Cellular origin and hormonal regulation of K(+)-ATPase activities sensitive to Sch-28080 in rat collecting duct

Glucagon actions on the kidney revisited: possible role in potassium homeostasis

It is now recognized that the metabolic disorders observed in diabetes are not, or not only due to the lack of insulin or insulin resistance, but also to elevated glucagon secretion. Accordingly, selective glucagon receptor antagonists are now proposed as a novel strategy for the treatment of diabetes. However, besides its metabolic actions, glucagon also influences kidney function. The glucagon receptor is expressed in the thick ascending limb, distal tubule, and collecting duct, and glucagon regulates the transepithelial transport of several solutes in these nephron segments. Moreover, it also influences solute transport in the proximal tubule, possibly by an indirect mechanism. This review summarizes the knowledge accumulated over the last 30 years about the influence of glucagon on the renal handling of electrolytes and urea. It also describes a possible novel role of glucagon in the short-term regulation of potassium homeostasis. Several original findings suggest that pancreatic α-cells may express a “potassium sensor” sensitive to changes in plasma K concentration and could respond by adapting glucagon secretion that, in turn, would regulate urinary K excretion. By their combined actions, glucagon and insulin, working in a combinatory mode, could ensure an independent regulation of both plasma glucose and plasma K concentrations. The results and hypotheses reviewed here suggest that the use of glucagon receptor antagonists for the treatment of diabetes should take into account their potential consequences on electrolyte handling by the kidney.

Primary molecular disorders and secondary biological adaptations in bartter syndrome

Bartter syndrome is a hereditary disorder that has been characterized by the association of hypokalemia, alkalosis, and the hypertrophy of the juxtaglomerular complex with secondary hyperaldosteronism and normal blood pressure. By contrast, the genetic causes of Bartter syndrome primarily affect molecular structures directly involved in the sodium reabsorption at the level of the Henle loop. The ensuing urinary sodium wasting and chronic sodium depletion are responsible for the contraction of the extracellular volume, the activation of the renin-aldosterone axis, the secretion of prostaglandins, and the biological adaptations of downstream tubular segments, meaning the distal convoluted tubule and the collecting duct. These secondary biological adaptations lead to hypokalemia and alkalosis, illustrating a close integration of the solutes regulation in the tubular structures.

Kidney collecting duct acid-base "regulon"

Kidneys 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".

Molecular identification of Sch28080-sensitive K-ATPase activities in the mouse kidney

Rat collecting ducts display either an ouabain-insensitive or an ouabain-sensitive K-ATPase activity inhibited by Sch28080 according as animals are fed a normal or a potassium-depleted diet (types I and III K-ATPase, respectively). Two isoforms of H,K-ATPase have been cloned from rat gastric mucosa and colon, respectively. Gastric and colonic H,K-ATPase are expressed in the kidney, suggesting that they might account for types I and III K-ATPases. However, this hypothesis is not fully supported by segmental expression of gastric and colonic H,K-ATPase along the rat collecting duct, as well as by comparison of the pharmacological properties of gastric and colonic H,K-ATPase expressed in Xenopus ovocyte and types I and III K-ATPases in rat collecting ducts. The aim of the present work is to address directly the molecular origin of types I and III K-ATPases in the mouse collecting duct by measuring K-ATPase activities in collecting ducts of wild-type mice and mice genetically deficient in either gastric or colonic H,K-ATPase fed either a regular or a potassium-depleted diet. Like the rat, mouse collecting ducts display type I or III K-ATPase activity when fed a regular or a potassium-depleted diet, respectively. Type I K-ATPase activity is detected in colonic H,K-ATPase-deficient mice but not in gastric H,K-ATPase-deficient animals. Conversely, type III K-ATPase activity disappears in colonic H,K-ATPase-deficient but not in gastric H,K-ATPase-deficient mice. In conclusion, types I and III K-ATPases measured in collecting ducts of normal and potassium-depleted mice reflect the functional expression of gastric and colonic H,K-ATPase, respectively.

Renal vacuolar H+-ATPase

Vacuolar H(+)-ATPases are ubiquitous multisubunit complexes mediating the ATP-dependent transport of protons. In addition to their role in acidifying the lumen of various intracellular organelles, vacuolar H(+)-ATPases fulfill special tasks in the kidney. Vacuolar H(+)-ATPases are expressed in the plasma membrane in the kidney almost along the entire length of the nephron with apical and/or basolateral localization patterns. In the proximal tubule, a high number of vacuolar H(+)-ATPases are also found in endosomes, which are acidified by the pump. In addition, vacuolar H(+)-ATPases contribute to proximal tubular bicarbonate reabsorption. The importance in final urinary acidification along the collecting system is highlighted by monogenic defects in two subunits (ATP6V0A4, ATP6V1B1) of the vacuolar H(+)-ATPase in patients with distal renal tubular acidosis. The activity of vacuolar H(+)-ATPases is tightly regulated by a variety of factors such as the acid-base or electrolyte status. This regulation is at least in part mediated by various hormones and protein-protein interactions between regulatory proteins and multiple subunits of the pump.

CREB trans-activates the murine H(+)-K(+)-ATPase alpha(2)-subunit gene

Despite its key role in potassium homeostasis, transcriptional control of the H(+)-K(+)-ATPase alpha(2)-subunit (HKalpha(2)) gene in the collecting duct remains poorly characterized. cAMP increases H(+)-K(+)-ATPase activity in the collecting duct, but its role in activating HKalpha(2) transcription has not been explored. Previously, we demonstrated that the proximal 177 bp of the HKalpha(2) promoter confers basal collecting duct-selective expression. This region contains several potential cAMP/Ca(2+)-responsive elements (CRE). Accordingly, we examined the participation of CRE-binding protein (CREB) in HKalpha(2) transcriptional control in murine inner medullary collecting duct (mIMCD)-3 cells. Forskolin and vasopressin induced HKalpha(2) mRNA levels, and CREB overexpression stimulated the activity of HKalpha(2) promoter-luciferase constructs. Serial deletion analysis revealed that CREB inducibility was retained in a construct containing the proximal 100 bp of the HKalpha(2) promoter. In contrast, expression of a dominant negative inhibitor (A-CREB) resulted in 60% lower HKalpha(2) promoter-luciferase activity, suggesting that constitutive CREB participates in basal HKalpha(2) transcriptional activity. A constitutively active CREB mutant (CREB-VP16) strongly induced HKalpha(2) promoter-luciferase activity, whereas overexpression of CREBdLZ-VP16, which lacks the CREB DNA-binding domain, abolished this activation. In vitro DNase I footprinting and gel shift/supershift analysis of the proximal promoter with recombinant glutathione S-transferase (GST)-CREB-1 and mIMCD-3 cell nuclear extracts revealed sequence-specific DNA-CREB-1 complexes at -86/-60. Mutation at three CRE-like sequences within this region abolished CREB-1 DNA-binding activity and abrogated CREB-VP16 trans-activation of the HKalpha(2) promoter. In contrast, mutation of the neighboring -104/-94 kappabeta element did not alter CREB-VP16 trans-activation of the HKalpha(2) promoter. Thus CREB-1, binding to one or more CRE-like elements in the -86/-60 region, trans-activates the HKalpha(2) gene and may represent an important link between rapid and delayed effects of cAMP on HKalpha(2) activity.

Mechanism of activation of ERK and H-K-ATPase by isoproterenol in rat cortical collecting duct

Isoproterenol stimulates H-K-ATPase activity in rat cortical collecting duct beta-intercalated cells through a PKA-dependent pathway. This study aimed at determining the signaling pathway underlying this effect. H-K-ATPase activity was determined in microdissected collecting ducts preincubated with or without specific inhibitors or antibodies against intracellular signaling proteins. Transient cell membrane permeabilization with streptolysin-O allowed intracellular access to antibodies. Isoproterenol increased phosphorylation of ERK in a PKA-dependent manner, and inhibition of the ERK phosphorylation prevented the stimulation of H-K-ATPase. Antibodies against the monomeric G protein Ras or the kinase Raf-1 curtailed the stimulation of H-K-ATPase by isoproterenol, whereas antibodies against the related proteins Rap-1 and B-Raf had no effect. Pertussis toxin and inhibition of tyrosine kinases with genistein also curtailed isoproterenol-induced stimulation of H-K-ATPase. It is proposed that activation of PKA by isoproterenol induces the phosphorylation of beta-adrenergic receptors and the switch from G(s) to G(i) coupling. In turn, betagamma-subunits released from G(i) would activate a tyrosine kinase-Ras-Raf-1 pathway, leading to the activation of ERK1/2 and of H-K-ATPase.

[Primary molecular changes and secondary biological problems in Bartter and Gitelman syndrome]

Bartter syndrome and Gitelman syndrome are primary hereditary diseases characterized by hypokaliemia, alkalosis, hypertrophy of the juxtaglomerular complex with secondary hyperaldoteronism and normal blood pressure. They result from molecular disorders leading to a defect of sodium reabsorption in respectively the Henle's loop and the distal convoluted tubule. Biological adaptations of downstream tubular segments, i.e. distal convoluted tubule and collecting duct, are responsible for hypokaliemia, alkalosis, renin-aldosterone activation, prostaglandins hypersecretion and dysregulation of the urinary excretion of calcium and magnesium, illustrating the close integration of the regulation of different solutes in the distal tubular structures.

The gamma-Na+,K+-ATPase subunit assembles selectively with alpha1/beta1-Na+,K+-ATPase but not with the colonic H+,K+-ATPase

BACKGROUND

The ubiquitous Na+-pump (Na+,K+-ATPase) assembles as a heterodimer of composition alpha/beta in some nephron segments, while in other segments it may exist as a heterotrimer of composition alpha/beta/gamma. The gamma-subunit has been reported to increase the affinity of the Na+-pump for adenosine 5'-triphosphate (ATP), and decrease affinity for both Na+ and K+. The alpha-subunit of the colonic H+,K+-ATPase (cHK) shares 75% sequence similarity with alpha1-Na+,K+-ATPase (alpha1) and assembles with beta1-Na+,K+-ATPase (beta1) in distal colon and renal medulla. Differences in pharmacological properties have been ascribed to when heterologously expressed function has been compared to function in vitro. The purpose of this study was to determine if cHK might associate with the gamma-subunit of the Na+,K+-ATPase (gamma) as a possible explanation for these variations in function.

METHODS

An antibody specific for the gamma was used in coimmunoprecipitation experiments to determine if the gamma assembles stably in vitro with cHK and beta1 in rat renal medulla or distal colon.

RESULTS

Our results demonstrate that the gamma-subunit assembles specifically with the Na+-pump, but not with cHK. Furthermore, the gamma-subunit assembly was specific for rat kidney and was not observed in distal colon.

CONCLUSION

Since the gamma-subunit did not assemble with the cHK/beta1 complex, gamma-subunit assembly cannot explain those variations in ex vivo and in vitro pharmacologic properties ascribed to cHK.

Novel Schering and ouabain-insensitive potassium-dependent proton secretion in the mouse cortical collecting duct

The intercalated (IC) cells of the cortical collecting duct (CCD) are important to acid-base homeostasis by secreting acid and reabsorbing bicarbonate. Acid secretion is mediated predominantly by apical membrane Schering (SCH-28080)-sensitive H(+)-K(+)- ATPase (HKA) and bafilomycin-sensitive H(+)-ATPase. The SCH-28080-sensitive HKA is believed to be the gastric HKA (HKAg). Here we examined apical membrane potassium-dependent proton secretion in IC cells of wild-type HKAg (+/+) and HKAg knockout (-/-) mice to determine relative contribution of HKAg to luminal proton secretion. The results demonstrated that HKAg (-/-) and wild-type mice had comparable rates of potassium-dependent proton secretion, with HKAg (-/-) mice having 100% of K(+)-dependent H(+) secretion vs. wild-type mice. Potassium-dependent proton secretion was resistant to ouabain and SCH-28080 in HKAg knockout mice but was sensitive to SCH-28080 in wild-type animals. Northern hybridizations did not demonstrate any upregulation of colonic HKA in HKAg knockout mice. These data indicate the presence of a previously unrecognized K(+)-dependent SCH-28080 and ouabain-insensitive proton secretory mechanism in the cortical collecting tubule that may play an important role in acid-base homeostasis.