DOWN-REGULATION OF L-TYPE CA2-DIHYDROXYVITAMIN D-3 - OSTEOBLASTIC CELLS EXPRESS L-TYPE ALPHA(1C) CA2+ CHANNEL ISOFORMS( CHANNEL TRANSCRIPT LEVELS BY 1,25)

Osteoblast Ca2+ channels play a fundamental role in controlling intrac ellular and systemic Ca2+ homeostasis. A reverse transcription-polymer ase chain reaction strategy was used to determine the molecular identi ty of voltage-sensitive calcium channels present in ROS 17/2.8 osteosa rcoma cells. The amino acid sequences encoded by the two resultant PCR products matched the alpha 1(C-a) and the alpha 1(C-d) isoforms. The ability of 1,25-dihydroxyvitamin D-3 (1,25(OH)(2)D-3) and structural a nalogs to modulate expression of voltage-sensitive calcium channel mRN A transcripts was then investigated. ROS 17/2.8 cells were cultured fo r 48 h in the presence of either 1,25(OH)(2)D-3, 1,24-dihydroxy-22-ene -24-cyclopropyl D-3 (analog BT) or 25-hydroxy-16-ene-23-yne-D-3 (analo g AT), and the levels of mRNA encoding alpha(1C) were quantitated usin g a competitive reverse transcription-polymerase chain reaction assay. We found that 1,25(OH)(2)D-3 and analog BT reduced steady state level s of alpha(1C) mRNA. Conversely, the Ca2+-mobilizing analog AT did not alter steady state levels of voltage-sensitive calcium channel mRNA. Since analog BT, but not analog AT, binds and transcriptionally activa tes the nuclear receptor for 1,25(OH)(2)D-3, these findings suggest th at the down-regulation of voltage-sensitive calcium channel mRNA level s may involve the nuclear receptor.