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Role of Adrenals and Estrogen in Regression of Mammary Tumors during Postpartum Lactation in the Rat¹

Department of Physiology, Neuroendocrine Research Laboratory, Michigan State University, East Lansing, Michigan 48824

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.

¹ Published with the approval of the Michigan Agricultural Experiment Station as Journal Article 8792.

² Present address: Department of Obstetrics and Gynecology, East Carolina University, Greenville, N. C. 27834.

³ Present address: Endocrine Sciences, 18418 Oxnard Street, Tarzana, Calif. 91356.

⁴ Aided in part by NIH Research Grants CA10771 from the National Cancer Institute and AM04784 from the National Institute for Arthritis, Metabolism, and Digestive Diseases. To whom requests for reprints should be addressed.

Received 11/ 2/78. Accepted 3/29/79.