Electroneutral absorption of NaCl by the aldosterone-sensitive distal nephron: implication for normal electrolytes homeostasis and blood pressure regulation

Cl- induces endothelium-dependent mesenteric arteriolar vasorelaxation through the NKCC1/TRPV4/NCX axis

AIMS

Although absorbed NaCl increases intestinal blood flow to facilitate absorption and transportation, it is unclear if it can directly mediate mesenteric arterial relaxation. We aimed to investigate and test our hypothesis that Cl- induces mesenteric arterial vasorelaxation via endothelium-dependent hyperpolarization (EDH).

MAIN METHODS

We used wire myograph to study NaCl-induced vasorelaxation of mesenteric arteries isolated from mice. Cl-, Ca2+ and K+ imaging was performed in human vascular endothelial cells pre-treated with pharmacological agents.

KEY FINDINGS

The Cl- concentration-dependently induced vasorelaxation of mesenteric arteries likely through EDH. The Cl--induced vasorelaxation was attenuated in TRPV4 KO mice and inhibited by selective blockers of Na+-K+-2Cl- cotransporter 1 (NKCC1) (bumetanide, 10 μM), transient receptor potential vanilloid 4 (TRPV4) (RN-1734, 40 μM), and small conductance Ca2+-activated K+ channels (SKCa) (apamin, 3 μM)/ intermediate conductance Ca2+-activated K+ channels (IKCa) (TRAM-34, 10 μM) and myoendothelial gap junction (18α-glycyrrhetinic acid, 10 μM), but enhanced by a selective activator of IKCa/SKCa (SKA-31, 0.3 μM). Cl- decreased intracellular K+ concentrations in endothelial cells, which was reversed by apamin (200 nM) plus TRAM-34 (500 nM). Extracellular Cl- raised intracellular Cl- concentrations in endothelial cells, which was attenuated by bumetanide (10 μM). Finally, Cl- induced a transient Ca2+ signaling via TRPV4 in endothelial cells, which became sustained when the Ca2+ exit mode of Na+-Ca2+ exchanger (NCX) was blocked.

SIGNIFICANCE

Cl- induces a pure EDH-mediated vasorelaxation of mesenteric arteries through activation of endothelial NKCC1/TRPV4/NCX axis. We have provided a novel insight into the role of Cl--induced vasorelaxation via EDH mechanism.

Pendrin-null mice develop severe hypokalemia following dietary Na+ and K+ restriction: role of ENaC

Pendrin is an intercalated cell Cl^-/[Formula: see text] exchanger thought to participate in K⁺-sparing NaCl absorption. However, its role in K⁺ homeostasis has not been clearly defined. We hypothesized that pendrin-null mice will develop hypokalemia with dietary K⁺ restriction. We further hypothesized that pendrin knockout (KO) mice mitigate urinary K⁺ loss by downregulating the epithelial Na⁺ channel (ENaC). Thus, we examined the role of ENaC in Na⁺ and K⁺ balance in pendrin KO and wild-type mice following dietary K⁺ restriction. To do so, we examined the relationship between Na⁺ and K⁺ balance and ENaC subunit abundance in K⁺-restricted pendrin-null and wild-type mice that were NaCl restricted or replete. Following a NaCl-replete, K⁺-restricted diet, K⁺ balance and serum K⁺ were similar in both groups. However, following a Na⁺, K⁺, and Cl^--deficient diet, pendrin KO mice developed hypokalemia from increased K⁺ excretion. The fall in serum K⁺ observed in K⁺-restricted pendrin KO mice was enhanced with ENaC stimulation but eliminated with ENaC inhibition. The fall in serum K⁺ observed in K⁺-restricted pendrin KO mice was enhanced with ENaC stimulation but eliminated with ENaC inhibition. However, reducing ENaC activity also reduced blood pressure and increased apparent intravascular volume contraction, since KO mice had lower serum Na⁺, higher blood urea nitrogen and hemoglobin, greater weight loss, greater metabolic alkalosis, and greater NaCl excretion. We conclude that dietary Na⁺ and K⁺ restriction induces hypokalemia in pendrin KO mice. Pendrin-null mice limit renal K⁺ loss by downregulating ENaC. However, this ENaC downregulation occurs at the expense of intravascular volume.NEW & NOTEWORTHY Pendrin is an apical Cl^-/[Formula: see text] exchanger that provides renal K⁺-sparing NaCl absorption. The pendrin-null kidney has an inability to fully conserve K⁺ and limits renal K⁺ loss by downregulating the epithelial Na⁺ channel (ENaC). However, with Na⁺ restriction, the need to reduce ENaC for K⁺ balance conflicts with the need to stimulate ENaC for intravascular volume. Therefore, NaCl restriction stimulates ENaC less in pendrin-null mice than in wild-type mice, which mitigates their kaliuresis and hypokalemia but exacerbates volume contraction.

Characterization of pendrin in urinary extracellular vesicles in a rat model of aldosterone excess and in human primary aldosteronism

Pendrin is a Cl^-/HCO₃^- exchanger selectively present in the intercalated cells of the kidney. Although experimental studies have demonstrated that pendrin regulates blood pressure downstream of the renin-angiotensin-aldosterone system, its role in human hypertension remains unclear. Here, we analyzed the quantitative changes in pendrin in urinary extracellular vesicles (uEVs) isolated from a total of 30 patients with primary aldosteronism (PA) and from a rat model of aldosterone excess. Western blot analysis revealed that pendrin is present in dimeric and monomeric forms in uEVs in humans and rats. In a rodent model that received continuous infusion of aldosterone with or without concomitant administration of the selective mineralocorticoid receptor (MR) antagonist esaxerenone, pendrin levels in uEVs, as well as those of epithelial Na⁺ channel (ENaC) and Na-Cl-cotransporter (NCC), were highly correlated with renal abundance. In patients with PA, pendrin levels in uEVs were reduced by 49% from baseline by adrenalectomy or pharmacological MR blockade. Correlation analysis revealed that the magnitude of pendrin reduction after treatment significantly correlated with the baseline aldosterone-renin ratio (ARR). Finally, a cross-sectional analysis of patients with PA confirmed a significant correlation between the ARR and pendrin levels in uEVs. These data are consistent with experimental studies showing the role of pendrin in aldosterone excess and suggest that pendrin abundance is attenuated by therapeutic interventions in human PA. Our study also indicates that pendrin analysis in uEVs, along with other proteins, can be useful to understand the pathophysiology of hypertensive disorders.

The Fundamental Role of Bicarbonate Transporters and Associated Carbonic Anhydrase Enzymes in Maintaining Ion and pH Homeostasis in Non-Secretory Organs

The bicarbonate ion has a fundamental role in vital systems. Impaired bicarbonate transport leads to various diseases, including immune disorders, cystic fibrosis, tumorigenesis, kidney diseases, brain dysfunction, tooth fracture, ischemic reperfusion injury, hypertension, impaired reproductive system, and systemic acidosis. Carbonic anhydrases are involved in the mechanism of bicarbonate movement and consist of complex of bicarbonate transport systems including bicarbonate transporters. This review focused on the convergent regulation of ion homeostasis through various ion transporters including bicarbonate transporters, their regulatory enzymes, such as carbonic anhydrases, pH regulatory role, and the expression pattern of ion transporters in non-secretory systems throughout the body. Understanding the correlation between these systems will be helpful in order to obtain new insights and design potential therapeutic strategies for the treatment of pH-related disorders. In this review, we have discussed the broad prospects and challenges that remain in elucidation of bicarbonate-transport-related biological and developmental systems.

Cl- as a bona fide signaling ion

Cl^- is the major extracellular (Cl^-_out) and intracellular (Cl^-_in) anion whose concentration is actively regulated by multiple transporters. These transporters generate Cl^- gradients across the plasma membrane and between the cytoplasm and intracellular organelles. [Cl^-]_in changes rapidly in response to cell stimulation and influences many physiological functions, as well as cellular and systemic homeostasis. However, less appreciated is the signaling function of Cl^-. Cl^- interacts with multiple proteins to directly modify their activity. This review highlights the signaling function of Cl^- and argues that Cl^- is a bona fide signaling ion, a function deserving extensive exploration.

The role of distal tubule and collecting duct sodium reabsorption in sunitinib-induced hypertension

OBJECTIVE

Antiangiogenic receptor tyrosine kinase inhibitors (RTKI) induce arterial hypertension which may limit their use. Renal fractional sodium excretion (FENa) is reduced in early RTKI-induced hypertension, whereas fractional lithium excretion is unaltered. Therefore, we tested the hypothesis that activated distal tubule and collecting duct sodium reabsorption contributes to RTKI-induced hypertension.

METHODS

Amiloride-sensitive and hydrochlorothiazide (HCTZ)-sensitive fractional sodium reabsorption (FRNa) and renal epithelial sodium channel (ENaC) as well as sodium chloride cotransporter (NCC) abundances were determined in sunitinib-treated and control rats. The antihypertensive effects of amiloride and HCTZ were investigated by radiotelemery.

RESULTS

After 4 days of treatment, mean arterial pressure was 20 mmHg higher, FENa was lower (0.32 ± 0.08% vs. 0.65 ± 0.14%; P < 0.05), and renal medullary-ENaC protein abundance was higher in sunitinib-treated rats than in controls. Amiloride-sensitive FRNa was 2.37 ± 0.52% in sunitinib-treated rats vs. 2.66 ± 0.44% in controls (n.s.). HCTZ increased FENa by a similar magnitude without affecting amiloride-sensitive FRNa in both groups. After 14 days of treatment, renal medullary β-ENaC protein abundance was higher in rats that received sunitinib than in controls, whereas α-ENaC, γ-ENaC, and NCC abundances were similar in both groups. Amiloride and HCTZ reduced the sunitinib-induced mean arterial pressure rise by 8 ± 3 mmHg (P < 0.05) and 12 ± 2 mmHg (P < 0.05), respectively, without additive effects when combined.

CONCLUSION

ENaC-dependent and thiazide-sensitive sodium-retaining mechanisms are not overactive in sunitinib-induced hypertension but ENaC blockers and in particular thiazides may be suitable for its treatment.

Modulation of Cl- signaling and ion transport by recruitment of kinases and phosphatases mediated by the regulatory protein IRBIT

IRBIT is a multifunctional protein that controls the activity of various epithelial ion transporters including NBCe1-B. Interaction with IRBIT increases NBCe1-B activity and exposes two cryptic Cl^--sensing GXXXP sites that enable regulation of NBCe1-B by intracellular Cl^- (Cl^- _in). Here, phosphoproteomic analysis revealed that IRBIT controlled five phosphorylation sites in NBCe1-B that determined both the active conformation of the transporter and its regulation by Cl^- _in Mutational analysis suggested that the phosphorylation status of Ser²³², Ser²³³, and Ser²³⁵ was regulated by IRBIT and determined whether NBCe1 transporters are in active or inactive conformations. The absence of phosphorylation at Ser²³², Ser²³³, or Ser²³⁵ produced NBCe1-B in the conformations pSer²³³/pSer²³⁵, pSer²³²/pSer²³⁵, or pSer²³²/pSer²³³, respectively. The activity of the pSer²³³/pSer²³⁵ form was similar to that of IRBIT-activated NBCe1-B, but it was insensitive to inhibition by Cl^- _in The properties of the pSer²³²/pSer²³⁵ form were similar to those of wild-type NBCe1-B, whereas the pSer²³²/pSer²³³ form was partially active, further activated by IRBIT, but retained inhibition by Cl^- _in Furthermore, IRBIT recruited the phosphatase PP1 and the kinase SPAK to control phosphorylation of Ser⁶⁵, which affected Cl^- _in sensing by the ³²GXXXP³⁶ motif. IRBIT also recruited the phosphatase calcineurin and the kinase CaMKII to control phosphorylation of Ser¹², which affected Cl^- _in sensing by the ¹⁹⁴GXXXP¹⁹⁸ motif. Ser²³², Ser²³³, and Ser²³⁵ are conserved in all NBCe1 variants and affect their activity. These findings reveal how multiple kinase and phosphatase pathways use phosphorylation sites to fine-tune a transporter, which have important implications for epithelial fluid and HCO₃ ^- secretion.

Acid-Base and Electrolyte Disorders in Patients with and without Chronic Kidney Disease: An Update

Kidneys play a pivotal role in the maintenance and regulation of acid-base and electrolyte homeostasis, which is the prerequisite for numerous metabolic processes and organ functions in the human body. Chronic kidney diseases compromise the regulatory functions, resulting in alterations in electrolyte and acid-base balance that can be life-threatening. In this review, we discuss the renal regulations of electrolyte and acid-base balance and several common disorders including metabolic acidosis, alkalosis, dysnatremia, dyskalemia, and dysmagnesemia. Common disorders in chronic kidney disease are also discussed. The most recent and relevant advances on pathophysiology, clinical characteristics, diagnosis, and management of these conditions have been incorporated.

Epithelial Na+ Channel Regulation by Extracellular and Intracellular Factors

Epithelial Na⁺ channels (ENaCs) are members of the ENaC/degenerin family of ion channels that evolved to respond to extracellular factors. In addition to being expressed in the distal aspects of the nephron, where ENaCs couple the absorption of filtered Na⁺ to K⁺ secretion, these channels are found in other epithelia as well as nonepithelial tissues. This review addresses mechanisms by which ENaC activity is regulated by extracellular factors, including proteases, Na⁺, and shear stress. It also addresses other factors, including acidic phospholipids and modification of ENaC cytoplasmic cysteine residues by palmitoylation, which enhance channel activity by altering interactions of the channel with the plasma membrane.

Dual regulation of the native ClC-K2 chloride channel in the distal nephron by voltage and pH

ClC-K2 is present on the basolateral membrane of kidney epithelial cells, but little is known about its single channel properties. Pinelli et al. record unitary ClC-K2 currents from intercalated cells of mouse connecting tubules and investigate their regulation by voltage, pH, Cl⁻, and Ca²⁺.

ClC-K2, a member of the ClC family of Cl⁻ channels and transporters, forms the major basolateral Cl⁻ conductance in distal nephron epithelial cells and therefore plays a central role in renal Cl⁻ absorption. However, its regulation remains largely unknown because of the fact that recombinant ClC-K2 has not yet been studied at the single-channel level. In the present study, we investigate the effects of voltage, pH, Cl⁻, and Ca²⁺ on native ClC-K2 in the basolateral membrane of intercalated cells from the mouse connecting tubule. The ∼10-pS channel shows a steep voltage dependence such that channel activity increases with membrane depolarization. Intracellular pH (pH_i) and extracellular pH (pH_o) differentially modulate the voltage dependence curve: alkaline pH_i flattens the curve by causing an increase in activity at negative voltages, whereas alkaline pH_o shifts the curve toward negative voltages. In addition, pH_i, pH_o, and extracellular Ca²⁺ strongly increase activity, mainly because of an increase in the number of active channels with a comparatively minor effect on channel open probability. Furthermore, voltage alters both the number of active channels and their open probability, whereas intracellular Cl⁻ has little influence. We propose that changes in the number of active channels correspond to them entering or leaving an inactivated state, whereas modulation of open probability corresponds to common gating by these channels. We suggest that pH, through the combined effects of pH_i and pH_o on ClC-K2, might be a key regulator of NaCl absorption and Cl⁻/HCO₃⁻ exchange in type B intercalated cells.

Aldosterone Modulates the Association between NCC and ENaC

Distal sodium transport is a final step in the regulation of blood pressure. As such, understanding how the two main sodium transport proteins, the thiazide-sensitive sodium chloride cotransporter (NCC) and the epithelial sodium channel (ENaC), are regulated is paramount. Both are expressed in the late distal nephron; however, no evidence has suggested that these two sodium transport proteins interact. Recently, we established that these two sodium transport proteins functionally interact in the second part of the distal nephron (DCT2). Given their co-localization within the DCT2, we hypothesized that NCC and ENaC interactions might be modulated by aldosterone (Aldo). Aldo treatment increased NCC and αENaC colocalization (electron microscopy) and interaction (coimmunoprecipitation). Finally, with co-expression of the Aldo-induced protein serum- and glucocorticoid-inducible kinase 1 (SGK1), NCC and αENaC interactions were increased. These data demonstrate that Aldo promotes increased interaction of NCC and ENaC, within the DCT2 revealing a novel method of regulation for distal sodium reabsorption.

Aldosterone regulates a 5' variant sgk1 transcript via a shared hormone response element in the sgk1 5' regulatory region

We previously identified a 5ʹ variant alternate transcript of Sgk1 (Sgk1_v3) encoding an NH₂‐terminal variant Sgk1 isoform, Sgk1_i3 that, like Sgk1, is expressed in the distal convoluted tubule, connecting tubule and collecting duct and can stimulate epithelial Na⁺ transport (Am J Physiol Renal Physiol 303: F1527–F1533, 2012). We now demonstrate that, similar to Sgk1, aldosterone and glucocorticoids stimulate Sgk1_v3 expression in cell lines from the collecting duct and airway epithelia. In mice, short term aldosterone infusion and maneuvers that increase endogenous aldosterone secretion including dietary Na⁺ deprivation and K⁺ loading increases distal nephron Sgk1_v3 expression in vivo. Although Sgk1_v3 has a different 5ʹ proximal regulatory region from Sgk1, the transcription start sites are less than 1000 bp apart. We cloned the 5ʹ regulatory region for Sgk1 and Sgk_v3 upstream of a luciferase gene and by deletion and reporter gene analysis we localized the corticosteroid regulatory region for Sgk1_v3 to a glucocorticoid response element (GRE) that had previously been identified for Sgk1 (Am J Physiol Endo Metab 283: E971–E979, 2002). We tested this element with MR in an MR‐null cell line and demonstrate that aldosterone stimulates Sgk1 and Sgk1_v3 via this GRE. We conclude that corticosteroids stimulate Sgk1 and Sgk1_v3 expression in epithelial cells via activation of a common conserved GRE in the 5ʹ flanking region of Sgk1.

Renal sodium transport in renin-deficient Dahl salt-sensitive rats

Objective:

The Dahl salt-sensitive rat is a well-established model of salt-sensitive hypertension. The goal of this study was to assess the expression and activity of renal sodium channels and transporters in the renin-deficient salt-sensitive rat.

Methods:

Renin knockout (Ren^−/−) rats created on the salt-sensitive rat background were used to investigate the role of renin in the regulation of ion transport in salt-sensitive hypertension. Western blotting and patch-clamp analyses were utilized to assess the expression level and activity of Na⁺ transporters.

Results:

It has been described previously that Ren^−/− rats exhibit severe kidney underdevelopment, polyuria, and lower body weight and blood pressure compared to their wild-type littermates. Here we found that renin deficiency led to decreased expression of sodium-hydrogen antiporter (NHE3), the Na⁺/H⁺ exchanger involved in Na⁺ absorption in the proximal tubules, but did not affect the expression of Na-K-Cl cotransporter (NKCC2), the main transporter in the loop of Henle. In the distal nephron, the expression of sodium chloride cotransporter (NCC) was lower in Ren^−/− rats. Single-channel patch clamp analysis detected decreased ENaC activity in Ren^−/− rats which was mediated via changes in the channel open probability.

Conclusion:

These data illustrate that renin deficiency leads to significant dysregulation of ion transporters.

SPAK and OSR1 play essential roles in potassium homeostasis through actions on the distal convoluted tubule

KEY POINTS

STE20 (Sterile 20)/SPS-1 related proline/alanine-rich kinase (SPAK) and oxidative stress-response kinase-1 (OSR1) phosphorylate and activate the renal Na(+) -K(+) -2Cl(-) cotransporter 2 (NKCC2) and Na(+) Cl(-) cotransporter (NCC). Mouse models suggest that OSR1 mainly activates NKCC2-mediated sodium transport along the thick ascending limb, while SPAK mainly activates NCC along the distal convoluted tubule, but the kinases may compensate for each other. We hypothesized that disruption of both kinases would lead to polyuria and severe salt-wasting, and generated SPAK/OSR1 double knockout mice to test this. Despite a lack of SPAK and OSR1, phosphorylated NKCC2 abundance was still high, suggesting the existence of an alternative activating kinase. Compensatory changes in SPAK/OSR1-independent phosphorylation sites on both NKCC2 and NCC and changes in sodium transport along the collecting duct were also observed. Potassium restriction revealed that SPAK and OSR1 play essential roles in the emerging model that NCC activation is central to sensing changes in plasma [K(+) ].

ABSTRACT

STE20 (Sterile 20)/SPS-1 related proline/alanine-rich kinase (SPAK) and oxidative stress-response kinase-1 (OSR1) activate the renal cation cotransporters Na(+) -K(+) -2Cl(-) cotransporter (NKCC2) and Na(+) -Cl(-) cotransporter (NCC) via phosphorylation. Knockout mouse models suggest that OSR1 mainly activates NKCC2, while SPAK mainly activates NCC, with possible cross-compensation. We tested the hypothesis that disrupting both kinases causes severe polyuria and salt-wasting by generating SPAK/OSR1 double knockout (DKO) mice. DKO mice displayed lower systolic blood pressure compared with SPAK knockout (SPAK-KO) mice, but displayed no severe phenotype even after dietary salt restriction. Phosphorylation of NKCC2 at SPAK/OSR1-dependent sites was lower than in SPAK-KO mice, but still significantly greater than in wild type mice. In the renal medulla, there was significant phosphorylation of NKCC2 at SPAK/OSR1-dependent sites despite a complete absence of SPAK and OSR1, suggesting the existence of an alternative activating kinase. The distal convoluted tubule has been proposed to sense plasma [K(+) ], with NCC activation serving as the primary effector pathway that modulates K(+) secretion, by metering sodium delivery to the collecting duct. Abundance of phosphorylated NCC (pNCC) is dramatically lower in SPAK-KO mice than in wild type mice, and the additional disruption of OSR1 further reduced pNCC. SPAK-KO and kidney-specific OSR1 single knockout mice maintained plasma [K(+) ] following dietary potassium restriction, but DKO mice developed severe hypokalaemia. Unlike mice lacking SPAK or OSR1 alone, DKO mice displayed an inability to phosphorylate NCC under these conditions. These data suggest that SPAK and OSR1 are essential components of the effector pathway that maintains plasma [K(+) ].

Caffeine intake antagonizes salt sensitive hypertension through improvement of renal sodium handling

High salt intake is a major risk factor for hypertension. Although acute caffeine intake produces moderate diuresis and natriuresis, caffeine increases the blood pressure (BP) through activating sympathetic activity. However, the long-term effects of caffeine on urinary sodium excretion and blood pressure are rarely investigated. Here, we investigated whether chronic caffeine administration antagonizes salt sensitive hypertension by promoting urinary sodium excretion. Dahl salt-sensitive (Dahl-S) rats were fed with high salt diet with or without 0.1% caffeine in drinking water for 15 days. The BP, heart rate and locomotor activity of rats was analyzed and urinary sodium excretion was determined. The renal epithelial Na⁺ channel (ENaC) expression and function were measured by in vivo and in vitro experiments. Chronic consumption of caffeine attenuates hypertension induced by high salt without affecting sympathetic nerve activity in Dahl-S rats. The renal α-ENaC expression and ENaC activity of rats decreased after chronic caffeine administration. Caffeine increased phosphorylation of AMPK and decrease α-ENaC expression in cortical collecting duct cells. Inhibiting AMPK abolished the effect of caffeine on α-ENaC. Chronic caffeine intake prevented the development of salt-sensitive hypertension through promoting urinary sodium excretion, which was associated with activation of renal AMPK and inhibition of renal tubular ENaC.

Adult nephron-specific MR-deficient mice develop a severe renal PHA-1 phenotype

Aldosterone is the main mineralocorticoid hormone controlling sodium balance, fluid homeostasis, and blood pressure by regulating sodium reabsorption in the aldosterone-sensitive distal nephron (ASDN). Germline loss-of-function mutations of the mineralocorticoid receptor (MR) in humans and in mice lead to the "renal" form of type 1 pseudohypoaldosteronism (PHA-1), a case of aldosterone resistance characterized by salt wasting, dehydration, failure to thrive, hyperkalemia, and metabolic acidosis. To investigate the importance of MR in adult epithelial cells, we generated nephron-specific MR knockout mice (MR(Pax8/LC1)) using a doxycycline-inducible system. Under standard diet, MR(Pax8/LC1) mice exhibit inability to gain weight and significant weight loss compared to control mice. Interestingly, despite failure to thrive, MR(Pax8/LC1) mice survive but develop a severe PHA-1 phenotype with higher urinary Na(+) levels, decreased plasma Na(+), hyperkalemia, and higher levels of plasma aldosterone. This phenotype further worsens and becomes lethal under a sodium-deficient diet. Na(+)/Cl(-) co-transporter (NCC) protein expression and its phosphorylated form are downregulated in the MR(Pax8/LC1) knockouts, as well as the αENaC protein expression level, whereas the expression of glucocorticoid receptor (GR) is increased. A diet rich in Na(+) and low in K(+) does not restore plasma aldosterone to control levels but is sufficient to restore body weight, plasma, and urinary electrolytes. In conclusion, MR deletion along the nephron fully recapitulates the features of severe human PHA-1. ENaC protein expression is dependent on MR activity. Suppression of NCC under hyperkalemia predominates in a hypovolemic state.

Roles and Regulation of Renal K Channels

More than two dozen types of potassium channels, with different biophysical and regulatory properties, are expressed in the kidney, influencing renal function in many important ways. Recently, a confluence of discoveries in areas from human genetics to physiology, cell biology, and biophysics has cast light on the special function of five different potassium channels in the distal nephron, encoded by the genes KCNJ1, KCNJ10, KCNJ16, KCNMA1, and KCNN3. Research aimed at understanding how these channels work in health and go awry in disease has transformed our understanding of potassium balance and provided new insights into mechanisms of renal sodium handling and the maintenance of blood pressure. This review focuses on recent advances in this rapidly evolving field.

Clopidogrel attenuates lithium-induced alterations in renal water and sodium channels/transporters in mice

Lithium (Li) administration causes deranged expression and function of renal aquaporins and sodium channels/transporters resulting in nephrogenic diabetes insipidus (NDI). Extracellular nucleotides (ATP/ADP/UTP), via P2 receptors, regulate these transport functions. We tested whether clopidogrel bisulfate (CLPD), an antagonist of ADP-activated P2Y(12) receptor, would affect Li-induced alterations in renal aquaporins and sodium channels/transporters. Adult mice were treated for 14 days with CLPD and/or Li and euthanized. Urine and kidneys were collected for analysis. When administered with Li, CLPD ameliorated polyuria, attenuated the rise in urine prostaglandin E2 (PGE2), and resulted in significantly higher urinary arginine vasopressin (AVP) and aldosterone levels as compared to Li treatment alone. However, urine sodium excretion remained elevated. Semi-quantitative immunoblotting revealed that CLPD alone increased renal aquaporin 2 (AQP2), Na-K-2Cl cotransporter (NKCC2), Na-Cl cotransporter (NCC), and the subunits of the epithelial Na channel (ENaC) in medulla by 25-130 %. When combined with Li, CLPD prevented downregulation of AQP2, Na-K-ATPase, and NKCC2 but was less effective against downregulation of cortical α- or γ-ENaC (70 kDa band). Thus, CLPD primarily attenuated Li-induced downregulation of proteins involved in water conservation (AVP-sensitive), with modest effects on aldosterone-sensitive proteins potentially explaining sustained natriuresis. Confocal immunofluorescence microscopy revealed strong labeling for P2Y(12)-R in proximal tubule brush border and blood vessels in the cortex and less intense labeling in medullary thick ascending limb and the collecting ducts. Therefore, there is the potential for CLPD to be directly acting at the tubule sites to mediate these effects. In conclusion, P2Y(12)-R may represent a novel therapeutic target for Li-induced NDI.

Acid-Base Homeostasis

Acid-base homeostasis and pH regulation are critical for both normal physiology and cell metabolism and function. The importance of this regulation is evidenced by a variety of physiologic derangements that occur when plasma pH is either high or low. The kidneys have the predominant role in regulating the systemic bicarbonate concentration and hence, the metabolic component of acid-base balance. This function of the kidneys has two components: reabsorption of virtually all of the filtered HCO3(-) and production of new bicarbonate to replace that consumed by normal or pathologic acids. This production or generation of new HCO3(-) is done by net acid excretion. Under normal conditions, approximately one-third to one-half of net acid excretion by the kidneys is in the form of titratable acid. The other one-half to two-thirds is the excretion of ammonium. The capacity to excrete ammonium under conditions of acid loads is quantitatively much greater than the capacity to increase titratable acid. Multiple, often redundant pathways and processes exist to regulate these renal functions. Derangements in acid-base homeostasis, however, are common in clinical medicine and can often be related to the systems involved in acid-base transport in the kidneys.

Effect of mineralocorticoid treatment in mice with collecting duct-specific knockout of endothelin-1

Aldosterone increases blood pressure (BP) by stimulating sodium (Na) reabsorption within the distal nephron and collecting duct (CD). Aldosterone also stimulates endothelin-1 (ET-1) production that acts within the CD to inhibit Na reabsorption via a negative feedback mechanism. We tested the hypothesis that this renal aldosterone-endothelin feedback system regulates electrolyte balance and BP by comparing the effect of a high-salt (NaCl) diet and mineralocorticoid stimulation in control and CD-specific ET-1 knockout (CD ET-1 KO) mice. Metabolic balance and radiotelemetric BP were measured before and after treatment with desoxycorticosterone pivalate (DOCP) in mice fed a high-salt diet with saline to drink. CD ET-1 KO mice consumed more high-salt diet and saline and had greater urine output than controls. CD ET-1 KO mice exhibited increased BP and greater fluid retention and body weight than controls on a high-salt diet. DOCP with high-salt feeding further increased BP in CD ET-1 KO mice, and by the end of the study the CD ET-1 KO mice were substantially hypernatremic. Unlike controls, CD ET-1 KO mice failed to respond acutely or escape from DOCP treatment. We conclude that local ET-1 production in the CD is required for the appropriate renal response to Na loading and that lack of local ET-1 results in abnormal fluid and electrolyte handling when challenged with a high-salt diet and with DOCP treatment. Additionally, local ET-1 production is necessary, under these experimental conditions, for renal compensation to and escape from the chronic effects of mineralocorticoids.

ClC-K chloride channels: emerging pathophysiology of Bartter syndrome type 3

The mutations in the CLCNKB gene encoding the ClC-Kb chloride channel are responsible for Bartter syndrome type 3, one of the four variants of Bartter syndrome in the genetically based nomenclature. All forms of Bartter syndrome are characterized by hypokalemia, metabolic alkalosis, and secondary hyperaldosteronism, but Bartter syndrome type 3 has the most heterogeneous presentation, extending from severe to very mild. A relatively large number of CLCNKB mutations have been reported, including gene deletions and nonsense or missense mutations. However, only 20 CLCNKB mutations have been functionally analyzed, due to technical difficulties regarding ClC-Kb functional expression in heterologous systems. This review provides an overview of recent progress in the functional consequences of CLCNKB mutations on ClC-Kb chloride channel activity. It has been observed that 1) all ClC-Kb mutants have an impaired expression at the membrane; and 2) a minority of the mutants combines reduced membrane expression with altered pH-dependent channel gating. Although further investigation is needed to fully characterize disease pathogenesis, Bartter syndrome type 3 probably belongs to the large family of conformational diseases, in which the mutations destabilize channel structure, inducing ClC-Kb retention in the endoplasmic reticulum and accelerated channel degradation.

Integrated compensatory network is activated in the absence of NCC phosphorylation

Thiazide diuretics are used to treat hypertension; however, compensatory processes in the kidney can limit antihypertensive responses to this class of drugs. Here, we evaluated compensatory pathways in SPAK kinase-deficient mice, which are unable to activate the thiazide-sensitive sodium chloride cotransporter NCC (encoded by Slc12a3). Global transcriptional profiling, combined with biochemical, cell biological, and physiological phenotyping, identified the gene expression signature of the response and revealed how it establishes an adaptive physiology. Salt reabsorption pathways were created by the coordinate induction of a multigene transport system, involving solute carriers (encoded by Slc26a4, Slc4a8, and Slc4a9), carbonic anhydrase isoforms, and V-type H⁺-ATPase subunits in pendrin-positive intercalated cells (PP-ICs) and ENaC subunits in principal cells (PCs). A distal nephron remodeling process and induction of jagged 1/NOTCH signaling, which expands the cortical connecting tubule with PCs and replaces acid-secreting α-ICs with PP-ICs, were partly responsible for the compensation. Salt reabsorption was also activated by induction of an α-ketoglutarate (α-KG) paracrine signaling system. Coordinate regulation of a multigene α-KG synthesis and transport pathway resulted in α-KG secretion into pro-urine, as the α-KG-activated GPCR (Oxgr1) increased on the PP-IC apical surface, allowing paracrine delivery of α-KG to stimulate salt transport. Identification of the integrated compensatory NaCl reabsorption mechanisms provides insight into thiazide diuretic efficacy.

Dietary potassium and the renal control of salt balance and blood pressure

Dietary potassium (K(+)) intake has antihypertensive effects, prevents strokes, and improves cardiovascular outcomes. The underlying mechanism for these beneficial effects of high K(+) diets may include vasodilation, enhanced urine flow, reduced renal renin release, and negative sodium (Na(+)) balance. Indeed, several studies demonstrate that dietary K(+) intake induces renal Na(+) loss despite elevated plasma aldosterone. This review briefly highlights the epidemiological and experimental evidences for the effects of dietary K(+) on arterial blood pressure. It discusses the pivotal role of the renal distal tubule for the regulation of urinary K(+) and Na(+) excretion and blood pressure and highlights that it depends on the coordinated interaction of different nephron portions, epithelial cell types, and various ion channels, transporters, and ATPases. Moreover, we discuss the relevance of aldosterone and aldosterone-independent factors in mediating the effects of an altered K(+) intake on renal K(+) and Na(+) handling. Particular focus is given to findings suggesting that an aldosterone-independent downregulation of the thiazide-sensitive NaCl cotransporter significantly contributes to the natriuretic and antihypertensive effect of a K(+)-rich diet. Last but not least, we refer to the complex signaling pathways enabling the kidney to adapt its function to the homeostatic needs in response to an altered K(+) intake. Future work will have to further address the underlying cellular and molecular mechanism and to elucidate, among others, how an altered dietary K(+) intake is sensed and how this signal is transmitted to the different epithelial cells lining the distal tubule.

Renal acid-base regulation: new insights from animal models

Because majority of biological processes are dependent on pH, maintaining systemic acid-base balance is critical. The kidney contributes to systemic acid-base regulation, by reabsorbing HCO3 (-) (both filtered by glomeruli and generated within a nephron) and acidifying urine. Abnormalities in those processes will eventually lead to a disruption in systemic acid-base balance and provoke metabolic acid-base disorders. Research over the past 30 years advanced our understanding on cellular and molecular mechanisms responsible for those processes. In particular, a variety of transgenic animal models, where target genes are deleted either globally or conditionally, provided significant insights into how specific transporters are contributing to the renal acid-base regulation. Here, we broadly overview the mechanisms of renal ion transport participating to acid-base regulation, with emphasis on data obtained from transgenic mice models.

Prostasin interacts with the epithelial Na+ channel and facilitates cleavage of the γ-subunit by a second protease

During maturation, the α- and γ-subunits of the epithelial Na+ channel (ENaC) undergo proteolytic processing by furin. Cleavage of the γ-subunit by furin at the consensus site γRKRR143 and subsequent cleavage by a second protease at a distal site strongly activate the channel. For example, coexpression of prostasin with ENaC increases both channel function and cleavage at the γRKRK186 site. We generated a polyclonal antibody that recognizes the region 144-186 in the γ-subunit (anti-γ43) to determine whether prostasin promotes the release of the intervening tract between the putative furin and γRKRK186 cleavage sites. Anti-γ43 precipitated both full-length (93 kDa) and furin-processed (83 kDa) γ-subunits from extracts obtained from oocytes expressing αβHA-γ-V5 channels, but only the full-length (93 kDa) γ-subunit from oocytes expressing αβHA-γ-V5 channels and either wild-type or a catalytically inactive prostasin. Although both wild-type and catalytically inactive prostasin activated ENaCs in an aprotinin-sensitive manner, only wild-type prostasin bound to aprotinin beads, suggesting that catalytically inactive prostasin facilitates the cleavage of the γ-subunit by an endogenous protease in Xenopus oocytes. As dietary salt restriction increases cleavage of the renal γ-subunit, we assessed release of the 43-mer inhibitory tract on rats fed a low-Na+ diet. We found that a low-Na+ diet increased γ-subunit cleavage detected with the anti-γ antibody and dramatically reduced the fraction precipitated with the anti-γ43 antibody. Our results suggest that the inhibitory tract dissociates from the γ-subunit in kidneys from rats on a low-Na+ diet.