Metabolic acidosis has dual effects on sodium handling by rat kidney

Exogenous ketosis suppresses diuresis and atrial natriuretic peptide during exercise

We have previously demonstrated that exogenous ketosis reduces urine production during exercise. However, the underlying physiological mechanism of this anti-diuretic effect remained unclear. Therefore, we investigated whether acute exogenous ketosis by oral ingestion of ketone ester (KE) during a simulated cycling race (RACE) affects the hormonal pathways implicated in fluid balance regulation during exercise. In a double-blind crossover design, 11 well-trained male cyclists participated in RACE consisting of a 3-h submaximal intermittent cycling (IMT_180') bout followed by a 15-minute time trial (TT_15') in an environmental chamber set at 28 °C and 60 % relative humidity. Fluid intake was adjusted to maintain euhydration. Before and during RACE, the subjects received either a control drink (CON) or the ketone ester (R)-3-hydroxybutyl (R)-3-hydroxybutyrate (KE), which elevated blood β-hydroxybutyrate to ~2-4 mM. Urine output during IMT_180' was ~20% lower in KE (1172 ± 557 ml) than in CON (1431 ± 548 ml, p < 0.05). Compared with CON, N-terminal pro-atrial natriuretic peptide (NT-pro ANP) concentration during RACE was ~20% lower in KE (p < 0.05). KE also raised plasma noradrenaline concentrations during RACE. Performance in TT_15' was similar between CON and KE. In conclusion, exogenous ketosis suppresses diuresis and downregulates α-natriuretic peptide activity during exercise.

Mechanistic insights into the role of serum-glucocorticoid kinase 1 in diabetic nephropathy: A systematic review

Aberrant expression of serum-glucocorticoid kinase 1 (SGK1) contributes to the pathogenesis of multiple disorders, including diabetes, hypertension, obesity, fibrosis, and metabolic syndrome. SGK1 variant is expressed in the presence of insulin and several growth factors, eventually modulating various ion channels, carrier proteins, and transcription factors. SGK1 also regulates the enzymatic activity of Na⁺ K⁺ ATPase, glycogen synthase kinase-3, ubiquitin ligase Nedd4-2, and phosphomannose mutase impacting cell cycle regulation, neuroexcitation, and apoptosis. Ample evidence supports the crucial role of aberrant SGK1 expression in hyperglycemia-mediated secondary organ damage. Diabetic nephropathy (DN), a dreadful microvascular complication of diabetes, is the leading cause of end-stage renal failures with high morbidity and mortality rate. The complex pathogenesis of DN encompasses several influencing factors, including transcriptional factors, inflammatory markers, cytokines, epigenetic modulators, and abnormal enzymatic activities. SGK1 plays a pivotal role by controlling various physiological functions associated with the occurrence and progression of DN; therefore, targeting SGK1 may favorably influence the clinical outcome in patients with DN. This review aimed to provide mechanistic insights into SGK1 regulated DN pathogenesis and summarize the evidence supporting the therapeutic potential of SGK1 inhibition and its consequences on human health.

Acetazolamide causes renal HCO 3 − wasting but inhibits ammoniagenesis and prevents the correction of metabolic acidosis by the kidney

Carbonic anhydrase (CAII) binds to the electrogenic basolateral Na+-HCO 3 − cotransporter (NBCe1) and facilitatesHCO 3 − reabsorption across the proximal tubule. However, whether the inhibition of CAII with acetazolamide (ACTZ) alters NBCe1 activity and interferes with the ammoniagenesis pathway remains elusive. To address this issue, we compared the renal adaptation of rats treated with ACTZ to NH4Cl loading for up to 2 wk. The results indicated that ACTZ-treated rats exhibited a sustained metabolic acidosis for up to 2 wk, whereas in NH4Cl-loaded rats, metabolic acidosis was corrected within 2 wk of treatment.NH 4 + excretion increased by 10-fold in NH4Cl-loaded rats but only slightly (1.7-fold) in ACTZ-treated rats during the first week despite a similar degree of acidosis. Immunoblot experiments showed that the protein abundance of glutaminase (4-fold), glutamate dehydrogenase (6-fold), and SN1 (8-fold) increased significantly in NH4Cl-loaded rats but remained unchanged in ACTZ-treated rats. Na+/H+ exchanger 3 and NBCe1 proteins were upregulated in response to NH4Cl loading but not ACTZ treatment and were rather sharply downregulated after 2 wk of ACTZ treatment. ACTZ causes renalHCO 3 − wasting and induces metabolic acidosis but inhibits the upregulation of glutamine transporter and ammoniagenic enzymes and thus suppresses ammonia synthesis and secretion in the proximal tubule, which prevented the correction of acidosis. This effect is likely mediated through the inhibition of the CA-NBCe1 metabolon complex, which results in cell alkalinization. During chronic ACTZ treatment, the downregulation of both NBCe1 and Na+/H+ exchanger 3, along with the inhibition of ammoniagenesis andHCO 3 − generation, contributes to the maintenance of metabolic acidosis.

ANP-stimulated Na+ secretion in the collecting duct prevents Na+ retention in the renal adaptation to acid load

We have recently reported that type A intercalated cells of the collecting duct secrete Na⁺ by a mechanism coupling the basolateral type 1 Na⁺-K⁺-2Cl^- cotransporter with apical type 2 H⁺-K⁺-ATPase (HKA2) functioning under its Na⁺/K⁺ exchange mode. The first aim of the present study was to evaluate whether this secretory pathway is a target of atrial natriuretic peptide (ANP). Despite hyperaldosteronemia, metabolic acidosis is not associated with Na⁺ retention. The second aim of the present study was to evaluate whether ANP-induced stimulation of Na⁺ secretion by type A intercalated cells might account for mineralocorticoid escape during metabolic acidosis. In Xenopus oocytes expressing HKA2, cGMP, the second messenger of ANP, increased the membrane expression, activity, and Na⁺-transporting rate of HKA2. Feeding mice with a NH₄Cl-enriched diet increased urinary excretion of aldosterone and induced a transient Na⁺ retention that reversed within 3 days. At that time, expression of ANP mRNA in the collecting duct and urinary excretion of cGMP were increased. Reversion of Na⁺ retention was prevented by treatment with an inhibitor of ANP receptors and was absent in HKA2-null mice. In conclusion, paracrine stimulation of HKA2 by ANP is responsible for the escape of the Na⁺-retaining effect of aldosterone during metabolic acidosis.

Zebrafish as a Model System for Investigating the Compensatory Regulation of Ionic Balance during Metabolic Acidosis

Zebrafish (Danio rerio) have become an important model for integrative physiological research. Zebrafish inhabit a hypo-osmotic environment; to maintain ionic and acid-base homeostasis, they must actively take up ions and secrete acid to the water. The gills in the adult and the skin at larval stage are the primary sites of ionic regulation in zebrafish. The uptake of ions in zebrafish is mediated by specific ion transporting cells termed ionocytes. Similarly, in mammals, ion reabsorption and acid excretion occur in specific cell types in the terminal region of the renal tubules (distal convoluted tubule and collecting duct). Previous studies have suggested that functional regulation of several ion transporters/channels in the zebrafish ionocytes resembles that in the mammalian renal cells. Additionally, several mechanisms involved in regulating the epithelial ion transport during metabolic acidosis are found to be similar between zebrafish and mammals. In this article, we systemically review the similarities and differences in ionic regulation between zebrafish and mammals during metabolic acidosis. We summarize the available information on the regulation of epithelial ion transporters during acidosis, with a focus on epithelial Na⁺, Cl- and Ca2+ transporters in zebrafish ionocytes and mammalian renal cells. We also discuss the neuroendocrine responses to acid exposure, and their potential role in ionic compensation. Finally, we identify several knowledge gaps that would benefit from further study.

Possible Mechanisms of Cardiac Contractile Dysfunction and Electrical Changes in Ammonium Chloride Induced Chronic Metabolic Acidosis in Wistar Rats

Metabolic acidosis could occur due to either endogenous acids accumulation or bicarbonate loss from the gastrointestinal tract or commonly from the kidney. This study aimed to investigate the possible underlying mechanism(s) of chronic acidosis-induced cardiac contractile and electrical changes in rats. Twenty four adult Wistar rats, of both sexes, were randomly divided into control group and chronic metabolic acidosis group, which received orally 0.28 M NH4Cl in the drinking water for 2 weeks. At the end of experimental period, systolic and diastolic blood pressure values were measured. On the day of sacrifice, rats were anesthetized by i.p. pentobarbitone (40 mg/kg b.w.), transthoracic echocardiography and ECG were performed. Blood samples were obtained from abdominal aorta for complete blood count and determination of pH, bicarbonate, chloride, sodium, potassium, troponin I, CK-MB, IL-6, renin and aldosterone levels. Hearts from both groups were studied for cardiac tissue IL-6 and aldosterone in addition to histopathological examination. Compared to control group, chronic metabolic acidosis group showed anemia, significant systolic and diastolic hypotension accompanied by significant reduction of ejection fraction and fraction of shortening, significant bradycardia, prolonged QTc interval and higher widened T wave as well as significantly elevated plasma levels of renin, aldosterone, troponin I, CK-MB and IL-6, and cardiac tissue aldosterone and IL-6. The left ventricular wall of the acidosis group showed degenerated myocytes with fibrosis and apoptosis. Thus, chronic metabolic acidosis induced negative inotropic and chronotropic effects and cardiomyopathy, possibly by elevated aldosterone and IL-6 levels released from the cardiac tissue.

Physiology and pathophysiology of potassium homeostasis

Total body potassium content and proper distribution of potassium across the cell membrane is of critical importance for normal cellular function. Potassium homeostasis is maintained by several different methods. In the kidney, total body potassium content is achieved by alterations in renal excretion of potassium in response to variations in intake. Insulin and beta-adrenergic tone play critical roles in maintaining the internal distribution of potassium under normal conditions. Despite homeostatic pathways designed to maintain potassium levels within the normal range, disorders of altered potassium homeostasis are common. The clinical approach to designing effective treatments relies on understanding the pathophysiology and regulatory influences which govern the internal distribution and external balance of potassium. Here we provide an overview of the key regulatory aspects of normal potassium physiology. This review is designed to provide an overview of potassium homeostasis as well as provide references of seminal papers to guide the reader into a more in depth discussion of the importance of potassium balance. This review is designed to be a resource for educators and well-informed clinicians who are teaching trainees about the importance of potassium balance.

Chronic Metabolic Acidosis Activates Renal Tubular Sodium Chloride Cotransporter through Angiotension II-dependent WNK4-SPAK Phosphorylation Pathway

The mechanism by which chronic metabolic acidosis (CMA) regulates sodium (Na(+))-chloride (Cl(-)) cotransporter (NCC) in the renal distal convoluted tubules remains unexplored. We examined the role of STE20/SPS1-related proline/alanine-rich kinase (SPAK) and with-no-lysine kinase 4 (WNK4) on expression of NCC in mouse models of CMA. CMA was induced by NH4Cl in wild type mice (WTA mice), SPAK, and WNK4 knockout mice. The quantities of Ncc mRNA, expression of total NCC, phosphorylated (p)-NCC, SPAK and WNK4 in the kidneys as well as NCC inhibition with hydrochlorothiazide and Na(+) balance were evaluated. Relative to WT mice, WTA mice had similar levels of Ncc mRNA, but increased expression of total and p-NCC, SPAK, and WNK4 and an exaggerated response to hydrochlorothiazide which could not be observed in SPAK or WNK4 knockout mice with CMA. In WTA mice, increased plasma renin activity, aldosterone and angiotensin II concentrations accompanied by a significantly negative Na(+) balance. High Na(+) diet abolished the enhanced NCC expression in WTA mice. Furthermore, an angiotensin II type 1 receptor blocker rather than a mineralocorticoid receptor antagonist exerted a marked inhibition on Na(+) reabsorption and NCC phosphorylation in WTA mice. CMA increases WNK4-SPAK-dependent NCC phosphorylation and appears to be secondary to previous natriuresis with volume-dependent angiotensin II activation.

Effects of acid challenges on type 2 angiotensin II receptor-sensitive ammonia production by the proximal tubule

Angiotensin II (ANG II) acting through its type 1 (AT1) receptor stimulates total ammonia (tNH3) production by the proximal tubule. The present studies explored the role of ANG II type 2 (AT2) receptors in modulating the stimulatory effects of ANG II on tNH3 production. Mouse S2 proximal tubule segments derived from 18-h and 7-day acid-loaded mice, and non-acid-loaded controls were dissected and microperfused in vitro. Adding ANG II to the luminal perfusion solution resulted in different increments in tNH3 production rates in tubules derived from 18-h vs. 7-day acid-loaded mice such that the increase in tNH3 production with ANG II was higher in tubules derived from 18-h acid-loaded mice compared with those derived from control and 7-day acid-loaded mice. Adding the AT2 receptor blocker PD123319 with ANG II increased ANG II-stimulated tNH3 production in S2 segments from control and 7-day acid-loaded mice but not in those from 18-h acid-loaded mice, and this increased effect of PD123319 was associated with higher AT2 receptor protein levels in brush-border membranes. Studies in cultured proximal tubule cells demonstrated that 2-h exposure to pH 7.0 reduced the modulating effect of PD123319 on ANG II-simulated tNH3 production and reduced cell surface AT2 receptor levels. We concluded that AT2 receptors reduce the stimulatory effect of ANG II on proximal tubule tNH3 production and that the time-dependent impact of AT2 receptor blockade on the ANG II-stimulated tNH3 production corresponded to time-dependent changes in AT2 receptor cell surface expression in the proximal tubule.

The sodium chloride cotransporter SLC12A3: new roles in sodium, potassium, and blood pressure regulation

SLC12A3 encodes the thiazide-sensitive sodium chloride cotransporter (NCC), which is primarily expressed in the kidney, but also in intestine and bone. In the kidney, NCC is located in the apical plasma membrane of epithelial cells in the distal convoluted tubule. Although NCC reabsorbs only 5 to 10% of filtered sodium, it is important for the fine-tuning of renal sodium excretion in response to various hormonal and non-hormonal stimuli. Several new roles for NCC in the regulation of sodium, potassium, and blood pressure have been unraveled recently. For example, the recent discoveries that NCC is activated by angiotensin II but inhibited by dietary potassium shed light on how the kidney handles sodium during hypovolemia (high angiotensin II) and hyperkalemia. The additive effect of angiotensin II and aldosterone maximizes sodium reabsorption during hypovolemia, whereas the inhibitory effect of potassium on NCC increases delivery of sodium to the potassium-secreting portion of the nephron. In addition, great steps have been made in unraveling the molecular machinery that controls NCC. This complex network consists of kinases and ubiquitinases, including WNKs, SGK1, SPAK, Nedd4-2, Cullin-3, and Kelch-like 3. The pathophysiological significance of this network is illustrated by the fact that modification of each individual protein in the network changes NCC activity and results in salt-dependent hypotension or hypertension. This review aims to summarize these new insights in an integrated manner while identifying unanswered questions.

Renal hemodynamic response to L-arginine in uncomplicated, type 1 diabetes mellitus: the role of buffering anions and tubuloglomerular feedback

According to the "tubulocentric" hypothesis of the glomerular hyperfiltration of diabetes mellitus (DM), tubuloglomerular feedback (TGF) is the critical determinant of the related renal hemodynamic dysfunction. To examine the role of TGF in human type 1 DM, 12 salt-replete healthy (C) and 11 uncomplicated DM individuals underwent measurements of glomerular filtration rate (GFR), renal blood flow (RBF), and lithium-derived absolute "distal" sodium delivery (DDNa). Measurements were made during two 3-h infusions of 0.012 mmol·kg(-1)·min(-1) l-arginine (ARG) buffered with either equimolar HCl (ARG.HCl) or citric acid (ARG.CITR). Our hypothesis was that changes in TGF signaling would be directionally opposite ARG.HCl vs. ARG.CITR according to the effects of the ARG-buffering anion on DDNa. Similar changes in C and DM followed ARG.CITR, with declines in DDNa (-0.26 ± 0.07 mmol/min C vs. -0.31 ± 0.07 mmol/min DM) and increases in RBF (+299 ± 25 vs. +319 ± 29 ml·min(-1)·1.73 m(-2)) and GFR (+6.6 ± 0.8 vs. +11.6 ± 1.2 ml·min(-1)·1.73 m(-2)). In contrast, with ARG.HCl, DDNa rose in both groups (P = 0.001), but the response was 73% greater in DM (+1.50 ± 0.15 mmol/min C vs. +2.59 ± 0.22 mmol/min DM, P = 0.001). RBF also increased (P = 0.001, +219 ± 20 ml·min(-1)·1.73 m(-2) C, +105 ± 14 DM), but ΔRBF after ARG.HCl was lower vs. ARG.CITR in both groups (P = 0.001). After ARG.HCl, ΔRBF also was 50% lower in DM vs. C (P = 0.001) and GFR, unchanged in C, declined in DM (-7.4 ± 0.9 ml·min(-1)·1.73 m(-2), P = 0.02 vs. C). After ARG.HCl, unlike ARG.CITR, DDNa increased in C and DM, associated with less ΔRBF and ΔGFR vs. ARG.CITR. This suggests that the renal hemodynamic response to ARG is influenced substantially by the opposite actions of HCl vs. CITR on DDNa and TGF. In DM, the association of ARG.HCl-induced exaggerated ΔDDNa, blunted ΔRBF, and the decline in GFR vs. C shows an enhanced TGF dependence of renal vasodilatation to ARG, in agreement with a critical role of TGF in DM-related renal hemodynamic dysfunction.

The hyperpolarization-activated cyclic nucleotide-gated HCN2 channel transports ammonium in the distal nephron

Recent studies have identified Rhesus proteins as important molecules for ammonia transport in acid-secreting intercalated cells in the distal nephron. Here, we provide evidence for an additional molecule that can mediate NH3/NH4 excretion, the subtype 2 of the hyperpolarization-activated cyclic nucleotide-gated channel family (HCN2), in collecting ducts in rat renal cortex and medulla. Chronic metabolic acidosis in rats did not alter HCN2 protein expression but downregulated the relative abundance of HCN2 mRNA. Its cDNA was identical to the homolog from the brain and the protein was post-translationally modified by N-type glycosylation. Electrophysiological recordings in Xenopus oocytes injected with HCN2 cRNA found that potassium was transported better than ammonium, each of which was transported significantly better than sodium, criteria that are compatible with a role for HCN2 in ammonium transport. In microperfused rat outer medullary collecting duct segments, the initial rate of acidification, upon exposure to a basolateral ammonium chloride pulse, was higher in intercalated than in principal cells. A specific inhibitor of HCN2 (ZD7288) decreased acidification only in intercalated cells from control rats. In rats with chronic metabolic acidosis, the rate of acidification doubled in both intercalated and principal cells; however, ZD7288 had no significant inhibitory effect. Thus, HCN2 is a basolateral ammonium transport pathway of intercalated cells and may contribute to the renal regulation of body pH under basal conditions.

Pendrin modulates ENaC function by changing luminal HCO3-

The epithelial Na(+) channel, ENaC, and the Cl(-)/HCO(3)(-) exchanger, pendrin, mediate NaCl absorption within the cortical collecting duct and the connecting tubule. Although pendrin and ENaC localize to different cell types, ENaC subunit abundance and activity are lower in aldosterone-treated pendrin-null mice relative to wild-type mice. Because pendrin mediates HCO(3)(-) secretion, we asked if increasing distal delivery of HCO(3)(-) through a pendrin-independent mechanism "rescues" ENaC function in pendrin-null mice. We gave aldosterone and NaHCO(3) to increase pendrin-dependent HCO(3)(-) secretion within the connecting tubule and cortical collecting duct, or gave aldosterone and NaHCO(3) plus acetazolamide to increase luminal HCO(3)(-) concentration, [HCO(3)(-)], independent of pendrin. Following treatment with aldosterone and NaHCO(3), pendrin-null mice had lower urinary pH and [HCO(3)(-)] as well as lower renal ENaC abundance and function than wild-type mice. With the addition of acetazolamide, however, acid-base balance as well as ENaC subunit abundance and function was similar in pendrin-null and wild-type mice. We explored whether [HCO(3)(-)] directly alters ENaC abundance and function in cultured mouse principal cells (mpkCCD). Amiloride-sensitive current and ENaC abundance rose with increased [HCO(3)(-)] on the apical or the basolateral side, independent of the substituting anion. However, ENaC was more sensitive to changes in [HCO(3)(-)] on the basolateral side of the monolayer. Moreover, increasing [HCO(3)(-)] on the apical and basolateral side of Xenopus kidney cells increased both ENaC channel density and channel activity. We conclude that pendrin modulates ENaC abundance and function, at least in part by increasing luminal [HCO(3)(-)] and/or pH.

Adaptation to metabolic acidosis and its recovery are associated with changes in anion exchanger distribution and expression in the cortical collecting duct

It is well known that acid/base disturbances modulate proton/bicarbonate transport in the cortical collecting duct. To study the adaptation further we measured the effect of three days of acidosis followed by the rapid recovery from this acidosis on the number and type of intercalated cells in the rabbit cortical collecting duct. Immunofluorescence was used to determine the expression of apical pendrin in β-intercalated cells and the basolateral anion exchanger (AE1) in α-intercalated cells. Acidosis resulted in decreased bicarbonate and increased proton secretion, which correlated with reduced pendrin expression and the number of pendrin-positive cells, as well as decreased pendrin mRNA and protein abundance in this nephron segment. There was a concomitant increase of basolateral AE1 and α-cell number. Intercalated cell proliferation did not seem to play a role in the adaptation to acidosis. Alkali loading for 6-20 h after acidosis doubled the bicarbonate secretory flux and reduced proton secretion. Pendrin and AE1 expression patterns returned to control levels, demonstrating that adaptive changes by intercalated cells are rapidly reversible. Thus, regulation of intercalated cell anion exchanger expression and distribution plays a key role in adaptation of the cortical collecting duct to perturbations of acid/base.

Targeting SGK1 in diabetes

Compelling evidence is accumulating indicating a pathophysiological role of the serum-and-glucocorticoid-inducible-kinase-1 (SGK1) in the development and complications of diabetes. SGK1 is ubiquitously expressed with exquisitely high transcriptional volatility. Stimulators of SGK1 expression include hyperglycemia, cell shrinkage, ischemia, glucocorticoids and mineralocorticoids. SGK1 is activated by insulin and growth factors via PI3K, 3-phosphoinositide dependent kinase PDK1 and mTOR. SGK1 activates ion channels (including ENaC, TRPV5, ROMK, KCNE1/KCNQ1 and CLCKa/Barttin), carriers (including NCC, NKCC, NHE3, SGLT1 and EAAT3), and the Na(+)/K(+)-ATPase. It regulates the activity of several enzymes (e.g., glycogen-synthase-kinase-3, ubiquitin-ligase Nedd4-2, phosphomannose-mutase-2), and transcription factors (e.g., forkhead-transcription-factor FOXO3a, beta-catenin and NF-kappaB). A common SGK1 gene variant ( approximately 3 - 5% prevalence in Caucasians, approximately 10% in Africans) is associated with increased blood pressure, obesity and type 2 diabetes. In patients suffering from type 2 diabetes, SGK1 presumably contributes to fluid retention and hypertension, enhanced coagulation and increased deposition of matrix proteins leading to tissue fibrosis such as diabetic nephropathy. Accordingly, targeting SGK1 may favourably influence occurrence and course of type 2 diabetes.

Chemoreceptor hypersensitivity, sympathetic excitation, and overexpression of ASIC and TASK channels before the onset of hypertension in SHR

RATIONALE

Increased sympathetic nerve activity has been linked to the pathogenesis of hypertension in humans and animal models. Enhanced peripheral chemoreceptor sensitivity which increases sympathetic nerve activity has been observed in established hypertension but has not been identified as a possible mechanism for initiating an increase in sympathetic nerve activity before the onset of hypertension.

OBJECTIVE

We tested this hypothesis by measuring the pH sensitivity of isolated carotid body glomus cells from young spontaneously hypertensive rats (SHR) before the onset of hypertension and their control normotensive Wistar-Kyoto (WKY) rats.

METHODS AND RESULTS

We found a significant increase in the depolarizing effect of low pH in SHR versus WKY glomus cells which was caused by overexpression of 2 acid-sensing non-voltage-gated channels. One is the amiloride-sensitive acid-sensing sodium channel (ASIC3), which is activated by low pH and the other is the 2-pore domain acid-sensing K(+) channel (TASK1), which is inhibited by low pH and blocked by quinidine. Moreover, we found that the increase in sympathetic nerve activity in response to stimulation of chemoreceptors with sodium cyanide was markedly enhanced in the still normotensive young SHR compared to control WKY rats.

CONCLUSIONS

Our results establish a novel molecular basis for increased chemotransduction that contributes to excessive sympathetic activity before the onset of hypertension.

The physiological impact of the serum and glucocorticoid-inducible kinase SGK1

PURPOSE OF REVIEW

The role of serum and glucocorticoid-inducible kinase 1 (SGK1) in renal physiology and pathophysiology is reviewed with particular emphasis on recent advances.

RECENT FINDINGS

The mammalian target of rapamycin complex 2 has been shown to phosphorylate SGK1 at Ser422 (the so-called hydrophobic motif). Ser397 and Ser401 are two additional SGK1-phosphorylation sites required for maximal SGK1 activity. A 5' variant alternate transcript of human Sgk1 has been identified that is widely expressed and shows improved stability, enhanced membrane association, and greater stimulation of epithelial Na+ transport. SGK1 is essential for optimal processing of the epithelial sodium channel and also regulates the expression of the Na+-Cl- cotransporter. With regard to pathophysiology, SGK1 participates in the stimulation of renal tubular glucose transport in diabetes, the renal profibrotic effect of both angiotensin II and aldosterone, and in fetal programing of arterial hypertension.

SUMMARY

The outlined recent findings advanced our understanding of the molecular regulation of SGK1 as well as the role of the kinase in renal physiology and the pathophysiology of renal disease and hypertension. Future studies using pharmacological inhibitors of SGK1 will reveal the utility of the kinase as a new therapeutic target.

Downregulation of claudin-2 expression in renal epithelial cells by metabolic acidosis

Chronic metabolic acidosis (CMA) is associated with an inhibition of fluid reabsorption in the renal proximal tubule. The effects of CMA on paracellular transport across the renal epithelial tight junction (TJ) is unknown. Claudin-2 is a transmembrane TJ-associated protein which confers TJ paracellular permeability to Na(+). We examined the effects of CMA on the expression of TJ transport proteins using both in vivo and in vitro models of CMA. The results showed downregulation of claudin-2 mRNA and protein expression in the cortex of rats subjected to the NH(4)Cl loading model of CMA. Madin-Darby canine kidney (MDCK) and HK-2 cells are models of renal epithelial cells and express claudin-2 protein in their TJ. We examined the effects of acidic pH exposure on the expression of claudin-2 in MDCK and HK-2 renal epithelial cells. Exposure of MDCK cells to pH 6.96 medium caused a significant and reversible decrease in claudin-2 protein abundance. A dose-response analysis of acidic medium exposure of MDCK and HK-2 cells demonstrated a downregulation of claudin-2 protein. The downregulation effect of acidic pH is specific to claudin-2 expression as the expression of other TJ-associated proteins (i.e., claudin-1, -3, -4, and -7, occludin, and zonula occludens-1) remained unchanged compared with control pH (7.40). Collectively, these data demonstrate that CMA downregulates the expression of claudin-2 likely through a direct effect of acidic pH. Potential physiological significance of these changes is discussed.