In several cell types 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) causes up-regulation of its receptor. The present study demonstrates that in the osteoblast-like cell line UMR 106 this up-regulation is inhibited by two different calcium channel blockers (nitrendipine, verapamil). Also with chelating extracellular calcium (EGTA) and by inhibition of calcium release from intracellular stores (TMB-8) comparable results were obtained. These findings indicate that calcium is functionally involved in this cellular response to the steroid hormone 1,25(OH)2D3. Moreover, data obtained with EGTA show that the 1,25(OH)2D3 receptor level is closely regulated by the extracellular calcium concentration.
