Metabolic clearance rate of insulin-like growth factor-I in fed and starved sheep

The metabolic clearance of insulin-like growth factor-I (IGF-I) has been examined in sheep using a radioiodinated hormone preparation (131I-labelled IGF-I). Following i.v. administration, 131I-labelled IGF-I was distributed in a volume equivalent to plasma (60 ml whole blood/kg liveweight) and demonstrated a triphasic pattern of clearance with apparent half-lives (t 1/2) of 4.0 +/- 0.4 (S.E.M.), 52.4 +/- 3.4 and 792 +/- 26.5 min (n = 10). No significant differences in the t1/2 of the three phases were identified in fed compared with starved animals (fed, n = 4, phase 1 = 3.1 +/- 0.64, phase 2 = 46 +/- 5.9 and phase 3 = 756 +/- 27 min; starved, n = 6, phase 1 = 4.6 +/- 0.58, phase 2 = 57 +/- 3.2 and phase 3 = 816 +/- 38.5 min). Similarly, no significant differences in the distribution volume (fed, n = 4, 44 +/- 4 ml/kg live-weight; starved, n = 6, 39 +/- 2 ml/kg liveweight) or metabolic clearance rate (fed, n = 4, 2.9 +/- 0.15 ml/min; starved, n = 6, 3.2 +/- 0.5 ml/min) of the IGF-I were found in fed compared with starved animals. High-performance gel filtration chromatography of sequential plasma samples following injection of 131I-labelled IGF-I revealed three clear peaks of radioactivity which demonstrated markedly different patterns of clearance. These correspond to hormone complexed to binding proteins of 150,000 and 50,000 daltons and to 'free' hormone.(ABSTRACT TRUNCATED AT 250 WORDS)