Live slow fibre bundles were isolated from the superficial region of the pectoral fin abductor superficialis muscle of common carp {Cyprinus carpio L.) acclimated to either 8 or 20°C.

The maximum tetanic tension (PO) of fibre bundles was similar when measured at the acclimation temperature of each group. However, at 8°C, PO was significantly higher in 8°C- than in 20°C-acclimated fish (202±8 versus 153±4kNm−2, respectively). For isometric tetani at 8°C, the times to 50% peak force and from peak force to 50 % relaxation were 15–20 % faster in preparations from cold- than from warm-acclimated carp. Force-velocity (P-V) curves were fitted using a hyperbolic-linear equation. The curvature of the P-V relationship was found to be independent of acclimation temperature. Unloaded contraction velocity (Vmax) was 17 % higher at 8°C in fibres from fish acclimated to 8°C than in fish acclimated to 20°C (1.18±0.04 and 0.98±0.04 muscle lengths s−1 respectively).Calculated values for maximum power output at 8°C were 26.5 W kg−1 for cold-acclimated and 18.0 W kg−1 for warm-acclimated fish.

Native myosin was purified from isolated fibre bundles using sodium pyrophosphate gel electrophoresis. The mobility of myosin heavy chains on 8% SDS-PAGE gels was similar for both acclimation groups. Myosin light chain subunits were separated on 15% SDS-PAGE gels. Fibre bundles from warm-acclimated fish contained almost exclusively slow myosin light chains (LC1s and LC2s). Preparations from cold-acclimated fish contained a significant proportion of fast myosin light chains (LC1f and LC2f) in addition to LC1s and LC2s. Histochemical studies revealed no differences in the fibre composition of preparations from warm- and cold-acclimated fish: both contained an average of 3% fast oxidative fibres in addition to slow fibres.

It is concluded that cold-acclimation results in modest improvements in the contractile performance of slow muscle fibres at low temperatures. The mechanism may involve the expression of myosin light chain isoforms normally associated with faster-contracting fibre types

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