The effects of levosimendan and OR-1896 on isolated hearts, myocyte-sized preparations and phosphodiesterase enzymes of the guinea pig

The concentration dependences of the Ca2+-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 EC50 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 Ca2+ sensitization (at pCa 6.2), by 51 +/- 7% and 52 +/- 6%, with EC50 values of 8 +/- 1 and 36 +/- 7 nM (P < 0.05), respectively. Thus, the two molecules could be defined as Ca2+ 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 (IC50 for phosphodiesterase III=2.5 nM, and IC50 for phosphodiesterase IV=25 muM, selectivity factor approximate to 10000) than OR-1896 QC(50) for phosphodiesterase III=94 nM, and IC50 for phosphodiesterase IV=286 muM, selectivity factor approximate to 3000). Hence, our data support the hypothesis that levosimendan and OR-1896 both exert positive inotropy via a Ca2+ -sensitizing mechanism and not via simultaneous inhibition of the phosphodiesterases III and IV isozymes in the myocardium. at their maximal free plasma concentrations. (C) 2004 Elsevier B.V. All rights reserved.