Interrelationships between ovarian follicles grown in culture and possible mediators.

Dominance or cooperation between ovarian follicles can determine the number of ovulations and fecundity, but interrelationships between follicles in mono- and poly-ovulatory species and their mechanisms are poorly understood. The goals of this work were to determine the existence and compare the character of mutual influence of cultured ovarian follicles from a mono-ovulatory species (cow) with established follicular dominance with those from a poly-ovulatory species (pig), in which interrelationship between follicles remain unknown, and to examine the role of ovarian cell proliferation, the insulin-like growth factor I (IGF-I)- oxytocin (OT) system, and steroid hormones in mediating interrelationships among ovarian follicles. Bovine and porcine ovarian follicles were isolated and cultured alone and in pairs, and the percentage of growing follicles was calculated. Porcine follicles were cultured alone and in pairs after addition of exogenous OT and IGF-I (100ngmL-1) or inactivation of endogenous OT and IGF-I by antisera against these hormones (1%). Proliferation of porcine follicular cells was assessed by SDS PAGE-Western immunoblotting, the release of IGF-I, progesterone, androstenedione and estradiol by cultured porcine ovarian follicles was analyzed by RIA/EIA. Overall, our observations suggest (1) competition/dominance (mutual suppression of growth) in bovine ovarian follicles, (2) cooperation (mutual support of growth) in porcine ovarian follicles, (3) that this mutual growth of porcine ovarian follicles was caused by the promotion of cell proliferation, (4) that this mechanism was probably not involved in bovine follicular dominance, (5) that communication between both porcine and bovine follicles affects their secretory activity, and (6) that both follicular dominance in cows and cooperation of follicles in pigs can be mediated by either down- or up-regulation of the IGF-I-OT system, which in turn affects follicular steroidogenesis and promotes follicular cell proliferation and follicular growth.

Oxytocin mediates some effects of insulin-like growth factor-I on porcine ovarian follicles.

The aims of the present study were (1) to investigate the influence of insulin-like growth factor-I (IGF-I) on follicular size, on the secretion of oxytocin (OT), progesterone (P), estradiol (E), IGF binding protein-3 (IGFBP-3), inhibin A, inhibin B and cAMP and on the expression of proliferation-associated peptide PCNA, ERK-related mitogen activated protein kinase (MAPK/ERK1, 2) and protein kinase A (PKA) in cultured porcine ovarian follicles; (2) to examine the effects of OT on IGF-I and on these functions; and (3) to determine whether the effects of IGF-I can be mediated by OT. To define the involvement of OT in mediating IGF-I action, we compared responses of porcine ovarian follicles to IGF-I and OT and examined whether blockade of endogenous OT by specific antiserum can affect IGF-I action. It was observed that IGF-I (1, 10 or 100 ng/ml) was able to prevent a decrease in the size of ovarian follicles during culture and caused an increase in the diameter of some follicles. It also stimulated the secretion of OT, P, IGFBP-3, inhibin A and cAMP, decreased the secretion of E and inhibin B (RIA/EIA/ELISA), and induced the expression of PCNA, PKA, MAPK/ERK1, but not MAPK/ERK2 (Western blotting). Like IGF-1, OT (100 ng/ml) prevented decrease in follicular size and increased the diameter of some follicles. It also stimulated the secretion of P and IGF-I, but not E. Antiserum against OT (1%), when given alone, did not affect the reduction of follicular size but slightly increased the percentage of follicles increasing their diameter during culture. The antiserum also inhibited secretion of OT and cAMP but not the secretion of P, E, IGFBP-3 or the expression of PKA, MAPK/ERK1 or 2. When given together with IGF-I, the antiserum prevented the stimulatory action of IGF-I on the proportion of enlarged follicles and on OT, IGFBP-3 and MAPK/ERK1. It augmented the effect of IGF-I on P, but not the effect on E, cAMP, PKA or MAPK/ERK2. These observations demonstrate the involvement of IGF-I and OT in the control of ovarian follicular size and follicular cell proliferation, progestagen, estrogen, IGFBP-3, inhibin A and B secretion and in cAMP/PKA- and MAPK/ERK1-dependent intracellular mechanisms. Furthermore, the reciprocal stimulation of IGF-I and OT and the similarity of some their effects, together with the prevention or augmentation of some IGF-I effects after OT blockade, suggest that IGF-I action can be mediated by OT.