Ultrastructural changes of the Gluteus medius muscle of untrained crossbred draught horses subjected to ploughing

The Chilean crossbred draught horse has been bred to perform heavy resistan ce work in order to have adequate physical fitness for the work they carry out during different agricultural tasks. Despite the significant contributi on of these animals to the rural economies of the under developed countries , little is known about their muscular adaptation to draught work. Generall y, these animals are subjected to intensive work after prolonged periods of rest, and, unlike sport horses, there is little concern about their physic al fitness and conditioning prior to starling the working season. The aim o f this study was to analyze the main ultra-structural changes observed in t he Gluteus medius muscle of Chilean crossbred draught horses during ploughi ng at the beginning of the working season after a long period of rest. For this purpose, muscle biopsies of the Gluteus medius from 4 Chilean crossbre d draught 3 months inactive horses between 10 to 12 years of age were taken by percutaneous puncture before and after 3 h of routine ploughing work. S amples were processed by transmission electron microscopy according to conv entional ultra-thin techniques. The ultra-structural analysis of the muscle biopsies from the animals studi ed after physical activity showed some significant changes such as: alterat ions in the contractile system (shortening of myofilaments), changes in the electron density of mitochondria (swollen mitochondria), frequent presence of lysosomal structures (principally granules of lipofucsin), thickening o f the basal membranes and partial occlusion of the intramuscular capillarie s due to the presence of macrophages, and extension of the basal membrane i nto the capillary lumen. These results suggest a probable direct relationsh ip between unaccustomed physical exercise in unconditioned draught horses a nd the ultra-structural changes observed in the skeletal muscle in response to intense physical activity.