Multiple molecular mechanisms of 1α,25(oh) 2-vitamin D 3 rapid modulation of three ion channel activities in osteoblasts

Abstract

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 D 3 [1α,25(OH) 2D 3] on three different ion channel types in rat osteoblasts, which include a voltage-gated L-type Ca 2+ channel, a mechanosensitive Cl − channel, and a stretch-activated cation (SA-Cat) channel. We found that physiological nanomolar concentrations of 1α,25(OH) 2D 3 rapidly modify the overall electrical activity of the membrane in ROS 17/2.8 cells. 1α,25(OH) 2D 3 increases the osteoblast L-type Ca 2+ 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) 2D 3 potentiates volume-sensitive Cl − currents through mechanisms that may involve a putative membrane receptor. We show for the first time that 1α,25(OH) 2D 3 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) 2D 3, which suppresses 1α,25(OH) 2D 3 activation of osteoblast Cl − currents, mimicked 1α,25(OH) 2D 3 agonist effects on Ca 2+ and SA-Cat channel activities. Cyclic AMP is involved in 1α,25(OH) 2D 3 effects on both Ca 2+ and SA-Cat channels, but not in Cl − channels. We conclude that 1α,25(OH) 2D 3 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 Ca 2+ channel molecule and, on the other hand, molecular pathways that may include a putative membrane receptor.