ADMINISTRATION OF GROWTH-HORMONE (GH), BUT NOT INSULIN-LIKE GROWTH-FACTOR-I (IGF-I), BY CONTINUOUS-INFUSION CAN INDUCE THE FORMATION OF THE150-KILODALTON IGF-BINDING PROTEIN-3 COMPLEX IN GH-DEFICIENT RATS

In the adult circulation, 70-90% of the serum insulin-like growth fact ors (IGFs) are carried by IGF-binding protein-3 (IGFBP-3), which exist s 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 uniquel y 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, w eight and feed and water intake were monitored, and on day 7, liver, k idney, spleen, heart, and lung were weighed, and sera were collected. Serum IGF-I was analyzed by immunoassay, and the molecular distributio n of the IGFBPs was determined by neutral size-exclusion chromatograph y combined with Western ligand blot and Western immunoblot. The GHD ra ts were 40-60% lighter than their normal littermates, and all organs e xamined were proportionately smaller. Serum IGF-I and IGFBP-3 levels w ere less than 10% of those in normal rats. Incubation of serum from GH D rats with I-125IGF-II showed that radiolabel was incorporated only into a 44-kDa IGFBP legion that contained the smaller IGFBPs. IGFBP-3 eluted around 60 kDa. No 150-kDa IGFBP region was detected. The admin istration 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 treat ment regimens significantly increased liver, spleen, and lung weight, whereas IGF-I therapy increased spleen, kidney, and heart. Administrat ion of GH twice daily did not increase serum IGF-I or IGFBP-3 concentr ations, 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 chromatog raphy combined with Western ligand blot analysis revealed that radioli gand was incorporated into 150- and 60-kDa regions, and that IGFBP-3 w as detectable in both regions. Thus, GH infusion was able to induce fo rmation of the 150-kDa ternary complex by increasing circulating level s 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 IGF BP-3 was only in the 60-kDa region of the chromatograph, suggesting th at IGF-I can induce neither ALS nor formation of the 150-kDa complex. These data suggest that continuous administration of GH is superior at restoring the IGF-IGFBP-3-ALS ternary complex in GH deficiency compar ed to twice daily GH injections. ALS and IGF-I are regulated by GH, wh ereas IGFBP-3 is regulated by IGF-I.