Ontogeny of a specific high-affinity binding site for ovine placental lactogen in fetal and postnatal liver

Ovine placental lactogen (oPL) exerts actions in sheep and rodent fetal tissues that growth hormone (GH) does not. However, in postnatal tissues, both oPL and GH possess these activities. Although a high-affinity binding site for oPL in ovine fetal liver has been reported, some investigators believe this to be the GH receptor. It was our objective to discriminate between oPL and GH binding to fetal liver microsomes using competitive saturation analyses. Microsomal membranes from fetal liver (Days 60, 90, 105, 120, and 135 of gestation) and postnatal liver (1 wk of age) were incubated with increasing amounts of ¹²⁵I]oPL in the absence or presence of a 100-fold molar excess of unlabeled oPL. Saturable binding of ¹²⁵I]oPL was observed with fetal liver and postnatal liver microsomes. The K_d of the oPL-binding site in fetal liver was 122.1 ± 8.2 pM (mean ± standard error), and receptor concentrations remained relatively constant (9.8 ± 1.1 fmol/mg of membrane protein) across gestation. The highest concentration of oPL binding was detected in 1-wk postnatal liver microsomes (53.0 fmol/mg of membrane protein). Saturation analyses using ¹²⁵I]GH and ¹²⁵I] prolactin (PRL) were also conducted with fetal liver membrane preparations. Although specific binding for these two radiolabeled ligands was observed in control tissues, no specific binding was observed in fetal liver. These data are in agreement with earlier reports that a high-affinity binding site for oPL exists in fetal tissues. The fact that saturable binding could not be demonstrated for either GH or PRL with fetal liver microsomes contradicts recent suggestions that oPL is binding the GH receptor.