Active immunization of ewes against inhibin (IMM) consistently increases ovulation rate but this response is not always accompanied by the expected 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, independent 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 > or = 3 mm were recovered to determine intrafollicular levels of inhibin A, activin A and follistatin by specific two-site immunoassay and oestradiol and testosterone by radioimmunoassay. Blood samples were collected to assess plasma FSH, oestradiol and inhibin antibody titres. Although plasma FSH levels were similar in IMM and CON ewes, IMM ewes had approximately 3-fold more follicles > or = 3 mm (P < 0.0001) and approximately 3-fold more oestrogenic follicle (P < 0.001) than CON ewes. Compared with CON ewes, follicles from IMM ewes had much higher concentrations of activin A (approximately 6-fold; P < 0.001) and inhibin A (approximately 3-fold; P < 0.001) but only slightly more follistatin (approximately 1.4-fold; not significant). The activin A:follistatin ratio in follicles from IMM ewes (approximately 1:1) was significantly higher (P < 0.001) than in follicles from CON ewes (approximately 0.3:1). Levels of inhibin antibody measured in follicular fluid (FF) from IMM ewes were similar to plasma levels. Given that activin A has been shown previously to up-regulate FSH receptors and aromatase activity in rat granulosa cells, the increase in intrafollicular activin A, unaccompanied by a rise in the concentration of its binding protein (follistatin), could explain how long-term IMM enhances follicle development and ovulation rate without necessarily promoting a sustained increase in FSH secretion.

This article has been cited by other articles:

				A. R Kayani, C. Glister, and P. G Knight Evidence for an inhibitory role of bone morphogenetic protein(s) in the follicular-luteal transition in cattle Reproduction, January 1, 2009; 137(1): 67 - 78. [Abstract] [Full Text] [PDF]

				T. M Lovell, S. L Al-Musawi, R. T Gladwell, and P. G Knight Gonadotrophins modulate hormone secretion and steady-state mRNA levels for activin receptors (type I, IIA, IIB) and inhibin co-receptor (betaglycan) in granulosa and theca cells from chicken prehierarchical and preovulatory follicles Reproduction, June 1, 2007; 133(6): 1159 - 1168. [Abstract] [Full Text] [PDF]

				C. Glister, N. P Groome, and P. G Knight Bovine follicle development is associated with divergent changes in activin-A, inhibin-A and follistatin and the relative abundance of different follistatin isoforms in follicular fluid J. Endocrinol., February 1, 2006; 188(2): 215 - 225. [Abstract] [Full Text] [PDF]

				M S Medan, S Akagi, H Kaneko, G Watanabe, C G Tsonis, and K Taya Effects of re-immunization of heifers against inhibin on hormonal profiles and ovulation rate Reproduction, October 1, 2004; 128(4): 475 - 482. [Abstract] [Full Text] [PDF]

				C. Glister, C F. Kemp, and P. G Knight Bone morphogenetic protein (BMP) ligands and receptors in bovine ovarian follicle cells: actions of BMP-4, -6 and -7 on granulosa cells and differential modulation of Smad-1 phosphorylation by follistatin Reproduction, February 1, 2004; 127(2): 239 - 254. [Abstract] [Full Text] [PDF]

				F. Jimenez-Krassel, M. E. Winn, D. Burns, J. L. H. Ireland, and J. J. Ireland Evidence for a Negative Intrafollicular Role for Inhibin in Regulation of Estradiol Production by Granulosa Cells Endocrinology, May 1, 2003; 144(5): 1876 - 1886. [Abstract] [Full Text] [PDF]

				C. Glister, N. P. Groome, and P. G. Knight Oocyte-Mediated Suppression of Follicle-Stimulating Hormone- and Insulin-Like Growth Factor-Induced Secretion of Steroids and Inhibin-Related Proteins by Bovine Granulosa Cells In Vitro: Possible Role of Transforming Growth Factor {alpha} Biol Reprod, March 1, 2003; 68(3): 758 - 765. [Abstract] [Full Text] [PDF]

				C. Glister, D. S. Tannetta, N. P. Groome, and P. G. Knight Interactions Between Follicle-Stimulating Hormone and Growth Factors in Modulating Secretion of Steroids and Inhibin-Related Peptides by Nonluteinized Bovine Granulosa Cells Biol Reprod, October 1, 2001; 65(4): 1020 - 1028. [Abstract] [Full Text] [PDF]

				T. M. Lovell, P. G. Knight, N. P. Groome, and R. T. Gladwell Changes in Plasma Inhibin A Levels During Sexual Maturation in the Female Chicken and the Effects of Active Immunization Against Inhibin {{alpha}}-Subunit on Reproductive Hormone Profiles and Ovarian Function Biol Reprod, January 1, 2001; 64(1): 188 - 196. [Abstract] [Full Text]

				G. W. Montgomery, D. L. Duffy, J. Hall, B. R. Haddon, M. Kudo, E. A. Mcgee, J. S. Palmer, A. J. Hsueh, D. I. Boomsma, and N. G. Martin Dizygotic Twinning Is Not Linked to Variation at the {alpha}-Inhibin Locus on Human Chromosome 2 J. Clin. Endocrinol. Metab., September 1, 2000; 85(9): 3391 - 3395. [Abstract] [Full Text]