Long QT syndrome in dogs with tick toxicity (Ixodes holocyclus)

OBJECTIVE

To evaluate cardiac electrical function in dogs with tick toxicity.

DESIGN

A prospective clinical investigation of 39 client-owned dogs treated for naturally occurring tick toxicity.

PROCEDURE

An ECG was performed on each dog on several occasions; at admission to hospital with tick toxicity, 24 h later, at discharge from hospital when clinically normal and approximately 12 months later.

RESULTS

The mean QT interval corrected for heart rate (QTc) was prolonged at admission, 24 h and at discharge compared to the QTc measured 12 months later. T wave morphology was altered in dogs at admission. All other parameters were within normal limits.

CONCLUSIONS

The prolonged QTc interval and altered T wave morphology of dogs with tick toxicity reflects delayed cardiac repolarisation and is comparable with long QT syndrome (LQTS) in people who are predisposed to polymorphic ventricular tachycardia and sudden death. Resolution of ECG changes lagged behind clinical recovery.

Proarrhythmia

Proarrhythmia is defined as the aggravation of an existing arrhythmia or the development of a new arrhythmia secondary to antiarrhythmic drug. Proarrhythmic events include drug-induced bradyarrhythmias, atrial and ventricular proarrhythmias. New onset sustained or incessant ventricular tachycardia and torsade de pointes can be life threatening. This article reviews the incidence, aggravating factors, and treatment of proarrhythmia.

Effects of BDF 9198 on action potentials and ionic currents from guinea-pig isolated ventricular myocytes

BDF 9198 (a congener of DPI 201 - 106 and BDF 9148) was found to be a positive inotrope on guinea-pig isolated ventricular muscle strips. The effects of BDF 9198 on action potentials and ionic currents from guinea-pig isolated ventricular myocytes were studied using the whole cell patch clamp method. In normal external solution, at 37 degrees C, action potential duration at 50% repolarization (APD(50)) was 167.4+/-8.36 ms (n=37). BDF 9198 produced a concentration-dependent increase in APD(50) (no significant increase at 1x10(-10) M; and APD(50) values of 273.03+/-35.8 ms at 1x10(-9) M; n=6, P<0.01 and 694.7+/-86.3 ms at 1x10(-7) M; P<0.001, n=7). At higher concentrations in the range tested, BDF 9198 also induced early and delayed and after-depolarizations. Qualitative measurements of I(Na) with physiological [Na](o) showed prolongation of the current by BDF 9198, and the appearance of transient oscillatory inward currents at high concentrations. Quantitative recording conditions for I(Na) were established using low external [Na] and by making measurements at room temperature. The current - voltage relation, activation parameters and time-course of I(Na) were similar before and after a partial blocking dose of Tetrodotoxin (TTX, 1 microM), despite a 2 fold difference in current amplitude. This suggests that voltage-clamp during flow of I(Na) was adequately maintained under our conditions. Selective measurements of I(Na) at room temperature showed that BDF 9198 induced a concentration-dependent, sustained component of I(Na) (I(Late)) and caused a slight left-ward shift in the current - voltage relation for peak current. The drug-induced I(Late) showed a similar voltage dependence to peak current in the presence of BDF 9198. Both peak current and I(Late) were abolished by 30 microM TTX and were sensitive to external [Na]. Inactivation of control I(Na) during a 200 ms test pulse to -30 mV followed a bi-exponential time-course. In addition to inducing a sustained current component, BDF 9198 left the magnitude of the fast inactivation time-constant unchanged, but increased the magnitude of the slow inactivation time-constant. Additional experiments with a longer pulse (1 s) raised the possibility that in the presence of BDF 9198, I(Na) inactivation may be comprised of more than two phases. No significant effects of 1x10(-6) M BDF 9198 were observed on the L-type calcium current, or delayed and inward rectifying potassium currents measured at 37 degrees C. It is concluded that the prolongation of APD(50) by BDF 9198 resulted from selective modulation of I(Na). Reduced current inactivation induced a persistent I(Na), increasing the net depolarizing current during the action potential. This action of the drug indicates a potential for 'QT prolongation' of the ECG. The observation of after-depolarizations suggests a potential for proarrhythmia at some drug concentrations.

Modulation of HERG potassium channels by extracellular magnesium and quinidine

Torsades de pointes is a polymorphic ventricular arrhythmia resulting from congenital or drug-induced (acquired) QT prolongation. Pharmacologic suppression of repolarizing potassium currents is one mechanism causing the acquired long QT (LQT) syndrome. Recent studies have linked mutations in a gene encoding a potassium channel subunit (HERG) to the LQT syndrome. Clinical experience indicates that intravenous magnesium sulfate is effective in reversing torsades de pointes, but the molecular basis of this effect is not understood. This study was designed to investigate the effects of extracellular magnesium (Mg2+) on HERG potassium currents. HERG potassium channels were expressed in Xenopus oocytes and in a human cell line and were examined by voltage-clamp methods. Extracellular Mg2+ (0.3-10 mM) caused a concentration-dependent shift in the membrane-potential dependence of HERG channel opening, causing a reduction in K+ current. This effect was much greater than that observed in another human delayed rectifier K+ channel, hKv1.5, suggesting a specific interaction with the HERG channel. Quinidine is an antiarrhythmic drug that also causes torsades de pointes under certain conditions. Quinidine (3 microM) inhibited HERG currents expressed in oocytes by 32.1 +/- 3.2% (n = 5), whereas 1 microM quinidine inhibited HERG currents in tsA201 cells by 75.8 +/- 2.4% (n = 12). Increasing extracellular Mg2+ did not relieve the inhibition by quinidine, but caused additional suppression. These results indicate that extracellular Mg2+ exerts a direct action on HERG potassium channels, resulting in suppression of outward repolarizing potassium current. It is concluded that modulation of this important K+ current is not the mechanism by which intravenous magnesium terminates drug-induced LQT and torsades de pointes. Potent suppression of HERG channel current by quinidine, compared with that of I(Ks) and I(Na), is a likely contributor to torsades de pointes arrhythmias.

Mechanisms and management of proarrhythmia

It is now well recognized that therapy with antiarrhythmic drugs can not only suppress cardiac arrhythmias, but also may increase their frequency or provoke new ones. Specific proarrhythmia syndromes, each with a distinct underlying mechanism and approach to therapy, have been described. The best-recognized examples are digitalis intoxication, proarrhythmia associated with sodium-channel block, and torsade de pointes occurring during QT-prolonging therapies. In the case of sodium-channel blockers, 2 forms of proarrhythmia are commonly recognized: slow atrial flutter with 1:1 atrioventricular conduction, and frequent ventricular tachycardia ([VT], most often found in patients with pre-existing VT reentrant circuits). In all cases, the best approach to therapy is to identify patients at risk (and thereby avoid therapy entirely), to recognize proarrhythmia when it occurs, to withdraw offending agent(s), and to use specific corrective therapies when available. Although most recognized episodes of proarrhythmia are thought to occur early in drug therapy, the increased mortality during chronic antiarrhythmic therapy demonstrated in large randomized trials suggests this phenomenon can also develop during long-term drug treatment. The recognition of proarrhythmia and the delineation of its underlying mechanisms should not only improve therapy with available drugs, but may also direct development of newer agents devoid of this potential.

Evidence for functional relevance of an enhanced expression of the Na(+)-Ca2+ exchanger in failing human myocardium

BACKGROUND

The present study aimed at investigating the expression of the Na(+)-Ca2+ exchanger and its functional role in human failing myocardium.

METHODS AND RESULTS

Na(+)-Ca2+ exchanger mRNA and protein levels were examined in nonfailing (NF, n = 8) and failing human myocardium (New York Heart Association functional class IV) with idiopathic dilated cardiomyopathy (DCM, n = 8) or ischemic heart disease (ICM, n = 6). The inotropic effect of the Na+ channel activator BDF 9148 was determined in electrically driven left ventricular papillary muscle strip preparations (1 Hz, 37 degrees C) from nonfailing (n = 8) and failing (n = 8) human hearts. Na(+)-Ca2+ exchanger mRNA levels were significantly increased, by 79% (P < .001) in DCM and by 58% (P < .01) in ICM compared with NF; protein levels increased by 36% (P < .001) and by 20% (P < .05), respectively. BDF 9148 increased the force of contraction concentration dependently, with a similar maximal effect in NYHA class IV and NF, but was more potent in NYHA class IV as demonstrated by a significantly smaller (P < .01) EC50 value (NYHA class IV, 0.18 [0.16 to 0.22] mumol/L; NF, 1.65 [1.3 to 3.0] mumol/L). In NYHA class IV, BDF 9148 (0.1 mumol/L) restored the positive force-frequency relationship and reduced the frequency-dependent increase in diastolic tension in relation to force of contraction.

CONCLUSIONS

The increased expression of the Na(+)-Ca2+ exchanger is a possible explanation for the increased inotropic potency of the Na+ channel activator BDF 9148 in failing human myocardium. The increase in exchanger molecules could be of functional relevance for the modulation of cardiac contractility by agents that increase the intracellular Na+ concentration. Enhancement of Na(+)-Ca2+ exchanger activity might be a powerful mechanism for increasing cardiac contractility in chronic heart failure.

Mechanism of block of a human cardiac potassium channel by terfenadine racemate and enantiomers

1. The cardiac toxicity of racemic terfenadine (marked QT prolongation and polymorphic ventricular arrhythmias) is probably due to potassium channel blockade. To test whether one of its enantiomers would be a less efficient potassium channel blocker, we compared the mechanism of action of the racemate with that of the individual enantiomers. 2. We synthesized the individual enantiomers of terfenadine and examined under whole cell voltage-clamp conditions the mechanism of action of the racemate, both enantiomers and a major metabolite on a cloned human cardiac potassium channel, hKv1.5. This delayed rectifier is sensitive to quinidine, clofilium and other 'class III' antiarrhythmic drugs at clinically relevant concentrations. 3. Upon depolarization, racemic terfenadine and its enantiomers induced a fast decline of hKv1.5 current towards a reduced steady state current level. During subsequent repolarization the tail currents deactivated more slowly than the control, resulting in a 'crossover' phenomenon. 4. The voltage-dependence of block was biphasic with a steep increase in block over the voltage range of channel opening (-30 to 0 mV), and a more shallow phase positive to 0 mV (where the channel is fully open). The latter was consistent with a binding reaction sensing 21% of the transmembrane electrical field (with reference to the cell interior). 5. The EC50 for hKv1.5 block by racemic terfenadine was 0.88 microM, while the values for R- and S-terfenadine were 1.19 microM and 1.16 microM, respectively. In contrast, the acid metabolite reduced hKv1.5 current by only 5% at a concentration of 50 microM. 6. These findings suggest that terfenadine blocks the hKvl.5 channel after it opens by entering into the internal mouth of the channel. We have previously shown that quinidine blocks hKvl.5 in a similar manner but with an apparent affinity of ~6 micro M. Thus, terfenadine and its enantiomers are approximately equipotent open state blockers of this human K+ channel and about 6 times more potent than quinidine. The similar state-, time-, and voltage-dependence of hKvl.5 block by both enantiomers also indicates that the chiral centre does not significantly constrain the orientation of critical binding determinants of terfenadine with respect to the receptor site.

Characterization of the effects of the new inotropic agent BDF 9148 in isolated papillary muscles and myocytes of the guinea-pig heart

1. The cardiotonic agent BDF 9148 (4-[3'-(1''-benzhydryl-azetidine-3''-oxy)-2'-hydroxypropoxy]-1H-indole- 2-carbonitrile) is structurally related to DPI 201-106 (4-[3'-(4''-benzhydryl-1''-piperazinyl) -2'-hydroxypropoxy]-1H-indole-2-carbonitrile) which is known to modify cardiac sodium channels. In guinea-pig papillary muscles, both compounds increase force of contraction with similar concentration-response curves. Like DPI 201-106, BDF 9148 prolongs the action potential duration in a tetrodotoxin-sensitive manner. With high concentrations (greater than 3 microM), however, the action potential duration shortens again. In order to elucidate the underlying changes in membrane currents, we have investigated the effects of BDF 9148 in isolated ventricular myocytes of the guinea-pig heart. 2. In isolated cells, a concentration of 1 microM BDF 9148 prolonged the action potential duration and markedly enhanced unloaded cell shortening, indicating that the procedure of cell isolation does not abolish the effect of the drug. 3. Membrane currents were studied with the single electrode voltage clamp technique. With clamp steps from -80 mV to -40 mV, BDF 9148 (1 microM) induced a slowly decaying inward current which was suppressed by tetrodotoxin. Therefore, like DPI 201-106, BDF 9148 slows the inactivation of the sodium channels. 4. In order to quantify the effects of BDF 9148 and DPI 201-106 on sodium current inactivation, we have measured the inward current amplitude still present at 100 ms after a depolarizing clamp step from -80 mV to -30 mV. Both drugs increased this current component in a concentration-dependent manner; however, BDF 9148 had a larger effect in the low concentration range.5. The calcium current was inhibited by BDF 9148 and DPI 201-106 in a concentration-dependent manner; the pD2 values were 5.70 and 5.95, respectively.6. The two compounds are thought to produce similar positive inotropic effects by imposing a sodium load on the muscle cells via modification of the sodium channels. The differences in action potential duration could be due to different contributions of ionic currents other than sodium or calcium currents and of pump and exchange currents. At present, there is not sufficient data to identify clearly distinct current components responsible for the differences in action potential prolongation.