Synergistic action of vasopressin and aldosterone on basolateral Na(+)-K(+)-ATPase in the cortical collecting duct

NHA2 promotes cyst development in an in vitro model of polycystic kidney disease

KEY POINTS

Significant and selective up-regulation of the Na+ /H+ exchanger NHA2 (SLC9B2) was observed in cysts of patients with autosomal dominant polycystic kidney disease. Using the MDCK cell model of cystogenesis, it was found that NHA2 increases cyst size. Silencing or pharmacological inhibition of NHA2 inhibits cyst formation in vitro. Polycystin-1 represses NHA2 expression via Ca2+ /NFAT signalling whereas the dominant negative membrane-anchored C-terminal fragment (PC1-MAT) increased NHA2 levels. Drugs (caffeine, theophylline) and hormones (vasopressin, aldosterone) known to exacerbate cysts elicit NHA2 expression. Taken together, the findings reveal NHA2 as a potential new player in salt and water homeostasis in the kidney and in the pathogenesis of polycystic kidney disease.

ABSTRACT

Autosomal dominant polycystic kidney disease (ADPKD) is caused by mutations in PKD1 and PKD2 encoding polycystin-1 (PC1) and polycystin-2 (PC2), respectively. The molecular pathways linking polycystins to cyst development in ADPKD are still unclear. Intracystic fluid secretion via ion transporters and channels plays a crucial role in cyst expansion in ADPKD. Unexpectedly, we observed significant and selective up-regulation of NHA2, a member of the SLC9B family of Na+ /H+ exchangers, that correlated with cyst size and disease severity in ADPKD patients. Using three-dimensional cultures of MDCK cells to model cystogenesis in vitro, we showed that ectopic expression of NHA2 is causal to increased cyst size. Induction of PC1 in MDCK cells inhibited NHA2 expression with concordant inhibition of Ca2+ influx through store-dependent and -independent pathways, whereas reciprocal activation of Ca2+ influx by the dominant negative membrane-anchored C-terminal tail fragment of PC1 elevated NHA2. We showed that NHA2 is a target of Ca2+ /NFAT signalling and is transcriptionally induced by methylxanthine drugs such as caffeine and theophylline, which are contraindicated in ADPKD patients. Finally, we observed robust induction of NHA2 by vasopressin, which is physiologically consistent with increased levels of circulating vasopressin and up-regulation of vasopressin V2 receptors in ADPKD. Our findings have mechanistic implications on the emerging use of vasopressin V2 receptor antagonists such as tolvaptan as safe and effective therapy for polycystic kidney disease and reveal a potential new regulator of transepithelial salt and water transport in the kidney.

Vasopressin regulation of sodium transport in the distal nephron and collecting duct

Arginine vasopressin (AVP) is released from the posterior pituitary gland during states of hyperosmolality or hypovolemia. AVP is a peptide hormone, with antidiuretic and antinatriuretic properties. It allows the kidneys to increase body water retention predominantly by increasing the cell surface expression of aquaporin water channels in the collecting duct alongside increasing the osmotic driving forces for water reabsorption. The antinatriuretic effects of AVP are mediated by the regulation of sodium transport throughout the distal nephron, from the thick ascending limb through to the collecting duct, which in turn partially facilitates osmotic movement of water. In this review, we will discuss the regulatory role of AVP in sodium transport and summarize the effects of AVP on various molecular targets, including the sodium-potassium-chloride cotransporter NKCC2, the thiazide-sensitive sodium-chloride cotransporter NCC, and the epithelial sodium channel ENaC.

Protein phosphatase 2A interacts with the Na,K-ATPase and modulates its trafficking by inhibition of its association with arrestin

BACKGROUND

The P-type ATPase family constitutes a collection of ion pumps that form phosphorylated intermediates during ion transport. One of the best known members of this family is the Na⁺,K⁺-ATPase. The catalytic subunit of the Na⁺,K⁺-ATPase includes several functional domains that determine its enzymatic and trafficking properties.

METHODOLOGY/PRINCIPAL FINDINGS

Using the yeast two-hybrid system we found that protein phosphatase 2A (PP2A) catalytic C-subunit is a specific Na⁺,K⁺-ATPase interacting protein. PP-2A C-subunit interacted with the Na⁺,K⁺-ATPase, but not with the homologous sequences of the H⁺,K⁺-ATPase. We confirmed that the Na⁺,K⁺-ATPase interacts with a complex of A- and C-subunits in native rat kidney. Arrestins and G-protein coupled receptor kinases (GRKs) are important regulators of G-protein coupled receptor (GPCR) signaling, and they also regulate Na⁺,K⁺-ATPase trafficking through direct association. PP2A inhibits association between the Na⁺,K⁺-ATPase and arrestin, and diminishes the effect of arrestin on Na⁺,K⁺-ATPase trafficking. GRK phosphorylates the Na⁺,K⁺-ATPase and PP2A can at least partially reverse this phosphorylation.

CONCLUSIONS/SIGNIFICANCE

Taken together, these data demonstrate that the sodium pump belongs to a growing list of ion transport proteins that are regulated through direct interactions with the catalytic subunit of a protein phosphatase.

Vasotocin has the potential to inhibit basolateral Na(+)/K (+)-pump current across isolated skin of tree frog in vitro, via its V(2)-type receptor/cAMP pathway

Adult frog skin transports Na(+) from the apical to the basolateral side across the skin. Antidiuretic hormone (ADH) is involved in the regulation of Na(+) transport in both mammals and amphibians. We investigated the effect of arginine vasotocin (AVT), the ADH of amphibians, on the short-circuit current (SCC) across intact skin and on the basolateral Na(+)/K(+)-pump current across apically nystatin-permeabilized skin of the tree frog, Hyla japonica, in which the V(2)-type ADH receptor is expressed in vitro. In intact skin, 1 pM AVT had no effect on the SCC, but 10 nM AVT was sufficient to stimulate the SCC since 10 nM and 1 microM of AVT increased the SCC 3.2- and 3.4-fold, respectively (P > 0.9). However, in permeabilized skin, AVT (1 microM) decreased the Na(+)/K(+)-pump current to 0.79 times vehicle control. Similarly, 500 microM of 8Br-cAMP increased the SCC 3.2-fold, yet 1 mM of 8Br-cAMP decreased the Na(+)/K(+)-pump current to 0.76 times vehicle control. Arachidonic acid (10(-5) M) tended to decrease the Na(+)/K(+)-pump current. To judge from these in vitro experiments, AVT has the potential to inhibit the basolateral Na(+)/K(+)-pump current via the V(2)-type receptor/cAMP pathway in the skin of the tree frog.

Prolactin increases Na+ transport across adult bullfrog skin via stimulation of both ENaC and Na+/K+-pump

PRL is involved in osmoregulation in lower vertebrates. Its serum concentration starts to increase during the metamorphosis of bullfrog tadpoles. Adult bullfrog skin transports Na(+) from the apical to the basolateral side across the skin. PRL is involved in the regulation of this transport. We investigated the effect of ovine PRL on the epithelial Na(+) channel (ENaC), Na(+)/K(+)-pump, and basolateral K(+) channels, which regulate Na(+) transport across adult bullfrog skin, by measuring the short-circuit current (SCC). At 0.1 microg/ml, PRL had no effect on the SCC. PRL (1 microg/ml) was sufficient to stimulate the SCC since 1 and 10 microg/ml of PRL each increased SCC 1.8-fold. Current-fluctuation analysis revealed that PRL (10 microg/ml) increased the density of active ENaC almost 1.8-fold. The effect of PRL on the Na(+)/K(+)-pump was investigated using apically nystatin-permeabilized skin with Ca-free Na-Ringers' solution on each side. PRL (10 microg/ml) increased SCC in this condition around 1.1-fold, suggesting that PRL stimulates the Na(+)/K(+)-pump [although PRL (1 microg/ml) had no effect on this SCC]. The effect of PRL on basolateral K(+) channels was investigated using apically nystatin-permeabilized skin with high-K Ringer's solution on the apical side. PRL (10 microg/ml) had no effect on the SCC, suggesting that PRL does not affect basolateral K(+) channels. Thus, although PRL stimulates the Na(+)/K(+)-pump, this effect probably contributes less than that on ENaC to the regulation of Na(+) transport across adult bullfrog skin.

Molecular mechanism of edema formation in nephrotic syndrome: therapeutic implications

Sodium retention and edema are common features of nephrotic syndrome that are classically attributed to hypovolemia and activation of the renin-angiotensin-aldosterone system. However, numbers of clinical and experimental findings argue against this underfill theory. In this review we analyze data from the literature in both nephrotic patients and experimental models of nephrotic syndrome that converge to demonstrate that sodium retention is not related to the renin-angiotensin-aldosterone status and that fluid leakage from capillary to the interstitium does not result from an imbalance of Starling forces, but from changes of the intrinsic properties of the capillary endothelial filtration barrier. We also discuss how most recent findings on the cellular and molecular mechanisms of sodium retention has allowed the development of an efficient treatment of edema in nephrotic patients.

Phylogenetic, ontogenetic, and pathological aspects of the urine-concentrating mechanism

The urine-concentrating mechanism is one of the most fundamental functions of avian and mammalian kidneys. This particular function of the kidneys developed as a system to accumulate NaCl in birds and as a system to accumulate NaCl and urea in mammals. Based on phylogenetic evidence, the mammalian urine-concentrating mechanism may have evolved as a modification of the renal medulla's NaCl accumulating system that is observed in birds. This qualitative conversion of the urine-concentrating mechanism in the mammalian inner medulla of the kidneys may occur during the neonatal period. Human kidneys have several suboptimal features caused by the neonatal conversion of the urine-concentrating mechanism. The urine-concentrating mechanism is composed of various functional molecules, including water channels, solute transporters, and vasopressin receptors. Abnormalities in water channels aquaporin (AQP)1 and AQP2, as well as in the vasopressin receptor V2R, are known to cause nephrogenic diabetes insipidus. An analysis of the pathological mechanism involved in nephrogenic diabetes insipidus suggests that molecular chaperones may improve the intracellular trafficking of AQP2 and V2R, and, in the near future, such chaperones may become a new clinical tool for treating nephrogenic diabetes insipidus.

Expression of androgen receptor and androgen regulation of NDRG2 in the rat renal collecting duct

Androgens are known to regulate gene expression in the renal proximal tubule. Whether the distal parts of the nephron, in particular the cortical collecting duct (CCD), where sodium reabsorption is controlled tightly by aldosterone, are also targets for these hormones is unknown. Real-time PCR on rat isolated renal tubules showed that androgen receptor mRNA is not only, as expected, expressed in the proximal tubule, but also in the CCD. We examined the effects of adrenalectomy (ADX) plus castration and in-vivo administration of the active metabolite of testosterone, dihydrotestosterone (DHT), on the intrarenal expression of N-myc downstream regulated gene 2 (NDRG2), an early aldosterone-induced gene located specifically in the CCD. NDRG2 belongs to a newly identified family of differentiation-related genes; although the function of these genes remains elusive, regulation of NDRG1 by androgens has been suggested. Castration plus ADX increased NDRG2 expression (RNase protection assay) significantly in the whole kidney, and a single i.p. injection of DHT caused a significant decrease in NDRG2 expression 4 h afterwards (up to 24 h). Furthermore, real-time PCR on microdissected tubules revealed that the decrease in NDRG2 expression caused by DHT is restricted to the CCD. Thus, aldosterone and androgens have opposite effects on NDRG2 expression in the renal CCD. These results are the first demonstration of androgen-dependent gene regulation in the rat renal CCD.

Stimulation of Na+ transport by AVP is independent of PKA phosphorylation of the Na-K-ATPase in collecting duct principal cells

Arginine-vasopressin (AVP) stimulates Na(+) transport and Na-K-ATPase activity via cAMP-dependent PKA activation in the renal cortical collecting duct (CCD). We investigated the role of the Na-K-ATPase in the AVP-induced stimulation of transepithelial Na(+) transport using the mpkCCD(c14) cell model of mammalian collecting duct principal cells. AVP (10(-9) M) stimulated both the amiloride-sensitive transepithelial Na(+) transport measured in intact cells and the maximal Na pump current measured by the ouabain-sensitive short-circuit current in apically permeabilized cells. These effects were associated with increased Na-K-ATPase cell surface expression, measured by Western blotting after streptavidin precipitation of biotinylated cell surface proteins. The effects of AVP on Na pump current and Na-K-ATPase cell surface expression were dependent on PKA activity but independent of increased apical Na(+) entry. Time course experiments revealed that in response to AVP, the cell surface expression of both endogenous Na-K-ATPase and hybrid Na pumps containing a c-myc-tagged wild-type human alpha(1)-subunit increased transiently. Na-K-ATPase cell surface expression was maximal after 30 min and then declined toward baseline after 60 min. Immunoprecipitation experiments showed that PKA activation did not alter total phosphorylation levels of the endogenous Na-K-ATPase alpha-subunit. In addition, mutation of the PKA phosphorylation site (S943A or S943D) did not alter the time course of increased cell surface expression of c-myc-tagged Na-K-ATPase in response to AVP or to dibutyryl-cAMP. Therefore, stimulation of Na-K-ATPase cell surface expression by AVP is dependent on PKA but does not rely on alpha(1)-subunit phosphorylation on serine 943 in the collecting duct principal cells.

AVP-induced VIT32 gene expression in collecting duct cells occurs via trans-activation of a CRE in the 5'-flanking region of the VIT32 gene

VIT32, a vasopressin-induced transcript, inhibits Na(+) transport when coexpressed with the epithelial sodium channel in Xenopus laevis oocytes (EMBO J 21: 5109-5117, 2002). To understand the mechanism of VIT32 gene regulation, we examined the effect of DDAVP and cAMP stimulation on VIT32 expression in M-1 mouse collecting duct cells and in H441 human airway epithelial cells. Elevation of cAMP with forskolin and IBMX increased VIT32 gene expression with a peak effect at 2 h. The increase in gene expression was abolished by H89 and by actinomycin D, suggesting that cAMP stimulates VIT32 mRNA expression by a PKA-mediated increase in gene transcription. An approximately 1.5-kb fragment of the 5'-flanking region of VIT32 was cloned and was able to confer cAMP-stimulated reporter gene activity when transfected into M-1 and H441 cells. By deletion analysis and site-directed mutagenesis, a cAMP response element (CRE) was identified within the proximal promoter region that was sufficient to account for the increase in VIT32 gene expression seen with DDAVP and elevation of cAMP. Furthermore, DDAVP-stimulated VIT32 promoter-reporter activity was inhibited by H89 and by a dominant negative CREB construct. Finally, we were able to identify CREB as a nuclear protein that bound to the VIT32 CRE in gel mobility shift assays. In summary, DDAVP stimulates transcription of VIT32 via a CRE within the proximal promoter region of the VIT32 gene.

"Drink at least eight glasses of water a day." Really? Is there scientific evidence for "8 x 8"?

Despite the seemingly ubiquitous admonition to "drink at least eight 8-oz glasses of water a day" (with an accompanying reminder that beverages containing caffeine and alcohol do not count), rigorous proof for this counsel appears to be lacking. This review sought to find the origin of this advice (called "8 x 8" for short) and to examine the scientific evidence, if any, that might support it. The search included not only electronic modes but also a cursory examination of the older literature that is not covered in electronic databases and, most importantly and fruitfully, extensive consultation with several nutritionists who specialize in the field of thirst and drinking fluids. No scientific studies were found in support of 8 x 8. Rather, surveys of food and fluid intake on thousands of adults of both genders, analyses of which have been published in peer-reviewed journals, strongly suggest that such large amounts are not needed because the surveyed persons were presumably healthy and certainly not overtly ill. This conclusion is supported by published studies showing that caffeinated drinks (and, to a lesser extent, mild alcoholic beverages like beer in moderation) may indeed be counted toward the daily total, as well as by the large body of published experiments that attest to the precision and effectiveness of the osmoregulatory system for maintaining water balance. It is to be emphasized that the conclusion is limited to healthy adults in a temperate climate leading a largely sedentary existence, precisely the population and conditions that the "at least" in 8 x 8 refers to. Equally to be emphasized, lest the message of this review be misconstrued, is the fact (based on published evidence) that large intakes of fluid, equal to and greater than 8 x 8, are advisable for the treatment or prevention of some diseases and certainly are called for under special circumstances, such as vigorous work and exercise, especially in hot climates. Since it is difficult or impossible to prove a negative-in this instance, the absence of scientific literature supporting the 8 x 8 recommendation-the author invites communications from readers who are aware of pertinent publications.

Low-dose desmopressin infusion: renal action in healthy women in moderate salt retention and depletion, and interactions with prostanoids

Studies were carried out to evaluate the influence of variations in sodium balance on the renal response to low-dose infusion of 1-desamino-8- D -arginine vasopressin (dDAVP), and the functional interaction between dDAVP and renal prostanoids. The studies were performed on healthy women in conditions of extracellular fluid volume expansion (SR group, n =9) and depletion (SD2 group, n=6), respectively. The study protocol included hypotonic polyuria (induced by oral water load) and subsequent antidiuresis (induced by low-dose infusion of dDAVP). Three 60-min clearance (cl.) periods were performed during polyuria (cl. P), early (cl. A1) and late (cl. A2) antidiuresis. The urinary concentrations of prostaglandin (PG) E(2) and the stable metabolites of PGI(2) and thromboxane (Tx) A(2), 6-keto-PGF(1alpha) (6KPGF) and TxB(2), were estimated. Paired renal functional explorations were performed in salt retention and salt depletion both in absence and presence of indomethacin (SR.I and SD2.I groups). In both paired and unpaired studies, the early and late effects of dDAVP on the functional excretory variables and the excretion of prostanoids were assessed as percentage variations, (A1-P)% P and (A2-A1)% A1. (I) dDAVP in salt retention and depletion. During early infusion dDAVP produced in both conditions a significant reduction in urinary flow rate, creatinine cl., absolute and fractional excretions of sodium, chloride and potassium; during late infusion dDAVP was effective in inducing a further significant reduction in urinary flow rate. In salt retention compared to depletion the early reductions in sodium and chloride (absolute and fractional) excretions were significantly lower. (II) Indomethacin pretreatment. During early infusion the dDAVP-induced reductions in the urinary flow rate and 6KPGF excretion were enhanced in both conditions. In salt depletion the dDAVP effects in reduction of creatinine cl. and urinary electrolyte excretions were also enhanced. During late infusion the antidiuretic effect of dDAVP was suppressed in salt retention, while in salt depletion creatinine cl., the urinary excretions of electrolytes and both 6KPGF and TxB(2) showed increases significantly different from the dDAVP effects in the absence of indomethacin. In conclusion, (a) the salt-retaining effect of dDAVP was less effective in salt retention compared to depletion. (b) Indomethacin pretreatment affected the renal action of dDAVP in a time-dependent pattern. The early effects in both conditions were consistent with an inhibited synthesis of modulator PGs. On the contrary, the late effects were consistent with the occurrence, at least in salt depletion, of an escape from dDAVP renal action. This escape phenomenon probably depended on a partial regression of the pharmacological inhibition of the modulating PGs.

The effects of dietary sodium loading on the activity and expression of Na, K-ATPase in the rectal gland of the European dogfish (Scyliorhinus canicula)

cDNA fragments of both the alpha- and beta-subunits of the Na, K-ATPase and a cDNA fragment of the secretory form of Na-K-Cl cotransporter from the European dogfish (Scyliorhinus canicula) were amplified and cloned using degenerate primers in RT-PCR. These clones were used along with a sCFTR cDNA from the related dogfish shark, Squalus acanthias to characterise the expression of mRNAs for these ion transporters in the dogfish rectal gland subsequent to an acute feeding episode. Following a single feeding event where starved dogfish were fed squid portions (20 g squid/kg fish), there was a delayed and transient 40-fold increase in the activity of Na, K-ATPase in crude rectal gland homogenates. Increases in enzyme activity were apparent 3 h after the feeding event and peaked at 9 h before returning to control values within 24 h. These increases in activity were accompanied by small and transient decreases in plasma sodium and chloride concentrations lasting up to 3 days. Significant increases in the expression of mRNAs for alpha- and beta-subunits of the Na, K-ATPase, the Na-K-Cl cotransporter and CFTR chloride channel were detected but not until 1-2 days after the feeding event. It is concluded that the transient increase in Na, K-ATPase activity is not attributable to increases in the abundance of alpha- and beta-subunit mRNAs but must be associated with some, as yet unknown, post-transcriptional activation mechanism.

Tetracycline-inducible gene expression in cultured rat renal CD cells and in intact CD from transgenic mice

The renal collecting duct (CD) plays a key role in the control of ion and fluid homeostasis. Several genetic diseases that involve mutations in genes encoding for ion transporters or hormone receptors specifically expressed in CD have been described. Suitable cellular or transgenic animal models expressing such mutated genes in an inducible manner should represent attractive systems for structure-function relationship analyses and the generation of appropriate physiopathological models of related diseases. Our first goal was to develop a CD cell line that allows inducible gene expression using the tetracycline-inducible system (Tet-On). We designed several strategies aimed at the development of a tight and highly inducible system in RCCD1 cells, a rat cortical collecting duct (CCD) cell line exhibiting several properties of the native CCD. Analysis of reporter gene expression demonstrated that the Tet-On system is suitable for inducible gene expression in these cells. In a second step, we have tested whether transgenic Tet-On mice expressing the tetracycline transactivator under the control of the human cytomegalovirus promoter were suitable for inducible gene expression in tubule epithelial cells. The results indicate that, in vivo, the inducible expression of the lacZ reporter gene appeared to be restricted to the CD. This particular strain of transgenic mice may therefore be useful for the expression of genes of interest in an inducible manner in the collecting duct.

Sodium-potassium-adenosinetriphosphatase-dependent sodium transport in the kidney: hormonal control

Tubular reabsorption of filtered sodium is quantitatively the main contribution of kidneys to salt and water homeostasis. The transcellular reabsorption of sodium proceeds by a two-step mechanism: Na(+)-K(+)-ATPase-energized basolateral active extrusion of sodium permits passive apical entry through various sodium transport systems. In the past 15 years, most of the renal sodium transport systems (Na(+)-K(+)-ATPase, channels, cotransporters, and exchangers) have been characterized at a molecular level. Coupled to the methods developed during the 1965-1985 decades to circumvent kidney heterogeneity and analyze sodium transport at the level of single nephron segments, cloning of the transporters allowed us to move our understanding of hormone regulation of sodium transport from a cellular to a molecular level. The main purpose of this review is to analyze how molecular events at the transporter level account for the physiological changes in tubular handling of sodium promoted by hormones. In recent years, it also became obvious that intracellular signaling pathways interacted with each other, leading to synergisms or antagonisms. A second aim of this review is therefore to analyze the integrated network of signaling pathways underlying hormone action. Given the central role of Na(+)-K(+)-ATPase in sodium reabsorption, the first part of this review focuses on its structural and functional properties, with a special mention of the specificity of Na(+)-K(+)-ATPase expressed in renal tubule. In a second part, the general mechanisms of hormone signaling are briefly introduced before a more detailed discussion of the nephron segment-specific expression of hormone receptors and signaling pathways. The three following parts integrate the molecular and physiological aspects of the hormonal regulation of sodium transport processes in three nephron segments: the proximal tubule, the thick ascending limb of Henle's loop, and the collecting duct.

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

The precise adaptation of renal sodium excretion to systemic needs is to a large extent achieved by the regulation of sodium re-absorption in the aldosterone-sensitive distal nephron. Transcellular sodium re-absorption by the segment-specific cells of the aldosterone-sensitive distal nephron (often called principal cells) is mainly controlled at the level of the expression and activity levels of the epithelial sodium channel, the apical amiloride-sensitive sodium influx pathway. Recent investigations have identified the first early aldosterone-induced proteins that act on epithelial sodium channel function in expression systems. Indirect evidence suggests that one of these aldosterone-induced proteins, the serum- and glucocorticoid-inducible protein kinase SGK1, plays a central integratory role in the control of epithelial sodium channel surface expression and activity, also in the mammalian kidney. Gene-modified animals lacking epithelial sodium channel subunits or expressing mutant subunits have substantiated the central role of the epithelial sodium channel in sodium re-absorption and blood pressure control, as well as for neonatal lung liquid clearance. Mice overexpressing or lacking specific hormones or their receptors have been used to study their role in sodium transport regulation, but the study of mouse physiology appears to lag behind the generation of gene-modified mice. Nonetheless, these new animal models have had a strong impact on research, by stimulating the integration of knowledge and techniques learned from reductionistic molecular approaches into tissue and animal studies, thus breaking down barriers and stimulating collaborations.

Expression of synaptotagmin VIII in rat kidney

The synaptotagmins are a family of integral membrane proteins proposed to function as regulators of both exocytosis and endocytosis. Here, we have used immunochemical techniques and RT-PCR to assess sites of renal expression of synaptotagmin VIII. A polyclonal antibody was raised to a synthetic peptide corresponding to the carboxy-terminal 21 amino acids of mouse synaptotagmin VIII. On immunoblots of membrane fractions from renal cortex and medulla (and in several other tissues), the antibody labeled a 52-kDa band (absent with preimmune IgG). Immunofluorescence localization was carried out in tissue sections from rat kidney. The synaptotagmin VIII antibody labeled early proximal tubules, thin ascending limbs, thick ascending limbs, connecting tubules, and collecting ducts. In collecting ducts, both type A and B intercalated cells exhibited basolateral labeling, whereas principal cells were labeled chiefly in the apical and subapical portion of the cells. Thick ascending limbs were labeled in both the basolateral and apical regions. RT-PCR experiments using total RNA extracted from cortex and medulla or microdissected inner medullary collecting ducts gave a single band of appropriate size. Sequencing of the PCR product confirmed that the amplified target is synaptotagmin VIII. We conclude that synaptotagmin VIII is broadly expressed among renal tubule epithelia, raising the possibility that it is involved in regulation of transport and/or cell remodeling at several sites in the nephron and collecting duct.

Transcriptional regulation of sodium transport by vasopressin in renal cells

We have examined whether arginine vasopressin (AVP) can induce a long-term modulation of transepithelial ion transport in addition to its well known short-term effect. In the RCCD1 rat cortical collecting duct cell line, an increase in both short-circuit current and 22Na transport was observed after several hours of 10(-8) M AVP treatment (a concentration above the in vivo physiological range). This delayed effect was partially prevented by apical addition of 10(-5) M amiloride and was blocked by 10(-6) M actinomycin D and 2 x 10(-6) M cycloheximide. The amounts of mRNA encoding the alpha1 (not beta1) subunit of Na+/K+-ATPase and the beta and gamma (not alpha) subunits of the amiloride-sensitive epithelial Na+ channel were significantly increased by AVP treatment. The increase in mRNA was blocked by actinomycin D, not by amiloride, suggesting a Na+-independent increase in the rate of transcription of these subunits. The translation rates of the alpha1 subunit of Na+/K+-ATPase and the beta and gamma subunits of the rat epithelial sodium channel increased significantly, whereas the translation rates of the other subunits remained unchanged. Finally, the number of Na+ channels present in the apical membrane of the cells increased, as demonstrated by enhanced specific [3H]phenamil binding.

Vasopressin potentiates mineralocorticoid selectivity by stimulating 11 beta hydroxysteroid deshydrogenase in rat collecting duct

Arginine vasopressin (AVP) and corticosteroid hormones are involved in sodium reabsorption regulation in the renal collecting duct. Synergy between AVP and aldosterone has been well documented, although its mechanism remains unclear. Both aldosterone and glucocorticoid hormones bind to the mineralocorticoid receptor (MR), and mineralocorticoid selectivity depends on the MR-protecting enzyme 11 beta hydroxysteroid deshydrogenase (11-HSD), which metabolizes glucocorticoids into derivatives with low affinity for MR. We have investigated whether the activity of 11-HSD could be influenced by AVP and corticosteroid hormones. This study shows that in isolated rat renal collecting ducts, AVP increases 11-HSD catalytic activity. This effect is maximal at 10(-8) M AVP (a concentration clearly above the normal physiological range of AVP concentrations) and involves the V2 receptor pathway, while activation of protein kinase C or changes in intracellular calcium are ineffective. The stimulatory effect of AVP on 11-HSD is largely reduced after adrenalectomy, and is selectively restored by infusion of aldosterone, not glucocorticoids. We conclude that this synergy between AVP and aldosterone in controlling the activity of 11-HSD is likely to play a pivotal role in resetting mineralocorticoid selectivity, and hence sodium reabsorption capacities of the renal collecting duct.