1,25(OH)(2)D-3 ENHANCES PTH-INDUCED CA2-TYPE CA2+ CHANNELS( TRANSIENTS IN PREOSTEOBLASTS BY ACTIVATING L)

We previously demonstrated electrophysiologically that 1,25-dihydroxyv itamin D-3 [1,25(OH)(2)D-3] shifts the activation threshold of L-type Ca2+ channels in osteoblasts toward the resting potential and prolongs mean open time. Presently, we used single-cell Ca2+ imaging to study the combined effects of 1,25(OH)(2)D-3 and parathyroid hormone (PTH) d uring generation of Ca2+ transients in fura 2-loaded MC3T3-E1 cells. P retreatment with 1,25(OH)(2)D-3 concentrations, which alone did not pr oduce Ca2+ transients, consistently enhanced Ca2+ responses to PTH. En hancement was dose dependent over the range of 1 to 10 nM and was bloc ked by pretreatment with 5 mu M nitrendipine during pretreatment. A 1, 25(OH)(2)D-3 analog that activates L-type channels and shifts their ac tivation threshold also enhanced PTH responses. In contrast, an analog devoid of membrane Ca2+ effects did not enhance PTH-induced Ca2+ tran sients. The PTH-induced Ca2+ transient involved activation of a dihydr opyridine-insensitive cation channel that was inhibited by Gd3+. Toget her, these data suggest that 1,25(OH)(2)D-3 increases osteoblast respo nsiveness to PTH through rapid modification of L-type Ca2+ channel gat ing properties, whose activation enhances Ca2+ entry through other cha nnels such as the PTH-responsive, Gd3+-sensitive cation channel.