EMD 53998 sensitizes the contractile proteins to calcium in intact ferret ventricular muscle.

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Abstract
EMD 53998 (a thiadiazinone) is an inotropic drug that produces a pronounced increase in the Ca2+ sensitivity of the contractile proteins in skinned cardiac fibers. The present study was undertaken to determine whether this effect on Ca2+ sensitivity could explain the increase in tension observed in intact ventricular muscle. The experiments were performed on isolated ferret papillary muscles that had been microinjected with aequorin to measure the intracellular Ca2+ concentration. Force and intracellular Ca2+ concentration were monitored before, during, and after application of EMD 53998. EMD 53998 (5 microM) increased developed tension by 230%; aequorin light transients increased by only 85%, and this increase was reduced and became insignificant in the presence of agents that prevent catecholamine release. When a similar increase in developed tension was produced by elevation of extracellular calcium, the aequorin light transients increased by 240%. Thus, EMD 53998 produces a substantial Ca2+ sensitization in intact ventricular muscle, and this can explain most of its inotropic effect. In addition, EMD 53998 caused a small prolongation of the time course of contraction and a small reduction of the time course of the aequorin light transient. A computer model is described that shows that both these effects can be explained by the effect of EMD 53998 on Ca2+ sensitivity. At much higher concentrations, EMD 53998 also caused an increase in resting tension. EMD 53998 is the first agent for which much of the inotropic effect in intact cardiac muscle can be accounted for by increased Ca2+ sensitivity of the contractile proteins. Inotropic agents with this mechanism of action cause increased force production with much less increase in the intracellular Ca2+ transients than conventional agents and, therefore, increase the energy efficiency of the myocardium and are less likely to cause Ca(2+)-activated arrhythmias.