Stimulation of Ca2+ release-activated Ca2+ channels as a potential mechanism involved in non-genomic 1,25(OH)(2)-vitamin D-3-induced Ca2+ entry in skeletal muscle cells

As in other vitamin D target cells, activation of voltage-dependent Ca2+ channels (VDCC) mediates the fast, non-genomic, 1,25(OH)(2)D-3 stimulation of Ca2+ influx in skeletal muscle cells (SMC). 1,25(OH)(2)D-3 has also been shown to rapidly induce the release of Ins(1,4,5)P-3 in SMC. Experiments were performed to investigate whether Ca2+ in the mechanism by which 1,25(OH)(2)D-3 regulates Ca2+ entry into these cells. In cultured chick SMC loaded with Fura-2/AM the hormone (10(-12)-10(-8) M) induced a rapid (30 sec) followed by a sustained (up to 5 min) increase in intracellular Ca2+ concentration ([Ca2+](i)) associated to Ca2+ mobilization from internal stores and influx of extracellular Ca2+, respectively. Thus, the initial, transient, 1,25(OH)(2)D-3-dependent increment in [Ca2+](i) could be observed in Ca2+-free medium and was abolished by the PLC inhibitor U73122. Readdition of Ca2+ to cells that had undergone the initial 1,25(OH)(2)D-3-induced [Ca2+](i) rise in Ca2+ free medium resulted in a fast increment in [Ca2+](i) indicating the existence of a hormone-activated CRAC entry pathway. The sustained phase of the Ca2+ response to 1,25(OH)(2)D-3 was only partially (60%) suppressed by nifedipine, whereas lanthanum (10 mu M) completely abolished the hormone effects. Accordingly, depletion of intracellular Ca2+ stores by thapsigargin re-produced 1,25(OH)(2)D-3-induced Ca2+ influx, inhibiting any further response to the sterol. 1,25(OH)(2)D-3 increased the rate of quenching of Fura-2 fluorescence by Mn2+, indicating activation of Mn2+ permeable channels. Altogether, these results provide the first evidence involving CRAC channels in the rapid modulation of Ca2+ entry in animal cells by 1,25(OH)(2)D-3. (C) 1997 Academic Press.