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

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 o f the oocyte. Follicular rupture is most likely caused, at least in part, b y decreased tensile strength at the follicular apex due to degradation of c ollagen fibres of the extracellular matrix. It has been debated whether cha nges in intrafollicular pressure occur during the ovulatory process and whe ther 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 preovulator y 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 propor tionality between electrical resistance and pressure within the tip of an i nserted micropipette. The basal intrafollicular pressure was 16.6 +/- 1.0 m m Hg at the preovulatory phase (48 h after eCG) and increased gradually thr oughout 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 ovulator y phase; significantly higher (P < 0.01) than the preovulatory phase). Shor t-term peaks of increased pressure, possibly representing contractility, we re not detected in follicles of the preovulatory phase, but were seen in mo st follicles of the mid- and late ovulatory phases. The mean amplitude of t he short-term pressure increases was 12.3 <plus/minus> 3.2 mm Hg and the in creases occurred at intervals of 24.7 +/- 3.6 s. These short-term increment s in intrafollicular pressure were still present after hysterectomy had bee n performed. The wall tension index was calculated by measuring the follicu lar size and estimating the thickness of: the follicle wall. The index incr eased from 93.9 +/- 13.3 at the preovulatory phase to 207.3 +/- 47.7 (mid-o vulatory phase) and to significantly higher values at the late ovulatory ph ase (320.9 +/- 33.5). In conclusion, this study shows that there is an incr ease in intrafollicular pressure in the ovulating follicle of the rat ovary during the late stages of the ovulatory process, and that short-term incre ases in intrafollicular pressure occur during the late phase of the ovulato ry process. These changes in pressure may be essential for follicular ruptu re to proceed normally.