Myosin heavy chain profile of equine Gluteus medius muscle following prolonged draught-exercise training and detraining

Fourteen 4-year old Andalusian mares were used to examine the plasticity of myosin heavy chain (MHC) composition in horse skeletal muscle with heavy d raught-exercise training and detraining. Seven horses underwent a training programme based on carriage exercises for 8 months. Afterwards, they were k ept in paddocks for 3 months. The remaining seven animals were used as cont rol horses. Three gluteus medius muscle biopsies were removed at depths of 20, 40 and 60 mm from each horse before (month 0), during the training (mon ths 3 and 8) and after detraining (month 11). Myosin heavy chain compositio n was analysed by electrophoresis and immunohistochemically with anti-MHC m onoclonal antibodies. Fibre areas, oxidative capacity and capillaries were studied histochemically. After 8 months of training, MHC-IIX and IIX fibres decreased whereas MHC-I and type I and I + IIA fibres increased. Neither M HC-IIA nor the percentage of IIA fibres changed when the data were consider ed as a whole, but the proportion of MHC-IIA increased in the superficial r egion of the muscle after 8 months of training. Mean areas of type II fibre s were not affected by training and detraining, but the cross-sectional of type I fibres increased after 3 month of training and not further increases were recorded afterward. The percentage of high-oxidative capacity fibres and the number of capillaries per mm(2) increased with training. Most of th ese muscular adaptations reverted after detraining. These results indicate that long term draught-exercise training induces a reversible transition of MHC composition in equine muscle in the order IIX --> IIA --> I. The physi ological implication of these changes is an impact on the velocity of short ening and fatigue resistance of muscle fibres.