热应激对奶牛泌乳性能的影响及其机制

随着温室效应加剧,奶牛热应激问题日益凸显,对乳业造成巨大经济损失。热应激是一复杂的生理应答过程,奶牛在温热环境下表现为呼吸和心率加快,直肠温度升高,采食量下降,对其内分泌系统和免疫系统造成负面影响,严重降低奶牛泌乳性能。目前关于奶牛热应激的报道多集中于生产试验,特别是饲喂功能性饲料添加剂对应激的缓解作用,但其具体作用机制尚不清晰。文章介绍了热应激对国内外不同地区奶牛产奶量和乳品质的广泛影响,并从能量代谢、内分泌、氧化应激、细胞凋亡和自噬等方面综述了热应激对奶牛泌乳性能的影响机制。能量代谢方面,从奶牛采食量减少、脂质分解和能量代谢紊乱等内在分子学机理角度解释了奶牛在热应激下处于能量负平衡状态的原因;内分泌方面,介绍了热应激对奶牛下丘脑-垂体-肾上腺轴/甲状腺轴/性腺轴/生长轴的调控,分析了激素变化对机体的影响及其作用机制;氧化应激方面,重点阐述了热应激通过影响机体内ROS水平从而产生氧化应激的分子机制及激活的相关防御信号通路;细胞凋亡和自噬方面,介绍了高温胁迫引起奶牛乳腺上皮细胞损伤,细胞凋亡相关基因表达引发的内源性和外源性细胞凋亡,而过度自噬引发的细胞损伤也对乳腺泌乳起负面调控作用。笔者指出,在可预见的未来,热应激将是奶牛养殖业面临的最大难题,应建立可控的奶牛热应激模型运用于生产实践研究,并加强奶牛乳腺上皮细胞水平的基础研究,结合高通量数据分析技术,系统揭示热应激的发病机制,为缓解热应激提供全面的理论依据。

Effect of Heat Stress on Lactation Performance in Dairy Cows

With the steady aggravation of global warming, dairy cows face increasing heat stress, which results increasing economic loss to the dairy industry. Heat stress occurs when cows are exposed to extreme heat and cannot maintain their core temperature. Under hot temperature environment, dairy cattle is characterized by increased respiration, heart rate, and rectal temperature and decreased feed intake, which have a negative impact on endocrine system and immune system and seriously reduce milk performance of dairy cows. Previous studies of heat stress in dairy cows focused on its effects on productive performance, especially to alleviate the effect of heat stress by functional feed additives, but the specific mechanism of heat stress is not clear. This paper summarizes the widespread impact of heat stress on milk production and quality of dairy cows in different regions, and introduces the known mechanisms of heat stress from aspects of energy metabolism, endocrine, oxidative stress, apoptosis and autophagy. In the aspect of energy metabolism, heat stress induces a negative energy balance in cows by reducing food intake and inducing disorder in lipid breakdown and energy metabolism; In terms of endocrine system, heat stress affects the hypothalamic-pituitary-adrenal axis / thyroid axis / gonadal axis / growth axis of dairy cows and results in hormonal changes. In oxidative stress, heat stress affects the ROS levels in vivo and activates related defense signal pathways. In the aspect of apoptosis and autophagy, heat stress contributes to the damage of mammary epithelial cells by the expression of apoptosis-related genes and by excessive autophagy. The paper put forward that in the foreseeable future, heat stress will be a major challenge for the dairy industry. It is necessary to establish cows heat stress models to comprehensively investigate the basic mechanisms of heat stress in dairy cows using advanced cellular and molecular techniques. In addition, efforts should be placed to develop new interventions to reduce the damage of hear stress to dairy cows.