Calcium activation produces a characteristic response to stretch in both skeletal and cardiac muscle

Abstract

RESTING frog skeletal muscle, when stretched at a constant velocity, exhibits a force response which, for length changes less than about 0.2%, is elastic in nature. First observed by Hill¹, this response was attributed to a short range elastic component (SREC) that was thought to be a property of a small number of cross bridges which, even in the resting state, extend from the myosin-containing thick filaments to interact with sites on the actin-containing thin filaments. This view was criticised² on the grounds that the SREC stiffness remained approximately constant as the amount of thick and thin filament overlap was decreased by increasing muscle length. On the basis of the sliding filament theory^3,4, SREC stiffness would be expected to decrease because reduced amounts of thick and thin filament overlap would result in a decreased number of cross bridges available in interact with thin filament sites. Flitney and Hirst⁵ have reported that in twitch contractions of frog skeletal muscle a short range elastic response is present and is similar in form to that of resting muscle. In this case, however, a directly proportional relationship was found between the short range stiffness and the amount of overlap between the thick and thin filaments, which suggests that this stiffness is a property of attached cross bridges. To investigate the possible presence of attached cross bridges in resting muscle, we have measured the calcium dependence of the short range stiffness in skeletal and cardiac muscle preparations in which the myofibrils were directly exposed to the bathing medium.