Intrafollicular pressure and biochemical mediators in ovulation

Abstract

Ovulation is the process whereby the preovulatory follicle ruptures to enable a fertilisable ovum to escape from the interior of the ovary to the oviduct. During the ovulatory process, from he onset of the preovulatory LH-surge to the follicular rupture, marked biochemical and structural changes in follicles to enable it to rupture. The present study has investigated some of the mediator pathways and mechanisms, that may be a part of the ovulatory cascade. It is well-known that the ovarian expression of some cytokines and growth factors are increased in the follicle after the preovulatory LH-surge. Most of these local mediators use protein tyrosine kinase (PTK) activity in the transmission of their signals to alter cellular functions. In an initial study, the role of PTKs in the ovulatory process was investigated. In vitro perfused rat ovaries were stimulated to ovulate by LH, and the ovaries responded by ovulations in vitro, and by an increase in th synthesis of intra-ovarian mediators such as steriods, prostaglandins and plasminogen activator. Presence of two structurally different PTK-inhibitors inhibited ovulation, without affecting the production of the mediators. Thus, the study indicates a role for PTK-activity in the local mechanisms, of the ovulation.An integral part of the ovulation seems to be the degradation of the collagen layers in the apex of the follicle. Responsible for this degradation are the matrix metalloproteinases (MMP)s. In one study the effects of the PTK-inhibitor genistein on gonadotrophin-stimulated perfused rat ovary regarding the expression of the MMPs and tissue inhibitors of the metalloproteinases (TIMP)s were assessed. The expression levels of collagenase-III, and TIMP-1 were increased after gonadotrophin treatment, and the increase of collagenase-III was blocked by genistein. The data indicates that PTK are involved in tissue remodelling of ovulation, by stimulating expression of collagenase-III.Several members of the eicosanoids have been shown to be increased by the preovulatory LH-increase and have thereby been implicated in the ovulatory process. In the present study, the participation of the leukotriene (LT)B4 in rat ovulation was investigated by use of the LTB4-receptor antagonist (L-ANT) administrated either in intrabursally in vivo, or in the in vitro perfused rat ovary. The L-ANT dose-dependently inhibited ovulation in vivo and in vitro, and the conversion of pro-MMP-2 to active MMP-2 was inhibited by the L-ANT. The result suggest a role for LTB4 in the ovulatory process of the rat. It is not clear whether the rupture of the follicle is due to only decrease of the tensile strength of the follicular wall, or also an increased intrafollicular pressure (IFP). In this study, methodology was developed for measurements of IFP in the rat in vivo. With an active micropressure monitoring system, it was found that the IFP at the preovulatory stage was around 17 mmHg, and that the pressure increased after gonadotropin-stimulation to the rat ovary. Around the same time as the increase in the IFP was observed, onset of a short-term increments in IFP were seen, possible representing ovarian contractions. It is suggested that the LH/hCG induced increase of IFP and contractions are needed for the follicular rupture.The mechanism for the regulation of the IFP, was investigated in vivo. It was found that systemic administration of phenylephrine (Phe) decreased the IFP, during late ovulatory phase. Topical administration of a nitric oxide synthase inhibitor NG-monomethyl-L-argine, decreased the IFP during mid and late ovulatory phases. The results suggests that the IFP is dependent on changes in the systemic blood pressure, and that locally acting NO may regulate the IFP. In summary, evidence is provided that a rise in IFP is important for the mechanism that lead to follicular rupture, and that PTK activity, and LTB4 are important in the mechanism for the breakdown of the follicular apex.