GABA coordinates with insulin in regulating secretory function in pancreatic INS-1 β-Cells

Abstract

Pancreatic islet β-cells produce large amounts of γ-aminobutyric acid (GABA), which is co-released with insulin. GABA inhibits glucagon secretion by hyperpolarizing α-cells via type-A GABA receptors (GABA ARs). We and others recently reported that islet β-cells also express GABA ARs and that activation of GABA ARs increases insulin release. Here we investigate the effects of insulin on the GABA-GABA AR system in the pancreatic INS-1 cells using perforated-patch recording. The results showed that GABA produces a rapid inward current and depolarizes INS-1 cells. However, pre-treatment of the cell with regular insulin (1 μM) suppressed the GABA-induced current (I GABA) by 43%. Zinc-free insulin also suppressed I GABA to the same extent of inhibition by regular insulin. The inhibition of I GABA occurs within 30 seconds after application of insulin. The insulin-induced inhibition of I GABA persisted in the presence of PI3-kinase inhibitor, but was abolished upon inhibition of ERK, indicating that insulin suppresses GABA ARs through a mechanism that involves ERK activation. Radioimmunoassay revealed that the secretion of C-peptide was enhanced by GABA, which was blocked by pre-incubating the cells with picrotoxin (50 μM, p&0.01) and insulin (1 μM, p&0.01), respectively. Together, these data suggest that autocrine GABA, via activation of GABA ARs, depolarizes the pancreatic β-cells and enhances insulin secretion. On the other hand, insulin down-regulates GABA-GABA AR signaling presenting a feedback mechanism for fine-tuning β-cell secretion. © 2011 Bansal et al.