1,25-Dihydroxyvitamin D₃-induced calcium efflux from calvaria is mediated by protein kinase C

1,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃) is an important regulator of bone metabolism involved in both formation and resorption. Traditionally it was assumed that vitamin D receptors are intracellular. Recent data indicate that vitamin D may also act through a membrane receptor, specifically raising intracellular calcium and inositol 1,4,5 triphosphate. The present study was undertaken to explore further the mechanism(s) of vitamin D-induced bone resorption in cultured bone. 1,25(OH)₂D₃ induced a dose-dependent increase of calcium efflux from cultured bone. This increase was completely obliterated by inhibition of protein kinase C (PKC) with either staurosporine or calphostin C. In cultured rat calvariae, 1,25(OH)₂D₃ also induced a dose-dependent translocation of PKC from cytosol to membrane. The activation of PKC by 1,25(OH)₂D₃ occurred following a 30-s incubation, peaked at 1 minute, and disappeared by 5 minutes. 1,25(OH)₂D₃ did not increase cAMP production in similarly cultured calvaria. These results suggest that the action of 1,25(OH)₂D₃ on calcium flux from cultured bone is mediated, in part, via activation of PKC.