Ovulation. Studies on intra-ovarian mediators influencing ovulation in the rat ovary

Abstract

Ovulation is the process whereby the preovulatory follicle ruptures and the oocyte is transported from the interior of the follicle onto the surface of the ovary and later into the Fallopian tube. This process is initiated by the preovulatory surge of luteinizing hormone (LH) and there now exist ample evidence that tissue remodeling processes and vascular changes in and around the follicle are important for this to take place. A network of several intra-ovarian mediators, induces these changes in a highly coordinated fashion. In the present study, the participation of a number of mediator systems in this ovulatory process in the rat ovary was investigated. The aims of the present thesis were in particular to investigate the presence and the participation of the eicosanoid system, the nitric oxide synthase (NOS) system, angiotensin II (ANG II), and a general inflammatory reaction of the ovary in ovulation. The contribution of lipoxygenase derivatives in ovulation was tested by the use of the lipoxygenase inhibitor NDGA. Both during in vivo and in vitro perfusion conditions, it was found that NDGA effectively inhibited ovulation and intra-ovarian leukotriene (LT) levels. The inhibition of ovulation was reversed by addition of LTB4, suggesting a key-role for this particular LT. Prostaglandins (PGs) are produced by the action of prostaglandin G/H endoperoxide synthase (PGS) enzymes, which exist in a constitutively expressed form (PGS-1) and an inducible form (PGS-2). The contribution of PGS-2-derived PGs in the ovulatory process was investigated both in vivo and in vitro in the rat by the use of the selective inhibitor NS398. This PGS-2 inhibitor dose-dependently and effectively depressed PG levels. Interestingly, the dose response curves concerning ovulation inhibition and PG inhibition indicated that there is not a parallel inhibition of these two entities but rather that there may exist a threshold level of PGs, which is determining if ovulation is optimally induced or not. Since a great number of inflammatory mediators have been implicated in ovulation, the effect of glucocorticoid receptor activation on ovulation was assessed in vitro. Dexamethasone did not affect ovulation rate in the in vitro perfused ovary in spite of effectively inhibiting both PG synthesis and plasminogen activator activity (PA) in the ovary. Two out of the three existing isoforms of NOS were localized in the rat ovary during the peri-ovulatory period. Endothelial NOS (eNOS) was found mostly in connection with the vasculature of the stroma and in the theca layer of the large follicles, but also inside the corpus luteum. Inducible NOS (iNOS) was detected in the same compartments, but was also demonstrated in granulosa cells during the ovulatory process. To evaluate the role of NO in ovulation and the possible contribution from iNOS and eNOS to the ovulatory response, experiments were carried out with the in vitro perfused rat ovary model. It was found that an unselective NOS inhibitor suppressed ovulation rate, but this effect was not seen by the more iNOS specific inhibitor. A surplus of NO, accomplished by addition of NO donor, did not change the LH-induced ovulatory response, indicating that a normal LH activation is sufficient for a full ovulatory response.A complete renin-angiotensin system has been localized to the ovary and ANG II may have an effect in ovulation by effects on either or both of the type 1 (AT1) and type 2 (AT2) receptors. In experiments it was found that the non-selective ANG II antagonists, saralasin, effectively inhibited LH-induced ovulation in vitro but that a specific antagonist to the AT2 receptor was ineffective. Saralasin also reduced PG levels. In conclusion, the study has shown that PG, LTs, ANG II and NO are active participants in the intra-ovarian ovulatory mechanisms.