饲料中淀粉和糊精对红鳍东方鲀生长和低氧耐受性的影响

Effects of dietary starch and dextrin on growth and hypoxia tolerance in tiger puffer (Takifugu rubripes)

中文关键词: 急性低氧  碳水化合物  代谢  低氧诱导因子  红鳍东方鲀

英文关键词: Acute hypoxia  Carbohydrate  Metabolism  Hypoxia inducible factors  Takifugu rubripes

中文摘要:

      在水产养殖鱼粉鱼油供应相对紧缺的背景下，充分挖掘碳水化合物(糖类)的营养功能具有重要意义。低氧是水产养殖中一种常见的环境应激。大量研究表明，鱼类在低氧条件下主要利用糖来提供能量，但饲料糖源对红鳍东方鲀(Takifugu rubripes)的生长和急性低氧耐受性尚未见报道。本研究分别在饲料中添加玉米淀粉或糊精(玉米淀粉水解物)作为糖源，养殖8周后，测定生长、急性低氧存活率、代谢产物和低氧诱导因子(HIF)信号通路等指标。结果显示，与淀粉组相比，糊精虽然不会影响红鳍东方鲀的生长性能和鱼体成分，但显著提高了其在急性低氧条件下的存活率。在常氧条件下，淀粉组和糊精组肝脏的糖原和乳酸含量均无显著差异(P>0.05)，但在低氧条件下，糊精组肝脏的乳酸含量和乳酸脱氢酶基因表达量均显著高于淀粉组(P<0.05)。同时，糊精组肝脏hif-1α (hypoxia inducible factor 1 subunit alpha)和hif-3α (hypoxia inducible factor 1 subunit alpha, like)、肌肉hif-1α和vegfa (vascular endothelial growth factor A)的表达量均显著高于淀粉组(P<0.05)。在低氧条件下，糊精组血清甘油三酯(TG)含量显著升高，但淀粉组血清和肝脏的TG含量在低氧后却显著下降(P<0.05)。与常氧组相比，糊精组在低氧后的肌肉总蛋白含量、肝脏蛋白激酶B (v-akt murine thymoma viral oncogene homolog 1, akt1)和哺乳动物雷帕霉素靶蛋白(mechanistic target of rapamycin kinase, mtor)基因表达量均显著降低，但淀粉组肝脏的mtor基因表达在低氧后却显著升高(P<0.05)。综上所述，用易消化的糊精替代玉米淀粉激活了HIF信号通路介导的无氧糖酵解来为鱼体提供更多的能量，同时还可以抑制脂肪的分解和蛋白质的合成，减少了O2的消耗，从而提高了红鳍东方鲀对急性低氧的耐受性。本研究结果为鱼类耐低氧饲料的设计和健康养殖提供了重要参考。

英文摘要:

      Fish meal and fish oil are important sources of protein and lipid in feeds. The lack of fish meal and fish oil supply has become an important limiting factor for the aquaculture industry. The proportion of fish oil and fish meal used in commercial feeds for farmed fish and crustaceans is continually decreasing, while the carbohydrate content is increasing. Carbohydrates are one of the three major nutrients that provide energy for the body. They have a much lower production price than protein and fat. Starch is a polymeric carbohydrate made from glucose molecules; it is the most commonly used in aquatic feeds and can replace fish meal and fish oil to reduce feed cost. Therefore, it is important to fully explore the nutritional function of carbohydrates for aquaculture. Hypoxia is a common stress in aquaculture, and acute hypoxia may lead to massive mortality of cultured fish in a short period of time resulting in serious economic losses to aquaculture. Therefore, it is crucial to find ways to improve the acute hypoxia tolerance of fish. Fish mainly use glucose for energy under acute hypoxia. Hypoxia inhibits oxidative phosphorylation in mitochondria and activates the anaerobic glycolysis pathway. Glucose degrades to produce lactate and ATP. Carnivorous fishes have limited absorption and utilization capacities to feed carbohydrates, unlike omnivorous fishes. Dextrin is an intermediate starch hydrolysis product, with a molecular weight between starch and glucose; it has good adhesion properties and is more easily digested and absorbed than starch. Therefore, we hypothesized that the use of easily digestible carbohydrate in the feed is an effective way to improve the acute hypoxia tolerance of fish. Takifugu rubripes is loved by Japanese and Korean consumers owing to its delicious taste and high nutritional value. It is a characteristic species of Chinese mariculture fish. It is mainly cultured in a high-density factory, and the dissolved oxygen in the water often relies on water exchange and an oxygenation pump; its gill cover is degraded and is at risk of acute hypoxia. This study determined the effect of the addition of corn starch or dextrin (corn starch hydrolysate) to the feed on growth performance, acute hypoxia survival rate, metabolite content, and the hypoxia inducible factor (HIF) signaling pathway of T. rubripes after 8 weeks of feeding. There were no significant differences in weight gain, feed conversion ratio, hepatosomatic index, viscerosomatic index, condition factor, and body composition between the starch group and the dextrin group (P>0.05). However, the survival rate during acute hypoxia of the dextrin group was significantly higher than that of the starch group (P<0.05). There were no significant differences in the liver glycogen and lactate contents, and lactate dehydrogenase A4 (ldha) gene expression between the starch group and dextrin group (P>0.05) during normoxia. However, the lactate content and ldha gene expression were significantly higher in the liver of the dextrin group than those in the starch group (P<0.05) during hypoxia. This indicated that feeding dextrin strongly activated anaerobic glycolysis to provide more energy under hypoxia. The serum triglyceride (TG) content significantly increased in the dextrin group compared with the normoxia groups, although the TG content in the serum and liver significantly decreased in the starch group after acute hypoxia (P<0.05). This suggested that feeding starch promoted lipid catabolism and oxygen consumption. There was no significant difference in the total soluble protein content of the muscle between the starch group and the dextrin group (P>0.05) during normoxia. However, the total soluble protein content of the muscle decreased in the dextrin group compared with the starch group (P<0.05) after acute hypoxia. Meanwhile, the total muscle soluble protein content, and the gene expression of liver v-akt murine thymoma viral oncogene homolog 1 (akt1), and mechanistic target of rapamycin kinase (mtor) significantly decreased in the dextrin group after hypoxia compared with the normoxia group. However, the liver mtor gene expression significantly increased in the starch group after hypoxia (P<0.05). This data demonstrated that protein synthesis was inhibited in the dextrin group under hypoxia. Hypoxia inducible factor (HIF) is the most critical transcription factor in cellular response to hypoxic stress. The dextrin group had higher hypoxia inducible factor 1 subunit alpha like (hif-3α) gene expression in the liver and lower hypoxia inducible factor 1 subunit alpha a (hif-1α) gene expression in the muscle (P<0.05) compared with the starch group during normoxia. The gene expression of liver hif-1α and hif-3α and muscle hif-1α and vascular endothelial growth factor A (vegfa) significantly increased in the dextrin group compared with the starch group (P<0.05) during hypoxia. This proved that feeding dextrin strongly activated the HIF signaling pathway under hypoxia. In summary, replacing starch with easily digestible dextrin in the feed did not affect the growth performance. Instead, it activated the HIF signaling pathway and anaerobic glycolysis to provide more energy for fish. Meanwhile, feeding dextrin inhibited lipid catabolism and protein synthesis, and reduced oxygen consumption to improve the acute hypoxia tolerance of T. rubripes. The study provides important guidance for the formulation design of hypoxia-tolerant feed and healthy development of aquaculture.