A Highly Sensitive Test for LH-Releasing Activity: The Ovariectomized, Estrogen Progesterone-Blocked Rat1

Abstract

The LH-releasing effect of crude acidic extracts of rat stalk-median eminence tissue was evaluated in chronically ovariectomized rats in which LH secretion had been partially blocked by injection of estrogen and progesterone. Ten min after intravenous administration of these extracts a significant increment in plasma LH was detectable, as measured by the ovarian ascorbic acid depletion test. The minimal effective dose (MED) of stalk-median eminence extract was approximately 40 μg wet weight of tissue extracted. By contrast, cerebral cortical extract was completely inactive at doses up to 5,000 μg wet weight of tissue extracted. Pitressin was active in inducing LH release in this test at doses of 200 or more mU. Thus, the quantity of Pitressin required to produce a positive response is approximately 200–400 times greater than the quantity of vasopressin found in the MED of hypothalamic extract. The MED for epinephrine was 5 μg, a quantity several thousand times the amount present in the MED of stalk-median eminence. This test for LH-releasing activity is much more sensitive than any other yet described.