Influence of infused prolactin on hormone binding to tissue slices

The influence of circulating prolactin on the binding of labeled hormone to tissue slices in vitro was investigated in rats bearing dimethylbenzanthracene-induced mammary tumors. Prolactin was infused into four animals over four 30-min periods to give serum levels ranging from 20 to 2000 ng/ml. Serial biopsies of liver and tumor were taken during infusion, and binding of labeled prolactin and insulin measured in tissue slices. Whereas insulin binding remained constant, prolactin binding was significantly depressed in both tissues when samples were obtained at serum prolactin levels greater than 300 ng/ml. This decrease was due to diminished numbers of available binding sites rather than a change in binding-site affinity for prolactin. The data indicate that under the conditions of this experiment, endogenous circulating prolactin may interfere with in vitro measurements of hormone uptake when serum levels excee 300 ng/ml.