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 (P0) increased from 14.5 +/− 1.1 N cm-2 at 2 degrees C to 19.1 +/− 1.8 N cm-2 at 15 degrees 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 (Vmax). There was a linear relation between log10 V0 and temperature below 15 degrees C (Q10 = 1.9, P less than 0.01). The force-velocity characteristics of the fibres were determined at 2 degrees C and 20 degrees C. Points below 0.6 P0 on the P-V curve could be fitted by a linear form of Hill's equation. Extrapolated Vmax values were 0.55 muscle lengths s-1 (L0 s-1) at 2 degrees C and 1.54 L0 s-1 at 20 degrees C. Curvature of the P-V relationship was independent of temperature. The Mg2+, 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 mmol l-1, pMgATP 2.5). A linear relationship between log10 ATPase and temperature was only obtained below 15 degrees C (P less than 0.001). Above 15 degrees C, the Q10 for ATPase decreased significantly. The Q10(0–15 degrees 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.