Rearing temperature enhances hepatic glucokinase but not glucose-6-phosphatase activities in European sea bass (Dicentrarchus labrax) and gilthead sea bream (Sparus aurata) juveniles fed with the same level of glucose P. Enes a,b, ⁎, S. Panserat c , S. Kaushik c , A. Oliva-Teles a,b a Departamento de Zoologia e Antropologia, Faculdade de Ciências, Universidade do Porto, 4099-002 Porto, Portugal b CIMAR/CIIMAR-Centro Interdisciplinar de Investigação Marinha e Ambiental, Universidade do Porto, 4050-123 Porto, Portugal c INRA, UMR1067 Nutrition Aquaculture & Genomique, Pôle d'hydrobiologie, CD918, F-64310 St-Pée-sur-Nivelle, France ABSTRACT ARTICLE INFO Article history: Received 21 January 2008 Received in revised form 16 April 2008 Accepted 16 April 2008 Available online 23 April 2008 Keywords: European sea bass Gilthead sea bream Glucokinase Glucose Glucose-6-phosphatase Temperature The aim of this work was to elucidate if the previous results observed in hepatic glucokinase (GK) and glucose-6- phosphatase (G6Pase) activities in European sea bass and gilthead sea bream are due to temperature per se or to differences in feed intake at different water temperatures. For that purpose triplicate groups of ﬁsh (30 g initial body weight) were kept at 18 °C or 25 °C during two weeks and fed a ﬁxed daily ration of a glucose-free or 20% glucose diet. At the end of the experimental period, plasma glucose levels in both species were not inﬂuenced by water temperature but were higher in ﬁsh fed the glucose diet. Higher hepatic GK activity was observed in the two ﬁsh species fed the glucose diet than the glucose-free diet. In the glucose fed groups, GK activity was higher at 25 °C than at 18 °C. Glucose-6-phosphatase activities in both species were not inﬂuenced by water temperature. In European sea bass and in contrast to gilthead sea bream it was observed an effect of dietary composition on G6Pase activities with surprising higher activities recorded in ﬁsh fed the glucose diet than in ﬁsh fed the glucose- free diet. Overall, our data strongly suggest that European sea bass and gilthead sea bream are apparently capable to strongly regulate glucose uptake by the liver but not glucose synthesis, which is even enhanced by dietary glucose in European sea bass. Within limits, increasing water temperature enhances liver GK but not G6Pase activities, suggesting that both species are more able to use dietary carbohydrates at higher rearing temperatures. © 2008 Elsevier Inc. All rights reserved. 1. Introduction Glucose plays a key role as energy source in mammals but its importance in ﬁsh appears limited (Wilson, 1994; Hemre et al., 2002; Stone, 2003). Although, ﬁsh have the enzymatic machinery and metabolic pathways required for carbohydrate metabolism (Cowey and Walton, 1989; Dabrowski and Guderley, 2002) a very high hyperglycemia is generally observed after feeding carbohydrate-rich diets or after the performance of glucose tolerance tests, particularly in carnivorous species (Wilson, 1994; Peres et al., 1999). Until now, the physiological basis for such apparent glucose intolerance is not fully understood (Moon, 2001; Hemre et al., 2002). Hepatic glucokinase (GK, EC 2.7.1.2) and glucose-6-phosphatase (G6Pase, EC 3.1.3.9) are key enzymes playing major roles in the regulation of glycolytic and gluconeogenic pathways, respectively (Printz et al., 1993; van de Werve et al., 2000). Glucokinase catalyses the phosphorylation of glucose to glucose-6-phosphate and thus plays a key role in hepatic glucose utilization, whereas G6Pase catalyses the last step of hepatic glucose production, by hydrolysing glucose-6-phosphate into glucose. Thus, a lack of signiﬁcant regulation of hepatic glucose utilization and production may possibly explain the relative inability of ﬁsh to efﬁciently utilize dietary glucose (Panserat et al., 2001). As in mammals, an increase of hepatic GK activity with the increase of dietary carbohydrate was reported in several ﬁsh species (Tranulis et al., 1996; Borrebaek and Christophersen, 2000; Panserat et al., 2000a) including gilthead sea bream (Panserat et al., 2000a; Caseras et al., 2002; Enes et al., 2008a) and European sea bass (Enes et al., 2006a; Moreira et al., 2008). Regarding G6Pase, although Shimeno et al. (1995) reported a decrease of hepatic enzyme activity with increasing dietary carbohydrate levels in common carp Cyprinus carpio, such effect was not observed in European sea bass (Enes et al., 2006a), gilthead sea bream (Caseras et al., 2002) and rainbow trout Oncorhynchus mykiss (Panserat et al., 2000b, 2001). Water temperature is an important environmental factor that modulates ﬁsh growth. It also affects among other factors, feed intake, efﬁciency of carbohydrate utilization as energy source or the activity of intermediary metabolism enzymes (Médale et al., 1991, 1999; Brauge et al., 1995; Shikata et al., 1995; Peres and Oliva-Teles, 1999; Enes et al., 2006b, 2008b; Moreira et al., 2008). For instance, in common carp increasing water temperature improves starch Comparative Biochemistry and Physiology, Part A 150 (2008) 355–358 ⁎ Corresponding author. Departamento de Zoologia e Antropologia, Faculdade de Ciências, Universidade do Porto, 4099-002 Porto, Portugal. Tel.: +351 22 340 1507; fax: +351 22 340 1511. E-mail address: enes@portugalmail.com (P. Enes). 1095-6433/$ – see front matter © 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.cbpa.2008.04.596 Contents lists available at ScienceDirect Comparative Biochemistry and Physiology, Part A journal homepage: www.elsevier.com/locate/cbpa