title: 1,25-Dihydroxyvitamin D3 evokes oscillations of intracellular calcium in a pancreatic beta-cell line.
abstract: The steroid hormone 1 alpha,25-dihydroxyvitamin D3 [1,25-(OH)2D3] 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)2D3 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)2D3 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)2D3 (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)2D3 of the Ca2+ entry pathway in the plasma membrane and analyzed the relationship between 1,25-(OH)2D3-stimulated Ca2+ entry and Ca2+ release from intracellular stores. The 1,25-(OH)2D3-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)2D3-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)2D3-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)2D3-mediated Ca2+ signaling pathway may be involved in the regulation of insulin secretion from the pancreatic beta-cell.
authors.affiliation: Department of Anatomy, Cell and Neurobiology, Marshall University School of Medicine, Huntington, West Virginia 25704-9388, USA.
authors.forename: I N
authors.initials: IN
authors.lastname: Sergeev
authors.forename: W B
authors.initials: WB
authors.lastname: Rhoten
mesh_terms: D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002117:Calcitriol / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000378:metabolism; D002460:Cell Line; D007515:Islets of Langerhans / Q000187:drug effects* / Q000378:metabolism; D007811:Lanthanum / Q000494:pharmacology; D008345:Manganese / Q000378:metabolism; D009543:Nifedipine / Q000494:pharmacology; D010507:Periodicity; D051381:Rats; D012433:Ryanodine / Q000494:pharmacology; D013729:Terpenes / Q000494:pharmacology; D019284:Thapsigargin
chemical_list: D015220:Calcium Channels; D013729:Terpenes; D012433:Ryanodine; D002110:Caffeine; D008345:Manganese; D019284:Thapsigargin; D007811:Lanthanum; D002117:Calcitriol; D009543:Nifedipine; D002118:Calcium