Fibre type specific transformations in the enzyme activity pattern of rat vastus lateralis muscle by prolonged endurance training

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Abstract
The alterations in activity patterns of representative enzymes in energy metabolism were investigated in the superficial (white) and deep (red) portions of the fast vastus lateralis muscle of the adult rat in response to prolonged endurance training. It was found that following 15 weeks of extreme training (final running duration: 210 min per day, 27 m/min at 15 degree grade), increases in the activities of marker enzymes of the citric acid cycle (citrate synthase), β-oxidation (3-hydroxyacyl CoA dehydrogenase), and ketone body utilization (3-ketoacid CoA transferase) as well as of glutamate pyruvate transaminase occurred in both regions of the muscle, with the geatest increase being observed in the superficial portion (2.6–4.2-fold). Pronounced increases were also seen for hexokinase which showed highest activities after 7 weeks of training. Conversely, decreases were noted for various glycogenolytic, glycolytic and gluconeogenic enzymes (phosphorylase, glyceraldehydephosphate dehydrogenase, pyruvate kinase, lactate dehydrogenase and fructose-1,6-diphosphatase). Reduction in the activities of these enzymes was most pronounced in the deep portion of the muscle. These results demonstrate a fundamental rearrangement of the energy metabolism of the muscle in response to prolonged, high intensity training. In the case of the deep portion of the vastus lateralis muscle, which has been shown to be composed of a large percentage of fast oxidative-glycolytic fibres (FOG), the enzyme profile becomes similar to the slow oxidative (SO) fibre. In the superficial portion which contains predominantly fast glycolytic fibres (FG), the enzyme profile becomes similar to FOG fibres. The magnitude of the observed changes in enzyme activities was greater than in previous animal studies. This suggests that there might be no limit to the metabolic adaptability of skeletal muscle to increased contractile activity. In this context, the adaptations observed here qualitatively resemble the metabolic transformations reported for chronic low frequency electrical stimulation.