PARTICIPATION OF INTRALUTEAL PROGESTERONE AND PROSTAGLANDIN-F2-ALPHA IN LH-INDUCED LUTEOLYSIS IN PREGNANT RAT

We examined the participation of the intraluteal levels of progesteron e (P-4) and prostaglandin F-2 alpha (PGF(2 alpha)) in the induction of luteolysis by LH and its relationship with the induction of the 20 al pha-hydroxysteroid dehydrogenase activity (20 alpha-HSD). Subcutaneous administration of four doses of 10 mu g ovine LH (oLH) at 0800, 0900, 1000 and 1100 h on day 19 of pregnancy induced a decrease in the acti vity of the enzyme 3 beta-HSD 24 and 48 h after treatment and an incre ase in luteal 20 alpha-HSD activity 48 h after oLH treatment when comp ared with control rats. Intraluteal and serum P-4 levels were lower th an control values 24 and 48 h after oLH treatment, with a significant increase in luteal PGF(2 alpha) content and a decrease in corpus luteu m (CL) weight 48 h after oLH treatment. Intrabursal ovarian (IB) treat ment with an inhibitor of PG's biosynthesis (diclofenac) (70 mu g/ovar y) or P-4 (3 mu g/ovary) on day 20 of pregnancy, prevented the increas e in 20 alpha-HSD activity observed 48 h after oLH treatment, without any effect on 3 beta-HSD activity. The IB administration of P-4 preven ted the increase in intraluteal PGF(2 alpha) content induced by oLH tr eatment and the increases in 20 alpha-HSD activity and intraluteal PGF (2 alpha) content observed in control animals on day 21 of pregnancy. The inhibition of PG biosynthesis also prevents the decrease in intral uteal and serum P-4 level induced by oLH. These results provide good e vidence of the important participation of intraluteal P-4 and PGF(2 al pha) on the oLH-induced luteolysis in pregnant rats. We also found tha t P-4 produced by the CL is involved, in part, in the regulation of lu teal PG synthesis. Thus, the early decline in 3 beta-HSD activity and the consequent fall in intraluteal P-4 content, may trigger the synthe sis of PGs and thereafter the increase in luteal 20 alpha-HSD activity to establish luteolysis.