Influence of prolactin and growth hormone on rat mammary tumors induced by N-nitrosomethylurea.

The effects of hypophysectomy and prolactin-suppressing drugs on the growth of mammary tumors induced in Sprague-Dawley rats by N-nitrosomethylurea and dimethylbenz(a)anthracene were compared. The influence of ovine prolactin and growth hormone administration on N-nitrosomethylurea-induced tumors was also studied in hypophysectomized animals. After hypophysectomy, all 13 tumors induced in 13 rats by N-nitrosomethylurea underwent regression, as did ten of 12 induced by dimethylbenz(a)anthracene. There were no new tumors. Pergolide mesylate, a long-acting ergoline derivative, was given in a dose of 80 micrograms twice daily by s.c. injection for 28 days. Only three of 12 N-nitrosomethylurea-induced tumors regressed, while four became static. However, only two new tumors developed in the 12 pergolide-treated rats, compared to 11 in the 12 untreated controls. Bromocriptine mesylate, at ten times the pergolide dose, was even less effective; one of 16 tumors regressed, two became static, and eight new tumors appeared in the 16 rats. In contrast, eight of 12 dimethylbenz(a)anthracene-induced tumors regressed during pergolide therapy, two became static, and there was only one new tumor among the 12 rats. Prolactin, 1 mg twice daily for 7 days by s.c. injection, was given to another eight rats bearing 11 N-nitrosomethylurea-induced tumors, commencing 7 days after hypophysectomy. Regression of five tumors borne by four rats was reversed but resumed when treatment was stopped. Regression of five tumors in the other four animals was arrested without regrowth; the sixth became inpalpable. All of these six grew rapidly when growth hormone, 2 mg twice daily, was administered in addition to prolactin.