Aims/hypothesis: The role of gamma-aminobutyric acid (GABA) and A-type GABA receptors (GABA A Rs) in modulating islet endocrine function has been actively investigated since the identification of GABA and GABA A Rs in the pancreatic islets. However, the reported effects of GABA A R activation on insulin secretion from islet beta cells have been controversial. Methods: This study examined the hypothesis that the effect of GABA on beta cell insulin secretion is dependent on glucose concentration. Results: Perforated patchclamp recordings in INS-1 cells demonstrated that GABA, at concentrations ranging from 1 to 1,000 μmol/l, induced a transmembrane current (I GABA ) which was sensitive to the GABA A R antagonist bicuculline. The current-voltage relationship revealed that I GABA reversed at −42±2.2 mV, independently of glucose concentration. Nevertheless, the glucose concentration critically controlled the membrane potential (V M ), i.e., at low glucose (0 or 2.8 mmol/l) the endogenous V M of INS-1 cells was below the I GABA reversal potential and at high glucose (16.7 or 28 mmol/l), the endogenous V M of INS-1 cells was above the I GABA reversal potential. Therefore, GABA dose-dependently induced membrane depolarisation at a low glucose concentration, but hyperpolarisation at a high glucose concentration. Consistent with electrophysiological findings, insulin secretion assays demonstrated that at 2.8 mmol/l glucose, GABA increased insulin secretion in a dose-dependent fashion (p<0.05, n=7). This enhancement was blocked by bicuculline (p<0.05, n=4). In contrast, in the presence of 28 mmol/l glucose, GABA suppressed the secretion of insulin (p<0.05, n=5). Conclusions/interpretation: These findings indicate that activation of GABA A Rs in beta cells regulates insulin secretion in concert with changes in glucose levels.

Background and purpose: Some non-steroidal anti-inflammatory drugs (NSAIDs) incidentally induce hypoglycemia, which is often seen in diabetic patients receiving sulphonylureas. NSAIDs influence various ion channel activities, thus they may cause hypoglycemia by affecting ion channel functions in insulin secreting beta cells. This study investigated the effects of the NSAID meclofenamic acid (MFA) on the electrical excitability and the secretion of insulin from pancreatic beta cells. Experimental approach: Using patch clamp techniques and insulin secretion assays, the effects of MFA on the membrane potential and transmembrane current of INS-1 cells, and insulin secretion were studied. Key results: Under perforated patch recordings, MFA induced a rapid depolarization in INS-1 cells bathed in low (2.8mM), but not high (28mM) glucose solutions. MFA, as well as acetylsalicylic acid (ASA) and flufenamic acid (FFA), excited the cells by inhibiting ATP-sensitive potassium channels (K ATP ). In whole cell recordings, K ATP conductance consistently appeared when intracellular ATP was diluted. Intracellular glibenclamide prevented the development of K ATP activity, whereas intracellular MFA had no effect. At low glibenclamide concentrations, MFA induced additional inhibition of the K ATP current. Live cell Ca 2 þ imaging displayed that MFA elevated intracellular Ca 2 þ at low glucose concentrations. Furthermore, MFA dose-dependently increased insulin release under low, but not high, glucose conditions. Conclusions and Implications: MFA blocked K ATP through an extracellular mechanism and thus increased insulin secretion. As some NSAIDs synergistically inhibit K ATP activity together with sulphonylureas, the risk of NSAID-induced hypoglycemia should be considered when glucose-lowering compounds are administered.