title: GABAB receptor activation inhibits exocytosis in rat pancreatic beta-cells by G-protein-dependent activation of calcineurin.
abstract: We have investigated the regulation of hormone secretion from rat pancreatic islets by the GABAB receptors (GABABRs). Inclusion of the specific GABABR 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 approximately 60%. Using RT-PCR, transcription of GABABR1a-c,f and GABABR2 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 < or = 80% and voltage-gated Ca2+ entry by only approximately 30%. Both effects were concentration-dependent with IC50 values of approximately 2 microm. 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 (Gi/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.
authors.affiliation: Department of Physiological Sciences, BMC B11, SE-221 84 Lund, Sweden. matthias.braun@mphy.lu.se
authors.forename: Matthias
authors.initials: M
authors.lastname: Braun
authors.forename: Anna
authors.initials: A
authors.lastname: Wendt
authors.forename: Karsten
authors.initials: K
authors.lastname: Buschard
authors.forename: Albert
authors.initials: A
authors.lastname: Salehi
authors.forename: Sabine
authors.initials: S
authors.lastname: Sewing
authors.forename: Jesper
authors.initials: J
authors.lastname: Gromada
authors.forename: Patrik
authors.initials: P
authors.lastname: Rorsman
mesh_terms: D000818:Animals; D001418:Baclofen / Q000494:pharmacology; D019703:Calcineurin / Q000378:metabolism*; D002478:Cells, Cultured; D004305:Dose-Response Relationship, Drug; D005089:Exocytosis / Q000187:drug effects / Q000502:physiology*; D058786:GABA-B Receptor Agonists; D058788:GABA-B Receptor Antagonists; D019204:GTP-Binding Proteins / Q000378:metabolism*; D007515:Islets of Langerhans / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D017208:Rats, Wistar; D018080:Receptors, GABA-B / Q000378:metabolism*
chemical_list: D058786:GABA-B Receptor Agonists; D058788:GABA-B Receptor Antagonists; D018080:Receptors, GABA-B; D019703:Calcineurin; D019204:GTP-Binding Proteins; D001418:Baclofen