Inotropic agent EMD-53998 weakens nucleotide and phosphate binding to cross bridges in porcine myocardium.

The effect of EMD-53998 (EMD) on elementary steps of the cross-bridge cycle in skinned porcine myocardium was studied to understand the positive inotropic mechanisms of EMD. The kinetic constants of the elementary steps were obtained by sinusoidal analysis and compared in the presence and absence of 50 microM EMD. In the presence of 50 microM EMD, the equilibrium constant of the cross-bridge detachment step decreases three times, and the equilibrium constant of the cross-bridge attachment (force generation) step increases two times. Our results further show that, in the presence of 50 microM EMD, the association constants of MgATP and MgADP decrease to one-ninth and one-third, respectively, and the association constant of phosphate decreases to one-third. These results indicate that EMD suppresses the nucleotide binding to cross bridges and increases cross-bridge resistance to phosphate accumulation in myocardium. These results predict that EMD decreases the number of detached cross bridges and increases the number of attached cross bridges. This prediction is consistent with the twofold increase of isometric tension with 50 microM EMD.