2. 北京农学院, 奶牛营养学北京市重点实验室, 北京 102206
2. Beijing Key Laboratory for Dairy Cow Nutrition, Beijing University of Agriculture, Beijing 102206, China
现代奶牛养殖过程中通过大量饲喂高精料饲粮增加能量摄入量,维持奶牛的高产奶量[1],实现生产效益最大化。然而,这种饲喂方式在提高生产水平的同时,也可能诱发亚急性瘤胃酸中毒(subacute ruminal acidosis,SARA),其特征是瘤胃液pH降低(5.2~5.8)且每天持续时间超过3 h[2]。SARA严重影响瘤胃发酵、破坏瘤胃上皮完整性[3]、诱发炎症[4]、降低奶牛干物质采食量和产奶量等。Kleen等[5]调查,约26%处于泌乳期的奶牛患有SARA,北美国家奶牛产业每年因SARA损失5亿~10亿美元[6];而在中国,SARA引起的产奶量下降、淘汰率和死亡率增加所造成的经济损失更为严重。因此,SARA已成为奶牛产业发展的重要制约因素。
选择合适的营养性添加剂可以有效预防和缓解SARA,从而维持瘤胃内环境稳态。过去研究认为成年奶牛瘤胃微生物可合成90%的硫胺素,不需从饲粮中获取[7-8]。然而,近年来大量研究发现,当奶牛饲喂高精料饲粮发生SARA时,瘤胃中硫胺素酶含量增多,微生物合成硫胺素的量降低,内毒素(lipopolysaccharides,LPS)和促炎抑制细胞因子共同抑制了硫胺素载体(THTR)-1和THTR-2的基因表达,从而出现硫胺素缺乏症[9-12],因此,在饲粮中添加硫胺素对缓解奶牛SARA有重要意义。本文综述了硫胺素对处于SARA状态下奶牛的瘤胃微生物区系和结构、代谢产物及瘤胃上皮屏障的影响,旨在进一步认知硫胺素对SARA的缓解机制及为生产中有效预防和控制SARA的发生提供理论参考。
1 硫胺素概述及在奶牛上的应用硫胺素又称为维生素B1,是奶牛和其他哺乳动物重要的营养物质。硫胺素是α-酮戊二酸脱氢酶、丙酮酸脱氢酶等酶的辅酶,参与碳水化合物代谢和能量代谢。硫胺素对神经元之间的传递、免疫系统的激活、细胞和组织间的信号传递也起到一定的调节和控制作用[13]。缺乏硫胺素会引起奶牛瘤胃代谢紊乱及中枢和外周神经的病理变化,如厌食症和脑灰质软化症(PEM)[14],因此,硫胺素是维持奶牛健康生长、发育、繁殖和免疫所必需的营养物质。
奶牛瘤胃中的硫胺素来源于饲粮和瘤胃微生物。饲粮中的精粗比不仅影响硫胺素的吸收,而且会影响瘤胃微生物合成硫胺素的量。Schwab等[15]报道,奶牛瘤胃硫胺素的表观合成量为50.6 mg/d,可满足奶牛的需要量;然而,最近几年的研究发现,硫胺素的表观合成量显著降低(0.8~39.8 mg/d),这可能与饲粮结构和营养成分有关[16-17]。Castagnin等[17]报道,瘤胃微生物合成硫胺素的量与可消化有机物的量呈负相关;Tafaj等[18]发现,饲喂奶牛40%和25%高精料时,瘤胃中硫胺素的含量升高,而高精料50%和60%组的瘤胃中硫胺素的含量降低。饲喂奶牛高精料饲粮可抑制硫胺素的合成,进而出现硫胺素缺乏症。因此,近几年硫胺素在奶牛产业上得到广泛运用。Shaver等[19]研究发现,在奶牛高精料饲粮中添加300 mg/d的硫胺素,产奶量和乳蛋白率显著提高,与Kholif等[20]报道的添加340 mg/d的效果一致;另有研究表明,在奶牛饲粮中添加适量的硫胺素可调节瘤胃微生物活性、提高瘤胃液pH、改善挥发性脂肪酸(VFA)构成以及降低瘤胃中乳酸和LPS的含量和调控瘤胃上皮的屏障功能,从而缓解SARA的发生[9, 21]。
2 硫胺素对奶牛瘤胃微生物区系的影响瘤胃内环境的稳定与微生物菌群结构和数量密切相关。患有SARA的奶牛不仅瘤胃液pH下降,而且瘤胃微生物的活力降低并破坏其菌群结构,导致瘤胃微生物菌群失衡。发生SARA后瘤胃内原虫数量和纤维分解菌数量减少[22-23],革兰氏阴性菌大量死亡[24]。瘤胃原虫可通过分解淀粉为宿主提供能量,然而原虫对瘤胃液pH的变化较敏感,瘤胃液pH降低可严重损害其生存环境。研究发现,奶牛发生SARA时,前3周瘤胃中的原虫数量保持稳定,之后显著下降,这可能与持续饲喂高精料加重了SARA有关[25],而添加硫胺素可以增加SARA状态下瘤胃原虫数量[26]。厚壁菌门和拟杆菌门是瘤胃中的优势菌群,饲喂高精料饲粮后,厚壁菌门数量显著升高,拟杆菌门数量降低,而厚壁菌门/拟杆菌门与瘤胃液中LPS的含量呈正相关[27]。奶牛发生SARA时,革兰氏阴性菌大量死亡可提高瘤胃中LPS的含量,高含量的LPS会进一步加重SARA。在饲粮中添加硫胺素可抑制厚壁菌门的生长,降低瘤胃中LPS的含量,从而缓解SARA[21]。
乳酸产生菌主要由乳酸杆菌、牛链球菌和溶纤维丁酸弧菌组成,其中,牛链球菌是主要的分解淀粉产生乳酸的细菌;乳酸利用菌主要由反刍兽新月单胞菌和埃氏巨型球菌组成,乳酸产生菌和乳酸利用菌数量失衡是导致SARA的重要原因。当奶牛发生SARA时,瘤胃液pH大幅下降,当瘤胃液pH低于5.5时,牛链球菌和乳酸杆菌仍能大量增殖,分解精料产生乳酸,而反刍兽新月单胞菌和埃氏巨型球菌等乳酸利用菌的生长受到抑制,代谢乳酸的能力下降,致使瘤胃内乳酸大量积累[28],加重了SARA的症状。研究发现,饲粮中添加硫胺素可显著降低奶牛瘤胃中牛链球菌的数量,但对乳酸杆菌和溶纤维丁酸弧菌的数量无显著影响[29],因此,硫胺素可抑制产乳酸菌的增殖进而降低乳酸的积累。此外,埃氏巨型球菌是一种耐酸菌,可在酸性环境下代谢瘤胃内70%的乳酸[30],潘晓花[29]研究表明,饲粮中添加硫胺素可以显著提高SARA奶牛瘤胃中埃氏巨型球菌的数量。
3 硫胺素对奶牛瘤胃代谢产物的影响 3.1 VFA瘤胃微生物通过糖酵解途径和磷酸戊糖途径将饲粮中的碳水化合物转化成丙酮酸和乙酰辅酶A[31],主要代谢为VFA、乳酸以及少量的二氧化碳和甲烷[32]。VFA是奶牛合成脂质和葡萄糖的主要原料。饲喂高精料饲粮诱导奶牛发生SARA时,总VFA含量显著升高,乙酸含量降低,丙酸和丁酸含量升高[33]。瘤胃中乙酸主要由白色瘤胃球菌、产琥珀酸丝状杆菌和黄化瘤胃球菌分解纤维产生,而硫胺素是纤维分解菌生长过程中重要的营养物质[4]。当奶牛发生SARA时,瘤胃中硫胺素含量下降可能会损害纤维分解菌的生长,导致瘤胃乙酸含量下降。同时,在Bryant等[34]的研究中发现,在奶牛高精料饲粮中添加硫胺素可增加瘤胃中乙酸含量,这可能与其对纤维分解菌的生长起促进作用有关。高精料饲粮在奶牛瘤胃中发酵积累大量的丙酮酸,而缺少硫胺素导致丙酮酸脱氢酶的活性降低,使瘤胃中的丙酮酸经琥珀酸途径和丙烯酸途径形成丙酸,增加了丙酸的含量。目前为止,没有发现丙酸的含量和硫胺素的变化有显著的关系[9]。这可能是因为丙酸的含量不仅由硫胺素的含量决定,而主要是由发酵底物的含量和类型决定。
3.2 乳酸乳酸是瘤胃中重要的代谢产物,是诱导瘤胃酸中毒发生的物质之一。因此,降低瘤胃中乳酸的含量是维持瘤胃液pH的稳定和预防SARA的重要措施。奶牛发生SARA时,瘤胃微生物合成硫胺素的量不能满足自身需要,出现硫胺素缺乏症。而低含量的硫胺素可提高乳酸脱氢酶的活性,促进丙酮酸生成乳酸,提高瘤胃液中乳酸含量[35]。研究发现,奶牛饲粮中添加硫胺素后,9 h时瘤胃中的乳酸含量极显著降低,而其他时间点乳酸含量均显著降低[37];此外,奶牛瘤胃内灌注硫胺素可增加瘤胃和血液中硫胺素的含量,降低瘤胃中乳酸含量,缓解瘤胃酸性环境[30];Wang等[35]研究表明,奶牛饲粮中添加180 mg/kg硫胺素可抑制乳酸积累,提高瘤胃液pH。硫胺素可降低瘤胃中乳酸产量的机制:第一,产乳酸菌(牛链球菌、乳酸杆菌等)的数量多于乳酸利用菌(反刍兽新月单胞菌和埃氏巨型球菌)的数量[36],饲粮中添加硫胺素显著降低牛链球菌菌群数量并增加艾氏巨型的菌群数量,进而降低瘤胃中乳酸含量;第二,奶牛瘤胃中较高含量的硫胺素可增加原虫数量[37],原虫可暂时储存可发酵的碳水化合物,防止瘤胃微生物短时间内发酵大量的碳水化合物产生乳酸;第三,硫胺素是碳水化合物代谢过程中丙酮酸脱氢酶和α-酮戊二酸脱氢酶的辅酶,饲粮中添加硫胺素可促进碳水化合物代谢,防止中间代谢产物乳酸积累,最终稳定瘤胃液pH。
3.3 组织胺组织胺是瘤胃微生物分泌的脱羧酶脱羧氨基酸产生,健康生理状态下的奶牛瘤胃中含有少量的组织胺。奶牛瘤胃液pH低于5.5时,脱羧酶的活性增强使氨基酸脱羧产生大量的组织胺[38],其中,腐胺和亚精胺显著增加[39-40]。诱导奶牛发生SARA时,瘤胃中革兰氏阴性菌死亡释放大量的LPS,LPS可抑制瘤胃上皮物质转运功能,阻止组织胺从瘤胃中进入血液[41-42],造成组织胺在瘤胃中积累。Wang等[24]研究发现,饲喂高精料饲粮组(精料为70%)的奶牛瘤胃和血液中的组织胺含量显著高于对照组(精料为40%),加重了瘤胃上皮的损伤。然而,在奶牛饲粮中添加240 mg/kg硫胺素可提高瘤胃液pH,抑制脱羧酶的活性,进而减少组织胺的产生[9]。硫胺素还可抑制Toll样受体4(TLR4)介导的核因子-κB(NF-κB)信号传导途径,缓解瘤胃上皮炎症,恢复转运功能,促使瘤胃内组织胺进入血液[21]。因此,在饲粮中添加硫胺素可降低瘤胃中组织胺含量。
4 硫胺素对奶牛瘤胃上皮屏障功能的影响近年来的研究发现,SARA可损害瘤胃黏膜上皮的完整性,使瘤胃上皮角质层出现不同程度的脱落和损伤,破坏上皮细胞间的紧密连接结构,导致瘤胃上皮通透性增加[3, 43-44],削弱了瘤胃上皮的屏障功能;同时,SARA使瘤胃黏膜上皮增殖细胞核抗原(PCNA)表达下调,显著减弱了瘤胃上皮细胞的增殖活性,破坏了上皮细胞增殖与凋亡的动态平衡,导致瘤胃上皮屏障功能受损。此外,奶牛发生SARA时瘤胃液的酸度增强,革兰氏阴性菌死亡释放的LPS含量是正常含量的20倍,LPS和pH之间的协同作用可破坏瘤胃上皮屏障功能[45],导致黏膜细胞由LPS/TLR4信号传导途径触发局部炎症,产生大量的促炎细胞因子[白细胞介素(IL)-1β、IL-6和肿瘤坏死因子-α(TNF-α)][46];而胃肠道上皮的屏障功能受损将进一步促进LPS从瘤胃流入血液中,诱发促炎细胞因子的释放而引发全身炎症[47]和LPS血症。
饲粮中添加硫胺素可促进犊牛瘤胃乳头发育进而增强瘤胃上皮屏障功能,有研究表明,在犊牛高精料饲粮中添加120 mg/kg硫胺素,高精料饲粮组瘤胃乳头长度、宽度、密度及表面积显著低于高精料饲粮中添加硫胺素组,且前者犊牛瘤胃上皮乳头出现囊肿、乳头分支及角质层的异常增大,而后者的犊牛乳头发育良好,未出现微脓肿,且角质层光滑,但硫胺素促进乳头发育作用机制尚不清楚[48]。此外,硫胺素可抑制促炎细胞因子的释放缓解奶牛炎症反应。一方面,硫胺素可通过抑制TLR4识别瘤胃中的LPS,抑制NF-κB的活化,减少促炎细胞因子基因表达[49]。另一方面,饲粮结构和营养水平也可能会改变瘤胃上皮微生物的丰度,瘤胃上皮微生物丰度降低会促进瘤胃上皮炎症[21]。然而,在饲粮中添加硫胺素后奶牛促炎细胞因子的水平降低和TLR4的表达下降是否与瘤胃上皮微生物的丰度有关还需要进一步验证。
5 小结SARA可影响奶牛的生长性能,并诱发一系列代谢病和炎症,严重威胁了奶牛养殖业的发展,受到了国内外学者的广泛关注。虽然饲粮中添加硫胺素来缓解SARA的研究报道很多,但在奶牛饲粮中还没有具体的推荐添加量。为了确定最适添加量:首先,必须测量饲粮中未被吸收的硫胺素含量和微生物合成硫胺素的量;其次,应该明确饲粮的成分和奶牛的不同生理阶段对吸收硫胺素的影响。另外,硫胺素不仅在碳水化合物和能量代谢中发挥重要作用,而其对细胞调节、免疫功能和氧化损伤的影响也应该被研究,发现硫胺素的未知功能及影响将有助于提高奶牛的健康和生产力,并提高牛奶和其他奶制品的营养价值。
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