Cross-bridge properties in single intact frog fibers studied by fast stretches.

Cross-bridges properties were measured under different experimental conditions by applying fast stretches to activated skeletal frog muscle fiber to forcibly detach the cross-bridge ensemble. This allowed to measure the tension needed to detach the cross-bridges, Pc, and the sarcomere elongation at the rupture force, Lc. These two parameters are expected to be correlated with cross-bridges number (Pc) and their mean extension (Lc). Conditions investigated were: tetanus rise and plateau under normal Ringer and Ringer containing different BDM concentrations, hyper (1.4T) and hypotonic (0.8T) solutions, 5 and 14°C temperature. Pc was linearly correlated with the tension (P) developed by the fibers under all the conditions examined, however the ratio Pc/P changed depending on conditions being greater at low temperature and higher tonicity. These results indicate that, (a) Pc can be used as a measure of attached cross-bridge number and (b) the force developed by the individual cross-bridge increases at high temperature and low tonicity. Lc was not affected by tension developed, however it changed under different conditions, being greater at low temperature and high tonicity. These findings, suggests, in agreement with Pc data, that cross-bridge extension is smaller at low temperature and high tonicity. By comparing these data with tetanic tension we concluded that potentiation or depression induced on tetanic force by tonicity or temperature changes are entirely accounted for by changes of the force developed by the individual cross-bridge.