Uterine metabolism of estradiol in vivo during early pseudopregnancy in the rat

Abstract

We measured the concentration of [ 3H]estradiol (E 2) and its conversion to [ 3H]estrone (E 1) in endometrium and myometrium at diestrus, estrus, and Days 2–6 of pseudopregnancy (PSP) in the rat. Animals ( N = 5–10/ group) were anesthetized with pentobarbital and a constant infusion of [ 3H]E 2 initiated via a jugular vein. Following attainment of isotopic equilibrium, a blood sample was obtained from the inferior vena cava, the endometrium and myometrium isolated, and [ 3H]E 2 and [ 3H]e 1 content determined after purification by paper chromatography. The metabolic clearance rate of E 2 and the peripheral conversion of E 2 to E 1 (%Cr E 2 → E 1), remained relatively constant throughout the study period. Regardless of the reproductive status of the rat, endometrial and myometrial [ 3H]E 2 and [ 3H]E 1 concentrations (cpm/g) were greater ( P < 0.001) than respective values (cpm/ml) in serum. At estrus, the concentration of [ 3H]E 2 in the endometrium was lower ( P < 0.05) than that at diestrus. In contrast, endometrial %Cr E 2 → E 1 and weight of the endometrium were both greater ( P < 0.05) at estrus than at diestrus. Similar patterns were measured in the myometrium. During PSP, the concentration of [ 3H]E 2 in the endometrium increased ( P < 0.05) from Day 2 to Day 6 whereas concentrations in the myometrium remained relatively constant. Endometrial weight increased ( P < 0.05) between Days 2–4 of PSP and was associated with a concomitant decrease ( P <0.05) in the %Cr E 2 → E 1 from 59 to 22%. The %Cr E 2 → E 1 in the myometrium (2–4%) was constant between Days 2–4 of PSP but increased to 10% by Day 6 in association with a decrease ( P < 0.05) in weight of the myometrium. We have demonstrated in vivo that growth and differentiation of the uterine endometrium during early pseudopregnancy in preparation for implantation/deciduogenesis is associated with decreased endometrial %Cr E 2 → E 1. Because ovarian E 2 production is markedly decreased during early PSP, we suggest that a decrease in uterine metabolism of E 2 may act, in part, to maintain endometrial E 2 at concentrations sufficient to permit receptor interaction.