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

The effect of temperature (5-35 degrees 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 microM acetylcholine induced a tonic contraction which had a magnitude of 65.4 +/- 4.0 N cm-2 (n = 30) at 23 degrees C. Activation by caffeine (20 mM) produced a force response which was 157.1 +/- 7.9% (n = 16) of the acetylcholine response at 23 degrees C and acetylcholine and caffeine together produced force which was not significantly different from the response to caffeine alone. At 5 degrees 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 degrees 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 degrees C to 5 degrees 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 degrees 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 Ca(2+)-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 degrees C to 15 degrees C or 5 degrees C decreased the maximum Ca(2+)-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 Ca(2+)-activated force increased at 15 degrees C and 5 degrees 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 degrees 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 degrees C and 5 degrees C respectively compared with that at 23 degrees C.(ABSTRACT TRUNCATED AT 400 WORDS)