Biochemical adaptations in skeletal muscle of trained thyroidectomized rats.

The cytochrome c concentrations of the different types of skeletal muscle of trained and nontrained normal and thyroidectomized rats were measured. Animals were trained by treadmill running 1 mph, at a 15% incline, 1 h/day, 5 days/wk for at least 12 wk. This training program induced an expected 50% increase in cytochrome c in the high-oxidative fast-twitch red (FTR) and slow-twitch red (STR) fibers and only a 25% increase in the low-oxidative fast-twitch white (FTW) fibers of the normal rats. This same training program caused a greater increase (100%) in the FTR and STR fibers of the thyroidectomized runners and a dramatic 243% increase in the FTW fiber. Even though the thyroidectomy procedure caused a reduction in oxidative capacity of all types of skeletal muscle fibers to about one-half normal, the absolute increase in cytochrome c in the muscles of the trained thyroidectomized animals was essentially the same or greater than that of the normal trained animals. These results indicate that the adaptive response to training of an increased oxidative capacity in skeletal muscle occurs in the absence of normal thyroid function. They also suggest that the exercise bouts of the thyroidectomized animals were performed with a relatively greater involvement of the FTW muscle fibers.