Influence of membrane potential changes on cytoplasmic Ca2+ concentration in an electrically excitable cell, the insulin-secreting pancreatic B-cell.

Glucose stimulation of insulin release involves metabolism of the sugar and elevation of cytoplasmic calcium (Ca2+i) in pancreatic B-cells. We compared the dynamic changes of metabolism (fluorescence of endogenous reduced pyridine nucleotides, NAD(P)H), membrane potential (intracellular microelectrodes), and Ca2+i (fura-2 technique), in intact mouse islets. Glucose (15 mM) sequentially triggered an increase in NAD(P)H fluorescence, a depolarization with electrical activity, and a rise in Ca2+i. The change in NAD(P)H was monophasic and regular, whereas the changes in membrane potential and Ca2+i were multiphasic, with steady-state regular oscillations of similar average frequencies (about 2.2/min). Digital image analysis revealed that Ca2+i oscillations were synchronous in all regions of the islets. Omission of extracellular Ca2+ abolished the rise in Ca2+i but not the increase in NAD(P)H. Both electrical and Ca2+i oscillations disappeared in low external Ca2+ (1 mM), and became larger but slower in high Ca2+ (10 mM). Sustained depolarization (by tolbutamide, arginine, or high K+) and hyperpolarization (by diazoxide) of B-cells caused sustained increases and decreases of Ca2+i, respectively. In conclusion, the changes in membrane potential induced by various secretagogues trigger synchronous changes in Ca2+i in all B-cells of the islets. The oscillatory pattern of the electrical and Ca2+i responses induced by glucose is not accompanied by and thus probably not due to similar oscillations of metabolism.