The presence and physiological significance of ovarian inhibin-like activity (ILA) in immature rats was studied by measuring ILA in ovarian homogenates and by studying the short-term changes in serum concentrations of FSH and LH after unilateral or bilateral ovariectomy. Inhibin-like activity in ovarian homogenates was estimated using an in-vitro bioassay system with dispersed rat anterior pituitary cells and subsequent measurement of FSH and LH levels in the spent pituitary cell medium. Inhibin-like activity, expressed in units/ovary, was undetectable in 13-day-old ovaries, was present in 18-day-old ovaries (5·8 units/ovary) and rose significantly to a value of 29·1 units/ovary on day 23. Another significant rise was seen between 28 and 33 days of age, with ILA reaching a value of 66·4 units/ovary, a value still well below that found in adult dioestrous ovaries (155·6 units/ovary). In the bioassay no systematic dose-dependent influence on LH secretion was found. Serum concentrations of FSH after unilateral or bilateral ovariectomy at 18 days of age showed no change at 5 h and a significant (P < 0·05) increase compared with sham-operated controls at 24 h after operation. At 23, 28 and 33 days of age significant increases in FSH concentration were seen at 5 and 8 h after both unilateral and bilateral ovariectomy. At 24 h after unilateral ovariectomy, FSH had returned to the control values in these rats, whereas after bilateral ovariectomy a further increase in FSH was seen. Concentrations of LH generally varied widely and, compared with the situation after sham-operation, did not show a systematic trend of change during the 24-h period after unilateral or bilateral ovariectomy.
It was concluded that ILA is present in immature rat ovaries from the age of 18 days and increases, to a still sub-adult value, until at least 33 days of age. A physiological role for ILA, as a short-term regulator of FSH secretion seems to be present from at least 23 days of age and may account for the decreasing FSH concentrations seen during the late-prepubertal period, when ovarian steroids alone cannot explain the low FSH levels measured.
J. Endocr. (1985) 107, 251–257
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