Adrenomedullin as a regulator of cardiac function

Adrenomedullin (AM) is a 52-amino acid peptide which is produced in many tissues, including adrenal medulla, lung, kidney and heart. Intravenous administration of AM causes a long-lasting hypotensive effect, accompanied with an increase in the cardiac output in experimental animals. This study was aimed to examine whether AM has any direct effects on myocardial function. In addition to the myocardial contractility, the effects of AM on coronary vascular tone and A-type natriuretic peptide (ANP) release from atria and B-type natriuretic peptide (BNP) gene expression in the ventricles were studied in the perfused rat heart preparation. In spontaneously beating hearts, AM had no effects on the heart rate, but dose-dependently increased the developed tension (DT) with an EC50 of 7 x 10 nmol/l, reflecting a potent positive inotropic effect. The lower the initial resting tension, the higher was the elevation in DT. In paced hearts, a protein kinase A inhibitor, H-89, had no effect on AM-induced inotropic effect, and AM did not increase the cAMP content of the ventricular myocardium. In contrast, the inhibitors of sarcoplasmic reticulum Ca stores, ryanodine and thapsigargin, as well as a protein kinase C inhibitor, staurosporine, significantly attenuated the inotropic response to AM. L-type Ca channel blocker, diltiazem, also suppressed the AM-induced elevation in DT. Moreover, AM increased the duration of myocyte action potentials between 10 mV and –50 mV in isolated rat atria, consistent with an increase in L-type Ca channel current during the plateau. Inotropic effect of endothelin-1 (ET-1), another locally acting peptide, was enhanced by inhibiting the myocardial nitric oxide (NO) synthesis by N-nitro-L-arginine methyl ester (LNAME) in perfused rat heart. The AM-induced inotropic action was unaltered by L-NAME treatment. When AM and ET-1 were administrated in combined infusion, the inotropic response was significantly smaller than that following the infusion of the peptides alone. This attenuated response was more than overcome by infusion of L-NAME, although the individual responses to AM and ET-1 were not modulated by L-NAME at the doses used in the combination. Consistent with its vasodilator action, AM dose-dependently dilated the coronary arteries of the perfused heart. The effect of AM was not dependent on NO under basal conditions or in coronary arteries constricted with ET-1. Furthermore, AM enhanced the stretch-induced release of ANP from the right atrium, but did not affect the ventricular BNP expression induced by ET-1. In conclusion, AM exerts regulatory actions on the heart by increasing cardiac contractility, dilating coronary arteries and modulating stretch-induced ANP release. The inotropic effect of AM was independent of cyclic AMP, but may involve activation of protein kinase C, Ca influx through L-type Ca channels and the release of Ca from the sarcoplasmic reticulum. Endogenous NO production did not modulate the inotropic effect of AM, although the effect of ET-1 was suppressed. Combined administration of AM and ET-1 produced a weak inotropic response most likely because of a potentiated synthesis of NO. Finally, AM had a coronary vasodilator effect and augmented the stretch-induced ANP release in the right atrium.