Differences in temperature dependence of muscle contractile properties and myofibrillar ATPase activity in a cold-temperature fish

Single muscle fibres were isolated from the fast myotomal muscle of the teleost Myoxocephalus scorpius L. and chemically skinned with 1 % Brij. Maximum Ca²⁺-activated force (Po) increased from 14·5 ± 1·1 N cm^-2 at 2°C to 19·1 ± 1·8 N cm^-2 at 15 °C (mean ± S.E.). Maximum contraction velocity was determined by Hill’s slack-test method (V₀) and by extrapolation from force-velocity (P-V) relationships (V_max). There was a linear relation between log₁₀ V₀ and temperature below 15°C (Q₁₀= 1·9, P<0·01). The force-velocity characteristics of the fibres were determined at 2 °C and 20°C. Points below 0·6 P₀ on the P-V curve could be fitted by a linear form of Hill’s equation. Extrapolated V_max values were 0·55 muscle lengths s^-1 (L₀s^-1) at 2°C and 1·54L₀s^-1 at 20°C. Curvature of the P-V relationship was independent of temperature.

The Mg^24-, Ca²⁺-ATPase activity of Triton-X 100 extracted myofibrils was determined under similar ionic conditions to those used in skinned fibre experiments. (Ionic strength 0·16 mmol1^-1, pMgATP2·5). A linear relationship between log₁₀ ATPase and temperature was only obtained below 15 °C {P< 0·001). Above 15 °C, the Q₁₀ for ATPase decreased significantly. The Q_10(0-15°C) for ATPase activity (3·9) was significantly higher than for unloaded contraction velocity. Supercontraction of isolated myofibrils to very short sarcomere lengths and differences in the mechanical constraints for crossbridge cycling between the preparations probably account for the lack of proportionality between these two parameters.