Placental lactogen and growth hormone receptors in human fetal tissues: relationship to fetal plasma human placental lactogen concentrations and fetal growth.

The specific binding of human placental lactogen (hPL) and human GH (hGH) to particulate cell membranes from human fetal liver and skeletal muscle at 12-19 weeks gestation was examined. Fetal liver and muscle specifically bound [125I]hPL. This binding was inhibited by increasing concentrations of unlabeled hPL (half-maximal concentrations, 2.2 and 3.4 nmol/L, respectively). Scatchard analysis of the hepatic membrane binding revealed curvilinear plots with higher (Kd, 2.2 nmol/L) and lower (Kd, 24 nmol/L) affinity sites, while binding to muscle involved a single receptor class of Kd 5.6 nmol/L. The binding capacities for the two hepatic sites correlated positively with fetal body weight. [125I]hGH specifically bound to liver, but not muscle, with higher (Kd, 1.6 nmol/L) and lower (Kd, 8.6 nmol/L) affinity sites. [125I]PRL bound to hepatic membranes, but was preferentially displaced by hPL or hGH. Between 4 and 500 micrograms/L (mean, 82 micrograms/L, 3.8 nmol/L) hPL were present in fetal plasma. The findings identify distinct hPL receptors in human fetal liver and skeletal muscle and a hepatic hGH receptor in midgestation.