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高温对母猪泌乳和相关调控激素影响的研究进展

来源:成都市蓄康生物科技有限公司  点击数:2757

(祝贺成都市蓄康生物科技有限公司专注围产期母猪研究4周年)


张浩洁 胡宝玉 熊佳丽 陈婷 孙加节 罗君谊 习欠云 江青艳 张永亮

华南农业大学动物科学学院

摘    要:

母乳的产量和质量对哺乳仔猪的成活率、生长速度及健康程度至关重要,而高温是影响家畜泌乳健康的一个重要环境因素。特别是夏季,动物因长期处于高温环境而容易引发热应激。此外,动物的泌乳过程受到机体内分泌系统的严格调控,涉及多种激素,包括催乳素(PRL)、生长激素(GH)、胰岛素样生长因子-1(IGF-1)等。很多生产数据和研究结果表明,高温会造成母猪采食量降低,泌乳性能下降以及内分泌状态的改变,进而对母猪、仔猪均产生不利影响。为了保证母猪的健康状态及仔猪良好的生长性能,了解高温对母猪泌乳及相关内分泌激素的影响十分必要。作者针对高温对母猪泌乳量、乳成分和泌乳相关激素的影响进行总结,得到高温会降低母猪泌乳量,改变乳糖、乳脂和乳蛋白的含量及泌乳调控激素水平的结论,为后续探索高温影响母猪泌乳的内分泌机制及寻找适宜的缓解高温对母猪泌乳危害的措施提供参考。

关键词:

高温; 母猪; 泌乳; 乳成分; 激素;

作者简介:

张浩洁(1989-),女,河南南阳人,博士生,研究方向:动物生理生化与分子营养,E-mail:zhanghj089@126.com

张永亮(1966-),男,河北黄骅人,教授,博士生导师,研究方向:动物生理生化与分子营养,E-mail:zhangyl@scau.edu.cn


Research Progress on Influence of High Temperature on Lactation and Related Regulation Hormones of Sows

ZHANG Haojie HU Baoyu XIONG Jiali CHEN Ting SUN Jiajie LUO Junyi XI Qianyun JIANG Qingyan ZHANG Yongliang

College of Animal Science,South China Agricultural University

Abstract:

The production and quality of breast milk is crucial for the survival rate,growth rate and health condition of piglets.Hyperthermia is one of the important environmental factors that affect the health of animal lactation.Especially in summer,animals are easily subjected to heat stress due to the long-time high temperature.In addition,the process of lactation is strictly regulated by the body’s endocrine system,involving a variety of hormones,such as prolactin (PRL),growth hormone (GH) and insulin-like growth factor 1 (IGF-1). Many production data and researches indicated that hyperthermia induced the decline of food intake and lactation performance and causes changes in the endocrine state of the animal,which seriously affected sows and piglets.Therefore,in order to guarantee the health status of sows and obtain a better growth performance of piglets,it is necessary to find out how hyperthermia influences the sows and related hormones during the process of lactation.In this review,the authors summarized the effects of hyperthermia on milk production,milk composition and hormones related to lactation,founding that hyperthermia will reduce milk production and change the contents of lactose,milk fat,and lactoprotein,as well as the level of hormones related to lactation.It is expected to provide some references for researches on endocrine regulation mechanisms of environmental factors in animal lactation,as well as explorations on appropriate mitigation measures.

Keyword:

hyperthermia; sow; lactation; milk composition; hormones;


属于恒温动物,可以通过自身体温调节能力维持体温恒定。但当猪长期处于其适温区上限的温度环境中,极易遭受热应激,母猪由于体型较大、代谢率较高[1]及蒸发散热差而对高温更加敏感。泌乳母猪的等热区是12~22℃[2],当环境温度达到28℃时就会使母猪出现中暑,使泌乳母猪的采食量[3]、产奶量[4]下降,延迟母猪断奶后的再发情[5]。下丘脑—垂体—肾上腺轴的激活和血浆糖皮质激素浓度的升高是家畜应对热应激的两个重要表现[6]。此外,瞬时热刺激或者长期高温会刺激动物内分泌腺,进而引起相关激素的分泌与释放。因此母猪泌乳性能的降低很可能是高温导致相关内分泌激素分泌的改变所造成的。作者主要总结了高温对母猪泌乳量、乳成分及参与泌乳调控相关激素的影响,旨在为后续探索高温影响母猪泌乳的内分泌机制以及寻找适宜的缓解措施提供参考。

1 高温对泌乳的影响

乳是初生仔畜获取营养的重要物质。乳成分的变化不仅影响幼仔的生长速度,还能影响其生长模式[7]。除了乳成分之外,泌乳量也是反映哺乳动物泌乳性能的关键指标。环境、品种、泌乳阶段、母畜的营养水平和健康状况及物理刺激如吮吸或挤乳等因素都会影响泌乳量和乳成分[8],其中环境因素对泌乳的影响非常大,很多研究结果表明夏季高温会影响母畜的泌乳量和乳成分。

1.1 高温对泌乳量的影响

高温会对哺乳母猪的泌乳量产生不利影响。高温导致母猪采食量降低,体重减轻,进而泌乳量也降低;影响母猪的正常新陈代谢,造成内分泌失调,增加产后炎症的概率,也会引起泌乳量降低,甚至无乳。研究发现,在炎热季节(22.7~29.4℃)泌乳母猪泌乳期(28d)的泌乳量与温暖季节(20.5~28.2℃)相比下降了1.3kg/d[9]。同样,在实验室条件下,将泌乳母猪分别置于20和29℃下连续哺乳28d,结果显示,相对于20℃,29℃下母猪的哺乳频率升高(39VS 34次/天),泌乳量也从10.43kg/d下降到7.35kg/d[10]。然而在另一项研究中,将泌乳母猪分为两组,第1组在哺乳中期(8~14d)暴露于28℃,哺乳后期(15~21d)转移至20℃;第2组处理与第1组相反,结果发现温度升高并不影响母猪泌乳量(11.0kg/d)[11],这可能与高温的持续时间有关。除了母猪之外,很多研究还报道了高温对其他哺乳动物泌乳量的影响。艾阳[12]选取6头相同泌乳期的荷斯坦奶牛,从6月底到8月初,连续35d在10∶00和18∶00记录气温,统计产奶量,结果发现,随着环境气温由26℃升高至38℃,试验奶牛日均产奶量由15.2 kg/d下降到10 kg/d。Peana等[13]发现,当最高环境温度>21~24℃,母羊的产奶量降低15%,如果最低环境温度从9~12℃上升到18~21℃,产奶量下降20%。由此可见,通常情况下高温会降低母猪及其他哺乳动物的泌乳量,但具体的影响会因温度差异和持续时间而有所不同。

1.2 高温对乳成分的影响

目前关于高温对牛乳成分影响的研究比较多,大部分结果表明高温会导致牛乳品质下降,乳脂、乳蛋白[14]和乳糖[15]的含量降低。高温对母猪乳成分影响的研究相对缺乏,在已有的报道中,早期研究发现温度高低(30和20℃)对母猪初乳和常乳中乳脂率和蛋白质含量没有任何影响[16-17]。随后Renaudeau等[10]在研究高温和日粮蛋白质水平对母猪产奶量和仔猪生长性能的影响时发现,日粮蛋白质水平的降低不会影响产奶量、乳成分及仔猪生长性能,但当温度从20℃上升到29℃,哺乳14d的母猪乳中干物质含量从18.1%升高到18.6%,能量含量提高4%。同样,Silva等[9]研究发现,在22.7~29.4℃且湿度为93.7%的环境下母猪乳中能量和脂肪含量均升高。而Farmer等[18]研究发现,母猪在29℃时乳中干物质和脂肪的含量要比20℃时的少,且乳糖含量不受温度的影响,这可能是由于高温下体内存储脂肪的分解可以维持动物血液内正常的葡萄糖水平的缘故。

总之,高温会降低母猪的泌乳量并改变乳成分。一方面可能是高温条件下母猪采食量降低,营养摄入不足而导致母猪乳汁分泌不足和质量不佳[4];另一方面可能是高温改变了通过血流进行的养分重分配而引起泌乳量的降低及乳成分的变化[19]。

2 高温对泌乳相关激素的影响

高温除了影响哺乳动物的泌乳量和乳成分,还会影响机体的激素水平。而动物乳腺的发育及泌乳的启动和维持都会受到一系列激素的调控[20],包括垂体前叶分泌激素、性腺激素和一些调节整体代谢的激素。此外,如上文所述,高温影响动物的泌乳量和乳成分与采食量减少和养分分配变化有关,而研究发现泌乳期的采食量和养分分配与同期激素的变化也相关[21]。因此高温可能通过相关激素直接或间接影响动物的泌乳量和乳成分,其对母猪内分泌激素的影响值得关注。

2.1 催乳素

催乳素(prolactin,PRL)是维持乳腺细胞新陈代谢所必需的激素,作为泌乳过程中的一个“综合信号”参与到泌乳的多个方面。PRL可以促进乳的生成,PRL浓度的降低会导致泌乳量下降。给山羊体内注射多巴胺颉颃剂抑制体内PRL的水平,会导致山羊产奶量下降28%[22]。研究发现,在热应激下,荷斯坦奶牛[23]或者摩拉水牛[24]的循环PRL水平增加。而大部分研究表明高温对母猪PRL基础水平没有影响,Barb等[25]将泌乳9d后的母猪分别置于30℃和20℃,在泌乳第24和25天时检测PRL含量,结果发现两个温度下的PRL含量没有差异。随后Messias等[4]和Farmer等[26]的研究也得到类似的结果。但是Farmer等[18]在另一项研究中指出,与常温组(20℃)相比,高温组母猪(30℃)PRL浓度有下降趋势(P=0.09)。虽然高温可导致动物产奶量的下降[27-28],且PRL是促进动物泌乳的主要激素,但是高温下,PRL水平与泌乳量的变化之间并无明显规律,因此PRL很可能并不是高温下动物泌乳能力降低的主要作用激素。

2.2 生长激素

生长激素(growth hormone,GH)是由垂体前叶的生长激素细胞分泌和释放的一种蛋白类激素。GH可以促进乳腺发育和维持泌乳,Kleinberg等[29]发现GH可以刺激牛乳腺上皮细胞的大量增殖。有研究表明利用垂体或重组牛生长激素可以提高产奶量[30]。高温会影响GH的水平,当环境温度从20℃提高到30℃,母猪GH的基础浓度提高,同时GH对促甲状腺激素释放激素(thyrotropin-releasing hormone,TRH)的反应也增强[25]。虽然高温影响GH的机制还不清楚,但高温对GH的影响可能与动物采食量降低有关,因为在限饲的泌乳母猪中同样出现了GH浓度升高的现象[31]。但也有不少研究指出热应激会降低奶牛体内循环的GH水平[32-33],而且这种影响的产生依赖于营养水平[34]。

2.3 促卵泡激素和促黄体生成素

促卵泡激素(follicle-stimulating hormone,FSH)和促黄体生成素(luteinsing hormone,LH)是由垂体前叶分泌的一类促性腺激素。LH和FSH在动物泌乳期的分泌是受抑制的,同时仔畜吮吸的次数与LH的脉冲频率呈负相关[35]。关于热应激对奶牛LH和FSH影响的报道比较多,但结果却不尽一致:热应激可能会不影响[36]、促进[37]或降低[38]奶牛LH浓度水平,不一致的原因可能是不同的研究中采样的频率不同或者热应激的程度不同(急性和慢性)。另外还发现热应激条件下奶牛FSH与雌二醇水平的变化相一致,Gilad等[39]报道在急性或慢性热应激下,低浓度的FSH伴随着低浓度的雌二醇,而在雌二醇浓度正常的奶牛中没有检测到FSH浓度的变化。Ronchi等[23]发现高温或常温暴露下奶牛FSH的脉冲频率和幅度及基础浓度都没有变化。此外,高温下奶牛也会出现FSH浓度升高的情况,这主要是抑制FSH分泌的激素浓度降低所引起的[40]。高温对母猪促性腺激素影响的报道相对较少,Barb等[25]发现,在泌乳第9天,处于22℃的母猪与处于30℃的母猪其LH平均水平没有明显差异,但在第24天时,30℃下的母猪其LH脉冲频率降低,这可能也是高温使母猪断奶后发情延迟的原因。

2.4 雌激素

雌激素(estrogen)主要是由卵巢和胎盘产生的固醇类激素,它不仅可以促进乳腺腺泡导管的发育,还可以促进腺泡的发育及乳汁的生成[41]。给小母牛连续注射雌激素,可以极高地提高母牛的产奶量[42]。关于高温对泌乳母猪雌激素水平影响的报道极少,已有的研究结果表明,泌乳母猪雌二醇-17β水平在高温环境中要低于凉爽环境中的[43],而雌二醇的降低又与高温环境下母猪断奶后再发情的延迟有关。这种情况在其他哺乳动物中也会出现,如高温会降低奶牛血浆雌二醇浓度[44]。还有研究发现,母羊暴露在36.8℃、70%湿度的环境中48h,卵泡雌二醇的合成活性也会降低[45]。相对于急性热应激,在长期慢性热应激中更容易引起雌二醇水平的下降[46]。

2.5 胰岛素样生长因子-1

胰岛素样生长因子-1(insulin-like growth factor-1,IGF-1)是GH诱导靶细胞产生的具有多种生理功能的活性蛋白多肽物质,也是一种有效的促泌乳激素[47]。有研究指出高温对母猪IGF-1的影响与GH相反,30℃下母猪具有更低的IGF-1水平[4],此外热应激会降低母猪在泌乳第2天和第21天的IGF-1浓度[18]。Lucy等[48]详细地记录了热应激条件下母猪妊娠期和泌乳期IGF-1水平的变化,结果显示,热应激会降低妊娠60~110d内的IGF-1水平,但不影响泌乳期IGF-1的水平。通常情况下,GH可以刺激肝脏合成和分泌大量IGF-1,但在高温条件下,IGF-1可能不受GH的影响,从而出现了与GH不一致的变化。事实上,关于IGF-1与热应激的关系还存在争议,一些研究者发现热应激条件下的奶牛,其IGF-1的水平可能出现轻微下降[49],也可能没有变化[50]。

2.6 皮质醇

皮质醇(cortisol,COR)是由肾上腺分泌的一种糖皮质类激素,该类激素对应激原刺激反应敏感。在奶牛上,COR可以促进乳腺叶泡系统分化发育,这是PRL促进乳腺蛋白合成的前提条件[51]。有研究指出高温可以刺激下丘脑—垂体—肾上腺轴的功能,使猪血浆COR浓度升高[52],同样,奶牛[53]和水牛[54]的血浆COR浓度也会随着环境温度的升高而显著上升。但也有研究发现,泌乳母猪在产后第4天和第19天以及断奶后第1天,高温组(30℃)COR浓度低于常温组(20℃)[4]。COR属于分解代谢激素,高温条件下,母猪COR浓度降低,会降低机体的分解代谢,进而减少体热的产生,导致高温条件下机体储备动用不足以及泌乳量减少。

2.7 甲状腺激素

甲状腺激素(thyroid hormones)是由甲状腺分泌的一种含碘激素,主要有甲状腺素(thyroxine,T4)和三碘甲腺原氨酸(triiodothyronine,T3)两种。甲状腺激素可以促进泌乳[51],同时对促乳类激素如PRL和GH也有促进作用[55]。在泌乳期间,甲状腺激素连同胰岛素和COR可使养分朝着乳生成和内脏蛋白的合成分布。有研究发现泌乳奶牛血浆中T3和T4的水平在热应激时下降[56],这种甲状腺激素的下降还会伴随着血浆GH水平的降低从而达到降低机体产热的共同作用[57]。高温对母猪的甲状腺激素也有影响,在热应激期间无论是泌乳母猪还是断奶后的母猪,T3的浓度都会显著降低[4-5],但高温对母猪T4浓度的影响并不稳定,在泌乳期间,与饲养于20和22℃的母猪相比,饲养于30℃的母猪T4的分泌变化不一致,有不受影响[5]、降低不显著[25]或者显著降低[4]3种结果。由于甲状腺激素是控制体热产生的重要激素,且T3相对于T4活性更高,所以T3在高温下的显著下降很可能与产热减弱有关。此外环境温度的升高可能会诱导脱碘酶活性降低,导致血液中T3/T4的比率降低[58]。

综上所述,高温确实会引起一系列内分泌激素水平的变化,而哺乳动物乳合成和分泌受到催乳素与其他多种激素的协同调控[59],因此高温可能通过引起相关激素水平的变化而影响哺乳动物的泌乳量和乳成分。但截至目前,不同的研究结果并没有明确高温对单一或多种激素的影响以及由此产生的对动物泌乳量和乳成分的影响。高温对内分泌激素的不确定影响可能与试验的测定时间、动物的生理状态和泌乳阶段及激素自身的分泌特点有关。因此高温通过内分泌激素影响泌乳的机制还需要更加细致和深入的研究。

3 小结

环境高温会引起泌乳母猪内分泌性的适应性变化,这对母猪自身的采食量、泌乳量和繁殖性能都会产生不利影响。很多研究已经证明高温会降低母猪的泌乳量,改变乳成分及泌乳调控的相关激素水平,并推测这些变化可能与高温造成的采食量下降及机体为减少产热而降低代谢率等有关,但高温影响泌乳的具体作用机制还不太清楚,尚需进一步研究。

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