Specific, high affinity binding of 125I-IGF-I to the type I IGF receptor on human fibroblast monolayers was not altered by varying feeding schedules, serum lots, washing procedures, or incubation times and temperatures. However, markedly different competitive binding curves were obtained when different iodinated IGF-I preparations were used. Five of six radioligands bound preferentially to the type I IGF receptor on human fibroblast monolayers, with 50% displacement at 4-8 μg/l unlabelled IGF-I; with one radioligand a paradoxical 20-200% increase in 125I-IGF-I binding was observed at low concentrations of unlabelled IGF-I, while concentrations as high as 100 μg/l IGF-I failed to displace this radioligand. The latter binding pattern cannot be accounted for by 125I-IGF-I binding to the type II IGF receptor. These data indicate that various radioligands may have preferential affinities for different IGF-I binding sites on human fibroblast monolayers.
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