Insulin-like growth factor binding proteins in rats respond to fasting and the protein and energy content of the re-feeding diet

Abstract

We investigated the effects of fasting and re-feeding various levels of protein and energy on serum levels of specific insulin-like growth factor binding proteins (IGFBPs) and serum IGF-I binding activity patterns in rats. Fasting (48 hr) caused serum IGF-I levels to decrease 62% from 400 ng/mL to 151 ng/mL ( P < 0.05). Re-feeding fasted animals (48 hr) a low protein-low energy diet (control diet, intake 66% lower than re-fed control group) maintained IGF-I at fasted levels whereas re-feeding a low protein-isocaloric (5% protein, energy-adequate) diet caused IGF-I levels to drop 50% below fasted levels to 76 ng/mL ( P < 0.05). IGF-I binding activity in the 155K region paralleled serum IGF-I levels. Fasting caused a 150% increase in IGF-I binding activity in the 40K region. Re-feeding a control diet restored both peaks of IGF binding activity to baseline levels. IGF binding activity in the 40K region of rats re-fed a low protein-isocaloric diet increased 60% above fasting (over 200% above control). Energy level with protein restriction was important since responses in both IGF binding peaks were greater in low proten-isocaloric than low protein-low energy re-fed groups. Ligand blotting revealed that protein restriction at both energy levels caused the 38–45K bands (IGFBP-3) to decrease, and the 32K band (IGFBP-1) to increase compared to baseline, fasted, or re-fed control levels. Signal intensity of the IGFBP 24K band was lower in fasted and low protein-low energy rats compared to groups receiving adequate energy. Thus, that IGFBPs are differentially regulated and acutely responsive to nutritional manipulations suggests that the IGFBPs may be markers of acute changes in nutritional status.