RAPID CLEARANCE OF HUMAN INSULIN-LIKE GROWTH-FACTOR BINDING PROTEIN-3FROM THE RAT CIRCULATION AND CELLULAR-LOCALIZATION IN LIVER, KIDNEY AND STOMACH

The gastrointestinal tract represents a major site for the trophic act ions of insulin-like growth factors (IGFs), which may be derived in vi vo from a large circulating pool. The high capacity binding protein fo r IGFs in blood is IGF binding protein-3 (IGFBP-3), which is largely c omplexed with an acid-labile subunit (ALS). However, we and others hav e shown that IGFBP-3 not complexed with ALS can rapidly leave the circ ulation, and may carry IGFs to peripheral tissues. In this study we in vestigated the transfer of recombinant, glycosylated human (h)IGFBP-3 from the rat circulation to the stomach and intestine, compared with l iver and kidney. [I-125]-labeled IGFBP-3 was administered into the tai l vein of conscious male rats, which were killed between 5 min and 2 h later. Blood was taken for the preparation of plasma, and the liver, kidneys, stomach and intestine were removed either for estimation of t he associated radioactivity, or fixed for autoradiographic analysis of histological sections. Following injection, [I-125]-labeled IGFBP-3 w as associated, in part, with a 150 kDa complex in plasma within 10 min when analyzed by gel filtration chromatography. However, 84% of the a dministered IGFBP-3 had already left the circulation, and 40% of the i nitial injected dose was accumulated in liver by 5 min, with a further 4% localized in the kidneys. Autoradiographic analysis shoveled that IGFBP-3 was selectively accumulated within Kupffer cells of the liver, and by the glomeruli and proximal tubules of the kidney. Little radio labeled IGFBP-3 was recovered from the small intestine, but 14% of the initial injected dose was found within the stomach after 2 h, and a f urther 12% within the stomach contents. Autoradiographic localization within the stomach showed that the [I-125]-labeled IGFBP-3 was primari ly associated with the mucosal lining and gastric glands. Separation o n sodium dodecyl sulphate polyacrylamide gel electrophoresis showed th at the majority of the radioactivity associated with the stomach conte nts represented small, degraded peptides. These results suggest that w hile a rapid clearance of IGFBP-3 is achieved by the liver and kidney, a longer term accumulation occurs in the stomach with a luminal secre tion. This may represent a delivery system by which circulating IGFs m ay reach gastric tissue.