Inhibin increases in the ovaries of female rats approaching first ovulation: Relationships with follicle growth and serum FSH concentrations

In order to relate various prepubertal events in a group of 95 late prepubertal female rats, the following data were obtained during the last 10 days before the day of first ovulation: (1) amounts of ovarian inhibin-like activity (ILA) in some animals (n = 47); (2) size and numbers of healthy (antral) follicles with a volume ≥ 100 x 10⁵ μm³ (or diameter ≥ 260 μm) present per ovary in their litter-mates (n = 48); (3) serum FSH concentrations in both groups. Rats were unilaterally ovariectomized to obtain an ovary for either estimation of ILA content or for histological procedures and counting of follicles. At the time of unilateral ovariectomy they were bled to obtain serum for estimation of FSH concentrations. Rats were kept until the day after the day of first ovulation to determine the time-interval between the day of unilateral ovariectomy and first ovulation. They were studied between 10 and 1 days (days -10 to -1, maturational age) before first ovulation. In addition, adult cyclic rats were bilaterally ovariectomized on different days of the oestrous cycle for estimation of ovarian ILA content. The amount of ovarian ILA was estimated in steroid-free ovarian cytosols using an in-vitro bioassay system with dispersed anterior pituitary cells and subsequent measurement of FSH and LH in the spent medium. The amount of ovarian ILA was about 83 units/ovary from days -10 to -5, and subsequently increased (P < 0.005) to reach a maximum on day -1, the day of pro-oestrus (213 units/ovary). Inhibin-like activity in adult rat ovaries at pro-oestrus amounted to 374 units/ovary. A significant relationship was found between ovarian ILA content and total volume of follicles of classes III-V (≥ 350 x 10⁵ μm³) (r = 0.9683, P < 0.005) except for the period between days -7 and -5 when this volume increased earlier than did the ILA content. The total volume of all follicles ≥ 100 x 10⁵ μm³ was steady from days -10 to -7. On day -6 this volume increased, mainly as a result of an increase of total volume of class II follicles. Thereafter, the total volume of follicles in classes III-V started to increase and was maximal on day -1, while the total volume of follicles in classes I plus II decreased and reached a minimum on day -1. The serum FSH concentration declined between days -10 and -1 from 400 to 100 μg/l (P < 0.001); the presence of follicles of classes III-V was always associated with FSH concentrations ≤200 μg/l (P < 0.005). The presence of class I and II follicles was not related to FSH concentrations. This suggested that mainly follicles of classes III-V contribute to ovarian ILA. The present data show that in immature rats ovarian ILA content increases towards the day of first pro-estrus, as it does later during pro-oestrus in adult cyclic rats. Inhibin-like activity seems to be produced mainly by follicles of classes III-V which are present in the ovaries during the last 5 days preceding first ovulation. In this time same period FSH concentrations are kept within narrow limits (<200 μg/l) as is the case during the adult cycle. Thus, ILA probably plays a role in the fine regulation of FSH secretion.