INSULIN-LIKE GROWTH-FACTOR-I (IGF-I) CONCENTRATION IN 150-KILODALTON COMPLEXES CONTAINING HUMAN IGF-BINDING PROTEIN-3 (HIGFBP-3) AFTER INTRAVENOUS-INJECTION OF ADULT-RATS WITH HIGFBP-3

After iv injection into adult rats, human insulin-like growth factor-b inding protein-3 (hIGFBP-3) forms 150-kDa complexes with excess endoge nous rat acid-labile subunit (ALS) within 2 min (Lewitt et ai., 1993, Endocrinology 133:1797). Because their previous in vitro studies indic ated that hIGFBP-3 only bound to ALS in the presence of IG F-I, and be cause little free TGF-I is present in plasma, the authors postulated t hat IGF-I bad been mobilized to the circulation to saturate the 150-kD a hIGFBP-3 complexes. We examined this hypothesis by determining wheth er the hIGFBP-3 that appears in 150-kDa complexes 2 min after iv injec tion is accompanied by an increase in IGF-I. Within 2 min, some of the injected hIGFBP-3 (similar to 30% as much as endogenous intact rat IG FBP-3) is present in complexes that are cleared slowly from the circul ation and presumed to be 150-kDa complexes. Gel filtration and immunop recipitation studies performed on blood collected 2 min after injectio n confirmed that the injected hIGFBP-3 (46-82% as much as rat IGFBP-3) was associated with ALS in 150-kDa complexes. The formation of 150-kD a complexes containing hIGFBP-3 was not accompanied by a corresponding change in the IGF-I content (determined by RIA) of whole serum or 150 -kDa serum fractions, suggesting that the hIGFBP-3 had rapidly associa ted with rALS in vivo without mobilizing IGF-I. Surprisingly, however, hIGFBP-3 was cleared much more rapidly from 150-kDa complexes formed after injection of hIGFBP-3 than after injection of hIGFBP-3:IGF-I com plexes, suggesting that the complexes observed after hIGFBP-3 injectio n might not have formed in vivo. In fact, 150-kDa complexes formed to a similar extent when hIGFBP-3 was added ex vivo to blood collected fr om rats that had not received hIGFBP-3. We conclude that hIGFBP-3 can rapidly associate with rALS to form 150-kDa complexes in vivo without the mobilization of IGF-I. Because 150-kDa complexes also are formed e x vivo, however, we are unable to resolve whether the complexes that f ormed in vivo formed as binary or ternary complexes.