EFFECTS OF ACTIVE IMMUNIZATION OF SHEEP AGAINST AN AMINO-TERMINAL PEPTIDE OF THE INHIBIN ALPHA(C) SUBUNIT ON INTRAFOLLICULAR LEVELS OF ACTIVIN-A, INHIBIN-A AND FOLLISTATIN

Active immunization of ewes against inhibin (IMM) consistently increas es ovulation rate but this response is not always accompanied by the e xpected rise in plasma FSH. Inhibin-related molecules also have local auto/paracrine effects within the ovary and the ovulatory response to IMM could be due to neutralization of one of these effects, independen t of changing FSH levels. To investigate this, ovaries were collected from long-term IMM (n=6) and control (CON; n=8) ewes killed 48 h after progestagen withdrawal (late follicular phase) and all follicles grea ter than or equal to 3 mm were recovered to determine intrafollicular levels of inhibin A, activin A and follistatin by specific two-site im munoassay and oestradiol and testosterone by radioimmunoassay. Blood s amples were collected to assess plasma FSH, oestradiol and inhibin ant ibody titres. Although plasma FSH levels were similar in IMM and CON e wes, IMM ewes had similar to 3-fold more follicles greater than or equ al to 3mm (P<0.0001) and similar to 3-fold more oestrogenic follicles (P<0.001) than CON ewes. Compared with CON ewes, follicles from LMM ew es had much higher concentrations of activin A (similar to 6-fold; P<0 .001) and inhibin A (similar to 3-fold; P<0.001) but only slightly mor e follistatin (similar to 14-fold; not significant). The activin A:fol listatin ratio in follicles from IMM ewes (similar to 1:1) was signifi cantly higher (P<0.001) than in follicles from CON ewes (similar to 0. 3:1). Levels of inhibin antibody measured in follicular fluid (FF) fro m IMM ewes were similar to plasma levels. Given that activin A has bee n shown previously to up-regulate FSH receptors and aromatase activity in rat granulosa cells, the increase in intrafollicular activin A, un accompanied by a rise in the concentration of its binding protein (fol listatin), could explain how long-term IMM enhances follicle developme nt and ovulation rate without necessarily promoting a sustained increa se in FSH secretion.