Response of urinary purine derivatives excretion to veast RNA infusion levels in the yak abomasum

本研究对3头牦牛施以皱胃瘘管手术,以酵母RNA为嘌呤碱基供体,连续注射4期,以期测定牦牛吸收嘌呤的回收率,为尿嘌呤衍生物估测牦牛瘤胃微生物氮产量的模型积累数据.结果表明,牦牛皱胃连续注射酵母RNA可使尿囊素(564～1 426 μmol/kg BW0.75)、总嘌呤衍生物(purine derivative,PD)排出量(629～1 507 μmol/kgBW0.75)及尿囊素占总PD比例线性提高(0.90～0.95)(P＜0.01)；嘌呤碱基注射水平对尿酸、肌酐及尿氮排出量影响不显著(P＞0.05).回归分析发现,牦牛皱胃嘌呤注射量(X,mmol/d)与尿嘌呤衍生物排出量(Y,mmol/d)间存在线性关系:Y=0.85X+33.02 (R2 =0.96,P＜0.001),牦牛吸收嘌吟在尿中的回收率为85％.     

牦牛瘤胃微生物氮估测模型

通过系统研究发现：青藏高原牦牛、本地黄牛及犏牛内源性嘌呤衍生物每日每千克代谢体重(BW~(0.75))总排出量分别为134、163、138 μmol;尿嘌呤衍生物排出量(PD,mmol/d)与可消化有机物食入量(DOMI,kg/d)线性相关[牦牛:PD=16.02 DOMI + 1.27 (R~2 = 0.75,P<0.001);本地黄牛:PD= 14.42 DOMI + 3.03 (R~2=0.89,P<0.001);犏牛：PD = 20.01 DOMI-0.71 (R~2=0.94,P<0.001)];牦牛皱胃连续注射酵母RNA测得牦牛吸收嘌呤在尿中的回收率为85%,则牦牛小肠吸收嘌呤(X, mmol/d)与尿嘌呤衍生物排出量(Y,mmol/d)的关系为:     

饲粮氮水平对牦牛尿嘌呤衍生物排出量与瘤胃微生物氮产量的影响

本试验旨在探讨牦牛尿中嘌呤衍生物(PD)排出量对饲粮氮水平的响应规律,并基于此估测了瘤胃微生物氮(MN)产量,以期为高寒牧区牦牛的科学饲养提供参考。选取4头体重[(192±12)kg]相近、年龄(3岁)相同的去势公牦牛,采用4×4拉丁方试验设计将牦牛分为4组,各组饲粮氮水平分别是1.03%、1.95%、2.85%和3.76%,每组1头;试验分为4期,每期21 d,包含15 d的预试期和6 d的正试期。结果表明,牦牛尿中PD主要由尿囊素和尿酸组成,尿囊素/PD和尿酸/PD分别为0.69~0.76、0.23~0.30,黄嘌呤与次黄嘌呤的含量极少。当饲粮氮水平升高时,尿中PD、尿囊素、尿酸以及马尿酸排出量均线性增加(P0.05),而尿酸/PD和嘌呤氮指数(PNI)均线性降低(P0.05)。瘤胃细菌嘌呤碱基(RNA当量)含量、瘤胃细菌氮含量以及瘤胃MN产量都随着饲粮氮水平升高而线性增加(P0.05),但饲粮氮用于合成MN的效率[即瘤胃MN/食入氮(NI)]却线性降低(P0.05)。基于尿中PD排出量(mmol/d)和瘤胃MN产量(g/d)与NI(g/d)之间良好的线性关系,构建了如下数学模型:PD=0.58NI+18.28,MN=0.18NI+22.18。综合得出,当牦牛饲粮氮水平为2.85%时,牦牛瘤胃M N产量最大,为42.60 g/d,而PNI以及饲粮氮用于合成M N的效率却在低氮(1.03%)条件下达到最高,这一结果揭示了牦牛对低氮饲粮中氮素营养高效利用的特点,解释了牦牛对青藏高原饲料营养匮乏的适应性的营养机理。     

燕麦干草对藏羊尿中嘌呤衍生物、肌酐及马尿酸排出量的影响

通过研究燕麦干草不同限饲水平对藏羊尿中嘌呤衍生物(purine derivative,PD)、肌酐和马尿酸排出量的影响,为估测藏羊瘤胃微生物蛋白产量及揭示氮代谢机制提供依据。试验采用4×4拉丁方设计,选取5只体况体重相近(45±2)kg、健康的20月龄去势公藏羊,随机分为4组(其中1组为2只藏羊),分限饲试验和绝食试验两阶段进行。4个限饲水平分别被设定为自由采食量(voluntary intake,VI)的30%、50%、70%和90%,试验正试期采用全收尿法以测定藏羊尿中每天嘌呤衍生物的排出量。结果表明:(1)随着限饲水平的降低,马尿酸、PD及各组分排出量线性升高(P0.05),而肌酐排出量不受影响(P0.05)。(2)限饲期,PD、马尿酸排出量均与可消化有机物采食量(digestible organic matter intake,DOMI)线性正相关(P0.05)(3)利用尿中PD与DOMI所建立的模型估测藏羊内源PD排出量为0.09mmol/(kg BW0.75·d)。(4)绝食期,藏羊内源PD排出量为0.11mmol/(kg BW0.75·d)。结果提示,限饲水平与马尿酸、PD及各组分排出量呈负相关(P0.05),但与肌酐排出量无相关性(P0.05);根据PD与DOMI建立的模型估测出内源PD排出量与内源PD实际排出量接近,验证了模型的准确性。     

限饲对东北梅花鹿消化代谢及尿嘌呤衍生物排出量的影响

试验选取4只体况相近的3岁东北梅花鹿公鹿,采用4×4拉丁方设计,饲喂同一种日粮,分4个不同限饲水平A(2.5kg/d)、B(2.0kg/d)、C(1.5kg/d)、D(1.0kg/d),利用尿嘌呤衍生物法并结合消化代谢试验,测定限制饲喂条件下东北梅花鹿尿嘌呤衍生物组成及排出量,可消化干物质和可消化有机物进食量与尿嘌呤衍生物排出量(PD)的关系及各种营养物质消化率。试验结果表明:东北梅花鹿尿中嘌呤衍生物包括49%～56%尿囊素、35%～38%尿酸、6%～15%黄嘌呤和次黄嘌呤;限饲条件下,东北梅花鹿尿中嘌呤衍生物、肌酐酸排出量无显著性差异(P>0.05);可消化干物质DDM(Ir=0.8592,n=16,P<0.001)和可消化有机物DOMI(r=0.8605,n=16,P<0.001)与尿中嘌呤衍生物排出量(PD)均存在显著的线性正相关;限饲使可消化干物质、可消化有机物质进食量和粗蛋白消化率显著降低(P<0.05),使干物质、有机物质、中性洗涤纤维、酸性洗涤纤维及钙、磷表观消化率均显著降低(P<0.05);限饲对尿氮和粪氮排出量无显著性影响(P>0.05),而显著影响氮沉积量(P<0.05),且随饲喂量降低,氮沉积逐渐减少,当饲喂量降到D水平时,出现了氮的负平衡,表明氮的摄入量已不能满足梅花鹿维持需要,需要动用内源氮来满足自身维持需要。     

不同营养水平对4～6月龄荷斯坦犊牛尿中嘌呤衍生物排出量的影响

本试验采用3头(120±5)日龄、体重为(72±2)kg的荷斯坦公犊牛,通过调整日粮营养水平来观察其尿中嘌呤衍生物(PD)的排出规律,比较其与成年反刍动物之间的差异,考察犊牛期能否用尿嘌呤衍生物法来估测瘤胃微生物蛋白。试验设计3种营养水平日粮,采用随机区组试验设计。结果发现,犊牛尿中嘌呤衍生物中尿酸大约为50%～75%,而在成年反刍动物尿酸不到15%。PD的排出量随可消化有机物(DOM)和可消化粗蛋白(DCP)进食量升高而上升,存在弱的相关性(R2=0.76,n=9)。其排出水平为0.12～0.24mmol/(BW0.75·d)。作为估测成年反刍动物微生物蛋白合成量的另一个指标[PD]×BW0.75/[C](其中[PD]为嘌呤衍生物浓度,[C]为尿中肌酐酸浓度,BW0.75为代谢体重,简称PDC),与DOM和DCP进食量存在负相关,这与成年反刍动物的研究结果是相反的。     

日粮不同精粗比对肉羊氮沉积和尿嘌呤衍生物排出量的影响

本试验旨在研究肉羊在不同日粮精粗比条件下氮沉积和尿嘌呤衍生物的排出规律。选择12只体况健康的9月龄、体重(47.21±3.35)kg杜×寒杂交绵羊公羊,试验日粮为12种不同精粗比的全混合颗粒饲料,采用12×4不完全拉丁方设计,进行4期消化代谢试验,试验每期19 d,其中预试期14 d,正试期5 d,采用全收粪、尿法。结果表明:随日粮精粗比的提高,尿氮排出量和氮沉积量均显著提高(P<0.05),日粮精粗比对粪氮排出量的影响不显著(P>0.05),氮沉积量与氮采食量之间存在线性相关(R2=0.72,P<0.05);尿中尿囊素排出量和总尿嘌呤衍生物排出量均随日粮精粗比升高而显著升高(P<0.05),尿囊素、尿酸、黄嘌呤和次黄嘌呤排出量占总尿嘌呤衍生物排出量的比例变化范围分别为69.92%~84.76%、2.89%~7.58%和9.39%~25.04%。日粮精粗比对粪氮与尿氮有不同的影响,对尿嘌呤衍生物排出量影响显著;尿嘌呤衍生物排出量与尿氮排出量存在着线性相关,相关方程为Y=0.429X+0.324(R2=0.85)。     

用尿中嘌呤衍生物估测瘤胃微生物蛋白产量的研究进展

反刍动物尿中嘌呤衍生物包括尿囊素、尿酸、黄嘌呤和次黄嘌呤,尿中嘌呤衍生物是一种很好的估测瘤胃微生物蛋白合成量的标记物。本研究就动物内源嘌呤排泄、尿中嘌呤衍生物排泄量与嘌呤吸收量关系模型建立、全收尿法和点尿法计算微生物蛋白供给以及最近对不同动物(包括绵羊、黄牛、山羊、水牛、牦牛、骆驼、驼羊)嘌呤衍生物排泄的研究等方面,阐述用尿中嘌呤代谢物估测瘤胃微生物蛋白产量的方法。     

尿嘌呤衍生物法估测瘤胃微生物蛋白质产量

尿嘌呤衍生物法具有非侵入性和操作方便等优点,是估测微生物蛋白质产量的一种有效方法.该方法能够准确估测瘤胃微生物蛋白质产量的变化,且其残差变异低于其他方法,估测结果的一致性较好.对其改良后的方法有点采样法和菊糖法.点采样法优点在于可以确定恰当的采样时间,保证尿嘌呤衍生物与肌酐的比例(PD/C)与尿嘌呤衍生物日排出量的高相...     

不同粗饲料组合全混合日粮对泌乳奶牛瘤胃液微生物蛋白浓度24h变化和小肠微生物蛋白流量的影响

本试验研究了不同粗饲料组合全混合日粮(TMR)对荷斯坦泌乳奶牛瘤胃液微生物蛋白(MCP)浓度24 h变化和嘌呤衍生物(PD)法测定小肠MCP流量的影响。选用5头体重(543±45)kg、年龄4岁、泌乳期和日均产奶量相近、安装有大口径瘤胃瘘管的奶牛作为试验动物。采用5×5拉丁方设计,TMR精粗比为61∶39,共包括5个粗饲料组合,分别为100%玉米秸(TMR1)、50%玉米秸+50%玉米秸黄贮(TMR2)、50%玉米秸黄贮+50%全株玉米青贮(TMR3)、50%羊草+50%全株玉米青贮(TMR4)以及34%羊草+33%全株玉米青贮+33%苜蓿(TMR5)。结果表明:每隔2 h采集的不同时间点瘤胃液测定的MCP浓度差异不显著(P>0.05)。不同TMR间瘤胃液MCP浓度存在显著差异(P<0.05),TMR3瘤胃液MCP浓度值最高(1.217 mg/mL),显著高于TMR1和TMR5(P<0.05),但与TMR2和TMR4相比差异不显著(P>0.05)。TMR1至TMR5尿中尿囊素、尿酸、嘌呤衍生物(PD)排出量及由此估计的流入小肠的微生物氮(MN)和MCP合成量依次升高,差异逐渐达到显著水平(P<0.05),5个处理尿囊素占PD排出量的83%~92%,尿酸排出量占PD排出量的8%~17%,且尿囊素(y=50.44x+35.58,R2=0.733)、尿酸(y=27.22x-28.77,R2=0.734)和PD排出量(y=77.67x+6.811,R2=0.734)均与TMR非纤维性碳水化合物/中性洗涤纤维(NFC/NDF)存在线性正相关关系。由此得出,饲喂羊草、全株青贮和苜蓿为TMR粗饲料的奶牛尿中PD排出量及由此估计的流入小肠的MCP合成量均高于其他各TMR。     

Endogenous fraction and urinary recovery of purine derivatives obtained by different methods in Nellore cattle

Two experiments were conducted to assess the endogenous fraction of purine derivative (PD) excretion, urinary recovery, and intestinal digestibility of purines in Nellore heifers. For both experiments, 8 Nellore heifers fitted with ruminal and abomasal cannulas were allocated to two 4 × 4 Latin squares. The diets were based on corn silage and concentrate (60 and 40% DM basis, respectively); feces and urine samples were obtained by total collection, and abomasal DM flow was estimated using indigestible NDF as an internal marker. In Exp. I, 4 of the 8 heifers (BW 258 ± 20 kg) were also fitted with ileal cannula. The planned treatments were 4 different DMI: 1.2, 1.6, 2.0, and 2.4% of BW (DM basis). The endogenous losses and purine recovery as urinary PD were estimated using linear regression between daily urinary PD excretion (Y) and daily abomasal flow of purine bases (X), expressed in millimoles per kilogram of BW(0.75). In Exp. II, the same 8 Nellore heifers (BW of 296 ± 15 kg) were fed at 1.37% BW (DM basis). The treatments were the infusion of purines (RNA from torula yeast, type VI, Sigma) into the abomasum in increasing amounts (0, 33, 66, and 100 mmol/d). All statistical analyses were performed using the PROC MIXED procedure in SAS. In Exp. I, the DMI range was 1.16 to 1.84% of BW and did not affect (P > 0.05) the apparent RNA digestibility in the small intestine, which had a mean of 75.6%, and a true digestibility of 93.0%. The mean ratio of the N-RNA to the total-N in the ruminal bacteria was 0.137. The daily urinary PD excretion (Y, mmol/kg of BW(0.75)) was a function of RNA flow in the abomasum (X, mmol/kg of BW(0.75)): Y = 0.860X + 0.460, where 0.860 and 0.460 were the PD recovery of purines and the endogenous fraction (in mmol/kg of BW(0.75)), respectively. In Exp. II, the daily urinary PD excretion was a function of RNA flow in the abomasum: Y = 0.741X + 0.301, where 0.741 and 0.301 were the recovery of PD in urine of infused purines and the endogenous losses (in mmol/kg of BW(0.75)), respectively. In conclusion, our data suggest that in Nellore heifers the respective values of endogenous PD excretion (mmol/kg of BW(0.75)), urinary recovery of the purines absorbed in the abomasum, and true digestibility of RNA in the small intestine were 0.30, 0.80, and 0.93.     

快速估测反刍动物瘤胃微生物蛋白在小肠流量的方法

选用瘤胃尚未发育的初生小尾寒羊羔羊及梅花仔鹿各4只,人工哺乳,随牛奶分别饲喂给羔羊和仔鹿5个水平的核酸。羔羊尿中嘌呤衍生物的总量(Y,mmol/d)随着摄入嘌呤量(X,mmol/d)的增加而增加。其线性方程为:Y=0.752+0.662X(r=0.995,P<0.001)。仔鹿尿中嘌呤衍生物的总量(Y,mmol/d)随着摄入嘌呤量(X,mmol/d)的增加也明显增加,线性方程为:Y=0.559+0.326X(r=0.990,P=0.001)。利用紫外分光光度计测定的羔羊尿中尿酸当量值与嘌呤摄入量呈强正相关(r=0.994,P<0.01)。由此推算出利用尿中尿酸当量值来估测羊小肠微生物氮流量(Y,g/d)的方程为:Y=0.417+0.396X;推算出的利用尿中尿酸当量值估测鹿小肠微生物氮流量(Y,g/d)的方程为:Y=-4.137+1.245X。X(mmol/d)为测得的羊或鹿尿中尿酸当量值。     

The effect of exogenous purine supply on the endogenous excretion of purine derivatives in the urine of growing lambs

An experiment was made to determine the absorption of purine metabolites in dietary nucleic acids through the digestive tract, and also to determine the utilization of nucleic acids absorbed in the body, using growing lambs. Two pairs of 120‐ and 180‐day‐old twin female lambs with a bodyweight of 18.2–19.0 kg were kept in metabolism crates and fed on purine‐free milk replacement (MR) with supplements of exogenous purine (purine base or purine nucleoside) at a level of 0.2 mmol/BW^0.75/d for 5 consecutive days, and thereafter they were maintained in the crates for 4 days. The daily amount of exogenous purine supply was calculated based on the urinary excreted purine derivatives (PD) in lambs fed on milk replacement alone. A urine sample was collected daily for 9 consecutive days, and the urinary excretion of PD was determined daily. Urinary PD excretion opened to increase within 24 h after the dose of purine bases, and the level was recovered on 3 days after ceasing the exogenous purine supply. The recovery of PD in the urine was about 70% of the purine supplement. When purine nucleosides were added to the feed, urinary PD excretion was initiated within 24 h after dosing, and the values were recovered after ending the purine nucleoside supply. The recovery rate of PD in the urine was only 30% of the supplemented purine. The plasma allantoin levels were almost similar after feeding purine bases and purine nucleosides, and the values were mostly in the range (40–60 µmol/L). These findings indicate that an exogenous purine can be directly incorporated into the body, and the purine as nucleoside is more effectively utilized for the synthesis of nucleic acids than as a purine base in the body of growing lambs.     

Estimation of endogenous phosphorus loss in growing and finishing pigs fed semi-purified diets

Thirty-six barrows were used in a series of 3 P-balance experiments in which growing and finishing pigs were fed highly digestible, semi-purified diets at or below the dietary available P requirement to estimate the effect of BW on endogenous P loss. Experiments 1, 2, and 3 were conducted with pigs averaging 27, 59, and 98 kg of BW, respectively. In each experiment, pigs were placed in metabolism crates and allotted by weight and litter to 3 dietary treatments. The basal diet consisted of sucrose, dextrose, cornstarch, and casein fortified with minerals (except P) and vitamins. Diets 1, 2, and 3 in Exp. 1 were the basal diet with 0, 0.078, or 0.157% added P, respectively, from monosodium phosphate. In Exp. 2 and 3, diets 1, 2, and 3 were the basal diet with 0, 0.067, and 0.134% added P, respectively, from monosodium phosphate. Within replicate, pigs were fed equal amounts of feed twice daily. Pigs were adjusted to treatments for 7 d before a 6-d, marker-to-marker collection of feces and urine. Phosphorus intakes for pigs fed the 3 diets ranged from 1.73 to 3.91 g/d in Exp. 1, from 2.18 to 5.32 g/d in Exp. 2, and from 1.96 to 6.26 g/d in Exp. 3. Fecal P excretion and P absorption increased linearly (P < 0.05) with increasing P intake. In the 3 experiments, urinary P excretion (g/d) was low for pigs fed diet 1 (0.010, 0.011, 0.019) and diet 2 (0.013, 0.058, 0.084) and was low for pigs fed diet 3 in Exp. 1 (0.037); however, urinary P was greater in pigs fed diet 3 in Exp. 2 and 3 (0.550 and 0.486, respectively). When P absorption (Y, g/d) was regressed on P intake (X, g/d) in Exp. 1, 2, and 3, the relationships were linear (P < 0.01): Y = -0.110 + 0.971X (R2 = 0.999), Y = -0.156 + 0.939X (R2 = 0.998), and Y = -0.226 + 0.8919X (R2 = 0.982), respectively. Thus, our estimates of endogenous P loss at zero P intake were 110, 156, and 226 mg/d for 27-, 59-, and 98-kg pigs, respectively. When these Y-intercepts were regressed on BW, the relationship was Y = 63.06 + 1.632X (R2 = 0.996), where Y = endogenous P loss in mg/d and X = BW in kg. Based on these data, we estimate the endogenous P loss of pigs fed highly digestible, semi-purified diets to increase by approximately 1.632 mg for each 1-kg increase in BW from 25 to 100 kg.     

Urinary purine derivates excretion as an indicator of in vivo microbial N flow in cattle: A review

Microbial protein flow to the duodenum may be regarded as the most important and sensitive indicator to optimise rumen metabolism in high-yielding dairy cows. In this review, the methodology and the sources of variation to estimate the duodenal microbial N flow with urinary excretion of purine derivatives (PD) as a non-invasive method is discussed. The urinary PD excretion was linearly related with the amount of purine bases (PB) infused in the abomasum or duodenum, but the recovery of PB in urine differed between experiments. The main sources of variation in the relationship between microbial N flow and urinary PD excretion are dietary contribution of nucleic acids to duodenal flow, varying N:purine ratio in duodenal digesta, differences in intestinal digestibility of nucleic acids and infused PB, and endogenous contribution of PD to urinary excretion. The recycling of PD to the rumen is negligible, and does not explain the incomplete urinary recovery of PD. A large proportion of the total PD is excreted as allantoin in urine. In some experiments this proportion was constant, whereas in others it varied with diet or physiological state of the animal. The excretion of PD in milk is not a suitable indicator of microbial N flow, due to mammary purine catabolism to uric acid and due to the strong positive correlation between milk allantoin excretion and milk yield. Instead of total urine collection, the molar ratio between urinary PD and creatinine can be used to estimate microbial N flow. However, a substantial between-animal variation in this ratio was found, and effects of changes in energy balance of dairy cows on urinary creatinine excretion should be determined. The urinary excretion of total PD and of allantoin provided lower estimates of duodenal microbial N flow than with measurements in the omasum or duodenum, but they closely reflected the changes observed with these measurements.     

Urinary excretion of purine derivatives and plasma allantoin level in sheep and goats during fasting

The relationship between blood plasma level and urinary excretion of allantoin (AN) was examined in sheep and goats during fasting to investigate the possible use of purine derivatives (PD) in urine and/or plasma for estimating the microbial protein production in the rumen, and the further digestion in the lower guts of ruminants. Urinary AN excretion decreased markedly during fasting (0.13 mmol/kgW^0.75 per day), although urinary levels of other PD, hypoxanthine + xanthine and uric acid did not differ irrespective of the feeding condition, that is, feeding, fasting and refeeding in both species. The AN concentration in blood plasma also decreased drastically in the starvation period, and was suddenly increased on refeeding in sheep and goats, and these phenomena were very similar to those of urinary AN excretion. Therefore, there was a high positive correlation between plasma AN level and urinary AN excretion, and the coefficient of correlation was statistically significant (P < 0.01). These results clearly indicate that changes in urinary AN reflect change in plasma AN, which is induced by the catabolism of purine base in the body.     

Splanchnic metabolism of nitrogenous compounds and urinary nitrogen excretion in steers fed alfalfa under conditions of increased absorption of ammonia and L-arginine supply across the portal-drained viscera

Effects of increased ammonia and/or arginine absorption across the portal-drained viscera (PDV) on net splanchnic (PDV and liver) metabolism of nitrogenous compounds and urinary N excretion were investigated in six catheterized Hereford x Angus steers (501 +/- 1 kg BW) fed a 75% alfalfa:25% (as-fed basis) corn-soybean meal diet (0.523 MJ of ME/[kg BW(0.75).d]) every 2 h without (27.0 g of N/kg of dietary DM) and with 20 g of urea/kg of dietary DM (35.7 g of N/kg of dietary DM) in a split-plot design. Net splanchnic flux measurements were obtained immediately before beginning and ending a 72-h mesenteric vein infusion of L-arginine (15 mmol/h). For 3 d before and during arginine infusion, daily urine voided was measured and analyzed for N composition. Feeding urea increased PDV absorption (P < 0.01) and hepatic removal (P < 0.01) of ammonia N, accounting for 80% of increased hepatic urea N output (P < 0.01). Numerical increases in net hepatic removal of AA N could account for the remaining portion of increased hepatic urea N output. Arginine infusion increased hepatic arginine removal (P < 0.01) and hepatic urea N output (P < 0.03) and switched hepatic ornithine flux from net uptake to net output (P < 0.01), but numerical changes in net hepatic removal of ammonia and AA N could not account fully for the increase in hepatic urea N output. Increases in urine N excretion equaled quantities of N fed as urea or infused as arginine. Estimated salivary urea N excretion was not changed by either treatment. Urea cycle regulation occurs via a complex interaction of mechanisms and requires N sources other than ammonia, but the effect of increased ammonia absorption on hepatic catabolism of individual AA in the present study was not significant.