Regulation of insulin-like growth factor-binding protein-3 ternary complexin feline diabetes mellitus

The 140 kDa ternary complex of insulin-like growth factor-binding protein-3 (IGFBP-3), IGFs and an acidlabile subunit (ALS) has previously been shown to be decreased in diabetes mellitus in humans and rats. We have studied IG F-I levels and ternary complex formation in normal and diabetic cats. Total IGF-I concentrations, measured by RIA using des(1-3)-IGF-I as tracer were (+/- S.E.M.) 54+/- 13nmol/l in eight normal and 227+/-57nmol/l in eight dia betic cats (P<0.01). The size-distribution of IGFBPs in the cat circulation was determined by incubation with I-125-IGF-II and Superose 12 chromatogra phy. In normal animals 26 +/- 2% of the I-125-IGF-II were in a 140kDa form compared with 48 +/- 5% in diabetic cats (P<0.01). When samples from normal and diabetic animals were co-incubated 52 +/- 3% were at 140 kDa. A simila r shift was seen when normal cat and normal human serum were co-incubated. A 2-fold increase in the 140 kDa form in diabetic cats was con-firmed first by size-fractionating samples and then performing a ligand-binding assay w ith I-125-IGF-I or -II and charcoal separation. SDS-PAGE and Western ligand blotting demonstrated a 45kDa doublet (presumably IGFBP-3) and 30-35 kDa f orms. There were no apparent differences between normal and diabetic profil es on SDS-PAGE, suggesting that a proportion of IGFBP-3 which circulates 'f ree' in normal cats forms a ternary complex in the diabetic circulation. We conclude that (i) in contrast to humans and rats, ALS is the limiting fact or for ternary complex formation in normal cats, (ii) ALS concentrations in crease in feline diabetes mellitus and, by promoting ternary complex format ion, this leads to an increase in total IGF-I concentrations, and (iii) tot al IGF-I concentrations may not be reliable in the diagnosis of acromegaly in diabetic cats.