Abstract
1alpha,25(OH)(2)-Vitamin D(3) (1,25D) modulates osteoblast gene expression of bone matrix proteins via a nuclear vitamin D receptor (VDR) and also modifies the electrical state of the plasma membrane through rapid nongenomic mechanisms still not fully understood. The physiological significance of 1,25D membrane-initiated effects remains unclear. To elucidate whether the VDR is required for 1,25D-promoted electrical responses, we studied 1,25D modulation of ion channel activities in calvarial osteoblasts isolated from VDR knockout (KO) and WT mice. At depolarizing potentials, Cl(-) currents were significantly potentiated (13.5 +/- 1.6-fold increase, n = 12) by 5 nM 1,25D in VDR WT but not in KO (0.96 +/- 0.3 fold increase, n = 11) osteoblasts. L-type Ca(2+) currents significantly shift their peak activation by -9.3 +/- 0.7 mV (n = 10) in the presence of 5 nM 1,25D in VDR WT but not in KO cells, thus facilitating Ca(2+) influx. Furthermore, we found that 1,25D significantly increased whole-cell capacitance in VDR WT (DeltaCap = 2.3 +/- 0.4 pF, n = 8) but not in KO osteoblasts (DeltaCap = 0.3 +/- 0.1 pF, n = 8); this corresponds to a rapid (1-2 min) fusion in WT of 71 +/- 33 versus in KO only 9 +/- 6 individual secretory granules. We conclude that, in calvarial osteoblasts, 1,25D modulates ion channel activities only in cells with a functional VDR and that this effect is coupled to exocytosis. This is a demonstration of the requirement of a functional classic steroid receptor for the rapid hormonal modulation of electric currents linked to secretory activities in a target cell.