Negative inotropic effect of adrenomedullin in isolated adult rabbit cardiac ventricular myocytes

Background Adrenomedullin (AM) is a potent vasodilator peptide. AM-induced vasodilatation is mediated by an increase of NO as well as cAMP. Both AM and binding sites for this peptide have been found in cardiac tissue, indicating the possible existence of an autocrine or paracrine system of AM in the heart. Methods and Results Myocytes were isolated by use of retrograde coronary perfusion with physiological solution containing collagenase and hyaluronidase from adult rabbit ventricles. Contraction of cardiac myocytes was traced with a video motion detector, and [Ca2+](i) was measured with indo 1 at 37 degrees C. The I-Ca was measured with a whole-cell patch clamp at 23 degrees C. AM and calcitonin gene-related peptide (CGRP), another member of the same peptide family, showed a concentration-dependent negative inotropic effect (10(-7) mol/L AM: contraction amplitude, 64+/-7% of control; [Ca2+](i), 52+/-5% of control; n=10; 10(-6) mol/L CGRP: contraction amplitude, 64+/-25%; [Ca2+](i), 70+/-3%; n=5; mean+/-SD). I-Ca was decreased to 60+/-39% by superfusion with AM after the cessation of N-G-monomethyl-L-arginine (L-NMMA), an NO synthase inhibitor. Pretreatment with L-NMMA (10 mu mol/L) abolished the negative inotropic effect of AM, whereas switching from AM+L-NMMA to AM+L-arginine (1 mmol/L) restored it. Superfusion with 8-bromo-cGMP also showed a negative inotropic effect. AM significantly increased the intracellular content of cGMP, a second messenger of NO, but not that of cAMP. AM (10 nmol/L) blunted the effect of 1 mu mol/L forskolin. Conclusions AM has a negative inotropic effect and decreased both [Ca2+](i) and I-Ca, with these effects being at least partly mediated via the L-arginine-NO pathway in adult rabbit ventricular myocytes.