动物营养学报  2014, Vol. 26 Issue (3): 585-590   PDF (1025KB)    
环境高温对猪脂肪代谢的影响
吴鑫, 冯京海 , 张敏红    
中国农业科学院北京畜牧兽医研究所, 动物营养学国家重点实验室, 北京 100193
摘要:猪胴体脂肪的沉积、分布以及脂肪酸组成不仅影响胴体的经济价值,还直接影响消费者的健康。基于对健康的关注,消费者更加愿意选择皮下、内脏脂肪含量低的猪肉。肌内脂肪的含量直接影响猪肉的嫩度、多汁性及风味,如何在不增加皮下、内脏脂肪沉积的基础上,提高肌内脂肪的含量成为当前畜牧学科的研究热点。猪的脂肪代谢主要受遗传、营养以及环境应激等因素的影响。本文主要针对环境高温对猪不同部位脂肪代谢的影响及其可能的调控机制进行综述。
关键词环境高温          脂肪代谢     调控机制    
Effects of High Ambient Temperature on Fat Metabolism of Pigs
WU Xin, FENG Jinghai , ZHANG Minhong    
State Key Laboratory of Animal Nutrition, Institute of Animal Sciences, Chinese Academy of Agricultural Sciences, Beijing 100193, China
Abstract: Carcass fat deposition, distribution and fatty acid composition not only affect the economic value of the carcass, but also directly affect consumers' health. Consumers are more willing to choose pork with less content of subcutaneous and visceral fat. The content of intramuscular fat directly affects the tenderness, juiciness and flavor of pork. It has been a hotspot to improve the content of intramuscular fat without increasing subcutaneous and visceral fat deposition. Fat metabolism is mainly affected by heredity, nutrition and environmental stress. This paper reviewed the effects of high ambient temperature on fat metabolism in different parts of pigs and underlying regulatory mechanisms.
Key words: high ambient temperature     pig     fat metabolism     regulatory mechanisms    

随着人们生活水平的提高和保健意识的增强,对猪肉品质的要求越来越高。消费者喜欢瘦肉率高、脂肪和胆固醇含量低的猪肉,同时又要求它的色泽好、嫩度高、多汁、风味鲜美。“杜×长×大”作为我国最为常用的商品杂交猪,其瘦肉率高、生长速度快,但肌内脂肪含量低,肉质欠佳。猪肉的风味和品质主要由肌内脂肪含量决定[1],当肌内脂肪含量低于2.0%时,肉的质地和口感都很差,而高于3.0%时,则肉的风味不再提高;同时,脂肪含量太高时不易被消费者接受,最佳的肌内脂肪含量为2.0%~3.0%[2,3]。但肌内脂肪含量的提高与皮下、内脏脂肪含量的提高不尽一致[4],如何在不增加皮下、内脏脂肪沉积的基础上,提高肌内脂肪的含量成为当前的研究热点和难点。猪的脂肪代谢主要受遗传、营养以及环境应激等因素的影响。南方诸省是我国生猪养殖的重要产区,这里夏季高温高湿天气持续时间较长,加之饲养密度的增大和全球变暖的加剧,热应激越来越成为制约养猪业发展的重要因素之一[5]。本文主要针对环境高温对猪不同脂肪组织脂肪代谢的影响及可能的机理进行综述,为今后在营养与环境方面对脂肪代谢进行调控提供一定的参考。

1 猪不同部位脂肪的发育、代谢及脂肪酸组成

猪胴体脂肪主要沉积在4个位置:皮下、内脏周围、骨骼肌组织间、骨骼肌组织内[6],其中沉积在骨骼肌组织内的脂肪称为肌内脂肪,由位于肌束间隙纵向排列的脂肪细胞、肌外膜、肌束膜和肌内膜上的膜脂以及肌浆中的甘油三酯(TG)液滴构成[7]。在猪生长过程中,不同部位脂肪组织的发育和代谢并不一致,其中皮下脂肪的快速生长期最早,然后是内脏脂肪和肌间脂肪,肌内脂肪发育最晚[8]。在同一时间,不同部位脂肪的沉积能力并不相同[9]。Mourot等[10]发现,即使均为皮下脂肪组织,但部位不同,脂肪合成相关酶的活性也存在较大的差异。不同部位脂肪中的脂肪酸组成及脂质的脂质含量也存在较大差别,Monziols等[11]发现,猪肌间脂肪含量最低,皮下脂肪含量略高,肾周脂肪含量最高。不同部位脂肪组织中不饱和脂肪酸的比例呈规律性变化,从皮下脂肪的外层到内层,然后是肌间脂肪,最后是肾周脂肪,脂肪中不饱和脂肪酸/饱和脂肪酸逐渐降低,单不饱和脂肪酸的含量也是同样趋势。总之,不同部位的脂肪发育顺序、脂肪合成能力、脂质含量以及脂肪酸组成都存在较大差异。

2 环境高温对猪不同部位脂肪沉积的影响

环境高温对不同部位脂肪沉积的影响存在较大差异。Christon[12]研究发现,高温不影响肥育猪背脂相对重,使肾周脂肪相对重升高26.7%(P<0.05);Katsumata等[13]研究发现,30 ℃持续高温使肩部皮下脂肪厚降低22.4%(P<0.05),腰部皮下脂肪厚降低21.9%(P<0.05),背脂厚降低18.9%(P<0.05),而内脏脂肪重有增加趋势(P<0.10)。上述试验结果表明,高温降低猪皮下脂肪的沉积,同时增强内脏脂肪的沉积,这也与其他人的结论[14,15]一致。上述研究均在自由采食的条件下进行,环境高温显著降低猪的采食量,因此上述研究得出的结论可能受到采食量不同的影响。为了研究环境高温对猪脂肪代谢的直接影响,Kouba等[16,17]增设了采食量配对组,研究发现,在采食量相同的条件下,高温组背脂相对重提高5.4%(P<0.05),肾周脂肪相对重提高127.0%(P<0.05),肾周脂肪/(背脂+肾周脂肪)的比例提高125.0%(P<0.05)。这表明在采食量相同的情况下,高温同时增加内脏脂肪和皮下脂肪的沉积,内脏脂肪增加的幅度更大。Le Dividich等[18]认为,长期热应激导致猪的脂肪沉积增强,并且呈现出外层脂肪向内层脂肪转移的趋势,以降低外层脂肪的隔热性,从而适应高温环境。高温对肌间脂肪和肌内脂肪影响的报道较为少见,Witte等[19]发现,和适温对照组相比,高温对猪背最长肌肌内脂肪含量无显著影响(P>0.05),这与Sugahaea等[20]的研究结果一致。高温对猪肌间脂肪和肌内脂肪沉积的影响还需进一步研究。

3 环境高温对猪不同部位脂肪代谢的影响

脂肪的沉积是合成和分解代谢动态变化的结果。猪脂肪组织合成TG所需的脂肪酸有2个来源,一是脂肪组织从头合成脂肪酸,另外一个是通过脂蛋白酯酶(LPL)从血浆中富含TG的脂蛋白[包括乳糜微粒和极低密度脂蛋白(VLDL)]中分解获得脂肪酸[21]。家禽脂肪组织中脂肪酸主要来源于肝脏合成分泌的VLDL[22],而猪脂肪组织中脂肪酸主要靠从头合成,从肝脏中获得的脂肪酸比例较小[21]

脂肪酸从头合成的关键酶包括乙酰辅酶A羧化酶(ACC)、苹果酸酶(ME)、葡萄糖-6-磷酸脱氢酶(G6PDH)、脂肪酸合成酶(FAS)等。Kouba等[16]研究发现,和采食量配对组相比,高温使猪背脂中ACC活性降低35.1%(P<0.01),肾周脂肪中ACC活性降低49.3%(P<0.01),肝脏中ACC活性降低25.2%(P<0.01),而肌肉中ACC活性增加38.6%(P<0.01)。Rinaldo等[23]发现,在31.5 ℃高温下,猪背脂、肾周脂肪和肝脏中ME和G6PDH的活性均显著下降(P<0.05)。上述结果表明,高温抑制猪背脂、肾周脂肪和肝脏中脂肪酸的从头合成。如前文所述,相同采食情况下,高温提高了肾周脂肪和背脂的沉积[17],这显然无法从脂肪酸的从头合成得到解释。Kouba等[17]研究发现,31 ℃高温下,背脂和肾周脂肪的LPL活性分别升高了144.6%和90.5%(P<0.01),Christon[12]和Rinaldo等[23]也有类似的发现。LPL可以分解血浆中乳糜微粒和VLDL携带的TG,释放出的脂肪酸,供脂肪组织合成TG贮存,是脂肪组织利用血浆中脂蛋白的关键控制因素[24]。上述结果表明,高温增强了脂肪组织对血浆脂蛋白的摄取和利用,从而促进猪脂肪的沉积。另外,Kouba等[17]还发现,高温导致血浆中游离脂肪酸(FFA)的浓度增加了1.6倍(P<0.01),血浆TG的浓度增加了2.6倍(P<0.01),推测环境高温可能抑制了骨骼肌脂肪酸氧化供能,导致血浆中FFA浓度升高,更多的FFA在肝脏中重新合成为TG,以VLDL的形式运送到脂肪组织,在LPL的作用下被脂肪组织摄取,环境高温是否影响骨骼肌脂肪酸的氧化还需进一步研究证实。

4 环境高温调控猪脂肪代谢的可能机制
4.1 调控内分泌

多种激素参与调节猪的脂肪代谢,如甲状腺激素、胰岛素和生长激素等。前人研究报道,高温显著降低猪血清中甲状腺激素的浓度[172325](P<0.05)。甲状腺激素能够增加脂肪组织对儿茶酚胺和胰高血糖素的敏感性,增加脂肪组织中腺苷酸环化酶的活性,使三磷酸腺苷(ATP)转化为环单磷酸腺苷(cAMP),cAMP作为第二信使激活cAMP-依赖性蛋白激酶,使无活性的激素敏感脂酶(HSL)转变为有活性的HSL,促进脂肪组织的分解,从而提高血液FFA浓度[26]。高温降低甲状腺激素的浓度,缓解甲状腺激素对脂肪的促分解作用,促进脂肪的储存。

Rinaldo等[23]报道,31.5 ℃高温极显著降低猪血清胰岛素的浓度(P<0.01),Chayoth等[27]在大鼠上也得到相似结果。胰岛素可以上调FAS基因在转录水平的表达[28,29],促进脂肪的合成。环境高温可能通过降低胰岛素的浓度,抑制脂肪的合成。

Marple等[30]报道,暴露在高温环境中,猪血浆中生长激素的水平显著降低(P<0.05),这与Parkhie等[31]在大鼠上的试验结果一致。生长激素可促进脂肪组织的分解代谢,使血浆中FFA浓度增加,脂肪沉积减少[32]。高温可能通过降低生长激素的水平,促进脂肪的沉积。

4.2 调控脂肪细胞因子

脂肪组织能分泌多种细胞因子,如瘦素(leptin)、肿瘤坏死因子-α(TNF-α)等,这些因子统称为脂肪细胞因子,它们在脂肪代谢和能量平衡中发挥着重要的作用[33]。其中leptin主要在脂肪组织中表达[34],血浆中leptin的含量与体内白色脂肪组织的含量紧密相关[35]。leptin通过与下丘脑的特定受体结合,抑制摄食,增加能量消耗,在脂肪储存、体重调节中起着重要作用[36]。leptin还可以通过自分泌或旁分泌途径作用于脂肪细胞,促进脂肪组织内TG的分解,抑制脂肪合成[37]。Collin等[25]研究表明,33 ℃高温下,断奶仔猪血清leptin的浓度显著降低(P<0.05),缓解了leptin的促脂肪分解和抑制脂肪合成的作用,促进了脂肪的沉积。

Warne[38]报道,TNF-α可以促进脂肪的分解。有研究报道,TNF-α可以极显著下调HSL基因的表达(P<0.01)[39,40],TNF-α还可以极显著下调脂肪组织甘油三酯酯酶(ATGL)和甘油三酯水解酶(TGH-2)的基因表达(P<0.01)[41,42]。但Green等[43]用TNF-α处理大鼠原代脂肪细胞后发现,HSL含量并未发生变化。由此推测,TNF-α可能不是通过直接调控上述脂解相关酶的表达来影响脂肪分解,而是通过增加其活性或其他途径来促进脂解。脂滴包被蛋白A(perilipin A)是定位于脂肪细胞脂滴表面的磷酸化蛋白质,作为屏障覆盖在脂滴的表面保护TG免受水解[44]。大量研究报道,TNF-α通过下调perilipin A的基因表达,使得脂滴失去屏障,促进了脂肪细胞的脂解[45,46,47,48]。除此之外,TNF-α还可以抑制脂肪的合成[49],Stephens等[50]用TNF-α处理小鼠的3T3-L1脂肪细胞24 h后,发现LPL基因表达量降低70%(P<0.01);Kern[51]测定绝食情况下LPL活性发现,TNF-α基因的表达量与LPL活性呈负相关,而且体重减轻后,脂肪组织中TNF-α的浓度极显著下降(P<0.01),LPL活性增加了41%(P<0.01)。Hauner等[52]也发现,TNF-α极显著下调LPL的合成(P<0.01)。Memon等[53]研究发现,TNF-α还抑制脂肪组织中FFA转运蛋白的基因表达,从而阻断FFA进入脂肪细胞,另外还发现,在脂质形成过程中,TNF-α可以抑制脂肪合成相关酶(如ACC)基因的表达。黄丹文等[54]研究发现,热应激极显著降低大鼠血浆中TNF-α浓度(P<0.01),表明高温可能通过抑制TNF-α的分泌,缓解其促进分解和抑制合成的作用,并最终促进脂肪的沉积。

5 小 结

在发育时间、脂肪合成能力和脂肪酸组成等方面,猪不同部位的脂肪存在差异。在采食量相同的情况下,环境高温促进皮下和内脏脂肪的沉积,且内脏脂肪增加幅度更大。环境高温抑制脂肪和肝脏组织从头合成脂肪酸的能力,但血浆中FFA和VLDL的浓度升高,脂肪组织LPL活性升高,推测环境高温可能抑制了骨骼肌脂肪酸的氧化,导致更多的FFA在肝脏中重新合成为TG,以VLDL的形式运送到脂肪组织,在LPL的作用下被脂肪组织摄取,最终促进了脂肪的沉积。环境高温可能通过降低猪血清中甲状腺激素、生长激素、胰岛素、leptin和TNF-α等的浓度,调控脂肪代谢。但是环境高温是否影响不同部位脂肪中相关激素和细胞因子受体的基因表达,导致不同部位脂肪沉积的差异化还需进一步研究。

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