In the present thesis the localization and activation of signaling proteins, known from human studies, in equine muscle were investigated under conditions of rest, after an acute bout of exercise and before and after a period of (intensified) training. Proteins of interest (protein kinase C (PKC), mitogen-activated protein kinases (MAPKs) and heat shock protein 27 (HSP27)) were studied using histological and biochemical techniques. PKC isoforms were differentially expressed in muscle fiber types and the subcellular localization at rest was different among PKC isoforms. MAPKs and HSP27 were differentially activated upon an acute bout of exercise in muscle of horses, whereas a normal or intensified training program did not result in clear long-term adaptations of these signaling proteins. In order to investigate training-induced fiber transitions, MHC content was determined as well. Changes were found in MHC content between muscles. However, training did not induce a change in MHC content which suggests no conversion of muscle fiber types upon training. In addition, for the first time in horses, proteomic technology was used in rhabdomyolysis and used to elucidate (intensified) training-induced muscle adaptation at the protein level. In cases of acute tying-up, horses expressed an altered form of creatine kinase M chain. Furthermore, the expression level of various proteins changed upon normal or intensified training. From the overall findings of this thesis we hypothesize that, like in human muscle, PKC isoforms, p38 MAPK, JNK and HSP27 play a major role in equine muscle cell signaling. Since these proteins are involved in the insulin signaling cascade, the findings emphasize the importance of the equine muscle in glucose homeostasis as well. The similarities between human and equine muscle cell signaling make the horse an excellent model to study biological markers for guiding training and preventing overtraining in humans and horses. Moreover, the application of such markers could increase the welfare of the equine athlete in clinical practice. The unraveling of the equine genome would facilitate the identification of new markers using proteomic technology. Despite the similarities between the human and equine athlete and the way they adapt to training, it remains to be elucidated whether or not other aspects (mental, neuro-endocrine, metabolic) act analogous in the process of developing overtraining
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