Tension responses to rapid (laser) temperature-jumps during twitch contractions in intact rat muscle fibres.

We examined the tension responses induced by rapid temperature-jumps (T-jumps) applied at different times during twitch and tetanic contractions in small intact fibre bundles (5-10 fibres) isolated from a fast foot muscle (flexor hallucis brevis) of the rat. A rapid T-jump of 2-4 degrees C was induced by a 0.2 ms infrared (lambda = 1.32 microm) laser pulse applied to the fibre bundle immersed in a 50 microl trough of physiological saline, the temperature of which was clamped at different steady temperatures ranging from 10 to 30 degrees C. In a tetanic contraction, the tension increased to the same steady level when a standard T-jump was applied at different intervals after the onset of stimulation; thus, with maximal activation, an enhanced force generation by T-jump leads to a new steady state. In a twitch contraction, a T-jump induced a large, potentiation of tension when it was applied during the rising phase. Whereas the twitch relaxation subsequent to a T-jump was faster in all cases, the amplitude of the twitch tension potentiation decreased as the T-jump was delayed with respect to the stimulus, and there was no increase of tension when a T-jump was placed on the relaxation phase of the twitch. The increase of tension induced by a T-jump applied on the rising phase resulted in peak tension that was greater than the tension in control twitches at the steady post-T-jump temperature; therefore tension was higher than that expected on the basis of steady state temperature dependence of twitch tension. Whether these effects on a twitch contraction arise from differential fibre-heating by a T-jump that leads to shortening and development of sarcomere length disorder etc remain unclear. However, the findings may be interpreted as indicating that twitch tension increment by a T-jump occurs when excitation (the action potential) leading to calcium release and thin filament activation occur at the low temperature, whereas the crossbridge force-generation processes (and Ca2+-uptake) proceed at the higher temperature.