Abstract
The locomotory muscles represent an important determinant of the athletic potential of the horse and hence the ability to compete at a high level. For efficient raising and training, it would be useful to predict the potential of a horse early in life based on the characteristics of the locomotory
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muscles evaluated by biopsy specimens. The muscle characteristics are partly determined at birth by genetic influences, but are still able to adapt to different demands, such as altered neuromuscular activity as occurs during development and training as well as in neurodegenerative disorders. Therefore, the aim of the present thesis was to obtain a better understanding of the equine muscular system and its ability to adapt to different (patho)physiological conditions. For this purpose small biopsies were taken and investigated from various muscles from different groups of horses at certain time points allowing longitudinal comparison of alterations induced by different exercise regimens or neuromuscular disease. The characteristics of skeletal muscle in general can be described by parameters such as power output and resistance to fatigue. Power is the product of contraction speed and force generated by the muscle and is reflected in the myosin heavy chain (MyHC) fiber type composition and the cross-sectional area (CSA) of the different fibers. Resistance to fatigue of a muscle depends on oxygen delivery by capillaries (quantified as diffusion index: i.e. the area one capillary has to supply) and on a number of enzymes for maintenance of energy metabolism (citrate synthase (CS) and succinate dehydrogenase (SDH)) and excitability (Na+, K+-ATPase). Methods were developed or adjusted to evaluate these parameters with maximum reproducibility in small muscle biopsies of the horses. It was demonstrated that the muscle of an immature horse is a heterogeneous tissue that achieves its adult form by a process of functional adaptation. Muscle biopsies are an important complementary tool for ante mortem diagnosis of lower motor neuron disorders, but are of limited value for the prediction of athletic potential, because the distinctive characteristics at a young age disappear throughout later life. During development the muscle shows an enhancement in power and fatigue related properties. For an optimal development exercise is considered to be essential, especially during the first year after birth. On the other hand, training a horse before conventional training starts does not have huge benefits for muscle performance. Apparently, the developmental adjustments reflected in the measured variables represent sufficient adaptation to meet the demands of training. The enhancement in membrane excitability and maintenance of force indicated that the training the foals were subjected to was intense enough to initiate reactions in the muscle. This means it is possible that alterations occur in parameters at another level. Therefore, it would be very interesting to further explore the adaptations to (patho)physiological conditions on, for example, the molecular level where subtle changes can be detected, i.e. the expression of mRNA for the relevant proteins.
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