A study of estrogen metabolic clearance rates and transfer factors

Abstract

We have attempted to measure the metabolic clearance rates (MCR) and the transfer factors of estradiol (E₂) and estrone (E₁) during 2-hr and 12-hr infusions. When estradiol-³H was infused for 2 hr, apparent equilibrium was reached at 70 min; the 12-hr infusions showed that plasma estradiol-³H levels increased slowly throughout the infusion. When estrone-³H was infused, constancy of estrone-³H levels was not attained in either the 2-hr infusions or in the two 12-hr infusions. The tritium level in the metabolite of the infused estrogen did not become constant in 50% of the short infusions and increased during all the long infusions. Thus, the conversion ratios C^E₁E₂ and C^E₂E₁ continually changed and transfer factors could not be calculated. The apparent “MCR'S” calculated on the basis of the 2-hr studies expressed as liters/24 hr per m² ±SD were: “MCR^E₁” (women) 980 ±94, (men) 1170 ±95; “MCR^E₂” (women) 615 ±17, (men) 830 ±30. The estradiol “MCR's” differed significantly between men and women. “MCR^E₂” was the same using either estradiol-¹⁴C or -³H and was unchanged by the infusion of 170 μg of estradiol daily. Postmenopausal women had estrogen “MCR's” in the same range as premenopausal women. Excess glucocorticoids increased the “MCR^E₂.”