LUTEINIZING HORMONE CONCENTRATIONS IN PITUITARY AND IN BLOOD PLASMA DURING THE ESTROUS CYCLE OF THE RAT

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Simultaneous measurement of luteinizing hormone (LH) in the pituitary and in circulating blood was carried out by the ovarian ascorbic acid depletion method during the estrous cycle in rats showing a regular 4-day cycle under the controlledenvironment of 12 hrs. light (8a.m.-8p.m.) and 12 hrs. darkness. The pituitary LH content was relatively high in the morning and early afternoon of proestrus as compared with that in the diestrus, but it depleted suddenly at 5 p.m. of proestrus and continued to decline until the next morning. The plasma LH was detectable only in the afternoon of proestrus and in the morning of estrus, and the maximal concentration was detected at 7 p.m. of proestrus. The rats hypophysectomized before 4 p.m. of proestrus did not ovulate at all, but some of the rats hypophysectomized at 5 p.m. or 6 p.m. did ovulate and all the rats operated at 7 p.m.or later ovulated completely. The concentration of ovarian ascorbic acid in normal rats depleted markedly at 7 p.m. of proestrus. These results indicate that the ovulatory surge of LH occurs in the late afternoon of proestrus and its critical period is around 5 to 7 p.m. under our laboratory conditions. The relationship between the LH release and the ovarian steroids secretion is discussed.
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