Net protein synthesis and amino acid uptake with testosterone injection

Testosterone (T) administration is known to increase muscle size and lean body mass, especially when combined with exercise Thus, we hypothesized that T causes net protein synthesis and amino acid uptake. Five young males were studied before and 5 days after injection of 200mg of testosterone enanthate. Stable isotopes of phenylalanine and lysine were infused to determine net amino acid flux. In addition, muscle fractional synthetic (FSR) and fractional breakdown (FBR) rates were determined in vivo by phenylalanine tracers. Net amino acid balance increased from a net efflux from the muscle pre injection to a net uptake into the muscle post injection (p<0.05) for both amino acids. FSR increased from 1.72 ± 0.20 (SE) %/day pre to 4.40 ± 0.66 %/day post injection (p<0.01). FBR also increased significantly from 2.11 ± 0.15 %/day to 2.88 ± 0.21 %/day (p<0.05). However, balance between FSR and FBR was negative (-0.39 ± 0.19 %/day) before injection and increased to 1.52 ± 0.82 %/day (p<0.05) after injection. These data indicate that increases in lean body mass and muscle size with T administration are likely the result of net muscle protein synthesis. In addition, these findings indicate that favorable changes in muscle protein synthesis can be realized with serum T values in the physiological range (425 ± 99 ng/dl pre to 953 ± 283 ng/dl post).