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  • Author: R. DENAMUR x
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Stricker & Gruether (1928, 1929) were the first to observe that whole pituitary extracts could initiate lactation in pseudopregnant ovariectomized rabbits. Greatly assisted by the many advantages of the specific pigeon crop sac response for isolation and quantitative assay of the effective protein, Riddle, Bates & Dykshorn (1933) isolated it and named it 'prolactin'. Since this early work, prolactin has been shown to have an important luteotrophic rôle in many species. In this rôle it is usually associated with LH.

In studying the part played by prolactin in reproductive processes, we have investigated its secretion throughout the oestrous cycle and during pregnancy in the sheep, and have studied the effects of replacement therapy in hypophysectomized and pituitary stalk-sectioned ewes.



All the experiments have been carried out on ewes of

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Twice-daily intramuscular injections of 0·5 mg oestradiol benzoate, starting on Day 3 of the cycle, were able to prolong the life of the corpora lutea in sheep, as judged by their weight, their DNA and RNA content, and the progesterone concentration in ovarian vein blood. This luteotrophic effect persisted even when the pituitary stalks of the animals were sectioned, but it was abolished when the animals were hypophysectomized.

Four possible sites of oestrogen action were considered; the hypothalamus, the pituitary, the corpus luteum and the uterus. It was concluded that oestrogen is luteotrophic principally because of its action on the uterus, where it seems to interfere with the normal luteolytic mechanism.

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The functional activity of ovine CL was assessed by their weight, DNA and RNA content, and the concentration of progesterone in ovarian venous blood.

If sheep were hysterectomized on Days 9 to 12 of the oestrous cycle, the CL were maintained in a fully functional state until at least Day 60, but their activity had begun to decline by Day 128 to 135. When hysterectomized sheep were hypophysectomized, there was a significant decline in luteal activity within 48 hr, regardless of whether or not the pituitary stalk and pars tuberalis were left intact. The CL had almost completely stopped secreting progesterone within 4 days of hypophysectomy.

Hysterectomized, hypophysectomized animals were therefore used in a series of experiments to test the luteotrophic properties of sheep pituitary gonadotrophins. Doses of up to 5 mg FSH/day were unable to prevent complete luteal regression; similarly, doses of up to 5 mg LH/ day were also without effect. When mixtures of FSH and LH were given, the results were no better. However, prolactin in doses of up to 1000 i.u./ day was invariably able to maintain functional CL for 12 days, although at a considerably reduced level of activity. When prolactin was combined with a small dose of LH (0·25 mg/day), the CL were maintained at a level of activity comparable to that seen in hysterectomized animals before hypophysectomy. The slight synergistic action of FSH with prolactin was probably due to LH contamination. No further beneficial effects were obtained by adding FSH to the prolactin—LH mixture.

We conclude that prolactin and LH are both necessary for the maintenance of the ovine CL, and that these two hormones together make up the `luteotrophic complex'. But whilst prolactin on its own has some luteotrophic activity, LH by itself is completely ineffective.