Metabolic clearance rates and interconversions of estrone and 17β-estradiol in normal males and females

Abstract

The continuous infusion of (3)H-6,7-estrone and (3)H-6,7-estradiol has been used to study the metabolic clearance rate (MCR), the interconversions, and the red cell uptake of these steroids in normal males and females. The whole blood MCR of estrone is 1,990 +/- 120 liters per day/m(2) (SE) in males and 1,910 +/- 100 liters per day/m(2) in females. The whole blood MCR of estradiol is 1,600 +/- 80 liters per day/m(2) in males and 1,360 +/- 40 liters per day/m(2) in females. The values in females do not vary significantly when studied in the follicular or luteal phase of the cycle. At least 35% of the total estrone metabolism in both sexes is extrasplanchnic and at least 25% of the total estradiol metabolism in males, and 15% in females is extrasplanchnic. The [rho](BB) (2,1) [transfer constant of estradiol to estrone, which is equivalent to the fraction of the precursor (estradiol) converted to the product (estrone) when both the infusion of the precursor and the measurement of the product are in peripheral blood] is 15%; and the [rho](BB) (1,2) [transfer constant of estrone to estradiol, which is equivalent to the fraction of the precursor (estrone) converted to product (estradiol) when both the infusion of the precusor and the measurement of the product are in peripheral blood] is 5% in both males and females. Our findings concerning the radioactivity in whole blood, as measured by our procedure, were the following: 15-20% of estrone in both sexes and 15% of estradiol in males is associated with red cells. Only 2% of the whole blood radioactivity of estradiol in females is associated with red cells. Changes in the distribution of radioactivity between plasma and red cells will influence the MCR as calculated from plasma, but not as calculated from whole blood.