In the adult circulation, 70-90% of the serum insulin-like growth factors (IGFs) are carried by IGF-binding protein-3 (IGFBP-3), which exists as part of a 150-kilodalton (kDa) ternary complex including IGF and an acid-labile subunit (ALS). We have examined the hormonal regulation and molecular distribution of IGFBP-3 in the circulation of a uniquely GH-deficient (GHD) rat model. For 7 days, GHD rats were given GH by either twice daily injections (1 mg/kg) or continuous infusion (2.4 mg/kg.day) or IGF-I by continuous infusion (1.4 mg/kg.day). Each day, weight and feed and water intake were monitored, and on day 7, liver, kidney, spleen, heart, and lung were weighted, and sera were collected. Serum IGF-I was analyzed by immunoassay, and the molecular distribution of the IGFBPs was determined by neutral size-exclusion chromatography combined with Western ligand blot and Western immunoblot. The GHD rats were 40-60% lighter than their normal littermates, and all organs examined were proportionately smaller. Serum IGF-I and IGFBP-3 levels were less than 10% of those in normal rats. Incubation of serum from GHD rats with [125I]IGF-II showed that radiolabel was incorporated only into a 44-kDa IGFBP region that contained the smaller IGFBPs. IGFBP-3 eluted around 60 kDa. No 150-kDa IGFBP region was detected. The administration of GH or IGF-I to GHD rats resulted in significant increases in weight gained, although food and water intake remained unaltered. Weight gain was observed in all three treatments groups. Both GH treatment regimens significantly increased liver, spleen, and lung weight, whereas IGF-I therapy increased spleen, kidney, and heart. Administration of GH twice daily did not increase serum IGF-I or IGFBP-3 concentrations, and the molecular distribution of IGFBP-3 remained unchanged. In contrast, continuous infusion of GH resulted in 5-fold increases in serum IGF-I and increases in IGFBP-3 levels. Size-exclusion chromatography combined with Western ligand blot analysis revealed that radioligand was incorporated into 150- and 60-kDa regions, and that IGFBP-3 was detectable in both regions. Thus, GH infusion was able to induce formation of the 150-kDa ternary complex by increasing circulating levels of IGF-I, IGFBP-3, and presumably ALS. Administration of IGF-I also increased serum IGF-I and IGFBP-3 levels, although the increase in IGFBP-3 was only in the 60-kDa region of the chromatograph, suggesting that IGF-I can induce neither ALS nor formation of the 150-kDa complex.(ABSTRACT TRUNCATED AT 400 WORDS).
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