Concentration of native prolactin and prolactin binding sites in hepatic subcellular fractions from...

Lactogen binding and prolactin content were measured in hepatic subcellular fractions from tumor-bearing rats (TBR; MtT/F4, MtT/W5, MtT/W10) with elevated prolactin and growth hormone levels and from control animals. Specific binding of 125I-oPRL to Golgi fractions from tumor-bearing animals was 2.5 to 7 fold greater than that from controls. Binding to plasmalemma was 6-fold greater in tumor-bearing rats. The specific binding of 125I-labelled bGH and insulin showed less marked differences between TBR and controls. Subcellular fractions were extracted with HCl to determine hormonal content. The content of prolactin and growth hormone in Golgi fractions from TBR was at least 20-fold that in fractions from controls. Rat prolactin extracted from Golgi heavy elements was 50% as effective as native material in binding to lactogen receptors as judged by radioreceptor assay. These studies demonstrate that the chronic elevation of prolactin was associated with an increase of receptors not only in the intracellular compartment but on the cell surface as well. Furthermore, they demonstrate that native prolactin is internalized and accumulated in rat liver Golgi fractions.