4.6 Article

GABAB receptor activation inhibits exocytosis in rat pancreatic β-cells by G-protein-dependent activation of calcineurin

Journal

JOURNAL OF PHYSIOLOGY-LONDON
Volume 559, Issue 2, Pages 397-409

Publisher

BLACKWELL PUBLISHING LTD
DOI: 10.1113/jphysiol.2004.066563

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We have investigated the regulation of hormone secretion from rat pancreatic islets by the GABA(B) receptors (GABA(B)Rs). Inclusion of the specific GABA(B)R antagonist CGP 55845 in the extracellular medium increased glucose-stimulated insulin secretion 1.6-fold but did not affect the release of glucagon and somatostatin. Conversely, addition of the GABABR agonist baclofen inhibited glucose-stimulated insulin secretion by similar to60%. Using RT-PCR, transcription of GABA(B)R(1a-c,f) and GABA(B)R(2) subunits was detected in beta-cells. Measurements of membrane currents and cell capacitance were applied to single beta-cells to investigate the mechanisms by which GABABR activation inhibits insulin secretion. In perforated-patch measurements, baclofen inhibited exocytosis elicited by 500-ms voltage-clamp depolarizations to 0 mV by less than or equal to 80% and voltage-gated Ca2+ entry by only similar to30%. Both effects were concentration-dependent with IC50 values of similar to2 muM. The inhibitory action of baclofen was abolished in the presence of CGP 55845. The ability of baclofen to suppress exocytosis was prevented by pre-treatment with pertussis toxin and by inclusion of GDPbetaS in the intracellular medium, and became irreversible in the presence of GTPgammaS as expected for a process involving inhibitory G-proteins (G(i/o)-proteins). The inhibitory effect of baclofen resulted from activation of the serine/threonine protein phosphatase calcineurin and pre-treatment with cyclosporin A or intracellular application of calcineurin autoinhibitory peptide abolished the effect. Addition of baclofen had no effect on [Ca2+](i) and electrical activity in glucose-stimulated beta-cells. These data indicate that GABA released from beta-cells functions as an autocrine inhibitor of insulin secretion in pancreatic islets and that the effect is principally due to direct suppression of exocytosis.

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