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Glucagon stimulation of hepatic Na(+)-pump activity and alpha-subunit phosphorylation in rat hepatocytes.

ABSTRACT: In this study the possible role of Na+ influx, arachidonate mediators and alpha-subunit phosphorylation in the stimulatory response of hepatic Na+/K(+)-ATPase to glucagon was examined. Glucagon stimulation of ouabain-sensitive 86Rb+ uptake in freshly isolated rat hepatocytes reached maximal levels in less than 1 min after hormone addition and was half-maximal (EC50) at a concentration of 2.4( +/- 1.3) x 10(-10) M. Analysis of the K(+)-dependence of this response indicates an effect on the apparent Vmax. for K+ with no significant change in the apparent kappa 0.5. Unlike monensin, glucagon stimulation of Na+/K(+)-ATPase-mediated transport activity was not associated with an increase in 22Na+ influx. This indicates that the stimulation of Na+/K(+)-ATPase by glucagon is not secondary to an increase in Na+ influx. A role for arachidonate mediators in this effect also appears unlikely because neither basal nor glucagon-stimulated ouabain-sensitive 86Rb+ uptake was significantly affected by supramaximal concentrations of cyclo-oxygenase, lipoxygenase, cytochrome p-450 or phospholipase A2 inhibitors. To study the possible role of protein kinase-mediated phosphorylation in the stimulation of ouabain-sensitive 86Rb uptake, hepatocytes were metabolically radiolabelled with [32P]P(i), Glucagon stimulated incorporation of 32P into a 95 kDa phosphoprotein that comigrates with Na+/K(+)-ATPase alpha-subunit immunoreactivity in two-dimensional gel electrophoresis. The alpha-subunit could be immunoprecipitated from detergent-solubilized particulate fractions of hepatocytes using an anti-(rat kidney Na+/K(+)-ATPase) serum. When hepatocytes were metabolically radiolabelled with [32P]P(i), the immunoprecipitated alpha-subunit contained 32P. Glucagon increased the incorporation of 32P into the immunoprecipitated subunit by 197 +/- 21% (n = 6). Similar results were observed with a rabbit anti-peptide serum ('anti-LEAVE' serum) prepared against an amino acid sequence in the alpha-subunit. The EC50 for glucagon-stimulated phosphorylation of the alpha-subunit (approximately 1 x 10(-10) M) was very close to that for glucagon stimulation of ouabain-sensitive 86Rb+ uptake. In conclusion, it appears that glucagon stimulation of hepatic Na+/K(+)-ATPase-mediated transport activity is not secondary to increases in Na+ influx or changes in the levels of an arachidonate mediator. The data provide support for the hypothesis that glucagon stimulation of Na(+)-pump activity in hepatocytes may be related to protein kinase-mediated changes in the phosphorylation state of the alpha-subunit.

SUBMITTER: Lynch CJ

PROVIDER: S-EPMC1217007 | biostudies-other | 1996 Feb

REPOSITORIES: biostudies-other

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Project description:Hepatocytes contain the Gi2 and Gi3 forms of the 'Gi-family' of guanine-nucleotide-binding proteins (G-proteins), but not Gi1. The anti-peptide antisera AS7 and I3B were shown to immunoprecipitate Gi2 and Gi3 selectively, and the antiserum CS1 immunoprecipitated the stimulatory G-protein Gs. Treatment of intact, 32P-labelled hepatocytes with one of glucagon, TH-glucagon ([1-N-alpha-trinitrophenylhistidine, 12-homoarginine]glucagon), Arg-vasopressin, angiotensin-II, the phorbol ester TPA (12-O-tetradecanoylphorbol 13-acetate) and 8-bromo-cyclic AMP elicited a time- and dose-dependent increase in the labelling of the alpha-subunit of immunoprecipitated Gi2 which paralleled the loss of ability of low concentrations of the non-hydrolysable GTP analogue guanosine 5'-[beta gamma-imido]triphosphate (p[NH]ppG) to inhibit forskolin-stimulated adenylate cyclase activity ('Gi'-function). The immunoprecipitation of phosphorylated Gi-2 alpha-subunit by the antiserum AS7 was blocked in a dose-dependent fashion by the inclusion of the C-terminal decapeptide of transducin, but not that of Gz (a 'Gi-like' G-protein which lacks the C-terminal cysteine group which is ADP-ribosylated by pertussis toxin in other members of the Gi family), in the immunoprecipitation assay. No labelling of the alpha-subunits of either Gi3 or Gs was observed. alpha-Gi2 was labelled in the basal state and this did not change over 15 min in the absence of ligand addition. In contrast to the monophasic dose-effect curves seen with vasopressin, angiotensin and TPA, the dose-effect curve for the glucagon-mediated increase in the labelling of alpha-Gi2 was markedly biphasic where the loss of Gi function paralleled the high-affinity component of the labelling of alpha-Gi2 caused by glucagon. TPA, TH-glucagon, angiotensin-II and vasopressin achieved similar maximal increases in the labelling of alpha-Gi2, which was approximately half that found after treatment of hepatocytes with either high glucagon concentrations (1 microM) or 8-bromocyclic AMP. Analysis of the phosphoamino acid content of immunoprecipitated alpha-Gi2 showed the presence of phosphoserine only. Incubation of hepatocyte membranes with [gamma-32P]ATP and purified protein kinase C, but not protein kinase A, led to the incorporation of label into immunoprecipitated alpha-Gi2. This labelling was abolished if membranes were obtained from cells which had received prior treatment with ligands shown to cause the phosphorylation of alpha-Gi2 in intact cells. We suggest that there are two possible sites for the phosphorylation of alpha-Gi2; one for C-kinase and the other for an unidentified kinase whose action is triggered by A-kinase activation.

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Glucagon stimulation of hepatic Na(+)-pump activity and alpha-subunit phosphorylation in rat hepatocytes.

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