Triamterene inhibits the delayed rectifier potassium current (IK) in guinea pig ventricular myocytes

Lengthening of cardiac repolarization in isolated guinea pigs hearts by sequential or concomitant administration of two IKr blockers

Block of I(Kr) is of major concern in drug safety. The objective of this study was to assess prolongation of cardiac repolarization during the combined use of two I(Kr) blockers when administered concomitantly or sequentially. (1) When isolated hearts from male guinea pigs were perfused concomitantly with two I(Kr) blockers, prolongation of monophasic action potential duration measured at 90% (MAPD(90)) was less than the summation of effects observed for each drug perfused alone. (2) In sequential administration, when ketoconazole or erythromycin was perfused first, they antagonized MAPD(90)-prolonging effects of domperidone. This effect was absent when domperidone or dofetilide was perfused first. Patch-clamp experiments confirmed that the order of sequential perfusion impacts the decrease in HERG tail amplitude. In conclusion, this study does not support the concept that potentiation of drug effects is observed during the combined administration of two I(Kr) blockers. Furthermore, order of administration of two I(Kr) blockers together may be an important factor in drug-induced long QT syndrome.

Cardiac slices as a predictive tool for arrhythmogenic potential of drugs and chemicals

IMPORTANCE OF THE FIELD

cardiac arrhythmia represents one of the primary safety pharmacological concerns in drug development. The most prominent example is drug induced ventricular tachycardia of the Torsade des Pointes type. The mechanism how this type of arrhythmia develops is a complex multi-cellular phenomenon. It can only be insufficiently reflected by cellular or molecular assays. However, organ models - such as Langendorff hearts - or in vivo experiments are expensive and time consuming and not suitable for assays requiring an increased throughput.

AREAS COVERED IN THIS REVIEW

here, we describe and review an assay bridging the gap between cardiomyocyte based assays and organ based systems - cardiac slices. This assay is reviewed in direct comparison with established safety pharmacological assays.

WHAT THE READER WILL GAIN

while slices have played an important role in brain research for > 2 decades, cardiac slices are experiencing a renaissance due to the novel challenges in safety pharmacology just in the last few years. Cardiac slices can be cultured and recorded over several days. It is possible to access electrophysiological data with a high number of electrodes - up to 256 electrodes - embedded in the surface of a microelectrode array.

TAKE HOME MESSAGE

cardiac slices close the gap between cellular and organ based assays in cardiac safety pharmacology. The tissue properties of a functional cardiac syncytium are more accurately reflected by a slice rather than a single cell.

Ischemic, genetic and pharmacological origins of cardiac arrhythmias: the contribution of the Quebec Heart Institute

Research in the field of basic electrophysiology at the Quebec Heart Institute (Laval Hospital, Quebec City, Quebec) has evolved since its beginning in the 1990s. Interests were focused on cardiac arrhythmias induced by drugs, allelic variants and metabolic factors produced during ischemia. The results have contributed to the creation of new standards in drug development, more specifically, testing all new drugs for their potential effects on cardiac potassium currents, which could produce life-threatening proarrhythmic effects. In a French-Canadian population, three heterozygous single nucleotide polymorphisms in hK(v)1.5, a gene encoding for a major atrial repolarizing current, were found. These variants affect the expression level of the hK(v)1.5 channel and change the inactivation process in the presence of its accessory beta subunit. Because these effects could shorten atrial action potential, their presence was tested in postcoronary bypass patients and a higher prevalence was found in patients with postoperative atrial fibrillation. Finally, three potentially proarrhythmic factors characteristic of ischemia were identified: pH decrease; oxygen free radicals, which both increase the flow of K(+) ions through human ether-a-go-go-related gene and hK(v)1.5, producing a reduction in action potential duration, frequently leading to cardiac arrhythmias; and lysophosphatidylcholine, a metabolite involved in the production of cardiac arrhythmias early during ischemia that was shown to be a major cause of electrical uncoupling. Over the past decade, the Quebec Heart Institute has provided a significant amount of original data in the field of basic cardiac electrophysiology, specifically concerning arrhythmias originating from pharmacological agents, genetic background and cardiac ischemia.

The Quebec Heart Institute: 50 years of excellence in cardiology

The Quebec Heart Institute was established in 1957 at the Laval Hospital in Sainte-Foy, Quebec. Since then, clinical and research activities have made this Institute one of the largest tertiary care cardiology centres in Canada. With its vast catchment area of more than 3,000,000 people, the Institute has developed a strong collaboration with referral physicians centred on clinical, teaching and research interests. The Institute pioneered several aspects of cardiac surgery, invasive cardiology, echocardiography, basic research and, more recently, a network of researchers and clinicians working in the field of 'metabolic cardiology'. The first 50 years of the Quebec Heart Institute are depicted in this overview, which will also introduce this special supplement to The Canadian Journal of Cardiology.

Impairment of hyperpolarization-activated, cyclic nucleotide-gated channel function by the intravenous general anesthetic propofol

Propofol (2,6-diisopropylphenol) is a widely used intravenous general anesthetic, which has been reported to produce bradycardia in patients at concentrations associated with profound sedation and loss of consciousness. Hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels conduct a monovalent cationic current I(h) (also known as I(q) or I(f)) that contributes to autorhythmicity in both the brain and heart. Here we studied the effects of propofol on recombinant HCN1, HCN2, and HCN4 channels and found that the drug inhibits and slows activation of all three channels at clinically relevant concentrations. In oocyte expression studies, HCN1 channel activation was most sensitive to slowing by propofol (EC(50) values of 5.6 +/- 1.0 microM for fast component and 31.5 +/- 7.5 microM for slow component). HCN1 channels also showed a marked propofol-induced hyperpolarizing shift in the voltage dependence of activation (EC(50) of 6.7 +/- 1.0 microM) and accelerated deactivation (EC(50) of 4.5 +/- 0.9 microM). Furthermore, propofol reduced heart rate in an isolated guinea pig heart preparation over the same range of concentrations. These data suggest that propofol modulation of HCN channel gating is an important molecular mechanism that can contribute to the depression of central nervous system function and also lead to bradyarrhythmias in patients receiving propofol during surgical anesthesia.

Use of diuretics in cardiovascular diseases: (1) heart failure

Diuretics are used extensively in hospitals and in community medical practice for the management of cardiovascular diseases. They are used frequently as the first line treatment for mild to moderate hypertension and are an integral part of the management of symptomatic heart failure. Although diuretics have been used for several decades, there is still some ambiguity and confusion regarding the optimal way of using these common drugs. In this paper, the classes and action of diuretics are reviewed, and the various indications, optimal doses, and recommendations on the effective use of these agents are discussed.

Droperidol lengthens cardiac repolarization due to block of the rapid component of the delayed rectifier potassium current

INTRODUCTION

Torsades de pointes have been observed during treatment with droperidol, a butyrophenone neuroleptic agent. Our objectives were (1) to characterize the effects of droperidol on cardiac repolarization and (2) to evaluate effects of droperidol on a major time-dependent outward potassium current involved in cardiac repolarization (I(K)r).

METHODS AND RESULTS

Isolated, buffer-perfused guinea pig hearts (n = 32) were stimulated at different pacing cycle lengths (150 to 250 msec) and exposed to droperidol in concentrations ranging from 10 to 300 nmol/L. Droperidol increased monophasic action potential duration measured at 90% repolarization (MAPD90) in a concentration-dependent manner by 9.8+/-2.3 msec (7.3%+/-0.7%) at 10 nmol/L but by 32.7+/-3.6 msec (25.7%+/-2.2%) at 300 nmol/L (250-msec cycle length). Increase in MAPD90 also was reverse frequency dependent. As noted previously, droperidol 300 nmol/L increased MAPD90 by 32.7+/-3.6 msec (25.7%+/-2.2%) at a pacing cycle length of 250 msec but by only 14.1+/-1.3 msec (13.6%+/-2.3%) at a pacing cycle length of 150 msec. Patch clamp experiments performed in isolated guinea pig ventricular myocytes demonstrated that droperidol decreases the time-dependent outward K+ current elicited by short depolarizations (250 msec; I(K)250) in a concentration-dependent manner. Estimated IC50 for I(K)250, which mostly underlies I(K)r, was 28 nmol/L. Finally, HERG K+ current elicited in HEK293 cells expressing high levels of HERG protein was decreased 50% by droperidol 32.2 nmol/L.

CONCLUSION

Potent block of I(K)r by droperidol is likely to underlie QT prolongation observed in patients treated at therapeutic plasma concentrations (10 to 400 nmol/L) of the drug.

Concomitant Block of the Rapid (I(Kr)) and Slow (I(Ks)) Components of the Delayed Rectifier Potassium Current is Associated With Additional Drug Effects on Lengthening of Cardiac Repolarization

BACKGROUND: The delayed rectifier potassium current, which comprises both a rapid (I(Kr)) and as slow (I(Ks)) component, is a major outward current involved in repolarization of cardiac myocytes. I(Kr) is the target of most drugs that prolong repolarization, whereas electrophysiological effects resulting from combined block of I(Kr) and I(Ks) still need to be characterized. METHODS AND RESULTS: Studies in isolated, buffer-perfused guinea pig hearts were undertaken to compare lengthening of cardiac repolarization under conditions of I(Kr) block alone, I(Ks) Block alone, or combined block of I(Kr) and I(Ks). In protocol A, isolated perfusion with N-acetylprocainamide (NAPA) (I(Kr) block), indapamide (I(Ks) block), or combined NAPA/indapamide was performed at a pacing cycle length of 250 msec. Increases in monophasic action potential duration measured at 90% polarization (MAPD(90)) from baseline after perfusion with NAPA 100 µmol/L (IC(50) for block of I(Kr)) was 19 +/- 6 msed (P <.05), after indapamide 100 µmol/L (EC(50) for block of I(Ks)) 13 +/- 2 msec (P <.05), but 42 +/- 5 msec after combined NAPA 100 µmol/L and indapamide 100 µmol/L (P <.05 vs. baseline and isolated administrations), suggesting the possibility of excessive lengthening of cardiac repolarization by blocking both I(Kr) and I(Ks). As well, in protocol B where sequential perfusions with dofetilide (I(Kr) blocker), dofetilide/indapamide, and indapamide in the same hearts were used, combined dofetilide/indapamide infusion showed a greater increase in MAPD(90) during all pacing cycles studied (250 to 150 msec). CONCLUSIONS: Combined I(Kr) and I(Ks) block may lead to excessive lengthening of cardiac repolarization. This may predispose patients to proarrhythmia during coadministration of drugs.

Ionic currents during sustained pacemaker activity in rabbit sino-atrial myocytes

1. The contribution of various ionic currents to diastolic depolarization (DD) in rabbit sinoatrial myocytes was evaluated by the action potential clamp technique. Individual currents were identified, during sustained pacemaking activity reproduced under voltage clamp conditions, according to their sensitivity to specific channel blockers. 2. The current sensitive to dihydropyridines (DHPs), blockers of L-type Ca2+ current (ICa,L), was small and outward during most of DD. Diastolic DHP-sensitive current was affected by changes in the driving force for K+, but it was insensitive to E-4031, which blocks the current termed IK,r; it was abolished by cell dialysis with a Ca2+ chelator. 3. The current sensitive to 2 mM Cs+ (ICs), a blocker of hyperpolarization-activated current (I(f)), was inward during the whole DD and it was substantially larger than the net inward current flowing during this phase. However, diastolic IK,r, identified in the same cells as the current sensitive to the blocker E-4031, exceeded ICs 2-fold. 4. These findings suggest that: (a) Ca2+ influx during the pacemaker cycle increases a K+ conductance, thus inverting the direction of the net current generated by L-type Ca2+ channel activity during DD; (b) the magnitude of I(f) would be adequate to account fully for DD; however, the coexistence of a larger IK,r suggests that other channels besides I(f) contribute inward current during this phase.

Modeling triggered cardiac activity: an analysis of the interactions between potassium blockade, rhythm pauses, and cellular coupling

It is known that under certain conditions, a combination of potassium channel blockade, sympathetic nervous activity, and pauses in sinus rhythm can increase the occurrence of cardiac arrhythmias. Although the arrhythmogenic interactions of these three factors are not completely understood, it is believed that the associated arrhythmias may be initiated by afterpotentials via a process that we refer to as propagated triggered activity. Using a two-cell computational model of ventricular action potential kinetics, we simulate nonuniform potassium blockade, sympathetic nervous activity, and pauses in sinus rhythm under conditions of hypokalemia. Under these conditions, the two-cell model suggests that (1) the arrhythmogenic interactions of potassium blockade and sympathetic nervous activity are highly dependent on heart rate; (2) triggered activity induced by potassium blockade would most likely occur during a pause in sinus rhythm; (3) during a sufficiently large pause in sinus rhythm, potassium blockade can induce triggered activity at normal levels of sympathetic activity; and (4) potassium blockade can increase the probability of triggered activity only if heart rate falls within a critical range. We also show that during pauses in sinus rhythm, two-cell triggering interactions between potassium blockade and sympathetic activity closely parallel the parametric displacement of the dynamic instability underlying the afterpotentials. Our results indicate that the behavior of the triggering mechanism studied here is consistent with that of pause-induced arrhythmias.

Angiotensin II modulates the delayed rectifier potassium current of guinea pig ventricular myocytes

Amplitude of the delayed rectifier (IK) tail current measured following long (5000msec) depolarizing pulses (-10 to +50mV) was decreased 19 +/- 3% (P < 0.05) in a voltage-independent manner by angiotensin II (AII) 100nM. In contrast, amplitude of tail current measured following short (250msec) depolarizing pulses to potentials > +10mV was increased 13 +/- 3% (P < 0.05) by AII 30nM. Deactivation kinetics of IK measured at -30mV were altered by AII 30nM and 100nM; time constant of the faster deactivating phase (tau 1) was decreased 1.4-fold. In summary, data obtained demonstrated that physiological concentrations of AII modulates major outward potassium currents involved in cardiac repolarization. Results suggest that AII increases amplitude of the rapid component of IK (IKr) but decreases its slow component IKs. Thus, we postulate that modulators of AII effects may exhibit direct cardiac electrophysiological properties.