Luteotropic Role of the Decidual Tissue in the Rat:Dependency on Intraluteal Estradiol*

Abstract

The decidual tissue (DT) of pregnant and pseudopregnant rats maintains progesterone secretion in the absence of PRL. However, the luteotropic activity of the decidua disappears in the absence of the pituitary or LH. Since the role of LH in the pregnant rat is mediated by estradiol, it was of interest to determine whether the luteotropic role of the DT is dependent upon intraluteal estradiol. Pseudopregnant rats, either hysterectomized or bearing DT, were hypophysectomized on day 9. Within 24 h, a precipitous reduction in serum progesterone was observed in the two groups. The intraluteal concentration of estradiol on day 12 also dropped after hypophysectomy from 4 ± 1 and 2.8 ± 0.5 pg/mg in DT-bearing and hysterectomized animals to 0.4 ± 0.3 and0.2 ± 0.1 pg/mg, respectively. Three days after hypophysectomy, the average weight of the decidualized uteri was 1.1 ± 0.3 g compared to 3.9 ± 0.3 g in intact controls. Insertion of a 1-cm testosterone (T1) capsule in DTbearing rats 24 h before hypophysectomy increased the intraluteal concentration of estradiol to 13.8 ± 1.9 pg/mg and maintained serum progesterone for 2 days at levels found in intact controls. On day 12, however, although the DT was fully maintained, serum progesterone declined. In hysterectomized rats also, T1 treatment elevated intraluteal estradiol to 6.6 ± 1.0 pg/mg after hypophysectomy. However, in contrast to results obtained in rats bearing DT, T1 was unable to sustain progesterone secretion. In DT-bearing rats, capsules filled with different amounts of testosterone maintained the intraluteal concentration of estradiol in proportion to their content. However, only when the levels of estradiol in the corpora lutea (CL) were at or above those in intact DT-bearing rats were progesterone levels maintained. Androsterone, which is not converted to estradiol by the CL, had no discernible luteotropic effect. In both hysterectomized and normal control pseudopregnant rats, the intraluteal concentration of estradiol was also increased proportionally to the level of testosterone treatment; however, progesterone secretion was not maintained in the absence of the DT. Daily administration of PRL (250 μg/day) plus testersterone to hysterectomized rats completely prevented the drop in progesterone levels that occurs after hypophysectomy or hypophysectomy plus testosterone treatment. The combination of PRL with testosterone treatment also prevented the decline in serum progesterone found on day 12 in DT-bearing rats treated with testosterone alone. While PRL alone maintained some progesterone secretion in hysterectomized rats, it has no discernible luteotropic effect in the presence of the DT. These results suggest 1) that CL of pseudopregnant rats readily convert testosterone to estradiol; 2) that the DT possesses luteotropic activity which is dependent on the availability of an aromatizable androgen; 3) that progesterone synthesis is controlled by the joint action of the DT and intraluteal estradiol; 4) that the tropic activity of the DT is present until day 11 only; and 5) that the DT inhibits the luteotropic effect of PRL.