IGF-I and IGFBP-3 transport in the rat heart.

Specific binding of IGF-binding protein (IGFBP)-3 was shown to be present in the isolated, beating rat heart. The uptake of perfused I-125-labeled IGF-I in the beating heart was decreased to 9% by blocking IGF-I binding sites with the IGF-I analog Long R-3 (LR3) IGF-I. When LR3 was perfused with complexes of I-125-IGF-I.IGFBP-3, uptake of I-125-IGF-I was decreased to 41%, which was significantly greater than LR3 and I-125-IGF-I (41 vs. 9%). These data suggest that both microvessel IGF-I and IGFBP-3 binding sites contribute to the transport of IGF-I in the perfused rat heart. This also suggests a novel and plausible mechanism whereby circulating IGFs reach sites of IGF bioactivity.