Estrogen-Progesterone Interactions in Uterus and Brain of Intact and Adrenalectomized Immature and Adult Rats

The uptake and metabolism of progesterone following estrogen pretreatment. was examined in immature and adult rats. In the first experiment 31, 45 and 60-day-old ovariectomized rats, half of which were pretreated with 0.325 μg estradiol/100 g body wt/day for 3 days, were given 40 μCi tritiated progesterone. With increasing age there was a decline in uterine uptake of radioactivity. Estradiol pretreatment reduced the uptake of radioactivity in the 31-day-old but not in the 45 or 60-day-old rats. Age differences could not be explained on the basis of uterotropic effects. The same effect was obtained in 31 and 60-day-old rats given estradiol and a higher dose (52 μCi/100 g) of tritiated progesterone. Greater uptake in immature rats and estrogen-induced inhibition of uptake in immature but not mature rats were again found when the last estrogen injection preceded the injection of tritiated progesterone by 12 hr instead of 24 hr. The proportion of radioactivity identified chromatographically as progesterone was greater in nonprimed immature rats than in estrogen-primed immature rats. However, the proportion of progesterone was the same in nonprimed and estrogen-primed adult rats. When immature and adult ovariectomized rats were administered tritiated estradiol the immature animals showed greater uptake and retention of radioactivity than did the mature animals. Because the estrogen could be exerting its differential effects on immature and adult animals through its action on the adrenal system, tritiated progesterone was administered to estrogen-primed and nonprimed rats of both ages, half of which had been adrenalectomized. Estrogen priming continued to inhibit the uterine uptake of radioactivity in immature rats. However, adult adrenalectomized rats pretreated with either estradiol or estrone showed an enhancement in the uptake of radioactivity. Thus, an interaction between age, estrogen pretreatment and adrenalectomy was found. For neural tissues, pituitary and plasma, regardless of adrenal condition and age of animals, there were no significant differences in uptake between estrogen-primed, progesterone-primed and nonprimed rats. Following tritiated estradiol there was a greater accumulation of radioactivity by the diencephalon than by the mesencephalon. The opposite was true following the administration of tritiated progesterone. (Endocrinology94: 214, 1974)