Tension transients in skeletal muscle fibres of the frog at varied tonicity of the extracellular medium.

Length steps (complete in 0.2 ms; amplitude < 2% of the fibre length) were applied during the tetanus plateau of intact frog muscle fibres (1.7-3.2 degrees C). The effects of varied tonicity on the early changes in tension in response to the length steps were studied. The solutions were made hypotonic by reduction of the NaCl concentration from 115.5 mM to 92.4 mM and hypertonic by addition of 98 mmol sucrose per litre of the normal Ringer fluid. In all solutions tested, the length step first caused tension to change simultaneously with the step reaching an extreme value T1. After completion of the length change, tension recovered quickly to an intermediate level T2 and, after a period with slowing or reversal of the recovery, it returned slowly to the steady-state value. The maximum isometric tension was significantly reduced by increases in tonicity. In contrast, there were only small effects of varied tonicity on the peak tension-change in response to a length step (the stiffness) and on the amplitude of the fast force recovery (T2-T1) after releases. The slope of the T2-curve (a plotting of T2 versus amplitude of the length step) was reduced for releases and increased for stretches when tonicity was raised. Furthermore, the T2-curve intersected the length axis for smaller releases at high tonicity levels. The reduced isometric tension to stiffness ratio at raised tonicity could be interpreted as a reduced average force per crossbridge. Simulations using the crossbridge model of Huxley and Simmons (1971) showed that the lack of change of the recovery amplitude (T2-T1) after releases and the changes in the T2-slope are in accordance with this interpretation. The shift of the T2 length intercept is consistent with the idea that the distance traversed by the crossbridges during the power-stroke is reduced by raised tonicity.