LUTEOLYTIC EFFECT OF LH - INHIBITION OF 3-BETA-HYDROXYSTEROID DEHYDROGENASE AND STIMULATION OF 20-ALPHA-HYDROXYSTEROID DEHYDROGENASE LUTEALACTIVITIES 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 pre gnancy was examined. Rat LH antiserum administered intrabursally at 10 00-1100 h on day 19 of pregnancy prevented the physiological decrease in 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD) activity, the incr ease in 20 alpha-hydroxysteroid dehydrogenase (20 alpha-HSD) activity and the fall in serum progesterone (P-4) level observed at 1800 h on d ay 21 of pregnancy. To see ii oLH has a direct effect on luteal steroi dogenesis, the gonadotrophin was injected into the periovarian bursa. The intrabursa treatment with 1 mu g oLH on day 19 of pregnancy at 080 0-0900 h did not modify corpus luteal function 36 h after treatment, b ut treatment with 4 mu g oLH per ovary induced a significant progressi ve decrease in luteal 3 beta-HSD activity scarring 12 h alter treatmen t, while a significant increase in 20 alpha-HSD activity, concomitant with a decrease in serum P-4 level, occurred 48 h after treatment. Lut eal P-4 content decreased with respect to control groups 36 and 48 h a fter intrabursal treatment with 4 mu g oLH. The intrabursal administra tion 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 in crease in serum P-4 levels without modifying 3 beta-HSD activity. In r ats 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-HS D activity and serum P-4 level. In conclusion, the luteal enzymatic ac tivity changes and the significant decrease in the intraluteal P-4 con centration induced by the intrabursal administration of oLH and the cl ear effect of AS-rLH preventing the physiological luteal changes prece ding parturition provide good evidence oi an intra-ovarian action oi L H during the normal progression of luteolysis in late pregnant rats.