Background Ca2+ influx mediated by a dihydropyridine- and voltage-insensitive channel in pancreatic beta-cells. Modulation by Ni2+, diphenylamine-2-carboxylate, and glucose metabolism

A stepwise increase in extracellular Ca2+ concentration ([Ca2+]o) can evoke insulin release from pancreatic islets in the absence of secretagogues. We have investigated the ionic mechanism underlying this secretory response by recording intracellular free Ca2+ concentration ([Ca2+]i) from single mouse islets of Langerhans using ratiometric fura-2 microfluorometry. In the presence of 11 mM glucose, the [Ca2+]i undergoes fast oscillations associated with bursting electrical activity. Nifedipine (10 microM) suppressed these oscillations and markedly lowered the [Ca2+]i. Raising the [Ca2+]o from 2.56 to 12.8 mM in the continued presence of 11 mM glucose and nifedipine evoked pronounced [Ca2+]i rises of variable amplitude and time course. This effect was dose-dependent (EC50 = 3.6 mM) and remained essentially unchanged in the absence of glucose or in the presence of 3 mM glucose and nifedipine, conditions where beta-cells are hyperpolarized by approximately -25 mV. Depleting the acetylcholine-mobilizable internal Ca2+ pools by repetitively challenging the islets with acetylcholine in the absence of Ca2+ actually potentiated the standard high Ca2+ responses. The latter were strongly reduced by millimolar concentrations of Ni2+ (70% reduction at 3 mM) and by diphenylamine-2-carboxylate (DPC; IC50 = 145 microM), a blocker of nonselective cation channels. The standard high Ca2+ responses were relatively insensitive to the glycolytic inhibitor mannoheptulose. It is proposed that the high Ca(2+)-evoked [Ca2+]i responses are primarily accounted for by Ca2+ influx through dihydropyridine- and voltage-insensitive, nonselective cation channels. These channels do not appear to be under the control of glucose metabolism. Although their function is unknown, they may be essential to supplying the beta-cells with Ca2+ in the absence of stimulatory levels of fuel secretagogues.