Multiple molecular mechanisms of 1α,25(oh)2-vitamin D3 rapid modulation of three ion channel activities in osteoblasts

Rapid nongenomic responses to steroids include modulation of ion channel activities on the cell membrane of target cells, but little is known about the molecular mechanisms involved. In this paper we investigate the mechanisms underlying the combined action of the secosteroid hormone 1α,25-dihydroxyvitamin D3 [1α,25(OH)2D3] on three different ion channel types in rat osteoblasts, which include a voltage-gated L-type Ca2+ channel, a mechanosensitive Cl− channel, and a stretch-activated cation (SA-Cat) channel. We found that physiological nanomolar concentrations of 1α,25(OH)2D3 rapidly modify the overall electrical activity of the membrane in ROS 17/2.8 cells. 1α,25(OH)2D3 increases the osteoblast L-type Ca2+ channel activity at low depolarizing voltages in a fashion similar to the 1,4-dihydropyridine (DHP) agonist Bay K8644. At highly depolarizing potentials 1α,25(OH)2D3 potentiates volume-sensitive Cl− currents through mechanisms that may involve a putative membrane receptor. We show for the first time that 1α,25(OH)2D3 also increases inward currents through SA-Cat channels at positive membrane voltages in a dose-dependent manner. Contrary to our expectations, the stereoisomer 1β,25(OH)2D3, which suppresses 1α,25(OH)2D3 activation of osteoblast Cl− currents, mimicked 1α,25(OH)2D3 agonist effects on Ca2+ and SA-Cat channel activities. Cyclic AMP is involved in 1α,25(OH)2D3 effects on both Ca2+ and SA-Cat channels, but not in Cl− channels. We conclude that 1α,25(OH)2D3 rapid effects on ion channel activities in ROS 17/2.8 cells occur through multiple mechanisms that, on the one hand, involve a possible direct interaction with the L-type Ca2+ channel molecule and, on the other hand, molecular pathways that may include a putative membrane receptor.