Different Mechanisms Involved in the Positive Inotropic Effects of Benzimidazole Derivative UD-CG 115 BS (Pimobendan) and Its Demethylated Metabolite UD-CG 212 Cl in Canine Ventricular Myocardium

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UD-CG 115 BS produced a positive inotropic effect in a concentration-dependent manner (EC50 = 9.2 x 10(-5) M, efficacy = 0.65) in isolated canine ventricular muscle. UD-CG 212 Cl also elicited a positive inotropic effect (EC50 = 1.9 x 10(-7) M, efficacy = 0.23); its potency was higher, but its efficacy was much less than that of UD-CG 115 BS. Although the effect of UD-CG 115 BS was not altered by a beta-adrenoceptor antagonist, bupranolol (3 x 10(-7) M), the response to UD-CG 212 Cl in high concentrations became transient in the presence of bupranolol: After reaching a peak, the force decreased gradually to the control level at greater than or equal to 10(-4) M. Both UD-CG 115 BS and UD-CG 212 Cl elevated the cyclic AMP level, but to a much smaller extent than other newly developed cardiotonic agents such as amrinone, milrinone, enoximone, and piroximone. Carbachol (3 x 10(-6) M) abolished the accumulation of cyclic AMP produced by these agents while it suppressed the maximum contractile response to UD-CG 115 BS by only 30%. The positive inotropic effect of UD-CG 212 Cl was converted to a negative effect by carbachol. Both UD-CG 115 BS and UD-CG 212 Cl produced a leftward shift in the concentration-response curve for the positive inotropic effect of isoproterenol. These results suggest that an elevation of cyclic AMP levels owing to cyclic AMP phosphodiesterase inhibition may be predominantly responsible for the positive inotropic effect of UD-CG 212 Cl but that a cyclic AMP-independent mechanism may contribute significantly to the positive inotropic effect of UD-CG 115 BS. UD-CG 212 Cl (greater than 3 x 10(-6) M) elicits a negative inotropic effect that is unmasked by beta-adrenoceptor blockade.