Abstract
Recent interest in the action of pituitary luteinizing hormone (LH) in enhancing the in vitro synthesis of progesterone in slices of luteal tissue(l) and its possible luteolytic role under certain conditions (2) make a systematic study of plasma LH levels in pregnant animals especially timely. The rabbit, in which ovulation is not spontaneous but induced (3) and in which luteal function is essential at all stages of pregnancy (4), presents particularly favorable conditions for such a study. It is well known that ovulation in the rabbit occurs about 10 hours after the coital stimulus and that the changes leading up to ovulation are probably dependent on the release of LH into the circulation, though it is not known to what extent other gonadotrophic hormones are involved. Hypophysectomy later than 60 minutes(5) after coitus will not prevent ovulation, indicating that enough gonadotrophic hormone is released from the pituitary gland within a short period to ensure ovulation. There is a coincident depletion of anterior pituitary gonadotrophic material(6) and a rise in plasma gonadotrophic activity(7). Other experiments have indicated that about twice the necessary amount of ovulating hormone is present within the first hour after coitus(8). There is a degranulation of pituitary basophiles and the appearance within a few hours of characteristic modified acidophile or carminophile cells of pregnancy, tentatively identified as sites of luteotrophic hormone synthesis(9). The pituitary gland, however, is essential at all stages of pregnancy in the rabbit(10).
Plasma gonadotrophins have not been demonstrable in the estrous or pregnant rabbit unless very large amounts of blood are injected(11) in which case ovulation and formation of corpora lutea occur. The hypertrophy of the interstitial cells of the ovary in the pregnant rabbit (12) also suggests the continued action of a luteinizing or interstitial cell-stimulating hormone.
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