Differential Dynamic Responses of Plasma LH and FSH to Ovariectomy and to a Single Injection of Estrogen in the Rat1

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Abstract
Comparative studies were made in the rat of the changes in plasma LH and FSH levels following ovariectomy (OVX) and the injection of a single dose of estradiol benzoate (Eb) to 48-hr OVX rats vs 3-week OVX rats. Serial bleedings after OVX revealed that plasma FSH rose at a much faster rate than LH (FSH elevated by 24 hr vs no significant changes in LH at 3 days post-OVX). A single subcutaneous injection of Eb (1 μg/100 g body wt) in the 48-hr OVX rat induced a clear ovulatory surge of LH 30 hr later but no change in FSH. A dose of Eb 5 times greater induced a still larger increment in LH 30 hr later, reaching levels comparable to those in chronic OVX rats. Twice-daily bleedings following this dose of Eb revealed low plasma LH levels in the mornings and high LH values in the afternoons of days 1–3 post-Eb, confirming a 24-hr rhythmicity in the LH-release mechanism. At the same time, a transient negative feedbackaction of the estrogen was observed in FSH. In 3-week OVX rats similar treatment with estrogen induced only inhibitory or negative feedback actions on both LH and FSH secretion. The contrast in LH and FSH secretion observed following OVX as well as the differential effect of a single injection of estrogen in the 48-hr OVX rat makes this animal a suitable model to study mechanisms controlling release of these gonadotropins. (Endocrinology94: 987, 1974)

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