Calcitonin gene-related peptide rapidly inhibits calcium uptake in osteoblastic cell lines via activation of adenosine triphosphate-sensitive potassium channels

In certain neurons, alternative RNA processing generates calcitonin gene related peptide (CGRP) from the same gene that encodes the hormone calcitonin. As CGRP-containing nerve fibers are prominent in skeleton, we evaluated the effects of CGRP on osteoblasts. Because the vasodilatory effect of neural CGRP in smooth muscle probably involves inhibition of unstimulated Ca²⁺ uptake, we examined the acute effects of CGRP on this parameter in rat osteoblastic cells. CGRP inhibits ¹⁵Ca²⁺ uptake in both UMR 106 osteosarcoma and RCOB-3 osteoblastic cells. This inhibition is rapid (0.5 min), occurs with an EC₅₀ of 1 nM, and cannot be demonstrated in the presence of 0.1 mM diltiazem, a blocker of voltage dependent Ca²⁺ channels. Depolarization of bone cells with high extracellular potassium (K⁺) also blocks the effects of CGRP on ⁴⁵Ca²⁺ uptake, suggesting a central role for K⁺ channels in mediating this action. In agreement with this hypothesis, the effect of CGRP is blocked by 1 μM glybenclamide, a specific inhibitor of ATP-sensitive potassium (K(ATP)) channels, or by pretreatment of cells with 1 mM iodoacetic acid to deplete intracellular ATP. Blocking Ca²⁺ activated potassium channels with 1 mM tetraethylammonium does not prevent CGRP's effect. Pinacidil, a specific activator of K(ATP) channels, mimics CGRP's effect. Both CGRP and pinacidil also produce a small significant stimulation of cellular Ca²⁺ efflux in UMR 106 cells. These data suggest that inhibition of diltiazem-sensitive Ca²⁺ channels occurs secondary to the hyperpolarization engendered by CGRP activation of K(ATP) channels in osteoblastic cells, an effect similar to that of CGRP on smooth muscle cells.