Dual Functional Role of Membrane Depolarization/Ca²⁺ Influx in Rat Pancreatic B-Cell

Transient exposure of rat pancreatic B-cell to 50 mM K⁺ ([K⁺₅₀]) makes exocytosis unresponsive to further depolarization, i.e., stimulation with 100 mM K⁺ or 1 uM glyburide, which closes the ATP-sensitive K⁺ (K⁺_ATP) channel, simultaneously with [K⁺₅₀] does not produce any greater insulin secretion compared with [K⁺₅₀] alone. In sharp contrast, 16.7 mM glucose ([G_16.7]) applied simultaneously with [K⁺₅₀] elicits an insulin response markedly greater than that produced by [K⁺₅₀] alone, which is not attenuated by 100 uM diazoxide, an inhibitor of K⁺_ATP channel closure. [G_16.7]-Induced Insulin secretion at the basal K⁺ concn of 4.7 mM was greatly (93%) suppressed by 100 uM diazoxide. Insulin secretion induced by [K⁺₅₀] plus [G_16.7] ([K⁺₅₀ + G_16.7]) was markedly suppressed (70%) by 1 uM nifedipine, a Ca²⁺-channel blocker and was completely abolished by 2 mM 2-cyclohexen-1-one, which reportedly decreases reduced glutathione level and blocks glucokinase. This finding indicates that insulin release induced by [K⁺₅₀ + G_16.7] is not due to leakage produced by toxic stimuli but to activation of exocytosis. When graded concentrations (25 and 50 mM) of K⁺ were applied simultaneously with [G_16.7] in the presence of 100 uM diazoxide, insulin response was clearly dependent on K⁺ concentration, indicating that the physiological range of membrane depolarization also activates the glucose-responsive effector. Membrane depolarization/Ca²⁺ influx directly stimulates hormone exocytosis on one hand and activates the K⁺_ATP channel-independent glucose-responsive effector or effectors on the other in the B-cell. The nature of the glucose-responsive effector or effectors remains to be established.