| Abstract: | The specific binding of 125I-labeled insulin, human hormone (125I]hGH), bovine growth hormone (125I]bGH), and ovine prolactin (125I]oPRL) was studied in mouse liver membranes. 125I]hGH and 125I]oPRL bound to adult liver membranes. Pregnancy increased the specific binding of 125I]hGH but not that of 125I]oPRL. 125I]hGH was displaced from membranes of pregnant mice by hGH, oPRL, and bGH, but only by hGH and oPRL from liver membranes of nonpregnant mice. Significant specific binding of 125I]bGH was seen only in pregnancy. The binding of 125I]bGH to pregnant mouse liver membranes increased with increasing concentration of either membrane protein or 125I]bGH. Both the specific binding and dissociation of 125I]bGH were greatly influenced by the time and temperature of incubation. Binding of 125I]bGH was inhibited by growth hormones, including hGH and rat GH, and not by lactogenic hormones (various prolactins and human placental lactogen), ACTH, glucagon, or insulin. The inhibition of 125I]hGH binding by hGH and bGH, in the presence of excess (2 mug/ml) of PRL, was very similar to that seen with 125I]bGH. Scatchard plots of displacement dose-response curves obtained under steady state conditions of 4C were nonlinear and very similar with either 125I]bGH or 125I]hGH. This contrasted with the linear Scatchard plots obtained from displacement dose-response curves of either 125I]oPRL or 125I]hGH in the presence of excess (2 mug/ml) bGH. Termination of pregnancy, either naturally or by hysterectomy, reduced 125I]bGH specific binding to nonpregnant levels by 24 to 36 h. Estrogen administration did not increase 125I]bGH binding in hepatic membranes. Nonpregnant mice possess hepatic lactogen binding sites which are uninfluenced by pregnancy. GH specific binding sites are markedly augmented during pregnancy. The close correlation between the level of these sites and pregnancy suggests that they are regulated by a product of the fetoplacental unit. |
