LUTEOLYSIS INDUCED BY A PROSTAGLANDIN-F2-ALPHA ANALOG OCCURS INDEPENDENTLY OF PROLACTIN IN THE RAT

The hypothesis that prolactin exerts a stimulatory dominance over the luteolytic effect of prostaglandin (FG) F-2 alpha on corpus luteum mai ntenance and progesterone production was experimentally tested. A dose -dependent effect of the stable PGF2 alpha analogue cloprostenol (dose range 200 ng-5 (mu g) was found 12 h after s,c, injection, in Day 9 a dult pseudopregnant rats: 1) LH receptor mRNA levels, as measured by R Nase protection assay, were dramatically decreased (by 67%) by a singl e s.c, dose of 200 ng cloprostenol; and 2) serum progesterone levels w ere significantly (p < 0.05) decreased (by 43%) whereas 20 alpha-dihyd roprogesterone significantly (p < 0.05) increased (by 80%) initially a t a 0,5-mu g dose of cloprostenol. To study the integrated response to prolactin and PCF2 alpha, we investigated the effect of cloprostenol treatment in sterile-mated female rats with or without circulating pro lactin. Prolactin secretion was inhibited by s,c. injection of bromocr iptine (1 mg) in the morning of the ninth day of pseudopregnancy, A gr oup of rats was left prolactin-depleted; in another group prolactin wa s reintroduced by adding 8 IU ovine prolactin. It was found that after injection of 0.5 mu g cloprostenol the LN receptor mRNA levels and th e serum progesterone/20 alpha-dihydroprogesterone ratio were not signi ficantly different whether the rats had circulating endogenous/exogeno us prolactin or were prolactin-depleted, Therefore, although prolactin exerts a stimulatory influence on both progesterone production and co rpus luteum LH receptor gene expression, the conclusion is reached tha t prolactin alone cannot antagonize the luteolytic effect of PGF(2 alp ha).