RATE OF FORCE GENERATION IN MUSCLE - CORRELATION WITH ACTOMYOSIN ATPASE ACTIVITY IN SOLUTION

Crossbridge models of muscle contraction based on biochemical studies predict that there may be a relationship between the rate-limiting step in the actomyosin ATPase cycle in vitro and the rate of force development in vivo. In the present study, we measured the rate of force redevelopment in skinned rabbit muscle fibers following unloaded isotonic shortening and a rapid restretch. For comparison, ATPase activity was measured under identical conditions, using myosin subfragment-1 chemically crosslinked to actin. We found that the time course of force redevelopment is well fitted by a single exponential function, implying that force redevelopment is a first-order process, described by a single rate constant. The magnitude of this rate constant is in close agreement with the rate constant necessary to simulate the experimental force-velocity relation on the basis of a crossbridge model of the type proposed by A. F. Huxley in 1957. In addition, the observed close correlation between the rate constant for force redevelopment and the maximal actin-activated actomyosin ATPase rate under a variety of conditions suggests that the step that determines the rate of force generation in the crossbridge cycle may be the physiological equivalent of the rate-limiting step in the actomyosin ATPase cycle in solution.