Force generation induced by rapid temperature jumps in intact mammalian (rat) skeletal muscle fibres

We examined the tension (force) responses induced by rapid temperature jumps (T-jumps) in electrically stimulated, intact fibre bundles (5-10 fibres, fibre length approximately 2 mm) isolated from a foot muscle (flexor hallucis brevis) of the rat; the muscle contains approximately 90 % type 2 fast fibres. In steady state experiments, the temperature dependence of the twitch tension was basically similar to that previously described from other fast muscles; the tetanic tension increased 3- to 4-fold in raising the temperature from approximately 2 to 35 degrees C and the relation between the tetanic tension and the reciprocal absolute temperature was sigmoidal with half-maximal tension at 9.5 degrees C. A rapid T-jump of 3-5 degrees C was induced during a contraction by applying an infrared laser pulse (lambda = 1.32 micro, 0.2 ms) to the 50 microl trough containing the fibre bundle immersed in physiological saline. At approximately 10 degrees C, a T-jump induced a large transient tension rise when applied during the rising phase of a twitch contraction, the amplitude of which decreased when the T-jump was delayed with respect to the stimulus; a T-jump probably perturbs an early step in excitation-contraction coupling. No transient increase was seen when a T-jump was applied during twitch relaxation. When applied during the plateau of a tetanic contraction a T-jump induced a tension rise to a higher steady tension level; the tension rise after a T-jump was 2-3 times faster than the corresponding phase of the initial tension rise in a tetanus. The approach to a new steady tension level after a T-jump was biphasic with a fast (phase 2b, approximately 35 s-1 at 10 degrees C) and a slow component (phase 3, < 10 s-1). The rates of both components increased (Q10 approximately 3) but their amplitudes decreased with increase of the steady temperature. These results from tetanized intact fibres are consistent with the thesis previously proposed from studies on Ca2+-activated skinned fibres, that the elementary force generation step in muscle is enhanced by increased temperature; the findings indicate that an endothermic molecular step underlies muscle force generation.