PROLACTIN BINDING IN OVARIECTOMY-RESPONSIVE AND OVARIECTOMY-NONRESPONSIVE RAT MAMMARY-CARCINOMA

Abstract

Growth of the transplantable mammary tumor, MTW9, in W/Fu rats is enhanced by elevated serum prolactin concentrations. This report compares prolactin binding to tumor membranes in 2 mammary tumor strains derived from MTW9. Maximum binding to membranes of both tumors occured at pH 7.6 after incubation for 30 h at 4.degree. C. The binding was inhibited only by polypeptide hormones that possess lactogenic activity. MTW9-P, an ovariectomy responsive tumor developed in rats maintained on daily perphenazine injections, had 4-fold higher prolactin binding than MTW9-MtT, an ovariectomy nonresponsive tumor developed in rats bearing the mammosomatotropic pituitary tumor, MtTW10. Withdrawal of perphenazine from rats bearing MTW9-P caused a fall to normal of plasma prolactin, no tumor regression and no significant change in prolactin binding. Resection of MtT resulted in tumor regression, a fall to normal of serum prolactin and a nearly 3-fold increase in prolactin binding. Scatchard plots of prolactin binding data yield an apparent affinity constant, Ka, of 1.2 .times. 109 l/mol for both tumors. The 4-fold higher prolactin binding in the ovariectomy responsive variant suggests a positive correlation between ovariectomy response and the number of membrane prolactin binding sites. No correlation between prolactin sensitivity and prolactin binding is apparent.