Mitochondrial reaction in skeletal muscle to induced activity.

The purpose of the present study was to investigate the structure of muscle mitochondria for a possible early response to electrically induced short- and medium-term activity. Rats were anesthetized with Na-pentobarbital. Both parts of the triceps surae muscle were stimulated by direct electrical impulses for periods between 2 and 60 min. Biopsies were taken and prepared for histological, enzyme-histochemical, and electron-microscopic examinations. Enzyme histochemistry demonstrated an increase of oxidative activity and an increase of RNA by NADH-tetrazolium reductase and modified Gomori's trichrome staining or acridine orange-induced fluorescence, respectively. These changes appeared especially within type I fibers, as shown by ATPase reactions, and corresponded to an increase in number and size of muscle mitochondria, followed by alterations of the mitochondrial structure. The mitochondria reacted immediately to the electrical stimulation of the muscle, and the degree of alteration depended on the duration of the experiments. These results suggest that mitochondria have a certain capacity for adapting rapidly to changes in the energy metabolism. Beginning energetic decompensation may be responsible for the alterations of the mitochondrial structure.