Effects of hormones on prolactin binding to normal mammary glands and pregnancy-dependent mammary tumors in GR/A female mice near parturition.

Pregnancy-dependent mammary tumors in GR/A female mice are different from normal mammary glands in hormone responsiveness and prolactin binding near parturition. The binding was modified by several hormones. Thus, prolactin binding under the hormonal conditions favorable for the progression of tumors was compared between normal and neoplastic mammary glands. Slices of the glands were incubated in 1 ml Medium 199 containing 125I-labeled ovine prolactin and the radioactivity incorporated was counted as the index of binding. Prolactin binding to either normal or neoplastic glands on day 19 of pregnancy was low and not affected by ovariectomy on day 15, and by daily injections of progesterone alone or in combination with estradiol benzoate between days 14 and 19 of pregnancy. One day after parturition (litter was removed immediately after parturition), the binding to tumors was raised significantly, while, the binding to normal glands remained low. Injections of progesterone alone or in combination with human placental lactogen twice a day from 1 day before parturition induced little changes in the binding to either normal or neoplastic glands. These results suggest no alteration of mammary response for prolactin binding to exogenous hormones in the process of transformation of the glands from normal to neoplastic states, and low predictability of membrane prolactin receptors for tissue responsiveness to the hormone.