ROLE OF ENDOTHELIUM AND HYPERPOLARIZATION IN CGRP-INDUCED VASODILATION OF RABBIT OPHTHALMIC ARTERY

The electromechanical effects of calcitonin gene-related peptide (CGRP) on intact and endothelium-denuded rabbit ophthalmic arteries were studied. CGRP inhibited norepinephrine (NE)-induced contractions. In intact arteries after washout of CGRP the contractile sensitivity to NE was increased. Conversely, in endothelium-denuded arteries, the relaxation induced by CGRP was prolonged, and after washout of CGRP the contractile sensitivity to NE was diminished. In intact arteries NE contractions were enhanced by N(G)-monomethyl-L-arginine (L-NMMA), an inhibitor of endothelium-derived relaxing factor (EDRF) synthesis, and in the presence of L-NMMA, CGRP-induced relaxations resembled those seen in endothelium-denuded arteries. This result suggests that there is an increased EDRF synthesis in intact arteries during NE stimulation and that CGRP may inhibit either the synthesis or the activity of EDRF. High concentrations of CGRP hyperpolarized the smooth muscle membrane both in intact and endothelium-denuded arteries. Hyperpolarizations were blocked by glibenclamide, indicating that they are mediated by activation of ATP-sensitive K+ channels. However, glibenclamide had little effect on the CGRP-induced relaxation. These results suggest that in normal physiological conditions CGRP-induced relaxation of the rabbit ophthalmic artery is mediated mainly by mechanisms other than hyperpolarization.