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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粗蛋白含量和玉米加工方法不同的羊饲料中尿素转运蛋白和水通道蛋白在尿素分泌到瘤胃中的全身尿素动力学和功能作用


目的是确定膳食粗蛋白 (CP) 含量和玉米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) 在浆膜至粘膜中的功能作用绵羊瘤胃上皮细胞中的尿素通量（Jsm-尿素）。将 32 只 Rideau-Arcott 公羊羔羊按体重分成 4 组，然后在 2 × 2 析因设计中随机分配到 4 种饮食中的 1 种（n = 8）。膳食因素包括 CP 含量（11% [LP] 与 16% [HP]）和玉米谷物加工（全壳 [WSC] 与蒸汽压片 [SFC] 玉米）。使用 4 天连续颈内输注 [15N15N]-尿素并同时收集四块羔羊 (n = 4) 的尿液和粪便来确定全身尿素动力学和氮平衡。节食23天后，处死羔羊，收集瘤胃上皮，安装在Ussing室中以确定Jsm-尿素并测量UT-B和AQP的mRNA丰度。浆膜和粘膜添加根皮素和 NiCl2 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 的羔羊具有更高的氮摄入量 (P < 0.01)（29.4 与 19.1 g/d）；然而，保留的氮（克/天或氮摄入量的百分比）没有不同。与饲喂 WSC 的羔羊相比，饲喂 SFC 的羔羊占氮摄入量的百分比 (P = 0.09) 往往具有较低的氮排泄（72.2% vs. 83.5%）和较高的氮保留（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-N 产量 (UER) 更高（29.9 vs. 20.6 g/d；P = 0.02），而尿素-N 则分泌到肠道（GER；g/d）和尿素中。用于合成代谢目的的-N（UUA；g/d）相似。饲喂 LP 的羔羊往往 (P = 0.05) 具有更大的 GER:UER（0.78 vs. 0.66）和 UUA:GER（0.23 vs. 0.13）比率，以及更大的 Jsm-尿素（144.7 vs. 116）。1纳摩尔/[cm2×h]； P = 0.07) 与那些喂 HP 的人相比。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），并且具有较低的根皮素不敏感 Jsm-尿素（87.1 vs. 143.1 nmol/[cm2]） × h]；P = 0.02) 与饲喂 WSC 的小鼠相比。与饲喂 WSC 的羔羊相比，饲喂 SFC 的羔羊中 UT-B（0.89 vs. 1.07；P = 0.08）和 AQP-3（0.90 vs. 1.05；P = 0.07）的 mRNA 丰度往往较低。总体而言，减少 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。虽然玉米谷物加工对 GER 没有影响，但饲喂 SFC 增加了通过 UT-B 和 AQP 介导的尿素分泌到瘤胃的部分。