Electrotonic Inhibition and Active Facilitation of Excitability in Ventricular Muscle

Inhibition and Facilitation in Cardiac Muscle. Introduction: The effects of subthreshold electrical pulses on the response to subsequent stimulation have been described previously in experimental animal studies as well as in the human heart. In addition, previous studies in cardiac Purkinje fibers have shown that diastolic excitability may decrease after activity (active inhibition) and, to a lesser extent, following subthreshold responses (electrotonic inhibition). However, such dynamic changes in excitability have not been explored in isolated ventricular muscle, and it is uncertain whether similar phenomena may play any role in the activation pal-terns associated with propagation abnormalities in the myocardium. Methods and Results: Experiments were performed in isolated sheep Purkinje fibers and papillary muscles, and in enzymatically dissociated guinea pig ventricular myocytes. In all types of preparations introduction of a conditioning subthreshold pulse between two subthreshold pulses was followed by a transient decay in excitability (electrotonic inhibition). The degree of inhibition was directly related to the amplitude and duration of the conditioning pulse and inversely related to the postconditioning interval. Yet, inhibition could be demonstrated long after (> 1 sec) the end of the conditioning pulse. Electrotonic inhibition was found at all diastolic intervals and did not depend on the presence of a previous action potential. In Purkinje fibers, conditioning action potentials led to active inhibition of subsequent responses. In contrast, in muscle cells, such action potentials had a facilitating effect (active facilitation). Electrotonic inhibition and active facilitation were observed in both sheep ventricular muscle and guinea pig ventricular myocytes. Accordingly, during repetitive stimulation with pulses of barely threshold intensity, we observed: (1) bistability (i.e., with the same stimulating parameters, stimulus: response patterns were either 1:1 or 1:0, depending on previous history), and (2) abrupt transitions between 1:1 and 1:0 (absence of intermediate wenckebach-like patterns). Simulations utilizing an ionic model of cardiac myocytes support the hypothesis that electrotonic inhibition in well-polarized ventricular muscle is the result of partial activation of I_k following subthreshold pulses. On the other hand, active facilitation may be the result of an activity-induced decrease in the conductance of I_K1. Conclusion: Diastolic excitability of well-polarized ventricular myocardium may be transiently depressed following local responses and transiently enhanced following action potentials. On the other hand, diastolic excitability decreases during quiescence. Active facilitation and electrotonic inhibition may have an important role in determining the dynamics of excitation of the myocardium in the presence of propagation abnormalities.     

Ionic currents and action potentials in rabbit,rat, and guinea pig ventricular myocytes

Summary Distinct differences exist in action potentials and ionic currents between rabbit, rat, and guinea pig ventricular myocytes. Data obtained at room temperature indicate that about half of the rabbit myocytes show prominent phase 1 repolarization and transient outward current. Action potentials in guinea pig ventricular myocytes resemble those from rabbit myocytes not exhibiting phase 1 repolarization; and guinea pig myocytes do not develop transient outward current. Rat ventricular action potentials are significantly shorter than those from rabbit and guinea pig ventricular myocytes. Unlike rabbit and guinea pig myocytes, rat ventricular myocytes also exhibit a prominent phase 1 and lack a well defined plateau phase during repolarization. All rat ventricular myocytes exhibit a transient outward current which can be best fitted by a double exponential relation. There are no significant differences between the amplitude, voltage dependence and inactivation kinetics of the inward calcium currents observed in rabbit, rat and guinea pig. The steady-state current-voltage relations between –120 mV and –20 mV, which mostly represent the inward rectifier potassium current are similar in rabbit and guinea pig. The amplitude of this current is significantly less in rat ventricular myocytes. The outward currents activated upon depolarization to between –10 and +50 mV are different in the three species. Only a negligible, or absent, delayed rectifier outward current has been observed in rabbit and rat; however, a relatively large delayed rectifier current has been found in guinea pig. These large interspecies variations in outward membrane currents help explain the differences in action potential configurations observed in rabbit, rat, and guinea pig.     

绝对不应期电刺激对正常豚鼠心室肌细胞舒缩的影响

为探讨绝对不应期电刺激对正常豚鼠心室肌细胞舒缩的影响 ,酶解分离心室肌细胞 ,在 0 .5Hz的基础刺激S1 (脉宽 4ms,电压 60V)的绝对不应期内给予刺激S2 (与S1 延迟 1 0ms,脉宽 1 0ms ,电压 60V) ,应用单细胞收缩动缘检测仪 ,同步观察细胞收缩幅度和钙瞬变的变化。结果 :①心室肌细胞收缩幅值增高 1 5 .45± 6.48% ,收缩速度峰值增加 1 5 .97± 8.37% ,舒张速度峰值增加 2 1 .63± 8.0 6% (n =1 0 ) ;②以荧光强度的比值 (F360 /F380 )反映细胞内Ca2 +浓度变化 ,即钙瞬变。心室肌细胞收缩过程其幅值增加 2 2 .5 5± 9.0 8% ,增高速度与减少速度分别增加36.75± 9.77%和 2 3.62± 4.47% (n =6)。结论 :适宜的绝对不应期电刺激可增强正常豚鼠心室肌细胞舒缩能力。     

Dynamics of the background outward current of single guinea pig ventricular myocytes. Ionic mechanisms of hysteresis in cardiac cells

Subthreshold potentials are thought to be mediated by time-independent, "passive" background currents. In this study, we show that the background current-voltage (I-V) relation of guinea pig ventricular myocytes is changed significantly by repetitive stimulation, in such a way that cell excitability becomes enhanced. Myocytes were used for whole-cell voltage-clamp experiments. A voltage-clamp ramp (100 mV/sec) to -50 mV was applied from a holding potential of -100 mV. Subsequently, a train of square voltage-clamp pulses to +10 mV (duration, 300 msec; interpulse interval, 300 msec) was delivered from a holding potential of -85 mV. A new ramp was applied again immediately after the train, and the resulting I-V curve was compared with that obtained before the train. Pulsing displaced the I-V relation to the right, the zero-current point becoming 1-2 mV less negative, and increased the degree of inward-going rectification. These changes were insensitive to tetrodotoxin (30 microM); disappeared during superfusion with cobalt (2 mM), verapamil (22 microM), or ryanodine (5 microM); and could not be mimicked by agonists of the protein kinase C system. In the presence of cesium (8 mM), pulsing still displaced the I-V curve to the right. However, the linear portion of the curve became steeper after the train. Subtraction of the cesium-sensitive current from control revealed that, although the zero-current point remained constant, the I-V relation showed a stronger inward-going rectification after pulsing. In accordance with these results, we have demonstrated hysteresis of excitability in ventricular myocytes. We conclude that the observed changes are mediated by an increase in intracellular calcium, which leads to an increase in rectification of IK1, as well as to activation of another membrane-conductance system, perhaps the Na-Ca exchange or the Ca(2+)-activated, nonselective current.     

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.     

Factors determining the susceptibility of the isolated guinea pig heart to ventricular fibrillation induced by sinusoidal alternating current at frequencies from 1 to 1000 Hz

We determined the threshold for ventricular fibrillation by means of sinusoidal alternating current (AC) from 1 to 1000 Hz on isolated perfused guinea pig hearts. Current was applied via electrodes located in the aorta and at the apex of the heart. The duration of current flow was kept constant at 1 s. Electrical activity was recorded with epicardial electrodes attached to the ventricles. Additional experiments were performed in isolated papillary muscles with intracellular microelectrodes. The fibrillation threshold, expressed as peak-to-peak current strength, attains a minimum at about 30 Hz and rises by a factor of 5 at 1 Hz, and by a factor of 14 at 1000 Hz. In the frequency range from 30 to 1000 Hz the rise of the fibrillation threshold can be attributed to the increase of the threshold for stimulation due to the progressive shortening of the AC periods. Thus no change of the fibrillation threshold occurs if DC pulses of constant duration are used in the same range of frequencies. Below 30 Hz there is only a slight increase of the threshold for stimulation, which cannot entirely account for the rise of the threshold for fibrillation. A likely cause of the reduced susceptibility of the heart to fibrillation at the lower frequencies is the reduced number of extrasystoles preceding the onset of fibrillation, which results in a less pronounced state of inhomogeneous excitability.     

Slow recovery of excitability and the Wenckebach phenomenon in the single guinea pig ventricular myocyte

The cellular mechanisms of Wenckebach periodicity were investigated in single, enzymatically dissociated guinea pig ventricular myocytes, as well as in computer reconstructions of transmembrane potential of the ventricular cell. When depolarizing current pulses of the appropriate magnitude were delivered repetitively to a well-polarized myocyte, rate-dependent activation failure was observed. Such behavior accurately mimicked the Wenckebach phenomenon in cardiac activation and was the consequence of variations in cell excitability during the diastolic phase of the cardiac cycle. The recovery of cell excitability during diastole was studied through the application of single test pulses of fixed amplitude and duration at variable delays with respect to a basic train of normal action potentials. The results show that recovery of excitability is a slow process that can greatly outlast action potential duration (i.e., postrepolarization refractoriness). Two distinct types of subthreshold responses were recorded when activation failure occurred: one was tetrodotoxin- and cobalt-insensitive (type 1) and the other was sensitive to sodium-channel blockade (type 2). Type 1 responses, which were commonly associated with the typical structure of the Wenckebach phenomenon (Mobitz type 1 block), were found to be the result of the nonlinear conductance properties of the inward rectifier current, IK1. Type 2 sodium-channel-mediated responses were associated with the so-called "millisecond Wenckebach." These responses may be implicated in the mechanism of Mobitz type 2 rate-dependent block. Single-cell voltage-clamp experiments suggest that variations in excitability during diastole are a consequence of the slow deactivation kinetics of the delayed rectifier, IK. Computer simulations of the ventricular cell response to depolarizing current pulses reproduced very closely all the response patterns obtained in the experimental preparation. It is concluded that postrepolarization refractoriness and Wenckebach periodicity are properties of normal cardiac excitable cells and can be explained in terms of the voltage dependence and slow kinetics of potassium outward currents. The conditions for the occurrence of intermittent activation failure during diastole will depend on the frequency and magnitude of the driving stimulus.     

Influence of electrical axis of stimulation on excitation of cardiac muscle cells

Orthogonal sequential shock can defibrillate the heart with greater efficacy compared with single shock defibrillation. In this study we tested the hypothesis that cardiac cells have a preferred orientation in their response to excitatory extracellular electric fields, so that orthogonal shocks may stimulate distinct populations of cells. A micropaddle electrode system was used to deliver rectangular pulses for extracellular field stimulation of individual heart cells. We found that single frog and guinea pig ventricular myocytes are excitable with rectangular pulse field stimulation over a wide range of pulse durations, ranging from 10 msec to as little as 20 microseconds. The excitation field strength varies inversely with pulse duration as described by the Weiss-Lapicque equation, although the frog myocytes show a significant "notch" at pulse durations of approximately 1-2 msec, and the guinea pig myocytes are more excitable than predicted for pulse durations of less than 0.2 msec. Every myocyte tested was more excitable when the long axis of the cell was oriented parallel to the stimulating field than when perpendicular to the field. For 2-msec pulses, the difference in field strength was a factor of 5.8 +/- 2.0 (n = 30) for frog and 2.6 +/- 0.5 (n = 23) for guinea pig myocytes. Complete excitation strength-duration curves were obtained in seven frog and 14 guinea pig cells for both parallel and perpendicular cell orientations.(ABSTRACT TRUNCATED AT 250 WORDS)     

丹参素对豚鼠心室肌细胞L-型钙通道的影响

目的　研究丹参素对豚鼠心室肌细胞膜L 型钙通道的影响 ,探讨丹参素在离子通道水平的药理机制。方法　急性酶解法获得豚鼠的单个心肌细胞 ,用标准的全细胞膜片钳技术记录钙电流。结果　在保持电位 (Holdingpoten tial,HP)为 - 80mV ,预先去极化至 - 4 0mV ,再去极化至0mV ,波宽为 30 0mS的刺激参数下 ,可记录到一具有电压和时间依赖性内向的电流 ,当用含 1μmol/L硝苯地平的台氏液灌流时该电流被迅速消除 ,在灌流液中加入丹参素 1mg/mL和 10mg/mL ,1mg/mL组对钙电流无明显改变 (P >0 0 5 ,n =5 ) ,10mg/mL组使钙电流减少 [- (7 76± 0 85 )pA/pFvs - (2 6 2 6± 0 5 9)pA/pFP <0 0 5 ,n =8],并使心肌细胞钙电流 -电压曲线上移 ,原有的电流 -电压依赖特征不变。结论　丹参素浓度依赖性地抑制心肌L 型钙通道     

Effects of sildenafil on cardiac repolarization

OBJECTIVES: Sudden death has occasionally been reported in patients taking sildenafil. The objective of this study was to investigate the effect of sildenafil on cardiac repolarization. METHODS: We used conventional microelectrode recording technique in isolated guinea pig papillary muscles and canine Purkinje fibers, whole-cell patch clamp techniques in guinea pig ventricular myocytes, and in vivo ECG measurements in guinea pigs. RESULTS: Action potential duration at 90% repolarization (APD(90)) was not affected by sildenafil in the therapeutic ranges (< or =1 microM), but shortened by higher concentration (> or =10 microM) in both guinea pig papillary muscles and canine Purkinje fibers. D-Sotalol prolonged APD(90) in the same preparations with concentrations > or =1 microM in a reverse frequency-dependent manner. Co-administration of sildenafil (10 and 30 microM) abolished the APD-prolonging effects of D-sotalol (30 microM) and amiodarone (100 microM). Sildenafil, with concentrations up to 30 microM, had no significant effect on both the rapid (I(Kr)) and the slow (I(Ks)) components of the delayed rectifier potassium currents in guinea pig ventricular myocytes. Sildenafil dose-dependently blocked L-type Ca(2+) current (I(Ca,L)), but had no effect on persistent Na(+) current in guinea pig ventricular myocytes. ECG recordings in intact guinea pigs revealed significant shortening of QTc interval by sildenafil (10 and 30 mg/kg orally). The QT-prolonging effects by D,L-sotalol (50 mg/kg) and amiodarone (100 mg/kg) were abolished by sildenafil (30 mg/kg). CONCLUSIONS: Sildenafil does not prolong cardiac repolarization. Instead, in supra-therapeutic concentrations, it accelerates cardiac repolarization, presumably through its blocking effect on I(Ca,L).     

Active components of excitability in K-depolarized ventricular muscle

The steady-state and dynamic characteristics of excitability were assessed in isolated guinea pig papillary muscles depolarized with elevated [K+]o to resting potentials near -60 mV. Transmembrane potentials were recorded from fibers during application of low-amplitude current pulses used to analyze net changes in active membrane components of excitability in terms of elicited local responses and measure threshold current (Ith). Generated local responses were blocked entirely by tetrodotoxin and lidocaine, which increased steady-state Ith by more than 200%. In the absence of Na+ channel-blocking agents, local responses showed marked but characteristic attenuation in a time- and voltage-dependent manner by preceding subthreshold depolarizations, which concomitantly reduced excitability. However, local responses and excitability were also modulated by small changes in [Ca2+]o (+/- 0.7 mmol) and reduced by exposure to slow channel blockers and to Cs+. Thus these data suggest that while the Na+ channel is the primary active component of excitability in partially depolarized ventricular muscle, Ca2+ -mediated and Cs+ -sensitive conductances may also participate, although to a lesser extent. These findings may help explain the frequency-dependence of excitability and conduction under conditions of ischemia in the intact heart.     

The min K channel underlies the cardiac potassium current IKs and mediates species-specific responses to protein kinase C.

A clone encoding the guinea pig (gp) min K potassium channel was isolated and expressed in Xenopus oocytes. The currents, gpIsK, exhibit many of the electrophysiological and pharmacological properties characteristic of gpIKs, the slow component of the delayed rectifier potassium conductance in guinea pig cardiac myocytes. Depolarizing commands evoke outward potassium currents that activate slowly, with time constants on the order of seconds. The currents are blocked by the class III antiarrhythmic compound clofilium but not by the sotalol derivative E4031 or low concentrations of lanthanum. Like IKs in guinea pig myocytes, gpIsK is modulated by stimulation of protein kinase A and protein kinase C (PKC). In contrast to rat and mouse IsK, which are decreased upon stimulation of PKC, myocyte IK and gpIsK in oocytes are increased after PKC stimulation. Substitution of an asparagine residue at position 102 by serine (N102S), the residue found in the analogous position of the mouse and rat min K proteins, results in decreased gpIsK in response to PKC stimulation. These results support the hypothesis that the min K protein underlies the slow component of the delayed rectifier potassium current in ventricular myocytes and account for the species-specific responses to stimulation of PKC.     

乙酰胆碱诱发的豚鼠心室肌反跳作用研究

观察乙酰胆碱 (ACh)诱发的豚鼠心室肌肌力及钙电流的反跳作用 ,并探讨其作用机制。采用豚鼠离体左室乳头肌观察 1μmol/LACh单独对心室肌收缩力 (Fc)的影响及在 10nmol/L异丙肾上腺素 (Iso)存在时对Fc的影响 ,并应用膜片钳全细胞记录方法分别观察 1μmol/LACh和 1μmol/LACh +10nmol/LIso及迅速洗脱ACh对豚鼠心室肌细胞L型钙通道电流 (ICa L)的影响。结果 :1μmol/LACh对心室肌Fc有直接抑制作用 ,抑制率为 30 .8%± 10 .7%(P <0 .0 5 ) ,而快速洗脱ACh后 ,Fc反跳性增强了 2 5 .5 %± 10 .3% (n =7,P <0 .0 1)。而在应用 10nmol/LIso后 1μmol/LACh对Fc有间接的抑制作用 ,快速冲洗ACh后亦引起Fc的反跳性增强 ,与Iso组相比增强了 2 7.1%±13.2 % (n =7,P <0 .0 1)。 1μmol/LACh对心室肌细胞基础峰电流无明显影响 (n =8,P >0 .0 5 ) ;而以基础ICa L峰值(931± 16 1pA)作对照 ,在加入 10nmol/LIso后 ,ICa L峰电流增强到 1889± 331pA(n =7,P <0 .0 1) ;再给予 10nmol/LIso +1μmol/LACh快速灌流 2min ,峰电流降低为 15 12± 2 0 2pA(P <0 .0 1) ,用含 10nmol/LIso的细胞外液快速洗脱ACh ,峰电流增强到 2 10 7± 2 0 5pA ,较Iso组反跳性增强了 15 .8%± 4 .0 % (n =7,P <0 .0 1)。结论 :ACh对豚鼠心室肌收缩     

L-type calcium current reactivation contributes to arrhythmogenesis associated with action potential triangulation

Introduction: The morphology of the mammalian cardiac action potential (AP) is an important factor in the susceptibility to drug-induced early afterdepolarizations (EADs) that may initiate Torsade de Pointes (TdP). AP triangulation has been shown to be an important predictor of drug-induced TdP.
Methods and Results: APs from guinea pig and rabbit left ventricular single myocytes were recorded using a microelectrode-recording technique. I_Ca-L currents were recorded in ventricular myocytes of guinea pig and rabbit using patch-clamping technique. At a stimulus frequency of 0.5 Hz, guinea pig ventricular myocytes displayed a square-like AP, whereas rabbit ventricular myocytes exhibited a triangle-like AP. Dofetilide-induced EADs were observed only in rabbit ventricular myocytes. Under the guinea pig AP clamping condition, the normalized I_Ca-L instant reactivation currents in guinea pig and rabbit myocytes at voltages of –40 mV were 0.13 ± 0.01 and 0.14 ± 0.01, respectively. However, when rabbit AP served as the first clamping voltage, the normalized I_Ca-L reactivation currents at –40 mV in guinea pig and rabbit myocytes were 0.20 ± 0.01, 0.21 ± 0.01, respectively, indicating that the I_Ca-L recovery from inactivation in the rabbit triangular AP condition was significantly faster than in the guinea pig square AP condition. Comparison of the voltage clamp using the triangular waveform with the square waveform further confirmed that triangulation accelerates I_Ca-L recovery from inactivation.
Conclusions: In rabbit ventricular myocardium, AP triangulation accelerates I_Ca-L channel recovery from inactivation, leading to instability of the cell membrane potential during repolarization, which is capable of initiating TdP.     

L-type calcium current recovery versus ventricular repolarization: preserved membrane-stabilizing mechanism for different QT intervals across species

BACKGROUND: Long QT syndrome is associated with early after-depolarization (EAD) that may result in torsade de pointes (TdP). Interestingly, the corrected QT interval seems to be proportional to body mass across species under physiologic conditions. OBJECTIVE: The purpose of this study was to test whether recovery of L-type calcium current (I(Ca,L)), the primary charge carrier for EADs, from its inactivated state matches ventricular repolarization time and whether impairment of this relationship leads to development of EAD and TdP. METHODS: Transmembrane action potentials from the epicardium, endocardium, or subendocardium were recorded simultaneously with a transmural ECG in arterially perfused left ventricular wedges isolated from cow, dog, rabbit, and guinea pig hearts. I(Ca,L) recovery was examined using action potential stimulation in isolated left ventricular myocytes. RESULTS: The ventricular repolarization time (action potential duration at 90% repolarization [APD(90)]), ranging from 194.7 +/- 1.8 ms in guinea pig to 370.2 +/- 9.9 ms in cows, was linearly related to the thickness of the left ventricular wall among the species studied. The time constants (tau) of I(Ca,L) recovery were proportional to APD(90), making the ratios of tau to APD(90) fall into a relatively narrow range among these species despite markedly different ventricular repolarization time. Drugs with risk for TdP in humans were shown to impair this intrinsic balance by either prolongation of the repolarization time and/or acceleration of I(Ca,L) recovery, leading to the appearance of EADs capable of initiating TdP. CONCLUSION: An adequate balance between I(Ca,L) recovery and ventricular repolarization serves as a "physiologic stabilizer" of ventricular action potentials in repolarization phases.     

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.     

Swelling-activated chloride current is activated in guinea pig cardiomyocytes from endotoxic shock

OBJECTIVE: Myocardial swelling occurs during endotoxic shock. The hypothesis that swelling-activated Cl- current (ICl,swell) activates during endotoxic shock was tested. METHODS: Endotoxic shock was induced by intravenous lipopolysaccharides (10 mg/kg) in guinea pigs. The effects of ICl,swell blockers on the cardiac action potentials in papillary muscles and on the ICl,swell in single ventricular myocytes were tested. RESULTS: Action potential duration (APD) at 90% of repolarization (APD90) was significantly shortened after 5-h endotoxic shock in guinea pig papillary muscles. I(Cl,swell) blockers, 9-anthracene carboxylic acid (9-AC) and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), dose-dependently prolonged the shortened APD90. Inducible nitric oxide synthase (iNOS) inhibitors, L-N6-(1-iminoethyl) lysine (L-NIL) and N-[[3-(aminomethyl)phenyl]methyl]-ethanimidamide (1400 W), also prolonged the APD90. Protein kinase C (PKC) activators, 4beta-phorbol 12-myristate 13-acetate (PMA) and phorbol 12,13-didecanoate (PDD), also prolonged the APD. The addition of glibenclamide (an ATP-sensitive K+ channel blocker) on top of these ICl,swell blockers hastened the recovery of APD90 compared to the use of ICl,swell blockers alone. Whole-cell voltage-clamp study in single ventricular myocytes from endotoxic shock heart disclosed activation of a DIDS- and 9-AC-sensitive current. These currents displayed outward rectification with reversal potentials similar to the calculated Nernst potential for Cl-. The reversal potentials tracked the ECl closely when the Cl- gradient was changed, suggesting that Cl- was the major charged carrier. CONCLUSIONS: We have shown for the first time that ICl,swell activates in guinea pig heart in endotoxic shock. The change in this membrane current, together with the activation of ATP-sensitive K+ current, contributes to the electrophysiological derangement in endotoxic shock.     

乙酰胆碱对离体豚鼠心室肌的直接作用机制探讨

目的 :研究乙酰胆碱 （ACh）对离体豚鼠心室肌的直接负性作用及机制。方法 :采用标准玻璃微电极细胞内记录技术记录动作电位 （AP）及肌力换能器记录心肌收缩力 （FC）的方法观察 ACh对离体豚鼠心室肌的作用 ,并观察几种受体或通道水平的阻断剂阿托品、氯化铯 （Cs Cl）、氯化镉 （Cd Cl2 ）对 ACh直接作用的影响。结果 :10 - 5 mol/LACh对心室肌动作电位持续时间 （APD）及 FC的抑制率分别为 7.31%和 37.5 7% （P<0 .0 5 ） ,10 - 5 mol/L阿托品和 2 0 m mol/L Cs Cl可阻断该作用 ,0 .1m mol/L Cd Cl2 对该作用无影响。结论 :10 - 5 m ol/L ACh对离体豚鼠心室肌有直接负性作用 ,ACh的作用与毒蕈碱型胆碱受体及 K+电流有关 ,而与 Ca2 +电流的关系可能不大。     

Role of membrane potential in Ba2+ induced automaticity in guinea pig cardiac myocytes.

STUDY OBJECTIVE--The aim was to study in isolated myocardial cells the role of membrane potential in barium induced spontaneous activity and the ionic mechanism of the underlying pacemaker current. DESIGN--The membrane potential and resistance of single myocytes were studied at different voltage levels by means of current and voltage clamp steps in the absence and presence of barium (Ba). EXPERIMENTAL MATERIAL--The membrane potentials and currents of single guinea pig ventricular myocytes were recorded by means of an intracellular microelectrode through which current could also be passed. MEASUREMENTS AND MAIN RESULTS--In the presence of Ba (0.1-0.2 mM), stepwise depolarisations induced a transient overshoot and initiated action potentials followed by an undershoot, diastolic depolarisation and spontaneous discharge. During progressive depolarisations, membrane resistance (Rm) increased, decreased transiently at the end of the action potential, and reincreased during diastole. Stepwise repolarisations had opposite effects. Hyperpolarisations reversed diastolic depolarisation and could unmask oscillatory potentials (Vos). Voltage clamp steps to +20 mV were followed by outward tail currents during which Rm increased. Larger or longer depolarisations were followed by larger outward tail currents at resting potential level. The outward tail current reversed at potentials negative to EK. CONCLUSIONS--In the presence of Ba, applied depolarisation facilitates the induction of spontaneous activity through an interplay between voltage dependent and time dependent Ba block and unblock of gK1, voltage dependent increase in Rm, increased potassium driving force, and negative shift in the slow inward current threshold and sometimes Vos. The pacemaker potential underlying spontaneous activity is due to the slow re-establishment of Ba block of IK1 during diastole.     

The slow component of the delayed rectifier potassium current in undiseased human ventricular myocytes

OBJECTIVE: The purpose of this study was to investigate the properties of the slow component of the delayed rectifier potassium current (I(Ks)) in myocytes isolated from undiseased human left ventricles. METHODS: The whole-cell configuration of the patch-clamp technique was applied in 58 left ventricular myocytes from 15 hearts at 37 degrees C. Nisoldipine (1 microM) was used to block inward calcium current (I(Ca)) and E-4031 (1-5 microM) was applied to inhibit the rapid component of the delayed rectifier potassium current (I(Kr)). RESULTS: In 31 myocytes, an E-4031 insensitive, but L-735,821 and chromanol 293B sensitive, tail current was identified which was attributed to the slow component of I(K) (I(Ks)). Activation of I(Ks) was slow (tau=903+/-101 ms at 50 mV, n=14), but deactivation of the current was relatively rapid (tau=122.4+/-11.7 ms at -40 mV, n=19). The activation of I(Ks) was voltage independent but its deactivation showed clear voltage dependence. The deactivation was faster at negative voltages (about 100 ms at -50 mV) and slower at depolarized potentials (about 300 ms at 0 mV). In six cells, the reversal potential was -81.6+/-2.8 mV on an average which is close to the K(+) equilibrium potential suggesting K(+) as the main charge carrier. CONCLUSION: In undiseased human ventricular myocytes, I(Ks) exhibits slow activation and fast deactivation kinetics. Therefore, in humans I(Ks) differs from that reported in guinea pig, and it best resembles I(Ks) described in dog and rabbit ventricular myocytes.