The Nonresponsiveness of Lactating Mammary Gland to Estradiol*

The effect of estradiol on mammary glucose metabolism and DNA synthesis has been studied. Estradiol causes a significant increase in both the rate of glucose oxidation and DNA synthesis in the mammary glands of virgin mice, but is without any stimulatory effect on the glands of lactating mice. The reason for the inability of lactating mammary gland to respond to estradiol appears to reside in the lactating tissue itself and does not seem to be related to the hormonal milieu of lactation. Previously, we have shown that, in contrast to mammary gland of virgin mice, lactating mammary glands do not respond to estradiol with an increase in progesterone receptor levels. Therefore, all of these studies taken together reveal that during lactation, mammary glands may be nonresponsive with respect to most known parameters of estrogen action.