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

Structural and functional basis for the long QT syndrome: relevance to veterinary patients

Long QT syndrome (LQTS) is a condition characterized by prolongation of ventricular repolarization and is manifested clinically by lengthening of the QT interval on the surface ECG. Whereas inherited forms of LQTS associated with mutations in the genes that encode ion channel proteins are identified only in humans, the acquired form of LQTS occurs in humans and companion animal species. Often, acquired LQTS is associated with drug-induced block of the cardiac K+ current designated I(Kr). However, not all drugs that induce potentially fatal ventricular arrhythmias antagonize I(Kr), and not all drugs that block I(Kr), are associated with ventricular arrhythmias. In clinical practice, the extent of QT interval prolongation and risk of ventricular arrhythmia associated with antagonism of I(Kr) are modulated by pharmacokinetic and pharmacodynamic variables. Veterinarians can influence some of the potential risk factors (eg, drug dosage, route of drug administration, presence or absence of concurrent drug therapy, and patient electrolyte status) but not all (eg, patient gender/genetic background). Veterinarians need to be aware of the potential for acquired LQTS during therapy with drugs identified as blockers of HERG channels and I(Kr).

Modulation of human ether-à-go-go-related K+ (HERG) channel inactivation by Cs+ and K+

Unlike many other native and cloned K+ channels, human ether-à-go-go-related K+ (HERG) channels show significant Cs+ permeability with a PCs/PK (the permeability of Cs+ relative to that of K+) of 0.36 +/- 0.03 (n = 10). Here, we find that raising the concentration of external Cs+ (Cs+o) dramatically slows HERG channel inactivation without affecting activation. Replacement of 5 mM K+o by 135 mM Cs+o increased both inactivation and recovery time constants and shifted the mid-point of the steady-state inactivation curve by 25 mV in the depolarized direction (n = 6, P < 0.01). Raising [Cs+]o also modulated the voltage sensitivity of inactivation gating. With 130 mM Cs+i and 135 mM NMDG+o, the inactivation time constant decreased e-fold per 47.5 +/- 1.1 mV (n = 5), and when 20 mM Cs+ was added to the bath solution, the inactivation time constant decreased e-fold per 20.6 +/- 1.3 mV (n = 5, P < 0.01). A quantitative analysis suggests that Cs+o binds to a site in the pore that is influenced by the transmembrane electrical field, so that Cs+o-induced slowing of HERG inactivation is less prominent at strong depolarizations. K+o has effects that are similar to Cs+o and their effects were additive, suggesting Cs+o and K+o may share a common mechanism of action. The strong effects of Cs+ on inactivation but not on activation highlight the importance of ion and channel interactions during the onset of inactivation in the HERG channel.

Cardiovascular disorders associated with cocaine use: myths and truths

Cocaine produces a pattern of cardiovascular responses that are associated with apparent myocardial ischemia, arrhythmias, and other life-threatening complications in some individuals. Despite recent efforts to better understand the causes of cocaine-induced cardiovascular dysfunction, there remain a number of unanswered questions regarding the specific mechanisms by which cocaine elicits hemodynamic responses. This review will describe the actions of cocaine on the cardiovascular system and the evidence for the mechanisms by which cocaine elicits hemodynamic and pathologic responses in humans and animals. The emphasis will be on experimental data that provide the basis for our understanding of the mechanisms of cardiovascular toxicity associated with cocaine. More importantly, this review will identify several controversies regarding the causes of cocaine-induced cardiovascular toxicity that as yet are still debated. The evidence supporting these findings will be described. Finally, this review will outline the obvious deficits in our current concepts regarding the cardiovascular actions of cocaine in hope of encouraging additional studies on this grave problem in our society.

Spironolactone and its main metabolite, canrenoic acid, block human ether-a-go-go-related gene channels

BACKGROUND

It has been demonstrated that spironolactone (SP) decreases the QT dispersion in chronic heart failure. In this study, the effects of SP and its metabolite, canrenoic acid (CA), on human ether-a-go-go-related gene (HERG) currents were analyzed.

METHODS AND RESULTS

HERG currents elicited in stably transfected Chinese hamster ovary cells were measured with the whole-cell patch-clamp technique. SP decreased HERG currents in a concentration-dependent manner (IC50=23.0+/-1.5 micromol/L) and shifted the midpoint of the activation curve to more negative potentials (Vh=-13.1+/-3.4 versus -18.9+/-3.6 mV, P<0.05) without modifying the activation and deactivation kinetics. SP-induced block (1 micromol/L) appeared at the range of membrane potentials coinciding with that of channel activation, and thereafter, it remained constant, reaching 24.7+/-3.8% at +60 mV (n=6, P<0.05). CA (0.01 nmol/L to 500 micromol/L) blocked HERG channels in a voltage- and frequency-independent manner. CA at 1 nmol/L shifted the midpoint of the activation curve to -19.9+/-1.8 mV and accelerated the time course of channel activation (tau=1064+/-125 versus 820+/-93 ms, n=11, P<0.01). The envelope of the tail test demonstrated that at the very beginning of the pulses to +40 mV (25 ms), a certain amount of block was apparent (31.3+/-9.9%). CA did not modify the voltage-dependence of HERG channel inactivation (Vh=-60.8+/-5.6 versus -62.9+/-3.1 mV, n=6, P>0.05) or the kinetics of the reactivation process at any potential tested. CA and aldosterone also blocked the native I(Kr) in guinea-pig ventricular myocytes.

CONCLUSIONS

At concentrations reached after administration of therapeutic doses of SP, CA blocked the HERG channels by binding to both the closed and open states of the channel.

Effects of levobupivacaine, ropivacaine and bupivacaine on HERG channels: stereoselective bupivacaine block

1 Levobupivacaine and ropivacaine are the pure S(-) enantiomers of N-butyl- and N-propyl-2',6'-pipecoloxylidide, developed as less cardiotoxic alternatives to bupivacaine. In the present study, we have analysed the effects of levobupivacaine, ropivacaine and bupivacaine on HERG channels stably expressed in CHO cells. 2 The three drugs blocked HERG channels in a concentration-, time- and state-dependent manner. Block measured at the end of 5 s pulses to -10 mV induced by 20 microM bupivacaine (52.7+/-2.0%, n=15) and ropivacaine (55.5+/-2.7%, n=13) was similar (P>0.05) and both lower than that induced by levobupivacaine (67.5+/-4.2%, n=11) (P<0.05). 3 Dextrobupivacaine (20 microM) was less potent (47.2+/-5.2%, n=10) than levobupivacaine (P<0.05), indicating stereoselective HERG channel block. 4. Block induced by the three local anaesthetics exhibited a steep voltage dependence in the range of channel activation. In all cases, block measured at the maximum peak current at a test potential of 0 mV after promoting recovery from inactivation (I-->O) was lower than that observed at the end of 5-s pulses (I+O). 5. Levobupivacaine, ropivacaine and bupivacaine accelerated HERG inactivation kinetics, slowed the recovery from inactivation and shifted the inactivation curve towards more negative membrane potentials. The three local anaesthetics induced a rapid time-dependent decline after using a protocol that quickly activates HERG channels. 6. All these results suggest that: (1) these drugs bind to the open and the inactivated states of HERG channels, (2) they stabilize HERG channels in the inactivated state, and (3) block induced by bupivacaine enantiomers is stereoselective.

Safety of non-antiarrhythmic drugs that prolong the QT interval or induce torsade de pointes: an overview

The long and growing list of non-antiarrhythmic drugs associated with prolongation of the QT interval of the electrocardiogram has generated concern not only for regulatory interventions leading to drug withdrawal, but also for the unjustified view that QT prolongation is usually an intrinsic effect of a whole therapeutic class [e.g. histamine H(1) receptor antagonists (antihistamines)], whereas, in many cases, it is displayed only by some compounds within a given class of non-antiarrhythmic drugs because of an effect on cardiac repolarisation. We provide an overview of the different classes of non-antiarrhythmic drugs reported to prolong the QT interval (e.g. antihistamines, antipsychotics, antidepressants and macrolides) and discusses the clinical relevance of the QT prolonging effect. Drug-induced torsade de pointes are sometimes considered idiosyncratic, totally unpredictable adverse drug reactions, whereas a number of risk factors for their occurrence is now recognised. Widespread knowledge of these risk factors and implementation of a comprehensive list of QT prolonging drugs becomes an important issue. Risk factors include congenital long QT syndrome, clinically significant bradycardia or heart disease, electrolyte imbalance (especially hypokalaemia, hypomagnesaemia, hypocalcaemia), impaired hepatic/renal function, concomitant treatment with other drugs with known potential for pharmacokinetic/pharmacodynamic interactions (e.g. azole antifungals, macrolide antibacterials and class I or III antiarrhythmic agents). This review provides insight into the strategies that should be followed during a drug development program when a drug is suspected to affect the QT interval. The factors limiting the predictive value of preclinical and clinical studies are also outlined. The sensitivity of preclinical tests (i.e. their ability to label as positive those drugs with a real risk of inducing QT pronglation in humans) is sufficiently good, but their specificity (i.e. their ability to label as negative those drugs carrying no risk) is not well established. Verapamil is a notable example of a false positive: it blocks human ether-a-go-go-related (HERG) K(+) channels, but is reported to have little potential to trigger torsade de pointes. Although inhibition of HERG K(+) channels has been proposed as a primary test for screening purposes, it is important to remember that several ion currents are involved in the generation of the cardiac potential and that metabolites must be specifically tested in this in vitro test. At the present state of knowledge, no preclinical model has an absolute predictive value or can be considered as a gold standard. Therefore, the use of several models facilitates decision making and is recommended by most experts in the field.

Cocaine-induced channelopathies: emerging evidence on the multiple mechanisms of sudden death

Sudden death due to cocaine in the absence of myocardial infarction has been attributed to the precipitation of life-threatening arrhythmias not unlike that due to antiarrhythmic drugs. Cocaine is a slow on-off sodium blocker and a fast on-off potassium blocker. Effects on repolarization are biphasic: At low concentrations, cocaine delays ventricular recovery, whereas at higher levels, cocaine hastens it. Two distinct clinical profiles emerge from case reports of electrocardiographically documented life-threatening arrhythmias attributed to cocaine. The first is monomorphic slow ventricular tachycardia or idioventricular rhythm that occurs in overdose situations and appears to reflect excessive sodium channel block; it may respond to sodium bicarbonate. The second is torsade de pointes that occurs in recreational users who have underlying risks for this tachycardia (such as fully or partially expressed congenital long QT syndrome) and reflects potassium channel blockade. These clinical observations can be explained by recent findings regarding the electrophysiologic effects of cocaine. Other patterns of severe arrhythmias due to cocaine may yet emerge.