Plasma clearance and tissue distribution of labelled chicken and human IGF-I and IGF-II in the chicken

The metabolic clearance of chicken IGF-I (cIGF-I), cIGF-II, human IGF-I (hIGF-I), and hIGF-II was examined in the chicken using I-125-labelled growth factors. Superose-12 chromatography of plasma collected at 7.5 min post-infusion revealed peaks of radioactivity corresponding to 150 and 43 kDa and unbound tracer. Statistical analysis of trichloroacetic acid (TCA)-precipitable radioactivity in sequential plasma samples as well as following chromatography of the same samples revealed that clearance of the radiolabelled peptides followed an apparent triphasic pattern. The close similarity of the individual chromatographically defined pools in their clearance rate compared with the three components described by TCA precipitation strongly suggested their identity. Both free I-125-labelled cIGF-II (3.11 min) and hIGF-II (3.01 min) were cleared at a greater rate than their IGF-I counterparts. Unbound hIGF-I was cleared at a greater rate than cIGF-I (4.45 vs 5.66 min respectively). A similar pattern for clearance was evident in the radiolabelled growth factors associated with the 43 kDa component, although at a longer half-life. There was no difference in the apparent clearance of the radiolabelled growth factors associated with the 150 kDa component between IGF-I or -II or between species. Analysis of the chromatographic profiles of radioactive IGF-I peptides complexed to serum proteins versus those bound to labelled IGF-II peptides revealed the presence of a large molecular mass binding protein in vivo. Ligand blotting of chicken serum determined that a binding protein with a mass of 70 kDa was detectable with I-125-IGF-II probes only, and was not present in pig serum. In addition, tissue uptake of I-I25-cIGF-I and -II was evaluated. Similar patterns of tissue distribution and uptake were observed for I-125-cIGF-I and -II, except that cIGF-II uptake by the liver exceeded that of I-125-cIGF-I at 15 min post-infusion. The rank order of tissue distribution was as follows: kidney>testis>heart>liver>pancreas>small intestine>cartilage>bursa>gizzard>leg muscle>breast muscle>brain. We conclude from these studies that the clearance of IGFs from the compartments identified in blood and the potential target tissues is dependent on their interactions with IGF-binding proteins and receptors.