The metabolism of estradiol 17 beta by the pregnant guinea pig uterus in vitro

Freeman, D. J.; Hobkirk, R.

The in vitro metabolism of [3H]estradiol-17.beta. by the uterus was studied in non-pregnant, pregnant (day 30-term) and post-parturant guinea pigs. Following incubation of tissue sections for 1 h in Krebs-Ringer phosphate buffer, 5 major metabolites could be extracted form the medium or tissue depending on age of gestation: estrone, estrone-3-glucuronide, estrone-3-sulfate, estradiol-3-glucuronide and estradiol-3-sulfate. Both sulfated estrogens were detected at each age of gestation studied; the glucuronides, mainly of estrone, were not detected until approximately day 50. Thereafter, as term (day 65-70) was approached, their percentage contribution to total radioactivity increased at the expense of estradiol and the sulfates. Following parturition, total metabolites of estradiol rapidly decreased, particularly the glucuronides. No conjugates were detected in uteri from nonpregnant guinea pigs. No conjugates were found in the pre-partum mouse, rat and hamster or in human endometrium obtained immediately after birth. In the guinea pig, a biochemical factor in the termination of normal pregnancy is the control of tissue levels of active estrogen (estradiol) by conjugation with glucuronic acid.