DEVELOPMENT OF A PROTEIN-BINDING ASSAY FOR TELEOST INSULIN-LIKE GROWTH-FACTOR (IGF)-LIKE - RELATIONSHIPS BETWEEN GROWTH-HORMONE (GH) AND IGF-LIKE IN THE BLOOD OF RAINBOW-TROUT (ONCORHYNCHUS-MYKISS)

Using rainbow trout plasma protein (IGF-BP) which specifically binds h uman insulin-like growth factor (IGF) (Niu and Le Bail 1993), we have developed an assay to measure plasma IGF-like levels in different tele ost species. Before the assay and to prevent interference by IGF-BP, I GF-like was extracted from all samples, using SP Sephadex C-25 in acid ic conditions. After this treatment, contamination of the IGF fraction by IGF-BP which was estimated by binding assay, was approximately 5%, and was not detectable by western ligand blot. Human IGF-I was used a s standard and labelled hormone. Sensitivity of the assay was 0.15-0.4 0 ng/ml (ED90) and ED50 was 1-3 ng/ml. hIGF-II crossreaction was parti al and no significant displacement was observed with Insulin from diff erent species or with other hormones. Inhibition curves were obtained with plasma IGF fractions (but not with tissue extracts) from teleost and mammals and are parallel to the standard curve. These results sugg est that the protein binding assay can quantify an IGF-like factor in the plasma of teleost and that the binding sites of IGF are well conse rved during vertebrate evolution. Using this IGF binding assay, IGF-li ke was measured in parallel with growth hormone (GH) in plasma from yo ung rainbow trout killed every 1.5h throughout one day. The daily prof iles for both hormones, which appear pulsatile, are similar. A signifi cant correlation was observed between GH levels and IGF-like levels wi th a 1.5h delay. Analogous observations were obtained in individual ca theterized adult rainbow trout. Although plasma GH levels differ great ly between fish, less variability is found with IGF-like. In a third e xperiment, rainbow trout were starved or submitted to bovine GH treatm ent for four weeks. Starved fish, in which plasma GH levels increased, had plasma IGF-like level significantly lower than in fed fish. In bG H injected fish, plasma IGF-like level was significantly higher than i n non-injected fish. These results suggest that, as in mammals, IGF-li ke secretion depends on plasma GH level and could be modulated by the nutritional status of fish.