HISTOCHEMICAL PROPERTIES AND ENZYME-ACTIVITIES OF SKELETAL-MUSCLE IN CHILEAN DRAFT HORSES

Biopsies of the gluteus medius muscle were taken at three different de pths from 16 adult (5 to 15 years old) Chilean crossbred draught horse s (8 males, 8 mares). The biopsy samples were analysed for muscle fibr e type composition, fibre sizes, and the activities of the enzymes cit rate synthase (an indicator of citric acid cycle activity), lactate de hydrogenase (an indicator of anaerobic metabolism) and creatine kinase (used as a marker of the metabolism of high energy phosphates). The p ercentage of type I fibres (high myosin adenosine triphosphatase activ ity at pH 4.5) increased by 100% and the proportion of type IIB fibres (moderate myosin adenosine triphosphatase activity at pH 4.5) decreas ed by 67% going from the superficial to the deep sampling sites of the muscle. The percentage of type IIA fibres (low myosin adenosine triph osphatase activity at pH 4.5) did not present variations as a function of sampling depth. There were no significant differences in the lesse r fibre diameter of fibre types between sampling sites. The activity o f the enzyme citrate synthase increased by 100% from the superficial t o the deep samples, whereas lactate dehydrogenase activity was 50% low er in the deepest portion of the muscle, In general, these re suits in dicated that Chilean crossbred draught horses had a high aerobic capac ity and a relatively low anaerobic capacity in the gluteus medius musc le, expressed by a high percentage of type I fibres, a low proportion of type IIB fibres and high activity of the enzyme citrate synthase, u sed as a marker of end terminal oxidative capacity. These muscle chara cteristics are highly consistent with the sort of work carried out by these animals, prolonged and sustained exercise of low intensity.