丹参酮ⅡA对豚鼠肥厚心肌延迟整流钾通道的影响

目的: 通过研究丹参酮ⅡA(TSN)对豚鼠肥厚心肌细胞快激活延迟整流钾电流(I_Kr)和慢激活延迟整流钾电流(I_Ks)的影响,在离子通道水平探讨丹参酮ⅡA抗肥厚心肌心律失常的机制。方法: 采用腹主动脉结扎技术制造心肌肥厚模型,将豚鼠随机分为假手术组(A组)、肥厚模型组(B组)、低剂量丹参组(C组,10 mg·kg^-1·d^-1)、高剂量丹参组(D组,20 mg·kg^-1·d^-1)和缬沙坦治疗组(E组,10 mg·kg^-1·d^-1),每组12只。通过应用标准的全细胞膜片钳技术记录各实验组心肌细胞膜上动作电位时程(APD)、I_Ks和I_Kr密度的变化。结果: (1)与A组相比,B组、C组、D组和E组手术后4周血压均明显升高,差异有显著差异(P＜0.01),B、C、D和E组间血压无显著差异(P＞0.05)。(2)与A组相比,B组心肌细胞的膜电容明显升高,APD显著延长(P＜0.01)。 (3)与B组相比,C、D和E组显著缩短肥大心肌细胞APD的延长,降低膜电容和阻断心肌细胞上I_Kr、I_Ks的密度(P＜0.01);C和D组间无显著差异(P＞0.05)。结论: 丹参酮Ⅱ-A能降低肥厚心肌细胞上I_Kr 和I_Ks的密度,可能是其干预肥厚心肌电生理异常的重要机制之一。     

家兔陈旧性心肌梗死心室肌细胞钾离子电流的变化

目的:探讨陈旧性心肌梗死(HMI)心律失常的发生机制,观察HMI非梗死区心肌细胞动作电位时程(APD)、瞬时外向钾电流(I_to)、延迟整流钾电流(I_K)和内向整流钾电流(I_K1)的变化。方法: 12只家兔随机分为2组,陈旧性心肌梗死组(HMI)开胸结扎冠状动脉左回旋支,假手术组开胸但不结扎冠状动脉。3个月后应用全细胞膜片钳技术记录非梗死区心肌细胞的APD、I_to、 I_K和I_K1。 结果: (1)HMI组心肌细胞的膜电容明显高于假手术组;(2)HMI组心肌细胞的APD显著延长,并有早期后除极(EAD)出现;(3)HMI组心肌细胞I_to 、I_K,tail和I_K1的电流密度分别为(4.03±0.33)pA/pF、(1.14±0.11)pA/pF和(17.6±2.3)pA/pF,显著低于假手术组的(6.72±0.42)pA/pF、(1.54±0.13)pA/pF和(25.6±2.6)pA/pF(P<0.01)。结论: HMI非梗死区心室肌细胞I_to 、I_K,tail、和I_K1的电流密度的降低是其APD延长和EAD出现的离子流基础,而APD延长和EAD的出现,可能在HMI恶性心律失常的发生中起着重要作用。     

兔在体左室肥厚心肌跨室壁电异质性的实验研究

目的：探讨兔在体左室肥厚心肌单相动作电位跨室壁的不均一性变化。方法：以腹主动脉缩窄术制备家兔高血压左室肥厚模型，并设假手术组（仅游离腹主动脉未缩窄）作为对照。采用自制复合式电极在兔左心室前壁同步记录心内膜、心肌中层、心外膜在体3层心肌单相动作电位（MAP），比较两组间跨室壁复极离散度(TDR)等各项参数的差异。结果：腹主动脉缩窄组平均动脉压、全心重量及其与体重比率、左心室游离壁厚度均大于假手术组。缩窄组3层心肌MAPD100［内膜：(191±19)ms；中层：(244±24)ms;外膜：(196±15)ms］均长于对照组［内膜：(170±18)ms ;中层：(172±15)ms;外膜：(168±16)ms，P<0.01］，以中层心肌MAPD100延长最为明显，缩窄组TDR（65±10)ms较对照组（4±3)ms明显增大（P<0.01）。结论：兔在体左室肥厚心肌跨室壁电不均一性明显增大，可能是肥厚心肌心律失常发生增多的原因之一。     

兔心肌梗死1周及2月后心室肌细胞瞬间外向钾电流的变化

目的:研究心肌梗死后心室肌细胞瞬间外向钾电流(I_to)的变化。方法:采用结扎兔冠状动脉左前降支的方法复制心肌梗死动物模型,酶解法分离单个心室肌细胞,应用膜片钳全细胞记录方法,观察心肌梗死后1周及2月心外膜梗死区及非梗死区心室肌细胞I_to的变化。结果:①非梗死区2月组心肌细胞膜电容明显大于对照组,P＜0.05。②梗死区I_to电流密度(+60mV时)的对比显示:对照组为(17.4±5.2)pA/pF(n=16),梗死区1周组为(7.5±2.4)pA/pF(n=12),明显低于对照组(P＜0.01)。梗死区2月组为(10.6±4.1)pA/pF(n=18),明显低于对照组(P＜0.01),但高于梗死区1周组(P＜0.05)。③非梗死区2月组I_to电流密度为(13.2±4.1)pA/pF(n=23),明显低于对照组(P＜0.05),但高于梗死区2月组(P＜0.05)。结论:心肌梗死可引起心室肌细胞I_to电流密度下降,而且不同区域之间(梗死区及远离梗死区)也存在电生理的异质性,可能是导致心肌梗死后出现折返性室性心律失常的原因。心肌梗死后2月梗死区I_to的下降有恢复倾向。     

磷酸肌酸对缺血大鼠心室中层心肌细胞瞬间外向钾电流的影响

目的:观察不同浓度外源性磷酸肌酸(phosphocreatine, PCr)对大鼠缺血心室中层心肌细胞(M细胞)瞬间外向钾通道(I_to)电流的影响,探讨其预防缺血性心律失常的电生理学机制。方法: 单个M细胞经酶解从大鼠左心室中层获得,采用膜片钳全细胞模式记录I_to电流,通过灌注模拟缺血液并充以95%N₂+5%CO₂的混合气体建立缺血模型,将PCr加入模拟缺血液中分别配成终浓度5、10、20和30 mmol/L。将细胞分成6组,分别给予模拟缺血液,含有5、10、20和30 mmol/L PCr的模拟缺血液以及台氏液灌流,后者充以95% O₂+5% CO₂的混合气体。10 min后记录各组的峰电流及电流密度。结果: 与台氏液组相比,单纯模拟缺血液组I_to峰电流密度降低(76.1±6.3)%(P<0.05),含有5、10、20和30 mmol/L PCr的模拟缺血液组I_to峰电流密度分别降低(57.1±9.6)%(P<0.05)、(40.3±10.3)%(P<0.05)、(34.3±9.6)%(P<0.05)和(32.1±10.6)%(P<0.05)。PCr为0、5、10 mmol/L时三者峰电流密度具有明显差异(P<0.05)。PCr为10、20、30 mmol/L对I_to峰电流密度的影响无显著差异(P>0.05)。结论: PCr能增加缺血时受抑制的M细胞I_to峰电流及电流密度,这可能是其预防缺血性心律失常的电生理学机制。低浓度 (0~10 mmol/L)PCr对I_to峰电流及电流密度的影响呈现明显的量效关系。     

大鼠骨髓间质干细胞诱导分化为心肌样细胞膜电流的研究

目的： 研究大鼠骨髓间质干细胞（MSCs）诱导分化为心肌样细胞膜电流的状态。方法： 按文献方法进行MSCs的诱导培养和鉴定。诱导后培养4周，用全细胞膜片钳技术，检测胞膜电流，并与未经诱导分化的MSCs进行比较。结果： 未经诱导分化的MSCs不表达内向电流，只表达外向钾电流。MSCs经5-aza诱导后表达心肌特异性肌钙蛋白T，记录到2种膜内向电流，分别为钠电流（I_Na）和L-型钙电流（I_Ca），以及3种外向钾电流，包括瞬间外向钾电流（I_to），超快速激活延迟整流钾电流（I_kur）及缓慢激活延迟整流钾电流（I_ks）。与未经诱导分化的MSCs相比，诱导后MSCs的钾电流以I_kur和I_ks为主。结论： 经5-aza诱导后，MSCs可分化成具有电压依赖性内向I_Na、I_Ca和外向I_to、I_kur、I_ks的心肌样细胞。     

急性坏死性胰腺炎对心室肌细胞钠和钙离子电流的影响

目的:研究急性坏死性胰腺炎(ANP)后心室肌细胞钠通道电流(I_Na)、L-钙通道电流(I_Ca-L)活性的变化。方法:采用牛磺胆酸钠逆行胰胆管注射建立鼠的ANP动物模型,应用膜片钳全细胞记录方法,观察ANP后1 h心肌细胞I_Na、I_Ca-L的变化。结果:I_Na和I_Ca-L电流-电压关系曲线在ANP组较对照组明显上移。ANP组I_Na电流密度峰值(-30 mV)为(12.45±2.26)pA／pF(n=16),明显低于假手术组(25.32±3.31)pA/pF(n=14),P<0.01;ANP组I_Ca-L电流密度峰值(+10 mV)为(3.63±0.65)pA／pF(n=16),显著低于假手术组(5.46±1.03)pA／pF(n=12),P<0.01。结论:ANP可导致心室肌细胞I_Na和I_Ca-L下降,引起心肌传导速度下降和动作电位时程缩短,可能是导致ANP后出现心律失常的原因。     

心脏复极储备电流IKs在糖尿病QT间期延长性别差异中的作用

目的: 观察不同性别糖尿病家兔QT间期延长病理条件下的缓慢延迟整流钾电流(I_Ks)以及蛋白变化,为探讨糖尿病性长QT综合征性别差异的离子机制做基础。方法: 取体重2-2.5 kg家兔,一次性注射预热(37 ℃)的四氧嘧啶(140 mg/kg),8周后造成1型糖尿病模型,测定血糖,记录标准II导联心电图,采用酶解法分离家兔单个心室肌细胞,应用全细胞膜片钳技术记录动作电位时程(APD)和I_Ks,并且运用Western blotting法检测KvLQT1和mink蛋白表达变化。结果: 雌雄糖尿病组QT间期和APD均较对照组延长,雄性延长明显,且延长百分比差异显著(P<0.05)。在+40 mV到+70 mV测试电压范围内,雄性糖尿病组I_Ks step电流密度均低于对照组(P<0.05),在+70 mV时,由对照组(3.08±0.67)pA/pF(n=17)降低到(1.27±0.20)pA/pF(n=16),在0 mV~+70 mV测试电压范围内,雌性糖尿病组I_Ks step电流密度均高于对照组(P<0.05),在+70 mV时,由对照组的(1.56±0.20)pA/pF(n=13)增加到(3.65±0.50)pA/pF(n=14)。Western blotting结果显示雄性糖尿病组KvLQT1和mink蛋白表达水平分别下 调21.6%和18.5%;雌性糖尿病组KvLQT1和mink蛋白表达水平分别上调42.3%和20.5%(P<0.05)。结论: I_Ks参与了糖尿病QT间期延长的发生,并且存在性别差异。在雌性家兔早期糖尿病模型中,作为一个复极储备,代偿性上调,限制了QT间期的过度延长。     

胡椒碱减轻H2O2 引起的兔心房肌细胞内向整流钾电流及超速激活延迟整流钾电流的异常改变

目的: 研究胡椒碱对H₂O₂引起的兔单个心房肌细胞内向整流钾电流(I_K1)及超速激活的延迟整流钾电流(I_KUr)异常的影响。方法: 采用全细胞膜片钳技术分析50 μmol/L H₂O₂对兔单个心房肌细胞I_K1和I_KUr的影响,并研究预先应用7 μmol/L胡椒碱对其的保护作用。结果: 7 μmol/L胡椒碱对正常兔心房肌细胞I_K1和I_KUr及其通道动力学无显著影响。在50 μmol/L H₂O₂作用下,兔心房肌细胞I_K1峰值由(-148.2±16.7)pA/pF降低至(-64.2±9.8)pA/pF (P<0.05),电流-电压曲线上移;而I_KUr峰值由(16.0±2.1)pA/pF降低至(6.1±1.4)pA/pF (P<0.05),电流-电压曲线下移,通道稳态激活曲线右移,通道稳态失活曲线左移及恢复时间减慢,而且存在频率依赖性特征。预先给予7 μmol/L胡椒碱,明显减轻H₂O₂对I_K1和I_KUr的抑制作用(P<0.01),并可减少H₂O₂对超速激活延迟整流钾通道动力学的异常影响。结论: 胡椒碱可减轻氧化应激对心房肌细胞I_K1和I_KUr的影响。     

灯盏花素对大鼠心室肌细胞瞬间外向和内向整流钾电流的影响

目的：研究灯盏花素对大鼠心室肌细胞膜瞬间外向钾电流（I_to）和内向整流钾电流(I_K1)的影响，在离子通道水平探讨灯盏花素的抗心律失常作用机制。方法： 用急性酶解法获得单个大鼠心室肌细胞，标准的全细胞膜片钳技术记录I_to和I_K1。结果：(1) 灯盏花素呈浓度依赖性抑制I_to，在+50 mV时，0.02、0.05、0.08、0.10 (g· L^-1)灯盏花素分别阻断Ito峰电流(10.07±0.30)%、(27.47±1.25)%、(42.72%±1.30)% 和(56.09±2.10)%，冲洗后可以完全恢复。在+50 mV时，0.10 g·L^-1灯盏花素使峰电流从（29.61±3.40）pA/pF 减少至（13.00±1.80）pA/pF (n=5，P<0.05)。（2）灯盏花素呈电压依赖性抑制I_to，在0～+50 mV,各浓度随电压增加抑制作用明显减弱。（3）0.10 g·L^-1灯盏花素对I_to失活、激活和复活曲线无明显影响。（4）0.10 g·L^-1灯盏花素对I_K1无明显影响。结论： 灯盏花素能够抑制心肌细胞I_to，呈浓度依赖性和电压依赖性，对I_K1无明显影响，这可能是其抗心律失常作用的重要机制之一。     

Sex differences in repolarization and slow delayed rectifier potassium current and their regulation by sympathetic stimulation in rabbits

Slow delayed rectifier potassium current (I_Ks) is important in action potential (AP) repolarization and repolarization reserve. We tested the hypothesis that there are sex-specific differences in I_Ks, AP, and their regulation by β-adrenergic receptors (β-AR’s) using whole-cell patch-clamp. AP duration (APD₉₀) was significantly longer in control female (F) than in control male (M) myocytes. Isoproterenol (ISO, 500 nM) shortened APD₉₀ comparably in M and F, and was largely reversed by β₁-AR blocker CGP 20712A (CGP, 300 nM). Inhibition of I_Ks with chromanol 293B (10 μM) resulted in less APD prolongation in F at baseline (3.0 vs 8.9 %, p?<?0.05 vs M) and even in the presence of ISO (5.4 vs 20.9 %, p?<?0.05). This suggests that much of the ISO-induced APD abbreviation in F is independent of I_Ks. In F, baseline I_Ks was 42 % less and was more weakly activated by ISO (19 vs 68 % in M, p?<?0.01). ISO enhancement of I_Ks was comparably attenuated by CGP in M and F. After ovariectomy, I_Ks in F had greater enhancement by ISO (72 %), now comparable to control M. After orchiectomy, I_Ks in M was only slightly enhanced by ISO (23 %), comparable to control F. Pretreatment with thapsigargin (to block SR Ca release) had bigger impact on ISO-induced APD shortening in F than that in M (p?<?0.01). In conclusion, we found that there are sex differences in I_Ks, AP, and their regulation by β-AR’s that are modulated by sex hormones, suggesting the potential for sex-specific antiarrhythmic therapy.     

Angiotensin II type 1 receptor mediates partially hyposmotic-induced increase of I Ks current in guinea pig atrium

A repolarizing conduction in the heart augmented by hyposmotic or mechanically induced membrane stretch is the slow component of delayed rectifier K⁺ current (I _Ks). I _Ks upregulation is recognized as a factor promoting appearance of atrial fibrillation (AF) since gain-of-function mutations of the channel genes have been detected in congenital AF. Mechanical stretch activates angiotensin II type 1 (AT₁) receptor in the absence of its physiological ligand angiotensin II. We investigated the functional role of AT₁ receptor in I _Ks enhancement in hyposmotically challenged guinea pig atrial myocytes using the whole-cell patch-clamp method. In atrial myocytes exposed to hyposmotic solution with osmolality decreased to 70% of the physiological level, I _Ks was enhanced by 84.1%, the duration of action potential at 90% repolarization (APD₉₀) was decreased by 16.8%, and resting membrane potential was depolarized (+4.9 mV). The hyposmotic-induced effects on I _Ks and APD₉₀ were significantly attenuated by specific AT₁ receptor antagonist candesartan (1 and 5 μM). Pretreatment of atrial myocytes with protein tyrosine kinase inhibitors tyrphostin A23 and A25 suppressed but the presence of tyrosine phosphatase inhibitor orthovanadate augmented hyposmotic stimulation of I _Ks. The above results implicate AT₁ receptor and tyrosine kinases in the hyposmotic modulation of atrial I _Ks and suggest acute antiarrhythmic properties of AT₁ antagonists in the settings of stretch-related atrial tachyarrhythmias.     

Cellular basis of sex disparities in human cardiac electrophysiology

Aim: Sex disparities in electrocardiogram variables and dysrhythmia susceptibility exist, notably in long QT syndrome (LQTS) and Brugada syndrome, but the underlying mechanisms in man are unknown. We studied the cellular basis of sex distinctions in human cardiac electrophysiology and dysrhythmia susceptibility using mathematical models of human ventricular myocytes. Methods: We implemented sex differences in the Priebe–Beuckelmann and ten Tusscher–Noble–Noble–Panfilov human ventricular cell models by modifying densities of the L‐type Ca²⁺ current (I_Ca,L), transient outward K⁺ current (I_to), and rapid delayed rectifier K⁺ current (I_Kr), according to experimental data from male and female hearts of various species. Sex disparities in transmural repolarization were studied in transmural strands of cells with ion current densities based on canine experimental data. Results: Female cells have longer action potential duration (APD), steeper APD‐heart rate relationship, larger transmural APD heterogeneity, and a greater susceptibility to pro‐dysrhythmogenic early afterdepolarizations (EADs) than male cells. Conversely, male cells have more prominent phase‐1 repolarization and are more susceptible to all‐or‐none repolarization. Conclusion: Sex differences in I_Ca,L, I_to and I_Kr densities may explain sex disparities in human cardiac electrophysiology. Female cells exhibit a limited ‘repolarization reserve’ as demonstrated by their larger susceptibility to EADs, which, combined with their larger transmural electrical heterogeneity, renders them more vulnerable to tachydysrhythmias in LQTS. Conversely, male cells have a limited ‘depolarization reserve’, as shown by their larger susceptibility to all‐or‐none repolarization, which facilitates tachydysrhythmias in Brugada syndrome. These general principles may also apply to dysrhythmia susceptibility in common disease.     

Mechanisms responsible for the altered cardiac repolarization dispersion in experimental hypothyroidism

Aims: To identify the causes for the inhomogeneity of ventricular repolarization and increased QT dispersion in hypothyroid mice. Methods: We studied the effects of 5‐propyl‐2‐thiouracil‐induced hypothyroidism on the ECG, action potential (AP) and current density of the repolarizing potassium currents I_to,fast, I_to,slow, I_K,slow and I_ss in enzymatically isolated myocytes from three different regions of mouse heart: right ventricle (RV), epicardium of the left ventricle (Epi‐LV) and interventricular septum. K⁺ currents were recorded with the patch‐clamp technique. Membranes from isolated ventricular myocytes were extracted by centrifugation. Kv4.2, Kv4.3, KChIP and Na/Ca exchanger proteins were visualized by Western blot. Results: The frequency or conduction velocity was not changed by hypothyroidism, but QTc was prolonged. Neither resting membrane potential nor AP amplitude was modified. The action potential duration (APD)₉₀ increased in the RV and Epi‐LV, but not in the septum. Hypothyroid status has no effect either on I_to,slow, I_k,slow or I_ss in any of the regions analysed. However, I_to,fast was significantly reduced in the Epi‐LV and in the RV, whereas it was not altered in cells from the septum. Western blot analysis reveals a reduction in Kv4.2 and Kv4.3 protein levels in both the Epi‐LV and the RV and an increase in Na/Ca exchanger. Conclusion: From these results we suggest that the regional differences in APD lengthening, and thus in repolarization inhomogeneity, induced by experimental hypothyroidism are at least partially explained by the uneven decrease in I_to,fast and the differences in the relative contribution of the depolarization‐activated outward currents to the repolarization process.     

Role of slow delayed rectifier K+-current in QT prolongation in the alloxan-induced diabetic rabbit heart

Aim: In diabetes mellitus, several cardiac electrophysiological parameters are known to be affected. In rodent experimental diabetes models, changes in these parameters were reported, but only limited relevant information is available in other species, having cardiac electrophysiological properties more resembling the human, including the rabbit. The present study was designed to analyse the effects of experimental type 1 diabetes on ventricular repolarization and the underlying transmembrane potassium currents in rabbit hearts. Methods: Diabetes was induced by a single injection of alloxan (145 mg kg⁻¹ i.v.). After the development of diabetes (3 weeks), electrophysiological studies were performed using whole cell voltage clamp and ECG measurements. Results: The QT_c interval in diabetic rabbits was moderately but statistically significantly longer than measured in the control animals (155 ± 1.8 ms vs. 145 ± 2.8 ms, respectively, n = 9–10, P < 0.05). This QT_c-lengthening effect of diabetes was accompanied by a significant reduction in the density of the slow delayed rectifier K⁺ current, I_Ks (from 1.48 ± 0.35 to 0.86 ± 0.17 pA pF⁻¹ at +50 mV, n = 19–21, P < 0.05) without changes in current kinetics. No differences were observed either in the density or in the kinetics of the inward rectifier K⁺ current (I_K1), the rapid delayed rectifier K⁺ current (I_Kr), the transient outward current (I_to) and the L-type calcium current (I_CaL) between the control and alloxan-treated rabbits. Conclusion: It is concluded that type 1 diabetes mellitus, although only moderately, lengthens ventricular repolarization. Diabetes attenuates the repolarization reserve by decreasing the density of I_Ks current, and thereby may enhance the risk of sudden cardiac death.     

Effects of potassium channel openers in the isolated perfused hypokalaemic murine heart

Aim: We explored the anti‐arrhythmic efficacy of K⁺ channel activation in the hypokalaemic murine heart using NS1643 and nicorandil, compounds which augment I_Kr and I_KATP respectively. Methods: Left ventricular epicardial and endocardial monophasic action potentials were compared in normokalaemic and hypokalaemic preparations in the absence and presence of NS1643 (30 μm ) and nicorandil (20 μm ). Results: Spontaneously beating hypokalaemic hearts (3 mm K⁺) all elicited early afterdepolarizations (EADs) and episodes of ventricular tachycardia (VT). Perfusion with NS1643 and nicorandil suppressed EADs and VT in 7 of 13 and five of six hypokalaemic hearts. Provoked arrhythmia studies using programmed electrical stimulation induced VT in all hypokalaemic hearts, but failed to do so in 7 of 13 and five of six hearts perfused with NS1643 and nicorandil respectively. These anti‐arrhythmic effects were accompanied by reductions in action potential duration at 90% repolarization (APD₉₀) and changes in the transmural gradient of repolarization, reflected in ΔAPD₉₀. NS1643 and nicorandil reduced epicardial APD₉₀ from 68.3 ± 1.1 to 56.5 ± 4.1 and 51.5 ± 1.5 ms, respectively, but preserved endocardial APD₉₀ in hypokalaemic hearts. NS1643 and nicorandil thus restored ΔAPD₉₀ from ?9.6 ± 4.3 ms under baseline hypokalaemic conditions to 3.9 ± 4.1 and 9.9 ± 2.1 ms, respectively, close to normokalaemic values. Conclusion: These findings demonstrate, for the first time, the anti‐arrhythmic efficacy of K⁺ channel activation in the setting of hypokalaemia. NS1643 and nicorandil are anti‐arrhythmic through the suppression of EADs, reductions in APD₉₀ and restorations of ΔAPD₉₀.     

Pharmacological separation of early afterdepolarizations from arrhythmogenic substrate in DeltaKPQ Scn5a murine hearts modelling human long QT 3 syndrome

Aim: To perform an empirical, pharmacological, separation of early afterdepolarizations (EADs) and transmural gradients of repolarization in arrhythmogenesis in a genetically modified mouse heart modelling human long QT syndrome (LQT) 3. Methods: Left ventricular endocardial and epicardial monophasic action potentials and arrhythmogenic tendency were compared in isolated wild type (WT) and Scn5a+/Δ hearts perfused with 0.1 and 1 μm propranolol and paced from the right ventricular epicardium. Results: All spontaneously beating bradycardic Scn5a+/Δ hearts displayed EADs, triggered beats and ventricular tachycardia (VT; n = 7), events never seen in WT hearts (n = 5). Perfusion with 0.1 and 1 μm propranolol suppressed all EADs, triggered beats and episodes of VT. In contrast, triggering of VT persisted following programmed electrical stimulation in 6 of 12 (50%), one of eight (12.5%), but six of eight (75%) Scn5a+/Δ hearts perfused with 0, 0.1 and 1 μm propranolol respectively in parallel with corresponding alterations in repolarization gradients, reflected in action potential duration (ΔAPD₉₀) values. Thus 0.1 μm propranolol reduced epicardial but not endocardial APD₉₀ from 54.7 ± 1.6 to 44.0 ± 2.0 ms, restoring ΔAPD₉₀ from ?3.8 ± 1.6 to 3.5 ± 2.5 ms (all n = 5), close to WT values. However, 1 μm propranolol increased epicardial APD₉₀ to 72.5 ± 1.2 ms and decreased endocardial APD₉₀ from 50.9 ± 1.0 to 24.5 ± 0.3 ms, increasing ΔAPD₉₀ to ?48.0 ± 1.2 ms. Conclusion: These findings empirically implicate EADs in potentially initiating spontaneous arrhythmogenic phenomena and transmural repolarization gradients in the re‐entrant substrate that would sustain such activity when provoked by extrasystolic activity in murine hearts modelling human LQT3 syndrome.     

Electrophysiological changes in rat ventricular and atrial myocardium at different stages of experimental diabetes

Action potential configuration in ventricular and atrial myocardium, as well as rate-dependent changes in ventricular action potential duration (APD) were studied and compared in healthy and diabetic rats. Diabetes was induced by a single injection of streptozotocin (STZ, 65 mg kg^–1 i.v.). Conventional microelectrode techniques were applied to record action potentials after the establishment of diabetes (2, 6, 10 and 18 weeks after STZ-treatment). Untreated age-matched animals were used as controls. Both depolarization and repolarization were significantly retarded following STZ-treatment. However, the time course of development of diabetic changes in atrial and ventricular myocardium was different. APD was significantly lengthened from week 2 of diabetes in ventricular, but only from week 6 in atrial preparations. In atrial myocardium, lengthening of APD was more pronounced at early rather than late phases of repolarization. The maximum rate of depolarization (V_max) was significantly reduced from the 6th week of diabetes in both preparations. No differences were observed in action potential amplitude (except at week 18) and in the resting membrane potential in diabetic rats. Diabetic ventricular preparations showed a positive APD-frequency relationship at any level of repolarization, in contrast to control muscles, where APD₂₅ and APD₅₀ values lengthened. But APD₇₅ and APD₉₀ values were not changed significantly with increase in the pacing frequency. The results indicate that development of diabetic alterations are not fully identical in atrial and ventricular myocardium of the rat, probably owing to differences in density and kinetics of ionic currents responsible for atrial and ventricular action potentials.     

Beta-adrenergic stimulation reverses the I Kr–I Ks dominant pattern during cardiac action potential

β-Adrenergic stimulation differentially modulates different K⁺ channels and thus fine-tunes cardiac action potential (AP) repolarization. However, it remains unclear how the proportion of I _Ks, I _Kr, and I _K1 currents in the same cell would be altered by β-adrenergic stimulation, which would change the relative contribution of individual K⁺ current to the total repolarization reserve. In this study, we used an innovative AP-clamp sequential dissection technique to directly record the dynamic I _Ks, I _Kr, and I _K1 currents during the AP in guinea pig ventricular myocytes under physiologically relevant conditions. Our data provide quantitative measures of the magnitude and time course of I _Ks, I _Kr, and I _K1 currents in the same cell under its own steady-state AP, in a physiological milieu, and with preserved Ca²⁺ homeostasis. We found that isoproterenol treatment significantly enhanced I _Ks, moderately increased I _K1, but slightly decreased I _Kr in a dose-dependent manner. The dominance pattern of the K⁺ currents was I _Kr?>?I _K1?>?I _Ks at the control condition, but reversed to I _Kr?I _K1?I _Ks following β-adrenergic stimulation. We systematically determined the changes in the relative contribution of I _Ks, I _Kr, and I _K1 to cardiac repolarization during AP at different adrenergic states. In conclusion, the β-adrenergic stimulation fine-tunes the cardiac AP morphology by shifting the power of different K⁺ currents in a dose-dependent manner. This knowledge is important for designing antiarrhythmic drug strategies to treat hearts exposed to various sympathetic tones.