Evidence of high and low affinity binding sites for basic fibroblast growth factor in mouse placenta

The placenta has been shown to contain bFGF, but the presence of specific binding sites for this growth factor in this tissue remained to be established. In order to study the role of bFGF in the placenta growth, we looked for specific binding sites on mouse placental cell membranes at days 12, 14, 16, and 18 of pregnancy. At day 12, Scatchard analyses indicated that two classes of specific interaction sites for bFGF were detected. One class of high affinity binding sites was characterized by an apparent Kd of 10 pM and a binding capacity of 10 fmoles per mg of membrane protein. A second class of low affinity binding sites was detected with an apparent Kd of 60 nM and a binding capacity of 26 pmoles per mg of membrane protein. At days 14, 16 or 18, Scatchard analyses only showed low affinity binding sites with an apparent Kd of 24 nM and a binding capacity of 230 pmoles per mg of membrane protein. The characterization of these binding sites was performed by cross linking experiments that revealed two forms of specific complexes. This result suggested that the high affinity binding sites correspond to putative receptors with relative molecular masses equal to 65,000 and 85,000. The dramatic decrease of the high affinity receptor number after the 12th day of pregnancy, which is synchronous with the 9-fold increase of the low affinity binding site number, suggests that the biological activity of bFGF could be regulated by a balance between both the numbers of high and low affinity binding sites on placenta cell membranes. Thus, as it was shown for other growth factors, bFGF could only be involved at specific pregnancy stages.