The effect of temperature on contractile activation of intact and chemically skinned ‘catch’ muscle fibre bundles of Mytilus edulis

Summary

The effect of temperature (5–35° C) on maximum force production was examined in intact and chemically skinned muscle fibre bundles (10–25 fibres) from the anterior byssus retractor muscle of Mytilus edulis. In intact fibre bundles, 10 μm acetylcholine induced a tonic contraction which had a magnitude of 65.4±4.0 N cm^-2 (n=30) at 23° C. Activation by caffeine (20 mm) produced a force response which was 157.1±7.9% (n=16) of the acetylcholine response at 23° C and acetylcholine and caffeine together produced force which was not significantly different from the response to caffeine alone. At 5° C the acetylcholine and caffeine responses were decreased by 9.6±3.4% (n=6) and 14.6±2.8% (n=8) compared with the respective responses at 23° C. However, there was no significant reduction of the response induced by the combined action of acetylcholine and caffeine when the temperature was decreased from 23° C to 5° C. The 20–80% of peak force activation time increased by about one order of magnitude for all acetylcholine, caffeine and combined acetylcholine-caffeine-induced responses when the temperature was decreased from 23–5° C. Repeated exposure of the intact preparation to caffeine caused a marked decrease in the caffeine-induced response (complete abolition of force after the third exposure to caffeine), but the response to caffeine could be fully restored following one acetylcholine-induced activation. The maximum Ca²⁺-activated force after skinning the preparation with saponin was not significantly different from the caffeine or combined acetylcholine-caffeine-induced responses before skinning. In the saponin skinned fibre preparation a drop in temperature from 23° C to 15° C or 5° C decreased the maximum Ca²⁺-activated force by 13.2±1.4% (n=8) and 41.4±3.1% (n=5) respectively. The activation time between 20–80% of the peak Ca²⁺-activated force increased at 15° C and 5° C by a factor of 1.5±0.1 (n=5) and 6.8±1.1 (n=5) respectively when compared to corresponding values at 23° C. The relaxation half-time decreased by a factor of 1.7±0.2 (n=5) and 3.0±0.2 (n=5) at 15° C and 5° C respectively compared with that at 23° C. It was possible to distinguish between the temperature effects on the contractile apparatus per se and the Ca²⁺ regulatory system with the results indicating that the contractile apparatus was more sensitive to a change in temperature than the Ca²⁺-regulatory system. Increasing the temperature to 35° C irreversibly affected the ability to develop and maintain force in both intact and skinned muscle preparations. These results indicate that: (1) acetylcholine does not fully activate the intact ‘catch’ muscle at 23° C; (2) acetylcholine is able to replenish the internal stores after depletion by caffeine; (3) compensatory mechanisms which oppose the inhibitory effect of lower temperatures on the contractile apparatus and the Ca²⁺-regulatory system must be operating in the intact fibre preparations.