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 beta-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.
Dukes
I D
ID
Glaxo Research Institute, Research Triangle Park, North Carolina 27709.
Cleemann
L
L
D000818:Animals; D001692:Biological Transport / Q000502:physiology; D002118:Calcium / Q000032:analysis / Q000493:pharmacokinetics* / Q000502:physiology; D015220:Calcium Channels / Q000502:physiology*; D002462:Cell Membrane / Q000502:physiology / Q000648:ultrastructure; D002478:Cells, Cultured; D005071:Evoked Potentials / Q000502:physiology; D016257:Fura-2; D007515:Islets of Langerhans / Q000737:chemistry / Q000166:cytology* / Q000502:physiology; D008564:Membrane Potentials / Q000502:physiology*; D051379:Mice; D012964:Sodium / Q000032:analysis / Q000378:metabolism / Q000493:pharmacokinetics; D013057:Spectrum Analysis
D015220:Calcium Channels; D012964:Sodium; D002118:Calcium; D016257:Fura-2