The Use of Constant Infusion of Unlabeled Dehydroepiandrosterone for the Assessmentof Its Metabolic Clearance Rate, Its Half-Life, and Its Conversion into Estrogens*

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Abstract
We have exploited the feasibility of utilizing constant infusion of unlabeled dehydroepiandrosterone (DHEA) for calculating its MCR, its half-life (t½), and its conversion ratio into estrogens. In nonpregnant women, results obtained by infusion of unlabeled DHEA were similar to those obtained by the infusion of either labeled steroid or a mixture of labeled and unlabeled steroid. An increase of 3-fold in the infusion rate did not change any of these results, indicating that enzyme availability was not a limiting step in the production of estradiol (E2) from DHEA. When compared to the nonpregnancy state, term pregnancy was not associated with a change in plasma DHEA concentrations, but the MCR of DHEA of 76.8 ± 18.8 (SE) liters/ kg-day was 2.5-fold higher and the t½ of 22.5 ± 3.6 min was 2- fold lower. The conversion ratio of DHEA into E2 increased during pregnancy by 500-fold, and DHEA contributed 9% of maternal circulating E2. In the first postpartum day, the MCR of DHEA and its t½ were not significantly different from those of term pregnancy, suggesting that the increased MCRof DHEA in pregnancy was not solely due to increased clearance of DHEA by the placenta.