Heterogeneous Expression of Potassium Currents and Pacemaker Currents Potentially Regulates Arrhythmogenesis of Pulmonary Vein Cardiomyocytes

Introduction: The relationship between the determining factors and beating rates of pulmonary vein (PV) pacemaker cardiomyocytes has not been fully elucidated. The purposes of this study were to compare the electrophysiological characteristics between PV fast and slow pacemaker cardiomyocytes. Methods: Whole‐cell patch clamp was used to investigate the action potentials, transient outward currents (I_to), sustained outward potassium currents (I_Ksus), rapid delayed rectifier potassium currents (I_Kr), inward rectifier potassium current (I_K1), pacemaker currents (I_f), and transient inward currents in isolated rabbit PV single fast (≥2.5 Hz) and slow (<2.5 Hz) pacemaker cardiomyocytes. Results: The fast PV pacemaker cardiomyocytes (n = 66) had lesser negative maximum diastolic potential (?53 ± 1 mV vs ?59 ± 1 mV, P < 0.001) and larger slope of diastolic depolarization (0.31 ± 0.02 V/sec vs 0.09 ± 0.01 V/sec, P < 0.001) than slow PV pacemaker cardiomyocytes (n = 65). Moreover, the PV beating rates correlated well with the slope of diastolic depolarization and maximum diastolic potential with a linear regression. As compared to those in slow PV pacemaker cardiomyocytes, the fast PV pacemaker cardiomyocytes had smaller I_to and I_K1, and larger I_f but with similar I_Ksus,I_Kr, and transient inward currents. However, only few fast (2/34) and slow (1/23) PV pacemaker cardiomyocytes contained barium‐sensitive hyperpolarization‐activated time‐dependent current (I_KH)_. Conclusions and Implications: The fast and slow PV pacemaker had different electrophysiological characteristics. I_to, I_K1, and I_f may regulate the beating rates of PV pacemaker cardiomyocytes.     

Contribution of <Emphasis Type="Italic">I</Emphasis><Subscript>Kr</Subscript> and <Emphasis Type="Italic">I</Emphasis><Subscript>K1</Subscript> to ventricular repolarization in canine and human myocytes: is there any influence of action potential duration?

Background  The aim of the present work was to study the profile of the rapid delayed rectifier potassium current (I _Kr) and the inward rectifier potassium current (I _K1) during ventricular repolarization as a function of action potential duration and rate of repolarization. Methods  Whole cell configuration of the patch clamp technique was used to monitor I _Kr and I _K1 during the action potential plateau and terminal repolarization. Action potentials recorded at various cycle lengths (0.4–5 s) and repolarizing voltage ramps having various slopes (0.5–3 V/s) were used as command signals. I _Kr and I _K1 were identified as difference currents dissected by E-4031 and BaCl₂, respectively. Results  Neither peak amplitudes nor mean values of I _Kr and I _K1 recorded during the plateau of canine action potentials were influenced by action potential duration. The membrane potential where I _Kr and I _K1 peaked during the terminal repolarization was also independent of action potential duration. Similar results were obtained in undiseased human ventricular myocytes, and also in canine cells when I _Kr and I _K1 were evoked using repolarizing voltage ramps of various slopes. Action potential voltage clamp experiments revealed that the peak values of I _Kr, I _K1, and net outward current during the terminal repolarization were independent of the pacing cycle length within the range of 0.4 and 5 s. Conclusions  The results indicate that action potential configuration fails to influence the amplitude of I _Kr and I _K1 during the ventricular action potential in dogs and humans, suggesting that rate-dependent changes in action potential duration are not likely related to rate-dependent alterations in I _Kr or I _K1 kinetics in these species. Returned for 1. Revision: 5 February 2008 1. Revision received: 11 April 2008 Returned for 2. Revision: 14 May 2008 2. Revision received: 15 May 2008     

Effects of benzyltetrahydropalmatine on two components of the delayed rectifier K+ current in Guinea pig ventricular myocytes

The effects of benzyltetrahydropalmatine (BTHP), a new class III antiarrhythmic agent, on the action potential in guinea pig papillary muscle and the rapidly activating component (I _Kr) and the slowly activating component (I _Ks) of the delayed rectifier potassium current (I _K) in isolated guinea pig ventricular myocytes were investigated. The action potentials of papillary muscles were studied using a standard microelectrode technique, while the K⁺ currents were recorded using the whole-cell patch clamp technique. The results showed that BTHP prolonged the action potential duration (APD) without altering other variables of the action potential in guinea pig papillary muscles. The 2 components of I _K were blocked by BTHP (1 100 mol·L^–1) in time-, concentration-, voltage-, and specifically frequency-dependent fashion. The IC₅₀ value for blockade ofI _Kr was 13.5 mol·L^–1, while the IC₅₀ value for blockade of I _Ks was 9.3 mol·L^–1. BTHP 30.0 mol·L^–1 reduced I _Kr and I _Kr,tail by 31 ± 4.3% and 36 ± 4.7% (n = 6, p < 0.01) and decreased I _Ks and I _Ks,tail by 40 ± 6.3% and 39 ± 4.6% (n = 7, p < 0.01) respectively. BTHP accelerated their deactivation course by reducing the time constants of deactivation of I _Kr and I _Ks. The activation kinetics of I _Kr or I _Ks were not affected by BTHP. It is concluded that BTHP prolonged the action potential duration with respect to its non-selective action on I _Kr and I _Ks in single guinea pig ventricular cell in a frequency-dependent fashion.     

Atrial-selective sodium channel block as a novel strategy for the management of atrial fibrillation

Pharmacological management of atrial fibrillation (AF) remains an important unmet medical need. Because available drugs for rhythm control of AF are often associated with a significant risk for development of ventricular arrhythmias or extracardiac toxicity, recent drug development has focused on agents that are atrial selective. Inhibition of the ultrarapid delayed rectifier potassium current (I_Kur), a current exclusive to atria, is an example of an atrial-selective approach. Recent studies, however, have shown that loss-of-function mutations in KCNA5, the gene that encodes K_V1.5, the α subunit of the I_Kur channel, is associated with the development of AF and that inhibition of I_Kur can promote the induction of AF in experimental models. Another potential atrial-selective approach has recently been identified. Experimental studies have demonstrated important atrioventricular differences in the biophysical properties of the sodium channel and have identified sodium channel blockers that can exploit electrophysiological distinctions between atria and ventricles. Atrial-selective/predominant sodium channel blockers such as ranolazine effectively suppress AF in experimental models involving canine-isolated right atrial preparations at concentrations that produce little to no effect on electrophysiological parameters in ventricular myocardium. Chronic administration of amiodarone was also found to exert atrial-selective depression of I_Na-dependent parameters and thus to prevent the induction of AF. Ranolazine and amiodarone have in common the ability to rapidly dissociate from the sodium channel and to prolong the atrial action potential duration via inhibition of I_Kr. Our observations suggest that atrial-selective sodium channel block may be a fruitful strategy for the management of AF.     

Specificities of atrial electrophysiology: Clues to a better understanding of cardiac function and the mechanisms of arrhythmias

The electrical properties of the atria and ventricles differ in several aspects reflecting the distinct role of the atria in cardiac physiology. The study of atrial electrophysiology had greatly contributed to the understanding of the mechanisms of atrial fibrillation (AF). Only the atrial L-type calcium current is regulated by serotonine or, under basal condition, by phosphodiesterases. These distinct regulations can contribute to I_Ca down-regulation observed during AF, which is an important determinant of action potential refractory period shortening. The voltage-gated potassium current, I_Kur, has a prominent role in the repolarization of the atrial but not ventricular AP. In many species, this current is based on the functional expression of K_V1.5 channels, which might represent a specific therapeutic target for AF. Mechanisms regulating the trafficking of K_V1.5 channels to the plasma membrane are being actively investigated. The resting potential of atrial myocytes is maintained by various inward rectifier currents which differ with ventricle currents by a reduced density of I_K1, the presence of a constitutively active I_KACh and distinct regulation of I_KATP. Stretch-sensitive or mechanosensitive ion channels are particularly active in atrial myocytes and are involved in the secretion of the natriuretic peptide. Integration of knowledge on electrical properties of atrial myocytes in comprehensive schemas is now necessary for a better understanding of the physiology of atria and the mechanisms of AF.     

Molecular determinants of repolarization time

This review analyzes recent data concerning the molecular determinants of repolarization time (RT) in normal and disease conditions. Considerations concerning the prognostic significance of RT were excluded. On a single normal cell, the duration of the action potential is the result of a balance between different ion currents. In vivo or on a multicellular preparation, the QT duration is modified by different transmural gradients, including the endo/epicardial gradient and the apex/base gradient. Spatial heterogeneity of the RT is not reflected by the range of the body surface QT dispersion. Inherited long QT syndrome is due to a gain or loss of function mutations located on the sodium current, the rapidly activating component of the delayed rectifier (I_Kr) and the slowly activating component of the delayed rectifier. So far, no mutations have been detected on the transient outward K⁺ current (I_tO). Drug-induced long QT is caused by drugs that act as potassium blockers, which interact on specific domains of K⁺ channel subunits, mainly on I_Kr. Several drugs may reveal ‘forme frustes’ of an inherited long QT. A prolonged RT is a well documented finding in cardiac hypertrophy and heart failure and is mostly caused by the noninduction and corresponding decreased density of the K⁺ channel responsible for I_tO. Hypertrophy can even reverse the trans-mural gradient. In humans and rats, isolated pressure overload prolongs the QT interval. The reduction in I_tO is likely to participate in the slowing of the cardiac cycle and reflects the re-expression of the fetal programme.     

Gender-related differences in ventricular myocyte repolarization in the guinea pig

Abstract. It is well established that gender-differences exist in cardiac electrophysiology and these are thought to contribute to the increased risk of women, compared to men, for the potentially lethal ventricular arrhythmia, torsades de pointes. Data from animal models with abbreviated estrus cycles suggest that androgens may play a protective role in males. However, the role of female sex hormones in gender-differences in cardiac electrophysiology is less clear. This report describes gender differences in ventricular electrophysiology, investigated using the guinea pig heart. Ionic currents and action potentials were compared between ventricular myocytes isolated from male guinea pig hearts and those from females on the day of estrus (day 0) and 4 days post-estrus (day 4). The density of inward rectifier K⁺ current (I_K1) at –120 mV was significantly greater in male myocytes than in female myocytes either at day 0 or day 4. The peak L-type Ca²⁺ current (I_Ca) at +10 mV was also significantly larger in male myocytes than in day 0 and day 4 female myocytes. Moreover, I_Ca differed significantly between day 0 and day 4 female myocytes, strongly suggesting that I_Ca density varies around the estrus cycle. Delayed rectifier (I_K) tail currents were significantly different between male and female day 4 myocytes. Action potential duration (at 90% repolarization; APD₉₀) was significantly shorter in male myocytes than in female myocytes at day 0, but not at day 4, broadly consistent with the combined differences in I_K and I_Ca between the three groups. Taken together, our data are consistent with the contribution of multiple factors, rather than a single hormone, to gender differences in ventricular repolarization. Since female guinea pigs possess a conventional estrus cycle, our data suggest that this species may be well suited to elucidating the modulatory influence of ovarian steroids on ventricular repolarization and arrhythmic risk. Our findings suggest that further work examining the basis to gender differences in ventricular repolarization in the guinea pig is warranted.     

Effects of nerve growth factor on the action potential duration and repolariz-ing currents in a rabbit model of myocardial infarction

Objective To investigate the effect of nerve growth factor (NGF) on the action potential and potassium currents of non-infarcted myocardium in the myocardial infarcted rabbit model. Methods Rabbits with occlusion of the left anterior descending coronary artery were prepared and allowed to recover for eight weeks (healed myocardial infarction, HMI). During ligation surgery of the left coronary artery, a polyethylene tube was placed near the left stellate ganglion in the subcutis of the neck for the purpose of administering NGF 400 U/d for eight weeks (HMI + NGF group). Cardiomyocytes were isolated from regions of the non-infarcted left ventricular wall and the action potentials and ion currents in these cells were recorded using whole-cell patch clamps. Results Compared with HMI and control cardiomyocytes, significant prolongation of APD₅₀ or APD₉₀ (Action potential duration (APD) measured at 50% and 90% of repolarization) in HMI + NGF cardiomyocytes was found. The results showed that the 4-aminopyridine sensitive transient outward potas?sium current (I_to), the rapidly activated omponent of delayed rectifier potassium current (I_Kr), the slowly activated component of delayed rectifier potassium current (I_Ks), and the L-type calcium current (I_CaL) were significantly altered in NGF + HMI cardiomyocytes compared with HMI and control cells. Conclusions Our results suggest that NGF treatment significantly prolongs APD in HMI cardiomyocytes and that a decrease in outward potassium currents and an increase of inward Ca²⁺ current are likely the underlying mechanism of action.     

Effects of allocryptopine on outward potassium current and slow delayed rec-tifier potassium current in rabbit myocardium

Objective Allocryptopine (ALL) is an effective alkaloid of Corydalis decumbens (Thunb.) Pers. Papaveraceae and has proved to be anti-arrhythmic. The purpose of our study is to investigate the effects of ALL on transmural repolarizing ionic ingredients of outward potassium current (I_to) and slow delayed rectifier potassium current (I_Ks). Methods The monophasic action potential (MAP) technique was used to record the MAP duration of the epicardium (Epi), myocardium (M) and endocardium (Endo) of the rabbit heart and the whole cell patch clamp was used to record I_to and I_Ks in cardiomyocytes of Epi, M and Endo layers that were isolated from rabbit ventricles. Results The effects of ALL on MAP of Epi, M and Endo layers were disequilibrium. ALL could effectively reduce the transmural dispersion of repolarization (TDR) in rabbit transmural ventricular wall. ALL decreased the current densities of I_to and I_Ks in a voltage and concentration dependent way and narrowed the repolarizing differences among three layers. The analysis of gating kinetics showed ALL accelerated the channel activation of I_to in M layers and partly inhibit the channel openings of I_to in Epi, M and Endo cells. On the other hand, ALL mainly slowed channel deactivation of I_Ks channel in Epi and Endo layers without affecting its activation. Conclusions Our study gives partially explanation about the mechanisms of transmural inhibition of I_to and I_Ks channels by ALL in rabbit myocardium. These findings provide novel perspective regarding the anti-arrhythmogenesis application of ALL in clinical settings.     

Ionic basis and analytical solution of the wenckebach phenomenon in guinea pig ventricular myocytes

The ionic mechanisms of slow recovery of cardiac excitability and rate-dependent activation failure were studied in single, enzymatically dissociated guinea pig ventricular myocytes and in computer simulations using a modified version of the Beeler and Reuter model for the ventricular cell. On the basis of our results, we developed a simplified analytical model for recovery of cell excitability during diastole. This model was based on the equations for current distribution in a resistive-capacitive circuit. A critical assumption in the model is that, in the voltage domain of the subthreshold responses, the sodium and calcium inward currents do not play a significant role, and only the two potassium outward currents, the delayed rectifier (IK) and the inward rectifier, are operative. The appropriate parameters needed to numerically solve the analytical model were measured in the guinea pig ventricular myocyte, as well as in the Beeler and Reuter cell. The curves of recovery of excitability and the rate-dependent activation patterns generated by numerical iteration of the analytical model equations closely reproduced the experimental results. Our analysis demonstrates that slow deactivation of the delayed rectifier current determines the observed variations in excitability during diastole, whereas the inward rectifier current determines the amplitude and shape of the subthreshold response. Both currents combined are responsible for the development of Wenckebach periodicities in the ventricular cell. The overall study provides new insight into the ionic mechanisms of rate-dependent conduction block processes and may have important clinical implications as well.     

Characterization of a Chinese KCNQ1 mutation (R259H) that shortens repolarization and causes short QT syndrome 2

Objectives To evaluate the association between a KCNQ1 mutation, R259H, and short QT syndrome (SQTS) and to explore the electrophysiological mechanisms underlying their association. Methods We performed genetic screening of SQTS genes in 25 probands and their family members (63 patients). We used direct sequencing to screen the exons and intron-exon boundaries of candidate genes that encode ion channels that contribute to the repolarization of the ventricular action potential, including KCNQ1, KCNH2, KCNE1, KCNE2, KCNJ2, CACNA1c, CACNB2b and CACNA2D1. In one of the 25 SQTS probands screened, we discovered a KCNQ1 mutation, R259H. We cloned R259H and transiently expressed it in HEK-293 cells; then, currents were recorded using whole cell patch clamp techniques. Results R259H-KCNQ1 showed significantly increased current density, which was approximately 3-fold larger than that of wild type (WT) after a depolarizing pulse at 1 s. The steady state voltage dependence of the activation and inactivation did not show significant differences between the WT and R259H mutation (P > 0.05), whereas the time constant of deactivation was markedly prolonged in the mutant compared with the WT in terms of the test potentials, which indicated that the deactivation of R259H was markedly slower than that of the WT. These results suggested that the R259H mutation can effectively increase the slowly activated delayed rectifier potassium current (I_Ks) in phase 3 of the cardiac action potential, which may be an infrequent cause of QT interval shortening. Conclusions R259H is a gain-of-function mutation of the KCNQ1 channel that is responsible for SQTS2. This is the first time that the R259H mutation was detected in Chinese people for the first time.     

Heat‐Stress Responses Modulate Beta‐Adrenergic Agonist and Angiotensin II Effects on the Arrhythmogenesis of Pulmonary Vein Cardiomyocytes

Effect of Heat Stress on Pulmonary Vein Cardiomyocytes. Introduction: Heat stress‐induced responses reduce the occurrence of atrial fibrillation (AF). Pulmonary vein (PV) cardiomyocytes with pacemaker activity play a critical role in the pathophysiology of AF. In this study, we examined whether heat‐stress responses alter the electrophysiological characteristics of PV cardiomyocytes and protect the PV against angiotensin II‐ or isoproterenol‐induced arrhythmogenesis. Methods and Results: We used whole‐cell patch clamp techniques to investigate the spontaneous activity and ionic currents in single isolated rabbit PV pacemaker cardiomyocytes with or without (control) exposure to heat stress (43°C, 15 minutes) 5 ± 1 hours before the experiments. Compared to control cardiomyocytes, heat‐stressed PV cardiomyocytes had slower beating rates. Heat‐stressed PV cardiomyocytes had larger L‐type calcium currents, transient outward currents, smaller inward rectifier potassium currents, but similar sodium‐calcium exchanger currents_. Additionally, heat‐stressed PV cardiomyocytes had a lower incidence of pacemaker currents than control PV cardiomyocytes. Moreover, isoproterenol increased the beating rate of control cardiomyocytes but not heat‐stressed PV cardiomyocytes. Similarly, angiotensin II also increased the beating rate of control cardiomyocytes, but not heat‐stressed PV cardiomyocytes, in association with decreased expression of the angiotensin II type 1 receptor. Conclusion: Heat‐stress responses altered the electrophysiological characteristics of PV cardiomyocytes and attenuated the effects of isoproterenol and angiotensin II on PV arrhythmogenesis, which may play a role in the protective potential of heat‐stress responses. (J Cardiovasc Electrophysiol, Vol. 22, pp. 183‐190, February 2011)     

Vascular effects of class-III antiarrhythmic drugs: chromanol 293B, but not dofetilide blocks the smooth muscle delayed rectifier K+ channel

Chromanol 293B and dofetilide are inhibitors of IK_s and IK_r, i.e., of the slow and the rapid component of the delayed rectifier potassium current. The specificity of these drugs was tested by investigating their effects on the delayed rectifier potassium current in vascular smooth muscle, regulating the tone of blood vessels. Using depolarizing step protocols with asymmetrical potassium concentrations (135/4.5 mM K⁺ in pipette/bath), voltage-dependent K⁺ currents (IK_v) of enzymatically dispersed guinea pig portal vein cells were studied in the whole-cell patch-clamp technique. Peak currents were obtained within 20 ms (at +50 mV) after activation. During a 10 s test pulse to +60 mV, these currents exhibited a relatively fast inactivation with time constants of 384 ms (τ_fast) and 4505 ms (τ_slow). Dofetilide was totally ineffective in modulating currents; in contrast, after application of chromanol 293B, a steady-state block of IK_v developed within 135 s. The block was concentration-dependent with an IC₅₀ of 7.4 μM. Chromanol did not produce any shift in the normalized steady-state activation and inactivation curves and the recovery from inactivation was not significantly changed. Chromanol 293B similarly inhibited delayed rectifier K⁺ channels whether in their closed or open state, and produced an “apparent” acceleration of inactivation, i.e., the drug accelerated the faster time constant of inactivation during a 10 s test pulse from 384 ms (control) to 149 ms (100 μM chromanol). In recent studies, chromanol was described as a specific blocker of slowly activating delayed rectifier potassium channels (IK_s) in cardiomyocytes. The results of this study, however, extend the inhibitory spectrum of the drug and demonstrate block of closed and open state delayed rectifier K⁺ currents in portal vein vascular smooth muscle. Such a block could possibly contribute to the generation of portal hypertension. Received: 2 March 2001, Returned for 1. revision: 22 March 2001, 1. Revision received: 9 May 2001, Returned for 2. revision: 16 May 2001, 2. Revision received: 3 August 2001, Accepted: 20 August 2001     

The Novel Antiarrhythmic Drug Dronedarone: Comparison with Amiodarone

Dronedarone is a noniodinated benzofuran derivative that has been developed to overcome the limiting iodine‐associated adverse effects of the commonly used antiarrhythmic drug, amiodarone. It displays a wide cellular electrophysiological spectrum largely similar to amiodarone, inhibiting the potassium currents I_kr, I_Ks, I_KI, I_KACh, and I_sus, as well as sodium currents and L‐type calcium currents in isolated cardiomyocytes. In addition, dronedarone exhibits antiadrenergic properties. In vivo, dronedarone has been shown to be more effective than amiodarone in several arrhythmia models, particularly in preventing ischemia‐ and reperfusion‐induced ventricular fibrillation and in reducing mortality. However, an increased incidence of torsades de pointes with dronedarone in dogs shows that possible proarrhythmic effects of dronedarone require further evaluation. The clinical trails DAFNE, EURIDIS, and ADONIS indicated safety, antiarrhythmic efficacy and low proarrhythmic potential of the drug in low‐risk patients. In contrast, the increased incidence of death in the dronedarone group of the discontinued ANDROMEDA trial raises safety concerns for patients with congestive heart failure and moderate to severe left ventricular dysfunction. Dronedarone appears to be effective in preventing relapses of atrial fibrillation and atrial flutter. Torsades de pointes, the most severe adverse effect associated with amiodarone, has not yet been reported in humans with dronedarone. Unlike amiodarone, dronedarone had little effect on thyroid function and hormone levels in animal models and had no significant effects on human thyroid function in clinical trials. In conclusion, dronedarone could be a useful drug for prevention of atrial fibrillation and atrial flutter relapses in low‐risk patients. However, further experimental studies and long‐term clinical trials are required to provide additional evidence of efficacy and safety of dronedarone.     

Defective “pacemaker” current (Ih) in a zebrafish mutant with a slow heart rate

At a cellular level, cardiac pacemaking, which sets the rate and rhythm of the heartbeat, is produced by the slow membrane depolarization that occurs between action potentials. Several ionic currents could account for this pacemaker potential, but their relative prominence is controversial, and it is not known which ones actually play a pacemaking role in vivo. To correlate currents in individual heart cells with the rhythmic properties of the intact heart, we have examined slow mo (smo), a recessive mutation we discovered in the zebrafish Danio rerio. This mutation causes a reduced heart rate in the embryo, a property we can quantitate because the embryo is transparent. We developed methods for culture of cardiocytes from zebrafish embryos and found that, even in culture, cells from smo continue to beat relatively slowly. By patch-clamp analysis, we discovered that a large repertoire of cardiac currents noted in other species are present in these cultured cells, including sodium, T-type, and L-type calcium and several potassium currents, all of which appear normal in the mutant. The only abnormality appears to be in a hyperpolarization-activated inward current with the properties of I_h, a current described previously in the nervous system, pacemaker, and other cardiac tissue. smo cardiomyocytes have a reduction in I_h that appears to result from severe diminution of one kinetic component of the I_h current. This provides strong evidence that I_h is an important contributor to the pacemaking behavior of the intact heart.     

Biphasic Response of Action Potential Duration to Metabolic Inhibition in Rabbit and Human Ventricular Myocytes: Role of Transient Outward Current and ATP-regulated Potassium Current

Inhibition of cell metabolism is associated with significant changes in action potential duration. The aim of this study was to investigate the time course of the changes in action potential duration during metabolic inhibition and to determine what changes in membrane currents are responsible. The amphotericin perforated patch clamp technique was used to study membrane currents and voltage in single rabbit and human ventricular myocytes. In all myocytes inhibition of cell metabolism, induced by hypoxia (P ₂<5 mmHg) or by addition of 100μ 2,4-dinitrophenol (DNP), resulted in action potential shortening, which was accompanied by an increase in outward current, likely to be carried by ATP-regulated potassium channels. In about 65% of the rabbit and 50% of the human ventricular myocytes, however, action potential shortening was preceded by an initial prolongation. During this action potential prolongation, the -type calcium current and the steady-state outward current remained unchanged. The transient outward current (I_to), however, was almost completely inhibited, suggesting that the action potential prolongation is caused by a decreased I_to. This interpretation was further supported by the observations that: (1) Action potential prolongation was found in all subepicardial myocytes, as was I_to, but only in a minority of the subendocardial myocytes. (2) Addition of DNP failed to cause action potential prolongation in subepicardial myocytes in the presence of 4-aminopyridine, a blocker of I_to. In conclusion, these data suggest that the phenomenon of action potential prolongation preceding action potential shortening during metabolic inhibition is mainly restricted to myocytes from subepicardial origin, and is due to a decrease in I_to     

促甲状腺激素释放激素对结肠平滑肌钾电流的影响

目的探讨促甲状腺激素释放激素(TRH)对大鼠结肠平滑肌细胞膜瞬时外向钾电流(Ito)和延迟整流钾电流(Ik)的影响。方法酶解法急性分离单个大鼠结肠平滑肌细胞,运用膜片钳方法检测10、20、50、100μmol/L的TRH对结肠平滑肌细胞膜Ito和Ik的影响。结果 TRH浓度依赖性减少Ito(P<0.05),10、20、50、100μmol/L的TRH可使峰值Ito降低13.4%、28.0%、39.5%、53.4%,但Ik降低不明显(P>0.05)。结论 TRH阻滞大鼠结肠平滑肌细胞的瞬时外向钾电流,但不能降低延迟整流钾电流,TRH可能通过调节细胞的兴奋性对肠道动力产生影响。     

Cardiac strong inward rectifier potassium channels

Cardiac I_K1 and I_KACh are the major potassium currents displaying classical strong inward rectification, a unique property that is critical for their roles in cardiac excitability. In the last 15 years, research on I_K1 and I_KACh has been propelled by the cloning of the underlying inwardly rectifying potassium (Kir) channels, the discovery of the molecular mechanism of strong rectification and the linking of a number of disorders of cardiac excitability to defects in genes encoding Kir channels. Disease-causing mutations in Kir genes have been shown experimentally to affect one or more of the following channel properties: structure, assembly, trafficking, and regulation, with the ultimate effect of a gain- or a loss-of-function of the channel. It is now established that I_K1 and I_KACh channels are heterotetramers of Kir2 and Kir3 subunits, respectively. Each homomeric Kir channel has distinct biophysical and regulatory properties, and individual Kir subunits often display different patterns of regional, cellular, and membrane distribution. These differences are thought to underlie important variations in the physiological properties of I_K1 and I_KACh. It has become increasingly clear that the contribution of I_K1 and I_KACh channels to cardiac electrical activity goes beyond their long recognized role in the stabilization of resting membrane potential and shaping the late phase of action potential repolarization in individual myocytes but extends to being critical elements determining the overall electrical stability of the heart.     

Toward an Understanding of the Molecular Mechanisms of Ventricular Fibrillation

A major goal of basic research in cardiac electrophysiology is to understand the mechanisms responsible for ventricular fibrillation (VF). Here we review recent experimental and numerical results, from the ion channel to the organ level, which might lead to a better understanding of the cellular and molecular mechanisms of VF. The discussion centers on data derived from a model of stable VF in the Langendorff-perfused guinea pig heart that demonstrate distinct patterns of organization in the left (LV) and right (RV) ventricles. Analysis of optical mapping data reveals that VF excitation frequencies are distributed throughout the ventricles in clearly demarcated domains. The highest frequency domains are usually found on the anterior wall of the LV, demonstrating that a high frequency reentrant source (a rotor) that remains stationary in the LV is the mechanism that sustains VF in this model. Computer simulations predict that the inward rectifying potassium current (I _K1) is an essential determinant of rotor stability and rotation frequency, and patch-clamp results strongly suggest that the outward component of the background current (presumably I _K1) of cells in the LV is significantly larger in the LV than in the RV. These data have opened a new and potentially exciting avenue of research on the possible role played by inward rectifier channels in the mechanism of VF and may lead us toward an understanding of its molecular basis and hopefully lead to new preventative approaches.