Relationship among fibre type, myosin ATPase activity and contractile properties.

At least two types of skeletal muscle myosin have been described which differ in ATPase activity and stability in alkaline or acidic media. Differences in ATPase characteristics distinguish Type I and Type II fibres histochemically. In this study, ATPase activity of myosin from muscles of several species with known histochemical and contractile properties has been determined to test the hypothesis that (1) myosin ATPase activity, (2) histochemical determination of fibre types and (3) maximum shortening velocity, all provide equivalent estimates of contractile properties in muscles of mixed fibre types. Maximum shortening velocity appears to be proportional to ATPase activity as expected from previous reports by Barany. However, both myosin ATPase and the maximum shortening velocity exhibit curvilinear relationships to the fraction of cross-sectional area occupied by Type II fibres. Therefore, we reject the hypothesis and conclude that histochemically determined myofibrillar ATPase does not accurately reflect the intrinsic ATPase activity or shortening velocity in muscles of mixed fibre types. Our data are consistent with the presence of more than two myosin isozymes or with a mixture of isozymes within single muscle fibres.