GRAMICIDIN-D IS A POTENT INSULIN SECRETAGOGUE - DEPENDENCE ON EXTRACELLULAR CALCIUM INFLUX

Gramicidin D, a sodium ionophore, was identified as a potent insulin s ecretagogue In the mouse beta-cell line, beta TC3. Gramicidin D stimul ated insulin secretion by 3.2-fold relative to control cells incubated with vehicle alone, Using ion-specific fluorescent probes, gramicidin D (1 mu M) increased the intracellular concentrations of Na+ ([Na+](i )) and Ca2+ ([Ca2+](i)). By contrast, no changes in pH, 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 sust ained second phase, The addition of EGTA (2 mM) to the extracellular m edium abolished gramicidin D-induced increase in [Ca2+](i) and insulin secretion. These parameters were also profoundly inhibited by the L-t ype Ca2+-channel inhibitor, verapamil (20 mu M). These findings sugges t that insulin secretion induced from beta TC3 cells by gramicidin D i s mediated via the promotion of Ca2+-influx. (C) 1995 Academic Press, Inc.