Comparative effects of sodium bicarbonate and sodium chloride on reversing cocaine-induced changes in the electrocardiogram

Cocaine abuse is associated with a number of cardiovascular complications that include arrhythmias and sudden cardiac death. Although the mechanism(s) remain unclear, cocaine-induced block of sodium channels resulting in slowed cardiac conduction is thought to play an important role. Several reports suggest that the effects of cocaine effects on cardiac sodium channels can be reversed by administration of sodium bicarbonate. Whether the beneficial effects of sodium bicarbonate are due to sodium ions or an increase in blood pH is unknown. Therefore the purpose of this study was to compare the effects of sodium loading alone (by using sodium chloride) versus sodium loading with an associated increase in arterial pH (by using sodium bicarbonate) on reversing cocaine-induced effects on the electrocardiogram (ECG) in a canine model. Seventeen anesthetized dogs received three i.v. injections of cocaine, 5 mg/kg, with each dose separated by 15 min. Two minutes after the third cocaine dose, each dog was randomly assigned to receive 2 mEq/kg i.v. sodium bicarbonate (1 mEq/ml) or 2 mEq/kg i.v. sodium chloride (1 mEq/ml). ECG, electrophysiologic, and hemodynamic data were recorded at baseline, after each cocaine injection, and after administration of sodium bicarbonate or sodium chloride. In both groups of animals, the first cocaine injection significantly (p < 0.05) prolonged the PR, QTc, AH, and HV intervals, and QRS duration compared with baseline. All intervals continued to lengthen in a dose-dependent manner after the second and third cocaine doses. Sodium bicarbonate significantly (p < 0.05) reduced cocaine-induced prolongation of PR [(147 +/- 5-130 +/- 5 ms), AH (81 +/- 6 - 72 +/- 6 ms), and HV intervals (55 +/- 2 - 39 +/- 1 ms). and QRS duration (96 +/- 6 - 66 +/- 4 ms), peak effect after third cocaine dose versus after sodium bicarbonate, respectively]. Sodium chloride had no effect on reversing cocaine-induced effects on the ECG. Cocaine produces dose-dependent slowing of cardiac conduction that is effectively reversed by sodium bicarbonate. The lack of efficacy of sodium chloride suggests that the increase in arterial pH associated with sodium bicarbonate is responsible for reversal of the effects of cocaine on the ECG. Therefore sodium bicarbonate may be clinically useful in the treatment of cocaine-induced cardiac arrhythmias, primarily as a result of its effects on arterial pH.     

Reversal of the Electrocardiographic Effects of Cocaine by Lidocaine. Part 1. Comparison With Sodium Bicarbonate and Quinidine

Based on modulated receptor concepts, an agent with fast on-off sodium channel binding properties (e.g., lidocaine) may reverse the effects of a drug with slow on-off kinetics (e.g., cocaine) through competition for a single receptor site on the sodium channel. We compared the effects of two drugs with different sodium channel-binding kinetics with those of sodium bicarbonate, a known antidote, on cocaine-induced slowing of ventricular conduction. Electrocardiographic (ECG) intervals were recorded before and after the addition of cocaine 30 μM in 26 isolated, Tyrode-perfused guinea pig hearts. The effects of the three potential antidotes were then analyzed: equimolar lidocaine (8 hearts), equimolar quinidine (6), and sodium bicarbonate (8). Cocaine significantly increased all ECG intervals. The addition of lidocaine to cocaine-containing perfusate decreased QRS duration from 42 ± 3 to 29 ± 3 msec (p<0.01), a 60% reversal. Addition of sodium bicarbonate to increase the pH of the perfusate from 7.37 ± 0.09 to 7.52 ± 0.08 (p<0.01) decreased the QRS duration from 38 ± 4 to 30 ± 6 msec (p<0.01), a 47% reversal. Addition of quinidine 30 pM augmented the effects of cocaine: QRS increased from 40 ± 6 msec to 54 ± 9 msec (p<0.01). Consistent with modulated receptor concepts, lidocaine reverses slowed ventricular conduction due to cocaine. The magnitude of this reversal is similar to that due to sodium bicarbonate. The potential of fast on-off agents to serve as antidotes for cocaine-induced arrhythmias requires further study.     

Stereospecific interaction of tocainide with the cardiac sodium channel

The antiarrhythmic action of type I antiarrhythmic drugs may be mediated via binding of the drugs to a receptor associated with the cardiac sodium channel. This suggested that the effects of type I drugs might be stereospecific. We measured the effect of the tocainide stereoisomers (which have stereospecific antiarrhythmic effects) on conduction time and on radioligand binding to the cardiac sodium channel. The concentration-dependent effects of the individual enantiomers of tocainide on interventricular conduction time measured during constant rate ventricular pacing at 350 msec were assessed in 47 isolated perfused rabbit heart preparations. Significant increases (p less than 0.05) in conduction time occurred for both R-(-)-tocainide (75 microM, 10 +/- 5 msec) and S-(+)-tocainide (150 microM, 4 +/- 1 msec). R-(-)-Tocainide was more potent than the S-(+)-tocainide in prolonging conduction time (p less than 0.05). This stereospecific prolongation of conduction time suggested a stereospecific interaction with the sodium channel. The affinities of the enantiomers for the channel were measured with a radioligand binding assay using [3H]batrachotoxinin benzoate and freshly isolated cardiac myocytes. Both enantiomers inhibited [3H]batrachotoxin benzoate binding, but the IC50 (+/- SD) values were different: R-(-)-tocainide 184 +/- 8 microM; S-(+)-tocainide, 546 +/- 37 microM (p less than 0.003). Tocainide isomers are stereospecific in terms of prolonging conduction time and in binding to the sodium channel. The stereospecific electrophysiologic effects of tocainide may result from binding to a receptor associated with the cardiac sodium channel.     

Cocaine blocks HERG, but not KvLQT1+minK, potassium channels

Cocaine causes cardiac arrhythmias, sudden death, and occasionally long QT syndrome in humans. We investigated the effect of cocaine on the human K(+) channels HERG and KvLQT1+minK that encode native rapidly (I(Kr)) and slowly (I(Ks)) activating delayed rectifier K(+) channels in the heart. HERG and KvLQT1+minK channels were heterologously expressed in human embryonic kidney 293 cells, and whole-cell currents were recorded. Cocaine had no effect on KvLQT1+minK current in concentrations up to 200 microM. In contrast, cocaine reversibly blocked HERG current with half-maximal block of peak tail current of 7.2 microM. By using a protocol to quickly activate HERG channels, we found that cocaine block developed rapidly after channel activation. At 0 mV, the time constants for the development of block were 38.2 +/- 2.1, 15.2 +/- 0.8, and 6.9 +/- 1.1 ms in 10, 50 and 200 microM cocaine, respectively. Cocaine-blocked channels also recovered rapidly from block after repolarization. At -100 mV, recovery from block followed a biphasic time course with fast and slow time constants of 3.5 +/- 0.7 and 100.3 +/- 15.4 ms, respectively. Using N-methyl-cocaine, a permanently charged, membrane-impermeable cocaine analog, block of HERG channels rapidly developed when the drug was applied intracellularly through the patch pipette, suggesting that the cocaine binding site on the HERG protein is located on a cytoplasmic accessible domain. These results indicate that cocaine suppresses HERG, but not KvLQT1+minK, channels by preferentially blocking activated channels, that it unblocks upon repolarization, and does so with unique ultrarapid kinetics. Because the cocaine concentration range we studied is achieved in humans, HERG block may provide an additional mechanism for cocaine-induced arrhythmias and sudden death.     

Terfenadine block of sodium current in canine atrial myocytes

Terfenadine, a histamine-1 receptor antagonist, is known to have direct effects on electrical activities in the heart. Studies have demonstrated an ability of terfenadine to suppress upstroke velocity of action potential, an indication of sodium channel blockade. To clarify whether terfenadine indeed blocks sodium current (I(Na)), we performed experiments to evaluate in detail the effects of terfenadine on I(Na) by applying whole-cell patch-clamp techniques to canine atrial myocytes. Terfenadine produced concentration-dependent inhibition of I(Na), with a median inhibitory concentration (IC50) of 0.93+/-0.12 microM. Significant effects were observed at a concentration of as low as 100 nM (approximately 15% reduction of I(Na)). The effects of terfenadine on I(Na) were voltage dependent, as indicated with greater inhibition at less-negative holding potentials and at more-positive test potentials. Terfenadine blockade of I(Na) was characterized by an important tonic block that accounted for approximately 50% of the total block. Use-dependent block also was observed and found to contribute to 26% of the total block, and this use dependence was accentuated with longer pulse duration. Our findings suggest that terfenadine is a potent sodium channel blocker. Terfenadine blocks I(Na) in both rested state and inactivated state of the channels, but preferentially interacts with the former. The I(Na)-blocking property of terfenadine may contribute to its cardiac side effects in patients.     

Slowing of the inactivation of voltage-dependent sodium channels by staurosporine, the protein kinase C inhibitor, in rabbit atrial myocytes

In this study, the effect of staurosporine, a potent protein kinase C (PKC) inhibitor, on Na+ current (I(Na)) was examined by whole-cell patch recording in rabbit atrial myocytes. The most prominent staurosporine effect was a slowing of I(Na) inactivation and 1 microM staurosporine reduced amplitude of I(Na) about 33%. Staurosporine decreased I(Na) at all potentials and slowed the I(Na) inactivation in a dose-dependent manner, with a Kd value of 1.107+/-0.162 microM. Staurosporine did not change the recovery kinetics and show use dependence. However, the activation and the steady-state inactivation curves were shifted toward more negative potentials (-5.5 and -5.1 mV, respectively). Two other PKC inhibitors, GF 109203X (1 microM) and chelerythrine (3 microM), did not show a slowing effect on I(Na) inactivation. In conclusion, our results indicate that the slowing of I(Na) inactivation by staurosporine seems not to be through blockade of PKC rather to act directly on the Na+ channels, and the direct blocking effects of staurosporine on the Na+ channel should be taken into consideration when staurosporine is used in functional studies of ion channel modulation by protein phosphorylation.     

Cocaine-induced changes in perfusion pressure and bile flow in perfused rat livers

Cocaine produces hepatotoxicity. To study the acute effect of cocaine on the liver, we used the isolated, single-pass perfused rat liver. When perfusion pressure was measured in a constant flow system, a 15-min infusion of cocaine (1.47 mM) increased perfusion pressure (136 +/- 15%), decreased bile flow (61 +/- 5%), and decreased oxygen uptake (82 +/- 5%). The vasoconstriction was concentration-dependent and reversible. The pressure increase elicited by cocaine was not inhibited by the alpha-receptor antagonists phentolamine, prazosin, or yohimbine. These antagonists did inhibit phenylephrine-induced increases in perfusion pressure. Neither serotonin at concentrations up to 1 mM nor lidocaine or procaine in concentrations equimolar to cocaine increased the perfusion pressure. Indomethacin (5 microM), SKF-525A, and chloramphenicol also failed to block vasoconstriction induced by cocaine. High concentrations of cocaine were cholestatic, while concentrations lower than 0.6 mM were choleretic. These results indicate that cocaine-induced vasoconstriction in the liver is not mediated by alpha-receptor activation or prostaglandins and does not require metabolic activation of cocaine. The acute effects of cocaine in the perfused liver are vascular (vasoconstriction) and functional (alteration in bile formation).     

The antidotal effect of alpha(1)-acid glycoprotein on amitriptyline toxicity in cardiac myocytes.

Tricyclic antidepressants in overdose cause toxicity marked by prolongation of the QRS interval of the electrocardiogram. These drugs are bound to alpha(1)-acid glycoprotein (AAG) with high affinity in plasma. Animal studies have shown that the administration of AAG shortens the QRS prolongation induced by tricyclic antidepressants. In order to clarify the pharmacological mechanism involved and to obtain clinically relevant evidence at the cellular level, whole-cell patch clamp techniques were performed in single guinea-pig ventricular myocytes to elicit the time and voltage-dependent fast sodium currents using both normal and modified physiological solutions. Cells stayed viable for much longer when they were placed in normal physiological solutions, providing sufficient recording time for consistently reproducible, clinically relevant toxicological results to be obtained. Amitriptyline (AMI) produced a concentration-dependent blockade of sodium currents with an approximate IC(50) of 0.69 microM. AAG reversed this blockade in a concentration-dependent fashion at concentrations ranging from 3.2 to 12.8 microM. Using the same experimental conditions, AAG also reversed the blockade of sodium current by quinidine, a class I antiarrythmic drug. Albumin did not reverse the blockade of sodium channels by AMI. The results indicate that AAG is a potential antidote for tricyclic antidepressant overdose.     

Characteristics of ginsenoside Rg3-mediated brain Na+ current inhibition

We demonstrated previously that ginsenoside Rg(3) (Rg(3)), an active ingredient of Panax ginseng, inhibits brain-type Na(+) channel activity. In this study, we sought to elucidate the molecular mechanisms underlying Rg(3)-induced Na(+) channel inhibition. We used the two-microelectrode voltage-clamp technique to investigate the effect of Rg(3) on Na(+) currents (I(Na)) in Xenopus laevis oocytes expressing wild-type rat brain Na(V)1.2 alpha and beta1 subunits, or mutants in the channel entrance, the pore region, the lidocaine/tetrodotoxin (TTX) binding sites, the S4 voltage sensor segments of domains I to IV, and the Ile-Phe-Met inactivation cluster. In oocytes expressing wild-type Na(+) channels, Rg(3) induced tonic and use-dependent inhibitions of peak I(Na). The Rg(3)-induced tonic inhibition of I(Na) was voltage-dependent, dose-dependent, and reversible, with an IC(50) value of 32 +/- 6 microM. Rg(3) treatment produced a 11.2 +/- 3.5 mV depolarizing shift in the activation voltage but did not alter the steady-state inactivation voltage. Mutations in the channel entrance, pore region, lidocaine/TTX binding sites, or voltage sensor segments did not affect Rg(3)-induced tonic blockade of peak I(Na). However, Rg(3) treatment inhibited the peak and plateau I(Na) in the IFMQ3 mutant, indicating that Rg(3) inhibits both the resting and open states of Na(+) channel. Neutralization of the positive charge at position 859 of voltage sensor segment domain II abolished the Rg(3)-induced activation voltage shift and use-dependent inhibition. These results reveal that Rg(3) is a novel Na(+) channel inhibitor capable of acting on the resting and open states of Na(+) channel via interactions with the S4 voltage-sensor segment of domain II.     

Inhibition of cardiac Na+ current by primaquine

The electrophysiological effects of the anti-malarial drug primaquine on cardiac Na(+) channels were examined in isolated rat ventricular muscle and myocytes. In isolated ventricular muscle, primaquine produced a dose-dependent and reversible depression of dV/dt during the upstroke of the action potential. In ventricular myocytes, primaquine blocked I(Na)(+) in a dose-dependent manner, with a K(d) of 8.2 microM. Primaquine (i) increased the time to peak current, (ii) depressed the slow time constant of I(Na)(+) inactivation, and (iii) slowed the fast component for recovery of I(Na)(+) from inactivation. Primaquine had no effect on: (i) the shape of the I - V curve, (ii) the reversal potential for Na(+), (iii) the steady-state inactivation and g(Na)(+) curves, (iv) the fast time constant of inactivation of I(Na)(+), and (v) the slow component of recovery from inactivation. Block of I(Na)(+) by primaquine was use-dependent. Data obtained using a post-rest stimulation protocol suggested that there was no closed channel block of Na(+) channels by primaquine. These results suggest that primaquine blocks cardiac Na(+) channels by binding to open channels and unbinding either when channels move between inactivated states or from an inactivated state to a closed state. Cardiotoxicity observed in patients undergoing malaria therapy with aminoquinolines may therefore be due to block of Na(+) channels, with subsequent disturbances of impulse conductance and contractility.     

Inhibition of sodium channel gating by trapping the domain II voltage sensor with protoxin II

ProTx-II, an inhibitory cysteine knot toxin from the tarantula Thrixopelma pruriens, inhibits voltage-gated sodium channels. Using the cut-open oocyte preparation for electrophysiological recording, we show here that ProTx-II impedes movement of the gating charges of the sodium channel voltage sensors and reduces maximum activation of sodium conductance. At a concentration of 1 microM, the toxin inhibits 65.3 +/- 4.1% of the sodium conductance and 24.6 +/- 6.8% of the gating current of brain Na(v)1.2a channels, with a specific effect on rapidly moving gating charge. Strong positive prepulses can reverse the inhibitory effect of ProTx-II, indicating voltage-dependent dissociation of the toxin. Voltage-dependent reversal of the ProTx-II effect is more rapid for cardiac Na(v)1.5 channels, suggesting subtype-specific action of this toxin. Voltage-dependent binding and block of gating current are hallmarks of gating modifier toxins, which act by binding to the extracellular end of the S4 voltage sensors of ion channels. The mutation L833C in the S3-S4 linker in domain II reduces affinity for ProTx-II, and mutation of the outermost two gating-charge-carrying arginine residues in the IIS4 voltage sensor to glutamine abolishes voltage-dependent reversal of toxin action and toxin block of gating current. Our results support a voltage-sensor-trapping model for ProTx-II action in which the bound toxin impedes the normal outward gating movement of the IIS4 transmembrane segment, traps the domain II voltage sensor module in its resting state, and thereby inhibits channel activation.     

Cardiotoxic effects of fenfluramine hydrochloride on isolated cardiac preparations and ventricular myocytes of guinea-pigs

The cardiotoxic effects of fenfluramine hydrochloride on mechanical and electrical activity were studied in papillary muscles, Purkinje fibres, left atria and ventricular myocytes of guinea-pigs. Force of contraction (f(c)) was measured isometrically, action potentials and maximum rate of rise of the action potential (V(max)) were recorded by means of the intracellular microelectrode technique and the sodium current (I(Na)) with patch-clamp technique in the cell-attached mode. For kinetic analysis (S)-DPI-201-106-modified Na(+) channels from isolated guinea-pig ventricular heart cells were used. Fenfluramine (1 - 300 microM) produced negative chronotropic and inotropic effects; additional extracellular Ca(2+) competitively antagonized the negative inotropic effect. Fenfluramine concentration-dependently reduced V(max) and showed tonic blockade of sodium channels, shortened the action potential duration in papillary muscles and Purkinje fibres. In cell-attached patches, fenfluramine decreased I(Na) concentration-dependently (10 - 100 microM), frequency-independently (0.1 - 3 Hz; 30 microM). The h(infinity) curve was shifted towards hyperpolarizing direction. At 30 microM, fenfluramine blocked the sodium channel at all test potentials to the same degree, and neither changed the threshold and reversal potentials nor the peak of the curve. No effect on single channel availability, but a significant decrease in mean open times and increase in mean closed times was observed. Mean duration of the bursts decreased and number of openings per record increased with increasing drug concentration. It is concluded that the effect on I(Na) plays an important role in the cardiotoxicity of fenfluramine in addition to primary pulmonary hypertension and valvular disorders.     

Determinants of stereospecific binding of type I antiarrhythmic drugs to cardiac sodium channels

The determinants of stereospecific binding of type I antiarrhythmic drugs to specific sites associated with the sodium channel were assessed using rat cardiac myocytes. The asymmetric carbon atoms of stereoisomers may be located at two sites within type I drugs. The structure of these drugs can be schematically illustrated as Aromatic-C1-link-C2-Amine, where C1 and C2 represent potentially asymmetric carbon atoms. We used enantiomeric pairs with either C1 or C2 asymmetric carbon atoms to assess the importance of conformation at these sites to drug binding. The affinities of enantiomers of seven sodium channel blockers were measured with a radioligand binding assay using [3H]batrachotoxinin benzoate [( 3H]BTXB) and freshly isolated cardiac myocytes. The enantiomers inhibited [3H]BTXB binding in a dose-dependent manner, with a mean Hill number of 1.0 +/- 0.1. The ratios of affinities [IC50 of (+)-isomer/IC50 of (-)-isomer] were, for the C1 pairs: quinidine, 0.29; cinchonidine, 0.55; disopyramide, 1.11; RAC 109, 5.33; and for C2 pairs: flecainide, 1.03; mexiletine, 2.15; tocainide, 3.01. The stereospecific differences in drug binding suggest that the orientations of both the aromatic and the amine groups to the rest of the drug molecule are important determinants of drug binding to the cardiac sodium channel. This also suggests the presence of at least two stereospecific domains within the binding sites for type I antiarrhythmic drugs.     

Electrophysiologic effects of an antiarrhythmic agent, bidisomide, on sodium current in isolated rat ventricular myocytes: comparison with mexiletine and disopyramide.

The effects of bidisomide, an antiarrhythmic agent, on sodium current (I(Na)) in isolated rat ventricular myocytes were investigated using a whole cell voltage clamp method. Bidisomide blocked I(Na) with a Ki of 214 microM at a holding potential of -140 mV. The blockade of I(Na) was enhanced at a less negative holding potential of -100 mV with a Ki of 21 microM. Bidisomide shifted the steady state inactivation curve to a negative potential direction by 20 mV without a significant change in the slope factor. Bidisomide slowed the time course of recovery of I(Na) at a holding potential of -140 mV with a slow recovery phase. The time constant of recovery phase for bidisomide, disopyramide and mexiletine were 2703, 1858 and 757 ms, respectively. The development of the block of I(Na) consisted of two phases in the presence of bidisomide. The fast and slow time constants were 11 and 648 ms. Bidisomide produced a use-dependent block of I(Na) when the depolarizing pulse was repeated at 1-3 Hz. Our results indicate that bidisomide binds to rat cardiac sodium channels and that the dissociation kinetics of bidisomide from the inactivated sodium channel is slower than that of disopyramide.     

Stereoselective interactions of (R)- and (S)-propafenone with the cardiac sodium channel.

The specific interactions of both (R)- and (S)-propafenone with the cardiac sodium channel were studied with patch clamp techniques in the whole-cell recording mode at reduced extracellular Na+ on guinea pig ventricular cells. Both (R)- and (S)-propafenone (10 microM) shifted the membrane potential required for half-maximal steady-state inactivation (E0.5) of the cardiac sodium channel to considerably more negative membrane potentials [E0.5 = -70.8 +/- 2.9 mV for controls vs. -85 +/- 3.1 mV for (R)-propafenone and -91.9 +/- 1.7 mV for (S)-propafenone]. (S)-Propafenone at a concentration of 10 microM is more effective in shifting the h infinity curve of the cardiac sodium channel. Recovery from inactivation of the cardiac sodium current is prolonged by orders of magnitude by both stereoenantiomeric forms [time constants were estimated to be 38 +/- 15 ms at -90 mV vs. 46.5 +/- 14.3 s for (R)-propafenone and 74.2 +/- 37.9 for (S)-propafenone]. Development of block occurs mainly through the inactivated channel conformation for both (R)- and (S)-propafenone. Development of block of inactivated cardiac sodium channels occurs with time constants of 15.9 +/- 3.9 s for (R)-propafenone and 19.7 +/- 7.3 s for (S)-propafenone at 10 microM. Action potential duration and possible stereoselective interaction with ion transport systems other than sodium channels may influence the block developed by either (R)- or (S)-propafenone at a given concentration and beating frequency indirectly through the membrane potential.(ABSTRACT TRUNCATED AT 250 WORDS)     

Preferential block of late sodium current in the LQT3 DeltaKPQ mutant by the class I(C) antiarrhythmic flecainide

Flecainide block of Na(+) current (I(Na)) was investigated in wild-type (WT) or the long QT syndrome 3 (LQT3) sodium channel alpha subunit mutation with three amino acids deleted (DeltaKPQ) stably transfected into human embryonic kidney 293 cells using whole-cell, patch-clamp recordings. Flecainide (1-300 mM) caused tonic and use-dependent block (UDB) of I(Na) in a concentration-dependent manner. Compared with WT, DeltaKPQ I(Na) was more sensitive to flecainide, and flecainide preferentially inhibited late I(Na) (mean current between 20 and 23.5 ms after depolarization) compared with peak I(Na). The IC(50) value of peak and late I(Na) for WT was 127 +/- 6 and 44 +/- 2 microM (n = 20) and for DeltaKPQ was 80 +/- 9 and 19 +/- 2 microM (n = 31) respectively. UDB of peak I(Na) was greater and developed more slowly during pulse trains for DeltaKPQ than for WT. The IC(50) value for UDB of peak I(Na) for WT was 29 +/- 4 microM (n = 20) and for DeltaKPQ was 11 +/- 1 microM (n = 26). For DeltaKPQ, UDB of late I(Na) was greater than for peak I(Na). Recovery from block was slower for DeltaKPQ than for WT. We conclude that DeltaKPQ interacts differently with flecainide than with WT, leading to increased block and slowed recovery, especially for late I(Na). These data provide insights into mechanisms for flecainide block and provide a rationale at the cellular and molecular level that open channel block may be a useful pharmacological property for treatment of LQT3.     

A proarrhythmic response to sodium channel blockade: modulation of the vulnerable period in guinea pig ventricular myocardium.

The vulnerable period (VP) is an interval of time during the cardiac cycle within which premature stimulation may lead to trains of responses (one: many stimulus-response coupling). Although the VP parallels the recovery of sodium channel availability, modulators of its boundaries remain unclear. Numerical studies of a uniform cable demonstrated that reduction in sodium channel availability increased the range of premature stimuli, resulting in unidirectional block, a precursor of reentrant activation. Consequently, we hypothesized that the kinetics of use-dependent sodium channel blockade could reflect one dimension of a drug's proarrhythmic potential. In strips from guinea pig right ventricle, we probed the boundaries of the VP in the presence of use-dependent sodium channel antagonists utilizing a train of stimuli followed by a premature stimulus. Under drug-free conditions when the sites of drive and premature stimulation were the same, the VP was less than 4 ms in duration. When the drive and premature sites were different, the drug-free VP was greater than 5 ms in 22 of 24 preparations and 0 in the other two, with an average VP duration of 16 +/- 10 ms (mean +/- SD). In the presence of 1 microM moricizine, VP = 17 +/- 4 ms; 12 microM moricizine, VP = 35 +/- 4 ms; 3 microM flecainide, VP = 50 +/- 17 ms; and 4 microM quinidine, VP = 2 +/- 1 ms. These results suggest that residual unsuppressed premature ventricular contractions (PVCs) in the presence of some class 1 drugs have a greater potential for initiating a proarrhythmic response than PVCs in the absence of a class 1 drug.     

Does QT widening in the Langendorff-perfused rat heart represent the effect of repolarization delay or conduction slowing?

It has been suggested that class I antiarrhythmic drugs and ischemia can widen the QT interval in the Langendorff-perfused rat heart preparation as a consequence of slowed ventricular conduction. If this were so, it would undermine the clinical relevance of the preparation and its effectiveness as an antiarrhythmic bioassay. To test this, the authors determined whether three different class I drugs could prolong QT in the preparation and whether this effect was augmented by ischemia and elevation of the potassium (K+) content of the perfusion solution. Baseline drug-free QT intervals correlated inversely with the K+ content (3 microM vs. 5 mM). QT intervals widened during the first 3-5 minutes of ischemia (P < 0.05), then returned gradually to baseline. Lidocaine (3.88 microM and 12.93 microM) had no effect on the QT interval before or during ischemia, whereas quinidine (7.90 microM but not 0.79 microM) and flecainide (1.48 microM but not 0.74 microM) caused QT widening before and during ischemia (P < 0.05). Elevating perfusion solution K+ content from 3 microM to 5 mM reduced the QT-widening effects of quinidine and flecainide (P < 0.05). Because lidocaine, a relatively selective sodium (Na+) channel blocker, failed to widen QT interval whereas quinidine and flecainide (combined Na+ and K+ channel blockers) did so, and because K+ elevation reduced rather than potentiated the drug-induced QT widening, it is unlikely that Na+ channel blockade and conduction slowing play any role in ischemia- or class I drug-induced QT widening in this model.     

Particular sensitivity to calcium channel blockers of the fast inward voltage-dependent sodium current involved in the invasive properties of a metastastic breast cancer cell line

1. A voltage-dependent sodium current has been described in the highly invasive breast cancer cell line MDA-MB-231. Its activity is associated with the invasive properties of the cells. The aim of our study was to test whether this current (I(Na)) is sensitive to three representative calcium channel blockers: verapamil, diltiazem and nifedipine. I(Na) was studied in patch-clamp conditions. 2. I(Na) was sensitive to verapamil (IC(50)=37.6+/-2.5 microM) and diltiazem (53.2+/-3.6 microM), while it was weakly sensitive to nifedipine. 3. The tetrodotoxin (TTX) concentration, which fully blocks I(Na) (30 microM), did not affect cell proliferation. Diltiazem and verapamil, at concentrations that do not fully block I(Na), strongly reduced cell proliferation, suggesting, regarding proliferation, that these molecules act on targets distinct from sodium channels. These targets are probably not other ionic channels, since the current measured at the end of a 500 ms long pulse in the voltage range between -60 and +40 mV was unaffected by verapamil and diltiazem. 4. We conclude that the sodium channel expressed in MDA-MB-231 cells is sensitive to several calcium channel blockers. The present study also underlines the danger of concluding to the possible involvement of membrane channel proteins in any phenomenon on the sole basis of pharmacology, and without an electrophysiological confirmation.     

Biochemical and biophysical studies of the interaction of class I antiarrhythmic drugs with the cardiac sodium channel

Class I antiarrhythmic drugs block the cardiac sodium channel and are now used widely for a variety of cardiac arrhythmias. The nature of the interaction of the drugs with these channels, and their mechanism of action, have been areas of considerable recent interest. Here we review several aspects of drug action. Evidence that sodium channel blockade itself is antiarrhythmic arises from experiments in which the sodium channel-specific toxin tetrodotoxin was shown to prevent ventricular fibrillation in rabbit hearts. A radioligand binding assay for the cardiac sodium channel was then developed using [³H]Batrachotoxinin (BTX) Benzoate and freshly isolated rat cardiac myocytes. Class I antiarrhythmic drug binding identified in this model fits conventional criteria for binding to a receptor. This receptor behaves like an allosteric protein; class I antiarrhythmic drugs appear to bind to and stabilize closed channels. The mechanism of drug binding and electrophysiologic consequences have been examined in a number of model systems by microelectrode studies, whole cell voltage clamping, and more recently voltage clamp studies of single isolated channels in lipid bilayers. Using the latter method, we have shown that lidocaine causes two distinct kinds of sodium channel blockade. The first is a very slow block which can be demonstrated with simple aryl compounds. The second kind of block is a very rapid block which can also be caused by simple alkyl amines. Thus, the two major structural moieties of lidocaine each cause a specific form of block. The structure-activity relationships of class I drugs were explored with homologs of lidocaine in the radioligand binding assay. With this method the existence of a number of receptor subdomains has been demonstrated, each of which recognizes specific structural moieties of class I drugs. Finally, we have explored both drug proarrhythmia and drug resistance. Drug proarrhythmia may be due to homogeneous slowing of conduction with little effect on refractoriness. One form of drug resistance may be due to induction of synthesis of new cardiac sodium channels by exposure to animals with chronic class I drug administration. Mexiletine induces a three-fold increase in rat cardiac sodium channels within 3 days due to increased sodium channel mRNA synthesis. Thus drug resistance might be due to drug-induced increases in the numbers of cardiac sodium channels. © 1994 Wiley-Liss, Inc.