Histochemical properties and enzyme activities of skeletal muscle in Chilean draught horses

Abstract

Biopsies of the gluteus medius muscle were taken at three different depths from 16 adult (5 to 15 years old) Chilean crossbred draught horses (8 males, 8 mares). The biopsy samples were analysed for muscle fibre type composition, fibre sizes, and the activities of the enzymes citrate synthase (an indicator of citric acid cycle activity), lactate dehydrogenase (an indicator of anaerobic metabolism) and creatine kinase (used as a marker of the metabolism of high energy phosphates). The percentage of type I fibres (high myosin adenosine triphosphatase activity at pH 4.5) increased by 100% and the proportion of type IIB fibres (moderate myosin adenosine triphosphatase activity at pH 4.5) decreased by 67% going from the superficial to the deep sampling sites of the muscle. The percentage of type IIA fibres (low myosin adenosine triphosphatase activity at pH 4.5) did not present variations as a function of sampling depth. There were no significant differences in the lesser fibre diameter of fibre types between sampling sites. The activity of the enzyme citrate synthase increased by 100% from the superficial to the deep samples, whereas lactate dehydrogenase activity was 50% lower in the deepest portion of the muscle. In general, these results indicated that Chilean crossbred draught horses had a high aerobic capacity and a relatively low anaerobic capacity in the gluteus medius muscle, expressed by a high percentage of type I fibres, a low proportion of type IIB fibres and high activity of the enzyme citrate synthase, used as a marker of end terminal oxidative capacity. These muscle characteristics are highly consistent with the sort of work carried out by these animals, prolonged and sustained exercise of low intensity.