THE EFFECT OF ACUTE IMMUNO-NEUTRALIZATION OF INHIBIN IN EWES DURING THE EARLY LUTEAL-PHASE OF THE ESTROUS-CYCLE ON OVARIAN HORMONE-SECRETION AND FOLLICULAR DEVELOPMENT

Ewes with ovarian autotransplants received either inhibin antiserum (1 0 mi i.v. raised in sheep against recombinant 32 kDa human inhibin; n= 6) or sheep serum (10 mi i.v.; n=5) on day 3 of the luteal phase with additional daily injections (1 mi i.v.) from 48 h after the initial bo lus until day 13. Jugular and ovarian venous blood samples were taken LC-hourly over days 2-13 of the luteal phase. Blood samples were also taken at more frequent intervals (every 10-15 min for 2-3 h) to examin e pulsatile secretory responses from the ovary to endogenous and gonad otrophin-releasing hormone-induced (150 ng i.m.) LH pulses on days 4, 6, 8, 10 and 12 of the luteal phase. Plasma FSH levels, ovarian steroi d secretion and ovarian follicular development were measured. The ovar ian follicle population was estimated daily by real time ultrasound sc anning. Immunisation against inhibin resulted in a 3- to 4-fold increa se (P<0.001) in plasma FSH levels within 8 h with levels remaining ele vated over controls for 6-7 days. Within 24 h of immunisation there wa s an increase in the number of small ovarian follicles (P<0.05) and by 3 days after treatment immunised ewes had 4-6 large ovarian follicles /ewe with this increase in the total number of large follicles being m aintained for the rest of the experimental period (P<0.05). Mean ovari an oestradiol secretion during intensive bleeds was not different from controls 24 h after immunisation, but by 3 days after immunisation it was elevated 4- to 5-fold (P<0.001) over controls with this increase being maintained throughout the experiment. Similar responses to immun isation against inhibin in androstenedione secretion were observed alt hough mean androstenedione secretion was not elevated until 7 days aft er treatment. In vitro antibody titres in immunised ewes remained elev ated but declined steadily (P<0.001) over the experimental period. We conclude that the initial stimulation of follicle development and ovar ian steroid secretion following passive immunisation against inhibin c an be attributed to increased blood FSH. However, the fact that with t ime FSH declined but increased follicle development was sustained, des pite maintenance of high circulating antibody titres, suggests that on a longer term basis inhibin immunisation may stimulate ovarian functi on by interfering with the modulation of follicle development by inhib in at an ovarian level.