Frontiers in Marine Science (Jul 2014)
Effects of dietary fish oil replacement by vegetable oil on the digestive enzymes activity and intestinal morphology in Meagre, <i>Argyrosomus regius</i> (Asso, 1801)
Abstract
Vegetable oil (VO) has gained importance in nutrition of aquatics organisms by being a strong substitute to fish oil in aquaculture diets. It is unknown, however, any VO blend that present a profile of long chain of fatty acids (PUFA-n3 e n-6) similar to those found in fish oil. On the other hand, there are vegetable oils that present a high proportion of fatty acids n-3, like linseed oil, and n-6, for example the rapeseed oil or soybean. The combination of these oils can be an alternative to replace fish oil in aquatic organisms’ diet. Furthermore, the use of vegetable oils decrease the price of diets, increase the utilization of ingested protein and provide essential fatty acids for the good development of animals [1]. Still the use of vegetable oils has induced intestinal alterations for other fish species with negative consequences on growth [2]. The selenium is an essential micromineral in aquatic organism diets and acts protecting the fatty acid of oxidative damages. Furthermore, problems like a decrease in digestive enzymes activity are attributed a dietary selenium deficiency [3]. This study aimed to analyze digestive enzymes activities of meagre (Argyrosomus regius) fed with different levels of vegetable oil blend (VO) in replacement of fish oil (FO), associated or not with selenium. A total of 600 meagre with an average weight of 20.45 ± 1.4g were distributed randomly in 24 tanks of 80 liters capacity, in a closed recirculation water system During the trial the water temperature was 20.7 ± 0.7ºC and oxygen 8.8 ± 1.7 mg L-1. The fishes were fed with eight experimental diets for 60 days, twice a day for six days per week. In this experiment, we studied experimental diets with two sources of oil (fish or vegetable blend oils with 45% of linseed, 35% of rapeseed and 20% of soybean oil), two lipid levels (12 and 17%) and selenium supplementation (presence or absence). For enzymatic determination of digestive enzymes, the intestine of six fish per treatment were kept and immediately stored in liquid nitrogen. Later, intestine homogenate was prepared with 50 mM Tris-HCl buffer pH 8 containing 200 mM NaCl (1:1 w/v) using homogenizer/blender. Posteriorly, the homogenate was centrifuged at 15.000 g for 30 minutes. The supernatant was collected and stored at -80ºC for posterior analysis. The digestive enzymes analyzed were: lipase (U/mg), amylase (U/mg) and chymotrypsin (U/mg). To evaluate intestinal morphology, portions from posterior intestine were collected and fixed in buffered formaldehyde for 24 hours, following embedment in paraffin. Sections of 6 µm were stained with H.E. and observed under binocular microscope. The results were analyzed by three way factorial. Amylase activity was bigger in FO when compared with VO (Table 1). The same result was observed in chymotrypsin activity. On the other hand, lipase activity was higher in VO. Regarding the levels of lipids, diets with 17% had higher amylase activity than diets with 12%. The inverse was observed in chymotrypsin activity. In relation to lipase activity, no differences were observed on the two levels of lipids studied. No differences in digestive enzymes activities were observed when diets were supplemented with selenium. Epithelium architecture of the posterior intestine was slightly affected by dietary treatments. Higher levels of lipids seem to induce enterocyte vacuolization, and vacuoles seem to be larger when a blend of vegetable oils was used instead of fish oil. No clear role can be attributed to selenium regarding intestinal morphology. In conclusion, our study showed that the source and levels of lipid in diets for meagre have influence in activity of digestible enzymes like amylase, lipase and chymotrypsin. Furthermore, levels of selenium do not cause an alteration in studied digestible enzymes.
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