The positive inotropic effect of adrenomedullin is mediated via cyclic amp-independent mechanisms in isolated perfused rat hearts.

We have shown recently that adrenomedullin (AM), a novel vasoactive peptide, exerts a dose-dependent positive inotropic effect in vitro. AM was reported to elevate intracellular adenosine 3′, 5′-cyclic monophosphate (cAMP) levels in various tissues and cAMP is known to mediate the positive inotropic effect of β-adrenergic agents, raising the possibility that cAMP is responsible for the cardiotonic action of AM. The aim of the present study was to elucidate the contribution of cAMP-dependent and -independent signal transduction mechanisms in the inotropic effect of AM using the isolated perfused rat heart preparation. Briefly, hearts (n=51) of male Sprague-Dawley rats (210-270 g) were excised and arranged for retrograde perfusion by the standard Langendorff technique. The hearts were perfused with a modified Krebs-Henseleit bicarbonate buffer under constant flow conditions (5 ml min−1), submitted to a resting tension of 2 g and paced 15-20 % above the spontaneous beating frequency. Contractile force (apicobasal displacement), perfusion pressure and heart rate were recorded. After equilibration (60 min) and control period (10 min) agents were infused into the coronaries for 30 min. Analysis of variance (ANOVA) followed by the Bonferroni t-test was used to assess statistical significance. Administration of AM (1 nM) increased developed tension by 44 ± 4 % (F=48.5, P<0.001). A protein kinase A inhibitor, H-89 (100 nM) did not affect the AM induced inotropic action (40 ± 3 % vs 44 ± 4 %, F=1.46, P=0.129). In the presence of staurosporine (10 nM), a potent protein kinase C inhibitor, the inotropic effect of AM was decreased (17± 6 %vs 44 ± 4 %, F=10.7, P<0.001). Diltiazem (1 μM), an L-type Ca2+ channel blocker significantly attenuated the AM induced developed tension elevation, too (27 ± 6 % vs 44 ± 4 %, F=2.5, P<0.01). The inotropy was unaffected by administration of vehicle or various inhibitors alone. None of the infusions caused change in heart rate (308 ± 1 beats min−1) and perfusion pressure (29 ± 0.3 mmHg). These results suggest that the inotropic effect of AM is mediated via cAMP-independent mechanism and includes activation of protein kinase C and extracellular Ca2+ influx through L-type Ca2+ channels.