Reversal of phosphate induced decreases in force by the benzimidazole pyridazinone, UD-CG 212 CL, in myofilaments from human ventricle

UD-CG 212 Cl, (Fig. 1:4,5-dihydro-6-[2-(4-hydroxyphenyl)-1H-benzimidazole-5-yl]-5-methyl-3(2H) -pyridazinone), is the primary metabolite of the positive inotropic agent pimobendan (UDCG 115 BS, Acardi®). Our previous studies showed in detergent extracted preparations of canine ventricular muscle that sub-nanomolar concentrations of UD-CG 212 Cl increased submaximal myofilament force, but only when the activation state had been altered by relatively high (5-10 mM) concentrations of inorganic phosphate (P(i)) or relatively low (20 μM) concentrations of MgATP. In the present study, we investigated the effects of UD-CG 212 Cl on the pCa-force relationship of detergent extracted bundles of human cardiac fibers before and after addition of P(i). As expected, treatment with 5 mM P(i) depressed maximal force at pCa 4.5 by 27.0 ± 0.4% (mean ± SEM). Force generated at the half-maximally activating Ca²⁺ concentration (pCa₅₀) of control fibers (5.98 ± 0.2) was significantly (p < .05) reduced following the addition of 5 mM P(i) (pCa₅₀ = 5.69 ± 0.3). The addition of UD-CG 212 Cl over a range of concentrations (10^-11-10⁶ M) had no effect on Ca²⁺-sensitivity under control conditions, but in the presence of 5 mM P(i), there was a 23.1 ± 0.1% increase in the percent maximal force at pCa5.9. Ca²⁺-sensitivity was also significantly increased in the presence of P(i) and 10^-8 M UD-CG 212 Cl (pCa₅₀ 5.74 ± 0.3, p < .05). We conclude that UD-CG 212 Cl potentially increases sub-maximal force of human ventricular myofilaments with an inotropic action depending on a state of myofilament activation associated with ischemic conditions.