γ-Aminobutyric Acid (GABA) Is an Autocrine Excitatory Transmitter in Human Pancreatic β-Cells

摘要

Objective-Paracrine signaling via γ-aminobutyric acid (GABA) and GABA_A receptors (GABA_ARs) has been documented in rodent islets. Here we have studied the importance of GABAergic signaling in human pancreatic islets.rnRESEARCH DESIGN AND METHODS-Expression of GABA_A Rs in islet cells was investigated by quantitative PCR, immunohistochemistry, and patch-clamp experiments. Hormone release was measured from intact islets. GABA release was monitored by whole-cell patch-clamp measurements after adeno-viral expression of α_1β_1 GABA_AR subunits. The subcellular localization of GABA was explored by electron microscopy. The effects of GABA on electrical activity were determined by perforated patch whole-cell recordings.rnRESULTS-PCR analysis detected relatively high levels of the mRNAs encoding GABA_AR α_3, β_3, γ_2, and π subunits in human islets. Patch-clamp experiments revealed expression of GABA_A R Cl~- channels in 52% of p-cells (current density 9 pA/pF), 91% of δ-cells (current density 148 pA/pF), and 6% of α-cells (current density 2 pA/pF). Expression of GABA_A R subunits in islet cells was confirmed by immunohistochemistry. β-Cells secreted GABA both by glucose-dependent exocytosis of insulin-containing granules and by a glucose-independent mechanism. The GABA_AR antagonist SR95531 inhibited insulin secretion elicited by 6 mmol/1 glucose. Application of GABA depolarized p-cells and stimulated action potential firing in β-cells exposed to glucose.rnCONCLUSIONS-Signaling via GABA and GABA_A R constitutes an autocrine positive feedback loop in human β-cells. The presence of GABA_AR in non-p-cells suggests that GABA may also be involved in the regulation of somatostatin and glucagon secretion.