CAN THE OVARIAN HORMONES INHIBIT THE MAMMARY RESPONSE TO PROLACTIN?1

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Abstract
A total of 63 albino rabbits of both sexes, intact and castrate, were injd. with 9601.U. of estrone and II. U. of progesterone daily for 25 days in order to induce optimal mammary development. From the 25th to 35th days, all except 3 rabbits were injd. daily with 2 mg. of prolactin (20-251.U. per mg.), with or without 1 or both of the steroid hormones. At the end of the 35th day, the rabbits were killed and the mammary glands were exposed and rated for intensity of lactation. The extent of mammary growth induced by these treatments was detd. by injecting radioactive P (P32) 4 hours prior to sacrifice of the rabbits, and then making radio-autographs of the mammary glands. The rabbits given prolactin and both steroids during the last 10 days of the 35_day exptl. period had practically no milk in their mammary glands (avg. rating of less than 1); those injd. with prolactin alone had mammary glands filled with milk (avg. rating of 3-4); those given prolactin and progesterone showed no inhibition in lactational response (avg. rating of 3-4); and the rabbits injd. with prolactin and estrone showed only a slight decrease in milk secretion (avg. rating of 2-3). The radioautographs indicated the presence of intensive mammary growth in the rabbits treated with both steroids throughout the entire 35_day period but not in the rabbits given both steroids only for the 1st 25 days. In the doses employed, estrone and progesterone together can effectively inhibit the milk-secreting action of prolactin on the mammary gland of the rabbit, whereas progesterone alone is ineffective and estrone alone is only slightly effective.