EFFECT OF PH, PHOSPHATE, AND ADP ON RELAXATION OF MYOCARDIUM AFTER PHOTOLYSIS OF DIAZO-2

The aim of this study was to examine the effect of the metabolites H+, ADP, and P-i on the rate of cardiac relaxation. We used guinea pig ri ght ventricular trabeculae that had been chemically skinned, allowing the myofilaments to be studied in isolation. Laser-flash photolysis of the caged Ca2+ chelator diazo 2, causing a rapid fall in intracellula r Ca2+, enabled investigation of relaxation independently of the rate of Ca2+ diffusion. On the photolysis of diazo 2, the trabeculae relaxe d biphasically with exponential rate constants (k(1) and k(2)) of 10.0 7 and 4.23 s(-1), respectively, at 12 degrees C and 18.35 and 2.52 s(- 1), respectively, at a nominal 20 degrees C. Increasing the concentrat ion of both protons (pH 7.2-6.8) and MgADP (0.5-3.4 mM) slowed the two phases of the relaxation transients. Raising the concentration of P-i from the control level of 1.36 mM to 15.2 mM increased the rate of bo th phases, with relaxation becoming monoexponential at 19.4 mM P-i (wi th a k of 20.31 s(-1) at 12 degrees C). Cardiac muscle was compared wi th skeletal muscle under identical conditions; in cardiac muscle 19.4 mM P-i increased the rate of relaxation, whereas in skeletal muscle th is concentration of P-i slowed relaxation. We conclude that the mechan ism of relaxation differs between cardiac and skeletal muscle. This st udy is a direct demonstration of the effects of ATP metabolites on car diac myofilament processes during relaxation.