Vol. 295, Issue 3, 1284-1290, December 2000

Different Effect of the Ca²⁺ Sensitizers EMD 57033 and CGP 48506 on Cross-Bridge Cycling in Human Myocardium¹

Klara Brixius, Uwe Mehlhorn, Wilhelm Bloch and Robert H. G. Schwinger

Laboratory of Muscle Research and Molecular Cardiology, Clinic III of Internal Medicine (K.B., R.H.G.S.), Clinic of Cardiothoracic Surgery (U.M.), and Institute I of Anatomy (W.B.), University of Cologne, Köln, Germany

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.