The elementary force generation process probed by temperature and length perturbations in muscle fibres from the rabbit

Single chemically permeabilized fibres from rabbit psoas muscle were activated maximally at 5-6degreesC and then exposed to a rapid temperature increase ('T-jump') up to 37degreesC by passing a high-voltage pulse (40 kHz AC, 0.15 ms duration) through the fibre length. Fibre cooling after the T-jump was compensated by applying a warming (40 kHz AC, 200 ms) pulse. Tension and changes in sarcomere length induced by the T-jumps and by fast length step perturbations of the fibres were monitored. In some experiments sarcomere length feedback control was used. After T-jumps tension increased from similar to55 kN m(-2) at 5-6degreesC to similar to270 kN m(-2) at 36-37degreesC, while stiffness rose by similar to15%, suggesting that at a higher temperature the myosin head generates more force. The temperature-tension relation became less steep at temperatures above 25 C, but was not saturated even at near-physiological temperature. Comparison of tension transients induced by the T-jump and length steps showed that they are different. The T-jump transients were several times slower than fast partial tension recovery following length steps at low and high temperature (phase 2). The kinetics of the tension rise after the T-jumps was independent of the preceding length changes. When the length steps were applied during the tension rise induced by the T-jump, the observed complex tension transient was simply the sum of two separate responses to the mechanical and temperature perturbations. This demonstrates the absence of interaction between these processes. The data suggest that tension transients induced by the T-jumps and length steps are caused by different processes in myosin cross-bridges.