Regulation of lactogen specific binding sites in rat liver: studies on the role of lactogens and estrogen.

Regulation of the lactogen specific binding sites of rat liver was studied in several different high lactogen states. The specific binding of [125I]iodo-hGH was determined as a measure of these sites. Hypophysectomy of pregnant rats produced a much smaller decrease in hepatic binding of [125I]iodo-hGH than was seen in nonpregnant females. Three different prolactin secreting tumors (MtT/F4, MtT/F45, MtT/W5) produced greatly elevated levels of both rPRL and hepatic binding of [125I]iodo-hGH in both male and female rats. The increased binding reflected an increase in the concentration of lactogen specific membrane binding sites. There was no change in the apparent affinity of binding. Hypophysectomy of tumor-bearing rats was not followed by a decrease in the concentration of hepatic sites. Pituitary transplants beneath the kidney capsules of hypophysectomized (hypox) rats produced elevated rPRL levels and increased [125I]iod-GH specific binding to hepatic membranes. The elevation of serum rPRL preceded the increase in hepatic binding. There was a direct correlation between rPRL levels attained by 8 days after implantation and the level of hepatic binding. Estrogen treatment did not alter this correlation. Hormonal replacement with prolactin in combination with various hormones failed to induce the lactogen specific hepatic sites in estrogen treated hypox males, and did not prevent the marked decrease in hepatic binding observed in females following hypophysectomy. It is suggested that chronic elevation of prolactin plays a role in inducing lactogen specific binding sites in rat liver. Estrogen's inductive action seems to be largely effected at the pituitary level.