Greater potency of IGF-I than IGF-I/BP-3 complex in catabolic parenterally fed rats

We compared the anabolic effects of recombinant human insulin-like growth factor I (rhIGF-I, 2.5 mg/kg) and equimolar amounts of rhIGF-I prebound to rhIGF binding protein-3 (rhIGF-I/BP-3) coinfused continuously with total parenteral nutrition (TPN) solution in dexamethasone (Dex, 70 microg/day ip)-treated male rats for 6 days. The four TPN groups included control, Dex, Dex + IGF-I, and Dex+IGF-I/BP-3. Pharmacokinetic analysis indicated reduced clearance of IGF-I when infused as IGF-I/BP-3 compared with free IGF-I (0.91 +/- 0.09 vs. 2.01 +/- 0.19 ml serum/min, P < 0.001) and this was associated with significantly greater serum IGF-I concentrations in the Dex+IGF-I/BP-3 group. Despite greater total serum IGF-I levels, infusion of free IGF-I produced greater anabolic responses than IGF-I/BP-3 based on body weight, nitrogen balance, and jejunal cellularity. Treatment with free IGF-I, but not IGF-I/BP-3, significantly reduced serum insulin and glucose levels that were elevated due to Dex. There were no significant differences in liver IGF-I mRNA levels between groups. Serum IGFBP-3 levels were elevated with infusion of IGF-I/BP-3 compared with IGF-I. These results indicate greater anabolic potency of IGF-I compared with IGF-I/BP-3 when administered by continuous parenteral infusion with TPN solution in catabolic rats.