EFFECTS OF ALTERING THE BALANCE BETWEEN PROLACTIN AND OVARIAN HORMONES ON INITIATION OF LACTATION IN RABBITS1

Abstract

Thirty-six young rabbits were ovariectomized, and after 10-14 days were injected daily with .096 mg of estrone and 1 mg of progesterone for 25 days to induce optimal mammary development. From the 25th to 35th days, 3 groups of 6 rabbits each were continued on the same doses of steroid hormones together with 2, 4 and 8 mg of prolactin daily, respectively; 3 other groups of 6 each were injected with 2 mg of prolactin daily together with none, 0.25 or 0.5 of the 2 steroid hormones used previously. On the 36th day the rabbits were killed and their mammary glands were exposed and rated visually for lactational response. In the first 3 groups lactation was practically completely inhibited when only 2 mg of prolactin was injected daily in the presence of the 2 steroid hormones, while 4 or 8 mg daily elicited good lactational responses. In the second 3 groups, the greatest amount of milk was secreted when 2 mg of prolactin was given daily without the steroid hormones, while the same dose of prolactin in the presence of either 0.25 or 0.5 of the steroid hormones produced a smaller flow of milk. It is concluded that the antagonism between mammary growth and lactation is relative, depending on the balance between the levels of prolactin and the 2 ovarian hormones in the body. When the mammary growth stimulus exerted by the 2 ovarian hormones is greater than the lactational stimulus of prolactin, milk flow will be inhibited, and vice versa.