeding a nominally K+ -free diet for 12 days decreased glutamine synthetase expression throughout the entire proximal tubule and in the distal convoluted tubule and simultaneously increased glutamine synthetase expression in type A intercalated cells in both the cortical and outer medullary collecting duct. We conclude that glutamine synthetase is widely and specifically expressed in renal epithelial cells and that the regulation of expression differs in specific cell populations. Glutamine synthetase is likely to mediate an important role in renal ammonia metabolism.

The role of 70-kDa heat shock protein in dDAVP-induced AQP2 trafficking in kidney collecting duct cells

It has been reported that several proteins [heat shock protein 70 (Hsp70 and Hsc70), annexin II, and tropomyosin 5b] interact with the Ser256 residue on the COOH terminus of aquaporin-2 (AQP2), where vasopressin-induced phosphorylation occurs for mediating AQP2 trafficking. However, it remains unknown whether these proteins, particularly Hsp70, play a role in AQP2 trafficking. Semiquantitative immunoblotting revealed that renal expression of AQP2 and Hsp70 was significantly increased in water-restricted or dDAVP-infused rats. In silico analysis of the 5′-flanking regions of AQP2, Hsp70-1, and Hsp70-2 genes revealed that transcriptional regulator binding elements associated with cAMP response were identified at both the Hsp70-1 and Hsp70-2 promoter regions, in addition to AQP2. Luciferase reporter assay demonstrated the significant increase of luminescence after dDAVP stimulation (10−8 M, 6 h) in the LLC-PK1 cells transfected with luciferase vector containing 1 kb of the 5′-flanking region of Hsp70-2 gene. Hsp70-2 protein expression was also increased in mpkCCDc14 cells treated by dDAVP in a concentration-dependent manner. Cell surface biotinylation analysis demonstrated that forskolin (10−5 M, 15 min)-induced AQP2 targeting to the apical plasma membrane was significantly attenuated in the mpkCCDc14 cells with Hsp70-2 knockdown. Moreover, forskolin-induced AQP2 phosphorylation (Ser256) was not significantly induced in the mpkCCDc14 cells with Hsp70-2 knockdown. In contrast, Hsp70-2 knockdown did not affect the dDAVP-induced AQP2 abundance. In addition, siRNA-directed knockdown of Hsp70 significantly decreased cell viability. The results suggest that Hsp70 is likely to play a role in AQP2 trafficking to the apical plasma membrane, partly through affecting AQP2 phosphorylation at Ser256 and cell viability.

Expression of the ammonia transporter family member, Rh B Glycoprotein, in the human kidney

The ammonia transporter family member, Rh B Glycoprotein (RhBG/Rhbg), is essential for ammonia transport by the rodent kidney, but in the human kidney mRNA but not protein expression has been reported. Because ammonia transport is fundamental for acid-base homeostasis, the current study addressed RhBG expression in the human kidney. Two distinct RhBG mRNA sequences have been reported, with different numbers of consecutive cytosines at nt1265 and thus encoding different carboxy-tails. Sequencing the region of difference in both human kidney and liver mRNA showed eight sequential cytosines, not seven as in some reports. Knowing the correct mRNA sequence for RhBG, we then assessed RhBG protein expression using antibodies against the correct amino acid sequence. Immunoblot analysis demonstrated RhBG protein expression in human kidney and immunohistochemistry identified basolateral RhBG in connecting segment (CNT) and the cortical and outer medullary collecting ducts. Colocalization of RhBG with multiple cell-specific markers demonstrated that that CNT cells and collecting duct type A intercalated cells express high levels of RhBG, and type B intercalated cells and principal cells do not express detectable RhBG. Thus, these studies identify the correct mRNA and thus protein sequence for human RhBG and show that the human kidney expresses basolateral RhBG protein in CNT, type A intercalated cells, and non-A, non-B cells. We conclude that RhBG can mediate an important role in human renal ammonia transport.

Intercalated cell-specific Rh B glycoprotein deletion diminishes renal ammonia excretion response to hypokalemia

The ammonia transporter family member, Rh B Glycoprotein (Rhbg), is an ammonia-specific transporter heavily expressed in the kidney and is necessary for the normal increase in ammonia excretion in response to metabolic acidosis. Hypokalemia is a common clinical condition in which there is increased renal ammonia excretion despite the absence of metabolic acidosis. The purpose of this study was to examine Rhbg's role in this response through the use of mice with intercalated cell-specific Rhbg deletion (IC-Rhbg-KO). Hypokalemia induced by feeding a K(+)-free diet increased urinary ammonia excretion significantly. In mice with intact Rhbg expression, hypokalemia increased Rhbg protein expression in intercalated cells in the cortical collecting duct (CCD) and in the outer medullary collecting duct (OMCD). Deletion of Rhbg from intercalated cells inhibited hypokalemia-induced changes in urinary total ammonia excretion significantly and completely prevented hypokalemia-induced increases in urinary ammonia concentration, but did not alter urinary pH. We conclude that hypokalemia increases Rhbg expression in intercalated cells in the cortex and outer medulla and that intercalated cell Rhbg expression is necessary for the normal increase in renal ammonia excretion in response to hypokalemia.

Renal ammonia metabolism and transport

Renal ammonia metabolism and transport mediates a central role in acid-base homeostasis. In contrast to most renal solutes, the majority of renal ammonia excretion derives from intrarenal production, not from glomerular filtration. Renal ammoniagenesis predominantly results from glutamine metabolism, which produces 2 NH4(+) and 2 HCO3(-) for each glutamine metabolized. The proximal tubule is the primary site for ammoniagenesis, but there is evidence for ammoniagenesis by most renal epithelial cells. Ammonia produced in the kidney is either excreted into the urine or returned to the systemic circulation through the renal veins. Ammonia excreted in the urine promotes acid excretion; ammonia returned to the systemic circulation is metabolized in the liver in a HCO3(-)-consuming process, resulting in no net benefit to acid-base homeostasis. Highly regulated ammonia transport by renal epithelial cells determines the proportion of ammonia excreted in the urine versus returned to the systemic circulation. The traditional paradigm of ammonia transport involving passive NH3 diffusion, protonation in the lumen and NH4(+) trapping due to an inability to cross plasma membranes is being replaced by the recognition of limited plasma membrane NH3 permeability in combination with the presence of specific NH3-transporting and NH4(+)-transporting proteins in specific renal epithelial cells. Ammonia production and transport are regulated by a variety of factors, including extracellular pH and K(+), and by several hormones, such as mineralocorticoids, glucocorticoids and angiotensin II. This coordinated process of regulated ammonia production and transport is critical for the effective maintenance of acid-base homeostasis.

Renal ammonia excretion in response to hypokalemia: effect of collecting duct-specific Rh C glycoprotein deletion

The Rhesus factor protein, Rh C glycoprotein (Rhcg), is an ammonia transporter whose expression in the collecting duct is necessary for normal ammonia excretion both in basal conditions and in response to metabolic acidosis. Hypokalemia is a common clinical condition associated with increased renal ammonia excretion. In contrast to basal conditions and metabolic acidosis, increased ammonia excretion during hypokalemia can lead to an acid-base disorder, metabolic alkalosis, rather than maintenance of acid-base homeostasis. The purpose of the current studies was to determine Rhcg's role in hypokalemia-stimulated renal ammonia excretion through the use of mice with collecting duct-specific Rhcg deletion (CD-Rhcg-KO). In mice with intact Rhcg expression, a K(+)-free diet increased urinary ammonia excretion and urine alkalinization and concurrently increased Rhcg expression in the collecting duct in the outer medulla. Immunohistochemistry and immunogold electron microscopy showed hypokalemia increased both apical and basolateral Rhcg expression. In CD-Rhcg-KO, a K(+)-free diet increased urinary ammonia excretion and caused urine alkalinization, and the magnitude of these changes did not differ from mice with intact Rhcg expression. In mice on a K(+)-free diet, CD-Rhcg-KO increased phosphate-dependent glutaminase (PDG) expression in the outer medulla. We conclude that hypokalemia increases collecting duct Rhcg expression, that this likely contributes to the hypokalemia-stimulated increase in urinary ammonia excretion, and that adaptive increases in PDG expression can compensate for the absence of collecting duct Rhcg.

The sodium-activated sodium channel is expressed in the rat kidney thick ascending limb and collecting duct cells and is upregulated during high salt intake

Increased dietary salt triggers oxidative stress and kidney injury in salt-sensitive hypertension; however, the mechanism for sensing increased extracellular Na(+) concentration ([Na(+)]) remains unclear. A Na(+)-activated Na(+) channel (Na sensor) described in the brain operates as a sensor of extracellular fluid [Na(+)]; nonetheless, its presence in the kidney has not been established. In the present study, we demonstrated the gene expression of the Na sensor by RT-PCR and Western blotting in the Sprague-Dawley rat kidney. Using immunofluorescence, the Na sensor was localized to the luminal side in tubular epithelial cells of collecting ducts colocalizing with aquaporin-2, a marker of principal cells, and in thick ascending limb, colocalizing with the glycoprotein Tamm-Horsfall. To determine the effect of a high-salt diet (HSD) on Na sensor gene expression, we quantified its transcript and protein levels primarily in renal medullas from control rats and rats subjected to 8% NaCl for 7 days (n = 5). HSD increased Na sensor expression levels (mRNA: from 1.2 ± 0.2 to 5.1 ± 1.3 au; protein: from 0.98 ± 0.15 to 1.74 ± 0.28 au P < 0.05) in the kidney medulla, but not in the cortex. These data indicate that rat kidney epithelial cells of the thick ascending limb and principal cells of the collecting duct possess a Na sensor that is upregulated by HSD, suggesting an important role in monitoring changes in tubular fluid [Na(+)].

Mechanisms of the effects of acidosis and hypokalemia on renal ammonia metabolism

Renal ammonia metabolism is the predominant component of net acid excretion and new bicarbonate generation. Renal ammonia metabolism is regulated by acid-base balance. Both acute and chronic acid loads enhance ammonia production in the proximal tubule and secretion into the urine. In contrast, alkalosis reduces ammoniagenesis. Hypokalemia is a common electrolyte disorder that significantly increases renal ammonia production and excretion, despite causing metabolic alkalosis. Although the net effects of hypokalemia are similar to metabolic acidosis, molecular mechanisms of renal ammonia production and transport have not been well understood. This mini review summarizes recent findings regarding renal ammonia metabolism in response to chronic hypokalemia.