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Temperature-dependent changes in the viscoelasticity of intact resting mammalian (rat) fast- and slow-twitch muscle fibres

G. Mutungi and K. W. Ranatunga

1. The tension and sarcomere length responses induced by ramp stretches (at amplitudes of 1-3 % fibre length (L_o) and speeds of 0·01-12 L_o s^-1) were examined at different temperatures (range, 10-35 °C) in resting intact muscle fibre bundles isolated from the soleus (a slow-twitch muscle) and extensor digitorum longus (a fast-twitch muscle) of the rat. Some observations are also presented on the effects of chemical skinning on passive viscoelasticity at 10 °C.

2. As previously reported, the tension response to a ramp stretch, in different preparations and under various conditions, could be resolved into a viscous (P₁), a viscoelastic (P₂) and an elastic (P₃) component and showed characteristic differences between slow and fast muscle fibres.

3. Chemical skinning of the muscle fibres led to a decrease in the amplitude of all three tension components. However, the fast-slow fibre differences remained after skinning. For example, the viscosity coefficient derived from P₁ tension data decreased from 0·84 ± 0·06 before skinning to 0·44 ± 0·06 kN s m^-2 after skinning in fast fibres; the corresponding values in slow fibres were 2·1 ± 0·08 and 0·87 ± 0·09 kN s m^-2, respectively.

4. Increasing the experimental temperature from 10 to 35 °C led to a decrease in all the tension components in both fast and slow muscle fibre bundles. The decrease of P₁ (viscous) tension was such that the viscosity coefficient calculated using P₁ data was reduced from 0·84 ± 0·1 to 0·43 ± 0·05 kN s m^-2 in fast fibres and from 2·0 ± 0·1 to 1·0 ± 0·1 kN s m^-2 in slow fibres (Q₁₀ of ~1·3 in both).

5. In both fast and slow muscle fibre preparations, the plateau tension of the viscoelastic component (P₂) decreased by 60-80 % as the temperature was increased from 10 to 35 °C giving P₂ tension a Q₁₀ of ~1·4 in slow fibres and ~1·7 in the fast fibres. Additionally, the relaxation time of the viscoelasticity decreased from 11·9 ± 1 ms (fast) and 43·1 ± 1 ms (slow) at 10 °C to 3 ± 0·5 ms (fast) at 25 °C and 8·7 ± 0·6 ms (slow) at 35 °C (Q₁₀ of ~2·0 in slow and ~2·5 in fast fibres).

6. The fast-slow fibre differences in passive viscoelasticity remained at the high physiological temperatures. The physiological significance of such fibre-type differences and their possible underlying mechanisms are discussed.

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