Muscle insulin binding and plasma levels in relation to liver glucokinase activity, glucose metabolism and dietary carbohydrates in rainbow trout Encarnacio ´n Capilla a , Franc ßoise Me ´dale b , Isabel Navarro a , Ste ´phane Panserat b , Christiane Vachot b , Sadasivam Kaushik b , Joaquim Gutie ´rrez a, * a De ´partement de Fisiologia, Facultat de Biologia, D. III, Universitat de Barcelona, Avda. Diagonal 645, Barcelona E-08028, Spain b Fish Nutrition Laboratory, Unite ´ mixte INRA – IFREMER, Station d’Hydrobiologie INRA, Saint-Pe ´e-sur-Nivelle 64310, France Received 11 March 2002; received in revised form 8 August 2002; accepted 30 August 2002 Abstract Rainbow trout were fed for 10 weeks with either a carbohydrate-free diet (C-free) or with four experimental diets containing various levels (20 or 40%) and sources of starch (extruded wheat or peas) in order to examine metabolic utilisation of dietary vegetable carbohydrates and its endocrine control. The study was focused on the parameters described as limiting in glucose metabolism in fish. Feeding trials were conducted at 8 and 18 jC to establish whether carbohydrate-rich diets can be used in trout farming irrespective of water temperature. At both temperatures, pea diets (especially the highest level) resulted in a feed efficiency as high as the C-free diet. Fish had similar growth rates except when fed the low wheat content diet. Glycaemia values 6 h after feeding were significantly higher in trout fed carbohydrate diets than those given the C-free diet, whereas plasma insulin levels were similar independently of the levels of dietary starch. This study provides the first evidence that glucokinase (GK) activity and mRNA level in trout liver increase in proportion to the content of dietary starch. Nevertheless, these changes were not correlated with plasma insulin levels. Insulin-like growth factor-I (IGF-I) binding and number of receptors in skeletal muscle were consistently higher than those for insulin but no diet-induced differences were found for any of these parameters. Temperature clearly affected the postprandial profile of glucose and insulin, which both showed lower levels 6 h after feeding at 8 jC than at 18 jC, which was consistent with a lower feed intake. Glucose and insulin levels decreased markedly 24 h after feeding at 18 jC, while they were still high at 8 jC, an observation concordant with delayed transit rate. These findings indicate satisfactory adaptation of rainbow trout to diets with a relatively high vegetable starch content, especially when provided as extruded peas, and indicate that diets with increased levels of carbohydrates can be used in this species even when it is reared at low temperature. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Dietary starch; Hexokinases; Insulin receptor; IGF-I receptor; Salmonids 1. Introduction Fish use part of the dietary protein supply to cover their energy needs. However, protein economy can be achieved by increasing non-protein energy in the diet, which leads to a reduction in nitrogen discharge into the environment [1,2]. In diets for salmonid, non-protein energy is currently supplied by lipid sources, which are more digestible than complex carbohydrates. The digestibility of starch mainly depends on its structural complexity [3,4]. Heat treatments such as extrusion increase the degree of starch gelatinization and improve digestibility [5,6]. Thus improved, digestible carbohydrates increase protein retention and reduce nitrogen excretion in rainbow trout [7–9]. However, ingestion of high levels of digestible carbohy- drates results in prolonged hyperglycaemia in most species of fish [3,10,11]. This hyperglycaemia lasts longer when rainbow trout is reared at low temperature [12]. Temper- ature modulates fish growth and feed intake and affects the efficiency of carbohydrates as energy suppliers [13]. Never- theless, the endocrine control of fish glucose utilisation at distinct temperatures is poorly documented. At the meta- bolic level, the limited ability of fish to regulate blood glucose level is an impairment in the first step of glycol- 0167-0115/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII:S0167-0115(02)00212-4 * Corresponding author. Tel.: +34-93-4021532; fax: +34-93-4110358. E-mail address: joaquim@porthos.bio.ub.es (J. Gutie ´rrez). www.elsevier.com/locate/regpep Regulatory Peptides 110 (2003) 123 – 132