24R,25-(OH)₂ vitamin D₃ inhibits 1α,25-(OH)₂ vitamin D₃ and testosterone potentiation of calcium channels in osteosarcoma cells

Calcium influx via L-type calcium channels in osteoblast cells causes a rapid (in seconds) elevation in intracellular calcium initiated by plasma membrane receptors for 1α,25-dihydroxyvitamin D₃ (1α,25-D₃). 24R,25- Dihydroxyvitamin D₃ (24,25-D₃) alone, in concentrations up to 200 nM, does not cause potentiation of calcium currents in osteoblasts, but it does inhibit the current potentiation by 1α,25-D₃. To determine how various steroids interact in their potentiation of calcium channels, the action of vitamin D₃ analogues and testosterone with calcium channels in the rat osteoblast-like cell line ROS 17/2.8 was investigated. Bath additions of both 1α,25-Da and testosterone at doses below K( 1/4 ) (the dose causing 50% left shift in the current-voltage relationship) are additive in their ability to potentiate calcium channels. When 1α,25-D₃ and testosterone are added together at concentrations that would cause a maximal shift in the current- voltage relationship by each agent alone (V(max)), the effect of these steroids is not additive. Taken together these data suggest one population of calcium channels is activated by 1α,25-D₃ or testosterone. The shift in the current-voltage relationship caused by 1α,25-D₃ is reduced by 1β,25- dihydroxyvitamin D₃ (1β,25-D₃), an agent which is thought to act specifically on the plasma membrane receptor for 1α,25-D₃, but the potentiation caused by testosterone is not blocked by 1β,25-D₃. However, 24,25-D₃ inhibits the left shift in the peak current-voltage relationship mediated by either 1α,25-D₃ and testosterone. This result implies that 1) 1β,25-D₃ directly displaces 1α,25-D₃ but not testosterone from its plasma membrane receptor, and 2) the rapid (in seconds) stimulatory effects of 1α,25-D₃ and testosterone on calcium channels are mediated by separate plasma membrane receptors for testosterone and 1α,25-D₃, which are blocked by another receptor for 24,25-D₃. The interaction of these three receptors with L-type calcium channels is pertussis toxin-sensitive.