1,25-DIHYDROXYVITAMIN D-3 EVOKES OSCILLATIONS OF INTRACELLULAR CALCIUM IN A PANCREATIC BETA-CELL LINE

The steroid hormone 1 alpha,25-dihydroxyvitamin D-3 [1,25-(OH)(2)D-3] can elicit biological responses via a nongenomic pathway that involves rapid opening of the plasma membrane Ca2+ channels. There is also evidence that 1,25-(OH)(2)D-3 influences insulin secretion in the pancreatic beta-cell, which is primarily mediated by a rapid rise in the concentration of intracellular free Ca2+ ([Ca2+](i)). We employed fluorescent digital ratiometric video imaging at the single cell level to study the effects of 1,25-(OH)(2)D-3 on [Ca2+](i) in a pancreatic beta-cell Line, RINr1046-38. In RIN cells equilibrated at a steady state glucose concentration (5.5 mM), 1,25-(OH)(2)D-3 (2-20 nM) rapidly, within 5-10 sec, increased [Ca2+](i) and evoked sinusoidal [Ca2+](i) oscillations with a frequency of 1.87 +/- 0.13 min(-1) and an amplitude of 236 +/- 3 nM (from the initial basal level of 110 +/- 2 nM). The [Ca2+](i) oscillations were acutely dependent on extracellular Ca2+, but not on extracellular glucose. Further, we investigated the mechanisms of activation by 1,25-(OH)(2)D-3 of the Ca2+ entry path way in the plasma membrane and analyzed the relationship between 1,25-(OH)(2)D-3-stimulated Ca2+ entry and Ca2+ release from intracellular stores. The 1,25-(OH)(2)D-3-evoked [Ca2+](i) oscillations were mediated by nonselective Ca2+ channels, which are permeable to Mn2+ and suppressed by extracellular La3+. Blockage of voltage-dependent Ca2+ channels by nifedipine significantly decreased the amplitude of the oscillations. Depletion of intracellular Ca2+ stores with thapsigargin did not affect the 1,25-(OH)(2)D-3-stimulated Ca2+ entry estimated by the Mn2+ entry and fura-2 fluorescence quench, which implies that the hormone directly activates nonselective Ca2+ channels. The 1,25-(OH)(2)D-3-evoked increase in the background Ca2+ influx appears to generate [Ca2+](i) oscillations by triggering Ca2+ release through the ryanodine receptor/Ca2+ release channel, but not through activation of the inositol 1,4,5-triphosphate receptor. Our findings are consistent with a role of the plasmalemmal vitamin D receptor coupled to the plasma membrane Ca2+ channels in mediating rapid effects of the hormone. We propose that the 1,25-(OH)(2)D-3-mediated Ca2+ signaling pathway may be involved in the regulation of insulin secretion from the pancreatic beta-cell.