ATPase activity of intact single muscle fibres of Xenopus laevis is related to the rate of force redevelopment after rapid shortening.

Five different fibre types have been recognized in the iliofibularis muscle of Xenopus laevis. The force-velocity and histochemical characteristics of these fibres vary considerably and differences are also found in their myosin composition. In this study a comparison was made between the rate of ATP hydrolysis estimated from the stable maintenance heat rate and the mechanical performance of fibres of type 1, 2 and 3. In the experiments, firstly the force-velocity relation of a fibre was determined, and subsequently, heat production during isometric tetanic contractions at 20 degrees C was measured. Force redevelopment following the fastest shortening used for the measurement of the force-velocity relationship was fitted to a single exponential. The rate of ATP hydrolysis, estimated from the heat production, was found to be roughly proportional to the rate of force redevelopment. Crossbridge attachment rate was determined by using a simulation of a four state model of the crossbridge cycle. It appears that crossbridge attachment rate is proportional to the in vivo actomyosin ATPase activity during an isometric tetanic contraction.