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

Summary.: To determine the mechanism by which oestrogen induces luteolysis, cyclic guinea-pigs were given a single subcutaneous injection of 10µg of oestradiol cyclopentylpropionate (OECP) on Day 1 and daily subcutaneous injections of various gonadotrophins on Days 1 to 9. The mean diameter of the corpora lutea and their histology were used as end points in assessing luteal status.

Ovine LH, prolactin, or a combination of both did not reverse the effect of OECP. Daily treatment with 200 µg of ovine FSH in normal saline partially counteracted the luteolytic effect of OECP on the size and morphology of the corpora lutea. A combination of 100 µg of FSH and 1 mg of prolactin or 10 µg of LH was less effective than 200 µg of FSH. Treatment with 10 or 25 i.u. of PMSG in saline or 200 µg of FSH in 5% beeswax-sesame oil suspension counteracted the luteolytic effect of OECP in the majority of animals. The corpora lutea regressed when the initial injection of PMSG was deferred until Day 4.

The results indicate that oestrogen induces luteal regression in the cyclic guinea-pig by inhibiting the pituitary secretion of FSH which is apparently necessary—at least in part—for the normal maintenance of corpora lutea.