Distribution of acute-phase alpha 2-macroglobulin in rat fetomaternal compartments

Rat acute-phase alpha 2-macroglobulin (AP alpha 2M) concentration was measured by radioimmunoassay in maternal serum, fetal plasma, maternal liver, fetal liver, and amniotic fluid as a function of gestational and neonatal age. The concentration profiles of AP alpha 2M in maternal serum and fetal plasma displayed two peaks, one in early gestation and another during late gestation. Synthesis of AP alpha 2M was confirmed by the immunoprecipitation of [35S]methionine incorporated into cultures of selected tissues. The following observations were made. 1) Maternal serum concentrations of AP alpha 2M were higher than those observed in fetal plasma in early gestation. This was attributable to a high level of maternal AP alpha 2M synthesis in metrial gland which was absent in liver and moderate in yolk sac. 2) In late gestation fetal plasma concentrations of AP alpha 2M greatly exceeded those observed in maternal serum. This could be explained by the pronounced synthesis of AP alpha 2M in fetal liver that was not apparent in maternal liver or yolk sac. 3) During labor, a transient increase in AP alpha 2M concentration was observed in maternal serum and fetal plasma. 4) During lactation a moderately elevated maternal serum concentration of AP alpha 2M was maintained. 5) Amniotic fluid concentration of AP alpha 2M was very low throughout gestation, which indicated that the fetal glomerulus was relatively impermeable to this large protein. It is concluded that in early gestation a principal maternal source of AP alpha 2M appears to be the metrial gland, whereas in late gestation fetal liver is a major source of AP alpha 2M appearing in fetal plasma from where some of this macroglobulin is speculated to be transported to the maternal circulation.