反刍动物体内尿素循环及其转运蛋白的分子调控机制研究进展

虽然在哺乳动物体内都存在尿素循环利用机制,但是反刍动物由于瘤胃的存在而使得尿素循环在维系机体氮平衡和提高氮素利用效率等方面发挥着更加重要的生物学意义。通过瘤胃壁扩散或转运,血液中的尿素可进入胃肠道,在脲酶的作用下转化为氨态氮,从而为瘤胃微生物蛋白合成提供氮源。研究表明,尿素在瘤胃上皮的自由扩散速度较慢,而尿素转运蛋白可以介导尿素分子高效地进行跨膜转运,其也被认为是反刍动物尿素循环最重要的调控因子。然而,相关报道已经证实,尿素转运蛋白的表达和功能发挥受到日粮营养水平与结构组成、瘤胃内环境、动物年龄等因素的影响。本文以尿素循环为出发点,重点阐述了反刍动物体内尿素循环的特点、影响因素以及尿素转运蛋白的表达特征及其分子调控过程,以期从分子生物学角度解析反刍动物尿素循环的作用机制,从而为生产实践中动物氮素的精准营养提供理论依据和技术支撑。

Progresses of Research on Urea Recycling and Molecular Regulation Mechanism of Urea Transportersin Ruminants

The mechanism of urea recycling is existing in all mammals. However,it plays a crucially biological role in ruminants for maintaining nitrogen balance and improving the efficiency of nitrogen utilization because of the urea transportation in rumen. This specific mechanism in ruminants is able to make blood urea from urinary excretionrepartitioned into the gastrointestinal tract by the free diffusion or transfer throughout the rumen epithelium. Subsequently, the urea would be converted to ammonia nitrogen upon the reaction of urease, which then provides nitrogen source for rumen microbial protein synthesis. Many previous studies have shown that it was rather slow of the speed of urea transferred across the rumen epithelium by free diffusion. However, urea transporters are known to be involved in urea transferring across rumen epithelium and thus facilitate efficiently the urea nitrogen recycling process. Consequently, urea transporters are considered as the most important regulators of the urea recycling mechanism in ruminants. Nevertheless,relevant publications have demonstrated that the expression and function characteristics of urea transporters were regulated by numerous factors,such as dietary nutrients content and structural constitutes,intraluminal environment,and the age of animals. Based on the above backgrounds,this paper was conducted to overview the characteristics of urea recycling in ruminants,and also for the introduction of the effect factors,expression characteristics and the molecular regulation of urea transporters. It is therefore,the present study was aimed to interpret the mechanism of urea recycling in ruminants from the perspective of molecular biology,and then to provide theoretical basis and technical support for the precise nitrogen nutrition of ruminant production practice.