PASSIVE-IMMUNIZATION AGAINST CIRCULATING INSULIN-LIKE GROWTH FACTOR-I (IGF-I) INCREASES PROTEIN CATABOLISM IN LAMBS - EVIDENCE FOR A PHYSIOLOGICAL-ROLE FOR CIRCULATING IGF-I

Primed constant infusions of [C-14] urea were used to determine the acute effect of passive immunization against circulating free and protein-bound insulin-like growth factor-I (IGF-I) on the rate of net protein catabolism (NPC) in castrated male lambs fasted for 48 li. Following an intravenous bolus of 50 ml IGF-I antiserum, the rate of NPC increased to a peak 30 min after injection of 1.69+/-0.16 g/kg per day from a baseline value of 1. 45+/-0.22 g/kg per day (P < 0.05, n = 4). In three animals given 50 ml equivalents of the purified immunoglobulin fraction, NPC increased from 1.31+/-0.20 to 1.59+/-0.16 g/kg per day (P < 0.05). A similar trend was observed in animals given 25 ml antiserum (n = 4). The rate of NPC did not increase following a bolus of non-immune serum in control animals and the rate of NPC in the treated lambs returned to control levels within 60 min of antibody injection. Plasma insulin and glucose concentrations in both the treated and control groups were unchanged throughout the study. These data suggest that circulating IGF-I has a physiological role in regulating whole body protein turnover during starvation and possibly other catabolic states. The effect of immunoneutralization of circulating IGF-I is transient and this suggests that while IGF-I has an endocrine role in the regulation of protein turnover, other regulatory mechanisms are involved.