AJP - Endocrinology and Metabolism, Vol 262, Issue 4 E406-E411, Copyright © 1992 by American Physiological Society

ARTICLES

Increased serum clearance and degradation of 125I-labeled IGF-I in protein-restricted rats

J. P. Thissen, M. L. Davenport, J. B. Pucilowska, M. V. Miles and L. E. Underwood
Department of Pediatrics, School of Medicine, University of North Carolina, Chapel Hill 27599.

Dietary protein restriction in young rats stunts growth and decreases serum insulin-like growth factor I (IGF-I) concentrations. To investigate the possibility of a diet-induced accelerated clearance of IGF-I, the clearance of 125I-labeled IGF-I was determined by measuring the radioactivity in serum obtained at different intervals after bolus injection in 5-wk-old rats that had received either a normal 15% protein diet (P15; n = 6) or a 5% protein isocaloric diet (P5; n = 7) for 1 wk. The clearance and volume of distribution of 125I-IGF-I were increased in P5 rats by 58 and 75%, respectively, whereas the terminal elimination half-life was not changed. The calculated IGF-I production rate was decreased by 40% in P5 rats. The intermediate distribution phase (alpha) was twofold faster in P5 than in P15 animals, and 125I-IGF-I was preferentially bound to IGF binding proteins in the 40-kDa complex in P5 rats. These observations might explain the increased clearance and degradation of IGF-I in the P5 rats. Dietary protein restriction appears to lower serum IGF-I concentrations by both decreasing its production rate and increasing its clearance.

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