Recovery of Growth Hormone Release from Suppression by Exogenous Insulin-Like Growth Factor I (IGF-I): Evidence for a Suppressive Action of Free Rather Than Bound IGF-I1

To determine the time course of recovery of GH release from insulin-like growth factor I (IGF-I) suppression, 11 healthy adults (18–29 yr) received, in randomized order, 4-h iv infusions of recombinant human IGF-I (rhIGF-I; 3 μg/kg·h) or saline (control) from 25.5–29.5 h of a 47.5-h fast. Serum GH was maximally suppressed within 2 h and remained suppressed for 2 h after the rhIGF-I infusion; during this 4-h period, GH concentrations were approximately 25% of control day levels [median (interquartile range), 1.2 (0.4–4.0) vs. 4.8 (2.8–7.9) μg/L; P < 0.05]. A rebound increase in GH concentrations occurred 5–7 h after the end of rhIGF-I infusion [7.6 (4.6–11.7) vs. 4.3 (2.5–6.0) μg/L; P < 0.05]. Thereafter, serum GH concentrations were similar on both days. Total IGF-I concentrations peaked at the end of the rhIGF-I infusion (432 ± 43 vs. 263 ± 44 μg/L; P < 0.0001) and remained elevated 18 h after the rhIGF-I infusion (360 ± 36 vs. 202 ± 23 μg/L; P = 0.001). Free IGF-I concentrations were approximately 140% above control day values at the end of the infusion (2.1 ± 0.4 vs. 0.88 ± 0.3 μg/L; P = 0.001), but declined to baseline within 2 h after the infusion. The close temporal association between the resolution of GH suppression and the fall of free IGF-I concentrations, and the lack of any association with total IGF-I concentrations suggest that unbound (free), not protein-bound, IGF-I is the major IGF-I component responsible for this suppression. The rebound increase in GH concentrations after the end of rhIGF-I infusion is consistent with cessation of an inhibitory effect of free IGF-I on GH release.