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 ge ne-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 vasodila tory effect of neural CGRP in smooth muscle probably involves inhibiti on of unstimulated Ca2+ uptake, we examined the acute effects of CGRP on this parameter in rat osteoblastic cells. CGRP inhibits Ca-45(2+) u ptake in both UMR 106 osteosarcoma and RCOB-3 osteoblastic cells. This inhibition is rapid (0.5 min), occurs with an EC(50) of 1 nM, and can not be demonstrated in the presence of 0.1 mM diltiazem, a blocker of voltage-dependent Ca2+ channels. Depolarization of bone cells with hig h extracellular potassium (K+) also blocks the effect of CGRP on Ca-45 (2+) uptake, suggesting a central role for K+ channels in mediating th is action. In agreement with this hypothesis, the effect of CGRP is bl ocked by 1 mu M glybenclamide, a specific inhibitor of ATP-sensitive p otassium (K-ATP) channels, or by pretreatment of cells with 1 mM iodoa cetic acid to deplete intracellular ATP, Blocking Ca2+-activated potas sium channels with 1 mM tetraethylammonium does not prevent CGRP's eff ect. Pinacidil, a specific activator of K-ATP channels, mimics CGRP's effect. Both CGRP and pinacidil also produce a small significant stimu lation of cellular Ca2+ efflux in UMR 106 cells. These data suggest th at 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 mu scle cells.