Effect Of Different Dietary Carbohydrate Levels On Growth Performance, Immunity And Carbohydrate Metabolic Enzymes In Wuchang Bream (Megalobrama Amblycephala)

Wuchang bream (Megalobrama amblycephala), belongs to Osteichthyes, Cypriniformes, Carp, Bream subfamily, is an important economic herbivorous fish, is one of China’s conventional freshwater fish, because of its wide feeding habits, low aquaculture cost, fast growth rate, high survival rate with high-quality meat and tastes delicious. In our study, we investigated the optimal carbohydrate level in the diet of Wuchang bream, the effect of high carbohydrate levels diet on the nutritional physiology of Wuchang bream, and the effect of high carbohydrate levels diet on the growth, blood and liver physiological function of Wuchang bream. This study includes six aspects as follows:1. Effects of different dietary carbohydrate levels on growth performance, plasma biochemical parameters, activities of hepatic carbohydrate metabolic enzymes and expression of hepatic growth hormone gene in Wuchang bream(Megalobrama amblycephala).The results showed that the weight gain rate and specific growth rate of the fish in 31% carbohydrate (CHO) group were significantly higher than those in other groups (P<0.05). The feed conversion ratio (FCR) was significantly lower than those in other groups(P<0.05). The feeding rate increased significantly (P<0.05), with the carbohydrate levels increased (0-31%), but then decline. The broken-line regression analysis of weight gain against dietary starch level indicated that the optimal dietary carbohydrate level for maximum weight gain of juvenile Wuchang bream is 34% of dry diet. The hepatosomatic index(HSI), viserosomatic index (VSI) and protein efficiency ratio (PER) of the fish in control group (no CHO) were significantly lower than those in other groups (P<0.05). With the levels of dietary carbohydrate increased, the HSI and VSI showed an increasing trend. Hepatic glycogen content increased with dietary carbohydrate levels increased (P<0.05). The body lipid content of the fish in control group was significantly lower than those in other groups (P<0.05). Different dietary carbohydrate levels significantly affected activities of hepatic glucokinase (GK) and glucose-6-phosphatase (G6Pase) (P<0.05). The activity of hepatic GK of the fish was highest in 38% and 47%CHO group. The activity of hepatic G6Pase of the fish were higher in control and 19%CHO groups. With dietary carbohydrate levels increased, the plasma glucose levels showed an increasing trend. The plasma cholesterol of the fish in 19% and 31%CHO groups were significantly lower than those in other groups (P<0.05). The expression of hepatic growth hormone gene of the fish in 38%CHO group was significantly higher than those in other groups (P<0.05).2. Effects of different dietary carbohydrate levels on plasma immune parameters, hepatic antioxidant capacities and HSP70 gene expression in Wuchang bream (Megalobrama amblycephala).The results showed that with the levels of dietary carbohydrate increased, the serum total protein, lysozyme, C3 (complement 3), alkaline phosphatase (AKP) and acid phosphatase (ACP) increased firstly and then decreased. The serum total protein, lysozyme, C3 (complement 3), AKP and ACP of the fish in 31%CHO group were higher than those in other groups (P<0.05). The plasma cortisol showed an increasing trend. The plasma lactate, insulin, blood leukocyte respiratory burst activity of the fish showed a trend similar to serum total protein. The plasma lactate, insulin, blood leukocyte respiratory burst activity of the fish in 38%CHO group were significantly lower than those in other groups (P<0.05).The plasma glutamic-oxaloacetic transaminase (GOT) of the fish showed a trend opposite to serum total protein. The GOT of the fish in 31%CHO group was lower than those in other groups (P<0.05). With the levels of dietary carbohydrate increased, hepatic superoxide dismutase (SOD) activity and expression of hepatic heat shock protein 70 (HSP70) of the fish were firstly increased and then decreased.The hepatic SOD activity and expression of hepatic HSP70 of the fish in 38%CHO group were higher than those in other groups (P<0.05). The hepatic total antioxidant capacity (T-AOC) showed the same trend as hepatic SOD activity. The hepatic T-AOC of the fish in 31%CHO group was higher than those in other groups. The hepatic malondialdehyde (MDA) showed a trend opposite to hepatic SOD activity. The hepatic MDA of the fish in 31%CHO group was lower than those in other groups (P<0.05).3. Effects of high dietary carbohydrate on the growth performance, plasma biochemical parameters, hepatic antioxidant abilities and HSP70 gene expression in Wuchang bream (Megalobrama amblycephala).The results showed that fish fed the high CHO diet had significantly lower specific growth rate but significantly higher feed conversion ratio than those fed the control diet. But the weight gain and feeding rate of the fish in two groups were not significantly different. No significant differences among protein, ash content in whole body were observed. However, the lipid content in whole body of Wuchang bream tended to increase with increasing dietary carbohydrate levels. Fish fed the high CHO diet had significantly higher plasma cortisol and triglyceride levels than fish fed the control diet, however, fish fed the high CHO diet had significantly lower hepatic SOD activity and hepatic T-AOC than those fed the control diet. Higher hepatic MDA levels and lower AKP activity were observed in fish fed the high CHO diet. No significant differences were observed in glucose, cholesterol and insulin between two groups. The hepatic glycogen content and the expression of hepatic HSP70 gene of Wuchang bream fed high CHO diet were significantly higher compared to those of fish fed the control diet. Transmission electron microscopy studies showed that hepatocytes of fish fed with high CHO diet contained large lipid droplets, causing displacement of cellular organelles to periphery of hepatocytes. Fish fed the high CHO diet had higher cumulative mortality than those fed the control diet after hypoxia challenge. The present study suggested that high dietary CHO influenced the growth performance, hepatic antioxidant abilities and health of Wuchang bream.4. Effects of high dietary carbohydrate on plasma biochemical parameters and hepatic antioxidant indexes metabolism in Wuchang bream(Megalobrama amblycephala).The results showed that the level of blood glucose in the high carbohydrate (CHO) group higher than that in the normal group during fast period, but not significant (P>0.05), the levels of plasma insulin, cortisol and plasma lactate in high CHO group were significantly higher than those in the normal group, but the plasma AKP level in high CHO group was significantly lower than that in the normal group (P<0.05). During the fast period, the levels of hepatic pyruvic acid, pyruvate kinase, MDA and the expression of HSP70 in the high CHO group were significantly higher than those in the normal group, but the levels of hepatic SOD and T-AOC in high CHO group were significantly lower than those in normal group (P<0.05).5. Effects of high dietary carbohydrate on growth performance, plasma biochemical parameters and expression of hepatic growth hormone gene in Wuchang bream (Megalobrama amblycephala) at two temperatures.The results showed that the crude protein, ash, serum cholesterol, triglycerides and hepatic G6Pase activity of the fish reared at 30℃ were significantly reduced, but the hepatic glycogen and GH gene expression levels of fish significantly increased (<0.05), compared to those of the fish reared at 25 ℃. The WG, SGR, protein efficiency ratio and hepatic G6Pase in control group (31% of CHO) were significantly higher than those in high CHO group (47% of CHO) at each temperature, but the FCR, HSI, body crude lipid, plasma glucose and hepatic GK activity of the fish in control group were significantly lower than those in high CHO group (P<0.05). For HIS, condition factors, hepatic GK activity and the level of GH gene expression of the fish respectively, there was an interaction between temperature and dietary carbohydrate (P<0.05).6. Effects of high dietary carbohydrate on plasma immune parameters, hepatic antioxidant capacity and HSP70 gene expression at two temperatures.The results showed that the plasma GOT, lactate, cortisol, hepatic MDA, hepatic pyruvate and the level of HSP70 gene expression of the fish in control group (31% CHO) were significantly lower than those in high CHO group (47% of CHO) at each temperature, but the plasma lysozyme, C3, AKP, ACP and hepatic T-AOC in the control group were significantly higher than those in high CHO group (P<0.05) at each temperature. Compared to those of the fish reared at 30℃, the plasma C3, leukocyte respiration burst activity and hepatic SOD of the fish reared at 25 ℃ significantly increased, but the hepatic MDA of the fish reared at 25℃ significantly decreased (P<0.05). For the plasma lysozyme, C3, GOT, AKP, blood leukocyte respiratory burst activity and hepatic SOD of the fish respectively, there was an interaction between temperature and dietary carbohydrate. The liver tissue of the fish in high CHO group reared at 30℃ showed that the shape of hepatocytes were irregular and a lot of lipid vacuoles in hepatocytes.