Effects of Exogenous Progesterone on the Ovarian Progestin Secretion and Plasma LH and Prolactin Levels in Cyclic Rats

Abstract

In the cyclic rats, progesterone secretion increased twice a cycle, on the afternoon of proestrus and on the first day of diestrus. 20α-Hydroxypregn-4-en-3-one (20α-OH-P) secretion elevated after LH surge and remained high by the first day of diestrus. The level of plasma LH increased once a cycle on the afternoon of proestrus and remained low through the estrous and diestrous stages. Plasma prolactin level also increased on the afternoon of proestrus, and it occurred earlier than the onset of LH release. A single injection with 5mg per rat of progesterone at 11.00hr on the day of proestrus (designated Day 1) caused the following effects; a) about 3hr advancement in the release of both LH and prolactin, either amount of which, however, was almost comparable with the controls, b) a marked increase and an advancement in preovulatory progesterone secretion, c) an increase in diestrous progesterone secretion on Day 3, and d) a lower preovulatory progesterone secretion on Day 5 than the respective controls, the occurrence of which delayed about 3hr. In the following estrous cycle, however, both increments of progesterone secretion at diestrus (Day 7) and proestrus (Day 9) and gonadotropin release occurred similarly as in the controls. 20α-OH-P secretion was high during the period from proestrus to early diestrus and low at late diestrus in both control and treated rats. The highest value in 20α-OH-P secretion was seen at 15.00 to 16.00hr on Day 1 in the treated rats. Numbers of ova and newly formed corpora lutea counted in the mornings of Day 2 and Day 6 were similar to those in the controls. An additional treatment with progesterone at 11.00hr or with exogenous LH at 15.00hr on Day 5 increased ovarian secretion of progesterone at 16.00hr, although the responses were lower.These data suggest that the exogenous progesterone given on the morning of proestrus not only advances the release of both LH and prolactin but also sensitizes the ovarian steroidogenic response to LH, which results in “supersecretion” of progesterone in the proestrous afternoon and larger output of progesterone in the diestrus stage. Endogenous progestins hypersecreted in the previous cycle slightly suppress the ovarian steroidogenic response, but neither the ovulatory response to gonadotropins nor the release of pituitary LH and prolactin is altered.