Abstract

Ca²⁺ sensitizers may be advantageous for treatment in human heart failure by increasing cardiac force without increasing the Ca²⁺ transient or energy consumption. To study the mode of action of the Ca²⁺ sensitizers EMD 57033 (EMD) and CGP 48506 (CGP), their influence on butanedione monoxime (BDM)-mediated depression of cross-bridge cycling was analyzed in human myocardium (explanted hearts, dilated cardiomyopathy, n = 19). In Triton X (1%)-skinned fiber preparations of left ventricular myocardium from patients suffering from dilated cardiomyopathy, troponin I was extracted by vanadate (10 mM) treatment, resulting in a Ca²⁺-independent contraction. In troponin I-depleted fibers BDM (5–50 mM) was applied in the absence and presence of EMD (10 μM) or CGP (10 μM). To analyze the influence on cross-bridge kinetics, tension cost (ratio of ATPase activity and tension development) was studied. BDM exerted a dose-dependent force inhibition in troponin I-depleted fibers (IC₅₀ = 7.22 mM), which was antagonized by EMD (IC₅₀ of BDM + EMD = 19.97 mM) and CGP (IC₅₀ of BDM + CGP = 15.30 mM). EMD increased Ca²⁺ sensitivity of force and maximal force in Triton X-skinned fibers. The Ca²⁺-sensitizing effect of CGP was accompanied by an increased Ca²⁺ sensitivity of myosin-ATPase activity, an increased slope of the Ca²⁺force and Ca²⁺ ATPase curve, as well as a reduced maximal myosin ATPase activity. CGP and EMD reduced tension cost. In conclusion, EMD and CGP antagonize the BDM-mediated relaxation in troponin I-depleted cardiac muscle fibers. The Ca²⁺-sensitizing effect of CGP seems to be dependent on an improvement of the myofilament cooperativity, whereas EMD seems to operate by increasing the force per cross-bridge.