Stimulation by calcitonin gene-related peptide of atrial natriuretic peptide secretion in vitro and its mechanism of action

Calcitonin gene-related peptide (CGRP) is present in nerve fibers within atrial tissue, raising the possibility that CGRP release may influence atrial natriuretic peptide (ANP) secretion. We, therefore, examined the effect of CGRP on immunoreactive ANP (ANP-IR) secretion. Isolated rat left atria paced at 2 Hz were superfused with 0.1 microM CGRP. A biphasic 2-fold increase in ANP-IR secretion occurred in response to CGRP. We next examined the mechanism of CGRP-stimulated secretion. The biphasic ANP-IR secretory response to CGRP was similar to that induced by superfusion with the beta-adrenergic agonist isoproterenol and (Bu)2cAMP, but distinct from that of the non-cAMP dependent stimuli phenylephrine, ouabain, and Bay K 8644. Superfusion with 0.1 microM CGRP for 4 min with 100 microM isobutylmethylxanthine increased atrial cAMP content from 4.29 +/- 1.21 to 10.32 +/- 2.14 pmol/mg atrial weight (P less than 0.001). Atria were next superfused with methacholine, an inhibitor of adenylyl cyclase activation. The addition of 0.1 microM isoproterenol or 0.1 microM CGRP to the superfusate containing 10 microM methacholine failed to stimulate ANP-IR secretion and lowered cAMP accumulation by 70%. Superfusion with 10 microM atropine negated the inhibitory effects of methacholine. We conclude that 1) CGRP stimulates ANP-IR secretion in vitro; and 2) CGRP-stimulated secretion appears to be mediated by cAMP. Thus, ANP-IR secretion may be modulated by atrial nerve fibers containing CGRP in vivo.