Rapid control of transmembrane calcium influx by 1α,25-dihydroxyvitamin D3 and its analogues in rat osteoblast-like cells

Abstract

1α,25-Dihydroxyvitamin D3 [1α,25(OH)2D3] has been shown to exert both its nuclear vitamin D receptor (mVDR)-mediated genomic actions and membrane vitamin D receptor (mVDR)-mediated nongenomic actions. In this study, the effects of 1α,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 (2x10-5 M), a well-known L-type Ca2+ channel agonist, 1α,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 1α,25(OH)2D3- induced Ca2+ influx was completely blocked by pre-treatment with nifedipine (2x10-5 M), an L-type Ca2+ channel antagonist. Two vitamin D analogues, 22-oxa-1α,25(OH)2D3 (OCT, 10-8 M) and 20-epi-22-oxa-24a,26a,27a-trihomo- 1α,25(OH)2D3 (KF1060, 10-8 M), which were 3.8 and 3600-fold more active than 1α,25(OH)2D3 in stimulating differentiation on human promyelocytic leukemic HL-60 cells, respectively, also increased intracellular Ca2+ concentration, while their C2+ channeling activities were similar to or significantly weaker than that of 1α,25(OH)2D3. Furthermore, the enhanced transmembrane Ca2+ influx induced by 1α,25(OH)2D3 (10-8 M) or OCT (10-8 M) was completely blocked by pre-treatment with the respective 1β epimer [1β,25(OH)2D3 and 1βOCT] at equal concentration. These findings suggest that 1α,25(OH)2D3 and its analogues modulate transmembrane Ca2+ influx in osteoblast-like cells by opening L-type Ca2+ channels which can recognize 1α-hydroxy analogues as agonists and 1β-hydroxy analogues as antagonists.