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

Abstract

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.

RESEARCH DESIGN AND METHODS

Expression of GABA _ARs 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 adenoviral 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.

RESULTS

PCR analysis detected relatively high levels of the mRNAs encoding GABA _AR α ₂, β _3, γ ₂, and π subunits in human islets. Patch-clamp experiments revealed expression of GABA _AR Cl ⁻ channels in 52% of β-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 _AR 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/l glucose. Application of GABA depolarized β-cells and stimulated action potential firing in β-cells exposed to glucose.

CONCLUSIONS

Signaling via GABA and GABA _AR constitutes an autocrine positive feedback loop in human β-cells. The presence of GABA _AR in non–β-cells suggests that GABA may also be involved in the regulation of somatostatin and glucagon secretion.