Gramicidin D is a potent insulin secretagogue: dependence on extracellular calcium influx

Gramicidin D, a sodium ionophore, was identified as a potent insulin secretagogue in the mouse beta-cell line, beta TC3. Gramicidin D stimulated insulin secretion by 3.2-fold relative to control cells incubated with vehicle alone. Using ion-specific fluorescent probes, gramicidin D (1 microM) increased the intracellular concentrations of Na+ ([Na+]i) and Ca2+ ([Ca2+]i). By contrast, no changes in pHi were detected in cells exposed to ionophore. The increase in [Ca2+]i was biphasic and characterized by an initial peak at 1-2 minutes followed by a sustained second phase. The addition of EGTA (2 mM) to the extracellular medium abolished gramicidin D-induced increase in [Ca2+]i and insulin secretion. These parameters were also profoundly inhibited by the L-type Ca(2+)-channel inhibitor, verapamil (20 microM). These findings suggest that insulin secretion induced from beta TC3 cells by gramicidin D is mediated via the promotion of Ca(2+)-influx.