New insights into sodium transport regulation in the distal nephron: Role of G-protein coupled receptors

International audienceThe renal handling of Na+ balance is a major determinant of the blood pressure (BP) level. The inability of the kidney to excrete the daily load of Na+ represents the primary cause of chronic hypertension. Among the different segments that constitute the nephron, those present in the distal part (i.e., the cortical thick ascending limb, the distal convoluted tubule, the connecting and collecting tubules) play a central role in the fine-tuning of renal Na+ excretion and are the target of many different regulatory processes that modulate Na+ retention more or less efficiently. G-protein coupled receptors (GPCRs) are crucially involved in this regulation and could represent efficient pharmacological targets to control BP levels. In this review, we describe both classical and novel GPCR-dependent regulatory systems that have been shown to modulate renal Na+ absorption in the distal nephron. In addition to the multiplicity of the GPCR that regulate Na+ excretion, this review also highlights the complexity of these different pathways, and the connections between them

Rôle du récepteur activé par les protéases 2 (PAR2) rénal dans l homéostasie du sodium et du potassium

Les récepteurs activés par les protéases de type 2 (PAR2) sont des récepteurs couplés aux protéines G trimériques dont la particularité est d être activés par clivage protéolytique de leur domaine N-terminal extracellulaire par des sérines protéases. Nous avons étudié le rôle de PAR2 dans le transport ionique dans deux segments du tubule rénal participant à la régulation fine de la balance du sodium et du potassium. Dans la branche large ascendante corticale de l anse de Henle, nous avons montré que l activation de PAR2 induit une augmentation de la réabsorption transcellulaire et paracellulaire de NaCl en stimulant la Na,K-ATPase et en augmentant la perméabilité paracellulaire au Na+. Dans le canal collecteur cortical, PAR2 est exprimé dans les cellules principales et les cellules intercalaires. Dans les premières, l activation de PAR2 induit une faible activation de la réabsorption de Na+ via le canal ENaC et une inhibition de la sécrétion de K+ via le canal ROMK. Dans les secondes, PAR2 induit l activation de la voie de réabsorption électroneutre de NaCl. Les effets de PAR2 dans le canal collecteur impliquent l activation des kinases WNK4, ERK et SPAK. In vivo, nous avons observé qu en réponse à un régime carencé en Na+, les souris PAR2-/- présentent un défaut de conservation rénale de Na+ et développent une hypotension, et qu en réponse à une carence en K+, elles présentent un défaut de conservation rénale du K+ conduisant à une hypokaliémie. Ces résultats montrent pour la première fois que PAR2 contrôle de façons opposées la réabsorption de Na+ et la sécrétion de K+ dans le néphron distal et participe ainsi au maintien de la pression artérielle et de la kaliémie.Proteinase activated receptor 2 (PAR2) are G protein-coupled receptor activated by proteolysis of their N-terminal extracellular domain. PAR2 are expressed in the kidney and we investigated their role in sodium and potassium transport in two segments of the distal nephron, which is the site of the fine tuning of sodium and potassium balance. In the cortical thick ascending limb, PAR2 activation increases Na+ reabsorption through both trans and para-cellular pathways: PAR2 stimulates the Na,K-ATPase and enhances the paracellular permeability to Na+ by triggering at least two signaling pathways, inducing a PLC/PKC/ERK cascade. In the cortical collecting duct, PAR2 is expressed in both principal and intercalated cells. In principal cells, activation of PAR2 induces a mild Na+ reabsorption through ENaC while inhibiting K+ secretion by ROMK. In intercalated cells, it activates the electroneutral NaCl reabsorption pathway. Effects of PAR2 in the collecting duct involve the kinases ERK, WNK4 and SPAK. To determine the physiologic relevance of these in vitro findings, we studied the renal function of PAR2-/- mice. In response to Na+ depletion, PAR2-/- mice show a defect in Na+ handling and develop hypotension and, in response to K+ depletion, they show a defect in K+ handling and hypokalemia. Taken together, our results show for the first time that PAR2 controls Na+ reabsorption and K+ secretion in opposite directions in the distal nephron, and participates in the control of blood pressure and kalemia.PARIS-BIUSJ-Biologie recherche (751052107) / SudocSudocFranceF

Kidney collecting duct acid-base "regulon".

International audienceKidneys are essential for acid-base homeostasis, especially when organisms cope with changes in acid or base dietary intake. Because collecting ducts constitute the final site for regulating urine acid-base balance, we undertook to identify the gene network involved in acid-base transport and regulation in the mouse outer medullary collecting duct (OMCD). For this purpose, we combined kidney functional studies and quantitative analysis of gene expression in OMCDs, by transcriptome and candidate gene approaches, during metabolic acidosis. Furthermore, to better delineate the set of genes concerned with acid-base disturbance, the OMCD transcriptome of acidotic mice was compared with that of both normal mice and mice undergoing an adaptative response through potassium depletion. Metabolic acidosis, achieved through an NH4Cl-supplemented diet for 3 days, not only induced acid secretion but also stimulated the aldosterone and vasopressin systems and triggered cell proliferation. Accordingly, metabolic acidosis increased the expression of genes involved in acid-base transport, sodium transport, water transport, and cell proliferation. In particular, >25 transcripts encoding proteins involved in urine acidification (subunits of H-ATPase, kidney anion exchanger, chloride channel Clcka, carbonic anhydrase-2, aldolase) were co-regulated during acidosis. These transcripts, which cooperate to achieve a similar function and are co-regulated during acidosis, constitute a functional unit that we propose to call a "regulon"

The renal cortical collecting duct: a secreting epithelium?

International audienceIn vitro microperfusion experiments have demonstrated that cortical collecting ducts (CCDs) reabsorb sodium via principal and type B intercalated cells under sodium-depleted conditions and thereby contribute to sodium and blood pressure homeostasis. However, these experiments were performed in the absence of the transepithelial ion concentration gradients that prevail in vivo and determine paracellular transport. The present study aimed to characterize Na+, K+ and Cl− fluxes in the mouse CCD in the presence of physiological transepithelial concentration gradients. For this purpose, we combined in vitro measurements of ion fluxes across microperfused CCDs of sodium-depleted mice with the predictions of a mathematical model. When NaCl transport was inhibited in all cells, CCDs secreted Na+ and reabsorbed K+; Cl− transport was negligible. Removing inhibitors of type A and B intercalated cells increased Na+ secretion in wild-type (WT) mice but not in H+/K+-ATPase type 2 (HKA2) knockout mice. Further inhibition of basolateral NaCl entry via the Na+-K+-2Cl− cotransporter in type A intercalated cells reduced Na+ secretion in WT mice to the levels observed in HKA2−/− mice. With no inhibitors, WT mouse CCDs still secreted Na+ and reabsorbed K+. In vivo, HKA2−/− mice excreted less Na+ than WT mice after switching to a high-salt diet. Taken together, our results indicate that type A intercalated cells secrete Na+ via basolateral Na+-K+-2Cl− cotransporters in tandem with apical HKA2 pumps. They also suggest that the CCD can mediate overall Na+ secretion, and that its ability to reabsorb NaCl in vivo depends on the presence of acute regulatory factors

Inhibition of K + secretion in the distal nephron in nephrotic syndrome: possible role of albuminuria

International audienceNephrotic syndrome features massive proteinuria and retention of sodium which promotes ascite formation. In the puromycin aminonucleoside-induced rat model of nephrotic syndrome, sodium retention originates from the collecting duct where it generates a driving force for potassium secretion. However, there is no evidence for urinary potassium loss or hypokalaemia in the nephrotic syndrome. We therefore investigated the mechanism preventing urinary potassium loss in the nephrotic rats and, for comparison, in hypovolaemic rats, another model displaying increased sodium reabsorption in collecting ducts. We found that sodium retention is not associated with urinary loss of potassium in either nephrotic or hypovolaemic rats, but that different mechanisms account for potassium conservation in the two models. Collecting ducts from hypovolaemic rats displayed high expression of the potassium-secreting channel ROMK but no driving force for potassium secretion owing to low luminal sodium availability. In contrast, collecting ducts from nephrotic rats displayed a high driving force for potassium secretion but no ROMK. Down-regulation of ROMK in nephrotic rats probably stems from phosphorylation of ERK arising from the presence of proteins in the luminal fluid. In addition, nephrotic rats displayed a blunted capacity to excrete potassium when fed a potassium-rich diet, and developed hyperkalaemia. As nephrotic patients were found to display plasma potassium levels in the normal to high range, we would recommend not only a low sodium diet but also a controlled potassium diet for patients with nephrotic syndrome

α-Ketoglutarate regulates acid-base balance through an intrarenal paracrine mechanism

Paracrine communication between different parts of the renal tubule is increasingly recognized as an important determinant of renal function. Previous studies have shown that changes in dietary acid-base load can reverse the direction of apical α-ketoglutarate (αKG) transport in the proximal tubule and Henle's loop from reabsorption (acid load) to secretion (base load). Here we show that the resulting changes in the luminal concentrations of αKG are sensed by the αKG receptor OXGR1 expressed in the type B and non-A-non-B intercalated cells of the connecting tubule (CNT) and the cortical collecting duct (CCD). The addition of 1 mM αKG to the tubular lumen strongly stimulated Cl(-)-dependent HCO(3)(-) secretion and electroneutral transepithelial NaCl reabsorption in microperfused CCDs of wild-type mice but not Oxgr1(-/-) mice. Analysis of alkali-loaded mice revealed a significantly reduced ability of Oxgr1(-/-) mice to maintain acid-base balance. Collectively, these results demonstrate that OXGR1 is involved in the adaptive regulation of HCO(3)(-) secretion and NaCl reabsorption in the CNT/CCD under acid-base stress and establish αKG as a paracrine mediator involved in the functional coordination of the proximal and the distal parts of the renal tubule

Performance of ion chromatography to measure picomole amounts of magnesium in nanolitre samples

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Renal Proteinase-activated Receptor 2, a New Actor in the Control of Blood Pressure and Plasma Potassium Level

International audienceProteinase-activated receptor 2 (PAR2) is a G protein-coupled membrane receptor that is activated upon cleavage of its extracellular N-terminal domain by trypsin and related proteases. PAR2 is expressed in kidney collecting ducts, a main site of control of Na(+) and K(+) homeostasis, but its function remains unknown. We evaluated whether and how PAR2 might control electrolyte transport in collecting ducts, and thereby participate in the regulation of blood pressure and plasma K(+) concentration. PAR2 is expressed at the basolateral border of principal and intercalated cells of the collecting duct where it inhibits K(+) secretion and stimulates Na(+) reabsorption, respectively. Invalidation of PAR2 gene impairs the ability of the kidney to control Na(+) and K(+) balance and promotes hypotension and hypokalemia in response to Na(+) and K(+) depletion, respectively. This study not only reveals a new role of proteases in the control of blood pressure and plasma potassium level, but it also identifies a second membrane receptor, after angiotensin 2 receptor, that differentially controls sodium reabsorption and potassium secretion in the late distal tubule. Conversely to angiotensin 2 receptor, PAR2 is involved in the regulation of sodium and potassium balance in the context of either stimulation or nonstimulation of the renin/angiotensin/aldosterone system. Therefore PAR2 appears not only as a new actor of the aldosterone paradox, but also as an aldosterone-independent modulator of blood pressure and plasma potassium

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