Specific binding sites for calcitonin gene-related peptide (CGRP) were demonstrated in the rat heart and spleen. Autoradiography revealed rat [125I]iodo CGRP binding associated with the intima and media of the aorta, the coronary arteries and the heart valves, and the red pulp of the spleen. Half-maximal inhibition of rat [125I]iodo-CGRP binding to membranes of the rat atria and the spleen was obtained with, respectively, 5 and 0.35 nM unlabeled rat CGRP; these values correspond to EC50 values of 3 and 0.14 nM for activation of adenylate cyclase by CGRP. In the isolated, spontaneously beating right atrium, the EC50 values of stimulation of the force and rate of contraction by rat CGRP were 120 and 70 nM, respectively. Rat CGRP caused relaxation of splenic strips, precontracted with noradrenaline; the EC50 was 50 nM. The beta-adrenergic blocking agent metoprolol, while obliterating the increase in the force and rate of contraction evoked by noradrenaline in the right atrium, did not significantly change the action of CGRP. Similarly, preserved action of CGRP in the presence of indomethacin as well as mepyramine and cimetidine argues against a role of prostaglandins or histamine in the functional responses of CGRP. Much like CGRP, capsaicin, which releases mediators from sensory neurons, caused stimulation of the force and rate of contraction of the isolated right rat atrium. After tachyphylaxis to CGRP, the response to noradrenaline was intact, while the positive chronotropic and inotropic effects of capsaicin were suppressed. The results indicate that the cardiac effects of capsaicin may be due to the release of endogenous CGRP through a local mode of action.

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