急性心肌梗死后一周对心室肌细胞瞬间外向钾电流的影响

研究急性心肌梗死 (AMI)心室肌细胞瞬间外向钾电流 (Ito)的变化。采用结扎兔冠状动脉左前降支的方法建立AMI动物模型 ,应用膜片钳全细胞记录方法 ,研究AMI后 1周心外膜梗死区心肌细胞Ito的变化。结果 :正常对照组 (n =16 )心肌细胞在 - 30mV激活 ,心肌梗死 (简称心梗 )组细胞在 - 2 0mV激活 ,均呈线性电压依赖性。心梗组梗死区细胞 (n =12 )Ito的电流密度明显下降 ,I V曲线明显下移。心梗组Ito电流密度 (去极化电位 +6 0mV时 )明显低于对照组 (7.4 7± 2 .39vs 17.39± 5 .2 4pA/pF ,P <0 .0 1)。结论 :AMI可引起心室肌细胞Ito电流密度下降 ,导致梗死区细胞动作电位平台期相对延长 ,复极异常 ,造成心肌细胞之间动作电位及不应期离散度增大 ,容易形成折返 ,此可能是导致心肌梗死后出现折返性室性心律失常的原因。     

Abnormalities in Ca(i)handling in myocytes that survive in the infarcted heart are not just due to alterations in repolarization

Studies from our laboratory have defined alterations in Ca(i)handling in the non-dialyzed subepicardial cells that have survived in the 5 day infarcted heart (IZs). To determine whether changes in the action potential profile contributed to the observed Ca(i)changes we have used a combined voltage clamp/epifluorescent technique to determine and compare changes in fura 2 ratios in IZs compared to those of epicardial cells from the non-infarcted canine hearts (NZs). We found that Ca(i)changes in voltage clamped IZs persisted. In NZs, Ca(i)transients showed the expected voltage dependence while IZs did not. To determine whether altered NaCa exchanger activity contributed to the observed changes in Ca(i)in IZs, we measured NaCa exchanger Ca(2+)fluxes (reverse and forward mode) and ionic currents in both cell types and under different Na(i)loads (10 and 20 m m). We found that there were no significant differences in resting, peak or magnitude of fura 2 ratio changes or in outward current densities between NZs and IZs even under the different Na(i)loads. Thus, we suggest that chronic up- or downregulation of the NaCa exchanger protein does not underlie observed Ca(i)changes in IZs. Additionally, Ca(2+)released with paced voltage steps represented 79% of that released by caffeine in NZs while, in IZs, caffeine releasable Ca(2+)was equivalent to that released with step depolarization. Thus, abnormalities in Ca(i)handling in IZs appear not to arise secondarily to changes in action potential configuration nor do they appear to be due to disease-induced alteations in NaCa exchanger function.     

Can PKA activators rescue Na+ channel function in epicardial border zone cells that survive in the infarcted canine heart?

OBJECTIVE AND METHODS: In this study, we investigated the effects of a PKA stimulating cocktail on sodium currents from normal epicardial cells (NZs) and on those from cells dispersed from the epicardial zone of the 5-day infarcted canine heart (IZs). To do so, we used whole-cell voltage-clamp techniques. RESULTS: During superfusion with the PKA activator cocktail, peak sodium current (I(Na)) density significantly increased by 32+/-5.3% (NZs) and 17+/-5.4% (IZs). However, despite this increase, IZ peak I(Na) still was not fully restored to NZ values. In both cell types, the density effect was accompanied by a shift in I/I(max) curves, as well as a slowing in recovery from inactivation. Inactivation from a closed state was accelerated. Furthermore, in the presence of chloroquine, which is known to interrupt intracellular vesicular traffic, PKA activator effects to augment I(Na) were only partially inhibited in NZs but abolished in IZs. To understand whether the phosphorylation status of basal Na(+) channels in the two cell groups differed, the effects of okadaic acid and PP2A1 were studied. Results suggest that in IZs, Na(+) channels in the basal state are already phosphorylated. CONCLUSIONS: PKA stimulation of I(Na) of the remodeled IZ does augment current density possibly by augmenting the trafficking of channels to an active site on the membrane. However, the resulting I(Na), while partially rescued, is not similar to the potentiated I(Na) of NZs. Specific kinetic changes also occur with the PKA stimulation of IZs and results with okadaic acid and PP2A1 suggest that in their remodeled state, Na(+) channels in IZs are already phosphorylated.     

Nonlinear relation between Vmax and INa in canine cardiac Purkinje cells

We studied the relation of the maximal upstroke velocity (Vmax) of action potentials to the peak sodium current (INa) under voltage clamp in single, internally perfused, canine cardiac Purkinje cells under conditions that ensured membrane action potentials due only to INa. Three different methods of altering sodium channel availability were investigated: voltage-dependent inactivation, tetrodotoxin (TTX) block, and use-dependent block by quinidine. Under all three conditions, the relation of Vmax to INa was nonlinear, and no relation was found that would allow prediction of INa results from Vmax measurements. With voltage-dependent inactivation or TTX block, sodium channel availability measured by Vmax was reduced less than availability measured by peak INa, so that Vmax overestimated sodium channel availability. This overestimation of sodium channel availability by Vmax could be attributed to greater sodium channel mobilization during the slowed action potential upstrokes. The overestimation varied with experimental temperature as a consequence of changes in sodium channel kinetics. Vmax also overestimated sodium channel availability during TTX exposure so that the Kd for TTX block was 4.5 micron from Vmax measurements but only 1.6 microM from INa measurements. Use-dependent block of INa by quinidine had a striking voltage-dependent component under voltage clamp that could not be appreciated from action potentials. Consequently, block could be underestimated or overestimated by Vmax measurements. We conclude that Vmax measurements represent a convenient index for INa, but Vmax is not a reliable method for quantitative studies of sodium channel behavior.     

兔急性心肌梗死后二月心室肌细胞钠离子通道活性的变化

研究急性心肌梗死 (AMI)后心室肌细胞钠离子通道活性的变化。采用结扎兔冠状动脉左前降支的方法建立AMI动物模型 ,应用膜片钳全细胞记录方法 ,观察AMI后 2个月心外膜梗死区心肌细胞钠通道电流 (INa)的变化。结果 :①正常对照组INa电流密度峰值 (去极化电位 - 30mV时 )为 45 .5± 5 .33pA/pF(n =12 ) ,心肌梗死 (简称心梗 )组为 16 .4± 4.43pA/pF(n =13) ,心梗组较对照组明显下降 ,P <0 .0 1。心梗组INa电流 电压关系曲线较对照组明显下移。②心梗组INa失活曲线较对照组明显左移 (即向超级化方向移动 ) ,对照组半数失活电压 (V0 .5)为 - 76 .2± 5 .3mV(n =5 ) ,心梗组V0 .5为 - 82 .4± 5 .6mV(n =12 ) ,P <0 .0 5。③心梗组钠通道灭活后恢复时程较对照组减慢 ,恢复曲线下移。结论 :AMI可导致梗死区心室肌细胞INa下降、钠通道动力学发生变化 ,引起心肌传导速度下降和不应性延长 ,此可能是导致AMI后出现折返性室性心律失常的原因。     

Altered Pharmacologic Responsiveness of Reduced L-Type Calcium Currents in Myocytes Surviving in the Infarcted Heart

Altered Pharmacology of I_ca,L in Myocytes From Infarcted Heart. The pharmacologic responses of macroscopic L-type calcium channel currents to the dihydropyridine agonist, Bay K 8644, and β-adrenergic receptor stimulation by isoproterenol were studied in myocytes enzymatically dissociated from the epicardial border zone of the arrhythmic 5-day infarcted canine heart (IZs). Calcium currents were recorded at 36° to 37° C using the whole cell, patch clamp method and elicited by applying step depolarizations from a holding potential of -40 mV to various test potentials for 250-msec duration at 8-second intervals. A Cs⁺ -rich and 10 mM EGTA-containing pipette solution and a Na⁺-and K⁺-free external solutions were used to isolate calcium currents from other contaminating currents. During control, peak I_ca,L, density was found to be significantly less in IZs (4.0 ± 1.1 pA/pF) than in myocytes dispersed from the epicardium of the normal noninfarcted heart (NZs; 6.5 ± 1.8 pA/pF). Bay K 8644 (I μM) significantly increased peak I_ca,L density 3.5-fold above control levels in both NZs (to 22.5 ± 6.2 pA/pF; n = 7) and IZs (to 12.8 ± 3.0 pA/pF; n = 5), yet peak I_ca,L density in the presence of drug was significantly less in IZs than NZs. The effects of Bay K 8644 on kinetics of current decay and steady-state inactivation relations of peak I_ca,L were similar in the two cell types. In contrast, the response of peak L-type current density to isoproterenol (1 μM) was significantly diminished in IZs compared to NZs regardless of whether Ba²⁺ or Ca²⁺ ions carried the current. Thus, these results indicate an altered responsiveness to β-adrenergic stimulation in cells that survive in the infarcted heart. Furthermore, application of forskolin (1 μM and 10 μM) or intracellular cAMP (200 μM), agents known to act downstream of the β-receptor, also produced a smaller increase in peak I_Ba density in IZs versus NZs, suggesting that multiple defects exist in the β-adrenergic signaling pathway of IZs. In conclusion, these studies illustrate that reduced macroscopic calcium currents of cells in the infracted heart exhibit an altered pharmacologic profile that has important implications in the development of drugs for the diseased heart.     

Sodium channel block produces opposite electrophysiological effects in canine ventricular epicardium and endocardium.

Using microelectrode techniques we compared the effects of tetrodotoxin (TTX, 2-3 microM), DL-propranolol (1-3 micrograms/ml), and flecainide acetate (10-15 microM) on isolated canine ventricular epicardial (epicardium) and endocardial (endocardium) tissues. Propranolol, TTX, and flecainide decreased Vmax and phase 0 amplitude in a use-dependent manner in both tissues. The effects of propranolol were slow to develop and wash out. TTX and propranolol always abbreviated action potential duration in endocardium. Action potential duration was abbreviated by 23.8 +/- 5.6 msec after propranolol (1 microgram/ml, basic cycle length [BCL] = 1,000 msec) and 10.8 +/- 12.9 msec after TTX (2 microM, BCL = 1,000 msec). In epicardium, the reduction of phase 0 and 1 amplitudes led to a slowing of the second action potential upstroke and an increase in the amplitude of phase 2. This accentuation of the notch resulted in a paradoxical prolongation of the epicardial action potential. Action potential duration was prolonged 34.4 +/- 11.3 msec after 4 hours of exposure to propranolol (1 microgram/ml, BCL = 1,000 msec), 11.1 +/- 6.3 msec after 15 minutes of exposure to TTX (2 microM, BCL = 1,000 msec), and 19.9 +/- 8.2 msec after 25-45 minutes of exposure to flecainide (15 microM, BCL = 500 msec). With stronger sodium block, phase 1 terminated at more negative potentials, the second upstroke often failed to appear, and an all-or-none repolarization ensued causing a marked abbreviation of the epicardial action potential. In some epicardial preparations, we observed marked abbreviation at some sites but prolongation at other sites after sodium blockade with flecainide. The dispersion of repolarization was often attended by reentrant activity. The differential response of epicardium and endocardium to sodium blockade was not observed when the preparations were pretreated with 4-aminopyridine or ryanodine, agents known to diminish the transient outward current and epicardial notch. Acceleration-induced prolongation of refractoriness was observed after sodium blockade in epicardium but not in endocardium. Postrepolarization refractoriness also occurred in epicardium but not in endocardium after TTX, propranolol, or flecainide exposure. The data indicate that propranolol, TTX, and flecainide, via their action to block sodium current, may exert opposite effects on action potential duration and refractoriness in cells spanning the ventricular wall. The presence of the transient outward current in epicardium but not in endocardium appears to contribute importantly to these differences.(ABSTRACT TRUNCATED AT 400 WORDS)     

Frequency and voltage dependent effects of AN-132, a new class I anti-arrhythmic agent, on Vmax of action potential upstroke in guinea pig ventricular muscles

The in vitro electrophysiological properties of a newly synthesised antiarrhythmic agent, AN-132, were evaluated by recording transmembrane action potentials from guinea pig papillary muscles. AN-132 (10-100 mumol.litre-1) caused a dose dependent decrease in the maximum upstroke velocity (Vmax) of the action potential without affecting the resting potential. In the presence of AN-132, trains of stimuli at rates greater than or equal to 0.1 Hz led to an exponential decline in Vmax. This use dependent block was enhanced at higher stimulation frequency. The time constant for the recovery of Vmax from the use dependent block was 39.5-41.2 s. The curves relating membrane potential and Vmax were shifted by AN-132 (100 mumol.litre-1) in the direction of more negative potentials (6.1 mV). In preparations treated with AN-132 (30 and 100 mumol.litre-1), the Vmax of test action potentials preceded by conditioning clamp pulses to 0 mV was progressively decreased with an increasing number of pulses. A single prolonged clamp pulse to 0 mV reduced Vmax much less than multiple brief clamp pulses. These findings suggest than AN-132 has use dependent inhibitory action on the fast sodium channel by binding to the channel mainly during its activated state and that the unbinding rate of the drug during diastole is very slow. This use dependency and its greater inhibition of Vmax in depolarised muscles through the increase in tonic block may play a major role in preventing ventricular arrhythmias.     

Reentrant ventricular arrhythmias in the late myocardial infarction period. 7. Effect of verapamil and D-600 and the role of the "slow channel".

Reentrant ventricular arrhythmias (RVA) were analyzed in dogs 3--7 days after ligation of the anterior descending coronary artery using averaged "composite" recordings of electrical activity of reentrant pathways (RP) from the epicardial surface of the infarction zone (IZ). Verapamil (V) and D-600 (D) (0.2--0.5 mg/kg i.v.) resulted in slight-to-moderate improvement of conduction in RP with abolition of spontaneous RVA and RVA initiated by premature depolarizations. The effect of V was not blocked by pretreatment with propranolol (0.5 mg/kg i.v.). Using a standard microelectrode technique and strips of epicardial muscle from the IZ, D (0.5--1 X 10(-6) g/ml) slightly improved the upstroke velocity and membrane responses of depressed ischemic cells. In contrast, tetrodotoxin (5 X 10(-7) g/ml) further depressed or abolished action potentials of ischemic cells. We conclude: 1) the moderate antiarrhythmic effect of V and D on RVA is the result of improved conduction in RP; 2) this action is partly explained by improvement of a depressed sodium channel and is not related to catecholamine release; 3) slow-response action potentials play no significant role in the genesis of ischemia-related RVA, which probably results from depression of the fast response.     

Effect of lidocaine on the early inward transient current in sheep cardiac Purkinje fibers.

We used two experimental techniques to study the effect of lidocaine hydrochloride on the early inward transient (sodium) current as it is reflected by the maximum rate of change of action potential phase 0 (Vmax). We assessed the effect of lidocaine on Vmax as Purkinje fibers were slowly depolarized by increasing the extracellular potassium concentration from 4.0 to 16.0 mM; these voltage-dependent effects were compared with lidocaine's effect on membrane responsiveness (which measures both the time and the voltage dependence of Vmax). We also used a voltage clamp technique to establish the effect of lidocaine on the voltage dependence of Vmax by measuring Vmax 800-1000 msec after transmembrane voltage (Vm) had been changed in small steps. We studied the effect of lidocaine on the time course of early inward transient current reactivation by depolarizing the membrane to -25 +/- 5 mv for 100 msec to inactivate this current, clamping Vm to a repolarized test voltage for various periods, and then measuring phase 0 Vmax of action potentials elicited immediately after termination of the voltage clamp. We showed that lidocaine at 5 mg/liter, but not a 1 mg/liter, shifted the steady-state Vmax- Vm relationship to a more negative position on its voltage axis by about 5 mv and markedly slowed the reactivation of the measure early inward transient current.     

Effects of quinidine on action potentials and ionic currents in isolated canine ventricular myocytes

We examined the effects of quinidine (5-20 microM) on transmembrane action potentials and ionic currents of isolated canine ventricular myocytes. Collagenase treatment of canine ventricular tissue produced a yield of 40-60% healthy cells. Myocytes had normal resting and action potentials as measured using conventional microelectrodes. Quinidine decreased Vmax, amplitude, overshoot, and the duration of action potentials stimulated by passage of brief current pulses through the recording pipette. Recovery was complete after washout except that action potential duration was prolonged compared with control. A discontinuous single microelectrode voltage ("switch") clamp was used to measure ionic currents. Quinidine irreversibly reduced steady-state outward current as measured with three different voltage clamp protocols. Quinidine reversibly decreased peak calcium current as well as the slowly inactivating and/or steady-state inward currents in the plateau voltage range, presumably both "late" sodium (tetrodotoxin-sensitive) and calcium (tetrodotoxin-insensitive) currents. The effect on calcium current showed both tonic and use-dependent block. Thus, quinidine has a multitude of actions on both inward and outward currents, which combine to produce the net effect of quinidine on action potential configuration.     

Block of cardiac sodium channels by antiarrhythmic drugs

The effects of seven Class-I antiarrhythmic drugs on the maximum upstroke velocity (Vmax) of action potential were examined in isolated guinea pig ventricular muscles in order to characterize their use- and state-dependent sodium channel blocking action. From the onset and offset kinetics of the use-dependent Vmax inhibition during stimulation trains, the seven drugs were subdivided into two groups; fast drugs (lidocaine, mexiletine, and tocainide), and slow drugs (quinidine, aprindine, disopyramide and flecainide). In experiments to assess the state-dependent sodium channel block, a conditioning clamp pulse to 0 mV was applied by using the single sucrose-gap voltage-clamp technique, and the Vmax of test action potential 100 msec after the clamp pulse was measured. The decrease in Vmax by 10 msec clamp pulse was defined as the activated channel block (ACB), and the decrease in Vmax as the clamp pulse duration was prolonged from 10 to 500 msec was defined as the inactivated channel block (ICB). The ratio of ICB to ACB was less than 1.0 for quinidine, disopyramide and flecainide, and much greater than 1.0 for aprindine, lidocaine, mexiletine, and tocainide. These characteristics may contribute to the differences in efficacy of each drug in treating various types of arrhythmias.     

Block of activated and inactivated sodium channels by class-I antiarrhythmic drugs studied by using the maximum upstroke velocity (Vmax) of action potential in guinea-pig cardiac muscles

The inhibitory effect of disopyramide, quinidine, mexiletine, lidocaine, tocainide and aprindine on the maximum upstroke velocity (Vmax) of action potential was examined in isolated guinea-pig cardiac muscles in terms of their sodium channel block during activated and/or inactivated state. In right ventricular papillary muscles, a conditioning clamp pulse was applied from the resting potential to 0 mV level using the single sucrose-gap voltage clamp technique, and Vmax of test action potential elicited 100 ms after termination of the clamp pulse was measured as an index of sodium channel availability. Such clamp pulses caused various Vmax decreases in the presence of the six drugs. The decrease in Vmax by 10 ms clamp pulse was defined as the activated channel block (ACB), and the decrease in Vmax as the clamp pulse duration was prolonged from 10 to 500 ms was defined as the inactivated channel block (ICB). The ratio of ICB to ACB was less than 1.0 (0.36-0.51) for disopyramide and quinidine, and much greater than 1.0 (2.61-11.23) for mexiletine, lidocaine, tocainide and aprindine. From these findings it was suggested that the former group of drugs may block the sodium channel mainly during the upstroke phase of action potential, while the latter do so mainly during the plateau phase. This assumption was confirmed by experiments comparing the potency of Vmax inhibition in atrial and ventricular muscles isolated from the same guinea-pig heart.     

Correlation between in vivo transmembrane action potential durations and activation-recovery intervals from electrograms. Effects of interventions that alter repolarization time

Classic cable theory was used to analyze the relation between the activation-recovery interval measured from unipolar electrograms and transmembrane action potential duration. Theoretic analysis demonstrated that the temporal derivative of the extracellular potential is proportional to a spatial weighting of the third temporal derivative of the transmembrane action potentials along a cable with uniform propagation in a homogeneous medium. Thus, the activation-recovery interval, measured as the interval between times of minimum derivative (Vmin) of the QRS and maximum derivative (Vmax) of the T wave, should be related to action potential duration, measured as the interval between times of Vmax of the upstroke and Vmin of the downstroke of the transmembrane action potential. This relation was examined experimentally in 12 anesthetized dogs. Unipolar electrograms and transmembrane action potentials were recorded simultaneously from sites within 2 mm of each other during control states, cardiac sympathetic nerve stimulation, localized epicardial warming, and graded reductions in myocardial perfusion. The heart was paced from several sites. There was close correlation between activation-recovery interval and action potential duration measurements taken during cardiac sympathetic nerve stimulation and local epicardial warming (r = 0.96 and 0.99 for cardiac sympathetic nerve stimulation and warming, respectively). In five animals in which coronary perfusion pressure was gradually lowered, the variables correlated closely over a range of values from 62 to 212 msec (r = 0.98). However, although the overall correlation was good and mean differences between activation-recovery interval and action potential duration were small, in individual cases there were differences up to 24 msec.(ABSTRACT TRUNCATED AT 250 WORDS)     

Combined effects of hypoxia, hyperkalemia and acidosis on membrane action potential and excitability of guinea-pig ventricular muscle

The effects of hypoxia (with and without acidosis) on membrane action potentials and recovery kinetics of their upstroke velocity (Vmax) were studied in isolated guinea-pig papillary muscles at various extracellular K+ concentrations. At 5 mM [K+]0, hypoxia (hypoxic and glucose-free perfusate) at pH 7.4 caused a progressive shortening of action potential duration and a slight decrease in Vmax and resting potential. The recovery kinetics of Vmax assessed by premature stimuli were not affected by hypoxia. At high [K+]0 of 10 or 12 mM, hypoxia caused a marked decrease in Vmax, while the shortening of the action potential and the decrease in resting potential were similar to those at 5 mM [K+]0. However, the recovery kinetics of Vmax were markedly slowed by hypoxia. When hypoxia was added in the presence of mild acidosis (pH 6.8), the shortening of the action potential due to hypoxia was appreciably less. However, other hypoxia-induced changes in action potential and in recovery kinetics of Vmax under normal and high [K+]0 were not influenced by the concomitant acidosis. These results show that the depressant effect of hypoxia on the action potential upstroke and on the recovery of excitability of ventricular myocardium is increased when the muscles are partly depolarized at high K+. Slight differences in extracellular K+ in the presence of hypoxia have a marked effect on the time course of recovery of excitability. This inhomogeneity in refractoriness could be important for the occurrence of re-entrant arrhythmias in ischemic myocardium.     

Structural and electrophysiological changes in the epicardial border zone of canine myocardial infarcts during infarct healing

Structural and electrophysiological properties of the epicardial muscle which survives on the surface of transmural infarcts of the canine heart (epicardial border zone) were studied at different times after occlusion of the left anterior coronary artery (LAD). Isolated preparations were superfused in vitro, transmembrane potentials recorded, and impulse propagation mapped. In preparations from subacute infarcts (1 and 5 days), resting potential, action potential amplitude, upstroke velocity, and duration were all significantly reduced. Well-defined directional differences in propagation occurred. Propagation was more rapid in the direction perpendicular to the left anterior coronary artery than in the direction perpendicular to the base of the heart, because of the uniform anisotropic structure of the surviving muscle fibers which were arranged in tightly packed bundles oriented perpendicular to the left anterior coronary artery. The only ultrastructural abnormalities found in these muscle fibers was an accumulation of large amounts of lipid droplets. As the infarcts healed, resting potential, action potential amplitude, and upstroke velocity returned to normal by 2 weeks, although action potential duration decreased further. Lipid droplets had disappeared, and connective tissue had invaded the epicardial border zone, separating the muscle bundles. By 2 months, action potentials were normal, but the muscle fibers were widely separated and disoriented by the connective tissue (parallel bundles no longer were found). In these regions with a nonuniform anisotropic structure, the well-defined directional differences in impulse propagation were lost. However, activation was very slow, perhaps because of diminished connections between cells. The persistence of slow conduction in healed infarcts may contribute to the occurrence of chronic arrhythmias.     

Determinants of action potential duration in neonatal rat ventricle cells.

STUDY OBJECTIVE--The aim was to study the currents that determine the action potential duration in ventricular cells from neonatal rats. DESIGN--Microelectrode measurements of action potentials from ventricle strips were compared with action potentials obtained from isolated myocytes with the whole cell patch clamp method in current clamp mode. Ionic currents were studied in myocytes in voltage clamp mode using recognised modulators of channel activity. EXPERIMENTAL MATERIAL--Neonatal rats (2 d old) were decapitated and myocytes were prepared from the apical third of collagenase treated hearts. MEASUREMENTS AND MAIN RESULTS--Modification of the action potential by 1.8-5.0 mM Ca, 2.0 mM Co, 8 mM 4-aminopyridine, 1.8 mM Sr, and 20 mM tetraethylammonium suggested the presence of the slowly inactivating Ca current ICa,L, an early outward current Ieo, and at least one other K current. Action potentials from myocytes and ventricular strips were comparable. Voltage clamp experiments were confirmatory and revealed currents with the following properties: (1) ICa,L: a Ca current with a current density of 21.7 microA.cm-2, activated between -30 and -20 mV, saturated at 1.8 Cao, inactivated faster at 5 than at 1.8 mM Cao, more permeable to Ba and Sr than to Ca, and with Sr as charge carrier blocked by Ca; (2) Ieo: the peak current had a linear I/V relation between 0 and 70 mV and was abolished by 4 mM 4-aminopyridine; (3) IK1: the current was an inward rectifier that showed a relaxation at potentials negative to -90 mV. CONCLUSIONS--Action potentials obtained from neonatal rat ventricle with microelectrodes are comparable with those measured in myocytes in current clamp mode. The action potential duration is mainly determined by ICa,L, Ieo, and IK1, and there is no evidence for the presence of a delayed rectifier.     

Effects of increasing intercellular resistance on transverse and longitudinal propagation in sheep epicardial muscle

Propagation in cardiac muscle is faster in the longitudinal than in the transverse axis of the cells. Yet, as a result of the larger upstroke velocity of action potentials propagating transversely, it has been suggested that longitudinal propagation is more vulnerable to block. To study the relation between conduction velocity and maximal upstroke velocity (Vmax), as well as the time course of conduction delay and block in the transverse vs. longitudinal direction, thin square pieces of sheep epicardial muscle were superfused with the cellular uncoupler heptanol (1.5 mM). Action potentials were recorded with microelectrodes at opposite corners of the preparation while stimulating alternately in the longitudinal or transverse direction with bipolar electrodes located at contralateral corners. In all cases, block occurred more promptly for transverse than for longitudinal propagation. The decrease in conduction velocity was greater than expected for Vmax decay and, in some cases, Vmax increased while conduction velocity decreased. In the presence of high grade conduction impairment, foot potentials appeared and the upstrokes became "notched." We conclude that when intercellular coupling is impaired, transverse propagation is more vulnerable to block, and need not be dependent on changes in Vmax.     

Mechanism of increased amplitude and duration of the plateau with sudden shortening of diastolic intervals in rabbit ventricular cells

Action potentials and membrane currents were recorded from isolated single ventricular cells from rabbit hearts using the suction pipette whole-cell clamp method. Action potentials elicited after short diastolic intervals of less than 2 seconds showed an increase and prolongation of the plateau compared to those elicited after a 10-second rest period. The recovery of the tetrodotoxin-insensitive secondary inward current revealed a transient increase at short diastolic intervals above the level of full recovery (after 10 seconds). The increased secondary inward current recovery, however, was voltage-dependent, and the period of its increase did not cover the entire diastolic intervals of the action potential overshoots, suggesting the contribution of another ionic current to the changes in potential. During depolarizing voltage steps, from + to -20 mV, a rapid activating and then inactivating outward current was elicited, which overlapped the calcium current. This outward current exhibited time- and voltage-dependent properties similar to those of the transient outward current in Purkinje and other cardiac preparations. The recovery of the transient outward current was slow, achieving only 75% of its full level at 2 seconds, whereas the same level of calcium current recovery was achieved at 200 milliseconds. The application of 4-aminopyridine suppressed most of the transient outward current, and the rest of the current was abolished by caffeine or Co2+. The 4-aminopyridine sensitive transient outward current exhibited slow recovery kinetics compared to those of the other or calcium current, and its inhibition caused elimination of the augmented plateau during electrical restitution. The application of verapamil or Co2+ for inhibition of secondary inward current also abolished the action potential overshoot. These results indicate that an increase and prolongation of the plateau at short diastolic intervals are produced by the slower recovery from inactivation in the 4-aminopyridine-sensitive transient outward current than that in the calcium current.     

Age-related appearance of outward currents may contribute to developmental differences in ventricular repolarization.

Ventricular repolarization significantly influences contractility, refractoriness, and ion channel state. Factors affecting repolarization will thus affect these secondary phenomena. To understand the influence of age on ventricular repolarization, we studied neonatal, young, and adult dogs using electrocardiogram, action potential, and whole-cell voltage-clamp recordings from single epicardial myocytes. Hearts of neonatal and 57-58-day-old dogs require a significantly longer time for repolarization than those of adult dogs, as determined by analysis of rate-corrected QT and JT (QT minus QRS) intervals. Epicardial action potentials of neonates are significantly longer than those of adults, as determined by measurements of duration at 50% and 90% repolarization. The adult action potential is characterized by a large phase 1 notch that is absent from neonatal recordings. This notch develops between 58 and 64 days of age, and by 64-68 days of age, it is equal to that in adults. In addition, action potentials recorded from adult and young epicardial muscle are more greatly affected by rapid pacing and superfusion of 2 mM 4-aminopyridine than are potentials recorded from neonatal tissue. Whole-cell voltage-clamp recordings reveal a 4-aminopyridine-sensitive transient outward current in adult myocytes that is absent from neonatal myocytes. The correlation between developmental changes in the 4-aminopyridine-sensitive current, the action potential, and the QT interval suggests that the transient outward current may be an important determinant in the relation between age and repolarization.