The maximal binding of 125I-IGF-II to the basal plasma membranes is similar to that found earlier for the muscle layer of the stem villi vessels of the human term placenta , but significantly (p < 0.0001) smaller than that for the microvillous membranes. These findings are corroborated by cross-linking experiments showing that the total 125I-IGF-II bound to microvillous membranes is 1.5 times that of 125I-IGF-II bound to basal plasma membranes.
The existence of differing patterns of IGF-II-binding sites in both trophoblastic membranes is shown by the specificity of IGF-II binding and the relative abilities of competitors to displace 125I-IGF-II: IGF-II > IGF-I > insulin for the basal plasma membranes and IGF-II > insulin > IGF-I for the microvillous membranes.
Type 2 IGF receptors were shown to occur on microvillous and basal plasma membranes by affinity cross-linking to 125I-IGF-II and SDS-PAGE. Both microvillous and basal plasma membranes contained three main labeled species, at 250 kDa, 135 kDa, and 130 kDa. buy diabetes drugs
Type 2 IGF receptors bind only 125I-IGF-II, and this ligand is not displaced by IGF-I or insulin, even at high concentrations. This indicates that the 250-kDa labeled band is the IGF-II-binding sites, as in mammalian cells. However, both membranes have a set of binding sites (F) that form the most rapidly migrating labeled complex. Cross-linking experiments with labeled IGF-I gave a complex in the same position that is displaced mainly by IGFI, poorly by IGF-II, and not at all by insulin. This 130-kDa band is probably the a subunits of the type 1 IGF receptor.