Sodium reabsorption in aldosterone-sensitive distal nephron: news and contributions from genetically engineered animals

Maintaining K+ balance on the low-Na+, high-K+ diet

A low-Na⁺, high-K⁺ diet (LNaHK) is considered a healthier alternative to the "Western" high-Na⁺ diet. Because the mechanism for K⁺ secretion involves Na⁺ reabsorptive exchange for secreted K⁺ in the distal nephron, it is not understood how K⁺ is eliminated with such low Na⁺ intake. Animals on a LNaHK diet produce an alkaline load, high urinary flows, and markedly elevated plasma ANG II and aldosterone levels to maintain their K⁺ balance. Recent studies have revealed a potential mechanism involving the actions of alkalosis, urinary flow, elevated ANG II, and aldosterone on two types of K⁺ channels, renal outer medullary K⁺ and large-conductance K⁺ channels, located in principal and intercalated cells. Here, we review these recent advances.

Interacting influence of diuretics and diet on BK channel-regulated K homeostasis

Large conductance, Ca-activated K channels (BK) are abundantly located in cells of vasculature, glomerulus, and distal nephron, where they are involved in maintaining blood volume, blood pressure, and K homeostasis. In mesangial cells and smooth muscle cells of vessels, the BK-α pore associates with BK-β1 subunits and regulates contraction in a Ca-mediated feedback manner. The BK-β1 also resides in connecting tubule cells of the nephron. BK-β1 knockout mice (β1KO) exhibit fluid retention, hypertension, and compromised K handling. The BK-α/β4 resides in acid/base transporting intercalated cells (IC) of the distal nephron, where they mediate K secretion in mammals on a high K, alkaline diet. BK-α expression in IC is increased by a high K diet via aldosterone. The BK-β4 subunit and alkaline urine are necessary for the luminal expression and function of BK-α in mouse IC. In distal nephron cells, membrane BK-α expression is inhibited by WNK4 in in vitro expression systems, indicating a role in the hyperkalemic phenotype in patients with familial hyperkalemic hypertension type 2 (FHHt2). β1KO and BK-β4 knockout mice (β4KO) are hypertensive because of exaggerated epithelial Na channels (ENaC) mediated Na retention in an effort to secrete K via only renal outer medullary K channels (ROMK). BK hypertension is resistant to thiazides and furosemide, and would be more amenable to ENaC and aldosterone inhibiting drugs. Activators of BK-α/β1 or BK-α/β4 might be effective blood pressure lowering agents for a subset of hypertensive patients. Inhibitors of renal BK would effectively spare K in patients with Bartter Syndrome, a renal K wasting disease.

Stimulation of calcium-sensing receptor increases biochemical H⁺-ATPase activity in mouse cortex and outer medullary regions

The aim of this project was to investigate the interaction between the calcium-sensing receptor (CaSR) and proton extrusion by the V-ATPase and gastric-like isoform of the H(+)/K(+)-ATPase in the mouse nephron. Biochemical activity of H(+)- ATPases was analysed using a partially purified membrane fraction of mouse cortex and outer medullary region. The V-ATPase activity (sensitive to 10(-7) mol·L(-1) bafilomycin) from the cortical and outer medullary region was significantly stimulated by increasing the [Formula: see text] (outside Ca(2+)), in a dose-dependent pattern. Gastric H(+)/K(+)-ATPase activity (sensitive to 10(-5) mol·L(-1) Schering 28080) was also sensitive to changes in [Formula: see text] levels. A significant increase in V-ATPase activity was also observed when CaSR was stimulated with agonists such as 300 μmol·L(-1) Gd(3+) and 200 μmol·L(-1) neomycin, both in the cortex and outer medulla. The cortical and outer medullary gastric H(+)/K(+)-ATPase activity was also stimulated by Gd(3+) and neomycin. Finally, cortical V-ATPase activity was significantly stimulated by 10(-9) mol·L(-1) angiotensin II, and the stimulation of CaSR in the presence of angiotensin significantly enhanced this effect, suggesting that an interaction in the intracellular signaling pathways is involved. In summary, CaSR stimulation enhances the biochemical activity of V-ATPase and gastric H(+)/K(+)-ATPase in both the cortical and outer medullary region of mouse kidney.

Aldosterone postnatally, but not at birth, is required for optimal induction of renal mineralocorticoid receptor expression and sodium reabsorption

Sodium wasting during the neonatal period is the consequence of a physiological aldosterone resistance, related to a low renal mineralocorticoid receptor (MR) expression at birth, both in humans and mice. To investigate whether aldosterone is involved in the neonatal regulation of MR expression, we compared aldosterone and corticosterone levels and renal MR expression by quantitative real-time PCR, between aldosterone synthase (AS) knockout, heterozygous, and wild type (WT) mice, at birth and postnatal d 8. Analysis of MR transcripts showed a similar expression profile in all genotypes, demonstrating that the lack of aldosterone does not modify either the low renal MR expression at birth or its postnatal induction. However, mRNA levels of the α-subunit of the epithelial sodium channel, a MR target gene, were significantly higher in WT compared with AS knockout mice, both at birth and postnatal d 8, despite high corticosterone levels in AS knockout mice, indicating that aldosterone is required for optimal renal induction of the epithelial sodium channel. Using organotypic cultures of newborn WT kidneys, we confirmed that aldosterone does not regulate MR expression at birth, but is instead capable of increasing MR expression in mature kidneys, unlike dexamethasone. In sum, we demonstrate both in vivo and in vitro, that, whereas aldosterone has no significant impact on renal MR expression at birth, it is crucial for optimal MR regulation in postnatal kidneys and for appropriate hydroelectrolytic balance. Understanding of MR-regulatory mechanisms could therefore lead to new therapeutic strategies for the management of sodium loss in preterms and neonates.

The renal connecting tubule: Resolved and unresolved issues in Ca(2+) transport

The renal connecting tubule (CNT) localizes to the distal part of the nephron between the distal convoluted tubule and the collecting duct, and consists of two different cell types: segment-specific and intercalated cells. The former reabsorb water (H(2)O), sodium (Na(+)) and calcium (Ca(2+)) ions to the blood compartment, while secreting potassium ions (K(+)) into the pro-urine. The latter cells contribute to the renal control of the acid-base balance. Several factors and hormones tightly regulate these transport processes. Although the CNT reabsorbs only ∼15% of filtered Ca(2+) load, this segment is finally decisive for the amount of Ca(2+) that appears in the urine. Impaired Ca(2+) transport across CNT can provoke severe urinary Ca(2+) excretion, called hypercalciuria. This review mainly focuses on the activity, abundance and expression of the epithelial Ca(2+) channel named Transient Receptor Potential Vanilloid 5 (TRPV5) that is the gatekeeper of active Ca(2+) reabsorption in the CNT.

Modulation of the renin-angiotensin-aldosterone system in sepsis: a new therapeutic approach?

IMPORTANCE OF THE FIELD

Severe sepsis is characterized by relative hypotension associated with a high cardiac output, peripheral vasodilation, and organ dysfunction. The renin-angiotensin-aldosterone system (RAAS) is primarily activated to increase blood pressure, but recently potential pro-inflammatory effects of angiotensin II have attracted interest because of the reported association between angiotensin II levels and organ failure and mortality in sepsis. RAAS antagonists could represent a new therapeutic option in this setting.

AREAS COVERED IN THIS REVIEW

The role of RAAS activation in severe sepsis and septic shock, and the potential benefits (and risks) of using RAAS antagonists.

WHAT THE READER WILL GAIN

Insight into RAAS function in severe sepsis and the potential for RAAS inhibitors to be used as an adjunctive therapy in patients with severe sepsis, with discussion of promising results from animal models of sepsis.

TAKE HOME MESSAGE

Use of RAAS antagonists is an emerging therapeutic option in severe sepsis because these agents may reduce endothelial damage, organ failure, and mortality. However, timing of administration of RAAS antagonists is important because reduced RAAS function may contribute to refractive hypotension later on in septic shock and benefits of RAAS antagonists seem to be restricted to the early phases of sepsis.

Differential regulation of H+-ATPases in MDCK-C11 cells by aldosterone and vasopressin

The objective of the present work was to characterize the biochemical activity of the proton pumps present in the C11 clone of Madin–Darby canine kidney (MDCK) cells, akin to intercalated cells of the collecting duct, as well as to study their regulation by hormones like aldosterone and vasopressin. MDCK-C11 cells from passages 78 to 86 were utilized. The reaction to determine H+-ATPase activity was started by addition of cell homogenates to tubes contained the assay medium. The inorganic phosphate (Pi) released was determined by a colorimetric method modified from that described by Fiske and Subbarow. Changes in intracellular calcium concentration in the cells was determined using the Ca2+-sensing dye fluo-4 AM. Homogenates of MDCK-C11 cells present a bafilomycin-sensitive activity (vacuolar H+-ATPase), and a vanadate-sensitive activity (H+/K+-ATPase). The bafilomycin-sensitive activity showed a pH optimum of 6.12. ATPase activity was also stimulated in a dose-dependent fashion as K+ concentration was increased between 0 and 50 mmol·L–1, with an apparent Km for the release of Pi of 0.13 mmol·L–1 and Vmax of 22.01 nmol·mg–1·min–1. Incubation of cell monolayers with 10−8 mol·L–1 aldosterone for 24 h significantly increased vacuolar H+-ATPase activity, an effect prevented by 10−5 mol·L–1 spironolactone. Vacuolar H+-ATPase activity was also stimulated by 10−11 mol·L–1 vasopressin, an effect prevented by a V1 receptor-specific antagonist. This dose of vasopressin determined a sustained rise of cytosolic ionized calcium. We conclude that (i) homogenates of MDCK-C11 cells present a bafilomycin-sensitive (H+-ATPase) activity and a vanadate-sensitive (H+/K+-ATPase) activity, and (ii) vacuolar H+-ATPase activity is activated by aldosterone through a genomic pathway and by vasopressin through V1 receptors.

Aldosterone regulates rapid trafficking of epithelial sodium channel subunits in renal cortical collecting duct cells via protein kinase D activation

Aldosterone elicits rapid physiological responses in target tissues such as the distal nephron through the stimulation of cell signaling cascades. We identified protein kinase D (PKD1) as an early signaling response to aldosterone treatment in the M1-cortical collecting duct (M1-CCD) cell line. PKD1 activation was blocked by the PKC inhibitor chelerythrine chloride and by rottlerin, a specific inhibitor of PKCdelta. The activation of PKCdelta and PKCepsilon coincided with PKD1 activation and while a complex was formed between PKD1 and PKCepsilon after aldosterone treatment, there was a concurrent reduction in PKD1 association with PKCdelta. A stable PKD1 knockdown M1-CCD-derrived clone was developed in which PKD1 expression was 90% suppressed by gene silencing with a PKD1-specific siRNA. The effect of aldosterone treatment on the subcellular distribution of enhanced cyan fluorescent protein (eCFP)-tagged epithelial sodium channel (ENaC) subunits in wild type (WT) and PKD1 suppressed cells was examined using confocal microscopy. In an untreated confluent monolayer of M1-CCD cells, alpha, beta, and gamma ENaC subunits were evenly distributed throughout the cytoplasm of WT and PKD1-suppressed cells. After 2 min treatment, aldosterone stimulated the localization of each of the ENaC subunits to discrete regions within the cytoplasm of WT cells. The translocation of eCFP-ENaC subunits in WT cells was inhibited by rottlerin and the mineralocorticoid receptor (MR) antagonist spironolactone. No subcellular translocation of eCFP-ENaC subunits was observed in PKD1-suppressed cells treated with aldosterone. These data demonstrate the involvement of a novel MR/PKCdelta /PKD1 signaling cascade in the earliest ENaC subunit intracellular trafficking events that follow aldosterone treatment.

SGK1 activates Na+-K+-ATPase in amphibian renal epithelial cells

Serum- and glucocorticoid-induced kinase 1 (SGK1) is thought to be an important regulator of Na+reabsorption in the kidney. It has been proposed that SGK1 mediates the effects of aldosterone on transepithelial Na+transport. Previous studies have shown that SGK1 increases Na+transport and epithelial Na+channel (ENaC) activity in the apical membrane of renal epithelial cells. SGK1 has also been implicated in the modulation of Na+-K+-ATPase activity, the transporter responsible for basolateral Na+efflux, although this observation has not been confirmed in renal epithelial cells. We examined Na+-K+-ATPase function in an A6 renal epithelial cell line that expresses SGK1 under the control of a tetracycline-inducible promoter. The results showed that expression of a constitutively active mutant of SGK1 (SGK1TS425D) increased the transport activity of Na+-K+-ATPase 2.5-fold. The increase in activity was a direct consequence of activation of the pump itself. The onset of Na+-K+-ATPase activation was observed between 6 and 24 h after induction of SGK1 expression, a delay that is significantly longer than that required for activation of ENaC in the same cell line (1 h). SGK1 and aldosterone stimulated the Na+pump synergistically, indicating that the pathways mediated by these molecules operate independently. This observation was confirmed by demonstrating that aldosterone, but not SGK1TS425D, induced an ∼2.5-fold increase in total protein and plasma membrane Na+-K+-ATPase α1-subunit abundance. We conclude that aldosterone increases the abundance of Na+-K+-ATPase, whereas SGK1 may activate existing pumps in the membrane in response to chronic or slowly acting stimuli.

GI262570, a Peroxisome Proliferator-Activated Receptor γ Agonist, Changes Electrolytes and Water Reabsorption from the Distal Nephron in Rats

Peroxisome proliferator-activated receptor-gamma (PPARgamma) agonists have been shown to have significant therapeutic benefits such as desirable glycemic control in type 2 diabetic patients; however, these agents may cause fluid retention in susceptible individuals. Since PPARgamma is expressed selectively in distal nephron epithelium, we studied the mechanism of PPARgamma agonist-induced fluid retention using male Sprague-Dawley rats treated with either vehicle or GI262570 (farglitazar), a potent PPARgamma agonist. GI262570 (20 mg/kg/day) induced a plasma volume expansion. The plasma volume expansion was accompanied by a small but significant decrease in plasma potassium concentration. Small but significant increases in plasma sodium and chloride concentrations were also observed. These changes in serum electrolytes suggested an activation of the renal mineralocorticoid response system; however, GI262570-treated rats had lower plasma levels of aldosterone compared with vehicle-treated controls. mRNA levels for a group of genes involved in distal nephron sodium and water absorption are changed in the kidney medulla with GI262570 treatment. In addition, due to a possible rebound effect on epithelial sodium channel (ENaC) activity, a low dose of amiloride did not prevent GI262570-induced fluid retention. On the contrary, the rebound effect after amiloride treatment potentiated GI262570-induced plasma volume expansion. This is at least partially due to a synergistic effect of GI262570 and the rebound from amiloride treatment on ENaCalpha expression. In summary, our current data suggest that GI262570 can increase water and sodium reabsorption in distal nephron by stimulating the ENaC and Na,K-ATPase system. This may be an important mechanism for PPARgamma agonist-induced fluid retention.

Forms of mineralocorticoid hypertension

Hypertension with hypokalemia, metabolic alkalosis, and suppressed plasma renin activity defines mineralocorticoid hypertension. Mineralocorticoid hypertension is the consequence of an overactivity of the epithelial sodium channel expressed at the apical membrane of renal cells in the distal nephron. This is usually the case when the mineralocorticoid receptor is activated by its physiologic substrate aldosterone. The best known form of mineralocorticoid hypertension is an aldosterone-producing adrenal tumor leading to primary aldosteronism. Primary aldosteronism can also be caused by unilateral or bilateral adrenal hyperplasia and rarely adrenal carcinoma. Interestingly, most of the inherited monogenic disorders associated with hypertension involve an excessive activation of the mineralocorticoid axis. In some of these disorders, mineralocorticoid hypertension results from activation of the mineralocorticoid receptor by other steroids (cortisol, deoxycorticosterone), by primary activation of the receptor itself, or by constitutive overactivity of the renal epithelial sodium channel. The present review addresses the physiology and significance of the key players of the mineralocorticoid axis, placing emphasis on the conditions leading to mineralocorticoid hypertension.

Targeted degradation of ENaC in response to PKC activation of the ERK1/2 cascade

Renal A6 epithelial cells were used to determine the mechanism by which protein kinase C (PKC) decreases epithelial Na(+) channel (ENaC) activity. Activation of PKC reduced relative Na(+) reabsorption to <20% within 60 min. This decrease was sustained over the next 24-48 h. In response to PKC signaling, alpha-, beta-, and gamma-ENaC levels were 0.97, 0.36, and 0.39, respectively, after 24 h, with the levels of the latter two subunits being significantly decreased. The PKC-mediated decreases in beta- and gamma-ENaC were significantly reversed by simultaneous addition of the mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase-1/2 inhibitors U-0126 and PD-98059. These inhibitors, in addition, protected Na(+) reabsorption from PKC, demonstrating that the MAPK1/2 cascade, in some instances, plays a central role in downregulation of ENaC activity. The effects of PKC on beta- and gamma-ENaC levels were additive with those of inhibitors of transcription (actinomycin D) and translation (emetine and cycloheximide), suggesting that PKC promotes subunit degradation and does not affect subunit synthesis. The bulk of whole cell gamma-ENaC was degraded within 1 h after treatment with inhibitors of synthesis; however, a significant pool was "protected" from inhibitors for up to 12 h. PKC affected this protected pool of gamma-ENaC. Moreover, proteosome inhibitors (MG-132 and lactacystin) reversed PKC effects on this protected pool of gamma-ENaC. Thus PKC signaling via MAPK1/2 cascade activation in A6 cells promotes degradation of gamma-ENaC.

Identification of cytoplasmic domains within the epithelial Na+ channel reactive at the plasma membrane

The activity of membrane proteins is controlled, in part, by protein-protein interactions localized to the plasma membrane. In the current study, domains within the epithelial Na(+) channel (ENaC) reactive at the plasma membrane were identified using a novel yeast one-hybrid screen. The cytosolic N terminus of alphaENaC and the cytosolic C termini of alpha-, beta-, and gammaENaC contained domains reactive at the plasma membrane. Fluorescent micrographs of epithelial cells overexpressing fusion proteins of enhanced green fluorescent protein and mENaC cytosolic domains were consistent with those in yeast. A novel membrane reactive domain within the cytosolic C terminus of gamma-mENaC was localized to the 17 amino acids between residues Thr(584)-Pro(600). Two overlapping internalization signals within the C terminus of gamma-mENaC, a WW-binding domain (PY motif) and a tyrosine-based endocytic signal, were additive with respect to decreasing complementation and expression levels of hybrid proteins. Decreases in expression levels of hybrid proteins containing the PY and endocytic motif were reversed with latrunculin A, an inhibitor of endosomal movement. Decreases in complementation and expression levels of hybrid proteins mediated by the combined PY and overlapping endocytic motif proceeded in the absence of established ubiquitination sites within ENaC. In addition, the endocytic motif was active in the absence of the PY motif, demonstrating that these two domains, while possibly interacting, also have discrete functions. The novel domains within the cytosolic N terminus of alphaENaC and the C termini of alpha-, beta-, and gammaENaC identified here are likely to be involved in protein-protein and/or protein-lipid interactions localized to the plasma membrane. We hypothesize that these newly identified domains play a role in modulating ENaC activity.