Interactions of 6,7-3H-17β-estradiol with Mammary Gland and Other Organs of the C3H Mousein Vivo

After injection of a physiological amount of 6,7-3H-17β-estradiol into ovariectomized C3H mice,the nonvolatile radioactivity in the mammary gland consists almost exclusively of “free” 17β-estradiol. The mammary gland is able to concentrate and retain 17β-estradiol against a large concentration gradient with the blood. On a dry weight basis, the amount of hormone retained by the mammary tissue is about 1/3 that of uterus, 2/3 that of vagina, 11 times that of omental adipose tissue, 17 times that of lung and 42 times that of muscle. Estrogenic or antiestrogenic substances but not steroids without estrogenic activity are able to compete with 17β-estradiol for the process of accumulation and retention, an indication that this process is highly specific. In all tissues the pool of hormone specifically retained shows a half-life (turnover ti.ne) of about 7 hr. These findings suggest that the mammary gland is able to bind 17β-estradiol in a highly specific manner, and furthermore that this ability is not a peculiarity of classic target organs for estrogen but, although to a very different degree, is widely distributed in the body. (Endocrinology85: 1, 1969)