黄芪总黄酮对豚鼠心室肌细胞动作电位和延迟整流钾电流的作用

目的:研究黄芪总黄酮(TFA)对豚鼠心室肌细胞动作电位(AP)和延迟整流钾电流(IK)的作用。方法:实验采用标准玻璃微电极细胞内记录心室肌细胞AP和全细胞膜片钳技术记录IK。结果:应用TFA 20 mg/kg后心室肌AP的幅度从给药前的(70.25±5.97)mV降低到(58.73±4.86)mV(n=6,P<0.05);AP时程(APD90)从给药前的(236.08±17.63)ms延长到(282.13±22.01)ms(n=6,P<0.05)。应用TFA 40 mg/kg后心室肌AP的幅度从给药前的(72.65±5.12)mV降低到(48.32±8.76)mV(n=6,P<0.05);AP时程(APD90)的从给药前的(265.34±14.82)ms延长到(315.46±24.33)ms(n=6,P<0.05)。而应用TFA 20 mg/kg后,在+50 mV指令电压下,心室肌细胞的IK从(1.21±0.24)nA降至(0.96±0.27)nA(n=6P<0.01),应用TFA 40 mg/kg后心室肌细胞的IK从(1.45±0.36)nA下降到(1.01±0.27)nA,差异有显著性(n=6P<0.01)。结论:TFA可以降低心室肌AP幅值,延长AP时程,抑制心室肌细胞的IK幅值,并且有一定的剂量依赖性。     

黄芪总黄酮对豚鼠心室肌细胞钾离子通道的作用

目的研究黄芪总黄酮(totalflavonoids of ast ragalus,TFA)对豚鼠心室肌细胞延迟整流钾电流(IK)、内向整流钾电流(IK1)的作用。方法胶原酶酶解法急性分离豚鼠心室肌细胞,利用全细胞膜片钳的方法记录心室肌细胞的延迟整流钾电流、内向整流钾电流。结果(1)应用TFA(20mg.kg-1)后IK从(1.21±0.24)nA降至(0.96±0.27)nA(P<0.01,n=6),而TFA(40mg.kg-1)组IK从(1.45±0.36)nA下降到(1.01±0.27)nA,差异有统计学意义(P<0.01,n=6)。(2)应用TFA(20mg.kg-1,40mg.kg-1)后与对照组比较,TFA(20mg.kg-1,40mg.kg-1)分别使内向整流钾电流(IK1)峰值从(-8.93±2.03)nA和(-8.83±1.56)nA下降到(-8.87±1.58)nA和(-8.54±1.37)nA,差异无统计学意义(P>0.05,n=6)。结论TFA可以降低豚鼠心室肌细胞延迟整流钾电流(IK)的幅度,呈剂量依赖性。TFA对豚鼠心室肌细胞内向整流钾通道(IK1)作用不明显。     

卡托普利对豚鼠心室肌细胞动作电位及外向延迟整流钾电流的作用

目的 探讨卡托普利对豚鼠心室肌细胞动作电位及外向延迟整流钾电流的作用。方法 采用内充3M KCL的标准微电极记录心肌动作电位。采用膜片钳全细胞技术，钳制电位-50mV，持续时间100ms，指令电位 40mV，记录外向延迟整流钾电流(Ik)最大峰电流。结果 与缺血组比较，卡托普利组APD30、APD50及ERP显著延长，APD50无显著变化。缺血组Ik幅度显著增高，而卡托普利组及卡托普利 缺血组显著降低。各组电流—电压关系曲线形态虽无显著变化，但缺血组显著上移，而卡托普利组、卡托普利 缺血组比缺血组下移。结论卡托普利降低外向钾电流及延长APD30、APD50和ERP。     

Independent modulation of L-type Ca2+ channel in guinea pig ventricular cells by nitrendipine and isoproterenol

Dihydropyridine (DHP) Ca2+ channel blockers decrease L-type Ca2+ channel current (I(CaL)) by enhancing steady-state inactivation, whereas beta-adrenergic stimulation increases I(CaL) with small changes in the kinetics. We studied the effects of DHP Ca2+ channel blockers on cardiac I(CaL) augmented by beta-adrenergic stimulation. We recorded I(CaL) as Ba2+ currents (I(Ba)) from guinea pig ventricular myocytes using the whole-cell patch clamp technique. and compared the effects of nitrendipine (NIT) in the absence and presence of isoproterenol (1 microM, ISO) or forskolin (10 microM, FSK). Maximal I(Ba) elicited from a holding potential of -80 mV were diminished to 69.4+/-13.5% (mean and SE, n=5) of control by NIT (100 nM) and the diminished I(Ba) were increased to 180.3+/-23.2% of control by ISO in the presence of NIT, which was similar to the enhancement seen in the absence of NIT. NIT shifted the V(1/2) of the I(Ba) inactivation curve from -34.6+/-1.9 mV (n=5) to -48.7+/-1.2 mV, enhancing I(Ba) decay with shortening T(1/2) at -10 mV from 164.6+/-24.2 ms (n=7) to 105.4+/-15.2 ms. ISO elicited a small additional shift in the V(1/2) of I(Ba) inactivation in the same direction. ISO and FSK each slowed I(Ba) decay in the absence of NIT, but not in its presence. Thus, beta-adrenergic agonists increase and DHP Ca2+ channel blockers decrease the amplitude of cardiac I(CaL) independently and the kinetics of I(CaL) is determined mainly by the latter when these drugs coexist.     

Late sodium current and its contribution to action potential configuration in guinea pig ventricular myocytes

We used the patch clamp technique to study the nature of the late sodium current in guinea pig ventricular myocytes. In a cell attached mode of single channel recording at room temperature (22-24 degrees C) two kinds of late (100 msec or more after beginning of the depolarizing pulse) sodium channel activities were recognized. One is isolated brief openings appearing once for about 120 depolarizations per channel (background type), while the other type is sustained openings with rapid interruptions (burst type) that occurred only once for 2,700 depolarizations per channel. The time constant obtained from the open time histogram of the burst type (1.05 msec) was about five times longer than that of background type (0.18 msec, measured at the potential 10 mV above the threshold). Magnitude of the late sodium current flowing through the entire surface of a myocyte was estimated with tetrodotoxin (60 microM), a specific inhibitor of sodium channels, in whole-cell clamp experiments. The steady tetrodotoxin-sensitive current of 12 to 50 pA was registered at -40 mV (26 +/- 14 pA, mean +/- SD, n = 5), in good agreement with the late sodium current calculated from the single channel recording. Tetrodotoxin produced small (congruent to 10%) but significant decreases in the action potential duration. These results suggest the presence of a small but significant late sodium current with slow inactivation kinetics and that this current probably plays a significant role in maintaining the action potential plateau and the duration in guinea pig ventricular myocytes.     

氟硒对豚鼠心室肌细胞动作电位的影响

应用灌流方法和细胞内微电极技术观察Ｆ－、Ｓｅ及Ｆ－加Ｓｅ对豚鼠心肌细胞电生理影响。结果用５．２６×１０－６ｍｏｌ／ＬＦ－灌流，随时间延长ＲＰ降低，ＡＰＤ５０、ＡＰＤ９０缩短，Ｖｍａｘ有下降趋势。用９．５０×１０－６ｍｏｌ／ＬＳｅ灌流，ＡＰＤ５０、ＡＰＤ９０明显延长。上述浓度的Ｆ－和Ｓｅ同时应用则灌流前后心肌电生理各参数无明显改变。此表明Ｆ－灌流可使心肌细胞膜电位降低，兴奋性降低，复极时间缩短，Ｓｅ可桔抗Ｆ－所致的此种心肌电生理异常。     

Regulation of ATP sensitive potassium channel of isolated guinea pig ventricular myocytes by sarcolemmal monocarboxylate transport.

OBJECTIVE: The aim was to describe the effects of extracellular application of monocarboxylates (pyruvate, lactate, or acetate) on current through KATP channels (iK,ATP) in isolated guinea pig ventricular myocytes. METHODS: The iK,ATP was elicited during whole cell voltage clamping by application of metabolic poisons, 2,4-dinitrophenol (150 microM) or glucose free cyanide (1 mM) and could be blocked by glibenclamide (3 microM). RESULTS: Extracellular application of monocarboxylates, pyruvate (0.1-10 mM), L-lactate (0.1-10 mM), and acetate (10 mM) led to a rapid inhibition of iK,ATP--an effect which was fully reversible upon washout. Substances without any effect on iK,ATP were (10 mM each) gluconate, citrate, glutamate, creatine, succinate, and glycine. The mechanism underlying the effects of monocarboxylates on iK,ATP was unlikely to be related to an increased ATP production, since D-lactate (10 mM) essentially had the same effect on iK,ATP as the L-isomer of lactate. Furthermore, with intracellular dialysis of alpha-cyano-4-hydroxycinnamate (0.1-0.5 mM), which inhibits pyruvate uptake into mitochondria, extracellular pyruvate exerted the same inhibitory effect on iK,ATP. High concentrations of extracellular alpha-cyano-4-hydroxycinnamate (4 mM), which blocks the sarcolemmal monocarboxylate carrier, prevented the effects on iK,ATP by pyruvate, L-lactate, D-lactate, and acetate. Furthermore, intracellular dialysis with D-lactate (10 mM) led to a more rapid onset of iK,ATP when activated by ATP free dialysis. Activity of isolated KATP channels, measured in isolated membrane patches in the inside out or outside out configuration, typically had a single channel conductance of around 80 pS and was blocked by glibenclamide (3-9 microM). No significant effect of pyruvate was observed in either patch configuration. CONCLUSIONS: In cardiac tissue there may be some modulatory role involving monocarboxylate transport on KATP channel activity, the nature of which is unclear at present but which may involve cytosolic pH changes. Physiological and pathophysiological implications of these findings are discussed.     

Effect of ethanol on guinea pig ventricular action potentials

The effects of low concentrations of ethanol on transmembrane potentials of guinea pig ventricular muscle were analyzed. Papillary muscles were superfused with Tyrode solution in vitro at 30°C while driven at 60 min.⁻¹ Transmembrane potentials were recorded by means of intracellular microelectrodes before, during and after a 15 min exposure to ethanol. Ethanol 2.5 mg/100 ml (0.55mM) did not affect transmembrane potentials. Ethanol 5 mg/100 ml enhanced the amplitude of the action potential (AAP) without affecting the membrane resting potential (MRP) and the maximum velocity of the upstroke (dV/dt). Ethanol 40 mg/100 ml also enhanced AAP without modifying MRP and dV/dt. Verapamil and propranolol blocked the augmenting effect of ethanol on AAP. Ethanol 40 mg/100 ml also enhanced the amplitude of responses restored by norepinephrine in fibers depolarized by high [K]₀ to a level of resting potential at which the fast component of the upstroke was abolished. The enhancement of AAP was accompanied by a shift of the plateau to more positive values, but the duration of the action potential measured at 50% of repolarization was not affected. The results indicate that ethanol in low concentrations augments, through a catechol-mediated effect, the slow component of the upstroke of ventricular action potentials.     

The effects of mexiletine on action potential duration and its restitution in guinea pig ventricular muscles

To clarify the mechanism of mexiletine-induced changes in action potential duration (APD), we studied the effects of mexiletine (2 micrograms/ml) on APD at 0 mV (APD0mV) and 90% (APD90%) repolarization and on restitution of premature responses in guinea pig ventricular muscle at three extracellular potassium concentrations [( K]0) and three stimulation rates using standard microelectrode techniques. The rates at which APD0mV and APD90% were shortened by mexiletine (S-APD0mV and S-APD90%) expressed as percent change from control were most pronounced at [K]0 = 5.4 mM. The percent changes at the three concentrations were 5.0 +/- 2.1% at a [K]0 of 2.7 mM, 6.0 +/- 3.2% at 5.4 mM and 1.8 +/- 1.2% at 10.0 mM for S-APD0mV and 2.0 +/- 1.9%, 4.7 +/- 2.0% and 1.3 +/- 1.5% for S-APD90% at the same concentrations, respectively. S-APD0mV and S-APD90% were more markedly affected when stimulated at a frequency of 1 Hz than when stimulated at 0.2 or 0.5 Hz. Mexiletine failed to produce any additional APD shortening beyond that produced by the introduction of tetrodotoxin (2.5 X 10(-6) M). Mexiletine and tetrodotoxin did not influence APD restitution that fitted a single exponential curve. We conclude that the shortening of APD by mexiletine results from inhibition of a tetrodotoxin-sensitive sodium current.     

溶血磷脂酸对豚鼠心室肌动作电位及延迟整流钾电流的影响

目的观察溶血磷脂酸（LPA）对离体豚鼠心室乳头肌动作电位及心室肌细胞延迟整流钾电流的影响。方法采用标准玻璃微电极技术记录豚鼠乳头肌动作电位。应用全细胞电压钳方法记录心室肌细胞延迟整流钾电流（Ik）。结果LPA0．1、1．0、10umol/L可浓度依赖性增加心室肌动作电位幅度（APA）（P〈0．05，P〈0．01，P〈0．01），延长动作电位50％、90％时程（APD50、APD90）（P〈0．05），钾通道阻断剂TEA可部分阻断LPA对APD50的延长作用。LPA0．1、1．0、10umol／L可明显抑制Ik（P〈0．05）。结论LPA可增加豚鼠心脏乳头肌动作电位幅值、延长动作电位时程，并抑制豚鼠心室肌细胞Ik。     

Distribution of Kir6.0 and SUR2 ATP-sensitive potassium channel subunits in isolated ventricular myocytes

The subcellular distribution of ATP-sensitive potassium (K(ATP)) channel subunits in rat-isolated ventricular myocytes was investigated using a panel of subunit-specific antisera. Kir6.1 subunits were associated predominantly with myofibril structures and were co-localized with the mitochondrial marker MitoFluor red (correlation coefficient (cc) = 0.63 +/- 0.05). Anti-Kir6.1 antibodies specifically recognized a polypeptide of 48 kDa in mitochondrial membrane fractions consistent with the presence of Kir6.1 subunits in this organelle. Both Kir6.2 and SUR2A subunits were distributed universally over the sarcolemma. Lower-intensity antibody-associated immunofluorescence was detected intracellularly, which was correlated with the distribution of MitoFluor red in both cases (cc, Kir6.2, 0.56 +/- 0.05; SUR2A, 0.61 +/- 0.06). A polypeptide of 40 kDa was recognized by anti-Kir6.2-subunit antibodies in western blots of both microsomal and mitochondrial membrane fractions consistent with the presence of this subunit in the sarcolemma and mitochondria. Similarly, SUR2A and SUR2B subunits were detected in western blots of microsomal membrane proteins consistent with a sarcolemmal localization for these polypeptides. SUR2B subunits were shown in confocal microscopy to co-localize strongly with t-tubules (cc, 0.81 +/- 0.05). Together, the results indicate that Kir6.2 and SUR2A subunits predominate in the sarcolemma of ventricular myocytes consistent with a Kir6.2/SUR2A-subunit combination in the sarcolemmal K(ATP)channel. Kir6.1, Kir6.2 and SUR2A subunits were demonstrated in mitochondria. Combinations of these subunits would not explain the reported pharmacology of the mitochondrial K(ATP) channel (Mol Pharmacol 59 (2001) 225) suggesting the possibility of further unidentified components of this channel.     

Exercise-induced expression of cardiac ATP-sensitive potassium channels promotes action potential shortening and energy conservation

Physical activity is one of the most important determinants of cardiac function. The ability of the heart to increase delivery of oxygen and metabolic fuels relies on an array of adaptive responses necessary to match bodily demand while avoiding exhaustion of cardiac resources. The ATP-sensitive potassium (K_ATP) channel has the unique ability to adjust cardiac membrane excitability in accordance with ATP and ADP levels, and up-regulation of its expression that occurs in response to exercise could represent a critical element of this adaption. However, the mechanism by which K_ATP channel expression changes result in a beneficial effect on cardiac excitability and function remains to be established. Here, we demonstrate that an exercise-induced rise in K_ATP channel expression enhanced the rate and magnitude of action potential shortening in response to heart rate acceleration. This adaptation in membrane excitability promoted significant reduction in cardiac energy consumption under escalating workloads. Genetic disruption of normal K_ATP channel pore function abolished the exercise-related changes in action potential duration adjustment and caused increased cardiac energy consumption. Thus, an expression-driven enhancement in the K_ATP channel-dependent membrane response to alterations in cardiac workload represents a previously unrecognized mechanism for adaptation to physical activity and a potential target for cardioprotection.     

Ventricular action potentials, ventricular extracellular potentials, and the ECG of guinea pig

Action potentials were recorded from different regions of the guinea pig ventricle to characterize regional differences in waveform configuration, and to acquire insight into the generation of the T-wave of the electrocardiogram. Isolated tissue preparations were driven at 1 Hz, and microelectrodes were used to map accessible surface regions of the epicardium, endocardium, and septum. There were minimal differences in regional resting potentials (mean -87 mV) and amplitudes (mean 122 mV), but Vmax in the epicardium (mean 110 V/sec) was much smaller than elsewhere (mean 247 V/sec). The action potential duration at the -80 mV repolarization level was longest in the papillary muscles (mean 154 msec), shortest in the septum (mean 126 msec), and generally 10-15 msec longer at the base than at the apex. The characteristics of intramural action potentials were inferred from measurements on enzymatically isolated myocytes, the rationale being that most dissociated myocytes originated from intramural cell layers. The action potentials in about 40% of the myocytes had durations similar to those recorded from the tissue surface (110-170 msec), and the remainder ranged from 170-290 msec long. The existence of longer-than-surface action potentials in the ventricle was also inferred from the body surface electrocardiogram and from bipolar electrograms of isolated left ventricles. In both cases, the Q-T intervals could be accounted for only by action potentials longer than those recorded from the ventricular surface.     

卡维地洛对模拟缺血时豚鼠乳头肌动作电位和心室肌细胞ATP敏感性钾电流的影响

目的　观察卡维地洛对模拟缺血豚鼠乳头肌动作电位和心室肌细胞ATP敏感性钾电流(IK·ATP)的影响 ,探讨卡维地洛对心室肌ATP敏感性钾通道 (KATP)调节及其在抗心肌缺血、抗心律失常中的作用机制。方法　应用标准玻璃微电极技术 ,观察 0 0 3μM、0 3μM和 3 0 μM 3种不同浓度的卡维地洛对模拟缺血时豚鼠心室乳头肌动作电位的影响 ,同时采用全细胞膜片钳方法 ,记录模拟缺血时上述 3个浓度的卡维地洛对豚鼠心室肌细胞IK·ATP的作用。结果　模拟缺血液灌流细胞 ,其中 1 5 1 %(n =5)的细胞KATP通道在 3min内完全开放 ,这些细胞均在KATP通道完全开放后 (1 2 3 0 0± 33 64)s死亡。模拟缺血液加入 0 0 3μM (n =9)、0 3μM (n =9)、3 0 μM (n =1 0 )卡维地洛灌流细胞的各组 ,KATP通道分别于灌流后 (9 0 0± 3 1 6)min、(9 77± 0 86)min和 (1 1 74± 4 41 )min开放。 +40mV时的IK·ATP(pA/pF)分别是 2 5 44± 1 8 48、2 0 1 7± 30 83和 3 95± 2 48,与对照组 32 65± 36 0 2比较 ,0 0 3μM组差异有显著性 (P <0 0 5) ,后两者差异呈高度显著性 (P <0 0 1 ) ;模拟缺血液灌流 5～ 2 0min(n =1 5)均可见动作电位时限 (APD)显著缩短 ,APD90 从正常对照组的 (2 31 0± 1 4 3)ms降至 (1 30 0±1 2 4     

Enhancement of potassium-sensitive current in heart cells by pinacidil. Evidence for modulation of the ATP-sensitive potassium channel

Pinacidil belongs to a novel group of compounds that enhance the potassium permeability of vascular smooth muscle. Evidence also exists that this drug enhances the potassium permeability of cardiac tissue. The purpose of the present investigation was to determine if pinacidil alters potassium-channel activity in heart and, if so, which potassium channel is the target. We used the whole-cell arrangement of the patch voltage clamp to record membrane currents from isolated guinea pig ventricular cells. In solutions designed to isolate potassium currents, pinacidil enhances a time-independent current positive to the potassium equilibrium potential. Current measured at voltages negative to the potassium equilibrium potential are essentially unaltered by the drug. The potassium sensitivity of outward current indicates that the target for the drug is a potassium channel. Experiments designed to test for voltage-dependent channel gating strongly suggest that the pinacidil-sensitive current is not voltage gated. Pinacidil-sensitive current is blocked by externally applied Ba2+, Cs+, and tetraethylammonium ion. In addition, it is potently blocked after external application of 100 nM glibenclamide. Taken along with the time- and voltage-independent properties of pinacidil-sensitive current, this pharmacology strongly suggests that the target for pinacidil in heart is the ATP-sensitive potassium channel.     

Modulation of hERG potassium channel gating normalizes action potential duration prolonged by dysfunctional KCNQ1 potassium channel

Long QT syndrome (LQTS) is a genetic disease characterized by a prolonged QT interval in an electrocardiogram (ECG), leading to higher risk of sudden cardiac death. Among the 12 identified genes causal to heritable LQTS, ～90% of affected individuals harbor mutations in either KCNQ1 or human ether-a-go-go related genes (hERG), which encode two repolarizing potassium currents known as I(Ks) and I(Kr). The ability to quantitatively assess contributions of different current components is therefore important for investigating disease phenotypes and testing effectiveness of pharmacological modulation. Here we report a quantitative analysis by simulating cardiac action potentials of cultured human cardiomyocytes to match the experimental waveforms of both healthy control and LQT syndrome type 1 (LQT1) action potentials. The quantitative evaluation suggests that elevation of I(Kr) by reducing voltage sensitivity of inactivation, not via slowing of deactivation, could more effectively restore normal QT duration if I(Ks) is reduced. Using a unique specific chemical activator for I(Kr) that has a primary effect of causing a right shift of V(1/2) for inactivation, we then examined the duration changes of autonomous action potentials from differentiated human cardiomyocytes. Indeed, this activator causes dose-dependent shortening of the action potential durations and is able to normalize action potentials of cells of patients with LQT1. In contrast, an I(Kr) chemical activator of primary effects in slowing channel deactivation was not effective in modulating action potential durations. Our studies provide both the theoretical basis and experimental support for compensatory normalization of action potential duration by a pharmacological agent.     

Blockade of the inward rectifying potassium current terminates ventricular fibrillation in the guinea pig heart

INTRODUCTION: Stable high-frequency rotors sustain ventricular fibrillation (VF) in the guinea pig heart. We surmised that rotor stabilization in the left ventricle (LV) and fibrillatory conduction toward the right ventricle (RV) result from chamber-specific differences in functional expression of inward rectifier (Kir2.x) channels and unequal IK1 rectification in LV and RV myocytes. Accordingly, selective blockade of IK1 during VF should terminate VF. METHODS AND RESULTS: Relative mRNA levels of Kir2.x channels were measured in LV and RV. In addition, LV (n = 21) and RV (n = 20) myocytes were superfused with BaCl2 (5-50 micromol/L) to study the effects on IK1. Potentiometric dye-fluorescence movies of VF were obtained in the presence of Ba2+ (0-50 micromol/L) in 23 Langendorff-perfused hearts. Dominant frequencies (DFs) were determined by spectral analysis, and singularity points were counted in phase maps to assess VF organization. mRNA levels for Kir2.1 and Kir2.3 were significantly larger in LV than RV. Concurrently, outward IK1 was significantly larger in LV than RV myocytes. Ba2+ decreased IK1 in a dose-dependent manner (LV change > RV change). In baseline control VF, the fastest DF domain (28-40 Hz) was located on the anterior LV wall and a sharp LV-to-RV frequency gradient of 21.2 +/- 4.3 Hz was present. Ba2+ significantly decreased both LV frequency and gradient, and it terminated VF in a dose-dependent manner. At 50 micromol/L, Ba2+ decreased the average number of wavebreaks (1.7 +/- 0.9 to 0.8 +/- 0.6 SP/sec x pixel, P < 0.05) and then terminated VF. CONCLUSION: The results strongly support the hypothesis that IK1 plays an important role in rotor stabilization and VF dynamics.     

Modification of the cardiac action potential by intracellular injection of adenosine triphosphate and related substances in guinea pig single ventricular cells

Effects of varying the intracellular adenosine triphosphate level on both the action potential and the membrane current were studied in single ventricular cells isolated from the guinea pig heart, using collagenase. Intracellular injection of adenosine triphosphate elevated the plateau potential level and prolonged the action potential duration. Similar results were obtained by injecting adenosine diphosphate, adenosine monophosphate, or creatine phosphate, i.e., substances considered to increase the intracellular concentration of adenosine triphosphate. In contrast, the action potential was depressed by procedures which could reduce the intracellular adenosine triphosphate level, such as an injection of creatine, superfusion of glucose-free Tyrode's solution containing 5.4 mM cyanide ion, or an injection of adenosine monophosphate into the cyanide-superfused cell. When the membrane current was recorded under the voltage clamp, it was found that the injection of adenosine triphosphate increased the amplitude of the slow inward current, whereas the superfusion of cyanide ion did not significantly decrease the slow inward current, although the action potential became considerably shorter. It was also found that the adenosine monophosphate injection decreased the amplitude of the net outward membrane current at the plateau level and increased it at around -40 mV, and thus intensified the N-shape of the isochronal 0.3-second current-voltage curve. The cyanide ion superfusion produced the opposite effect; in response to depolarizing clamp pulses more positive to the plateau level, the membrane current increased significantly with cyanide ion, but increased only slightly with adenosine triphosphate. These results suggest that intracellular adenosine triphosphate modifies the membrane currents at the plateau potential range, thus altering the action potential duration.