Infused IGF-I/IGFBP-3 complex causes glomerular localization of IGF-I in the rat kidney.

Insulin-like growth factor I (IGF-I) increases renal blood flow, glomerular filtration rate (GFR), and proximal tubule reabsorption of phosphate in humans and rodents. The biological effects of IGF-I are likely to be influenced by cellular localization of IGF-I within the kidney. We therefore tested whether the renal localization of infused IGF-I could be altered if given with selected IGF-binding proteins (IGFBPs). Rats were treated with intravenous injections of125I-labeled IGF-I,125I-IGFBP-3, or125I-IGFBP-4 alone or with complexes of 125I-IGF-I and IGFBP-3 or IGFBP-4. The cellular localization of IGF and the IGFBP within the kidney was then determined.125I-IGF-I,125I-IGFBP-4, and125I-IGF-I/IGFBP-4 complexes were found almost exclusively in vacuolar structures (endosomes) of proximal renal tubules. In contrast, about one-third of renal125I-IGFBP-3 and125I-IGF-I/IGFBP-3 was localized to glomeruli. When 125I-IGF-I was given alone, 3% was found in glomeruli and 89% in proximal tubules. When given as 125I-IGF-I/IGFBP-3, 29% was in glomeruli and 65% in proximal tubules. We conclude that the cellular localization of IGF-I within the kidney can be directed to glomerular elements if the IGF-I is given with IGFBP-3.