李小龍 萬征

慢性心力衰竭(簡稱心衰)患者常伴發(fā)各種室性心律失常,其死亡率約占心衰患者總死亡率的13%;而隨著冠狀動脈性心臟病(簡稱冠心病)發(fā)病率和心臟重構(gòu)的增加,缺血性心肌病(Ischemic Cardiomyopathy,ICM)已逐漸成為心衰的主要病因[1-2]。心臟重構(gòu)是心臟在病理環(huán)境中的一種適應(yīng)性改變,包括心肌重構(gòu)、血管重構(gòu)、能量代謝重構(gòu)和電重構(gòu)等[3-4]。而其中電重構(gòu)又是心律失常的基礎(chǔ),通常發(fā)生在心衰早期,從細胞層面開始逐漸累及整個心臟。近年來,心臟電重構(gòu)已成為ICM和心衰的研究焦點。

一、心室肌細胞

心衰時心室肌細胞的適應(yīng)性變化會導(dǎo)致細胞電生理特性的改變,即細胞電重構(gòu)。其中,離子通道重構(gòu)是主體,包括膜電流、通道蛋白表達和通道調(diào)節(jié)的異常改變等,這將引起細胞動作電位(Action Potential,AP)的改變,如細胞動作電位時程(Action Potential Duration,APD)延長等,這是心律失常發(fā)生的直接原因之一。見圖1。

比較正常心室肌細胞,衰竭心臟心室肌細胞APD明顯延長[5]。

圖1 來自正常和衰竭心臟的心室肌細胞AP示意圖

1.離子通道和膜電流的重構(gòu):在心衰模型中,心肌細胞的電生理特征主要表現(xiàn)為可記錄到的APD延長[6],涉及到多種離子通道重構(gòu)和膜電流變化(表1)。在ICM中,左室心肌細胞快鈉電流(Transient Sodium Current,INaT)在缺血時明顯減少[7],而INaT的減少將導(dǎo)致AP 0期上升速率降低,使興奮傳導(dǎo)速度減慢,進而使再灌注區(qū)受損或正在恢復(fù)功能的細胞和正常細胞間興奮的傳導(dǎo)速度產(chǎn)生差異,從而誘發(fā)”折返”等異常電生理現(xiàn)象。同時,由于晚鈉電流(Late Sodium Current,INaL)參與AP平臺期的形成,且當(dāng)發(fā)生ICM或心衰時,INaL明顯增大[8],從而使平臺期顯著延長。在心衰模型中,內(nèi)向整流鉀電流(Inwardly Rectifying Potassium Current,Ik1)減少,延長復(fù)極,降低復(fù)極后不應(yīng)性,從而易誘發(fā)遲發(fā)后除極(Delayed Afterdepolarizations,DADs)和功能性折返[9]。

近年來,一些新發(fā)現(xiàn)的離子通道也逐漸成為治療和延緩心衰進展的研究熱點之一,如ATP敏感性鉀(ATP-sensitive Potassium,KATP)通道、小電導(dǎo)鈣激活鉀(Small Conductance Calcium-activated Potassium,SK)通道以及雙孔鉀(two-pore-domain potassium,K2P)通道。在正常心臟中,細胞膜上KATP通道的高表達有助于心肌細胞快速復(fù)極,在機體耗氧量增多的情況下,有助于增加心率和心輸出量;K2P通道也是促進心肌細胞復(fù)極,穩(wěn)定細胞膜電位[10]。而SK通道在正常心臟中的表達量很少,因此在正常生理狀態(tài)下,其并不會對心室肌細胞的電活動產(chǎn)生較大影響[11]。在心衰模型中,KATP通道的表達量減少了35%～40%,使心肌細胞復(fù)極減慢,在心率加快的情況下,易引起心肌細胞耗氧量明顯增加,導(dǎo)致了心肌細胞氧需求與氧供應(yīng)之間的不平衡,引起心肌缺血性損傷及心功能的進一步失調(diào)[12],進而加速心衰進展,加重電生理紊亂,引起惡性循環(huán);K2P2.1通道在心衰時表達減少,易引起心室電傳導(dǎo)紊亂及室性心律失常,但其機制尚未明確[13];而SK通道同樣可以誘發(fā)室性心律失常。有研究認為在衰竭心室肌細胞中,ISK增多[14];同時, 單相動作電位復(fù)極90%的時程(Monophasic Action Potential Duration at 90% Repolarization,MAPD90) 縮短,并表現(xiàn)出致室性心律失常作用;而當(dāng)ISK被阻滯時,縮短的MAPD90會恢復(fù)正常,產(chǎn)生心臟保護作用[15]。

2.離子通道與轉(zhuǎn)運體的重構(gòu):少量的Ca2+通過L型鈣通道(LTCC)進入細胞內(nèi),并刺激蘭尼堿受體(RyRs),使RyR2開放,進而使肌漿網(wǎng)釋放大量Ca2+進入細胞質(zhì),并刺激肌絲蛋白,從而引發(fā)收縮活動。通過關(guān)閉L型鈣通道(L-type Ca2+channel,LTCC)和蘭尼堿受體(Ryanodine Receptors,RyRs),肌漿網(wǎng)鈣

表1 心衰模型心室肌不同通道電流變化概況

注:“—”為無明顯變化;“↑”為延長;“↓”為縮短;“Ik1”為內(nèi)向整流鉀電流;“Ito”為瞬時外向鉀電流;“Ik”為延遲整流鉀電流;“INaL”為晚鈉電流;“ICa-L”為L型鈣電流;“IKATP”為ATP敏感性鉀電流;“IKCa”鈣激活鉀電電流。

ATP酶(SERCA)回攝Ca2+及通過鈉鈣交換體(Na+/Ca2+Exchanger,NCX)的Ca2+流出,從而建立細胞質(zhì)低濃度Ca2+環(huán)境,引發(fā)舒張活動。

近年來研究顯示離子通道電流的改變通常提示通道蛋白及構(gòu)象方面的重構(gòu),在這一層面的研究和認識逐步深入。

在正常心臟中,Ca2+信號(Ca2+signaling)是維持心臟正常電活動和收縮舒張功能的核心環(huán)節(jié)之一。在該環(huán)節(jié)中,由心肌細胞細胞膜上和細胞內(nèi)的離子通道和轉(zhuǎn)運體共同參與,如鈉鈣交換體(Na+/Ca2+exchanger,NCX)和蘭尼堿受體(Ryanodine Receptors,RyRs)等,它們在電傳導(dǎo)、興奮收縮耦聯(lián)等方面發(fā)揮重要作用。見圖2。鈣調(diào)蛋白激酶Ⅱ(Ca2+/Calmodulin-dependent Protein Kinase Ⅱ,CaMKⅡ)則在其中居重要地位,在ICM所致的心衰中,CaMKⅡ的表達量及活性均明顯增加,使得RyRs發(fā)生過磷酸化,肌漿網(wǎng)Ca2+泄露增加[31],誘發(fā)鈣超載,激活NCX并產(chǎn)生內(nèi)向電流,引發(fā)DADs,進而觸發(fā)AP及室性心律失常。近期有研究表明可通過阻滯NOD1(Nucleotide-binding Oligomerization Domain-containing Protein 1)信號通路來減少異常收縮期Ca2+釋放和RyRs的過磷酸化來減少上述Ca2+調(diào)控異常的發(fā)生[32],以減少NCX激活所介導(dǎo)的內(nèi)向電流的產(chǎn)生,進而減少AP的觸發(fā)及心律失常的發(fā)生。在生理條件下,NCX1(Na+/Ca2+exchanger 1,NCX1)是鈣穩(wěn)態(tài)必需的調(diào)節(jié)因子,其表達增加可以減弱壓力過載介導(dǎo)的心臟結(jié)構(gòu)重構(gòu)[33]。因此,NCX1可能是延緩心衰進展的潛在治療靶點。

有研究報道稱CaMKⅡ抑制劑SMP-114主要通過減少Ca2+火花(Ca2+sparks)的發(fā)生,從而使Ca2+泄露明顯減少以維持鈣穩(wěn)態(tài),且可明顯減少INaL[34],進而減少室性心律失常的發(fā)生。在ICM,盡管Ca2+火花的發(fā)生減少,但RyRs對Ca2+敏感性增強,促進Ca2+火花向Ca2+波(Ca2+waves)的擴展,使Ca2+波的發(fā)生頻率增加,產(chǎn)生致心律失常作用[33]。此外,鉀離子相互作用蛋白在心衰時表達減少,引起L型鈣電流增大,延長APD,同時明顯減少鈣瞬變幅度及延長瞬變時間[33],進而誘發(fā)早發(fā)后除極和(或)DADs,形成了心律失常的電生理基礎(chǔ)。

圖2 鈣調(diào)蛋白與T管及肌漿網(wǎng)的關(guān)系模型圖[30]

二、細胞外基質(zhì)和細胞間連接

在衰竭心臟中,心室肌的細胞構(gòu)成和細胞外基質(zhì)(extracellular matrix,ECM)的改變、縫隙連接的表達和分布異常,將引起細胞間電傳導(dǎo)速度和路徑的改變,增加電傳導(dǎo)的各向異性(anisotropic),構(gòu)成了各種折返性心律失常的電生理基礎(chǔ)(圖3)。

圖3 心肌纖維化的病因及后果示意圖

顯微圖像顯示結(jié)締組織(膠原:紅色)在正常心肌(左)和廣泛左心室纖維化患者心肌(右)的分布情況[35]。比例尺=100 μm。

1.細胞外基質(zhì):衰竭的心臟可發(fā)生一系列引起電生理異常的病理改變,包括心肌肌束間及血管周圍膠原纖維顯著增生,可見灶性纖維瘢痕,心肌細胞萎縮或壞死、排列紊亂和間質(zhì)血管擴張充血,使得細胞外基質(zhì)(Extracellular Matrix,ECM)發(fā)生重度重構(gòu)[36]。

在衰竭心臟心室成纖維細胞中,SOCE/CRAC通道(Store-operated Ca2+Entry/Ca2+Release-activated Channels)介導(dǎo)的Ca2+電流明顯增大,且該通道成孔亞單位ORAI表達增多,可導(dǎo)致衰竭心臟膠原蛋白分泌增多[37];同時,ECM中收縮蛋白的表達增多[38],二者共同促進心肌間質(zhì)纖維化;此外,過量的ECM蛋白可阻斷心肌細胞束的連續(xù)性,而這會使心肌電傳導(dǎo)的各向異性增加,導(dǎo)致局部電傳導(dǎo)紊亂和折返性心律失常[39](圖4)。目前,有研究人員發(fā)現(xiàn)可以通過抑制G蛋白βγ亞單位及其與G蛋白耦聯(lián)受體激酶2之間的聯(lián)系來消除上述病理性心肌纖維化[40],進而改善心臟結(jié)構(gòu)重構(gòu)和電重構(gòu),延緩心衰進展,減少心律失常的發(fā)生;此外,有研究報道,在心衰時心臟賴氨酰氧化酶的表達量及活性均進行性增加[41]以及心臟成纖維細胞特異激活轉(zhuǎn)錄因子3的表達上調(diào)[42],均可削弱心肌纖維化。

(a)正常心肌組織。(b)與心肌細胞束平行的纖維化(如發(fā)生反應(yīng)性纖維化)?？v向細胞-細胞連接完整,且縱向傳導(dǎo)不受影響。(c)平行纖維化和穿過心肌束的纖維化(如發(fā)生梗死后修復(fù)性纖維化)[43]。由于死亡的心肌細胞被成纖維細胞和纖維化細胞外基質(zhì)蛋白替換而產(chǎn)生的物理分離,導(dǎo)致縱向傳導(dǎo)的局部異常。

2.細胞間連接:在正常心臟中,縫隙連接是心肌細胞之間連接的主要方式,而在心室肌中,縫隙連接蛋白主要為Cx43。有報道ICM患者的心室肌組織中,Cx43的數(shù)量明顯減少[44],且從閏盤到細胞邊界重新分布,由細胞的端-端連接轉(zhuǎn)變?yōu)閭?cè)-側(cè)連接,偏側(cè)化增加,且大部分Cx43發(fā)生脫磷酸化改變,導(dǎo)致細胞間興奮耦聯(lián)下降[45]。這些異常都可導(dǎo)致心肌發(fā)生不均一性電傳導(dǎo)改變,使心肌細胞間傳導(dǎo)速度變慢,APD延長,形成折返環(huán)路等,從而誘發(fā)惡性室性心律失常[46]。此外,Cx43的重構(gòu)可能也會導(dǎo)致心肌細胞內(nèi)鈣超載與ATP濃度降低[47],影響心室肌興奮收縮耦聯(lián)。因此,抑制Cx43重構(gòu),可以恢復(fù)心肌細胞間電耦聯(lián),改善沖動在細胞間傳導(dǎo),使心臟電活動趨于同步,減少心律失常發(fā)生。

此外,心肌ECM中的整合素、基質(zhì)細胞蛋白和金屬蛋白酶在胞間連接中同樣發(fā)揮著重要作用[48],可以作為治療心臟重構(gòu)和心律失常的直接靶點。

三、展望

目前,隨著分子生物學(xué)的進展,心衰研究已進入分子時代,著眼于衰竭細胞中離子通道亞單位及相關(guān)構(gòu)型構(gòu)象等,從而在分子水平闡述電重構(gòu),可以全方位立體化解讀ICM的電重構(gòu)現(xiàn)象,進而為ICM合并心衰患者心律失常的治療提供新思路,為研發(fā)分子靶向藥物提供新方向,提供心衰患者心律失常“上游治療”基礎(chǔ)。

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