Effect of triiodothyronine and reserpine on induction and growth of mammary tumors in rats by 3-methylcholanthrene.

This study was designed to investigate the effects of exogenous L-triiodothyronine (LT3) and reserpine administration on the induction and stimulation of 3-methylcholanthrene (3-MC) mammary tumors. Two hundred-fifty-eight 30-day-old virgin 100 gm Sprague Dawley rats were divided into eight groups. Groups I-IV received either reserpine, LT3, both, or saline 22 days after tumor induction with 3-MC. Groups V-VIII were pretreated with either reserpine, LT3, or both for 29 days prior to tumor induction, and treatment was continued after tumor induction in all except Group VII. All rats were observed for tumor growth and sacrificed at seven months. Reserpine administration resulted in a protective effect against tumor induction in all groups in which it was used, with the most striking effects observed in the pretreated groups. Early LT3 stimulation of tumor formation was reversed by simultaneous administration of reserpine. Prolonged administration was unnecessary when both agents were administered prior to 3-methylcholanthrene.