日粮能量水平对藏羊体内尿素循环的影响及其调控机制研究

The Qinghai-Tibetan Plateau was characterized by high altitude, low temperature, hypoxia, strong UV radiation and short growing season, Tibetan sheep and yaks were the most important indigenous ruminants inhabited in this region. Our previous studies have founded that the Tibetan sheep were low in N maintenance requirement and high in dietary N utilization efficiency, which might be related to some special mechanisms evolved in Tibetan sheep by living a long time in low N stress during the cold season. When the dietary energy was sufficient but N was deficient, the urea recycling in Tibetan sheep was more efficient than the ruminants in low land, such as to maintain a better N balance. However, the Tibetan sheep lose weight seriously when under the traditional grazing management, it was probably due to the inefficient urea recycling while lack of energy. Therefore, a study was conducted to investigate the urea recycling characters and urea transporter B (UT-B) expressions in tissues of digestion systems and its regulation to urea kinetics, in which the dietary CP concentrations were simulated the grass CP levels (6.0% or 10.0%) in cold season while energy level was 60%, 80%, 100%, 120% times of maintenance requirement in sheep. Based on the above study, we will try to explore best protein and energy content feeds for Tibetan sheep, and then to provide some suitable theory and skills for supplements in cold season.

青藏高原具有高海拔、低温、缺氧、强紫外线和牧草生长期短的特点，藏羊和牦牛是栖息在该区域最重要的土著反刍家畜。近年来我们的研究发现，藏羊具有较低的氮维持需要量和高效的日粮氮利用效率的特点，这可能与藏羊长期适应冷季日粮氮素营养胁迫而进化形成的特殊生理代谢机制有关。当日粮能量充足而氮素匮乏时，藏羊的尿素循环利用效率显著高于其他低海拔地区的反刍家畜，从而用以维持体内氮素平衡。然而，藏羊在传统放牧条件下还是发生掉膘严重的现象，这可能是因为冷季能量摄入严重不足而导致尿素循环效率低下。本课题日粮模拟冷季牧草中CP水平（6.0%或10.0%），通过调控每个氮水平下的能量（60%、80%、100%和120%能量维持需要）梯度，研究日粮能量水平对藏羊体内尿素循环特征的影响以及消化系统组织器官中尿素转运蛋白对其的调控机制。通过上述研究，开发藏羊最佳蛋白能量配比饲料，为其在冬季的科学补饲提供理论指导和技术支撑。

本课题经过三年的野外试验和室内分析工作，结合动物饲养试验、消化代谢试验、屠宰试验和体外发酵试验等方法，利用同位素示踪、RT-PCR、 Western blot等技术，从日粮能量水平对藏羊营养物质表观消化率、瘤胃发酵参数、抗逆性氮素代谢特征、尿素循环与转运和体外发酵参等方面进行解析，主要结果如下：1）在采食相同日粮时，藏羊日增重大于小尾寒羊；与小尾寒羊相比，藏羊具有更高的DM、OM和NDF消化率、VFA产量、瘤胃液铵态氮、尿素氮和可溶性蛋白浓度。2）藏羊尿PD排出量和MN合成量高于小尾寒羊；藏羊的日粮可消化氮高于小尾寒羊，粪氮排出量显著低于小尾寒羊。藏羊氮沉积和氮沉积率高于小尾寒羊，且都随能量水平升高而线性增加。3） 藏羊肝脏尿素氮合成量（UER）、进入消化道尿素氮量(GER)、返回鸟氨酸循环的尿素氮量(ROC)和用于合成代谢的尿素氮量(UUA) 、血浆和唾液尿素氮浓度、肾脏尿素氮重吸收率均高于小尾寒羊。4）UT-B mRNA在藏羊和小尾寒羊腮腺、瘤胃和肾脏组织中均有表达；38 kDa可能为UT-B的糖基化蛋白；随日粮能量水平升高，藏羊和小尾寒羊腮腺UT-B mRNA和蛋白表达量均线性降低，藏羊肾髓UT-B蛋白表达量则线性增加。5）藏羊瘤胃背囊和后背盲囊AQP3 mRNA表达丰度显著高于小尾寒羊。随能量升高，瘤胃背囊、瘤胃腹囊、肾脏皮质和髓质AQP3 mRNA表达丰度均线性增加。6）藏羊瘤胃总SCFA产量、瘤胃上皮吸收表面积和与SCFA转运吸收相关的基因表达量高于小尾寒羊；在维持瘤胃上皮细胞内pH稳态过程中，藏羊更依赖于νH⁺-ATPase来保持NHE蛋白活性，而小尾寒羊则更依赖于Na⁺/K⁺-ATPase。综上所述， 本课题结果揭示了不同能量日粮对藏羊低氮条件下营养代谢机制，为藏羊冷季科学补饲提供了理论依据和技术支撑。