Abstract
Six isonitrogenous and isocaloric diets were formulated to contain 0% (control), 0.4, 0.8, 1.2, 1.6, or 2.4% dietary cholesterol and fed to juvenile Nile tilapia (Oreochromis niloticus) (2.20 ± 0.12 g) twice daily to apparent satiation for 8 weeks in triplicate at a salinity of 16. Fish fed 0.4% cholesterol showed a higher weight gain and specific growth rate and a lower feed coefficient ratio than fish fed other diets. No difference was found in the survival of Nile tilapia fed various levels of cholesterol. Cholesterol in the serum and liver and low-density lipoprotein cholesterol in the serum increased with the increase in the dietary cholesterol content. Relative to the control, no significant difference was found in the expression of head kidney P450scc mRNA between treatment groups. The expression of head kidney 11β-HSD2 mRNA was the highest in the control group, and it decreased significantly with increasing levels of diet cholesterol. Fish fed 0.4 or 1.2% cholesterol had a higher 20β-HSD2 mRNA expression in the head kidney than those fed other diets. Fish fed 0.8% cholesterol had higher expressions of GR1 and GR2B mRNA in the liver than other groups. Fish fed 0.4% cholesterol had the highest activity of gill Na+/K+-ATPase. Fish fed 0.8 to 2.4% cholesterol had higher serum cortisol contents than the fish in the control group and the fish fed 0.4% cholesterol. This study suggests that dietary cholesterol is not essential for Nile tilapia survival in brackish water, but 0.4% cholesterol supplementation in the Nile tilapia diet contributes to the improvement of hyperosmotic adaptation and increases in gill Na+/K+-ATPase activity and serum cortisol content by regulating the hypothalamic-pituitary-interrenal stress axis.
Highlights
Cholesterol, as a main structural component of animal cell membranes and a precursor for biosynthesis of vitamin D3, prostaglandins, steroids and bile acids, is an essential nutrient for eukaryotic animals (Steffens, 1989; Fast and Boyd, 1992; Sheen et al, 1994)
The contents of cholesterol and low-density lipoprotein cholesterol (LDL-C) in the serum and the content of cholesterol in the liver significantly increased with the increasing percentage of dietary cholesterol, and the fish fed 2.4% cholesterol had the highest contents of cholesterol and LDL-C in the serum (P < 0.01) (Table 4)
The concentration of cortisol in the serum initially increased significantly with increasing levels of dietary cholesterol, but it leveled off when the cholesterol level exceeded 0.8% (P < 0.05) (Figure 4). This is the first report on the effects of dietary cholesterol on the growth performance and osmoregulation of Nile tilapia in a brackish water environment
Summary
Cholesterol, as a main structural component of animal cell membranes and a precursor for biosynthesis of vitamin D3, prostaglandins, steroids and bile acids, is an essential nutrient for eukaryotic animals (Steffens, 1989; Fast and Boyd, 1992; Sheen et al, 1994). Since bony fish have the cholesterol synthesis ability, it is not essential to add dietary cholesterol for the Growth Response to Brackish Water health of bony fish (Sealey et al, 2001; Jobling, 2011). Recent research indicates that cholesterol supplements to a fish diet formulated with soybean meal as a protein source can improve the growth performance of tongue sole (Cynoglossus semilaevis) larvae (Han, 2013) and promote the growth of channel catfish (Ictalurus punctatus) (Twibell and Wilson, 2004). The addition of 0.5–1.5% cholesterol to a fluecured soybean meal diet can significantly improve the activity of digestive enzymes and promote digestion and absorption in Japanese flounder (Paralichthys olivaceus) (Chen, 2006). Our knowledge is limited on other physiological roles that cholesterol can play to improve fish health under stress conditions, cholesterol is closely linked to fish stress through the hypothalamic-pituitaryinterrenal axis (HPI axis) (Mormède et al, 2007)
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