Calcium-stimulated myofibrillar ATPase activity correlates with shortening velocity of muscle fibres in Xenopus laevis.

The iliofibularis muscle of Xenopus laevis is reported to contain five types of fibres which have different force-velocity relationships. Ten fibres of each type were selected on the basis of succinate dehydrogenase activity, cross-sectional area and location in the muscle, in order to assess the validity of the fibre type classification. Maximum calcium-stimulated myofibrillar ATPase activity (Vmax) and apparent Michaelis constant (Km) for ATP were determined for these 50 fibres from serial sections. The values obtained varied according to the type of fibre. Type 1 had the highest and type 5 the lowest values for Km and Vmax. In a separate experiment, single freeze-dried fibres were used to determine the relationship between their ATP content and apparent Km for ATP. There was a tendency for high ATP concentrations in fibres with high Km values. When myofibrillar ATPase activity was related to the maximum velocity of shortening of the five fibre types, a significant correlation was found. It is concluded that calcium-stimulated myofibrillar ATPase histochemistry allows an estimate of the maximum shortening velocity of muscle fibres from Xenopus laevis.