The in vivo metabolism of androgens by muscle and adipose tissue of normal men.

Androstenedione and testosterone labeled with 3H and 14C were fused simultaneously at constant rates into the brachial arm vein of 10 normal men. During the infusions blood samples were obtained from the brachial artery, a deep vein draining primarily muscle and a superficial vein draining primarily adipose tissue of the arm contra-lateral to the infusion. In the 10 men the mean +/- SE value for the fractional metabolism of adrostenedione by muscle is 0.20 +/- 0.30 which is not different from the mean value for the fractional metabolism by androstenedione by adipose tissue, 0.29 +/- 0.04. The mean value for the metabolism of testosterone by muscle, 0.04 +/- *.01, is significantly less than the metabolism by adipose tissue, *.11 +/- 0.01. Interconversion between adrostenedione and testosterone occurs in both tissues. The mean value for pA,T A,M is 0.024 + 0.005 and for pA,T A,AT is 0.024 +/- 0.005. The mean value for pT,A A,M is 0.005 +/- 0.003 and for pT,A A,AT is 0.008 +/- 0.003. The fractional metabolism of these androgens by these tissues is similar to the fractional metabolism of estrone and estradiol by these same tissues. Muscle appears to contribute about 5-12% of the overall metabolism of androstenedione and testosterone and 10-15% to theoverall conversion of androstenedione to testosterone. Adipose tissue contributes about 2-7% of the overall metabolism of these androgens and 5-10% of the overall conversion of androstenedione to testosterone, but less than 2% to the overall conversion of testosterone to androstenedione. In normal men, muscle appears to be more important to the metabolism of androstenedione and testosterone than is adipose tissue.