POTASSIUM CURRENTS AND EFFECTS OF VITAMIN D-3 METABOLITES AND CYCLIC-GMP IN RAT OSTEOBLASTIC CELLS

A K+ current (I(K1)), activated by depolarization above - 20 mV, showing voltage-dependent inactivation within a few seconds and reduced by 40% by 1 mM TEA, was observed in all cells. In a few cells, we also observed a progressive K+-current increase during cell dialysis. The developping current (I(K2)) was not sensitive to 1 mM TEA and did not inactivate. It was detectable over the whole voltage range and slowly increased during 10 s depolarizations. 1,25-(OH)2D3 and 24,25-(OH)2D3 did not affect I(K1), but induced a small K+-current increase in some cells showing no I(K2). This effect was not mimicked by cyclic GMP analogs which, on the contrary, induced a K+-current decrease. 24,25-(OH)2D3 (even at 10(-11) M), but not 1,25-(OH)2D3, strongly reduced I(K2). The results further document the diversity of voltage-gated currents of osteoblastic cells, confirm the existence of immediate effects of vitamin D-3 metabolites, which are independent of classical 1,25-(OH)2D3 receptors.