Effects of castration and testosterone substitution on body composition and muscle metabolism in rats.

Male weanling Wistar rats were castrated or sham-operated and followed for 12 weeks without substitution or with large (2 mg . 14 days-1) or small (0.2 mg . 14 days-1) intramuscular dose of testosterone enantate. Castration without substitution was associated with lower body weight and smaller fat cell sizes in different adipose tissue depots. The epididymal and caudal subcutaneous depots were the most sensitive to castration. The percentage of fast-twitch high-oxidative (type II A) muscle fibers decreased in the non-substituted castrated animals. There was a decrease in phosphorylase and lactate dehydrogenase activities in the white portion of the gastrocnemius muscle of the castrates. These changes were reversed by the large dose of testosterone. Removal of testosterone by castration thus seems to "feminize" male rats with respect to body composition and muscle metabolism.