Abstract
We found a very interesting contraction in flexor carpi radialis muscle (FCR) of forelimb in the frog, Rana japonica, during the breeding season. This contraction has a characteristic 'long-lasting relaxation phase after contraction'. In the present experiment, we investigated the mechanical properties of FCR by measuring the change in force and stiffness of the muscle contraction at 4°C. The total time from the end of stimulation to the end of relaxation was 1797±732 s (n=13) and the relaxation was composed of three phases, which were separated clearly by two changing points on the force curve. The time of first changing point was 4.0±1.0 s (n=13) and the second one was 77±23 s (n=13) after the end of stimulation. When a quick release was applied to each phase, the force recovered slowly but not completely. The magnitude of stiffness decreased quickly in the first relaxation phase, but in the second and third phases they decreased slowly, and the third phase was the slowest. Another specific property of FCR was that it contracted spontaneously, resulting in the appreciable increase in stiffness without stimulation. Present results indicate that cross-bridge cycles would turn slowly in the relaxation and that three different types of relaxations would exist at least in the muscle fibers of FCR, and these might be related to the mechanism of the change in the intracellular Ca concentration. [Jpn J Physiol 54 Suppl:S118 (2004)]
