Luteolytic effect of LH: Inhibition of 3 beta-hydroxysteroid dehydrogenase and stimulation of 20 alpha-hydroxysteroid dehydrogenase luteal activities in late pregnant rats

The mechanisms associated with the onset of luteolysis in the pregnant rat are not well known. The effect of a specific rat LH antiserum (AS-rLH) and of ovine LH (oLH) on luteal steroidogenesis on day 19 of pregnancy was examined. Rat LH antiserum administered intrabursally at 1000-1100 h on day 19 of pregnancy prevented the physiological decrease in 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD) activity, the increase in 20 alpha-hydroxysteroid dehydrogenase (20 alpha-HSD) activity and the fall in serum progesterone (P-4) level observed at 1800 h on day 21 of pregnancy. To see ii oLH has a direct effect on luteal steroidogenesis, the gonadotrophin was injected into the periovarian bursa. The intrabursa treatment with 1 mu g oLH on day 19 of pregnancy at 0800-0900 h did not modify corpus luteal function 36 h after treatment, but treatment with 4 mu g oLH per ovary induced a significant progressive decrease in luteal 3 beta-HSD activity scarring 12 h alter treatment, while a significant increase in 20 alpha-HSD activity, concomitant with a decrease in serum P-4 level, occurred 48 h after treatment. Luteal P-4 content decreased with respect to control groups 36 and 48 h after intrabursal treatment with 4 mu g oLH. The intrabursal administration of 8 mu g oLH induced an increase in 20 alpha-HSD activity and a decrease ill 3 beta-HSD activity 36 h after treatment. Administration of 4 mu g oLH per ovary on day 8 oi pregnancy induced a significant increase in serum P-4 levels without modifying 3 beta-HSD activity. In rats treated with oLH on day 19 of pregnancy the decrease in 3 beta-HSD activity occurred 36 h before the significant increase in 20 alpha-HSD activity and serum P-4 level. In conclusion, the luteal enzymatic activity changes and the significant decrease in the intraluteal P-4 concentration induced by the intrabursal administration of oLH and the clear effect of AS-rLH preventing the physiological luteal changes preceding parturition provide good evidence oi an intra-ovarian action oi LH during the normal progression of luteolysis in late pregnant rats.