Exercise-induced fibre type transitions with regard to myosin, parvalbumin, and sarcoplasmic reticulum in muscles of the rat

Effects of a long-term, high intensity training program upon histochemically assessed myofibrillar actomyosin ATPase, myosin composition, peptide pattern of sarcoplasmic reticulum (SR), and parvalbumin content were analysed in muscles from the same rats which were used in a previous study (Green et al. 1983). Following 15 weeks of extreme training, an increase in type I and type IIA fibres and a decrease in type IIB fibres occurred both in plantaris and extensor digitorum longus (EDL) muscles. In the deep portion of vastus lateralis (VLD), there was a pronounced increase from 10±5% to 27±11% in type I fibres. No type I fibres were detected in the superficial portion of vastus lateralis (VLS) both in control and trained animals. An increase in slow type myosin light chains accompanied the histochemically observed fibre type transition in VLD. Changes in the peptide pattern of SR occurred both in VLS and VLD and suggested a complete transition from type IIB to IIA in VLS and from type IIA to I in VLD. A complete type IIA to I transition in the VLD was also suggested by the failure to detect parvalbumin in this muscle after 15 weeks of training. Changes in parvalbumin content and SR tended to precede the transitions in the myosin light chains. Obviously, high intensity endurance training is capable of transforming specific characteristics of muscle fibres beyond the commonly observed changes in the enzyme activity pattern of energy metabolism. The time courses of the various changes which are similar to those in chronic nerve stimulation experiments, indicate that various functional systems of the muscle fibre do not change simultaneously.