Immediate postnatal rise in whole body androgen content in male rats: correlation with increased testicular content and reduced body clearance of testosterone.

Whole body content of androgen (testosterone + 5 alpha-dihydrotestosterone) was invariably higher in male than in female rat pups killed 1 or 3 h after natural delivery, whereas androgen content was equivalent in males and females killed immediately or 6, 12, and 24 h after birth. Testicular content of androgen was significantly elevated in males killed 1 and 24 h after birth, compared with levels in males killed immediately, or 3, 6, and 12 h after birth. Thus, heightened testicular androgen content was only initially associated with increased systemic levels of androgen in males during the immediate postpartum period. A second study assessed the possibility that the body's clearance (i.e., metabolism plus excretion) of testosterone is lower in newborn rats upon separation from the placental circulation than in slightly older pups. Rats of both sexes killed 1 and 3 h after s.c. injection of [3H] testosterone had significantly higher plasma concentrations of [3H] testosterone as well as several 5 alpha-reduced androgens (5 alpha-dihydrotestosterone, 3 alpha-androstanediol, and androsterone) when injections were given within minutes as opposed to 24 h after birth. This suggests that in both sexes the clearance of testosterone is slower immediately after birth than at later ages. This phenomenon together with a brief postnatal elevation in the testicular synthesis and secretion of testosterone may explain the temporary rise in circulating androgen concentrations that occurs in the newborn male rat.