Changes in pituitary and plasma LH concentrations after ovariectomy alone or in combination with single injections of various dosages of estrogen were evaluated. Investigations were conducted in immature and adult normal and sterile female rats to study changes with age and differences in thresholds of response to estrogen on LH synthesis vs. release. In immature rats (30 days of age) plasma LH was not detectable. Ovariectomy at 20 days of age, with autopsy 10 days later, resulted in an increase in plasma LH to 11.3 μg/100 ml although pituitary LH concentrations remained unaltered. Injection of 0.05 μg estradiol benzoate (EB)/100 g body wt 7 days after ovariectomy (day 27 of age) failed to alter significantly pituitary and plasma LH concentrations. With the injection of 0.1 fig EB/100 g body wt, a significant increase in pituitary LH occurred, while plasma LH concentrations were unaffected. Estrogen injected as 0.5 μg/100 g body wt inhibited LH release although pituitary LH synthesis was not suppressed and concentration increased to 6.89 μg/mg (intact control 2.45 μg/mg). In intact sterile rats at 30 days of age, pituitary LH values were low (0.89 μg/nag) and plasma concentrations remarkably high (17.5 μg/100 ml). Ten days after ovariectomy, plasma LH decreased by 1/2 to 8.6 μg/100 ml but pituitary LH failed to increase in concentration. Injection of 0.05 μg EB/100 g body wt doubled pituitary LH concentrations without altering plasma LH values, and when 0.5 μg EB was injected plasma LH decreased to undetectable levels. In adult normal, ovariectomized rats, changes in plasma and pituitary LH synthesis and release occurred similar to those in the normal immature rat. After 0.2 μg EB/100 g body wt, plasma LH decreased to nondetectable levels, while pituitary concentration increased from a control value of 2.85 to 7.8 μg/mg. In the adult sterile rat, patterns of pituitary and plasma LH concentrations closely paralleled the changes observed in the immature. However, when compared to the adult normal rat 5× the estrogen dosage was required to inhibit release and at this dosage pituitary LH synthesis also was inhibited. A hypothesis is presented on the possible positive and negative feedback effects that estrogen may exert on LH synthesis, storage and release. (Endocrinology86: 542, 1970)
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Barraclough et al. (1970) studied this question.