抄録
We investigated the mechanical properties of flexor carpi radialis muscle (FCR), one of the forelimb muscles in a male frog used for long-lasting mating posture, so-called "amplexus", by measuring the change in force and stiffness during contraction at 4ºC in breeding season. We found that this contraction has a characteristic 'long-lasting, more than 30 min, relaxation phase' after the end of stimulation. In the course of the relaxation, higher stiffness than in resting state was observed, indicating that some myosin molecules were binding to actin filaments. In the present experiment, we further measured the contractile force and the shortening velocity of FCR in breeding (Feb.-Mar.) and non-breeding seasons (Jul.-Sep.) and compared them. It was noted that the rate of force decrease in the relaxation in non-breeding season was remarkably faster than that in breeding season, while the shortening velocities on all loads were not significantly different in two seasons. Present results indicate that the extremely slow relaxation observed in breeding season was caused by the decrease in the very slow rate of force fall. Amplexus may be brought about by this phenomenon. Further it is inferred from the present experiment that the slow relaxation may not be induced by the change in muscle fiber types but by the change in the regulation of the intracellular Ca++ concentration. [J Physiol Sci. 2007;57 Suppl:S97]
