Binding of prolactin by fetal rhesus cell membrane fractions

Interest in possible physiologic roles of prolactin in the developing fetus led to the present study of affinity of human prolactin to fetal rhesus cell membrane fractions. The fractions were prepared by ultracentrifugation and their composition confirmed by electron microscopy. The cell membrane preparations were derived from the fetal placenta, liver, lung, myocardium and brain. Kinetic studies showed apparent binding affinity constants for 125I-labeled human prolactin (hPRL) of the order of 10(9)M-1 and capacities of from 3 ng to over a microgram per organ in late pregnancy for all of these tissues except brain. The calculated binding capacities, although probably underestimated, were found sufficiently high to insure that a portion of maternal and fetal prolactin exists in bound form to fetal tissues, consistent with possible roles in the regulation of fetal tissue cellular function, and provided they have access to circulating PRL.