The influence of calcium on shortening velocity of skinned frog muscle cells

Conclusions

The experiments reviewed here attempted to determine whether the kinetics of crossbridge cycling in intact myofibrils are directly affected by the level of activating calcium. In view of the conflicting evidence available to date, it is not yet possible to answer this question. Maximum shortening velocity, used as an index of crossbridge cycling kinetics, appears to depend directly on the concentration of activating calcium in some experiments, and to be independent of calcium in others. The differences between the experimental results appear to be genuine, and not an artifact of the experimental technique. Several criticisms of the techniques are discussed in detail to show that the error produced by any individual imperfection in technique would not be sufficient to account for the large differences in the results. It is possible that the effects of several such technical imperfections acted in consort to produce the large differences. If the large differences were the result of an accumulation of several small errors in both sets of experiments, so that the correct experimental result was a compromise, Julian's main conclusion would still be correct. That is, any genuine difference in the relative force-velocity curves, regardless of its magnitude, would indicate that calcium must have some effect on the actomyosin complex in addition to its effect on the regulatory proteins. A more likely explanation of the different results is that some physiological mechanism was operative in one set of experiments and not in the other. Two possible mechanisms, light chain phosphorylation and the influence of shortening on the cooperative binding of calcium to thin filaments, are discussed.