BACKGROUND CA2-INSENSITIVE AND VOLTAGE-INSENSITIVE CHANNEL IN PANCREATIC BETA-CELLS - MODULATION BY NI2+, DIPHENYLAMINE-2-CARBOXYLATE, AND GLUCOSE-METABOLISM( INFLUX MEDIATED BY A DIHYDROPYRIDINE)

A stepwise increase in extracellular Ca2+ concentration ([Ca2+](o)) ca n evoke insulin release from pancreatic islets in the absence of secre tagogues. We have investigated the ionic mechanism underlying this sec retory response by recording intracellular free Ca2+ concentration ([C a2+](i)) from single mouse islets of Langerhans using ratiometric fura -2 microfluorometry. In the presence of 11 mM glucose, the [Ca2+](i) u ndergoes fast oscillations associated with bursting electrical activit y. Nifedipine (10 mu M) suppressed these oscillations and markedly low ered 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 (EC(50) = 3.6 mM) and remained essentially unchanged in the absence of glucose or in the presence of 3 mM glucose and nifedip ine, 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 lat ter were strongly reduced by millimolar concentrations of Ni2+ (70% re duction at 3 mM) and by diphenylamine-2-carboxylate (DPC; IC50 = 145 m u M), a blocker of nonselective cation channels. The standard high Ca2 + responses were relatively insensitive to the glycolytic inhibitor ma nnoheptulose. It is proposed that the high Ca2+-evoked [Ca2+](i) respo nses are primarily accounted for by Ca2+ influx through dihydropyridin e- and voltage-insensitive, nonselective cation channels. These channe ls do not appear to be under the control of glucose metabolism. Althou gh their function is unknown, they may be essential to supplying the b eta cells with Ca2+ in the absence of stimulatory levels of fuel secre tagogues.