Two forms of rat placental lactogen revealed by radioimmunoassay

A radioimmunoassay (RIA) has been developed to the species of lactogenic hormone (rPL) present in late pregnant rat placenta. Partially purified rPL calibrated against oPRL in the rat liver receptor assay (RRA) was used as standard. None of the PL's, PRL's, and GH's from other species cross-reacted in the RIA when tested at 1000 ng/ml. Rat serum PL levels remained stable in samples stored at -70C but not at -20C. In previous studies, using RRA, two peaks of rPL were found in rat serum, one at Day 11-13 and one at Day 20. By RIA, however, only the 20,000 dalton species of rPL present in late pregnant serum was immunoreactive. The RIA did not detect the larger, 40-50,000 dalton molecular species of rPL predominant in mild-pregnant serum, thus revealing the presence of two possibly unrelated forms of rPL. Serum rPL concentrations at Day 21 of pregnancy increased exponentially as the number of fetuses increased until a maximum of nine fetus.