The regulation of insulin-like growth factors in barramundi, Lates calcarifer\ud

A knowledge of the factors which regulate insulin-like growth factors (IGFs) in teleost fish is important for understanding the physiological process of growth and thus devising strategies to improve growth in culture systems. In mammals, two IGF molecules have been identified. Whilst IGF-I is predominantly regulated by growth hormone (GH) and the nutritional status of the animal, GH does not appear to regulate IGF-II which is thought to be important for fetal development. Although the IGF system has been well characterised in mammals, there is a paucity of data on IGFs in teleost fish.\ud \ud The detection of IGFs in teleost species has generally been determined using non-homologous competitive binding assays, often without complete assay validation. In addition, there are few studies which investigate the mechanisms of IGF regulation. The aim of the present study was to determine the effect of nutritional status and water temperature on the growth and regulation of IGFs in juvenile barramundi, Lates calcarifer.\ud \ud Following acidic size exclusion chromatography, circulating IGF-I and IGF-II were detected in the serum of juvenile barramundi using type I and type II radioreceptor (RRA) assays, respectively. Both RRA were rigorously validated using the recommended protocol for the measurement of IGFs in biological fluids (Bang et al., 1994). Peaks containing the IGF molecules were serially diluted and demonstrated parallelism to human IGF-I and IGF-II standard reference curves, in the type I and type II RRA, respectively. IGF binding proteins were identified as a false peak of immunoreactivity, ranging in molecular size from 12.3 - 66 kDa. The IGF binding proteins (IGFBPs) were further characterised using an IGF binding protein assay with subsequent neutral size exclusion chromatography and Western ligand blots. High quantitative recovery was demonstrated in the type I (99 ± 2.7 %) and type II (97 -± 3.4 %) RRA by the addition of unlabelled IGF-I or IGF-II respectively to the serum prior to analysis. Infra- and inter-coefficients of variation were within acceptable literature ranges being 1.7 ± 0.3 % and 8.2 ± 2.1 % respectively for the type I RRA and 3.9 ± 0.6 % and 8.3 ± 2.8 % respectively for the type II RRA. These findings satisfied the requirements of IGF assay validation and thus provided a method for detecting both IGF-I and IGF-II in the serum of juvenile barramundi.\ud \ud The present study demonstrated that ration size regulates the growth, level of circulating IGF-I and expression of hepatic IGF-I mRNA in juvenile barramundi. In contrast, the expression of IGF-I mRNA in the brain, the ratio of the alternatively spliced Ea-4 : Ea-2 IGF-I mRNA transcripts and the concentration of circulating IGF-II were not significantly affected by ration size. As hepatic IGF-I mRNA and circulating levels of IGF-I were reduced during starvation and there was an accompanying decreased growth, it is likely that systemic IGF-I of hepatic origin is important for somatic growth in this species. The response of IGF-I, both pre- (hepatic only) and post-translational, but not IGF-II, to ration size in juvenile barramundi is similar to findings in gilthead seabream, thereby providing further evidence for the general principle of regulation of the GH:IGF-I axis in fish by nutritional status.\ud \ud Dietary protein and energy regulated the growth and level of circulating IGF-I in juvenile barramundi, providing support for the theory of nutritional regulation of IGF-I, but not IGF-II, in this species. Since decreased dietary protein and energy caused a reduction in the concentration of circulating IGF-I, which was accompanied by decreased growth, it is likely that systemic IGF-I is affected by protein and energy restriction in this species. Although the mechanisms of this regulation remain unknown, results from studies conducted in other teleost fish demonstrate that protein and energy restriction result in an insensitivity of the liver to GH. Although the role of IGF-II remains somewhat uncertain in teleost fish, it is reasonable to suggest that IGF-II in fish, as in mammals, is not directly influenced by nutrition or GH.\ud \ud Although water temperature affected growth in both trials there was no clear effect of temperature on circulating IGF-I or IGF-II.\ud \ud The results of this study provide a technique for detecting changes in circulating IGF levels and indicate that environmental parameters, including ration size, dietary protein and energy content and water temperature affect the growth and the synthesis of insulin-like growth factor-I in juvenile barramundi, Lates calcarifer