Osteoblast Ca2+ permeability and voltage-sensitive Ca2+ channel expression is temporally regulated by 1,25-dihydroxyvitamin D3

The cardiac subtype of the L-type voltage-sensitive Ca2+ channel (VSCC) Cav1.2 ({alpha}1C) is the primary voltage-sensitive channel responsible for Ca2+ influx into actively proliferating osteoblasts. This channel also serves as the major transducer of Ca2+ signals in growth-phase osteoblasts in response to hormone treatment. In this study, we have demonstrated that 24-h treatment of MC3T3-E1 preosteoblasts with 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], a coupling factor for bone resorption, coordinately downregulates Cav1.2 ({alpha}1C) and uniquely upregulates T-type channel Cav3.2 ({alpha}1H). No other voltage-sensitive channel {alpha}-subunit of the 10 that were surveyed was upregulated by 1,25(OH)2D3. The shift from predominantly L-type to T-type channel expression has been demonstrated to occur at both mRNA and protein levels detected using quantitative PCR and immunohistochemistry with antibodies specific for each channel type. Functional and pharmacological studies using specific inhibitors have revealed that treatment with 1,25(OH)2D3 also alters the Ca2+ permeability properties of the osteoblast membrane from a state of primarily L-current sensitivity to T-current sensitivity. We conclude that the L-type channel is likely to support proliferation of osteoblast cells, whereas T-type channels are more likely to be involved in supporting differentiated functions after 1,25(OH)2D3-mediated reversal of remodeling has occurred. This latter observation is consistent with the unique expression of the T-type VSCC Cav3.2 ({alpha}1H) in terminally differentiated osteocytes as we recently reported. [PUBLICATION ABSTRACT]