EVIDENCE FOR CAMP-INDEPENDENT MECHANISMS MEDIATING THE EFFECTS OF ADRENOMEDULLIN, A NEW INOTROPIC PEPTIDE

Background-Adrenomedullin (ADM), a new vasorelaxing and natriuretic pe ptide, may function as an endogenous regulator of cardiac function, be cause ADM and its binding sites have been found in the heart. We chara cterize herein the cardiac effects of ADM as well as the underlying si gnaling pathways in vitro, Methods and Results-In isolated perfused, p aced rat heart preparation, infusion of ADM at concentrations of 0.1 t o 1 nmol/L for 30 minutes induced a dose-dependent, gradual increase i n developed tension, whereas proadrenomedullin N-20 (PAMP; 10 to 100 n mol/L), a peptide derived from the same gene as ABM, had no effect. Th e ADM-induced positive inotropic effect was not altered by a calcitoni n gene-related peptide (CGRP) receptor antagonist, CGRP(8-37), or H-89 , a cAMP-dependent protein kinase inhibitor. ADM also failed to stimul ate ventricular cAMP content of the perfused hearts. Ryanodine (3 nmol /L), a sarcoplasmic reticulum Ca2+ release channel opener, suppressed the overall ADM-induced positive inotropic effect. Pretreatment with t hapsigargin (30 nmoI/L), which inhibits sarcoplasmic reticulum Ca2+ AT Pase and depletes intracellular Ca2+ stores, attenuated the early incr ease in developed tension produced by ADM. In addition, inhibition of protein kinase C by staurosporine (10 nmol/L) and blockade of L-type C a2+ channels by diltiazem (1 mu mol/L) significantly decreased the sus tained phase of ADM-induced increase in developed tension. Superfusion of atrial myocytes with ADM (1 nmol/L) in isolated left atrial prepar ations resulted in a marked prolongation of action potential duration between 10 and -50 mV transmembrane voltage, consistent with an increa se in L-type Ca2+ channel current during the plateau. Conclusions-Our results show that ADM enhances cardiac contractility via cAMP-independ ent mechanisms including Ca2+ release from intracellular ryanodine-and thapsigargin-sensitive Ca2+ stores, activation of protein kinase C, a nd Ca2+ influx through L-type Ca2+ channels.