REVERSAL BY GONADOTROPHINS OF THE LUTEOLYTIC EFFECT OF OESTROGEN IN THE CYCLIC GUINEA-PIG

Reversal by gonadotropins of the luteolytic effect of estrogen in the cyclic guinea-pig was investigated in 83 virgin animals. A single s ubcutaneous injection of 10 mcg of estradiol cyclopentylpropionate (OECP ) was administered on Day 1 and daily subcutaneous injections of various gonadotropins on Days 1-9. Mean diameter of the corpora lutea and their histology were used as end points in assessing luteal status. OECP caused a significant (p less than .0005) reduction in the size of the corpora lutea which were seen to be in an advanced stage of regression characterized by cyloplasmic vacuolization in luteal cells a pycnotic appearance of the nuclei absence of mitotic figures and loss of discrete cell boundaries. 1 of 4 OECP-treated animals ovulated prematurely. Ovine luteinizing hormone (LH) prolactin or a combination of both did not reverse the effect of OECP. 200 mcg of ovine follicle stimulating hormone (FSH)/day partially counteracted the luteolytic effect of OECP on the size and morphology of the corpora lutea. 100 mcg of FSH plus 1 mg of prolactin or 10 mcg of LH was less effective than 200 mcg of FHS. 10 or 25 IU of pregnant mare serum globulin or 200 mcg of FSH in 5% beeswax-sesame oil counteracted the luteolytic effect of OECP in the majority of animals. These results indicate that estrogen induces luteal regression in the cyclic guinea-pig by inhibiting the pituitary secretion of FSH which is apparently necessary for the normal maintenance of corpora lutea.