Rapid Control of Transmembrane Calcium Influx by 1.ALPHA.,25-Dihydroxyvitamin D3 and Its Analogues in Rat Osteoblast-Like Cells

1alpha,25-Dihydroxyvitamin D3 [1alpha,25(OH)2D3] has been shown to exert both its nuclear vitamin D receptor (nVDR)-mediated genomic actions and membrane vitamin D receptor (mVDR)-mediated nongenomic actions. In this study, the effects of 1alpha,25(OH)2D3 and its analogues on transmembrane Ca2+ influx were examined in the growth phase of rat osteosarcoma ROS17/2.8 cells. Like BAYK8644 (2 x 10(-5)M), a well-known L-type Ca2+ channel agonist, 1alpha,25(OH)2D3 (10(-8)M) increased transmembrane influx of Ca2+ through voltage-dependent Ca2+ channels and increased intracellular Ca2+ concentration within 2 min of addition to the medium. The 1alpha,25(OH)2D3-induced Ca2+ influx was completely blocked by pre-treatment with nifedipine (2 x 10(-5)M), an L-type Ca2+ channel antagonist. Two vitamin D analogues, 22-oxa-1alpha,25(OH)2D3 (OCT, 10(-8) M) and 20-epi-22-oxa-24a, 26a,27a-trihomo-1alpha,25(OH)2D3 (KH1060, 10(-8)M), which were 3.8 and 3600-fold more active than 1alpha,25(OH)2D3 in stimulating differentiation on human promyelocytic leukemic HL-60 cells, respectively, also increased intracellular Ca2+ concentration, while their Ca2+ channeling activities were similar to or significantly weaker than that of 1alpha,25(OH)2D3. Furthermore, the enhanced transmembrane Ca2+ influx induced by 1alpha,25(OH)2D3 (10(-8)M) or OCT (10(-8)M) was completely blocked by pre-treatment with the respective 1beta epimer [1beta,25(OH)2D3 and 1beta-OCT] at equal concentration. These findings suggest that 1alpha,25(OH)2D3 and its analogues modulate transmembrane Ca2+ influx in osteoblast-like cells by opening L-type Ca2+ channels which can recognize 1alpha-hydroxy analogues as agonists and 1beta-hydroxy analogues as antagonists.