Exp Clin Cardiol Vol 15 No 4 2010 e116 Impaired oxidative phosphorylation in overtrained rat myocardium Lumme Kadaja PhD 1 , Margus Eimre PhD 1 , Kalju Paju MD PhD 1 , Mart Roosimaa MD 1 , Taavi Põdramägi MD 1 , Priit Kaasik PhD 2 , Ando Pehme PhD 2 , Ehte Orlova MD PhD 1 , Margareeta Mudist BSc 1 , Nadezhda Peet BSc 1 , Andres Piirsoo PhD 1 , Teet Seene PhD 2 , Frank N Gellerich PhD 3 , Enn K Seppet MD PhD 1 1 Institute of General and Molecular Pathology, Faculty of Medicine; 2 Laboratory of Functional Morphology, Faculty of Exercise and Sport Sciences, University of Tartu, Tartu, Estonia; 3 Department for Behavioral Neurobiology, Leibniz Institute for Neurobiology, Magdeburg, Germany Correspondence and reprints: Dr Enn K Seppet, Institute of General and Molecular Pathology, Faculty of Medicine, University of Tartu, 19 Ravila Street, 50411 Tartu, Estonia. Telephone 372-7374371, fax 372-7374372, e-mail enn.seppet@ut.ee I t is generally acknowledged that regularly performed endur- ance exercise has many health benefits including improve- ment of skeletal and cardiac muscle function in normal and diseased states. The underlying mechanisms include increased protein synthesis leading to muscle hypertrophy, altered isoen- zyme profile of contractile proteins, and stimulation of biogen- esis and functional parameters of mitochondria (1-12). However, exercising is not always favourable; it can also dam- age muscle cells. According to the contemporary concept, any successful training program aimed at increasing physical perfor- mance should achieve supercompensation, ie, development of increased muscle function following a single exercise stimulus (bout). For this purpose, the intensity of the exercise bout must increase stepwise and the periods of rest between the bouts must be sufficiently long to ensure regeneration of the muscle functions but short enough not to cause regression of super- compensation (13). Thus, every advantageous training pro- gram comprises a component of repetitive overloading, which in the case of lack of recovery may produce undesired effects such as the absence of performance improvement, chronic fatigue and muscle damage due to the destruction of myofibrils and atrophy of muscle fibres – all of which result in overtrain- ing syndrome (14). In fact, these unfavourable changes take place rather frequently and explain why many athletes, although undergoing high-volume training, do not improve their performance. It has been suggested that training volume, rather than intensity, may be the major factor contributing to the development of overtraining syndrome (15). Thus, the main challenge is to select the optimal regime for exercising in terms of volume, intensity and periods between exercise to avoid damaging effects in muscle. Studies in skeletal muscles have suggested a potential role of mitochondrial impairment in outcomes of overtraining. Acute exhaustive exercise (EE) stimulates generation of reactive oxygen species (ROS), which damage muscle cells, being side products of oxidative phosphorylation (OXPHOS) due to uni- valent reduction of molecular oxygen by electrons leaking from the respiratory chain (16-19). Mitochondria are the source and targets of ROS because complexes I and III of the electron transport chain are the main sites of mitochondrial superoxide production (20,21); ROS, in turn, induce damage of the res- piratory chain (22). The magnitude of changes in exercise- induced ROS likely depends on the muscle type (23). Mitochondrial impairment is strongly linked to the develop- ment of cellular Ca 2+ overload, which, besides increased ROS, is another condition underlying exercise-induced muscle fibre injury. Suppressed function of Ca 2+ ATPase of the sarcoplasmic reticulum (SR) or sarcolemma compromises the removal of Ca 2+ and thereby elevates the cytosolic Ca 2+ concentration. This process eventually activates degradation of structural and contractile proteins and membrane phospholipids via Ca 2+ - dependent proteolytic and phospholipolytic pathways (24). At the same time, because Ca 2+ overload results in uptake of Ca 2+ into mitochondria (25), excessive accumulation of that ion in the matrix reduces mitochondrial capacity to synthesize ATP and causes direct damage of mitochondrial membranes. The data regarding the effects of exercise on myocardial per- formance and mitochondrial structure and function are scarce and controversial. Early studies have revealed that a single bout of rigorous exercise leads to mitochondrial swelling and cristae disruption in cardiomyocytes (26). However, Ji and Mitchell (27) reported an increase in state 3 respiration in cardiac mito- chondria after an acute (single) bout of maximal exercise. Repeated bouts of rigorous exercise (training) are known to depress state 3 and 4 respiration rates and increase mitochon- drial susceptibility to oxidative stress (28). Terblanche et al (29) found that endurance training combined with an EE bout led to reduction in palmitoylcarnitine oxidation in rat heart tissue. In orIgInal artIcle ©2010 Pulsus Group Inc. All rights reserved L Kadaja, M Eimre, K Paju, et al. Impaired oxidative phosphorylation in overtrained rat myocardium. Exp Clin Cardiol 2010;15(4):e116- e127. The present study was undertaken to characterize and review the changes in energy metabolism in rat myocardium in response to chronic exhaustive exercise. It was shown that a treadmill exercise program applied for six weeks led the rats into a state characterized by decreased performance, loss of body weight and enhanced muscle catabolism, indicating development of over- training syndrome. Electron microscopy revealed disintegration of the car- diomyocyte structure, cellular swelling and appearance of peroxisomes. Respirometric assessment of mitochondria in saponin-permeabilized cells in situ revealed a decreased rate of oxidative phosphorylation (OXPHOS) due to diminished control over it by ADP and impaired functional cou- pling of adenylate kinase to OXPHOS. In parallel, reduced tissue content of cytochrome c was observed, which could limit the maximal rate of OXPHOS. The results are discussed with respect to relationships between the volume of work and corresponding energy metabolism. It is concluded that overtraining syndrome is not restricted to skeletal muscle but can affect cardiac muscle as well. Key Words: Mitochondria; Overtraining; Oxidative phosphorylation; Rat myocardium