EFFECTS OF LEVOSIMENDAN, A CARDIOTONIC AGENT TARGETED TO TROPONIN-C, ON CARDIAC-FUNCTION AND ON PHOSPHORYLATION AND CA2-RETICULUM IN GUINEA-PIG HEART( SENSITIVITY OF CARDIAC MYOFIBRILS AND SARCOPLASMIC)

A new cardiotonic agent, 6-tetrahydro-4-methyl-6-oxo-3-pyridazinyl)-ph enyl] hydrazono]propanedinitrile (Levosimendan), has been developed an d screened for its ability to bind to cardiac troponin C. In perfused hearts, low concentrations of 0.03 or 0.1 mu mol/L Levosimendan increa sed +dP/dt, but did not affect the speed of relaxation and produced on ly a slight increase in spontaneous heart rate in the hearts perfused with 0.1 mu mol/L of the drug. In these same hearts, perfusion with 0. 03 mu mol/L Levosimendan did not alter the P-32 incorporation into tro ponin I or C protein, whereas a slight but significant increase was no ted for phospholamban, with no detectable change in tissue cAMP levels . Administration of 0.1 or 0.3 mu mol/L Levosimendan significantly inc reased myocardial cAMP levels as well as the phosphorylation of phosph olamban, troponin I, and C protein. Levosimendan (0.03 to 10 mu mol/L) reversibly increased force generated by detergent-extracted fiber bun dles over a range of submaximally activating free Ca2+ concentrations with no significant effect on maximum force or on Ca2+ binding to myof ilament troponin C. There was no direct effect of Levosimendan on Ca2 uptake by vesicles of sarcoplasmic reticulum (SR). In contrast, under conditions optimal for cAMP-dependent phosphorylation, Levosimendan s lightly but significantly lowered the concentration of Ca2+, yielding half-maximal uptake rates by the SR vesicles. Our results indicate tha t at low concentrations Levosimendan acts preferably as a Ca2+ sensiti zer, whereas at higher concentrations its action as a phosphodiesteras e inhibitor contributes to the positive inotropic effect.