Percentage binding of testosterone and dihydrotestosterone and unbound testosterone and dihydrotestosterone in rabbit maternal and fetal plasma during sexual organogenesis.

The percentages of bound testosterone (17 beta-hydroxy-4-androsten-3-one; T) and dihydrotestosterone (17 beta-hydroxy-5 alpha-androstan-3-one; DHT) and their unbound concentrations were determined in pregnant rabbits and their fetuses from the 18th day of gestation to birth. T and DHT were also measured in fetal testes. In the testis, the total T/total DHT ratio, very high at 22 days (73.7 +/- 15.2), decreased until birth (6.7 +/- 0.8). In male fetuses the concentrations of total and unbound circulating T and DHT were always low and did not show any peak during sexual organogenesis. The percent binding of T (from 73.0 +/- 0.5 to 77.6 +/- 0.6) and DHT (from 76.5 to 83.7 +/- 1.1) in fetuses were similar in both sexes and significantly lower than those measured in mothers (T: from 87.2 +/- 0.6 to 91.6 +/- 0.9; DHT: from 87.3 +/- 0.9 to 93.8 +/- 0.9).