The role of exercise and exercise-related factors in the control of mitochondrial oxidative function

Abstract: The effects of exercise and exercise-related factors on the control of mitochondrial oxidative function were investigated in human and rat skeletal muscle. Oxidative function was assessed through the measurement of oxygen consumption in chemically permeabilized (skinned) fibers. Mitochondria in skinned muscle fibers remain in their natural structural environment, permitting sophisticated mechanisms of respiratory control to be studied while allowing the surrounding milieu to be manipulated. Specifically, the acute effects of eccentric and high intensity exercise, the chronic effects of exercise training, and the role of phosphocreatine (PCr), creatine (Cr), inorganic phosphate (Pi), and H+ in the control mitochondrial oxidative function were studied. Two potentially important metabolites that undergo large changes in concentration in the intracellular milieu during exercise are Cr and PCr. While the importance of these metabolites for temporal energy buffering has long been understood, they also have been suggested to play a role in spatial energy buffering and in the regulation of mitochondrial function in vivo. It is known that Cr, in the presence of submaximally stimulating concentrations of ADP, stimulates respiration in skinned fibers from oxidative muscle. However, the effect of PCr on oxidative function is unknown. It was shown for the first time that PCr decreases the sensitivity of mitochondrial respiration to ADP in skeletal muscle. Additionally, the role of in vivo concentrations of PCr + Cr in respiratory control was demonstrated. The rate of submaximally ADP-stimulated respiration increased two fold when the concentrations of PCr + Cr were increased from those occurring in vivo at rest to those occurring during high intensity (HI) exercise. During HI exercise [Pi] and [H+] can be increased several fold in the intracellular milieu. Although these metabolites play an integral role in oxidative metabolism, it has also been suggested that, in high concentration, they can negatively affect mitochondrial function. Therefore, the effect of these metabolites on respiration in skeletal muscle skinned fibers was investigated. It was shown that increased concentrations of Pi and W, either independently or in combination, decrease the stimulatory effect of Cr on mitochondrial respiration. Although these metabolites inhibit the control of respiration by PCr/Cr, they did not affect maximally ADP-stimulated respiration (Vmax). In order to study the effects of exercise on mitochondrial function, respiratory measurements in skinned fibers were performed in an identical medium prior to and following several types of physical activity. HI exercise (3 bouts of cycling until fatigue at 130% of V02 max) caused a transient decrease in the sensitivity of mitochondrial respiration to ADP. In contrast to previous studies in horse, where Vmax decreased following HI exercise, the maximal respiratory capacity of skeletal muscle increased immediately following HI exercise and remained elevated after 110 min recovery. Eccentric exercise (EE) has been shown to cause structural damage and functional impairment in muscle and has been suggested to deteriorate mitochondrial function. To test this hypothesis, the effect of 30 min of eccentric cycling on skeletal muscle oxidative function was studied. The results demonstrate that EE, despite causing a high degree of delayed onset muscle soreness, does not alter mitochondrial function. It is well known that endurance training results in an up-regulation of both oxygen transport capacity and peripheral oxidative potential. In order to determine more precisely the qualitative and quantitative effects of endurance training on skeletal muscle oxidative function, mitochondrial function was assessed in skinned muscle fibers before and after six weeks of cycle training. After the training period, the rates of non-coupled and maximally ADP-stimulated respiration were increased by roughly 40%, whereas the sensitivity of the individual mitochondrion to ADP decreased. It is concluded that, in addition to increases in mitochondrial density, important qualitative changes in the control of mitochondrial oxidation function occur following endurance training. This dissertation has explained an important mechanism of mitochondrial respiratory control in skeletal muscle by PCr/Cr and demonstrated an inhibition of this mechanism by Pi and H+. Additionally, it was shown that mitochondrial function remains intact or is increased following HI concentric or eccentric exercise. Following endurance training, the sensitivity of the mitochondrion to ADP is decreased whereas the total oxidative capacity of skeletal muscle is increased.