As in other vitamin D target cells, activation of voltage-dependent Ca2+ channels (VDCC) mediates the fast, non-genomic, 1,25(OH)2D3 stimulation of Ca2+ influx in skeletal muscle cells (SMC). 1,25(OH)2D3 has also been shown to rapidly induce the release of Ins(1,4,5)P3 in SMC. Experiments were performed to investigate whether Ca2+ release-activated Ca2+ channels (CRAC) also participate in the mechanism by which 1,25(OH)2D3 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)2D3-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)2D3-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)2D3 was only partially (60%) suppressed by nifedipine, whereas lanthanum (10 microM) completely abolished the hormone effects. Accordingly, depletion of intracellular Ca2+ stores by thapsigargin reproduced 1,25(OH)2D3-induced Ca2+ influx, inhibiting any further response to the sterol. 1,25(OH)2D3 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)2D3.
Copyright 1997 Academic Press.