Late sodium current block for drug-induced long QT syndrome: Results from a prospective clinical trial.

Drug-induced long QT syndrome has resulted in many drugs being withdrawn from the market. At the same time, the current regulatory paradigm for screening new drugs causing long QT syndrome is preventing drugs from reaching the market, sometimes inappropriately. In this study, we report the results of a first-of-a-kind clinical trial studying late sodium (mexiletine and lidocaine) and calcium (diltiazem) current blocking drugs to counteract the effects of hERG potassium channel blocking drugs (dofetilide and moxifloxacin). We demonstrate that both mexiletine and lidocaine substantially reduce heart-rate corrected QT (QTc) prolongation from dofetilide by 20 ms. Furthermore, all QTc shortening occurs in the heart-rate corrected J-Tpeak (J-Tpeak c) interval, the biomarker we identified as a sign of late sodium current block. This clinical trial demonstrates that late sodium blocking drugs can substantially reduce QTc prolongation from hERG potassium channel block and assessment of J-Tpeak c may add value beyond only assessing QTc.

Allosteric effects of erythromycin pretreatment on thioridazine block of hERG potassium channels.

BACKGROUND AND PURPOSE: The prevalence of concurrent use of two or more drugs that block human ether-a-go-go-related gene product (hERG) K(+) channels is not uncommon, but is not well characterized. This study defined the effects of concurrent exposure of two hERG-blocking drugs on hERG current amplitude. Experiments were conducted to determine if concomitant exposure to two potent pore hERG blockers, thioridazine and terfenadine and a weak hERG blocker, erythromycin, would result in an additive, synergistic or inhibitory effect. EXPERIMENTAL APPROACH: hERG currents from stably transfected HEK cells were measured using the whole-cell variant of the patch-clamp method at physiological temperatures. Concentration-response relationships for thioridazine or terfenadine were obtained with cells pre-exposed to erythromycin. KEY RESULTS: Pre-exposure of cells to erythromycin resulted in an approximately 14-22-fold rightward shift in the hERG concentration-response curve for thioridazine and terfenadine respectively. This reduction in affinity was not the result of a change in the voltage-dependent characteristics of the channel. Results suggest an external binding site for erythromycin. CONCLUSIONS AND IMPLICATIONS: Pretreatment with erythromycin induced an approximately 14-22-fold reduction in hERG affinity for pore-binding drugs at concentrations of erythromycin, which by themselves only block hERG by 10% or less. These results suggest distinct, allosterically linked binding sites on opposite sides of the hERG channel. Occupancy of the external site by erythromycin reduces the affinity of the pore binding site. Furthermore, these results suggest that co-administration of erythromycin may provide some reduction in cardiac liability of potent hERG-blocking drugs.

The selective sphingosine 1-phosphate receptor modulator BAF312 redirects lymphocyte distribution and has species-specific effects on heart rate.

BACKGROUND AND PURPOSE: BAF312 is a next-generation sphingosine 1-phosphate (S1P) receptor modulator, selective for S1P(1) and S1P(5 ) receptors. S1P(1) receptors are essential for lymphocyte egress from lymph nodes and a drug target in immune-mediated diseases. Here, we have characterized the immunomodulatory potential of BAF312 and the S1P receptor-mediated effects on heart rate using preclinical and human data. EXPERIMENTAL APPROACH: BAF312 was tested in a rat experimental autoimmune encephalomyelitis (EAE) model. Electrophysiological recordings of G-protein-coupled inwardly rectifying potassium (GIRK) channels were carried out in human atrial myocytes. A Phase I multiple-dose trial studied the pharmacokinetics, pharmacodynamics and safety of BAF312 in 48 healthy subjects. KEY RESULTS: BAF312 effectively suppressed EAE in rats by internalizing S1P(1) receptors, rendering them insensitive to the egress signal from lymph nodes. In healthy volunteers, BAF312 caused preferential decreases in CD4(+) T cells, T(naïve) , T(central memory) and B cells within 4-6 h. Cell counts returned to normal ranges within a week after stopping treatment, in line with the elimination half-life of BAF312. Despite sparing S1P(3) receptors (associated with bradycardia in mice), BAF312 induced rapid, transient (day 1 only) bradycardia in humans. BAF312-mediated activation of GIRK channels in human atrial myocytes can fully explain the bradycardia. CONCLUSION AND IMPLICATIONS: This study illustrates species-specific differences in S1P receptor specificity for first-dose cardiac effects. Based on its profound but rapidly reversible inhibition of lymphocyte trafficking, BAF312 may have potential as a treatment for immune-mediated diseases.

Effects of cocaine and its major metabolites on the HERG-encoded potassium channel.

Cocaine abuse has been reported to result in QT prolongation in humans; however, the mechanisms underlying this effect are still poorly understood. In this study we compared the direct effects of cocaine and its major metabolites in human embryonic kidney 293 cells stably transfected with human ether-a-go-go-related gene (HERG). Cocaine blocked HERG-encoded potassium channels with an IC50 of 4.4 +/- 1.1 microM (22 degrees C). Cocaethylene (a metabolite formed in the presence of ethanol) had a significantly lower IC50 of 1.2 +/- 1.1 microM (P < 0.0001), and cocaine's primary pyrolysis metabolite methylecgonidine blocked HERG with a higher IC50 of 171.7 +/- 1.2 microM. In contrast, 1 mM ecgonine methylester or benzoylecgonine produced only a minimal block (21 +/- 4 and 15 +/- 8%, respectively). Blockade of HERG by cocaine, cocaethylene, and methylecgonidine increased significantly over the voltage range where HERG activates, but became constant at voltages where HERG activation was maximal, indicating that all three drugs block open channels, but by a mechanism that is not highly sensitive to voltage per se. Cocaine and cocaethylene also significantly slowed the time course of deactivation at -60 mV, an effect consistent with open channel block. We conclude that cocaethylene is slightly more potent than cocaine as a blocker of HERG, whereas methylecgonidine has much lower potency, and both benzoylecgonine and ecgonine methyl ester are essentially inactive at clinically relevant concentrations.

Loratadine blockade of K(+) channels in human heart: comparison with terfenadine under physiological conditions.

Recently, there has been considerable attention focused on drugs that prolong the QT interval of the electrocardiogram, with the H(1)-receptor antagonist class of drugs figuring prominently. Albeit rare, incidences of QT prolongation and ventricular arrhythmias, in particular torsade de pointes, have been reported with the antihistamines astemizole and terfenadine and more recently with loratadine. The most likely mechanism for these drug-related arrhythmias is blockage of one or more ion channels involved in cardiac repolarization. Several studies have demonstrated block of multiple cardiac K(+) channels by terfenadine, including I(to), I(sus), I(K1), and I(Kr) or human ether-a-go-go-related gene (HERG). In contrast to terfenadine, previous studies have shown the antihistamine loratadine to be virtually free of cardiac ion channel-blocking effects. This disparity in the lack of any significant cardiac ion channel-blocking effect and the existence of numerous adverse cardiac event reports for loratadine prompted the comparison of the human cardiac K(+) channel-blocking profile for loratadine and terfenadine under physiological conditions [37 degrees C, holding potential (V(hold)) = -75 mV] with the whole-cell patch-clamp method. Isolated human atrial myocytes were used to examine drug effects on I(to), I(sus), and I(K1), whereas HERG was studied in stably transfected HEK cells. In contrast to previous studies in nonhuman systems and/or under nonphysiological conditions, terfenadine (1 microM) had no effect on I(to), I(sus), or I(K1) at pacing rates up to 3 Hz. Similar results were found for 1 microM loratadine. However, both drugs potently blocked HERG current amplitude, with a mean IC(50) of 173 nM for loratadine and 204 nM for terfenadine (pacing rate, 0.1 Hz). Neither drug exhibited any significant use-dependent blockage of HERG (pacing rates = 0.1-3 Hz). These results point to a similarity in the human cardiac K(+) channel-blocking effects of loratadine and terfenadine and provide a possible mechanism for the arrhythmias associated with the use of either drug.

Rate-dependent blockade of a potassium current in human atrium by the antihistamine loratadine.

The antihistamine loratadine is widely prescribed for the treatment of symptoms associated with allergies. Although generally believed to be free of adverse cardiac effects, there are a number of recent reports suggesting that loratadine use may be associated with arrhythmias, in particular atrial arrhythmias. Nothing is known regarding the potassium channel blocking properties of loratadine in human cardiac cells. Using the whole-cell patch clamp technique, the effects of loratadine on the transient outward K current (Ito), sustained current (Isus), and current measured at -100 mV (IK1 and Ins), the major inward and outward potassium currents present in human atrial myocytes, were examined in order to provide a possible molecular mechanism for the observed atrial arrhythmias reported with loratadine use. Loratadine rate-dependently inhibited Ito at therapeutic concentrations with 10 nM loratadine reducing Ito amplitude at a pacing rate of 2 Hz by 34.9+/-6.0%. In contrast, loratadine had no effect on either Isus or current measured at -100 mV. These results may provide a possible mechanism for the incidences of supraventricular arrhythmias reported with the use of loratadine.

Fatty acid block of the transient outward current in adult human atrium.

Fatty acids represent an essential source of fuel for the heart and play an important role in the mechanical, electrical, and synthetic activities of cardiac cells. Under pathological conditions, such as ischemia followed by reperfusion, the myocardium is exposed to very high levels of fatty acids, in particular the monounsaturated fatty acid, oleic acid. Elevated plasma fatty acids have been linked to an increased risk for cardiac arrhythmias. In other species, fatty acids have been shown to modulate several cardiac ion channels, most notably potassium channels. Virtually nothing is known about the actions of oleic acid on potassium channels in human heart. We therefore characterized the effects of oleic acid on the transient outward current, sustained current, and inwardly rectifying current, some of the major potassium channels present in human atrium, using the whole-cell patch clamp method. Exposure of cells to oleic acid (5 microM) reduced the transient outward potassium current to 3.7 +/- 0.8 pA/pF (n = 4) compared with 7.0 +/- 0.7 pA/pF (n = 4) (P <. 05) for cells not exposed. In contrast, oleic acid had little effect on either the sustained current (4.3 +/- 0.3 pA/pF, n = 4 for oleic acid versus 4.8 +/- 0.5, n = 5 for control) present after the decay of the transient outward current or on the amplitude of IK1 measured at -100 mV (1.4 +/- 0.4 pA/pF, n = 4 for oleic acid versus 1.3 +/- 0. 4 pA/pF, n = 6 for control). In addition, oleic acid significantly slowed the rate of recovery of the transient outward current, which is predicted to result in a use-dependent reduction in current amplitude in the beating heart. These results suggest a possible contributing role for oleic acid block of the transient outward current in the pathological consequences of myocardial ischemia.

Quinidine interactions with human atrial potassium channels: developmental aspects.

Clinical studies have suggested that quinidine is less effective when used for the treatment of atrial arrhythmias in pediatric patients compared with its clinical effectiveness in the adult patient population. Age-related changes in the cardiac actions of quinidine on action potential duration and interaction with potassium channels in several mammalian species also have been reported. We investigated the effects of postnatal development on quinidine's interaction with major repolarizing currents (Ito, IKur, Ins, and IK1) in human atrial myocytes, using the whole-cell configuration of the voltage-clamp technique. Our results indicate that there are age-related changes in both the IC50 for quinidine blockade of Ito, as well as the mechanism of quinidine unblocking. In contrast, quinidine was found to inhibit both adult and pediatric IK1 and IKur in an age-independent manner, whereas the nonselective cation current (Ins), which contributes to the sustained outward current (Isus), was insensitive to quinidine. The results from this study help to clarify the electrophysiological mechanism by which quinidine elicits its antiarrhythmic effect in the pediatric and adult human population.

Stable expression and characterization of the human brain potassium channel Kv2.1: blockade by antipsychotic agents.

We have cloned the cDNA encoding the voltage-dependent K+ channel Kv2.1 from human brain (hKv2.1). RNase protection and RT-PCR (reverse transcriptase-PCR) experiments reveal abundant Kv2.1 transcripts in human brain with virtually no expression detectable in human heart. hKv2.1 has been stably transfected into a human glioblastoma cell line, and transformed cells display large, slowly activating outward currents. The kinetics, steady-state activation and inactivation parameters, and external tetraethylammonium sensitivity were all similar to those described previously for hKv2.1 channels transiently expressed in Xenopus oocytes or other mammalian cell lines. A number of dopamine receptor antagonist/antipsychotic agents were shown to block hKv2.1. Trifluoperizine, trifluperidol and pimozide produced time-dependent blockade of hKv2.1 with IC50 values of approx. 1-2 microM. The diphenylbutylpiperidine fluspirilene was shown to be 4-5-fold more potent than the other agents tested inhibiting hKv2.1 current with an IC50 value of 297 nM. The block produced by fluspirilene was both time- and frequency-dependent. Furthermore, fluspirilene (1 microM) shifted the midpotential of the hKv2.1 steady-state inactivation curve by approx. 15 mV in the hyperpolarizing direction. These results demonstrate the usefulness of this transfection system for the pharmacological characterization of hKv2. 1. Fluspirilene proved to be a relatively potent blocker of hKv2.1 and may provide a useful starting point for the development of more potent and selective agents active against this brain K+ channel.

L-type calcium current in pediatric and adult human atrial myocytes: evidence for developmental changes in channel inactivation.

Animal studies have documented the presence of marked, species-dependent, developmental changes in the properties of the L-type calcium current in cardiac myocytes. In an effort to understand the postnatal changes which occur in the calcium current in human heart, we characterized the calcium current in atrial myocytes isolated from 17 pediatric and older children (ages 3 d to 14 y) and 12 adult (ages 43-79 y) human hearts using the whole-cell patch clamp technique. In contrast to animal models, we found no evidence for age-related changes in calcium current density, steady-state inactivation, or kinetics of recovery from inactivation, suggesting that, in human atrium, calcium channels are in many aspects functionally mature at the time of birth. However, statistically significant differences were found in the kinetics of calcium current inactivation, with calcium current measured in cells isolated from pediatric human atria inactivating approximately 2-fold faster than cells isolated from adult hearts. These results suggest a possible role for age-related changes in calcium current inactivation in the shortened action potential duration observed in pediatric compared with adult human atrium.

Description of a nonselective cation current in human atrium.

Ion currents were examined in isolated human atrial myocytes by using the whole-cell patch-clamp technique. When currents were recorded with a K(+)-containing pipette solution, depolarizing voltage pulses elicited a rapidly activating outward current that decayed to an apparent steady state. Exposure of cells to 10 mmol/L 4-aminopyridine markedly reduced current amplitude; however, a rapidly activating current that was approximately 30% of the steady state current amplitude remained. When pipette K+ was replaced with Cs+, a similar rapidly activating current that reversed polarity at approximately 0 mV was recorded. This current was seen in 100% of the cells tested from 17 different hearts (n = 142), and its amplitude was approximately 40% of the amplitude of the steady state current recorded in the presence of pipette K+. The current amplitude was not significantly different in cells isolated from adult (6.31 +/- 1.35 pA/pF, n = 8) and pediatric (5.54 +/- 1.04 pA/pF, n = 9) hearts. Studies designed to determine the charge-carrying species indicated that changes in bath Cl- concentration had no effect on either the amplitude or the reversal potential of this current, whereas removal of pipette Cs+ and bath Na+ dramatically reduced this current. In addition, this current was not modulated by either isoproterenol (1 mumol/L, 22 degrees C) or cell swelling. This study provides the first description of a nonselective cation current in human atrial myocytes, which may play an important role in repolarization in human atria.

The pH dependence of cocaine interaction with cardiac sodium channels.

Previous in vitro and in vivo studies have provided evidence implicating cocaine block of cardiac sodium channels as a putative mechanism for cocaine-induced arrhythmias and sudden death. Cocaine also has been shown to cause seizures which can result in respiratory and/or metabolic acidosis. In this study we investigated how changes in both internal pH (pHi) and external pH (pHo) over the range of 6.6 to 9.2 modify the sodium channel blocking properties of cocaine in isolated guinea pig ventricular myocytes by using the whole-cell variant of the patch clamp technique. Use-dependent block produced by a train of 1-sec pulses to -20 mV was not affected by changes in pHi, but both the amplitude and time constant for approaching steady-state block were significantly affected by changes in pHo. Characterization of the time course of cocaine binding during a depolarizing pulse indicated that the kinetics of drug interaction with inactivated channels were independent of pHi, but were significantly affected by changes in pHo. The rate of recovery from channel block at a holding potential of -140 mV also was independent of pHi, but strongly dependent on pHo, with the unblocking time constant decreasing exponentially as pHo was increased. The results of this study indicate that cocaine's effect on cardiac sodium channels can be modulated significantly by changes in pHo, and provide further support for previously poorly tested assumptions of the modulated receptor hypothesis.

Comparison of Ito in young and adult human atrial myocytes: evidence for developmental changes.

In an effort to understand the ionic basis for the developmental changes that have been reported to occur in the configuration of the human atrial action potential, we characterized the transient outward current (Ito) and the inward rectifier current in atrial myocytes isolated from 20 young (ages 1 day-2.5 yr) and 8 adult (11-68 yr) human hearts using the whole cell patch-clamp technique. We found evidence for statistically significant (P < 0.05) age-related changes in the Ito, including 1) the presence of an Ito in only 67% of the cells isolated from young hearts vs. 100% of the cells isolated from adult hearts, 2) an almost twofold increase in the current density of Ito in adult cells vs. young cells, and 3) recovery kinetics that are approximately twofold slower in young myocytes relative to adult myocytes. In contrast, there were no age-related changes found in the current density of the inward rectifier current or the sustained current measured after the decay of Ito. These results suggest important current-dependent changes that occur with age in human atria.

Blockade of multiple human cardiac potassium currents by the antihistamine terfenadine: possible mechanism for terfenadine-associated cardiotoxicity.

Use of the antihistamine terfenadine has been associated with QT prolongation and torsade de pointes. One possible mechanism is blockade of cardiac potassium channels. We therefore characterized the effects of terfenadine on potassium currents recorded from isolated human cardiac myocytes. We demonstrated terfenadine block of the transient outward current and a novel, ultra-rapidly activating, delayed rectifier K+ current (IKur), which is very sensitive to 4-aminopyridine. IKur is probably produced by the protein product of Kv1.5a, a Shaker-like potassium channel cDNA cloned from human heart. We also compared terfenadine blockade of fHK (Kv1.5a) currents stably expressed in a human embryonic kidney cell line with terfenadine blockade of IKur in human atrial myocytes. Using the patch-clamp technique, we found that terfenadine produced a time-dependent reduction in Kv1.5a current that was consistent with blockade from the cytoplasmic side of the channel. The terfenadine-sensitive Kv1.5a current in human embryonic kidney cells was similar to the 4-aminopyridine-sensitive current in human atrial myocytes. In addition to blockade of the transient outward current and IKur, terfenadine at clinically relevant concentrations blocked both the rapidly and slowly activating components of the delayed rectifier in human atrial myocytes. Blockade of these K+ currents may contribute to the cardiotoxicity associated with terfenadine usage.

Cocaethylene, a metabolite of cocaine and ethanol, is a potent blocker of cardiac sodium channels.

Cocaethylene is an active metabolite of cocaine believed to play a causative role in the increased incidence of sudden death in individuals who coadminister ethanol with cocaine. However, the direct effects of cocaethylene on the heart have not been well defined. In this study, we defined the effects of cocaethylene on the cardiac Na current (INa) in guinea pig ventricular myocytes at 16 degrees C using the whole-cell patch-clamp method. Cocaethylene (10-50 microM) produced both a significant tonic block and a rate-dependent block of INa at cycle lengths between 2 and 0.2 sec. Cocaethylene produced a significantly greater tonic block than cocaine at a concentration of 50 microM and produced a significantly greater use-dependent block over a 5-fold range of drug concentrations (10-50 microM) and cycle lengths (0.2-1.0 sec). Analysis of channel-blocking characteristics revealed that cocaethylene had a significantly higher affinity for inactivated channels (Kdi = 5.1 +/- 0.6 microM, n = 15) compared with cocaine (Kdi = 7.9 +/- 0.5 microM, n = 10) (P < .01) and that cocaethylene produced a significantly greater hyperpolarizing shift of the steady-state INa inactivation curve (P < .05). Cocaethylene also had a significantly longer time constant for recovery from channel block at -140 mV (12.24 +/- 0.88 sec, n = 16) compared with cocaine (8.33 +/- 0.56 sec, n = 14) (P < .01).(ABSTRACT TRUNCATED AT 250 WORDS)

Variability of the QTc interval: impact on defining drug effect and low-frequency cardiac event.

Prolongation of the QT interval corrected for heart rate (QTc) can lead to the development of torsades de pointes, a life-threatening form of polymorphic ventricular tachycardia. However, the QTc interval duration exhibits a high degree of spontaneous variability and is not necessarily a direct predictor of the risk of torsades. This observation holds implications for the assessment of the potential proarrhythmic effects of noncardiac pharmacologic agents. To date, the antihistamine terfenadine is the only noncardiac drug that has undergone a comprehensive and systematic evaluation related to the consequences of its causing QTc prolongation. The results suggest that QTc prolongation resulting solely from terfenadine at clinical doses does not have an important impact on clinically relevant endpoints. The risk of serious ventricular arrhythmias with terfenadine using epidemiologic data is the same or less than that associated with traditional first-generation antihistamines. The risk of a clinical cardiac event (QTc prolongation, ventricular arrhythmias, syncope, or sudden death) with terfenadine is similar to that of other antihistamines. Factors associated with increased risk in patients taking terfenadine include significant liver disease, hypokalemia, overdose, and concomitant administration of ketoconazole-like agents or erythromycin; use of terfenadine is relatively contraindicated in these settings. No increased risk of serious arrhythmias has been confirmed in conjunction with the use of terfenadine in patients with cardiac disease.

Characterization of the sodium channel blocking properties of the major metabolites of cocaine in single cardiac myocytes.

Clinical reports indicate that almost 30% of cocaine overdose-related deaths occur 2 to 5 hr after administration, when an appreciable amount of cocaine can be expected to have been converted to metabolites. We investigated the effects of the three major metabolites of cocaine: benzoylecgonine, ecgonine methyl ester and norcocaine on sodium channels in isolated guinea pig myocytes using the whole cell variant of the patch clamp technique at 16 +/- 0.5 degrees C. Benzoylecgonine and ecgonine methyl ester did not produce a significant block of sodium current at a concentration of 100 microM. In contrast, 30 microM norcocaine was found to reduce sodium current in a use-dependent manner qualitatively similar to cocaine. The time course of sodium current block development and recovery were characterized. With 30 microM norcocaine, two phases of block development were defined: a rapid phase (tau = 11.9 +/- 11.6 msec) and a slow phase (tau = 2.2 +/- 0.5 sec). Recovery from drug-induced block at -140 mV was approximately twice as fast for norcocaine (tau = 4.6 +/- 1.7 sec, n = 7) compared to cocaine (tau = 8.4 +/- 0.9 sec, n = 6). Norcocaine was found to have a higher affinity for inactivated cardiac sodium channels (Kdi = 5.7 +/- 0.9 microM) than cocaine (Kdi = 7.8 +/- 1.2 microM) (P less than .01); however, norcocaine produced less use-dependent block due to its faster unbinding kinetics. These data indicate that although norcocaine and cocaine are potent sodium channel blockers, benzoylecgonine and ecgonine methyl ester are ineffective blockers at clinically relevant concentrations (i.e., less than or equal to 100 microM).

Electrocardiographic evidence for cocaine cardiotoxicity in cat.

Recent case studies suggest that cocaine overdose may produce life-threatening cardiac arrhythmias. We therefore investigated its effects on the electrocardiogram (leads II and V1) and arterial blood pressure in cats anesthetized with pentobarbital. Cocaine was administered by intravenous infusion over a 2-min interval at 1 mg/kg in 10 cats. In 5 out of 10 cats an additional infusion of 3 mg/kg cocaine was also administered after hemodynamic and electrocardiographic parameters had returned to control values (i.e., within 10 min). During and following infusion of 1 mg/kg cocaine, no significant change in heart rate or systolic or diastolic blood pressure were found, however the QRS duration increased by 38% (from 46 +/- 5 to 64 +/- 12 ms) (p less than 0.01). Evidence for bundle branch block and (or) premature ventricular beats was observed in 9 out of 10 cats after 1 mg/kg cocaine. Infusion of a further 3 mg/kg cocaine in five cats significantly lowered diastolic blood pressure (from 98 +/- 18 to 64 +/- 28 mmHg; 1 mmHg = 133.3 Pa) (p less than 0.01), and further prolonged QRS to 79 +/- 14 ms, a 75% increase from the mean control value (p less than 0.01). In addition, 1st and 2nd degree atrioventricular block, ventricular extrasystoles, and ectopic rhythms (AV junctional or idioventricular) were observed in four out of five cats given 3 mg/kg cocaine. Mean plasma concentrations of cocaine were 1.37 +/- 0.39 micrograms/mL (4.28 +/- 1.22 microM) (n = 5) at the end of a 1 mg/kg infusion and 2.93 +/- 0.43 micrograms/mL (9.16 +/- 1.34 microM) after a 3 mg/kg infusion (n = 3).(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of cocaine-induced block of cardiac sodium channels.

Recent evidence suggests that cocaine can produce marked cardiac arrhythmias and sudden death. A possible mechanism for this effect is slowing of impulse conduction due to block of cardiac Na channels. We therefore investigated its effects on Na channels in isolated guinea pig ventricular myocytes using the whole-cell variant of the patch clamp technique. Cocaine (10-50 microM) was found to reduce Na current in a use-dependent manner. The time course for block development and recovery were characterized. At 30 microM cocaine, two phases of block development were defined: a rapid phase (tau = 5.7 +/- 4.9 ms) and a slower phase (tau = 2.3 +/- 0.7 s). Recovery from block at -140 mV was also defined by two phases: (tau f = 136 +/- 61 ms, tau s = 8.5 +/- 1.7 s) (n = 6). To further clarify the molecular mechanisms of cocaine action on cardiac Na channels, we characterized its effects using the guarded receptor model, obtaining estimated Kd values of 328, 19, and 8 microM for channels predominantly in the rested, activated, and inactivated states. These data indicate that cocaine can block cardiac Na channels in a use-dependent manner and provides a possible cellular explanation for its cardiotoxic effects.