Enzyme levels in pools of microdissected human muscle fibres of identified type: Adaptive response to exercise

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Abstract
Enzymes activities were determined in pools of type I (slow twitch) and II A and II B (fast twitch) fibers of the thigh muscle from individuals engaged to a higher degree in physical training of an endurance character and from nonendurance-trained controls. The endurance-trained (ET) group had significantly higher activity levels of the mitochondrial enzymes citrate synthase, malate dehydrogenase and 3-OH-acylCoA dehydrogenase both in type I (2.1 .times., 1.7 .times., 1.4 .times.) and in type II A (2.3 .times., 1.8 .times., 1.4 .times.) and II B fibers (2.0 .times., 1.5 .times., 1.5 .times.) than the nonendurance-trained (NET) group. Of the glycolytic enzymes, phosphofructokinase (PFK) in type I fibers was significantly higher (1.8 .times.) in the ET than in the NET group, whereas glyceraldehydephosphate dehydrogenase (GAPDH) in type I fibers was similar in the 2 groups. In type II fibers both PFK and GAPDH levels tended to be higher in the ET group. Lactate dehydrogenase (LDH) of both fiber types were not different in the 2 groups. Type I fibers differed significantly from type II fibers for all the 6 enzymes measured in both groups. No significant difference between fibers of types II A and II B was found. Fibers of types I, II A and II B in human skeletal muscle all possess great adaptability with regard to their oxidative capacity. Extensive endurance training may enhance the glycolytic capacity in both type I and type II fibers, although the glycolytic capacity of the muscle as a whole generally is low in endurance trained subjects owing to a predominance of type I fibers. Further studies are needed to determine whether there is a metabolic distinction between fibers of types II A and IIB.