Relaxation from rigor by photolysis of caged-ATP in different types of muscle fibres from Xenopus laevis.

Using chemically skinned fast and slow fibres from the iliofibularis muscle of Xenopus laevis, we measured the force changes following laser pulse photolysis of caged-ATP at 4 degrees C in the presence and absence of added calcium. The time course of the early force change in the absence of calcium was used to derive an apparent second order rate constant for crossbridge detachment. These values were compared with previous model-dependent estimates stemming from force-velocity experiments. For fast muscle fibres, the value obtained here was equal to that obtained in the previous study, namely 4 x 10(5) M-1 S-1. For slow fibres, the value obtained from caged-ATP experiments was 1.5 x 10(4) M-1 S-1, whereas the value from force-velocity experiments was 20 times greater (2.9 x 10(5) M-1 S-1). The different values for slow fibres indicate that the model assumptions inherent in the analysis of the force-velocity experiments may not hold for all muscle types. For example, the process of dissociation of the actomyosin complex of slow myosins may be different from that of fast myosins. All observed or calculated kinetic transitions for the crossbridge cycle were slower in slow muscle fibres than in fast muscle fibres. These include the forward and backward rate constants for crossbridge attachment which were lower by a factor of three in slow fibres compared with fast fibres.