Supplementing the diet with functional ingredients is a key strategy to improve fish performance and health in aquaculture. The amino acids of the urea and nitric oxide (NO) cycles - arginine, ornithine and citrulline - perform crucial roles in the immune response through the generation of NO and the synthesis of polyamine used for tissue repair. We previously found that citrulline supplementation improves and maintains circulating free arginine levels in rainbow trout more effectively than arginine supplementation. Here, to test whether supplementation of urea cycle amino acids modulates the immune response in rainbow trout (Oncorhynchus mykiss), we supplemented a commercial diet with high levels (2% of total diet) of either arginine, ornithine or citrulline during a 7-week feeding trial, before challenging fish with the bacterium Aeromonas salmonicida. We carried out two separate experiments to investigate fish survival and 24 h post-infection to investigate the immediate response of free amino acid levels, and transcriptional changes in genes encoding urea cycle, NO cycle and polyamine synthesis enzymes. There were no differences in percentage fish mortality between diets, however there were numerous highly significant changes in free amino acid levels and gene expression to both dietary supplementation and infection. Out of 26 amino acids detected in blood plasma, 8 were significantly changed by infection and 9 by dietary supplementation of either arginine, ornithine or citrulline. Taurine, glycine and aspartic acid displayed the largest decreases in circulating levels in infected fish, while ornithine and isoleucine were the only amino acids that increased in concentration. We investigated transcriptional responses of the enzymes involved in arginine metabolism in liver and head kidney; transcripts for polyamine synthesis enzymes showed highly significant increases in both tissues across all diets following infection. The paralogous arginase-encoding genes, Arg1a, Arg1b, Arg2a and Arg2b, displayed complex responses across tissues and also due to diet and infection. Overall, these findings improve our understanding of amino acid metabolism following infection and suggests new potential amino acid targets for improving the immune response in salmonids.

中文翻译：

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虹鳟鱼中精氨酸，鸟氨酸和瓜氨酸的补充：对细菌感染的游离氨基酸动力学和基因表达反应。

用功能性成分补充饮食是改善水产养殖鱼类性能和健康的一项关键策略。尿素和一氧化氮（NO）循环中的氨基酸-精氨酸，鸟氨酸和瓜氨酸-通过产生NO和合成用于组织修复的多胺来在免疫反应中发挥关键作用。我们先前发现瓜氨酸补充剂比精氨酸补充剂能更有效地改善和维持虹鳟鱼中的循环游离精氨酸水平。在这里，为了测试补充尿素周期氨基酸是否能调节虹鳟鱼（Oncorhynchus mykiss）的免疫反应，我们在7周内补充了商业饮食中的精氨酸，鸟氨酸或瓜氨酸含量高（占总饮食的2％）喂养试验 然后用沙门氏菌气旋杆菌挑战鱼类。我们进行了两个单独的实验来研究鱼类的存活和感染后24小时，以研究游离氨基酸水平的即时反应以及编码尿素循环，NO循环和多胺合成酶的基因的转录变化。两种饮食之间鱼死亡率的百分比没有差异，但是在饮食补充和感染方面，游离氨基酸水平和基因表达有许多非常显着的变化。在血浆中检测到的26种氨基酸中，有8种因感染而发生显着变化，而9种因饮食中补充精氨酸，鸟氨酸或瓜氨酸而发生了显着变化。牛磺酸，甘氨酸和天冬氨酸在受感染的鱼类中循环水平下降最大，鸟氨酸和异亮氨酸是唯一浓度增加的氨基酸。我们调查了精氨酸代谢在肝脏和头肾中涉及的酶的转录反应。多胺合成酶的转录本在感染后在所有饮食中的两个组织中均显示出非常显着的增加。同源的精氨酸酶编码基因Arg1a，Arg1b，Arg2a和Arg2b在整个组织以及饮食和感染中均表现出复杂的反应。总体而言，这些发现改善了我们对感染后氨基酸代谢的理解，并提出了改善鲑鱼免疫反应的潜在氨基酸新靶标。多胺合成酶的转录本在感染后在所有饮食中的两个组织中均显示出非常显着的增加。同源的精氨酸酶编码基因Arg1a，Arg1b，Arg2a和Arg2b在整个组织以及饮食和感染中均表现出复杂的反应。总体而言，这些发现改善了我们对感染后氨基酸代谢的理解，并提出了改善鲑鱼免疫反应的潜在氨基酸新靶标。多胺合成酶的转录本在感染后在所有饮食中的两个组织中均显示出非常显着的增加。同源的精氨酸酶编码基因Arg1a，Arg1b，Arg2a和Arg2b在整个组织以及饮食和感染中均表现出复杂的反应。总体而言，这些发现改善了我们对感染后氨基酸代谢的理解，并提出了改善鲑鱼免疫反应的潜在氨基酸新靶标。