Mechanics and energetics of contraction in thick and in thin filament regulated muscles.

The striated adductor muscle of the sea scallop, P magellanicus, and the semi-tendinosus muscle of the frog, R pipiens, exhibited certain similarities and differences in mechanical and energetic properties. Scallop and frog striated muscle exhibited similar isometric: (a) mechanical twitch kinetics, (b) twitch to tetanus ratios, and (c) contractile economies. There were significant differences between scallop and frog muscle in that scallop muscle demonstrated: (a) a 30% lower maximum isometric force, (b) an inability to maintain force during a tetanus, and (c) a transient potentiation of twitch force after a tetanus associated with a transient decrease in contractile economy. Finally, it is clear that the present study, although contributing mechanical and energetic information of the scallop striated adductor muscle, does not consider the type of contraction normally induced by the scallop in vivo, a shortening, working contraction. It has been suggested (3 17) that swimming in the scallop may be most efficient when the load is high (approximately one-half of the force that can be generated) and the velocity of shortening of the striated adductor is low. This situation is somewhat different than would be predicted in frog muscle where efficiency is highest when velocity of shortening is relatively high and load is relatively low (about one-third of the maximum force that can be generated). A comparison of the mechanical efficiency versus load relation during working contractions in scallop and frog muscle might produce further interesting results.