Oscillations of cardiac wave length and proarrhythmia.

Drug-induced action potential duration (APD) prolongation was first proposed to be antiarrhythmic, but is now widely presumed to be torsadogenic. To elucidate this paradox, we tested the effect of APD upon liability for torsade de pointes. In addition, torsadogenicity is commonly associated with disturbances of repolarization, but at least in theory, it could also result from disturbances of conduction. These possibilities were tested in female rabbit hearts. Dofetilide, ATX II, and sodium channel blockers that did not prolong the action potential duration were used to modulate the APD and induce disturbances of conduction and disturbances of repolarization. Torsadogenicity could be induced by dofetilide and ATX II starting at normal APD (210 ms), reaching a peak incidence around a doubling of APD (400 to 450 ms), to then sharply decline with further APD prolongation, until torsade de pointes disappeared above 725 ms. Early afterdepolarizations (EAD) were regular triggers for torsade de pointes; while most of the EADs occurred in the plateau range, their incidence declined with repolarization but their potential for torsadogenicity increased. Sodium channel blockers that shorten the APD, even when devoid of hERG blocking properties, can yield torsade de pointes. Torsade de pointes can occur at normal, prolonged, and shortened APD, so that QT prolongation is an incomplete predictor of torsadogenicity. Torsade de pointes can result not only from disturbances of repolarization but also from disturbances of conduction.

Ultrafast sodium channel block by dietary fish oil prevents dofetilide-induced ventricular arrhythmias in rabbit hearts.

Several epidemiologic and clinical studies show that following myocardial infarction, dietary supplements of omega-3 polyunsaturated fatty acids (omega3FA) reduce sudden death. Animal data show that omega3FA have antiarrhythmic properties, but their mechanisms of action require further elucidation. The effects of omega3FA supplementation were studied in female rabbits to analyze whether their antiarrhythmic effects are due to a reduction of triangulation, reverse use-dependence, instability, and dispersion (TRIaD) of the cardiac action potential (TRIaD as a measure of proarrhythmic effects). In Langendorff-perfused hearts challenged by a selective rapidly activating delayed rectifier potassium current inhibitor that has been shown to exhibit proarrhythmic effects (dofetilide; 1 to 100 nM), omega3FA pretreatment (30 days; n=6) prolonged the plateau phase of the monophasic action potential; did not slow the terminal fast repolarization; reduced the dofetilide-induced prolongation of the action potential duration; reduced dofetilide-induced triangulation; and reduced dofetilide-induced reverse use-dependence, instability of repolarization, and dispersion. Dofetilide reduced excitability in omega3FA-pretreated hearts but not in control hearts. Whereas torsades de pointes (TdP) were observed in five out of six in control hearts, none were observed in omega3FA-pretreated hearts. Docosahexaenoic acid (DHA) inhibited the sodium current with ultrafast kinetics. Dietary omega3FA supplementation markedly reduced dofetilide-induced TRIaD and abolished dofetilide-induced TdP. Ultrafast sodium channel block by DHA may account for the antiarrhythmic protection of the dietary supplements of omega3FA against dofetilide-induced proarrhythmia observed in this animal model.

Phase 2 prolongation, in the absence of instability and triangulation, antagonizes class III proarrhythmia.

OBJECTIVE: To evaluate whether prolongation of the plateau of the action potential duration, in the absence of instability and triangulation, can reverse the proarrhythmia elicited by a class III antiarrhythmic agent. METHODS: The effects of almokalant, erythromycin and their combination, on cardiac electrophysiological parameters (action potential duration (APD), instability, triangulation and ectopics) were evaluated in isolated hearts from female albino rabbits. In this study, proarrhythmia was estimated quantitatively by number of ectopic beats. RESULTS: Erythromycin lengthened the APD primarily by a prolongation of the plateau, while having only minor effects upon phase 3 repolarization. The prolongation did not induce much instability, triangulation or reverse use dependence and, as expected, erythromycin did not induce significant proarrhythmia. Almokalant also lengthened APD, but it did not lengthen the plateau; instead, it prolonged phase 3 repolarization. The prolongation markedly triangulated the action potential, elicited much instability and marked reverse use dependence. This combination of effects induced very marked proarrhythmia. When almokalant and erythromycin were combined, their effects upon APD appeared additive: both the plateau and the repolarization phase were prolonged. However, the larger prolongation of APD did not lead to more proarrhythmia; this suggests that a prolongation of APD is not proarrhythmic per se. On the contrary, proarrhythmia as a function of APD prolongation was reduced in the presence of erythromycin (P<0.05). CONCLUSION: Instability plus triangulation consistently lead to serious proarrhythmia especially when combined with reverse use dependence, but prolongation of APD in itself is not necessarily proarrhythmic. In fact, APD prolongation in the absence of instability and triangulation can be antiarrhythmic.

Instability and triangulation of the action potential predict serious proarrhythmia, but action potential duration prolongation is antiarrhythmic.

BACKGROUND: Prolongation of action potential duration (APD) is considered a major antiarrhythmic mechanism (class 2I), but paradoxically, it frequently is also proarrhythmic (torsade de pointes). METHODS AND RESULTS: The cardiac electrophysiological effects of 702 chemicals (class 2I or HERG channel block) were studied in 1071 rabbit Langendorff-perfused hearts. Temporal instability of APD, triangulation (duration of phase 3 repolarization), reverse use-dependence, and induction of ectopic beats were measured. Instability, triangulation, and reverse use-dependence were found to be important determinants of proarrhythmia. Agents that lengthened the APD by >50 ms, with induction of instability, triangulation, and reverse use-dependence (n=59), induced proarrhythmia (primarily polymorphic ventricular tachycardia); in their absence (n=19), the same prolongation of APD induced no proarrhythmia but significant antiarrhythmia (P<0.001). Shortening of APD, when accompanied by instability and triangulation, was also markedly proarrhythmic (primarily monomorphic ventricular tachycardia). In experiments in which instability and triangulation were present, proarrhythmia declined with prolongation of APD, but this effect was not large enough to become antiarrhythmic. Only with agents without instability did prolongation of APD become antiarrhythmic. For 20 selected compounds, it was shown that instability of APD and triangulation observed in vitro were strong predictors of in vivo proarrhythmia (torsade de pointes). CONCLUSIONS: Lengthening of APD without instability or triangulation is not proarrhythmic but rather antiarrhythmic.

Stereoselective block of cardiac sodium channels by bupivacaine in guinea pig ventricular myocytes.

BACKGROUND: Bupivacaine is a potent local anesthetic widely used for prolonged local and regional anesthesia. However, accidental intravascular injection of bupivacaine can produce severe arrhythmias and cardiac depression. Although used clinically as a racemic mixture, S(-)-bupivacaine appears less toxic than the R(+)-enantiomer despite at least equal potency for local anesthesia. If the R(+)-enantiomer is more potent in blocking cardiac sodium channels, then the S(-)-enantiomer could be used with less chance of cardiovascular toxicity. Therefore, we tested whether such stereoselectivity existed in the bupivacaine affinity for the cardiac sodium channel. METHODS AND RESULTS: The inhibitory effects on the cardiac sodium current (INa) of 10 mumol/L R(+)- and S(-)-bupivacaine were investigated by use of the whole-cell voltage clamp technique in isolated guinea pig ventricular myocytes. Both enantiomers produced similar but limited levels of tonic block (6% and 8%). During long depolarizations (5 seconds at 0 mV), R(+)-bupivacaine induced a significantly larger inhibition of INa: 72 +/- 2% versus 58 +/- 3% for the S(-)-enantiomer (P < .01). Development of block was slow, but its rate was faster for R(+)-bupivacaine [time constant, 1.84 +/- 0.16 versus 2.56 +/- 0.26 seconds for the S(-)-enantiomer, P < .05]. The voltage dependence of the availability of the Na+ current was shifted to more hyperpolarizing potentials compared with the control; R(+)-bupivacaine induced a larger shift than S(-)-bupivacaine (37 +/- 2 versus 30 +/- 2 mV, P < .05). These data indicate stereoselective interactions with the inactivated state. In addition, both enantiomers induced substantial use-dependent block during 2.5-Hz pulse trains with medium (100-ms) and short (10-ms) depolarizations but without stereoselective difference. A stepwise approach was used to model these experimental results and to derive apparent affinities and rate constants. We initially assumed that bupivacaine interacted only with the rested and inactivated states of the Na+ channel. The apparent affinities of the inactivated state for S(-)- and R(+)-bupivacaine were 4.8 and 2.9 mumol/L, respectively. With the derived binding and unbinding rate constants, this model reproduced the stereoselective block during long depolarizations but failed to predict the use-dependent block induced by trains of short (10-ms) depolarizations. To account for the observed use-dependent interactions, it was necessary to include interactions with the activated state, which resulted in adequate reproduction of the experimental results. The apparent affinities of the activated or open state for S(-)- and R(+)-bupivacaine were 4.3 and 3.3 mumol/L, respectively. CONCLUSIONS: Both the large level of pulse-dependent block and the failure of the pure inactivated-state block model indicate that bupivacaine interacts with the activated (or open) state of the cardiac sodium channel in addition to its block of the inactivated state. The bupivacaine-induced block of the inactivated state of the Na+ channel displayed stereoselectivity, with R(+)-bupivacaine interacting faster and more potently. Both enantiomers also bind with high affinity to the activated or open state of the channel, but this interaction did not display stereoselectivity, although the binding to the activated or open state was faster for S(-)- than for R(+)-bupivacaine. The higher potency of R(+)-bupivacaine to block the inactivated state of the cardiac Na+ channel may explain its higher toxicity because of the large contribution of the inactivated-state block during the plateau phase of the cardiac action potential. These results would support the use of the S(-)-enantiomer to reduce cardiac toxicity.

Computer aided development of antiarrhythmic agents with class IIIa properties.

Most antiarrhythmic agents were discovered accidentally. In the last decade, the understanding of the mechanisms of action of agents with electrophysiologic activity has progressed greatly. As a result, it was possible to compute, before the CAST trial, that the agents selected for the trial would not be effective against tachycardias and that the drugs would be unsafe. Extension of these computations to existing Class I agents indicated that they were all poor suppressors of ventricular tachycardia. Furthermore, a Class I agent with an optimal electrophysiologic profile still computes to be a two-edged sword, possessing both antiarrhythmic and proarrhythmic properties. Fortunately, it is possible to conceive of drug profiles that would be purer antiarrhythmic agents. For example, a drug that only upon the development of a tachycardia lengthens action potential duration in a use-dependent manner until the refractory period exceeds the tachycardia cycle length will render continuation of the tachycardia impossible. Recognition of chemicals that have Class IIIa properties with the appropriate kinetics is a challenging task. However, today's microprocessors have become powerful enough to characterize the Class III kinetics. A system that fully automatically screens for effective antiarrhythmic agents is described. It is expected that chemicals selected for optimal basic electrophysiologic properties will yield safer and more effective antiarrhythmic agents.

Development of class III antiarrhythmic agents.

Currently available antiarrhythmic agents that act by lengthening the action potential duration and refractory period exert most of their effect during bradycardia (class IIIB). Unfortunately, when these agents are most needed, i.e., during tachycardia, they lose most of their effect. In my opinion, ideally, antiarrhythmic agents need not change the electrophysiology of the normal sinus beat, but should lengthen refractoriness upon acceleration (class IIIA) of the heart until the refractory period exceeds the cycle length of the tachycardia. Three examples of how such class IIIA effects can be achieved are: (i) an upstroke-dependent activator of inward currents could form a universal class IIIA agent, (ii) a use-dependent blocker of iTO could be a great class IIIA agent in the atria, and (iii) a frequency-dependent blocker of late repolarizing currents (e.g., iKs) could be a good target for a ventricular agent.

Suppression of time-dependent outward current in guinea pig ventricular myocytes. Actions of quinidine and amiodarone.

Prolongation of cardiac action potentials may mediate some of the arrhythmia-suppressing and arrhythmia-aggravating actions of antiarrhythmic agents. In this study, suppression of time-dependent outward current by quinidine and amiodarone was assessed in guinea pig ventricular myocytes. The net time-dependent outward current contained at least two components: a slowly activating, La(3+)-resistant delayed rectifier current (IK) and a rapidly activating, La(3+)-sensitive current. Quinidine block of total time-dependent outward current during clamp steps to positive potentials was relieved as a function of time, whereas that induced by amiodarone was enhanced. In contrast, at negative potentials, suppression of current, whereas amiodarone reduced IK but not the La(3+)-sensitive current, suggesting that differential block of the two components of time-dependent current underlies the distinct effects of the two agents. In contrast to these disparate effects on total time-dependent outward current, steady-state reduction of IK by both drugs increased at positive voltages and saturated at approximately +40 mV; the voltage dependence of block by quinidine (17% per decade, +10 to +30 mV) was steeper than that by amiodarone (5% per decade, +10 to +20 mV). Block by quinidine was time dependent at negative potentials: on stepping from +50 to -30 mV, block initially increased very rapidly, and subsequent deactivation of IK was slowed. This effect was not seen with amiodarone. At -80 mV, quinidine block was relieved with a time constant of 40 +/- 15 msec (n = 4, twin-pulse protocol). The effects of quinidine on IK were compatible with neither a purely voltage-dependent model of quinidine binding nor a model incorporating both voltage- and state-dependent binding of quinidine to delayed rectifier channels having only one open state. The voltage- and time-dependent features of quinidine block were well described by a model in which quinidine has greater affinity for one of two open states of the channel. We conclude that the effects of quinidine and amiodarone on time-dependent outward current reflects block of multiple channels. Quinidine block of IK was far more voltage dependent than that produced by amiodarone, suggesting the drugs act by different mechanisms.

Voltage clamp of the cardiac sodium current at 37 degrees C in physiologic solutions.

The cardiac sodium current was studied in guinea pig ventricular myocytes using the cell-attached patch voltage clamp at 37 degrees C in the presence of 145 mM external sodium concentration. When using large patch pipettes (access resistance, 1-2 M omega), the capacity current transient duration was typically 70 microseconds for voltage clamp steps up to 150 mV. At 37 degrees C the maximum inward sodium current peaked in approximately 200 microseconds after the onset of a clamp step and at this strong depolarization, less than 10% of the sodium current developed during the capacity transient. The sodium current developed smoothly and the descending limb of the current-voltage relationship usually spanned a range of 40 mV. Moreover, currents reduced by inactivation of sodium channels could be scaled to superimpose on the maximum current. Current tails elicited by deactivation followed a monoexponential time course that was very similar for currents of different sizes. Data obtained over a range of temperatures (15 degrees-35 degrees C) showed that the steady-state inactivation and conductance-voltage curves were shifted to more negative voltages at lower temperatures. These results demonstrate the feasibility of investigating the sodium current of mammalian cardiac cells at 37 degrees C in normal physiological solutions.

Interactions of flecainide with guinea pig cardiac sodium channels. Importance of activation unblocking to the voltage dependence of recovery.

Effects of flecainide, a potent antiarrhythmic agent, on sodium channel availability was investigated in guinea pig single cardiac cells by the whole-cell voltage-clamp technique. Sodium current (INa) experiments were performed at 17 degrees C, and maximum upstroke velocity (Vmax) experiments were performed at 37 degrees C. Flecainide (3 microM) caused little tonic block, but reduced sodium channel availability in a use-dependent manner. The latter effect was accentuated by depolarization and attenuated by hyperpolarization. Long (200-msec) and short (10-msec) depolarizations yielded similar use-dependent block. These results indicate that flecainide has a low affinity for rested (R) and inactivated (I) channels but a high affinity for activated ones (A). In each of these states, the channels can bind to drug to form the corresponding RD, ID, and AD states. Recovery from flecainide block consisted of two components. An initial fast component was strongly voltage dependent: with increasing hyperpolarization, recovery developed more quickly and to a larger extent. At 17 degrees C, the mean time constant shortened from 132 +/- 81.6 msec at -120 mV to 46.9 +/- 34.1 msec at +/- 160 mV (kinetics were too fast for accurate measurement at 37 degrees C). A later slow component was largely voltage independent: at 37 degrees C, the mean time constant was 9.8 +/- 3.2 seconds at -100 mV and 10.7 +/- 3.8 seconds at -75 mV. The slow component of recovery was similarly independent of voltage at 17 degrees C. In terms of the modulated-receptor theory, our results indicate that the fast recovery depends on availability for unblocking (RD) but occurs during activation (AD----A). Indeed, when the RD state is maximized by strong hyperpolarization, activation unblock was also maximized. However, during depolarization to -100 mV, availability for activation unblock declined with a time constant of 98 +/- 12 msec (RD----ID). Therefore, the voltage-dependent fast unblocking is mostly due to priming of the RD state (RD----ID), and the voltage-independent slow unblock reflects dissociation of flecainide from closed states (RD----R and ID----I). We conclude that flecainide interacts with sodium channels preferentially in the activated state, whereas unblocking occurs via two separate pathways: activated and closed states. Furthermore, drug association with channels shifts the voltage dependence of closed-state transitions (RD in equilibrium ID) and their kinetics toward more negative potentials.

Stretch-induced arrhythmias in the isolated canine ventricle. Evidence for the importance of mechanoelectrical feedback.

Alterations in loading conditions and muscle length influence the electrophysiology of ventricular myocardium and may play a role in arrhythmogenesis in globally dilated or dyskinetic ventricles. To test the hypothesis that stretch can initiate arrhythmias in normal myocardium, the response to graded mechanical stretch was studied in seven isolated blood-perfused canine ventricles. After eight conditioning contractions produced by His bundle pacing (2 Hz), global stretch of the ventricle was produced by a servocontrolled pump that abruptly increased ventricular volume by a precise amount (delta V) during early diastole and then returned ventricular volume to the initial holding volume (Vi). Ventricular premature contractions were readily produced; ventricular couplets and short runs of ventricular tachycardia were occasionally elicited. The probability of a stretch-induced arrhythmia was determined from multiple alternating sequences in which a stretch of known amplitude (delta V) or no stretch was delivered. As delta V was increased, the probability of a stretch-induced arrhythmia was low initially, increased sharply after a threshold was exceeded, and approaching 100% with physiological volumes. With Vi set to a standard value of 20 ml, corresponding to end-diastolic pressure of 5.3 +/- 5.2 mm Hg (mean +/- SD), the delta V resulting in a 50% chance of a stretch-induced arrhythmia (delta V50) was 15.0 +/- 1.6 ml. A decline in delta V50 was consistently observed when Vi was increased. While delta V50 values were remarkably similar (10.7% coefficient of variation), the pressure at the time the ventricular premature depolarization was triggered was highly variable for different ventricles; this finding suggests that myocardial strain is more important than absolute level of wall stress in the initiation of these arrhythmias. These results demonstrate that myocardial stretch predictably initiates arrhythmias and that the susceptibility to stretch-induced arrhythmias is enhanced by ventricular dilatation. Thus, ventricular ectopy in patients with regionally or globally dilated hearts may arise, in part, by a mechanism of myocardial stretch.

Effects of quinidine on the sodium current of guinea pig ventricular myocytes. Evidence for a drug-associated rested state with altered kinetics.

In guinea pig cardiac myocytes quinidine (20 microM) caused less than 10% tonic block reduction of the sodium current at -120 mV, but a fast pulse train reduced it more than 90%. Recovery from use-dependent block was time and voltage dependent, and was always slow (tau = 34 +/- 10 seconds at -160 mV; tau = 90 +/- 35 seconds at -120 mV; n = 15, mean +/- SD, p less than 0.001, paired t test). However, in association with repeated activation a fast component of recovery from block was observed: use-dependent unblocking. Availability of sodium channels for use-dependent unblocking was enhanced by hyperpolarization until a plateau was reached near -160 mV. Compared with the availability of drug-free sodium channels (h-curve), the voltage dependence of availability for use-dependent unblocking (h'-curve) was shifted by about 30 mV to more negative potentials, and its slope was reduced 2.5-fold. At -160 mV, the kinetics of development of availability of sodium channels for use-dependent unblocking were rapid (tau less than 10 msec). Depolarization to -120 mV reduced the availability of sodium channels for fast unblocking with a time constant of 191 +/- 46 msec (n = 14). Finally, block established by frequent brief depolarizations (activations) declined during prolonged inactivation. From these results we concluded that the time and voltage dependence of the availability of sodium channels for unblocking are considerably different from the availability for activation of drug-free channels, that rested drug-associated channels do exist, and that drug-associated channels do not conduct (or at least have a greatly reduced conductance) upon activation unless they first unblock. Furthermore, activated and inactivated channels have a different affinity for quinidine, and since quinidine can occupy the channel receptor even when "guarded," our results are incompatible with the guarded receptor hypothesis but can be explained within the framework of the modulated receptor hypothesis.

Class III antiarrhythmic agents have a lot of potential but a long way to go. Reduced effectiveness and dangers of reverse use dependence.

With regard to currently available class III agents, although their class III effect may reduce the likelihood of tachycardia initiation, their reverse use-dependent prolongation of action potential duration reduces their effectiveness during tachycardias and may even render them proarrhythmic, especially after long diastolic intervals. In contrast, agents that exhibit normal use-dependent prolongation of refractoriness hold great promise: While having relatively less effects on the normal heart beat, they could induce self-termination of a tachycardia. Prolongation of refractoriness can be achieved by lengthening of action potential duration and delaying recovery of excitability. Combination of these drug actions may yield important clinical applications.

Temperature and voltage dependence of sodium channel blocking and unblocking by O-demethyl encainide in isolated guinea pig myocytes.

O-demethyl encainide (ODE) is a metabolite of encainide with potent antiarrhythmic effects. We studied block of sodium channels by ODE in isolated guinea pig ventricular myocytes, using both direct measurement of whole cell sodium current and Vmax. Specifically, we examined whether block by ODE was use-dependent, whether ODE blocked activated or inactivated channels, and whether ODE's effect depended on membrane potential. At cold temperatures (15-20 degrees C), ODE was a potent blocker of activated cardiac sodium channels, with little or no effect on inactivated channels. At these temperatures, there was no detectable diastolic recovery from block. Repeated depolarizations produced use-dependent unblocking, which increased as the holding potential was made more negative. At warmer temperatures, use-dependent unblocking was increased, the availability curve of sodium channels for use-dependent unblocking was less shifted toward negative potentials, and diastolic recovery from block became detectable, albeit slow (tau approximately 25 s). Our findings demonstrate that electrophysiologic effects of agents such as ODE may be both quantitatively and qualitatively different at cold temperatures than at normal body temperature. One should be cautious about extrapolating electrophysiologic data obtained at cold temperatures to clinical situations.

Evidence for voltage-dependent block of cardiac sodium channels by tetrodotoxin.

The effects of tetrodotoxin (TTX) on cardiac sodium channels in guinea-pig ventricular muscle were investigated. Membrane potential was controlled using a single sucrose gap voltage clamp method, and the maximum upstroke velocity of the ventricular action potential (Vmax) was used as an indicator of drug-free sodium channels. Reduction of Vmax by TTX was found to be both voltage- and time-dependent, similar to the effects of many local anesthetic drugs, with the exception that TTX concentrations high enough to produce significant use-dependent block (e.g. 2 microM), also produced significant tonic block, even at potentials negative to -85 mV. The mechanism underlying use-dependent block was determined by defining the time course of block development at potentials between -40 and +20 mV, and the time course of recovery at -85 mV. In 2 microM TTX, the time course of block development at +20 mV contained two phases, a fast phase (tau less than 3 ms) having a mean amplitude of 8.1 +/- 3.2% of control Vmax, and a slow phase (tau = 429 +/- 43 ms) having an amplitude of 35 +/- 2% of control Vmax (n = 5). Recovery from use-dependent block at -85 mV occurred with a time constant of 324 +/- 58 ms (n = 5). The effects of TTX could be well-described by a modulated receptor model with an estimated 12 mV drug-induced shift of inactivation, and state-dependent dissociation constants of 10, 4 and 0.3 microM for rested, activated and inactivated channels. These same drug rate constants could also be used to adequately simulate the reported effects of TTX on plateau sodium currents in a variant model with slow inactivation kinetics.

Evidence for two components of sodium channel block by lidocaine in isolated cardiac myocytes.

The effects of lidocaine on sodium current in cardiac myocytes isolated from cat and guinea pig were investigated using the whole-cell variation of the patch-clamp technique. Lidocaine (43-200 microM) reduced sodium current during repetitive depolarizing pulses in a use-dependent manner. To clarify the nature of the use-dependent block, we characterized the time course of block development using a two-pulse protocol. Two distinct phases of block development were found: a rapid phase (tau = 1-6 msec) having a time course concurrent with the time course of channel activation, and a slower phase (tau = 100-900 msec), which developed after channels inactivated. The amplitude of the block during the rapid phase of development was a steep function of transmembrane voltage over the range of -70 to +20 mV. The voltage-dependence was similar to that for sodium channel activation (sodium conductance) but was too steep to be attributed solely to the passive movement of a singly charged molecule under the influence of the transmembrane voltage gradient. These results suggest that use-dependent block of sodium channels in cardiac tissue may result from an interaction of lidocaine with sodium channels in the activated as well as the inactivated channel states. Possible mechanisms underlying the fast component of block are discussed.

Competition between lidocaine and one of its metabolites, glycylxylidide, for cardiac sodium channels.

The modulated receptor hypothesis states that sodium channels have a specific receptor for antiarrhythmic drugs. Therefore, two agents that block sodium channels by binding to this receptor are expected to compete for occupancy. Glycylxylidide (GX) is a deethylated metabolite of lidocaine that accumulates in patients on lidocaine therapy. In single, voltage-clamped cardiocytes, GX, like lidocaine, blocked cardiac sodium channels in a use-dependent manner. However, its kinetics of recovery from block were markedly different from lidocaine: at potentials between -80 and -100 mV, GX-blocked channels recovered faster and more completely than lidocaine-blocked channels but recovered more slowly at more negative potentials (-120 to -140 mV). If lidocaine and GX compete for a common receptor, then there are conditions in which addition of a "faster" drug to a "slower" drug will produce less block than the slower drug alone. At potentials between -120 and -140 mV, addition of GX (slower drug) to lidocaine always increased the level of block, but addition of lidocaine to GX decreased the block in four of nine experiments and did not increase it in three of nine experiments. Conversely, at potentials between -80 and -100 mV, addition of lidocaine (slower drug) to GX always increased block, whereas addition of GX to lidocaine reduced the level of block in five of 16 experiments and did not increase it in seven of 16 experiments. Thus, upon addition of more blocker, the sodium current increased in 36% of cases or did not decline in 76% of cases. These results can be explained by the modulated receptor hypothesis with two drugs competing for the same receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Quinidine delays IK activation in guinea pig ventricular myocytes.

A major action of the antiarrhythmic agent quinidine is prolongation of cardiac repolarization. In these experiments, the time-dependent effects of quinidine on the delayed rectifier potassium current, IK, a current contributing to cardiac repolarization, were investigated in acutely disaggregated guinea pig ventricular myocytes using the whole-cell recording configuration of the patch-clamp method. The effect of quinidine on IK was dependent on the duration of depolarization. After long (2,000 msec) pulses, IK was reduced by 30 +/- 27% (SD; n = 8, paired) by 10 microM quinidine; in contrast, after short (100 msec) pulses, the drug decreased IK 65 +/- 35% (p less than 0.05). This effect was found both in paired experiments as well as when quinidine-pretreated cells were compared to non-pretreated cells. Quinidine significantly delayed IK activation (9 +/- 20 msec at baseline vs. 44 +/- 25 msec in drug, p less than 0.05), but did not alter the subsequent time course of activation (time constant 659 +/- 118 msec). These findings are consistent with the hypothesis that quinidine promotes occupancy of a channel state from which opening does not occur.