THE EFFECT OF EMD 57033, A NOVEL CARDIOTONIC AGENT, ON THE RELAXATION OF SKINNED CARDIAC AND SKELETAL-MUSCLE PRODUCED BY PHOTOLYSIS OF DIAZO-2, A CAGED CALCIUM CHELATOR

EMD 57033 is thought to produce its potentiating effect by increasing the apparent calcium sensitivity of myofibrils. We have investigated the effect of 10 mu M EMD 57033 on relaxation speed, induced by flash photolysis of 2mM diazo-2 (a caged Ca2+ chelator), in skinned semitendinosus frog muscle fibres and guineapig trabeculae. 10 mu M EMD 57033 has no effect on the relaxation speed of semitendinosus fibres. In trabeculae, EMD 57033 slightly increases the relaxation speed slightly, in contrast to ADP which produces a slowing. 1mM ADP combined with 10 mu M EMD 57033 slows relaxation but not to the degree seen with ADP alone. Like ADP, EMD 57033 increases the number of cross-bridges in the force producing state, but unlike ADP does not affect the transition rates involved in relaxation.