THE EFFECT OF LOWERED PH ON THE CA2-ACTIVATED CONTRACTILE CHARACTERISTICS OF SKELETAL-MUSCLE FIBERS FROM ENDURANCE-TRAINED RATS()

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 s ubjected 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 i n many studies to contribute to the process of muscular fatigue, was m imicked by lowering the pH (from pH 7.1 to 6.6) of the Ca2+-buffered s olutions bathing isolated, skinned (demembranated) muscle fibres. Acti vation at both levels of pH enabled comparisons between muscle contrac tile properties under normal and simulated 'fatigue' conditions. Endur ance swimming was shown to significantly improve contractile character istics of single fibres, activated at lowered pH, as evidenced by an i ncrease in Ca2+ sensitivity in the swim-trained rat muscle fibres comp ared to those sampled from sedentary control animals. This improvement in contractile properties was demonstrated in both fast- and slow-twi tch fibres. It was concluded that the endurance swimming programme sig nificantly improved skinned fibre contractile properties under acidoti c conditions.