Increased mammary gland response to pituitary mammotropic hormones by estrogen in rats.

The local effects of estradiol benzoate (EB) on mammary growth were investigated. The pellet of EB mixed with cholesterol at several ratios was implanted in the fat pad of the right third thoracic gland of the ovariectomized 3-month-old Sprague-Dawley female rat. The contralateral gland was served as the control. All the rats given EB pellets showed estrous smears for several days and subsequently continued diestrous smears. Seven-10 days after the onset of diestrus, some rats were killed by decapitation. The other rats were further grafted with the isologous pituitaries under the right kidney capsule and were killed after 20 days. By the observation of the whole mount preparation and the determination of mammary DNA, the mammary gland was found to be promoted in the growth only around the implanted pellet in the rat bearing both EB pellet at the ratio of 1: 5,000 and graft of 2 pituitaries. On the other hand, at all dose levels examined, EB implant alone did not induce mammary growth. These results indicate that EB increases the mammary response to pituitary mammotropic hormones even in its amount which did not promote by itself the mammary growth at all, and provide evidence that one of the actions of estrogen on mammary growth is to sensitize the gland to pituitary mammotropic hormones.