Testosterone injection stimulates net protein synthesis but not tissue amino acid transport

Abstract

Testosterone administration (T) increases lean body mass and muscle protein synthesis. We investigated the effects of short-term T on leg muscle protein kinetics and transport of selected amino acids by use of a model based on arteriovenous sampling and muscle biopsy. Fractional synthesis (FSR) and breakdown (FBR) rates of skeletal muscle protein were also directly calculated. Seven healthy men were studied before and 5 days after intramuscular injection of 200 mg of testosterone enanthate. Protein synthesis increased twofold after injection (P less than 0.05), whereas protein breakdown was unchanged. FSR and FBR calculations were in accordance, because FSR increased twofold (P less than 0.05) without a concomitant change in FBR. Net balance between synthesis and breakdown became more positive with both methodologies (P less than 0.05) and was not different from zero. T injection increased arteriovenous essential and nonessential nitrogen balance across the leg (P less than 0.05) in the fasted state, without increasing amino acid transport. Thus T administration leads to an increased net protein synthesis and reutilization of intracellular amino acids in skeletal muscle.