Parathyroid hormone-stimulated resorption in calvaria cultured in serum-free medium is enhanced by the calcium-mobilizing activity of 1,25-Dihydroxyvitamin D3

1,25(OH)2D3 enhances parathyroid hormone (PTH)-induced Ca2+ signaling in osteoblasts by activating plasma membrane voltage-sensitive Ca2+ channels (VSCCs). The ability of 1,25(OH)2D3 or the VSCC-activating analog AT (25-hydroxy-16-ene-23-yne-D3) to enhance parathyroid hormone-stimulated 45Ca2+ release from cultured new-born rat calvaria was measured. Analog BT (1,24-dihydroxy-22-ene-24-cyclopropyl-D3), that does not mobilize Ca2+, also was tested along with PTH. Control experiments were performed with and without PTH and with and without serum. Individual calvaria labeled in utero with 45Ca2+ were cultivated in serum-free medium on filters at the medium/air interface of 24-well culture plates and 45Ca2+ release followed over 72 h. The results demonstrated that 1,25(OH)2D3 and the Ca2+-mobilizing analog, AT, but not the nuclear receptor-binding analog, BT, enhanced PTH-stimulated 45Ca2+ release under serum-free conditions. This enhancement effect of the seco-steroids was not evident in the presence of 10% fetal calf serum. The effect of analog AT was faster than that of 1,25(OH)2D3. Nitrendipine, a specific L-type VSCC blocker, attenuated enhancement by vitamin D compounds, indicating that the high-threshold L-type VSCC is a molecular transducer of costimulation. These results emphasize the synergy between the calcitropic hormones 1,25(OH)2D3 and PTH in cultures containing osteoblasts and osteoclasts, and suggest that the Ca2+-mobilizing activity of 1,25(OH)2D3 enhances Ca2+ release from bone.