徐連彬　林雪彥　胡志勇　王　云　王中華

(山東農(nóng)業(yè)大學(xué)動(dòng)物科技學(xué)院，泰安271018)

?

氨基酸供給與內(nèi)分泌互作調(diào)控乳蛋白合成的研究進(jìn)展

徐連彬林雪彥胡志勇王云王中華*

(山東農(nóng)業(yè)大學(xué)動(dòng)物科技學(xué)院，泰安271018)

限制性氨基酸研究是泌乳奶牛研究的熱點(diǎn)之一，關(guān)于其影響乳蛋白合成途徑而進(jìn)行的一系列體內(nèi)外研究表明：底物效應(yīng)可能不是主要原因，更重要的是氨基酸及其構(gòu)成作為信號(hào)因子對(duì)神經(jīng)內(nèi)分泌和細(xì)胞內(nèi)信號(hào)通路的影響。本文從乳蛋白合成的影響因素入手，概括了氨基酸供給、激素分泌及二者相互作用對(duì)乳蛋白合成的調(diào)節(jié)，以期為闡明氨基酸供給影響乳蛋白合成的途徑提供一定理論參考。

氨基酸；內(nèi)分泌；互作調(diào)控；乳蛋白合成；研究進(jìn)展

氨基酸供給包括氨基酸種類、比例和水平3個(gè)方面，其對(duì)乳蛋白合成有重要的影響。有關(guān)改變必需氨基酸(EAA)供給影響乳蛋白產(chǎn)量的試驗(yàn)已證實(shí)了這一點(diǎn)[1-2]。傳統(tǒng)的氨基酸營(yíng)養(yǎng)理論用“木桶原理”來(lái)解釋氨基酸構(gòu)成對(duì)乳蛋白合成的影響。該理論認(rèn)為，相對(duì)于需要量供給最為短缺的EAA在補(bǔ)充到一定水平前，補(bǔ)充其他氨基酸沒(méi)有效果。事實(shí)上，無(wú)論在生長(zhǎng)動(dòng)物還是泌乳動(dòng)物上，均發(fā)現(xiàn)存在“共限制”氨基酸現(xiàn)象，即補(bǔ)充不同EAA均促進(jìn)了乳蛋白的合成，從而證實(shí)了該理論的局限性[3-5]。與生長(zhǎng)動(dòng)物不同，泌乳反芻動(dòng)物對(duì)單一EAA供給變化響應(yīng)不敏感。Bequette等[6]發(fā)現(xiàn)，奶山羊真胃灌注混合氨基酸中缺失組氨酸(His)，其動(dòng)脈濃度下降了近90%，而乳蛋白產(chǎn)量卻變化不顯著。盡管如此，作為構(gòu)成乳蛋白的基本單位，氨基酸的底物功能仍然不能忽視。Appuhamy等[7]觀察到，蛋氨酸(Met)和蘇氨酸(Thr)不影響乳腺上皮細(xì)胞內(nèi)的信號(hào)途徑，但影響酪蛋白合成速度，其作用途徑可能是底物效應(yīng)。Eif等[8]研究表明，限制單一EAA供給量，所限制EAA的細(xì)胞內(nèi)受體tRNA載荷可降至零，而其他氨基酸的受體tRNA仍滿載荷，各密碼子的翻譯速度因可利用底物數(shù)量的變化出現(xiàn)很大差異。乳腺氨基酸代謝研究表明，單一EAA缺乏可以通過(guò)提高乳腺血流量(MBF)和缺乏EAA的乳腺提取效率來(lái)增加對(duì)乳腺組織的供應(yīng)[6]。因此，限制性氨基酸乳腺供給量減少可能不是引起乳蛋白產(chǎn)量下降的主要原因，已有的研究表明存在2種其他的途徑。一是氨基酸供給影響乳腺上皮細(xì)胞內(nèi)調(diào)節(jié)蛋白質(zhì)合成的信號(hào)通路，主要包括：1)哺乳動(dòng)物雷帕霉素靶蛋白(mammalian target of rapamycin,mTOR)通路。細(xì)胞氨基酸營(yíng)養(yǎng)改變信號(hào)刺激該途徑對(duì)核糖體S6蛋白激酶1(ribosomal protein S6 kinase 1,rpS6K1)和真核細(xì)胞翻譯啟動(dòng)因子4E結(jié)合蛋白1(eukaryotic translation initiation factor 4E binding protein 1,4EBP1)的磷酸化，從而促進(jìn)胞內(nèi)蛋白質(zhì)合成的翻譯過(guò)程[9]。2)整合應(yīng)激(integrated stress response,ISR)網(wǎng)絡(luò)。細(xì)胞氨基酸營(yíng)養(yǎng)不足時(shí)，增加的空載tRNA刺激真核細(xì)胞翻譯啟動(dòng)因子2(eukaryotic translation initiation factor 2,eIF2)的磷酸化，從而抑制胞內(nèi)蛋白質(zhì)合成的翻譯起始[10]。二是限制或增加EAA供給引起血漿激素濃度的變化，后者可通過(guò)組織代謝、細(xì)胞增殖和轉(zhuǎn)錄翻譯等多個(gè)水平調(diào)控乳蛋白合成。一個(gè)比較一致的規(guī)律是限制單一或一組EAA供給，血漿胰島素(INS)和胰高血糖素(GLN)濃度同時(shí)升高。2006—2012年發(fā)表的6個(gè)相關(guān)研究[1,11-15]都觀察到了這一現(xiàn)象。因此，內(nèi)分泌變化是影響乳蛋白合成的重要因素，EAA供給-內(nèi)分泌變化-乳蛋白合成是一個(gè)值得關(guān)注的EAA供給影響乳蛋白合成的途徑，本文將對(duì)與此相關(guān)的研究進(jìn)行綜述。

1　氨基酸供給對(duì)內(nèi)分泌激素的影響及作用途徑

泌乳奶牛的下丘腦-垂體系統(tǒng)及乳腺細(xì)胞分泌的激素影響體內(nèi)游離氨基酸的利用，反之游離氨基酸也可能對(duì)神經(jīng)內(nèi)分泌系統(tǒng)產(chǎn)生反饋性調(diào)節(jié)。陳智梅[16]根據(jù)氨基酸平衡指數(shù)調(diào)整培養(yǎng)基中Met、賴氨酸(Lys)和亮氨酸(Leu)的比例，結(jié)果發(fā)現(xiàn)不同氨基酸模式影響離體乳腺組織中激素的合成與分泌。體外一系列的EAA灌注試驗(yàn)支持這一結(jié)論，例如：靜脈灌注分別缺失Met、Lys、His和支鏈氨基酸(BCAA)的混合氨基酸顯著改變了泌乳奶牛血漿中幾種激素的濃度[1]；小鼠腹腔注射精氨酸(Arg)后，INS和GLN的分泌量均顯著增加[12]。由此看出，激素分泌的刺激效果與氨基酸種類存在一定的關(guān)系。Kuhara等[17]通過(guò)綿羊靜脈灌注試驗(yàn)發(fā)現(xiàn)，酸性氨基酸主要促進(jìn)生長(zhǎng)激素(GH)的分泌，中性直鏈氨基酸主要促進(jìn)INS和GLN的分泌，而B(niǎo)CAA趨于抑制GLN的分泌。除作用于分泌外，氨基酸還能影響激素的組織敏感性及其受體的表達(dá)。Xiao等[18]發(fā)現(xiàn)，BCAA缺乏可以提高肝臟INS的敏感性。EAA缺乏未必影響血漿GH和胰島素樣生長(zhǎng)因子Ⅰ(IGF-Ⅰ)的濃度，但可以影響肝細(xì)胞GH和IGF-Ⅰ受體的表達(dá)[19]。

無(wú)論是缺失還是增加EAA，均可引起氨基酸的不平衡，因而可能是氨基酸不平衡而非單一氨基酸信號(hào)刺激了激素的分泌。關(guān)于氨基酸調(diào)節(jié)激素分泌機(jī)制的報(bào)道較少，根據(jù)前人的研究結(jié)果概括如下：首先，某些特殊氨基酸本身作為底物參與小分子質(zhì)量激素的合成。酪氨酸(Tyr)和苯丙氨酸(Phe)是腎上腺素和甲狀腺激素合成的前體。其次，氨基酸可能通過(guò)細(xì)胞內(nèi)信號(hào)途徑直接調(diào)節(jié)激素的分泌。研究表明氨基酸混合物對(duì)INS分泌的刺激作用50%以上通過(guò)谷氨酰胺(Gln)來(lái)實(shí)現(xiàn)，其胞內(nèi)信號(hào)途徑為環(huán)磷酸腺苷-蛋白激酶A(cyclic adenosine monophosphate-protein kinase A,cAMP-PKA)通路[20-21]。馬秀玲[22]指出，Arg可以在轉(zhuǎn)錄和翻譯水平對(duì)肝臟IGF-Ⅰ的合成進(jìn)行調(diào)節(jié)，涉及的可能環(huán)節(jié)包括轉(zhuǎn)錄速率、核-胞漿運(yùn)輸以及轉(zhuǎn)錄后的加工修飾。Xiao等[18]發(fā)現(xiàn)，BCAA缺乏通過(guò)mTOR和腺苷酸活化蛋白激酶(adenosine monophosphate activated protein kinase,AMPK)通路增加肝臟中INS的敏感性。在組織細(xì)胞感受氨基酸信號(hào)的過(guò)程中，氨基酸載體可能起到關(guān)鍵的作用，即所謂的“轉(zhuǎn)運(yùn)感受子”(transceptor)[23]，如Leu等中性氨基酸載體可與胞內(nèi)mTOR通路發(fā)生交互作用而傳遞信號(hào)[24]。最后，單一氨基酸或氨基酸平衡可能通過(guò)血漿代謝物間接發(fā)揮調(diào)節(jié)作用。王建發(fā)[25]體外培養(yǎng)奶牛垂體細(xì)胞時(shí)發(fā)現(xiàn)，Arg通過(guò)生成一氧化氮(NO)促進(jìn)胞外Ca2+內(nèi)流，從而調(diào)節(jié)GH的分泌；Tyr通過(guò)生成多巴胺并以自分泌或旁分泌的方式抑制催乳素(PRL)的分泌。與外源灌注速率增加相比，肝臟對(duì)EAA的利用速率增幅更大，其糖異生的代謝產(chǎn)物葡萄糖對(duì)INS和GLN的分泌具有直接的刺激作用[26-27]。此外，有研究報(bào)道氨基酸供給可能通過(guò)增加腸道細(xì)胞分泌胃饑餓素(ghrelin)來(lái)促進(jìn)INS和GH的分泌[11]。ghrelin是一種由胃黏膜內(nèi)分泌細(xì)胞分泌的腦腸肽，當(dāng)其與GH分泌細(xì)胞膜表面的生長(zhǎng)激素促分泌物受體(growth hormone secretagogue receptor,GHSR)結(jié)合后激活蛋白激酶C(protein kinase C,PKC)并提高核內(nèi)環(huán)磷腺苷效應(yīng)元件結(jié)合蛋白(cAMP response element binding protein,CREB)的磷酸化水平，后者與GH基因的啟動(dòng)子區(qū)特異性結(jié)合后促進(jìn)GH基因的轉(zhuǎn)錄[28]。

2　內(nèi)分泌激素對(duì)乳蛋白合成的影響及作用途徑

Cant等[29]對(duì)乳腺氨基酸代謝的相關(guān)研究進(jìn)行了綜述，并指出：乳蛋白合成速度決定于動(dòng)物的生理狀態(tài)、營(yíng)養(yǎng)狀況和擠奶制度，而非胞外氨基酸濃度。大量數(shù)據(jù)證實(shí)，泌乳奶牛體內(nèi)存在一系列針對(duì)氨基酸缺乏而優(yōu)先滿足泌乳的機(jī)制，其中內(nèi)分泌系統(tǒng)是一個(gè)重要的方面。綜合以往的研究結(jié)果，我們發(fā)現(xiàn)激素對(duì)乳蛋白合成的調(diào)控是在細(xì)胞、組織、器官和機(jī)體各個(gè)層次的集成調(diào)控，其具體途徑可能包括：1)調(diào)節(jié)機(jī)體代謝并促進(jìn)營(yíng)養(yǎng)物質(zhì)向乳腺的分配。奶牛機(jī)體自身存在營(yíng)養(yǎng)調(diào)控功能，當(dāng)氨基酸缺乏時(shí)，其可以通過(guò)協(xié)調(diào)分配在一定時(shí)間內(nèi)保持穩(wěn)態(tài)，激素在維護(hù)機(jī)體這一功能方面起到非常重要的作用。吳慧慧等[30]指出，反芻動(dòng)物泌乳期營(yíng)養(yǎng)分配的主要調(diào)節(jié)因子是激素，其影響穩(wěn)態(tài)信號(hào)應(yīng)答的機(jī)制可能包括改變組織受體及其動(dòng)力學(xué)、細(xì)胞內(nèi)信號(hào)傳導(dǎo)體系及生化通路中關(guān)鍵酶的表達(dá)和活性；此外，不同組織在氨基酸補(bǔ)償調(diào)控這一過(guò)程中可以相互協(xié)調(diào)，共同促進(jìn)營(yíng)養(yǎng)物質(zhì)向乳腺的轉(zhuǎn)移。2)促進(jìn)乳腺的發(fā)育并維持泌乳過(guò)程。3)調(diào)節(jié)氨基酸載體的表達(dá)和活性[31]。4)促進(jìn)乳蛋白合成相關(guān)基因的轉(zhuǎn)錄和翻譯，如圖1所示。下面以PRL、INS、IGF-Ⅰ、GH和GLN為例，從分子、細(xì)胞、組織、器官和機(jī)體多個(gè)水平對(duì)其具體的作用機(jī)制加以論述。

IGFBP：胰島素樣生長(zhǎng)因子結(jié)合蛋白 insulin-like growth factor binding protein；TGFα：轉(zhuǎn)化生長(zhǎng)因子α transforming growth factor α；TGFβ：轉(zhuǎn)化生長(zhǎng)因子β transforming growth factor β；Leptin：瘦素；FGF：纖維母細(xì)胞生長(zhǎng)因子 fibroblast growth factor；stromal cells：基質(zhì)細(xì)胞；IRS-1：胰島素受體底物1 insulin receptor substrate 1；RAS：大鼠肉瘤蛋白 rat sarcoma protain；β-Catenin：β-連環(huán)蛋白；Cell survival：細(xì)胞存活；Cell proliferation：細(xì)胞增殖；SOCS：細(xì)胞信號(hào)因子抑制因子 suppressors of cytokine signaling；JAK/STAT5 Signaling：Janus激酶/信號(hào)轉(zhuǎn)導(dǎo)與轉(zhuǎn)錄激活子5信號(hào)；Prl：催乳素 prolactin；GH：生長(zhǎng)激素 growth hormone；Milk Protein Gene Expression：乳蛋白基因表達(dá)；KEY：圖例；Prl or GH receptor：催乳素或生長(zhǎng)激素受體；IGF-Ⅰ receptor：胰島素樣生長(zhǎng)因子Ⅰ受體；Hybrid receptor：混合受體；Insulin receptor：胰島素受體；Fibroblast or adipocyte：纖維母細(xì)胞或脂肪細(xì)胞；ECM protain：細(xì)胞外基質(zhì)蛋白 extracellular matrix proteins。

圖1IGF-Ⅰ、PRL、GH和其他生長(zhǎng)因子對(duì)奶牛乳腺上皮細(xì)胞的作用

Fig.1Actions of IGF-Ⅰ, PRL, GH and selected growth factors in a bovine mammary epithelial cell[42]

2.1PRL

PRL是由腺垂體催乳素細(xì)胞合成并分泌的一類單鏈多肽類蛋白激素，由199個(gè)氨基酸組成，廣泛分布于垂體以外的組織器官。PRL在發(fā)動(dòng)并維持泌乳、刺激乳腺生長(zhǎng)發(fā)育和促進(jìn)乳蛋白合成方面具有十分重要的作用。楊建英等[32]發(fā)現(xiàn)，泌乳奶牛乳蛋白含量增加的同時(shí)會(huì)伴隨著血漿PRL濃度的升高。添加PRL顯著提高了離體奶牛乳腺上皮細(xì)胞的乳蛋白分泌量[33]。Boutinaud等[34]采用PRL抑制劑處理泌乳奶牛，發(fā)現(xiàn)κ-酪蛋白和PRL受體(PRLR)的mRNA表達(dá)豐度均顯著下降。

PRL調(diào)控乳蛋白合成的機(jī)理可能有以下幾個(gè)方面：一是PRL促進(jìn)乳腺發(fā)育并影響泌乳的維持。妊娠末期，PRL通過(guò)增加基因轉(zhuǎn)錄和延長(zhǎng)mRNA壽命控制小葉腺泡的發(fā)育及小葉上皮細(xì)胞的增殖[35]，參與調(diào)控該過(guò)程的是細(xì)胞內(nèi)絲裂原活化蛋白激酶(mitogen activated protein kinase,MAPK)信號(hào)通路。喹高利特，一種PRL抑制劑，可以顯著降低泌乳高峰期奶牛的產(chǎn)奶量[36],這可能與PRL功能抑制阻礙了乳腺上皮細(xì)胞的增殖，使正常泌乳細(xì)胞數(shù)減少有關(guān)。二是PRL與受體結(jié)合后通過(guò)胞內(nèi)磷脂酰肌醇3激酶-絲氨酸/蘇氨酸激酶(phosphatidylinositol 3 kinase-serine/threonine kinase,PI3K-Akt)通路和Janus激酶-信號(hào)轉(zhuǎn)導(dǎo)與轉(zhuǎn)錄激活子5(Janus kinases-signal transducer and activator of transcription 5,JAK-STAT5)通路誘導(dǎo)乳蛋白基因的轉(zhuǎn)錄[37]。PRLR是具有細(xì)胞因子受體超家族結(jié)構(gòu)特性的跨膜蛋白，包括膜外域、跨膜域和胞內(nèi)域3個(gè)部分。當(dāng)PRLR與PRL結(jié)合后發(fā)生二聚化，其胞內(nèi)結(jié)構(gòu)域與酪氨酸激酶進(jìn)行交叉磷酸化并激活Janus激酶(Janus kinases,JAK)和信號(hào)轉(zhuǎn)導(dǎo)與轉(zhuǎn)錄激活子(signal transducer and activator of transcription,STAT)，后者進(jìn)入細(xì)胞核后與乳蛋白基因上的特定序列相結(jié)合，啟動(dòng)基因的轉(zhuǎn)錄[38]。三是PRL可以調(diào)節(jié)氨基酸的轉(zhuǎn)運(yùn)[39]。Lacasse等[36]發(fā)現(xiàn)，PRL調(diào)節(jié)L-氨基酸轉(zhuǎn)運(yùn)載體的活性。研究發(fā)現(xiàn)，用PRL阻斷劑溴隱亭處理泌乳高峰期的大鼠后改變了幾種氨基酸的乳腺動(dòng)靜脈濃度差，而且這種差異可被外源性的PRL處理所恢復(fù)[40]。此外，PRL與INS、孕激素、糖皮質(zhì)激素和雌激素等多種激素存在協(xié)同作用，共同調(diào)節(jié)乳蛋白的合成[41]。

2.2INS

INS是一種由胰臟內(nèi)胰島β細(xì)胞分泌的蛋白質(zhì)激素，分子質(zhì)量為5 808 ku。作為體內(nèi)唯一降低血糖濃度的激素，其對(duì)乳蛋白合成有重要的影響。研究表明，乳蛋白合成的內(nèi)分泌調(diào)控主要是INS起作用[42]。Mackle等[43]采用高胰島素-正常葡萄糖鉗夾技術(shù)使血漿INS濃度增加4倍而血糖濃度維持恒定，并發(fā)現(xiàn)乳蛋白產(chǎn)量增加了15%，若再額外灌注酪蛋白，則乳蛋白產(chǎn)量進(jìn)一步提高。小鼠靜脈灌注Leu后，血漿INS濃度短暫升高，且這種變化促進(jìn)了Leu誘導(dǎo)的蛋白質(zhì)合成[44]。Menzies等[45]通過(guò)全基因表達(dá)分析篩選出了28個(gè)受INS刺激并與乳蛋白合成直接相關(guān)的基因，并證實(shí)INS在細(xì)胞內(nèi)通過(guò)多個(gè)水平刺激乳蛋白合成。以上這些結(jié)果說(shuō)明INS可以促進(jìn)乳腺對(duì)氨基酸的利用，進(jìn)一步發(fā)揮奶牛的泌乳潛能。

關(guān)于INS促進(jìn)蛋白質(zhì)合成的機(jī)理可以從2個(gè)角度加以闡述。細(xì)胞分子水平上，INS通過(guò)刺激胰島素受體底物1(insulin receptor substrate 1，IRS-Ⅰ)并借助配體的約束力和磷酸化誘導(dǎo)INS受體蛋白位點(diǎn)的生成，進(jìn)而促進(jìn)胞內(nèi)蛋白質(zhì)的合成[46]。首先，INS與乳腺上皮細(xì)胞上的INS受體結(jié)合后激活PI3K-Akt-mTOR信號(hào)通路促進(jìn)蛋白合成的翻譯[47]。mTOR對(duì)下游信號(hào)通路的作用具體表現(xiàn)為：刺激rpS6K1磷酸化并促進(jìn)蛋白質(zhì)翻譯的起始和延伸過(guò)程[48]；刺激4EBP1磷酸化并促進(jìn)真核生物起始因子4F(eukaryotic translation initiation factor 4F,eIF4F)復(fù)合物的形成，進(jìn)而啟動(dòng)含帽子結(jié)構(gòu)的mRNA翻譯起始[49]；刺激真核細(xì)胞翻譯延長(zhǎng)因子2(eukaryotic elongation factor 2,eEF2)磷酸化并促進(jìn)蛋白翻譯的延伸過(guò)程[50]。泌乳發(fā)動(dòng)后，乳腺乳蛋白基因mRNA表達(dá)量穩(wěn)定，但Akt基因表達(dá)上調(diào)，提高了INS促進(jìn)mRNA翻譯的效率[51]。其次，INS通過(guò)JAK-STAT5信號(hào)通路誘導(dǎo)相關(guān)基因的轉(zhuǎn)錄[52]。作為一種重要的泌乳信號(hào)轉(zhuǎn)導(dǎo)因子，STAT5可與激素介導(dǎo)和磷酸化的JAK信號(hào)通路偶聯(lián)，活化的STAT以二聚體的形式進(jìn)入細(xì)胞核內(nèi)與乳蛋白基因上的啟動(dòng)子結(jié)合，啟動(dòng)基因的轉(zhuǎn)錄，從而把細(xì)胞外信號(hào)與基因表達(dá)調(diào)控直接聯(lián)系起來(lái)[53]。組織器官水平上，INS通過(guò)對(duì)機(jī)體物質(zhì)代謝、能量平衡及流向乳腺的底物濃度和轉(zhuǎn)運(yùn)進(jìn)行調(diào)節(jié)，間接影響乳蛋白合成[39]。其中MBF的變化是一個(gè)重要途徑，Mackle等[43]報(bào)道，INS可以提高M(jìn)BF。Cant等[29]在其綜述中認(rèn)為MBF的改變可以解釋為乳腺面對(duì)機(jī)體循環(huán)血液中能量底物濃度不足時(shí)，嘗試恢復(fù)細(xì)胞內(nèi)ATP平衡的結(jié)果。吳慧慧等[30]證實(shí)，INS可以通過(guò)增加肝臟葡萄糖的合成滿足泌乳的能量需要，從而促進(jìn)乳蛋白合成。此外，INS還與其他激素如GH、PRL等存在信號(hào)轉(zhuǎn)導(dǎo)交叉，共同構(gòu)成一個(gè)信號(hào)網(wǎng)絡(luò)參與調(diào)控乳蛋白的合成。

2.3IGF-Ⅰ

IGF-Ⅰ是由GH誘導(dǎo)靶細(xì)胞產(chǎn)生的一種與INS有高度同源性的多功能堿性多肽，由70個(gè)氨基酸組成，分子質(zhì)量為7.5 ku，是細(xì)胞增殖與分化的重要調(diào)控因子。循環(huán)血液中95%的IGF-Ⅰ由肝臟分泌，其合成依賴于垂體分泌的GH和機(jī)體營(yíng)養(yǎng)狀況。IGF-Ⅰ通過(guò)與血液中的胰島素樣生長(zhǎng)因子結(jié)合蛋白(insulin-like growth factor binding protein，IGFBP)結(jié)合后被運(yùn)輸?shù)酵庵芙M織發(fā)揮作用。Rose等[54]發(fā)現(xiàn)，血液中IGF-Ⅰ濃度通常與泌乳量呈正相關(guān)，提示其對(duì)乳蛋白合成有一定的促進(jìn)作用。Prosser等[55]向泌乳山羊陰外動(dòng)脈注射IGF-Ⅰ后發(fā)現(xiàn)乳分泌速率提高了25%。

已經(jīng)證實(shí)乳腺組織中存在IGF-Ⅰ受體，提示其可直接作用于乳腺發(fā)揮作用[56]。IGF-Ⅰ調(diào)控泌乳的機(jī)理可能包括：1)調(diào)控乳腺上皮細(xì)胞的增殖、分化和凋亡。IGF-Ⅰ可以加快細(xì)胞周期進(jìn)程，促進(jìn)細(xì)胞分裂并刺激細(xì)胞的生長(zhǎng)[34]。青春期至成熟期的小鼠IGF-Ⅰ基因的過(guò)表達(dá)促進(jìn)了乳腺導(dǎo)管分支和腺泡的形成[57]。關(guān)于其作用途徑，F(xiàn)orsyth等[58]發(fā)現(xiàn)，IGF-Ⅰ以內(nèi)分泌、自分泌和旁分泌的方式調(diào)控細(xì)胞的生長(zhǎng)與分化。當(dāng)IGF-Ⅰ與其受體的α亞基結(jié)合時(shí)，引起β亞基的磷酸化并激活下游信號(hào)轉(zhuǎn)導(dǎo)通路，包括PI3K-Akt和MAPK通路[59]。MAPK在靜止期細(xì)胞處于去磷酸化狀態(tài)，當(dāng)IGF-Ⅰ與其受體結(jié)合后解除了β亞基上的酪氨酸激酶抑制，促使MAPK激活并將信號(hào)傳遞到核內(nèi)，啟動(dòng)有絲分裂過(guò)程[60]。2)促進(jìn)乳蛋白合成的轉(zhuǎn)錄和翻譯過(guò)程。Burgos等[61]證實(shí)，IGF-Ⅰ通過(guò)PI3K-Akt通路促進(jìn)細(xì)胞內(nèi)mRNA的翻譯，且其作用效果與濃度相關(guān)。季昀等[62]向不含血清的奶牛乳腺上皮細(xì)胞培養(yǎng)液中補(bǔ)充IGF-Ⅰ，發(fā)現(xiàn)其可單獨(dú)通過(guò)影響關(guān)鍵激酶及調(diào)節(jié)因子基因的轉(zhuǎn)錄來(lái)調(diào)控乳蛋白合成。3)促進(jìn)動(dòng)物體內(nèi)營(yíng)養(yǎng)物質(zhì)向乳腺的分配。例如，IGF-Ⅰ能通過(guò)增加MBF來(lái)促進(jìn)泌乳奶牛乳汁的分泌[63]。

2.4GH

GH是由動(dòng)物腦垂體前葉嗜酸性細(xì)胞產(chǎn)生的一種不含糖的單鏈多肽，由191個(gè)氨基酸構(gòu)成。GH在哺乳動(dòng)物乳腺發(fā)育和泌乳維持方面同樣具有促進(jìn)作用。Eppard等[64]發(fā)現(xiàn)，腦垂體中GH濃度與奶牛泌乳量呈正相關(guān)。GH可以顯著促進(jìn)離體奶牛乳腺上皮細(xì)胞系中αs1-酪蛋白和α-乳白蛋白mRNA的表達(dá)[65]。用GH處理泌乳早期奶牛，產(chǎn)奶量與對(duì)照組相比提高了36%[66]。Flint等[67]同時(shí)抑制大鼠PRL和GH并給予外源GH處理，發(fā)現(xiàn)泌乳量顯著增加，證實(shí)GH在促進(jìn)乳蛋白合成方面具有獨(dú)立的調(diào)控作用。

GH促進(jìn)泌乳的途徑大致分為2類：一是GH通過(guò)與其受體(GHR)結(jié)合而直接作用于乳腺上皮細(xì)胞[68-69]。GHR是單次跨膜蛋白質(zhì)，屬于Ⅰ類細(xì)胞因子受體超家族。Glimm等[70]運(yùn)用Northern雜交發(fā)現(xiàn)了奶牛乳腺中GHR的存在，并證實(shí)GHR基因主要在腺泡上皮細(xì)胞中表達(dá)。二是GH通過(guò)刺激肝臟細(xì)胞產(chǎn)生IGF-Ⅰ，進(jìn)而通過(guò)其內(nèi)分泌和旁分泌的介導(dǎo)發(fā)揮作用[71]。Kleinberg等[72]發(fā)現(xiàn)GH可以促進(jìn)乳腺組織自身IGF-Ⅰ mRNA的表達(dá)。因此，GH促進(jìn)乳蛋白合成的作用機(jī)制與IGF-Ⅰ相類似，包括：1)促進(jìn)乳腺細(xì)胞的增殖和發(fā)育[73]。GH可以誘導(dǎo)小鼠乳腺腺泡的發(fā)育[74]。青年母牛注射GH后乳腺細(xì)胞數(shù)增加了近50%[75]。2)促進(jìn)乳蛋白合成相關(guān)基因的轉(zhuǎn)錄和翻譯。Hayashi[53]發(fā)現(xiàn)，GH介導(dǎo)PI3K-Akt通路和胞外信號(hào)調(diào)節(jié)激酶(extracellular signal regulated kinases,ERK)通路影響胞內(nèi)mTOR的磷酸化。Malewski等[76]發(fā)現(xiàn)，GH通過(guò)JAK-STAT5信號(hào)通路促進(jìn)乳蛋白基因的表達(dá)。3)促進(jìn)體內(nèi)營(yíng)養(yǎng)物質(zhì)向乳腺的分配[77]。一方面，GH通過(guò)調(diào)節(jié)其他組織的代謝過(guò)程滿足奶牛泌乳的需求。Knapp等[78]指出，GH可以提高肝臟糖異生和氧化速率來(lái)支持乳蛋白合成的能量需要。另一方面，GH通過(guò)提高M(jìn)BF促進(jìn)乳腺對(duì)前提物的提取。Chaiyabutr等[79]發(fā)現(xiàn)，注射GH可以改變奶牛的MBF。

2.5GLN

GLN是一種由胰腺胰島α細(xì)胞分泌、29個(gè)氨基酸構(gòu)成的直鏈多肽。與INS相反，GLN是一種促進(jìn)分解代謝并增加血糖濃度的激素，其可以通過(guò)刺激磷酸化酶促進(jìn)肝臟對(duì)循環(huán)氨基酸的攝取和利用。已有研究證實(shí)GLN對(duì)乳蛋白合成有抑制作用，例如：Bobe等[80]發(fā)現(xiàn)，無(wú)論限制飼喂還是自由飼喂，給泌乳奶牛靜脈灌注GLN均極顯著降低了乳蛋白產(chǎn)量；She等[81]給產(chǎn)后21 d的奶牛連續(xù)14 d靜脈灌注GLN，乳產(chǎn)量顯著降低。

有報(bào)道奶牛泌乳的發(fā)動(dòng)與循環(huán)GLN濃度升高有一定的關(guān)系[82]。Donkin等[83]指出，GLN可以促使肝臟攝取更多的氨基酸用于糖異生和尿素循環(huán)，因此肝臟游離氨基酸濃度下降，從而降低了乳蛋白產(chǎn)量。以往的研究很少關(guān)注GLN對(duì)乳蛋白合成的具體影響，但有趣的是，單一EAA缺乏試驗(yàn)多數(shù)情況下觀察到了INS和GLN濃度的同時(shí)升高，目前還不清楚GLN濃度升高是否會(huì)干擾INS對(duì)乳蛋白合成的促進(jìn)作用，有關(guān)這方面的內(nèi)容值得進(jìn)一步探究。

3　小結(jié)與展望

綜上所述，氨基酸及其構(gòu)成可以作為信號(hào)分子參與各種代謝過(guò)程。PRL、INS、IGF-Ⅰ、GH和GLN在調(diào)控乳蛋白合成方面具有重要作用，其分泌受到氨基酸供給的調(diào)節(jié)。關(guān)于氨基酸構(gòu)成影響激素分泌及激素影響乳蛋白合成的機(jī)制，目前的研究雖建立了一定基礎(chǔ)，但還不是十分明確。未來(lái)的研究可采用體外法從分子水平關(guān)注這2個(gè)方面，重點(diǎn)是已經(jīng)報(bào)道的幾種信號(hào)氨基酸。其結(jié)果有助于構(gòu)建氨基酸影響乳蛋白合成的神經(jīng)內(nèi)分泌調(diào)控網(wǎng)絡(luò)，為探索飼糧理想氨基酸構(gòu)成及提高奶牛氮泌乳轉(zhuǎn)化效率提供一定的幫助。

[1]WEEKES T L,LUIMES P H,CANT J P.Responses to amino acid imbalances and deficiencies in lactating dairy cows[J].Journal of Dairy Science,2006,89(6):2177-2187.

[2]DOEPEL L,LAPIERRE H.Changes in production and mammary metabolism of dairy cows in response to essential and nonessential amino acid infusions[J].Journal of Dairy Science,2010,93(7):3264-3274.

[3]KAPOOR A C,GUPTA Y P.Biological evaluation of soybean protein and effect of amino acid supplementation[J].Journal of Food Science,1975,40(6):1162-1164.

[4]CLARK R M,CHANDLER P T,PARK C S.Limiting amino acids for milk protein synthesis by bovine mammary cells in culture[J].Journal of Dairy Science,1978,61(4):408-413.

[5]HANIGAN M D,FRANCE J,CROMPTON L A,et al.Evaluation of a representation of the limiting amino acid theory for milk protein synthesis[M]//MCNAMARA J P,FRANCE J,BEEVER D E.Modelling nutrient utilization in farm animals.Wallingford:CABI,2000:127-144.

[6]BEQUETTE B J,HANIGAN M D,CALDER A G,et al.Amino acid exchange by the mammary gland of lactating goats when histidine limits milk production[J].Journal of Dairy Science,2000,83(4):765-775.

[7]APPUHAMY J A D R N,KNOEBEL N A,NAYANANJALIE W A D,et al.Isoleucine and leucine independently regulate mTOR signaling and protein synthesis in MAC-T cells and bovine mammary tissue slices[J].The Journal of Nutrition,2012,142(3):484-491.

[8]EIF J,NILSSON D,TENSON T,et al.Selective charging of tRNA isoacceptors explains patterns of codon usage[J].Science,2003,300(5626):1718-1722.

[9]KIMBALL S R,JEFFERSON L S,NGUYEN H V,et al.Feeding stimulates protein synthesis in muscle and liver of neonatal pigs through an mTOR-dependent process[J].American Journal of Physiology:Endocrinology and Metabolism,2000,279(5):E1080-E1087.

[10]PROUD C G.eIF2 and the control of cell physiology[J].Seminars in Cell & Developmental Biology,2005,16(1):3-12.

[11]FUKUMOR R,YOKOTANI A,SUGINO T,et al.Effects of amino acids infused into the vein on ghrelin-induced GH,insulin and glucagon secretion in lactating cows[J].Animal Science Journal,2011,82(2):267-273.

[12]ISHIZUKA N,TANAKA K,SUZUKI Y,et al.Masked function of amino acid sensors on pancreatic hormone secretion in ventromedial hypothalamic (VMH) lesioned rats with marked hyperinsulinemia[J].Obesity Research & Clinical Practice,2012,6(3):e225-e232.

[13]LEMOSQUET S,DELAMAIRE E,LAPIERRE H,et al.Effects of glucose,propionic acid,and nonessential amino acids on glucose metabolism and milk yield in Holstein dairy cows[J].Journal of Dairy Science,2009,92(7):3244-3257.

[14]BERTHIAUME R,THIVIERGE M C,PATTON R A,et al.Effect of ruminally protected methionine on splanchnic metabolism of amino acids in lactating dairy cows[J].Journal of Dairy Science,2006,89(5):1621-1634.

[15]KIM C H,KIM T G,CHOUNG J J,et al.Effects of intravenous infusion of amino acids and glucose on the yield and concentration of milk protein in dairy cows[J].Journal of Dairy Research,2001,68(1):27-34.

[16]陳智梅.不同氨基酸模式對(duì)奶牛α-酪蛋白合成和激素分泌及脂肪合成影響的研究[D].碩士學(xué)位論文.呼和浩特:內(nèi)蒙古農(nóng)業(yè)大學(xué),2009.

[17]KUHARA T,IKEDA S,OHNEDA A,et al.Effects of intravenous infusion of 17 amino acids on the secretion of GH,glucagon,and insulin in sheep[J].American Journal of Physiology:Endocrinology and Metabolism,1991,260(1):E21-E26.

[18]XIAO F,YU J J,GUO Y J,et al.Effects of individual branched-chain amino acids deprivation on insulin sensitivity and glucose metabolism in mice[J].Metabolism,2014,63(6):841-850.

[19]BRAMELD J M,GILMOUR R S,BUTTERY P J.Glucose and amino acids interact with hormones to control expression of insulin-like growth factor-Ⅰ and growth hormone receptor mRNA in cultured pig hepatocytes[J].The Journal of Nutrition,1999,129(7):1298-1306.

[20]KELLY A,LI C,GAO Z,et al.Glutaminolysis and insulin secretion:from bedside to bench and back[J].Diabetes,2002,51(Suppl.3):S421-S426.

[21]LI C H,BUETTGER C,KWAGH J,et al.A signaling role of glutamine in insulin secretion[J].The Journal of Biological Chemistry,2004,279(14):13393-13401.

[22]馬秀玲.精氨酸對(duì)肝臟IGF-Ⅰ分泌的調(diào)控及其促進(jìn)免疫作用機(jī)制的研究[D].博士學(xué)位論文.天津:中國(guó)人民解放軍軍事醫(yī)學(xué)科學(xué)院,2003.

[23]PONCET N,TAYLOR P M.The role of amino acid transporters in nutrition[J].Current Opinion in Clinical Nutrition & Metabolic Care,2013,16(1):57-65.

[24]TAYLOR P M.Role of amino acid transporters in amino acid sensing[J].The American Journal of Clinical Nutrition,2014,99(1):223S-230S.

[25]王建發(fā).乳脂肪和乳蛋白主要前體物對(duì)DCAPCs中GH和PRL的影響及機(jī)制研究[D].博士學(xué)位論文.長(zhǎng)春:吉林大學(xué),2013.

[26]ZARRIN M,WELLNITZ O,BRUCKMAIER R M.Conjoint regulation of glucagon concentrations via plasma insulin and glucose in dairy cows[J].Domestic Animal Endocrinology,2015,51:74-77.

[27]BERTHIAUME R,THIVIERGE C,LOBLEY G E,et al.Effect of a jugular infusion of essential amino acids on splanchnic metabolism in dairy cows fed a protein deficient diet[J].Journal of Dairy Science,2002,85(Suppl.1):72-73.

[29]CANT J R,BERTHIAUME R,LAPIERRE H,et al.Responses of the bovine mammary glands to absorptive supply of single amino acids[J].Canadian Journal of Animal Science,2003,83(3):341-355.

[30]吳慧慧,劉建新,張才喬.反芻動(dòng)物泌乳期的營(yíng)養(yǎng)分配及其調(diào)控機(jī)制[J].中國(guó)畜牧雜志,2005,41(5):64-66.

[31]HANIGAN M D,CALVERT C C,DEPETERS E J,et al.Kinetics of amino acid extraction by lactating mammary glands in control and sometribove-treated Holstein cows[J].Journal of Dairy Science,1992,75(1):161-173.

[32]楊建英,張勇法,王艷玲,等.大豆黃酮對(duì)奶牛產(chǎn)奶量和乳中常規(guī)成分的影響[J].飼料研究,2005(6):30-31.

[33]佟慧麗,高學(xué)軍,李慶章,等.胰島素、催乳素對(duì)奶山羊乳腺上皮細(xì)胞泌乳功能的影響[J].畜牧獸醫(yī)學(xué)報(bào),2008,39(6):721-725.

[34]BOUTINAUD M,LOLLIVIER V,FINOT L,et al.Mammary cell activity and turnover in dairy cows treated with the prolactin-release inhibitor quinagolide and milked once daily[J].Journal of Dairy Science,2012,95(1):177-187.

[35]苗培,楊國(guó)宇,惠永華.催乳素及其受體對(duì)乳腺發(fā)育研究進(jìn)展[J].畜牧獸醫(yī)雜志,2007,26(1):36-38.

[36]LACASSE P,LOLLIVIER V,BRUCKMAIER R M,et al.Effect of the prolactin-release inhibitor quinagolide on lactating dairy cows[J].Journal of Dairy Science,2011,94(3):1302-1309.

[37]JAHCHAN N S,WANG D,BISSELL M J,et al.SnoN regulates mammary gland alveologenesis and onset of lactation by promoting prolactin/Stat5 signaling[J].Development,2012,139(17):3147-3156.

[38]DEVITO W J,STONE S.Ethanol inhibits prolactin-induced activation of the JAK/STAT pathway in cultured astrocytes[J].Journal of Cellular Biochemistry,1999,74(2):278-291.

[39]文靜,卜登攀,王建發(fā),等.激素調(diào)控乳蛋白合成的作用及其分子機(jī)制[J].華北農(nóng)學(xué)報(bào),2012,27(Suppl.1):111-115.

[41]生冉,閆素梅.催乳素與其他激素對(duì)乳腺內(nèi)乳成分合成的協(xié)同調(diào)節(jié)作用[J].動(dòng)物營(yíng)養(yǎng)學(xué)報(bào),2014,26(6):1435-1443.

[42]SPORNDLY E.Effects of diet on milk composition and yield of dairy cows with specical emphasis on milk protein content[J].Swedish Journal of Agricultural Research,1989,19(2):99-106.

[43]MACKLE T R,DWYER D A,INGVARTSEN K L,et al.Effects of insulin and postruminal supply of protein on use of amino acids by the mammary gland for milk protein synthesis[J].Journal of Dairy Science,2000,83(1):93-105.

[44]ANTHONY J C,LANG C H,CROZIER S J,et al.Contribution of insulin to the translational control of protein synthesis in skeletal muscle by leucine[J].American Journal of Physiology:Endocrinology and Metabolism,2002,282(5):E1092-E1101.

[45]MENZIES K K,LEFVRE C,MACMILLAN K L,et al.Insulin regulates milk protein synthesis at multiple levels in the bovine mammary gland[J].Functional & Integrative Genomics,2009,9(2):197-217.

[46]APPUHAMY J A D R N.Regulatory roles of essential amino acids,energy,and insulin in mammary cell protein synthesis[D].Ph.D.Thesis.Virginia:Virginia Polytechnic Institute and State University,2010.

[47]AKERS R M.Major advances associated with hormone and growth factor regulation of mammary growth and lactation in dairy cows[J].Journal of Dairy Science,2006,89(4):1222-1234.

[48]MAGNUSON B,EKIM B,FINGAR D C.Regulation and function of ribosomal protein S6 kinase (S6K) within mTOR signalling networks[J].Biochemical Journal,2012,441(1):1-21.

[49]FINGAR D C,SALAMA S,TSOU C,et al.Mammalian cell size is controlled by mTOR and its downstream targets S6K1 and 4EBP1/eIF4E[J].Genes & Development,2002,16(12):1472-1487.

[50]WANG X M,PROUD C G.The mTOR pathway in the control of protein synthesis[J].Physiology,2006,21(5):362-369.

[51]LEMAY D G,NEVILLE M C,RUDOLPH M C,et al.Gene regulatory networks in lactation:identification of global principles using bioinformatics[J].BMC Systems Biology,2007,1:56.

[52]王皓宇,秦彤,郝海生,等.胰島素對(duì)體外培養(yǎng)奶牛乳腺上皮細(xì)胞乳蛋白、乳脂肪合成相關(guān)基因mRNA表達(dá)的影響[J].畜牧獸醫(yī)學(xué)報(bào),2013,44(5):710-718.

[53]HAYASHI A A.Regulation of protein synthesis in the mammary gland[D].Ph.D.Thesis.Palmerston North:Massey University,2007.

[54]ROSE M T,WEEKES T E C,ROWLINSON P.Correlation of blood and milk components with the milk yield response to bovine somatotropin in dairy cows[J].Domestic Animal Endocrinology,2005,28(3):296-307.

[55]PROSSER C G,FLEET I R,CORPS A N,et al.Increase in milk secretion and mammary blood flow by intra-arterial infusion of insulin-like growth factor-Ⅰ into the mammary gland of the goat[J].Journal of Endocrinology,1990,126(3):437-443.

[56]WALL E H,MCFADDEN T B.The effects of milk removal or four-times-daily milking on mammary expression of genes involved in the insulin-like growth factor-Ⅰ axis[J].Journal of Dairy Science,2010,93(9):4062-4070.

[57]JONES R A,CAMPBELL C I,GUNTHER E J,et al.Transgenic overexpression of IGF-ⅠR disrupts mammary ductal morphogenesis and induces tumor formation[J].Oncogene,2007,26(11):1636-1644.

[58]FORSYTH I A.The insulin-like growth factor and epidermal growth factor families in mammary cell growth in ruminants:action and interaction with hormones[J].Journal of Dairy Science,1996,79(6):1085-1096.

[59]SAMANI A A,YAKAR S,LEROITH D,et al.The role of the IGF system in cancer growth and metastasis:overview and recent insights[J].Endocrine Reviews,2007,28(1):20-47.

[60]DENLEY A,CARROLL J M,BRIERLEY G V,et al.Differential activation of insulin receptor substrates 1 and 2 by insulin-like growth factor-activated insulin receptors[J].Molecular and Cellular Biology,2007,27(10):3569-3577.

[61]BURGOS S A,CANT J P.IGF-1 stimulates protein synthesis by enhanced signaling through mTORC1 in bovine mammary epithelial cells[J].Domestic Animal Endocrinology,2009,38(4):211-221.

[62]季昀,龐學(xué)燕,田青,等.生長(zhǎng)激素和胰島素樣生長(zhǎng)因子Ⅰ對(duì)奶牛乳蛋白合成關(guān)鍵激酶及調(diào)節(jié)因子mRNA表達(dá)量的影響[J].動(dòng)物營(yíng)養(yǎng)學(xué)報(bào),2013,25(1):198-207.

[63]王春陽(yáng),王秋芳.胰島素樣生長(zhǎng)因子與泌乳[J].動(dòng)物醫(yī)學(xué)進(jìn)展,1999,20(1):16-19.

[64]EPPARD P J,BAUMAN D E,MCCUTCHEON S N.Effect of dose of bovine growth hormone on lactation of dairy cows[J].Journal of Dairy Science,1985,68(5):1109-1115.

[65]JOHNSON T L,FUJIMOTO B A S,JIMéNEZ-FLORES R,et al.Growth hormone alters lipid composition and increases the abundance of casein and lactalbumin mRNA in the MAC-T cell line[J].Journal of Dairy Research,2010,77(2):199-204.

[66]MACRINA A L,KAUF A C W,KENSINGER R S.Effect of bovine somatotropin administration during induction of lactation in 15-month-old heifers on production and health[J].Journal of Dairy Science,2011,94(9):4566-4573.

[67]FLINT D J,GARDNER M.Evidence that growth hormone stimulates milk synthesis by direct action on the mammary gland and that prolactin exerts effects on milk secretion by maintenance of mammary deoxyribonucleic acid content and tight junction status[J].Endocrinology,1994,135(3):1119-1124.

[68]TONNER E,QUARRIE L,TRAVERS M,et al.Does an IGF-binding protein (IGFBP) present in involuting rat mammary gland regulate apoptosis?[J].Progress in Growth Factor Research,1995,6(2/3/4):409-414.

[69]ILKBAHAR Y N,THORDARSON G,CAMARILLO I G,et al.Differential expression of the growth hormone receptor and growth hormone-binding protein in epithelia and stroma of the mouse mammary gland at various physiological stages[J].Journal of Endocrinology,1999,161(1):77-87.

[70]GLIMM D R,BARACOS V E,KENNELLY J J.Molecular evidence for the presence of growth hormone receptors in the bovine mammary gland[J].Journal of Endocrinology,1990,126(3):R5-R8.

[71]SAKAMOTO K,YANO T,KOBAYASHI T,et al.Growth hormone suppresses the expression of IGFBP-5,and promotes the IGF-Ⅰ-induced phosphorylation of Akt in bovine mammary epithelial cells[J].Domestic Animal Endocrinology,2007,32(4):260-272.

[72]KLEINBERG D L,RUAN W F,CATANESE V,et al.Non-lactogenic effects of growth hormone on growth and insulin-like growth factor-Ⅰ messenger ribonucleic acid of rat mammary gland[J].Endocrinology,1990,126(6):3274-3276.

[73]BALDI A,MODINA S,CHELI F,et al.Bovine somatotropin administration to dairy goats in late lactation:effects on mammary gland function,composition and morphology[J].Journal of Dairy Science,2002,85(5):1093-1102.

[74]KUMARESAN P,TURNER C W.Effect of growth hormone and thyroxine on mammary gland growth in the rat[J].Journal of Dairy Science,1965,48(5):592-595.

[75]RADCLIFF R P,VANDEHAAR M J,CHAPIN L T,et al.Effects of diet and injection of bovine somatotropin on prepubertal growth and first-lactation milk yields of Holstein cows[J].Journal of Dairy Science,2000,83(1):23-29.

[76]MALEWSKI T,GAJEWSKA M,ZEBROWSKA T,et al.Differential induction of transcription factors and expression of milk protein genes by prolactin and growth hormone in the mammary gland of rabbits[J].Growth Hormone & IGF Research,2002,12(1):41-53.

[77]MOLENTO C F M,BLOCK E,CUE R I,et al.Effects of insulin,recombinant bovine somatotropin,and their interaction on insulin-like growth factor-Ⅰ secretion and milk protein production in dairy cows[J].Journal of Dairy Science,2002,85(4):738-747.

[78]KNAPP J R,FREETLY H C,REIS B L,et al.Effects of somatotropin and substrates on patterns of liver metabolism in lactating dairy cattle[J].Journal of Dairy Science,1992,75(4):1025-1035.

[79]CHAIYABUTR N,BOONSANIT D,CHANPONGSANG S.Effects of cooling and exogenous bovine somatotropin on hematological and biochemical parameters at different stages of lactation of crossbred Holstein Friesian cow in the tropics[J].Asian-Australasian Journal of Animal Sciences,2011,24(2):230-238.

[80]BOBE G,HIPPEN A R,SHE P,et al.Effects of glucagon infusions on protein and amino acid composition of milk from dairy cows[J].Journal of Dairy Science,2009,92(1):130-138.

[81]SHE P,HIPPEN A R,YOUNG J W,et al.Metabolic responses of lactating dairy cows to 14-day intravenous infusions of glucagon[J].Journal of Dairy Science,1999,82(6):1118-1127.

[82]SARTIN J L,CUMMINS K A,KEMPPAINEN R J,et al.Glucagon,insulin,and growth hormone responses to glucose infusion in lactating dairy cows[J].American Journal of Physiology:Endocrinology and Metabolism,1985,248(1):E108-E114.

[83]DONKIN S S,ARMENTANO L E.Insulin and glucagon regulation of gluconeogenesis in preruminating and ruminating bovine[J].Journal of Animal Science,1995,73(2):546-551.

(責(zé)任編輯菅景穎)

, professor, E-mail: zhwang@sdau.edu.cn

Recent Advances in Regulating Milk Protein Synthesis by Interaction of Amino Acid Supply and Endocrine

XU LianbinLIN XueyanHU ZhiyongWANG YunWANG Zhonghua*

(College of Animal Science and Technology, Shandong Agricultural University, Tai’an 271018, China)

Limiting amino acid is one of hot topics in dairy cows studies. Previous researches, bothinvitroandinvivo, show that amino acid supply and profile play a major role in milk protein synthesis by endocrine and intracellular signaling pathways rather than substrate. Focusing on the influencing factors of milk protein synthesis, this paper reviewed the regulation of amino acid supply, hormone secretion and their interaction on milk protein synthesis, which might provided a theoretical reference for elucidating the pathways of milk protein synthesis affected by amino acid supply.[ChineseJournalofAnimalNutrition, 2016, 28(8)：2324-2333]

amino acid; endocrine; regulation of interaction; milk protein synthesis; recent advances

10.3969/j.issn.1006-267x.2016.08.002

2016-03-03

國(guó)家自然科學(xué)基金面上項(xiàng)目(31372340)

徐連彬(1988—)，男，山東莒南人，博士研究生，從事反芻動(dòng)物營(yíng)養(yǎng)與生理研究。E-mail: lbxu0814@163.com

王中華，教授，博士生導(dǎo)師，E-mail: zhwang@sdau.edu.cn

S852.2

A

1006-267X(2016)08-2324-10