Vasopressin-stimulated electrogenic sodium transport in A6 cells is linked to a Ca(2+)-mobilizing signal mechanism

Vasotocin- and mesotocin-induced increases in short-circuit current across tree frog skin

In adult amphibian skin, Na(+) crosses from outside to inside. This Na(+) transport can be measured as the amiloride-blockable short-circuit current (SCC) across the skin. We investigated the effects of arginine vasotocin (AVT) and mesotocin (MT), and those of antagonists of the vasopressin and oxytocin receptors, on the SCC across Hyla japonica skin. (1) Both AVT (100 pmol/L or more) and MT (1 nmol/L or more) increased the SCC. (2) The AVT- and MT-induced increases in SCC recovered with time (downregulation). (3) These AVT/MT-induced effects were blocked by application of OPC-31260 (vasopressin V(2)-receptor antagonist). (4) The OPC-31260 concentration needed to block the AVT-induced response was lower upon post-application (after application of agonist) than upon pre-application (before application of agonist), suggesting the number of receptors may have decreased after AVT application. (5) Upon repeated application of AVT (100 pmol/L), the induced SCC increase did not differ significantly between the 1st and 2nd applications. (6) The time to reach the half-maximum value of the AVT-induced or MT-induced increase in SCC was not significantly different between washout and post-application of OPC-31260, suggesting that post-application of OPC-31260 cleared AVT and MT from their receptors. The effects of AVT, MT, and their antagonists in H. japonica, which is adapted to a terrestrial habitat, are compared with our previously published data on Rana catesbeiana (=Lithobates catesbeianus), which is adapted to a semiaquatic habitat.

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.

Protein kinase A-independent cAMP stimulation of progesterone in a luteal cell model is tyrosine kinase dependent but phosphatidylinositol-3-kinase and mitogen-activated protein kinase independent

Comprehensive understanding of the cellular mechanisms utilized by luteal cells in response to extracellular hormonal signals resulting in the normal synthesis and secretion of their steroid and peptide products has yet to be achieved. Previous studies have established that cAMP functions as a second messenger in mediating gonadotropin stimulated luteal progesterone secretion. Classically, increased intracellular concentrations of cAMP result in activation of protein kinase A (PKA), which in turn phosphorylates gene regulatory transcription factors. Recent studies demonstrate that non-PKA mediated actions of cAMP exist, yet the mechanisms are not well understood. In addition to gonadotropic hormones, such growth factors as insulin, insulin-like growth factor 1, and epidermal growth factor have been shown to modulate luteal steroid hormone synthesis and steroidogenic enzyme expression as either independent effects or via amplification or modulation of the action of gonadotropic hormones or cAMP. Thus, mechanisms independent of cAMP and also downstream to cAMP that do not involve PKA are likely to be important in steroidogenesis in mammalian cells. The present studies were performed to help define the cellular mediators involved in cAMP-stimulated progesterone expression. Our data demonstrate that, in an in vitro steroidogenic cell model, 1) cAMP-stimulated progesterone occurs in a manner that is independent of PKA, 2) neither phosphatidylinositol-3-kinase nor mitogen-activated protein kinase are involved in PKA-independent cAMP-stimulated progesterone production, 3) tyrosine kinase activity does mediate cAMP-stimulated progesterone production, and 4) cAMP directly activates the Ras protein. These data suggest novel mediators of cAMP-stimulated progesterone production.

Role of SGK in hormonal regulation of epithelial sodium channel in A6 cells

The purpose of this study was to examine the role of the serum- and glucocorticoid-induced kinase (SGK) in the activation of the epithelial sodium channel (ENaC) by aldosterone, arginine vasopressin (AVP), and insulin. We used a tetracycline-inducible system to control the expression of wild-type (SGK(wt)(T)), constitutively active (S425D mutation; SGK(S425D)(T)), or inactive (K130M mutation; SGK(K130M)(T)) SGK in A6 cells independently of hormonal stimulation. The effect of SGK expression on ENaC activity was monitored by measuring transepithelial amiloride-sensitive short-circuit current (I(sc)) of transfected A6 cell lines. Expression of SGK(wt)(T) or SGK(S425D)(T) and aldosterone stimulation have additive effects on I(sc). Although SGK could play some role in the aldosterone response, our results suggest that other mechanisms take place. SGK(S425D)(T) abrogates the responses to AVP and insulin; hence, in the signaling pathways of these hormones there is a shared step that is stimulated by SGK. Because AVP and insulin induce fusion of vesicles to the apical membrane, our results support the notion that SGK promotes incorporation of channels in the apical membrane.

Basolateral and apical A1 adenosine receptors mediate sodium transport in cultured renal epithelial (A6) cells

There are conflicting reports in the literature regarding the adenosine receptor that mediates the increase in sodium transport in the A6 cell. In this study we used specific A1 and A2 adenosine receptor agonists and antagonists, as well as two different subclones of the A6 cell, to determine which adenosine receptor mediates the increase in sodium transport. In the A6S2 subclone, basolateral and apical N6-cyclohexyladenosine (CHA), a selective A1 receptor agonist, stimulated sodium transport at a threshold concentration <10(-7) M, whereas CGS-21680, a selective A2 receptor agonist, had a threshold concentration that was at least 10(-5) M. The A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) was found to have a nonspecific effect on CHA-stimulated sodium transport, whereas the A2 receptor antagonist 8-(3-chlorostyryl)caffeine (CSC) had no effect. As with the A6S2 subclone, basolateral and apical CHA stimulated sodium transport at a nanomolar concentration in the A6C1 subclone and the threshold concentration for CGS-21680 was in the high micromolar range. Concurrent with the increase in 1 receptor in different subclones of the A6 cell, including a subclone capable of anion secretion.

Stimulatory effects on Na+ transport in renal epithelia induced by extracts of Nigella arvensis are caused by adenosine

Effects of the extract of Nigella arvensis (NA) seeds on transepithelial Na+ transport were studied in cultured A6 toad kidney cells by recording short-circuit current (Isc),transepithelial conductance (GT), transepithelial capacitance (CT) and fluctuation in Isc. Apical application of NA extract had merely a small stimulatory effect on Na+ transport, whereas basolateral administration markedly increased Isc, GT and CT. A maximal effect was obtained at 500 μl l-1 of lyophilized NA extract. The increase in CT suggests that the activation of Isc occurs through the insertion of transport sites in the apical membrane. In experiments performed in the absence of Na+transport [apical Na+ was replaced by N-methyl-D-glucamine(NMDG+)], basolateral NA extract did not affect Isc and GT, indicating that Cl- conductance was not influenced. Noise analysis of Isc using 6-chloro-3,5-diaminopyrazine-2-carboxamide(CDPC) showed that NA extract reduced single-channel current(iNa) and decreased channel open probability(Po) but evoked a threefold increase in channel density(NT), which confirms the insertion of Na+channels. The separation of the compounds in the crude extract of NAwas performed by fast protein liquid chromatography (FPLC) on a Superdex 200 gel-filtration column and by reverse-phase high-pressure liquid chromatography(RPHPLC) on an μRPC C2/C18 SC2.1/10 column connected to a SMART system. Analysis of the purified active fraction by mass spectrometry demonstrated the presence of adenosine as the single organic compound in the extract that had a stimulatory effect on Na+ transport. In a separate series of experiments, we confirmed that 1 μmol l-1 adenosine had similar effects on the parameters of Na+ transport as did the NAextract. The action of adenosine was further identified by experiments in which NA extract was added after adenosine. In these experiments, NA extract did not affect Isc, GT or CT. These results clearly demonstrate an essential role of adenosine in the stimulatory action of NA extract.

Effects of aldosterone on biosynthesis, traffic, and functional expression of epithelial sodium channels in A6 cells

The collecting duct regulates Na(+) transport by adjusting the abundance/activity of epithelial Na(+) channels (ENaC). In this study we have investigated the synthesis, degradation, endocytosis, and activity of ENaC and the effects of aldosterone on these processes using endogenous channels expressed in the A6 cell line. Biochemical studies were performed with a newly raised set of specific antibodies against each of the three subunits of the amphibian ENaC. Our results indicate simultaneous transcription and translation of alpha, beta, and gamma subunits and enhancement of both processes by aldosterone: two- and fourfold increase, respectively. The biosynthesis of new channels can be followed by acquisition of endoglycosidase H-resistant oligosacharides in alpha and beta subunits and, in the case of alpha, by the appearance of a form resistant to reducing agents. The half-life of the total pool of subunits (t(1/2) 40-70 min) is longer than the fraction of channels in the apical membrane (t(1/2) 12-17 min). Aldosterone induces a fourfold increase in the abundance of the three subunits in the apical membrane without significant changes in the open probability, kinetics of single channels, or in the rate of degradation of ENaC subunits. Accordingly, the aldosterone response could be accounted by an increase in the abundance of apical channels due, at least in part, to de novo synthesis of subunits.

Regulation of the epithelial Na(+) channel by extracellular acidification

The effect of extracellular acidification was tested on the native epithelial Na(+) channel (ENaC) in A6 epithelia and on the cloned ENaC expressed in Xenopus oocytes. Channel activity was determined utilizing blocker-induced fluctuation analysis in A6 epithelia and dual electrode voltage clamp in oocytes. In A6 cells, a decrease of extracellular pH (pH(o)) from 7.4 to 6.4 caused a slow stimulation of the amiloride-sensitive short-circuit current (I(Na)) by 68.4 +/- 11% (n = 9) at 60 min. This increase of I(Na) was attributed to an increase of open channel and total channel (N(T)) densities. Similar changes were observed with pH(o) 5.4. The effects of pH(o) were blocked by buffering intracellular Ca(2+) with 5 microM 1, 2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid. In oocytes, pH(o) 6.4 elicited a small transient increase of the slope conductance of the cloned ENaC (11.4 +/- 2.2% at 2 min) followed by a decrease to 83.7 +/- 11.7% of control at 60 min (n = 6). Thus small decreases of pH(o) stimulate the native ENaC by increasing N(T) but do not appreciably affect ENaC expressed in Xenopus oocytes. These effects are distinct from those observed with decreasing intracellular pH with permeant buffers that are known to inhibit ENaC.

Functional parathyroid hormone receptors are present in an umbilical vein endothelial cell line

Acute parathyroid hormone exposure induces vascular smooth muscle relaxation. In contrast, continuous infusion of parathyroid hormone leads to vasoconstriction and an elevation of blood pressure. Despite the known effects of parathyroid hormone on vascular smooth muscle, possible direct effects on the vascular endothelium have not previously been investigated. Using a human umbilical vein endothelial cell line, we found that parathyroid hormone increased both intracellular calcium and cellular cAMP content in these endothelial cells. Furthermore, exposure of these cells to increasing concentrations of parathyroid hormone stimulated both [(3)H]thymidine incorporation and endothelin-1 secretion. Parathyroid hormone/parathyroid hormone-related peptide receptor mRNA could be detected at low levels in these cells. In summary, these data demonstrate that endothelium-derived cells contain functional parathyroid hormone receptors. The potential physiological role of these receptors remains to be determined.

Toward a comprehensive molecular model of active calcium reabsorption

The fine tuning of Ca(2+) excretion in the kidney takes place in the distal nephron, which consists of the distal convoluted tubule, connecting tubule, and initial portion of the cortical collecting duct. In these segments, Ca(2+) is reabsorbed through an active transcellular pathway. The apical influx of Ca(2+) into the distal renal cell is presumably the rate-limiting step in this process, and its molecular identity has remained obscure so far. The recently discovered epithelial Ca(2+) channel (ECaC) exhibits the expected properties for being the gatekeeper in transcellular Ca(2+) reabsorption. The characteristics and potential physiological role of ECaC will be discussed in this review. Our knowledge of the mechanisms involved in the regulation of transcellular Ca(2+) transport has advanced rapidly since the development of cell models originating from distal tubular cells. Studies using these models indicate that hormones including arginine vasopressin, PGE(2), adenosine, ATP, and atrial natriuretic peptide should be considered as calciotropic hormones controlling renal Ca(2+) handling. Evidence is now beginning to emerge that the stimulating calciotropic hormones utilize new cAMP-independent pathways to stimulate Ca(2+) reabsorption. These new findings allow the development of a comprehensive and detailed model of the process of transcellular calcium transport in the kidney whereby the individual contribution of the participating transporters can now be fully appreciated.

Intracellular pH shifts in cultured kidney (A6) cells: effects on apical Na+ transport

We report, for the epithelial Na+ channel (ENaC) in A6 cells, the modulation by cell pH (pHc) of the transepithelial Na+ current (INa), the current through the individual Na+ channel (i), the open Na+ channel density (No), and the kinetic parameters of the relationship between I(Na) and the apical Na+ concentration. The i and N) were evaluated from the Lorentzian INa noise induced by the apical Na+ channel blocker 6-chloro-3, 5-diaminopyrazine-2-carboxamide. pHc shifts were induced, under strict and volume-controlled experimental conditions, by apical/basolateral NH4Cl pulses or basolateral arrest of the Na+/H+ exchanger (Na+ removal; block by ethylisopropylamiloride) and were measured with the pH-sensitive probe 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein. The changes in pHc were positively correlated to changes in INa and the apically dominated transepithelial conductance. The sole pHc-sensitive parameter underlying INa was No. Only the saturation value of the INa kinetics was subject to changes in pHc. pHc-dependent changes in No may be caused by influencing Po, the ENaC open probability, or/and the total channel number, NT = No/Po.

Hormone-stimulated Ca2+ reabsorption in rabbit kidney cortical collecting system is cAMP-independent and involves a phorbol ester-insensitive PKC isotype

BACKGROUND

Hormones such as parathyroid hormone (PTH), arginine vasopressin (AVP), and prostaglandin E2 (PGE2) are generally believed to act through cAMP to stimulate active Ca2+ reabsorption in the distal part of the nephron.

METHODS

This study investigates the relationship between intracellular cAMP levels and the rate of Ca2+ reabsorption in immunodissected rabbit connecting and cortical collecting tubules cultured to confluence on permeable supports.

RESULTS

Basolateral PTH, AVP, and PGE2 and apical adenosine dose dependently increased Ca2+ reabsorption from 48 to 110 nmol. hr-1. cm-2. Measurement of intracellular cAMP levels revealed that in the case of PTH and AVP, the dose-response curve for the increase in cAMP virtually matched that for transcellular Ca2+ transport. By contrast, with PGE2, this curve was shifted two decades to the right, whereas in the case of adenosine, no increase in cAMP was observed. The results with the latter two hormones disagree with the classic concept that Ca2+ reabsorption is stimulated via a cAMP-dependent mechanism. Furthermore, the potent adenylyl cyclase inhibitor 2', 5'-dideoxyadenosine (DDA; 100 micrometers) suppressed the PTH- and AVP-induced increase in cAMP completely without affecting Ca2+ reabsorption. Similarly, concentrations of PGE2, which maximally stimulated Ca2+ reabsorption without increasing cAMP, were not inhibited by DDA. The specific protein kinase C (PKC) inhibitor chelerythrine (5 micrometers) inhibited PTH-, AVP-, PGE2-, and adenosine-stimulated Ca2+ reabsorption by 77%, 67%, 79%, and 100%, respectively. Down-regulation of phorbol ester-sensitive PKC isotypes by prolonged (120 hr) treatment with 0.1 micrometers 12-O-tetradecanoylphorbol 13-acetate did not interfere with the inhibitory action of chelerythrine on hormone-stimulated Ca2+ transport.

CONCLUSION

PTH, AVP, PGE2, and adenosine stimulate Ca2+ reabsorption via a pathway that is independent of cAMP and that involves a phorbol ester-insensitive PKC isotype.

Vasopressin-induced activation of protein kinase C in renal epithelial cells

Recent studies indicate that the actions of arginine vasopressin (AVP) and other agonists that stimulate electrogenic sodium transport in renal epithelial A6 cells are linked to a Ca(2+)-mobilizing signal transduction mechanism that involves generation of inositol trisphosphate. Since diacylglycerol is the other product in this pathway, studies were performed to determine the possible role of PKC in the stimulation of sodium transport. AVP induced a biphasic increase in diacylglycerol generation, characterized by an initial rapid rise and then a sustained elevation, and PKC activation, reflected by phosphorylation of a specific 80 kDa myristoylated alanine-rich PKC substrate (MARCKS). To determine the PKC isoform(s) involved in this process, immunoblot analysis was performed using antisera that recognize both classical PKC isoforms, XPKC-I and XPCK-II, cloned from Xenopus oocytes. The transcripts of both isoforms were expressed in the A6 cell. Since protein recognized by antisera was translocated from cytosol to the particulate fraction after exposure to AVP, one or both isoforms were activated in the A6 cell. Further studies showed that cyclohexyladenosine and insulin, additional agonists of sodium transport in A6 cells, also stimulated phosphorylation of MARCKS. These results argue that Ca(2+)-dependent PKC is involved in the action of AVP, and that of other agonists, which stimulate sodium transport.

Cytochemical localization of adenylate cyclase in cultured renal epithelial (A6) cells

To characterize the vasopressin-adenylate cyclase (AC) signaling pathway in control of Na+ reabsorption in cultured renal (A6) cells, we determined the distribution of AC with a cytochemical technique using 5'-adenylylimidodiphosphate as substrate and cerium chloride as capturing agent. The addition of forskolin to the medium to stimulate AC activity increased the production of reaction deposits at the enzyme sites. To ensure that the cells were close to their physiological states, cytochemical reactions were performed on unfixed tissues. Subsequent postfixation adequately preserved the morphological features of the cells. AC was mainly restricted to the lateral folds of the cells while the apical membranes were devoid of any deposits. This result provided evidence that the V2-AC pathway is not present in the apical membrane and, hence, any vasopressin action on apical Na+ channels from the luminal side of the cell must involve other signaling pathways. The cytochemical results provided further morphological evidence of the functional coupling between the basolateral and apical membranes of renal cells. We examined the idea that highly variable basal rates of Na+ transport in young differentiating cell cultures may be related to the degree of AC activity. Cytochemical results apparently revealed highly variable amounts of deposits in these cells, but by quantitative analysis of AC activity we could find no significant differences between cells of 6, 14, and 21 days.

Action of insulin on the surface morphology of hepatocytes: role of phosphatidylinositol 3-kinase in insulin-induced shape change of microvilli

In previous studies we have shown that the insulin-responding glucose transporter isoform of 3T3-L1 adipocytes, GluT4, is almost completely located on microvilli. Furthermore, insulin caused the integration of these microvilli into the plasma membrane, suggesting that insulin-induced stimulation of glucose uptake may be due to the destruction of the cytoskeletal diffusion barrier formed by the actin filament bundle of the microvillar shaft regions [Lange et al. (1990) FEBS Lett. 261, 459-463; Lange et al. (1990) FEBS Lett. 276, 39-41]. Similar shape changes in microvilli were observed when the transport rates of adipocytes were modulated by glucose feeding or starvation. Here we demonstrate that the action of insulin on the surface morphology of hepatocytes is identical to that on 3T3L1 adipocytes; small and narrow microvilli on the surface of unstimulated hepatocytes were rapidly shortened and dilated on top of large domed surface areas. The aspect and mechanism of this effect are closely related to "membrane ruffling" induced by insulin and other growth factors. Pretreatment of hepatocytes with the PI 3-kinase inhibitor wortmannin (100 nM), which completely prevents transport stimulation by insulin in adipocytes and other cell types, also inhibited insulin-induced shape changes in microvilli on the hepatocyte surface. In contrast, vasopressin-induced microvillar shape changes in hepatocytes [Lange et al. (1997) Exp. Cell Res. 234, 486-497] were insensitive to wortmannin pretreatment. These findings indicate that PI 3-kinase products are necessary for stimulation of submembrane microfilament dynamics and that cytoskeletal reorganization is critically involved in insulin stimulation of transport processes. The mechanism of the insulin-induced cytoskeletal reorganization can be explained on the basis of the recent finding of Lu et al. [Biochemistry 35(1996) 14027-14034] that PI 3-kinase products exhibit much higher affinity for the profilin-actin complex than the primary products, PIP and PIP2. Thus, activated PI 3-kinase may direct a flux of profilin-actin complexes to the membrane locations of activated insulin receptors, where, due to the release of actin monomers after binding of profilactin to PI(3,4)P2 and PI(3,4,5)P3, massive actin polymerization is initiated. As a consequence, PI 3-kinase activation initiates a vectorial reorganization of the cellular actin system to membrane sites neighboring activated insulin receptors, giving rise to local membrane stress as visualized by extensive surface deformations and shortening of microvilli. In addition, extensive high-affinity binding of F-actin-barbed endcapping proteins enhances the cytoplasmic concentration of rapidly polymerizing filament ends. Consequently, the actin monomer concentration is lowered and the (cytoplasmic) pointed ends of the microvillar shaft bundle depolymerize and become shorter. The observations presented strengthen the previously postulated diffusion-barrier concept of glucose- and ion-uptake regulation and provide a mechanistic basis for explaining the action of insulin and other growth factors on transport processes across the plasma membrane.

Spontaneous calcium oscillations in embryonic stem cell-derived primitive endodermal cells

In vitro differentiation of mouse embryonic stem cells within three-dimensional cell aggregates called embryoid bodies parallels the development of postimplantation embryos at the egg cylinder stage, where visceral and parietal endoderm diverge from the primitive endoderm. We have investigated spontaneous [Ca2+]i oscillations by means of confocal laser-scanning microscopy in primitive endodermal cell layers of embryoid bodies during their differentiation to parietal and visceral endoderm. The frequency of [Ca2+]i oscillations increased from day 4 to day 19 of development, whereas their duration decreased from day 3 to days 16-17. Oscillations depended on both extracellular Ca2+ and Ca2+ release from intracellular stores as they were abolished in Ca(2+)-free solution and in the prescence of Ni2+ and thapsigargin. Signal transduction operated via the phospholipase C (PLC)-mediated inositol 1,4,5-triphosphate (InsP3) pathway with a negative feedback loop via protein kinase C (PKC) as U73,122, a blocker of PLC; bisindolylmaleimide 1, staurosporine, and H-7, blockers of PKC; and 10 mM caffeine totally inhibited [Ca2+]i spiking. Thimerosal, which hypersensitizes the InsP3 receptor, as well as vasopressin and bradykinin, which act via the InsP3 pathway, increased the frequency of [Ca2+]i spikes. In the prescence of brefeldin A (50 microM) or monensin (20 microM), which both inhibit endo/exocytotic vesicle pathways, an immediate transient increase in spiking activity was followed by a decline within 1 to 2 h. In the presence of brefeldin A or thapsigargin or in the absence of extracellular Ca2+, endocytotic vesicles were absent, suggesting that oscillating [Ca2+]i transients are involved in the exo/endocytotic vesicle shuttle.

Identification and characterization of MARCKS in Xenopus laevis

MARCKS proteins are widely distributed in mammalian cells and subserve an important role as probes in the examination of signal transduction processes because they are specific endogenous phosphoreceptors for activated protein kinase C. Experiments were performed to determine whether MARCKS proteins are present in amphibia and to show their usefulness as substrates for stimulated PKC activation, using cultured renal epithelial cells (A6) derived from Xenopus laevis as an experimental model.

Novel effect of insulin: insulin-stimulated Na+ transport is mediated by hydrolysis of phosphoinositides

Previous studies showed that insulin stimulation of electrogenic Na+ transport in renal epithelial cells is mediated by a calcium-dependent signal transduction mechanism. The present study was performed to determine whether the insulin-induced increase in intracellular Ca2+ (Cai2+) was mediated by hydrolysis of phosphatidylinositol and release of inositol trisphosphate. Experiments were conducted with cultured A6 cells, derived from Xenopus Laevis, grown on permeable supports. Addition of insulin resulted in 2 to 3 fold increases in inositol trisphosphate and a 50% increase in 1,2 diacylglycerol within 10s, which corresponded to the time-course, previously reported, of insulin stimulated increases in Na+ transport and Cai2+. Further studies showed that aldosterone, previously shown to stimulate an increase in 1,4,5-inositol trisphosphate at onset of the rise in Na+ transport, also increased DAG levels during the initial phase of stimulation of Na+ transport. These studies provide the first evidence that a biological response induced by insulin is mediated by hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) which results in two products, inositol trisphosphate which causes the release of Ca2+ from intracellular stores and 1,2 diacylglycerol. In addition this study provides further support for the proposal that a common signal transduction mechanism mediates electrogenic Na+ transport by multiple agonists.