GABA Coordinates with Insulin in Regulating Secretory Function in Pancreatic INS-1 β-Cells

Pancreatic islet beta-cells produce large amounts of c-aminobutyric acid (GABA), which is co-released with insulin. GABA inhibits glucagon secretion by hyperpolarizing alpha-cells via type-A GABA receptors (GABA(A)Rs). We and others recently reported that islet beta-cells also express GABA(A)Rs and that activation of GABA(A)Rs increases insulin release. Here we investigate the effects of insulin on the GABA-GABA(A)R 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 mu M) 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(A)Rs 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 mu M, p<0.01) and insulin (1 mu M, p<0.01), respectively. Together, these data suggest that autocrine GABA, via activation of GABA(A)Rs, depolarizes the pancreatic beta-cells and enhances insulin secretion. On the other hand, insulin down-regulates GABA-GABA(A)R signaling presenting a feedback mechanism for fine-tuning beta-cell secretion.