Plasticity of human skeletal muscle with special reference to effects of physical training on enzyme levels of the NADH shuttles and phenotypic expression of slow and fast myofibrillar proteins.

The adaptation of human skeletal muscle to endurance training and detraining has been investigated. The following variables were monitored: phenotypic expression of slow and fast isoforms of myofibrillar ATPase, as well as contractile and regulatory proteins, capillary supply and fibre areas, levels of enzymes in the main metabolic pathways and the NADH shuttles. For the latter purpose, several methodological surveys were undertaken. The main findings and conclusions are: Endurance training can induce a transformation of type II (fast-twitch) fibres into myofibrillar ATPase intermediate fibres (IM fibres: types IIC, IIC-IB and IB). Using immunohistochemical techniques, a co-existence of slow and fast isoforms of whole myosin, myosin heavy chains, and myosin lights chains as well as troponin C, T and I components, was demonstrated in the training-induced IM fibres. Furthermore, a co-existence of slow and fast isoforms of myofibrillar ATPase in the IM fibres, can be anticipated from the stainings for myofibrillar ATPase. No neonatal myosin heavy chains could be detected in any of the trained muscle fibres. The IM fibres were intermediate between type I (slow-twitch) and type II also with regard to morphological and metabolic characteristics. Along with other lines of evidence, the occurrence of IM fibres in conjunction with endurance training demonstrates that transformation of fibre type II to type I can occur in response to endurance training. On the basis of findings of a decreased spread of fibre areas among individuals in connection with extensive endurance training, it is suggested that fibre sizes are determined by two conflicting demands: good diffusion conditions and high force development. The existence of a mechanism that can elicit decreases in fibre size, despite extensive use of the fibres, is suggested. The magnitude by which levels of oxidative enzymes and capillary supply are enhanced by endurance training is dependent on both the exercise intensity and the duration. However, if the intensity is below a certain critical point, its inefficiency in stimulating to adaptive changes can not be compensated for by even a very long duration of exercise. The patterns of training-induced increases in CS, MDH and HAD indicate that the levels of these enzymes can be regulated independently. It appears possible that the levels of the malate-aspartate shuttle enzymes can vary in relation to citric acid cycle enzymes depending on the extent to which oxidation of fatty acids contributes to the metabolism. Detraining results in rather rapidly regressing levels of oxidative enzymes and capillary supply.(ABSTRACT TRUNCATED AT 400 WORDS)