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Radioimmunoassay of a 26,000-Dalton Plasma Insulin-Like Growth Factor-Binding Protein: Control by Nutritional Variables^*

WALKER H. BUSBY, DAVID K. SNYDER and DAVID R. CLEMMONS

Department of Medicine, University of North Carolina Chapel Hill, North Carolina 27599

Insulin-like growth factor I (IGF-I) is a peptide growth factor that circulates bound to carrier proteins. One form of carrier protein (mol wt, 26K) is not believed to be GH dependent, is relatively unsaturated, and modulates the cellular response to IGF-I. This study was undertaken to determine the variables that control the plasma concentration of this protein, which was measured using a specific RIA. The mean plasma 26K IGF-binding protein (IGF-BP) concentration in 15 normal fasting subjects at 0800 h was 9.4 ± 4.4 (±SD) µg/L. The mean value in GH-deficient patients was increased to 19.5 ± 10.1 µg/L (n = 60; P < 0.05), and it was 7.3 ± 4.3 µg/L in patients with acromegaly (n = 31). The GH dependency of these changes is further supported by the observation that subjects who received GH injections had a 51% reduction in their fasting values. Nutritional intake appeared to be a more important controlling variable than GH. During an overnight fast plasma 26K IGFBP values increased approximately 4-fold in 6 normal subjects. After 2 days of fasting, the mean value in 7 obese subjects rose progressively from 6.5 ± 2.3 to 11.7 ± 5.4 µg/L (P < 0.001), and it increased further to 19.2 ± 5.9 µg/L by day 4 of fasting; after 2 days of refeeding it returned to the prefasting level of 6.8 ± 1.9 µg/L. Likewise, ingestion of a standard test meal resulted in a significant decrease in mean plasma 26K IGF-BP from a fasting value of 8.4 ± 2.9 to 5.6 ± 2.8 µg/L 4 h postprandially (P < 0.05). In summary, the plasma concentrations of the 26K IGFI-BP fluctuate widely in response to dietary manipulation, whereas GH status appears to be a secondary controlling variable.

^* This work was supported by NIH Grants AG-02331, HL-36313, and RR-00046.

To whom all correspondence and requests for reprints should be addressed.

Received April 29, 1988.

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