Velocity-induced modifications in the crossbridge and/or the actin filament behavior during shortening of muscle fibers.

It has been shown experimentally that, in single muscle fibers, the force-velocity relation is more complicated than usually found (Edman, 1988a. Adv. expl. Med. Biol. 226, 643-652; 1988b, J. Physiol., Lond. 404, 301-321). In particular, there is a discontinuity in this relation for high loads (P/P0 greater than or equal to 0.8), i.e. for low velocities of shortening. Here the mathematical approach of the crossbridge behavior is used, independent of their mechanical role(s). This approach has been already presented in previous papers (Morel, 1984a,b, Prog. Biophys. molec. Biol. 44, 47-96, and references therein). It was found that, at P/P0 approximately 0.80, the force-velocity relation presents a reversal of curvature, which was compatible with the previous results obtained by Edman et al. (1976, Acta Physiol. Scand. 98, 143-156). However, by using extremely elegant techniques, Edman (1988a,b) found a different relation. Here, this problem is studied and it is shown that it is possible to fit the new relationship, provided it is assumed that shortening per se modifies the mechanical properties of the crossbridges and/or the actin filaments. For instance, the interval of attachment of a crossbridge increases with V and the constants of attachment f and detachment g decrease with V. Moreover, it is shown that this approach can predict the approximate constancy of the proportion of crossbridges attached to actin, irrespective of V. This is an old result presented by Podolsky et al. (1976, Proc. natn. Acad. Sci. U.S.A. 73, 813-817). This proportion is approximately 0.97.(ABSTRACT TRUNCATED AT 250 WORDS)