DETERMINANTS OF RELAXATION RATE IN SKINNED FROG SKELETAL-MUSCLE FIBERS

The influences of sarcomere uniformity and Ca2+ concentration on the k inetics of relaxation were examined in skinned frog skeletal muscle fi bers induced to relax by rapid sequestration of Ca2+ by the photolysis of the Ca2+ chelator, diazo-2, at 10 degrees C. Compared with an inta ct fiber, diazo-2-lnduced relaxation exhibited a faster and shorter in itial slow phase and a fast phase with a longer tail. Stabilization of the sarcomeres by repeated releases and restretches during force deve lopment increased the duration of the slow phase and slowed its kineti cs, when force of contraction was decreased by lowering the Ca2+ conce ntration, the overall kinetics of relaxation was accelerated, with the slow phase being the most sensitive to Ca2+ concentration. Twitchlike contractions were induced by photorelease of Ca2+ from a caged Ca2+ ( DM-Nitrophen), with subsequent Ca2+ sequestration by intact sarcoplasm ic reticulum or Ca2+ rebinding to caged Ca2+. These twitchlike respons es exhibited relaxation kinetics that were about twofold slower than t hose observed in intact fibers. Results suggest that the slow phase of relaxation is influenced by the degree of sarcomere homogeneity and r ate of Ca2+ dissociation from thin filaments. The fast phase of relaxa tion is in part determined by the level of Ca2+ activation.