Single fibres were isolated from the fast myotomal muscle of the short-horned sculpin (Myoxocephalus scorpius L.). Fish were acclimated to either 5 °C (10 h:14 h light:dark) or 15 °C (12 h:12 h light:dark) for 1­2 months. Isometric tension (Fmax) and unloaded contraction velocity (Vslack) were determined in maximally activated skinned fibres over the range 0 to 20 °C. Fibres isolated from 5 °C-acclimated and 15 °C-acclimated fish failed to relax completely following activations at 15 °C and 20 °C respectively. In 5 °C-acclimated fish, Fmax increased from 75 kN m-2 at 0 °C to 123 kN m-2 at 10 °C and was not significantly higher at 15 or 20 °C. The relationship between Fmax and temperature was not significantly different for cold- and warm-acclimated fish. Vslack was around 2.8 fibre lengths s-1 in both acclimation groups at 0 °C, but it increased at a significantly faster rate with temperature in 15 °C- than in 5 °C-acclimated fish. At 20 °C, Vmax was significantly higher in 15 °C-acclimated (8.7 fibre lengths s-1) than in 5 °C-acclimated fish (5.3 fibre lengths s-1). In order to investigate the molecular mechanism(s) underlying changes in Vmax, myosin was purified by ion-exchange chromatography. No difference in myosin heavy chain composition could be detected on the basis of peptide mapping with four different proteolytic enzymes. Two-dimensional polyacrylamide gel electrophoresis revealed no myofibrillar protein isoforms unique to either acclimation temperature. However, the ratio of myosin alkali light chain contents (LC3f:LC1f), as determined by capillary electrophoresis, was significantly lower in muscle from 15 °C-acclimated (0.73) than from 5 °C-acclimated fish (1.66). The results suggest that changes in Vmax are achieved via altered expression of myosin light chains independently of myosin heavy chain composition. In support of this hypothesis, the myofibrillar ATPase activity of fast muscle was not altered by temperature acclimation.

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