IMPAIRMENT IN FOLLICLE-STIMULATING-HORMONE SECRETION BY DIFFERENT TREATMENTS WITH FOLLICULAR-FLUID AS A SOURCE OF INHIBIN BLOCKS THE ABILITY OF PULSATILE LUTEINIZING-HORMONE TO INDUCE PREOVULATORY SURGES OF GONADOTROPINS IN PREPUBERTAL EWE LAMBS

This study was conducted to investigate whether FSH can affect the abi lity of pulsatile exogenous LH to induce a preovulatory surge of gonad otropins in prepubertal ewe lambs. March-born ewe lambs were randomly assigned in November to the following treatments: 1) control; 2) 1.5 m l charcoal-extracted bovine follicular fluid (bFF) for 240 h; 3) 1.5 m i bFF for 72 h; and 4) 3 mt bFF for 72 h. Follicular fluid was adminis tered as a source of inhibin to reduce secretion of FSH. Starting at 0 h, all animals were given (i.v.) 10 mu g ovine LH every h for a 48-h period. Lambs in the control group did not receive further treatment. Females in group 2 were administered (s.c.) 1.5 mt bFF every 8 h from -184 h to 56 h. In groups 3 and 4, lambs received (s.c.) 1.5 mt bFF an d 3 mi bFF, respectively, every 8 h from -16 h to 56 h. Preovulatory s urges of LH and FSH occurred concomitantly in 4 of 6 control lambs at a mean time of 34 h (range 20-42 h). In contrast, none of the females in groups 2, 3, or 4 showed a preovulatory surge of gonadotropins in r esponse to pulsatile LH (p < 0.05). At -16 h, FSH was reduced in the e we lambs of group 2 (p < 0.01). Thereafter, mean concentrations and ar ea under the curve for FSH were lower (p < 0.001), and variability in the pattern of secretion was reduced (p < 0.05), in lambs of groups 2, 3, and 4 as compared to controls. LH, apart from the preovulatory sur ge, was not affected. Females in groups 2, 3, and 4 secreted less estr adiol than controls (p < 0.01). Mean concentrations and area under the curve for estradiol during the 20 h preceding preovulatory surges (20 h = earliest surge) in controls were greater than those during the 20 h preceding 34 h (34 h = mean time for surges) in lambs that did not show a surge (p < 0.001). The induced luteal phases were of short dura tion, and no animal continued to cycle. Ovarian follicular development was evident in the majority (8 of 10) of the randomly selected lambs examined by laparoscopy during the expected midluteal phase. In summar y, long- and short-term administration of bFF resulted in reduced conc entrations of FSH, preventing the ability of pulsatile LH to induce a preovulatory surge of gonadotropins in prepubertal ewe lambs. The decr ease in estradiol associated with the bFF-induced reduction in FSH may have been the mechanism responsible for the lack of LH and FSH surges . It is proposed that unimpaired secretion of FSH is required for trea tment with LH to induce preovulatory surges of gonadotropins in this m odel.