Quantitative determination of calcium-activated myosin adenosine triphosphatase activity in rat skeletal muscle fibres.

A quantitative histochemical technique was developed for determining the kinetics of the calcium-activated myosin ATPase (Ca(2+)-myosin ATPase) reaction in rat skeletal muscle fibres. Using this technique, the maximum velocity (Vmax) and the apparent Michaelis-Menten rate constant for ATP (K(app)) of the Ca(2+)-myosin ATPase reaction were measured in type-identified fibres of the rat medial gastrocnemius (MG) muscle. The Vmax and the K(app) of the Ca(2+)-myosin ATPase reaction were lowest in type I fibres and highest (i.e., approx. two times greater) in type IIb fibres. The K(app) in type IIa fibres was similar to that in type I. However, the Vmax was 1.5 times greater in type IIa fibres, compared to type I fibres. Evidence is presented to suggest that the type IIb fibre population in the MG does not represent a single myosin isozyme. In addition, the broad range of Vmax and K(app) values indicates that there is marked heterogeneity in the myosin heavy chain and myosin light chain composition of myosin isozymes among individual fibres.