1 Acetylcholine and rat calcitonin gene-related peptide (CGRP) produced a relaxation in rat isolated aortic rings which was entirely dependent on the presence of the endothelium. 2 In the absence of any exogenous vasodilator agent, the cyclic guanosine monophosphate (cyclic GMP) content was higher in rings with endothelium than in those without. 3 The vasorelaxation produced by acetylcholine and sodium nitroprusside was accompanied by increases in cyclic GMP in the smooth muscle, whereas that produced by CGRP was not accompanied by cyclic GMP accumulation. 4 Therefore, it appears unlikely that CGRP releases an endothelium-derived relaxing factor similar to that released by acetylcholine.

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