The concentration dependences of the Ca(2+)-sensitizing and the phosphodiesterase-inhibitory effects of levosimendan (the (-) enantiomer of [[4-(1,4,5,6-tetrahydro-4-methyl-6-oxo-3-pyridazinyl)phenyl]hydrazono]propanedinitrile) and its active metabolite, OR-1896 (the (-) enantiomer of N-[4-(1,4,5,6-tetrahydro-4-methyl-6-oxo-3-pyridazinyl)phenyl] acetamide), were compared with their positive inotropic effects to reveal their mechanisms of action in guinea pig hearts. In Langendorff-perfused hearts, left ventricular +dP/dt(max) increased by 26+/-4% and 25+/-3% (mean+/-S.E.M.), with EC(50) values of 15+/-2 and 25+/-1 nM for levosimendan and OR-1896, respectively. In permeabilized myocyte-sized preparations, levosimendan and OR-1896 both increased isometric force production via Ca(2+) sensitization (at pCa 6.2), by 51+/-7% and 52+/-6%, with EC(50) values of 8+/-1 and 36+/-7 nM (P<0.05), respectively. Thus, the two molecules could be defined as Ca(2+) sensitizers and positive inotropes with very similar concentration dependences. However, major differences appeared when the phosphodiesterase-inhibitory effects of levosimendan and OR-1896 were probed on the two phosphodiesterase isoforms (phosphodiesterases III and IV) dominant in the left ventricular cardiac tissue. Levosimendan was a 40-fold more potent and a 3-fold more selective phosphodiesterase III inhibitor (IC(50) for phosphodiesterase III=2.5 nM, and IC(50) for phosphodiesterase IV=25 microM, selectivity factor approximately 10000) than OR-1896 (IC(50) for phosphodiesterase III=94 nM, and IC(50) for phosphodiesterase IV=286 microM, selectivity factor approximately 3000). Hence, our data support the hypothesis that levosimendan and OR-1896 both exert positive inotropy via a Ca(2+)-sensitizing mechanism and not via simultaneous inhibition of the phosphodiesterases III and IV isozymes in the myocardium at their maximal free plasma concentrations.