选取144头生长猪（初始体重为28.2±1.9 kg），采用完全随机区组设计，分为9个处理，每个处理8栏，每栏2头猪。采用3（3个水平的亮氨酸（101%、200%或299%SID Leu:Lys）） × 3（3个水平的色氨酸（18%、23%或28% SID Trp:Lys））析因法配制9种试验日粮，配制满足SID Leu和 SID Trp需求的基础日粮，并在基础日粮中添加L-Leu和(或)L-Trp配制其他8种试验日粮。在试验开始和21 d结束时记录猪的体重。试验最后一天，每个猪圈宰杀1头猪，采集血液和下丘脑样本，测定血浆尿素氮、血浆血清素和下丘脑血清素浓度。
结果表明，日粮Trp水平的提高增加了下丘脑血清素水平（P < 0.05），而平均日增重（ADG）和平均日采食量（ADFI）的提高仅在Leu水平超标时显著（P < 0.05）。增加日粮Leu降低了ADG、ADFI和下丘脑血清素（P < 0.05）。但添加299% SID Leu:Lys时，日粮Trp对ADG和ADFI的影响大于添加101% SID Leu:Lys时（交互作用，P < 0.05）。日粮Leu对血浆Leu浓度有正向影响，Trp对血浆Leu浓度有负向影响，但添加299% SID Leu:Lys对血浆Leu浓度的负向影响大于添加101% SID Leu:Lys 组（交互作用，P < 0.05）。日粮Trp和Leu水平的提高对血浆Trp浓度有显著的正向影响，但Leu水平在101% SID Leu:Lys时，血浆Trp浓度的提高高于299% SID Leu:Lys时（交互作用，P < 0.05）。
Effects of dietary leucine and tryptophan on serotonin metabolism and growth performance of growing pigs
An experiment was conducted to test the hypothesis that increased dietary Trp is needed in high-Leu diets for growing pigs to prevent a drop in plasma serotonin and hypothalamic serotonin concentrations and to maintain growth performance of animals. A total of 144 growing pigs (initial weight: 28.2 ± 1.9 kg) were assigned to 9 treatments in a randomized complete block design with 2 blocks, 2 pigs per pen, and 8 replicate pens per treatment. The 9 diets were formulated in a 3 × 3 factorial with 3 levels of dietary Leu (101%, 200%, or 299% standardized ileal digestible [SID] Leu:Lys), and 3 levels of dietary Trp (18%, 23%, or 28% SID Trp:Lys). A basal diet that met requirements for SID Leu and SID Trp was formulated and 8 additional diets were formulated by adding crystalline L-Leu and (or) L-Trp to the basal diet. Individual pig weights were recorded at the beginning of the experiment and at the conclusion of the 21-d experiment. On the last day of the experiment, 1 pig per pen was sacrificed, and blood and hypothalamus samples were collected to measure plasma urea N, plasma serotonin, and hypothalamic serotonin concentrations. Results indicated that increasing dietary Trp increased (P < 0.05) hypothalamic serotonin, whereas increases (P < 0.05) in average daily gain (ADG) and average daily feed intake (ADFI) were observed only in pigs fed diets containing excess Leu. Increasing dietary Leu reduced (P < 0.05) ADG, ADFI, and hypothalamic serotonin. However, the increase in ADG and ADFI caused by dietary Trp was greater if 299% SID Leu:Lys was provided than if 101% SID Leu:Lys was provided (interaction, P < 0.05). Plasma Leu concentration was positively affected by dietary Leu and negatively affected by dietary Trp, but the negative effect of Trp was greater if 299% SID Leu:Lys was provided than if 101% SID Leu:Lys was provided (interaction, P < 0.05). Plasma concentration of Trp was positively affected by increased dietary Trp and increased dietary Leu, but the increase in plasma concentration of Trp was greater if Leu level was at 101% SID Leu:Lys ratio than at 299% SID Leu:Lys ratio (interaction, P < 0.05). In conclusion, increased dietary Leu reduced ADG, ADFI, and hypothalamic serotonin concentration, and influenced metabolism of several indispensable amino acids, but Trp supplementation partly overcame the negative effect of excess Leu. This demonstrates the importance of Trp in regulation of hypothalamic serotonin, and therefore, feed intake of pigs.