BACKGROUND AND PURPOSELevosimendan is known as a calcium sensitizer, although it is also known to inhibit PDE3. We aimed to isolate each component and estimate their contribution to the increased cardiac contractility induced by levosimendan. EXPERIMENTAL APPROACHContractile force was measured in electrically stimulated ventricular strips from explanted failing human hearts and left ventricular strips from normal male Wistar rats. PDE activity was measured in a two-step PDE activity assay on failing human ventricle. KEY RESULTSLevosimendan exerted a positive inotropic effect (PIE) reaching maximum at 10 −5 M in ventricular strips from failing human hearts. In the presence of the selective PDE3 inhibitor cilostamide, the PIE of levosimendan was abolished. During treatment with a PDE4 inhibitor and a supra-threshold concentration of isoprenaline, levosimendan generated an amplified inotropic response. This effect was reversed by β-adrenoceptor blockade and undetectable in strips pretreated with cilostamide. Levosimendan (10 −6 M) increased the potency of β-adrenoceptor agonists by 0.5 log units in failing human myocardium, but not in the presence of cilostamide. Every inotropic response to levosimendan was associated with a lusitropic response. Levosimendan did not affect the concentration-response curve to calcium in rat ventricular strips, in contrast to the effects of a known calcium sensitizer, EMD57033 [5-(1-(3,4-dimethoxybenzoyl)-1,2,3,4-tetrahydroquinolin-6-yl)-6-methyl-3,6-dihydro-2H-1,3,4-thiadiazin-2-one]. PDE activity assays confirmed that levosimendan inhibited PDE3 as effectively as cilostamide. CONCLUSIONS AND IMPLICATIONSOur results indicate that the PDE3-inhibitory property of levosimendan was enough to account for its inotropic effect, leaving a minor, if any, effect to a calcium-sensitizing component. AbbreviationsCRC, contraction-relaxation cycles; cTnC, cardiac troponin C; dF/dtmax, maximal development of force; Fmax, maximal developed force; PIE, positive inotropic effect; RT, relaxation time; TPF, time to peak force; TR80, time to 80% relaxation.

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