The endocrinology of growth in carp and related species

Abstract

The focus of this review is on various aspects of the endocrine control of growth by the brain neuroendocrine-growth hormone (GH)-insulin-like growth factor (IGF) axis in carp and other cyprinids, with reference to other species, including mammals, if there are particular gaps in knowledge. The neuroendocrine regulation of GH secretion in goldfish and other carp is multifactorial, with a balance of stimulatory and inhibitory inputs to somatotrophs. Somatostatin is the primary inhibitor of basal and stimulated growth hormone (GH) secretion. GH secretion is stimulated by GH-releasing factor, gonadotropin-releasing hormone, dopamine, neuropeptide Y, thyrotropin-releasing hormone and cholecystokinin. Sex steroids, in particular estradiol, influence the responsiveness of the somatotrophs to neuroendocrine factors; the responsiveness to gonadotropin-releasing hormone, neuropeptide Y, and thyrotropin-releasing hormone is increased by estradiol, whereas the responsiveness to dopamine and cholecystokinin is greatest in sexually regressed goldfish. Growth hormone, a 188-amino-acid peptide with 5 cysteine residues, stimulates growth through direct actions on some tissues, as well as by stimulation of production of insulin-like growth factor (IGF) production. The liver has a high number of GH receptors and is a primary target organ; GH binding sites have also been demonstrated in gill, intestine, kidney and gonads. GH increases the efficiency of food conversion; GH stimulates intestinal amino acid transport and intestinal mass, which may be one mechanism for the effects on food conversion. A number of factors influence GH receptor number, including GH itself, nutritional status and other hormones. IGFs have been characterized in several salmonid species using molecular biology techniques. Production of IGFs is under stimulation of GH, insulin and other hormones, and is also influenced by nutritional status and metabolic factors. The liver contains the highest concentrations of IGF, although IGFs have been found in a number of other tissues. IGFs appear to travel in the blood bound to specific binding proteins. IGF receptors have been demonstrated in only one teleost species to date. IGF-I stimulates cartilage proteoglycan synthesis; GH is dependent on IGF-I for this action. Growth rates of cultured fish may be stimulated by neuroendocrine factors added to food, administration of GH or recombinant GH preparations, or by producing GH transgenic lines of fish. Techniques for enhancing growth rates of cultured fish are in an experimental stage.