Acceleration of the ATPase activity of glycerol-treated muscle fibers by repeated stretch-release cycles.

Abstract

Glycerol-treated muscle fiber bundles were fixed at their rest length in 50 mM KC1, 2 mM MgC1(2), and 10 micron CaC1(2) at pH 7.8 and 0 degrees C in the presence of sufficient amounts of ATP, creatine kinase, and creatine phosphate. The fiber bundles were stretched linearly with time for 0.3 s at a constant amplitude, suddenly released, then fixed at the rest length for a constant time interval (alpha seconds). The stretch-release cycle was repeated, and the ATPase activity (the rate of ADP liberation) [EC 3.6.1.3] was measured. It was found that: 1. ATPase was activated by repeated stretch-release. As repetitive stretch-release of 1--2% of the rest length caused maximum activation, we usually selected a value of 2.5% of the rest length. The activation of ATPase was found to be a function of the duration, alpha, of the isometric phase after sudden release from stretching. The ATPase activity of fiber bundles was almost unaffected when they were oscillated by a simple stretch-release without an isometric phase after the sudden release (alpha=0). 2. The ATPase activity of oscillated muscle fibers increased with increase in the value of alpha, reached a maximal level, then decreased gradually with further increase of alpha to a value slightly larger than that of static fibers. At 0 degrees C, the value of alpha for the maximum activation was observed at about 2 s, and the maximum activity was about 2.5 times that of static fibers. At 20 degrees C, the alpha value for maximum activation was about 0.5 s, and the maximum activity was about 1.8 times that of static fibers. 3. The time course of ADP liberation after one stretch-release cycle could be easily calculated from the ATPase activity of the summed durations of the isometric phase, alpha, assuming that the ATPase activation was turned off and on by the stretching and release, respectively, and that the state of cross-bridges immediately after the stretch-release was independent of alpha of the cycle. The rate of ADP liberation after stretch-release thus obtained showed a short lag phase, a sigmoidal increase, a decrease to almost zero, then a return to nearly the original level (the rate of static fibers). About 1.3 mol of ATP per mol of myosin was hydrolyzed at both 0 degrees C and 20 degrees C during one cycle of the changes in the rate of ADP liberation.