Interactions between estrogens, prolactin, and growth hormone on the growth of N-nitrosomethylurea-induced rat mammary tumors.

Rats bearing N-nitrosomethylurea-induced mammary tumors regressing after ovariectomy were assigned to control or treatment groups. All control tumors continued to regress, while the serum prolactin levels were subnormal. Estrogen replacement stimulated tumor regrowth, and increased the serum prolactin. Pergolide further suppressed the postovariectomy serum prolactin, and all tumors continued to regress. When estrogen and pergolide were given together, 32% of tumors progressed despite low serum prolactins. Ovine prolactin, delivered to ovariectomized rats at a rate of 40 micrograms/hr, caused 3 of 10 tumors to progress, 3 to become static, and 4 to continue regression. Prolactin also maintained the growth of 4 of 8 tumors after hypophysectomy, and arrested regression of 3 others. Posthypophysectomy regression was also prevented and growth maintained by the simultaneous administration of estrogen and growth hormone.