THE EFFECT OF EMD 57033, A NOVEL CARDIOTONIC AGENT, ON THE RELAXATIONOF 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 t he effect of 10 mu M EMD 57033 on relaxation speed, induced by flash p hotolysis of 2mM diazo-2 (a caged Ca2+ chelator), in skinned semitendi nosus frog muscle fibres and guineapig trabeculae. 10 mu M EMD 57033 h as no effect on the relaxation speed of semitendinosus fibres. In trab eculae, 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 alon e. Like ADP, EMD 57033 increases the number of cross-bridges in the fo rce producing state, but unlike ADP does not affect the transition rat es involved in relaxation.