Role of adrenals and estrogen in regression of mammary tumors during postpartum lactation in the rat.

The effects of bilateral adrenalectomy or estradiol benzoate treatment were observed on growth of 7,12-dimethylbenz(a)-anthracene-induced mammary tumors during postpartum lactation. In the control and estradiol benzoate-treated postpartum lactating rats, the mammary tumors decreased approximately 40% in size by Day 5 postpartum and continued to regress to 50% of their average original diameter by Day 25 postpartum. Adrenalectomy on Day 3 postpartum prevented mammary tumor regression and resulted in renewed mammary tumor growth. By Day 10 postpartum, average mammary tumor size in the adrenalectomized rats reached prepartum diameter and continued to increase in size until Day 25. Although serum prolactin concentrations were significantly higher in the lactating rats with mammary tumors than in the nonlactating rats with mammary tumors, there were no significant differences in serum corticosterone values. Adrenalectomy resulted in a significant increase in serum prolactin levels and in a marked fall in serum corticosterone levels. It is concluded that in rats adrenocortical activity is primarily responsible for reduced mammary tumor growth during postpartum lactation.