Contribution of cAMP-Phosphodiesterase Inhibition and Sensitization of the Contractile Proteins for Calcium to the Inotropic Effect of Pimobendan in the Failing Human Myocardium

Previous studies have shown reduced effects of cAMP-dependent positive inotropic agents in the failing human myocardium; thus other cAMP-independent mechanisms of action may be useful to increase force of contraction in this condition. The purpose of this investigation was to determine whether a positive inotropic effect of the cAMP-phosphodiesterase (PDE) inhibitor pimobendan is observed in the failing human myocardium and to study whether other factors, such as an increase in the Ca sensitivity of myofilaments, play a functional role in the increase in force of contraction. Pimobendan produced a positive inotropic effect in isolated preparations from nonfailing donor hearts; however, in moderately (New York Heart Association class II-III, NYHA II-III) and severely (NYHA IV) failing myocardium, this effect was reduced. In addition, in NYHA IV specimens pimobendan inhibited the crude cAMP-PDE (crude PDE) and the isoenzymes I-III (PDE I-III) in a concentration-dependent way. As judged from the IC50 values found in this tissue for the inhibition of PDE III and of crude PDE, the potency of the compound was 18.1 times greater on PDE III. Consistent with a cAMP-PDE-dependent mechanism of action, the positive inotropic effect was potentiated by isoproterenol and inhibited by adenosine in failing myocardium. In failing myocardium, pimobendan also increased the sensitivity of skinned cardiac fibers to Ca and shifted the Ca-tension relation to the left. This sensitizing effect began at 0.01 μmol/l in NYHA II-III and NYHA IV and rose to about 200% at 300 μmol/l in both groups. In contrast, the demethylated metabolite UD-CG 212 Cl failed to produce positive inotropic effects in failing myocardium alone, but in the presence of isoproterenol, it exerted an increase in force of contraction. The potency of UD-CG 212 Cl for PDE III inhibition in NYHA IV was greater than that of pimobendan. The metabolite pronouncedly decreased the sensitivity of skinned cardiac fibers to Ca at 30–300 μmol/l in NYHA II-III and NYHA IV. It is concluded that in the failing human heart pimobendan inhibited PDE III and sensitized contractile proteins for Ca. Both effects appear to be involved in the positive inotropic effect of the compound, because its metabolite, UD-CG 212 Cl, had no effect on force of contraction and on the Ca sensitivity of skinned cardiac fibers but inhibited PDE III even more potently than pimobendan. These findings suggest that PDE inhibition alone by the benzimidazole