GABA B receptor activation inhibits exocytosis in rat pancreatic β‐cells by G‐protein‐dependent activation of calcineurin

Abstract

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 GABA B R agonist baclofen inhibited glucose‐stimulated insulin secretion by ∼60%. Using RT‐PCR, transcription of GABA B R 1a‐c,f and GABA B R 2 subunits was detected in β‐cells. Measurements of membrane currents and cell capacitance were applied to single β‐cells to investigate the mechanisms by which GABA B R activation inhibits insulin secretion. In perforated‐patch measurements, baclofen inhibited exocytosis elicited by 500‐ms voltage‐clamp depolarizations to 0 mV by ≤ 80% and voltage‐gated Ca 2+ entry by only ∼30%. Both effects were concentration‐dependent with IC 50 values of ∼2 μ m . 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 GDPβS in the intracellular medium, and became irreversible in the presence of GTPγS 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 [Ca 2+ ] i and electrical activity in glucose‐stimulated β‐cells. These data indicate that GABA released from β‐cells functions as an autocrine inhibitor of insulin secretion in pancreatic islets and that the effect is principally due to direct suppression of exocytosis.