CALCIUM CURRENT REGULATION OF DEPOLARIZATION-EVOKED CALCIUM TRANSIENTS IN BETA-CELLS (HIT-T15)

Glucose-induced insulin secretion by beta-cells is linked to phasic increases in intracellular Ca2+ concentration ([Ca2+]i) arising from membrane depolarization. We examined the source of this Ca2+ in cultured beta-cells using rapid dual-wavelength spectroscopy of fura-2 under voltage-clamp conditions. Depolarization of the 6-cell initiated a sustained rise in [Ca2+]i that was dependent on the activation of L-type Ca2+ current that exhibited very slow inactivation. Neither release of internally stored Ca2+ nor Na+-Ca2+ exchange contributed significantly to this calcium rise, as evidenced by the suppressive effect of rapid application of Cd2+ and the lack of effect of elevations of intracellular Na+ concentration. Restoration of control Ca2+ levels was primarily dependent on Ca2+ channel closure, but both a voltage-dependent and voltage-independent Ca2+ efflux system also contributed. Both the fluorescence-based and charge-based estimates of the rise in [Ca2+]i were in broad agreement, indicating that Ca current activation was the primary source of the Ca2+ transient. The results suggest that nutrient-induced changes in beta-cell membrane potential tightly regulate [Ca2+]i, and thereby insulin release, primarily via alterations in the conductive state of slowly inactivating Ca2+ channels.