The objectives were to determine the effects of dietary crude protein (CP) content and corn grain processing on whole-body urea kinetics and the functional roles of urea transporter-B (UT-B) and aquaporins (AQP) in serosal-to-mucosal urea flux (Jsm-urea) in ovine ruminal epithelia. Thirty-two Rideau-Arcott ram lambs were blocked by bodyweight into groups of 4 and then randomly allocated within blocks to 1 of 4 diets (n = 8) in a 2 × 2 factorial design. Dietary factors were CP content (11% [LP] vs. 16% [HP]) and corn grain processing (whole-shelled [WSC] vs. steam-flaked [SFC] corn). Whole-body urea kinetics and N balance were determined using 4-d continuous intrajugular infusions of [15N15N]-urea with concurrent collections of urine and feces with four blocks of lambs (n = 4). After 23 d on diets, lambs were killed to collect ruminal epithelia for mounting in Ussing chambers to determine Jsm-urea and the measurement of mRNA abundance of UT-B and AQP. Serosal and mucosal additions of phloretin and NiCl2 were used to inhibit UT-B- and AQP-mediated urea transport, respectively. Lambs fed HP had a greater (P < 0.01) N intake (29.4 vs. 19.1 g/d) than those fed LP; however, retained N (g/d or % of N intake) was not different. As a % of N intake, lambs fed SFC tended (P = 0.09) to have a lower N excretion (72.2 vs. 83.5%) and a greater N retention (27.8 vs. 16.6%) compared to those fed WSC. Endogenous urea-N production (UER) was greater in lambs fed HP compared to those fed LP (29.9 vs. 20.6 g/d; P = 0.02), whereas urea-N secreted into the gut (GER; g/d) and urea-N used for anabolic purposes (UUA; g/d) were similar. Lambs fed LP tended (P = 0.05) to have greater GER:UER (0.78 vs. 0.66) and UUA:GER (0.23 vs. 0.13) ratios, and a greater Jsm-urea (144.7 vs. 116.1 nmol/[cm2 × h]; P = 0.07) compared to those fed HP. Lambs fed SFC tended to have a lower NiCl2-insensitive Jsm-urea (117.4 vs. 178.4 nmol/[cm2 × h]; P = 0.09) and had a lower phloretin-insensitive Jsm-urea (87.1 vs. 143.1 nmol/[cm2 × h]; P = 0.02) compared to those fed WSC. The mRNA abundance of UT-B (0.89 vs. 1.07; P = 0.08) and AQP-3 (0.90 vs. 1.05; P = 0.07) tended to be lower in lambs fed SFC compared to those fed WSC. Overall, reducing CP content tended to increase the GER:UER ratio with no changes in the expression or function of UT-B and AQP. Although corn grain processing had no effects on GER, feeding SFC increased the portion of urea secretion into the rumen that was mediated via UT-B and AQP.

中文翻译：

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尿素转运蛋白和水通道蛋白在粗蛋白含量和玉米籽粒加工方法不同的饲喂性日粮中尿素分泌到瘤胃中的全身尿素动力学和功能作用


目的是确定日粮粗蛋白 （CP） 含量和玉米籽粒加工对全身尿素动力学的影响，以及尿素转运蛋白-B （UT-B） 和水通道蛋白 （AQP） 在绵羊瘤胃上皮细胞血清-粘膜尿素通量 （Jsm-urea） 中的功能作用。将 32 只 Rideau-Arcott 公羊羔羊按体重分成 4 组，然后在 2 × 2 因子设计中随机分配到 4 种饮食中的 1 种 （n = 8）。饮食因素是 CP 含量（11% [LP] 对 16% [HP]）和玉米粒加工（全壳 [WSC] 与蒸汽片 [SFC] 玉米）。使用 4 天连续颈静脉内输注 [15N15N]-尿素，同时收集尿液和粪便与 4 块羔羊 （n = 4） 来确定全身尿素动力学和 N 平衡。日粮 23 天后，杀死羔羊以收集瘤胃上皮细胞，以安装在 Ussing 室中，以确定 Jsm-尿素以及 UT-B 和 AQP 的 mRNA 丰度的测定。根皮素和 NiCl2 的浆膜和粘膜添加分别用于抑制 UT-B 和 AQP 介导的尿素转运。饲喂 HP 的羔羊比饲喂 LP 的羔羊摄入更多的 （P < 0.01） 氮摄入量（29.4 vs. 19.1 g/d）;然而，保留的 N （g/d 或 N 摄入量的百分比） 没有差异。就氮摄入量的百分比而言，与饲喂 WSC 的羔羊相比，饲喂 SFC 的羔羊往往 （P = 0.09） 具有较低的氮排泄量（72.2 vs. 83.5%）和较高的氮保留率（27.8 vs. 16.6%）。与饲喂 LP 的羔羊相比，饲喂 HP 的羔羊的内源性尿素氮产量 （UER） 更高（29.9 vs. 20.6 g/d;P = 0.02），而分泌到肠道中的尿素 N （GER;g/d） 和用于合成代谢目的的尿素 N （UUA;g/d） 相似。饲喂 LP 的羔羊往往 （P = 0.05） 具有更高的 GER：UER （0.78 vs. 0.66） 和 UUA：GER （0.23 vs. 0.13） 比率，以及更高的 Jsm-尿素 （144.7 vs. 116.1 nmol/[cm2 × h];P = 0.07）与饲喂 HP 的羔羊相比。饲喂 SFC 的羔羊往往具有较低的 NiCl2 不敏感 Jsm-尿素（117.4 vs. 178.4 nmol/[cm2 × h];P = 0.09），并且根皮素不敏感的 Jsm-尿素较低 （87.1 vs. 143.1 nmol/[cm2 × h];P = 0.02）。UT-B 的 mRNA 丰度 （0.89 vs. 1.07;P = 0.08）和 AQP-3 （0.90 vs. 1.05;P = 0.07）与饲喂 WSC 的羔羊相比，饲喂 SFC 的羔羊的产量往往较低。总体而言，降低 CP 含量往往会增加 GER：UER 比率，而 UT-B 和 AQP 的表达或功能没有变化。尽管玉米谷物加工对 GER 没有影响，但饲喂 SFC 增加了通过 UT-B 和 AQP 介导的尿素分泌进入瘤胃的部分。