Validity of Metabolic Clearance and Interconversion Rates of Estrone and 17β-Estradiol in Normal Adults

Infusions of 3H-estrone or 3H-estradiol were started in 18 normal subjects 30 min after a priming dose and continued at a constant rate for 510–750 min. The concentration in plasma of free radioactivity as the infused, and product steroids, were measured in blood samples obtained throughout the course of the infusions. The concentrations of radioactivity as free estrone remained relatively constant throughout the infusions of 3H-estrone, lasting up to 750 min. When the concentrations expressed as a per cent of the mean concentration for each infusion are related to time, no trend in the values is evident (slope is −0.008%/100 min with 95% confidence limits of −2.3 to +2.3%/100 min). The concentration of radioactivity as free estradiol remained relatively constant throughout the course of infusions of 3H-estradiol lasting up to 630 min. When the concentrations, expressed as a per cent of the mean value, were related to time, no trend in the values is evident (slope is −1.8%/100 min with 95% confidence limits of -3.8 to +0.2%/100 min). During infusions of 3H-estrone, the concentration of radioactivity as the product free estradiol also remained relatively constant throughout. When the concentrations were expressed as a per cent of the mean value and related to time, no trend is evident (slope is +2.1%/100 min with 95% confidence limits of −1.0 to +5.3%/100 min). During infusions of 3H-estradiol the concentration of the product free estrone rose slightly but significantly. When the concentrations were expressed as a per cent of the mean value, there was a significant rise from 150 to 510 min, and, when the values were related to time, an upward trend is evident (slope is +6.7%/100 min with 95% confidence limits of +4.4 to +9.0%/100 min). When 3H-estrone or 3H-estradiol is infused into normal subjects an isotopic steady state for the infused steroid is reached within 2 hr, and theoretically correct metabolic clearance rates can be calculated. When 3H-estradiol is infused, an isotopic steady state for the product free estrone does not appear to be reached within 2 hr, but the rise is slight and gradual over the 8–10 hr infusions and does not appear to alter markedly the [ρ]BB2.1 values.