Continuous 14 day infusion of insulin-like growth factor (IGF)-II increases the growth of normal rats, but exhibits a lower potency than IGF-I

Abstract

The effects of continuous 14 day infusion of recombinant human IGF-I (104 or 260 micrograms/day) or IGF-II (104, 260 or 650 micrograms/day) via s.c. implanted osmotic pumps were compared in young female rats in order to establish the relative efficacies of these two growth factors. Significant increase in body weight gain and feed conversion efficiency were achieved by 260 micrograms/day of IGF-I or 650 micrograms/day of IGF-II. These treatments were associated with increased nitrogen retention and increases in the fractional weights of kidneys, spleen, total gut and individual gut regions. There was an increase in the size of villi and muscularis lining the jejunum, suggesting an increased absorptive capacity of the gut. However there was no significant change in the amount of faecal nitrogen excretion when expressed as a percentage of nitrogen intake. Interestingly, IGF-II was at least as potent as IGF-I in increasing the depth of jejunal crypts. Infusion of equivalent doses of either IGF-I or IGF-II resulted in similar increases in circulating concentrations of the respective peptides, though IGF-II infusion dose-dependently decreased plasma IGF-I concentrations from those of the controls. Plasma IGF-binding protein levels were increased by both IGF-I and IGF-II treatments, though IGF-I elicited greater responses. In summary, IGF-II can promote the growth of young female rats, although generally less potently than IGF-I.