Novel method for intrafollicular pressure measurements in the rat ovary: increased intrafollicular pressure after hCG stimulation

Abstract

The ovulatory process in the rat comprises a period of about 12-15 h, from the time of the preovulatory LH surge to follicular rupture and extrusion of the oocyte. Follicular rupture is most likely caused, at least in part, by decreased tensile strength at the follicular apex due to degradation of collagen fibres of the extracellular matrix. It has been debated whether changes in intrafollicular pressure occur during the ovulatory process and whether such changes facilitate rupture of the follicle. In the present study, rats were primed with equine chorionic gonadotrophin (eCG, 10 iu) followed by hCG (10 iu) 48 h later. The intrafollicular pressure in the preovulatory follicle was recorded during 1 h at distinct time phases of the ovulatory process by use of an active servo-null pressure system based on the proportionality between electrical resistance and pressure within the tip of an inserted micropipette. The basal intrafollicular pressure was 16.6 +/- 1.0 mm Hg at the preovulatory phase (48 h after eCG) and increased gradually throughout the ovulatory process to 21.4 +/- 2.4 mm Hg at 4-7 h after hCG (mid-ovulatory phase) and 23.9 +/- 1.9 mm Hg at 8-12 h after hCG (late ovulatory phase; significantly higher (P < 0.01) than the preovulatory phase). Short-term peaks of increased pressure, possibly representing contractility, were not detected in follicles of the preovulatory phase, but were seen in most follicles of the mid- and late ovulatory phases. The mean amplitude of the short-term pressure increases was 12.3 +/- 3.2 mm Hg and the increases occurred at intervals of 24.7 +/- 3.6 s. These short-term increments in intrafollicular pressure were still present after hysterectomy had been performed. The wall tension index was calculated by measuring the follicular size and estimating the thickness of the follicle wall. The index increased from 93.9 +/- 13.3 at the preovulatory phase to 207.3 +/- 47.7 (mid-ovulatory phase) and to significantly higher values at the late ovulatory phase (320.9 +/- 33.5). In conclusion, this study shows that there is an increase in intrafollicular pressure in the ovulating follicle of the rat ovary during the late stages of the ovulatory process, and that short-term increases in intrafollicular pressure occur during the late phase of the ovulatory process. These changes in pressure may be essential for follicular rupture to proceed normally.

The ovarian stroma as a new frontier

“…This process is further amplified by the release of tumor necrosis factor-alpha (TNF-a) from the degraded ECM to promote collagenase production and apoptosis of ovarian epithelial cells (Curry & Smith 2006). The weakened cellular and ECM components at the apical region, along with pressure from the follicular fluid and increased vascular pressure, facilitate follicular rupture and expulsion of the oocyte into the periovarian space (Matousek et al 2001).…”

Loss of Function of Endothelin-2 Leads to Reduced Ovulation and CL Formation

“…Importantly, systemic administration of tezosentan prior to ovulation inhibits follicle rupture in superovulation-induced rats and mice [13], [22]. It has been previously proposed that follicular or ovarian contraction, which EDN2 may induce, serves as an ovulatory trigger for a follicle to rupture [23]. This idea is given credence by the presence of a smooth muscle network in the theca-interstitial layer of the ovary.…”

Production and binding of endothelin-2 (EDN2) in the rat ovary: endothelin receptor subtype A (EDNRA)-mediated contraction

“…Using rats as the animal model, Matousek et al (2001) applied new techniques to the paradigm of changing intra-follicular pressure during the ovulatory cascade and we proposed that endothelin-2 (EDN2) was the final contractile signal facilitating ovulation (Ko et al 2006). Evidence suggests that these two seemingly intertwined processes are, however, independent factors within the ovulatory cascade.…”

“…Evidence suggests that these two seemingly intertwined processes are, however, independent factors within the ovulatory cascade. Changes in intra-follicular pressure are observed as early as midway through the periovulatory cascade (Matousek et al 2001), coincident with increases in factors affecting vascular permeability (Gómez et al 2003;Kitajima et al 2004;Miyabayashi et al 2005) and the array of proteolytic enzymes known to be integral to the breakdown of the extracellular matrix before ovulation (reviewed in Curry and Smith 2006). In contrast, expression of mRNA for Edn2 is only increased very late in the periovulatory period, ∼12 h after the induction of ovulation with hCG in the rat (Ko et al 2006), well after the first observed changes in intra-follicular pressure.…”