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: amounts of ovarian inhibin-like activity (ILA) in some animals (n = 47); size and numbers of healthy (antral) follicles with a volume greater than or equal to 100 X 10(5) microns3 (or diameter greater than or equal to 260 microns) present per ovary in their litter-mates (n = 48); 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 less than 0.005) to reach a maximum on day -1, the day of pro-oestrus (213 units/ovary).(ABSTRACT TRUNCATED AT 250 WORDS)