64 SEPTEMBER 2013 • WORLD AQUACULTURE • WWW.WAS.ORG in vitro (Niki et al. 1985). Similarly, in some fish species, the presence of a vitamin C/E sparing mechanism has been suggested (Lovell et al. 1984, Hamre et al. 1997) and reported to influence growth, tissue composition or immune responses. In this sense, higher contents of vitamin E were measured in larvae fed a diet supplemented with vitamin C, suggesting that vitamin E was being recycled by vitamin C or that the latter was being preferentially employed as an antioxidant, sparing vitamin E (Betancor et al. 2013a). Moreover, vitamin C is considered the most important antioxidant in extracellular fluids (Sies 1997) and is able to inhibit lipid peroxidation. Finally, although the vitamin C levels employed are elevated (1800-3600 mg/kg), levels of ascorbic acid higher than those required for growth are necessary to satisfy the demands of larvae in the case of other nutrients, such as vitamin E (Hamre et al. 1997). On the other hand, selenium exerts a certain effect on AOE expression, observed as a decrease in SOD gene expression. In addition, selenium gives rise to several selenoproteins, some of them with marked antioxidant capacity. To date, 38 selenoprotein genes have been identified in fish, larger than for all terrestrial vertebrates, including humans (Mariotti et al. 2012). Among selenoproteins, GPX represent a major class of functionally important selenoproteins with eight GPX isoforms currently characterized in humans (Mariotti et al. 2012) at different cellular and organic localizations. As observed in several juvenile and adult fish species, a synergism between vitamin E and selenium (Poston et al. 1976, Bell et al. 1985) and a sparing mechanism between vitamin E and C is observed. The type of radical generated endogenously in a given disease state is not known. Therefore, these nutrients may be suitable when high levels of DHA are included in the diet, but may not counteract free radicals properly when other prooxidants are employed. Response of Muscle to Oxidative Stress Skeletal muscle is one of the main organs affected by oxidative stress (Betancor et al. 2011, 2012a, 2012b, 2013a, 2013b). However, the kind and degree of incidence of muscular lesions associated with dietary vitamin E imbalances differ among fish species and according to nutritional status, age, size, diet quality and feeding period (Moccia et al. 1984). For instance, young Atlantic salmon developed severe muscular dystrophy when fed a vitamin E deficient diet (Poston et al. 1976), whereas no lesions were found in adult fish (Bell et al. 1985). The reason why muscle was primarily affected could be related to the high metabolism of this organ, which results in a great consumption of oxygen, with the DHA-containing phospholipids very susceptible to damage by free radicals (Hulbert et al. 2002). On the other hand, vitamin E may be necessary to promote membrane repair (Howard et al. 2011). Thus, if fish are not provided enough vitamin E, muscular lesions will appear, regardless of the dietary DHA content. This could explain why larvae fed diets with low DHA content (1 percent) developed muscular lesions when fed 1500 mg of vitamin E (Betancor et al. 2011). In European seabass larvae, muscular lesions had the typical histological features of nutritional muscle dystrophy, characterized by a segmental necrosis affecting red and white fibers (Betancor et al. 2012a). Semithin sections provided more information about the chronology of nutritional muscular dystrophy (Figure 1A, Betancor et al. 2012a,b, Bentancor et TABLE 2. Some pathological effects of free radicals on several adult and juvenile fish species and tissues. Sign Species Source of oxidation Author Jaundice Seriola quinqueradiata Unknown Sakai et al. 1989. 1998 Nutritional muscular dystrophy Cyprinus carpio VitE DEF Watanabe et al. 1970 Cyprinus carpio OO+VitE DEF Miyazaki 1986 Ictalurus punctatus Se+VitE DEF Gatlin et al. 1986 Ictalurus punctatus ↑ FA Lewis et al. 1985 Ictalurus punctatus OO+VitE DEF Murai and Andrews 1974 Lates calcarifer VitE DEF Bowater 2007 Salmo gairdneri OFO Cowey et al. 1984 Salmo gairdneri VitE+Se DEF Bell et al. 1985 Oncorhynchus mykiss VitE VitC VitE+C DEF Frischknecht et al. 1994 Haemolysis Salvelinus fontanalis Kawatsu 1969 Salmo gairdneri OO+VitE/C DEF Smith 1979 Salmo gairdneri OFO+VitE/Ethoxyquin Moccia et al. 1984 Salmo salar VitE+Se DEF Poston et al. 1976 Skeletal abnormalities Hippoglossus hippoglossus OFO Lewis McCrea and Lall 2007 Ceroid pigment Seriola quinqueradiata Unknown Sakai et al. 1989 Salmo salar OO Roald et al. 1981 DEF: Deficiency; OO: Oxidized oil; OFO: Oxidized fish oil
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