Differential Inactivation of Prolactin by Mammary Tissue From Pregnant and Parturient Rats

Abstract

The inguinal mammary glands were removed from 17 rats on the 12-15th days of gestation and from 19 rats on the 4th day postpartum. The glands were homogenized and 100-mg. samples were incubated with 0.2 mg. of Squibb prolactin (20-25 LU./mg.) at 38 [plus or minus] 0.1[degree]C for 1 hr. For controls, the same amt. of prolactin was similarly incubated without mammary tissue or with boiled mammary tissue. After incubation, the tissues were removed and assayed in White Carneau pigeons for prolactin activity. The mammary homog-enate from the lactating rats inactivated 65.6% of the prolactin compared with 19.5% for the tissue from the pregnant rats. When corrected for milk content, by lactose detns., the former tissue showed 8 times as much capacity to remove prolactin as the latter. These data suggest that the mammary glands of pregnant rats cannot utilize prolactin to any marked extent, and that this contributes to the absence of copious lactation during gestation.