Post-partum prolongation of the atrial repolarization in rabbit.

Female sexual steroids are known to modify the expression of various K+ channels and thus they can alter cardiac repolarization. In the present work, using conventional microelectrode techniques, action potential characteristics were studied in atrial myocardium isolated from virgin, late pregnant, early (1-3 days) post-partum and late (2-3 weeks) post-partum rabbits. No changes in action potential configuration were observed during pregnancy. However, the duration, overshoot and amplitude of action potentials were significantly increased in the early (1-3 days) post-partum period. Resting potential and maximum rate of depolarization remained unchanged. The observed changes were transient, normal action potential characteristics were obtained at weeks 2-3 post-partum. 4-aminopyridine (1 mmol L(-1)). caused a marked lengthening of action potential duration in all preparations obtained from non-pregnant and pregnant rabbits, whereas this 4-aminopyridine-induced prolongation was moderate in those preparations excised from the hearts of early post-partum animals. Action potential configuration was not affected by pinacidil (10 micromol L(-1)) or glibenclamide (5 micromol L(-1)) in non-pregnant or pregnant animals. In preparations obtained from early post-partum rabbits, pinacidil significantly shortened action potential duration, which was reverted by glibenclamide. The lengthening of action potential duration together with the decreased sensitivity to 4-aminopyridine observed in early post-partum animals may probably be caused by reduction of the transient outward K+ current at this stage. The results also suggest that electrophysiological alterations in the early post-partum period may probably be more pronounced than those associated with pregnancy itself.     

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.     

瑞芬太尼对兔心肌动作电位及跨室壁复极离散度的影响

目的:观察瑞芬太尼对兔心肌动作电位及跨室壁复极离散度的影响。方法:成年家兔18只,体重2.0~2.5 kg,制备Langendorff离体心脏灌注模型,K-H液平衡灌注15 min后随机分为3组(n=6):正常对照组(C组)继续灌注37℃K-H液60 min;瑞芬太尼组(R组)灌注含12μg/L瑞芬太尼的K-H液60 min;瑞芬太尼+氨茶碱组(RA组)灌注含12μg/L瑞芬太尼+30 mg/L氨茶碱的K-H液60 min。记录平衡灌注15 min(T0)、继续灌注15 min(T1)、30 min(T2)和60 min(T3)时心率(HR)和左心室前壁3层心肌单相动作电位(MAP),计算单相动作电位复极90%的时程(MAPD90)和跨室壁复极离散度(TDR)。记录早期后除极、延迟后除极及心律失常的发生情况。结果:与T0比较,R组T1~T3时HR减慢,MAPD90延长,TDR增大(P0.05)。与C组和RA组比较,R组HR减慢时,MAPD90延长,TDR增大(P0.05)。结论:瑞芬太尼减慢HR时,MAPD90延长,TDR增大,折返激动易于发生;氨茶碱增快HR,缩短MAPD90,从而使TDR减小。     

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.     

Ventricular repolarization pattern under heart cooling in the rabbit

Aim: Prolongation of ventricular repolarization is characteristic of myocardial cooling. In the present study, we investigated whether this prolongation is uniform or not throughout ventricular epicardium and how these hypothermia‐induced changes express in the body surface potential distribution. Methods: Epicardial and body surface potential mapping from 64 unipolar leads was carried out in 18 anaesthetized adult rabbits. Mild hypothermia documented by lowering the mediastinal and rectal temperature from 38 to 32 °C was elicited by perfusion of the mediastinum with cooled saline. Activation times, repolarization times and activation–recovery intervals were determined in each epicardial lead. Results: Baseline activation–recovery intervals distributed non‐uniformly on the ventricular epicardium, increasing progressively from the apex to the base and from the left ventricular (LV) sites to the right ventricular (RV) sites (P < 0.05), governing the repolarization sequence of ventricular epicardium. Heart cooling from 38 to 32 °C produced the heterogeneous prolongation of activation–recovery intervals which was more pronounced at the apex than at the base, and in the LV areas compared to the RV areas (P < 0.05). These nonuniform changes in local repolarization durations resulted in the development of base‐to‐apex repolarization sequence and inversion of the body surface potential distribution during the T wave. Conclusion: Thus, under cooling the rabbit heart from 38 to 32 °C, the nonuniform prolongation of local repolarization durations resulted in the reversal of ventricular epicardial repolarization sequence which, in turn, was responsible for the inversion in the body surface potential distribution during the T wave.     

Isolation of Toxins from Buthus Occitanus Sp. Scorpion and Their Action on Excitability of Myelinated Nerve

A series of short neurotoxins (molecular weight 3500-5000 D) was isolated from Vietnamese scorpion B. occitanus sp. All these toxins blocked generation of action potentials (this effect depended on their molecular weight), but did not change conduction velocity and excitation threshold of the nerve.     

Cell swelling causes the action potential duration to shorten in guinea-pig ventricular myocytes by activating I KATP

 In guinea-pig ventricular myocytes, cell swelling by incubation in hypotonic solution caused a pronounced shortening of the action potential duration (APD₉₀: 15.5±14.6% compared to control; mean ± SD) after a latency of 12 min when the intracellular ATP concentration was 2 mM. This shortening was partially reversible within 10 min after reperfusion with isotonic solution (APD₉₀: 80.5±12.1% compared to control). With 5 mM intracellular ATP in the pipette electrode, the effect of cell swelling on the action potential was significantly reduced. Incubation with 1 μM glibenclamide, a blocker of the ATP-dependent K⁺ current (I _KATP), abolished the swelling-induced shortening of the action potential duration, whereas incubation with 0.5 mM 4,4’-diisothiocyanatostilbene-2,2’-disulphonic acid (DIDS), a blocker of the swelling-induced Cl^– current (I _Cl,swell), had no effect on the action potential duration in hypotonic solution. Simultaneous measurements of membrane currents substantiate that I _KATP is the current that underlies this effect. These results suggest that in the ischaemic myocardium I _KATP may be partially activated by cell swelling, resulting in a shortening of the action potential duration before the intracellular ATP concentration has fallen below 2 mM. Received: 30 March 1998 Received after revision: 7 July 1998 Accepted: 25 July 1998     

Repolarization of epicardial ventricular surface of rabbit heart in acute stenosis of the aortic arch

Overload of the cardiac left ventricle causes opposite local changes in repolarization duration (activation-recovery intervals) on the right-and left-ventricular epicardium, which depend on the duration of overload. The activation-recovery intervals on the right-ventricular lateral surface decrease after 1-min overload, but increase after 10-min overload. The length of activation-recovery intervals on the lateral surface of the left-ventricular apex increases after 10-min aortic stenosis more markedly in comparison with that after 1-min overload. Decrease/increase of local lengths of activation-recovery intervals results in modification of the general sequence of cardiac ventricular surface repolarization. Translated from Byulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 146, No. 8, pp. 144–146, August, 2008     

家兔高钾血症心电生理机制的实验研究

目的观察家兔高钾血症心电生理特征,研究家兔高钾血症对心脏的毒性作用机制。方法对照组输注0．9％NaCl,记录家兔正常的心电图特征及血钾浓度,实验组分别输注1％、4％、10％KCl溶液（2mL／kg）复制高钾血症模型,观察家兔心电生理的变化特征,记录输钾前后血钾浓度、吸光度、心率、心电图异常率及各指标之间的变化关系。结果实验组输钾前后血钾浓度、吸光度、心率、心电图异常率、心电图各波形时限及振幅差异有显著变化（P〈0．05）,随着输钾浓度增高发生高血钾时间缩短,差异具有显著性（P〈0．05）,实验家兔血钾浓度与心电图诊断高钾血症的一致性良好（P〈0．01）,在ROC曲线上心电图变异率诊断水平最高（P〈0．01）,其次为血钾浓度（P〈0．05）,而心率诊断水平最差（P〉0．05）,血钾浓度与心电图变异率高度相关（P〈0．05）,血钾浓度与心率相关性较差（P〉0．05）。结论实验家兔高钾血症心电生理机制的研究对临床急性高钾血症的鉴别诊断、及时治疗有重要参考价值。     

Effects of celecoxib on ionic currents and spontaneous firing in rat retinal neurons

Accumulating evidence suggests that the side effects of celecoxib, widely used to treat muscle and joint pain, may be mediated in part through cyclooxygenase-2 (COX-2) independent mechanisms, such as inhibition of ion channels. In this study we report effects of celecoxib on ionic currents and neuronal activity in isolated rat retinal neurons. We found that celecoxib suppressed voltage-gated potassium currents in retinal bipolar cells with an effective concentration to inhibit 50% of function (EC(50)) of 5.5 muM. In retinal amacrine and ganglion cells, celecoxib inhibited voltage-dependent sodium channels with an EC(50) of 5.2 muM, and voltage-dependent transient and sustained potassium currents with EC(50)s of 16.3 and 9.1 muM, respectively. Notably, the rate of spontaneous spike activity was dramatically suppressed in ganglion and amacrine cells with an EC(50) of 0.76 muM. All actions of celecoxib on ionic currents and action potentials occurred from the extracellular side and were completely reversible. These findings indicate that inhibition of ion channels by celecoxib in the CNS may affect neuronal function at clinically relevant concentrations.     

Contribution of different Ca2+‐activated K+ channels to the first phase of the response to TRH in clonal rat anterior pituitary cells

Aims: Thyrotropin‐releasing hormone (TRH) induces biphasic changes in electrical activity, cytosolic free Ca²⁺ level ([Ca²⁺]_i), and prolactin secretion from both clonal GH cells and native lactotrophs. The first phase of the TRH response is characterized by hyperpolarization because of activation of Ca²⁺‐activated K⁺ channels (K_Ca). In the present study, the relative contribution of BK, SK, and IK channels to the first phase of the TRH response in GH₄ cells was assessed. Methods: The expression of IK channels was confirmed by PCR with specific primers for SK4 (IK). The response to TRH was studied using the perforated patch technique and Ca²⁺ microfluoromety (fura‐2). The involvement of different K_Ca channels was estimated by employing the specific channel blockers iberiotoxin (BK), apamin (SK) and clotrimazole (IK). Results: Application of 100 nm iberiotoxin, 1 μm apamin, and 10 μm clotrimazole reduced the peak value of the outward K⁺ current during the first phase of the TRH response by 33, 26, and 33%, respectively. Clotrimazole also shortened the duration of the outward current response by 60%, causing a reduction of total charge movement by 73%. All these toxin‐induced reductions were significant (P < 0.05). A combination of all three toxins abolished the current response almost completely. Conclusion: All the three main types of K_Ca channels are involved in the first phase of the TRH response, with IK as the major contributor. This is the first demonstration of a dominant role of IK compared with BK and SK channels in excitable cells.     

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。     

Pacemaker currents in mouse locus coeruleus neurons

We have characterized the currents that flow during the interspike interval in mouse locus coeruleus (LC) neurons, by application of depolarizing ramps and pulses, and compared our results with information available for rats. A tetrodotoxin (TTX)-sensitive current was the only inward conductance active during the interspike interval; no TTX-insensitive Na⁺ or oscillatory currents were detected. Ca²⁺-free and Ba²⁺-containing solutions failed to demonstrate a Ca²⁺ current during the interspike interval, although a Ca²⁺ current was activated at membrane potentials positive to −40 mV. A high- tetraethylammonium chloride (TEA) (15 mM) sensitive current accounted for almost all the K⁺ conductance during the interspike interval. Ca²⁺-activated K⁺, inward rectifier and low-TEA (10 μM) sensitive currents were not detected within the interspike interval. Comparison of these findings to those reported for neonatal rat LC neurons indicates that the pacemaker currents are similar, but not identical, in the two species with mice lacking a persistent Ca²⁺ current during the interspike interval. The net pacemaking current determined by differentiating the interspike interval from averaged action potential recordings closely matched the net ramp-induced currents obtained either under voltage clamp or after reconstructing this current from pharmacologically isolated currents. In summary, our results suggest the interspike interval pacemaker mechanism in mouse LC neurons involves a combination of a TTX-sensitive Na⁺ current and a high TEA-sensitive K⁺ current. In contrast with rats, a persistent Ca²⁺ current is not involved.     

Ionic mechanisms of desflurane on prolongation of action potential duration in rat ventricular myocytes

Purpose

Despite the fact that desflurane prolongs the QTC interval in humans, little is known about the mechanisms that underlie these actions. We investigated the effects of desflurane on action potential (AP) duration and underlying electrophysiological mechanisms in rat ventricular myocytes.

Materials and Methods

Rat ventricular myocytes were enzymatically isolated and studied at room temperature. AP was measured using a current clamp technique. The effects of 6% (0.78 mM) and 12% (1.23 mM) desflurane on transient outward K⁺ current (I_to), sustained outward current (I_sus), inward rectifier K⁺ current (I_KI), and L-type Ca²⁺ current were determined using a whole cell voltage clamp.

Results

Desflurane prolonged AP duration, while the amplitude and resting membrane potential remained unchanged. Desflurane at 0.78 mM and 1.23 mM significantly reduced the peak I_to by 20±8% and 32±7%, respectively, at +60 mV. Desflurane (1.23 mM) shifted the steady-state inactivation curve in a hyperpolarizing direction and accelerated inactivation of the current. While desflurane (1.23 mM) had no effects on I_sus and I_KI, it reduced the L-type Ca²⁺ current by 40±6% (p<0.05).

Conclusion

Clinically relevant concentrations of desflurane appear to prolong AP duration by suppressing I_to in rat ventricular myocytes.