Membrane actions of calcitonin gene-related peptide in cardiac and smooth muscle myocytes.
Calcitonin gene-related peptide is a 37-amino acid neuropeptide acting as a transmitter of nonadrenergic, noncholinergic nerves in the heart. Binding sites of high affinity have been reported in coronary arteries, in atria, and, of minor density, in ventricular myocardium. These sites are likely linked to G-proteins mediating modifications of ion channel opening probability and duration and to stimulation of adenylate cyclase activity and cAMP-mediated alterations of ion channel activities. In isolated and perfused guinea pig hearts, low concentrations of CGRP (1-3 nM) exerted no chronotropic effect, but increased coronary flow slightly. Atrioventricular conduction duration and effective refractory period of atrioventricular conduction were prolonged by 3 nM of CGRP. The higher concentration of 10 nM increased the sinus rate, and the effects on the atrioventricular node were counterbalanced. HV and QRS duration of the ECG remained essentially unchanged, but persistent ventricular fibrillation was inducible by burst stimulation in all CGRP-treated hearts. Results in human myometrial myocytes indicate that CGRP exerted direct G protein-mediated activation of potassium channels, leading to hyperpolarization and smooth muscle relaxation. Activation of potassium channels, most prominent in smooth muscle relaxation, is likely an additional factor in the cardiostimulatory profile of CGRP.