Common carp (Cyprinus carpio L.), 1 kg body weight, were acclimated for 1–2 months to water temperatures of either 7–8 degrees C (cold-acclimated group) or 23–24 degrees C (warm-acclimated group). Single fast fibres and small bundles of slow fibres were isolated from the myotomal muscles and chemically skinned. Force-velocity (P-V) characteristics were determined at 7 degrees C and 23 degrees C. The contractile properties of carp muscle fibres are dependent on acclimation temperature. In the warm-acclimated group maximum isometric tensions (P0, kN m-2) are 47 +/− 6 and 64 +/− 5 for slow muscle fibres and 76 +/− 10 and 209 +/− 21 for fast muscle fibres at 7 degrees C and 23 degrees C, respectively. Maximum contraction velocities (Vmax, muscle lengths-1), are 0.4 +/− 0.05 and 1.5 +/− 0.1 at 7 degrees C (slow fibres) and 0.6 +/− 0.04 and 1.9 +/− 0.4 at 23 degrees C (fast fibres). All values represent mean +/− S.E. P0 and Vmax at 7 degrees C are around 1.5-2.0 times higher for slow and fast muscle fibres isolated from the cold-acclimated group. Fibres from 7 degrees C-acclimated carp fail to relax completely following maximal activations at 23 degrees C. The resulting Ca-insensitive force component (50–70% P0) is associated with the development of abnormal crossbridge linkages and very slow contraction velocities. Activities of enzymes associated with energy metabolism were determined at a common temperature of 15 degrees C. Marker enzymes of the electron transport system (cytochrome oxidase), citric acid cycle (citrate synthase), fatty acid metabolism (carnitine palmitoyl transferase, beta-hydroxyacyl CoA dehydrogenase) and aerobic glucose utilization (hexokinase) have 30–60% higher activities in slow muscle from cold-acclimated than from warm-acclimated fish. Activities of cytochrome oxidase and citrate synthase in fast muscle are also elevated following acclimation to low temperature. It is concluded that thermal compensation of mechanical power output by carp skeletal muscle is matched by a concomitant increase in the potential to supply aerobically-generated ATP at low temperatures.

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