Adaptations to training and exercise can be measured in blood, muscle, and bones and are also reflected in behavior. Since equine total muscle mass represents approximately 42% of total body weight, a major role for skeletal muscle in adaptation to training can be expected. The plasticity of skeletal muscle to adapt to changes in activity or in working conditions is extremely high. A comparison between expression profiles of proteins and their corresponding mRNAs in equine skeletal muscle during development, training or disease would provide us with valuable fundamental knowledge on how adaptations to these different stages or interventions take place in this tissue. This thesis describes the expression profiles of myosin heavy chain (MyHC) and Na+,K+-ATPase isoforms in equine skeletal muscles. Besides basal expression levels, adaptations of the different isoforms due to exercise, training or disease were investigated. The data obtained could provide us with important knowledge on the outcome of training and the effects of disease, which in turn could help elucidate the mechanism behind these changes. Furthermore the results may be useful in the diagnostic analysis of a disease, such as LMND. The MyHC is the backbone of the contractile machinery and it comprises 25% of the total protein content in skeletal muscle. The expression of myosin heavy chains is transcriptionally controlled, as individual muscle fibers always express one mRNA isoform, but simultaneously can express two different proteins. This is the result of differences in half life of MyHC mRNA and protein. It is complicated to define adaptations to exercise and training using the MyHC expression as a parameter. It is suggested to measure, next to MyHC fiber type composition, other parameters that are more sensitive to training. This thesis showed that parameters such as capillaries per fiber ratio and Na+,K+-ATPase expression may be good candidates to test. We recommend taking biopsies from at least two muscles, because adaptations in muscles are not necessarily the same in different parts of the body. In contrast to training, measuring MyHC fiber type composition and fiber size is very useful to support the diagnosis of a neuromuscular disease like LMND. To indicate the importance of Na+,K+-ATPase, it has been estimated that roughly 23% of all cytoplasmic ATP is hydrolyzed by sodium pumps in a resting human body. All Na+,K+-ATPase isoforms present in muscles of other mammalian species are also detected in equine muscle. The Na+,K+-ATPase isoforms at the mRNA level have been shown to be a good parameter to measure effects of acute exercise in both muscles of the horse, while measuring isoform protein expression is a solid parameter to confirm training adaptations. Finally, measurement of Na+,K+-ATPase isoform mRNAs in muscles of diseased horses was found to be a very sensitive method to detect changes in muscles of horses affected by LMND. Further investigation is required to determine whether the measurement Na+,K+-ATPase expression at both the mRNA and protein level could be a useful tool for diagnosing LMND in horses
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