The effects of acidosis on the contractile characteristics of single, skinned fast- and slow-twitch skeletal muscle fibres were investigated in both sedentary and endurance-trained rats. The trained rats were subjected to a long-term 16 week programme of high-intensity endurance swimming, and carried a load (corresponding to 2% of body mass) during all 2 h sessions. Intracellular acidification, which has been shown in many studies to contribute to the process of muscular fatigue, was mimicked by lowering the pH (from pH 7.1 to 6.6) of the Ca(2+)-buffered solutions bathing isolated, skinned (demembranated) muscle fibres. Activation at both levels of pH enabled comparisons between muscle contractile properties under normal and simulated 'fatigue' conditions. Endurance swimming was shown to significantly improve contractile characteristics of single fibres, activated at lowered pH, as evidenced by an increase in Ca2+ sensitivity in the swim-trained rat muscle fibres compared to those sampled from sedentary control animals. This improvement in contractile properties was demonstrated in both fast- and slow-twitch fibres. It was concluded that the endurance swimming programme significantly improved skinned fibre contractile properties under acidotic conditions.

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