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Skeletal myosin heavy chain composition and carriage training.

Three different myosin heavy chain (MHC) isoforms have been identified in the equine gluteus medius muscle: the slow or MHC-I and the 2 fast MHC-IIA and MHC-IIX isoforms. They are distributed in 3 fibre types containing a single MHC (I, IIA, IIX) and 2 hybrid types co-expressing 2 isoforms (I + IIA, IIA + IIX). The aim of this study was to determine if heavy carriage training alters skeletal MHC composition in horses. Fourteen Andalusian mares age 42-46 months were used. Seven horses were used as controls to estimate the effects of growth on muscle. The remaining 7 horses underwent a training programme based on carriage exercise for 8 months. The intensity of exercise was individually adjusted to each horse according to a standardised exercise test. Gluteus medius muscle biopsies were analysed biochemically for MHC composition by electrophoresis, immunohistochemically for fibre types with specific anti-MHC monoclonal antibodies, and histochemically for fibre type areas, fibre oxidative capacity and capillaries. After training, MHC-IIX decreased and MHC-I increased. The percentages of type IIX and IIAX (i.e. fibres co-expressing MHCs IIA and IIX) fibres decreased, whereas the percentage of type I fibres increased. Neither MHC-IIA composition nor type IIA fibre percentage changed with training. The training had no significant effect on fibre areas and capillaries, but the percentage of fibres with high oxidative capacity increased. The control group showed no changes in muscle variables after the 8 month training period. These results suggest that carriage training alters MHC composition in equine skeletal muscle, reflecting a conversion of MHC isoforms in the order IIX-->IIA-->I and suggesting a reduction in the velocity of shortening of the muscle, but an increase in fatigue resistance.