Electrocardiographic interactions between pinacidil, a potassium channel opener and class I antiarrhythmic agents in guinea-pig isolated perfused heart

Potassium channel activators differentially modulate the effect of sodium channel blockade on cardiac conduction

AIMS

Diminished repolarization reserve contributes to the arrhythmogenic substrate in many disease states. Pharmacological activation of K(+) channels has been suggested as a potential antiarrhythmic therapy in such conditions. Having previously demonstrated that I(K1) and I(Kr) can modulate cardiac conduction, we tested here the effects of pharmacological I(KATP) and I(Ks) activation on cardiac conduction and its dependence on the sodium current (I(Na)).

METHODS AND RESULTS

Bath electrocardiograms (ECGs) recorded from Langendorff-perfused guinea pig ventricles revealed QRS prolongation during I(KATP) activation by pinacidil but not during I(Ks) activation by R-L3 relative to control. In contrast, when I(Na) was partially blocked by flecainide, R-L3 but not pinacidil prolonged the QRS relative to flecainide alone. Conduction velocity (θ) was quantified by optical mapping during epicardial pacing. Both longitudinal (θ(L)) and transverse (θ(T)) θ were reduced by pinacidil (by 10 ± 1 and 9 ± 3%, respectively) and R-L3 (by 11 ± 2% and 15 ± 4%, respectively). Flecainide decreased θ(L) by 33 ± 4% and θ(T) by 36 ± 5%. Whereas pinacidil did not further slow θ relative to flecainide alone, R-L3 decreased both θ(L) and θ(T).

CONCLUSION

Pharmacological activation of I(KATP) and I(Ks) slows cardiac conduction; however, they demonstrate diverse effects on θ dependence on I(Na) blockade. These findings may have significant implications for the use of K(+) channel activators as antiarrhythmic drugs and for patients with Na(+) channel abnormalities or being treated with Na(+) channel blockers.

Does Cl-/HCO3- exchange play an important role in reperfusion arrhythmias in rats?

The protective effects of Cl(-)/HCO(3)(-) exchange inhibitors, 4,4'-diisothiocyano-stilbene-2,2'-disulfonic acid (DIDS) and 4-acetamido-4'isothiocyanato-stilbene-2,2'-disulfonic acid (SITS), against reperfusion-induced arrhythmias were investigated in anesthetized rats. Rats were subjected to 5-min occlusion of the left coronary artery followed by 10-min reperfusion. All drugs were intravenously administered 5 min before the onset of occlusion. DIDS (75 mg/kg) reduced the incidence of ventricular fibrillation and mortality to 0%, whereas SITS (75 mg/kg) only decreased these parameters to 60%. DIDS simultaneously decreased the mean blood pressure and heart rate, and prolonged PQ and QRS intervals, whereas SITS produced a weaker effect on these parameters and no change in QRS interval. Mexiletine (5 mg/kg), which had been demonstrated to suppress the arrhythmias and reduce the heart rate and mean blood pressure in this model, was shown to prolong PQ and QRS intervals. Verapamil (0.5 mg/kg) or diltiazem (0.4 mg/kg) suppressed the arrhythmias, simultaneously decreasing the heart rate and mean blood pressure and prolonging PQ interval. The results indicate that the protective effect of DIDS on reperfusion arrhythmias in the anesthetized rats is unlikely to be attributed to the inhibitory action on Cl(-)/HCO(3)(-) exchange, but possibly mediated by its blocking effects on cardiac ion channels, such as Na(+) or Ca(2+) channels.

Cardiac effects of quinidine on guinea-pig isolated perfused hearts after in vivo quinidine pretreatment

1 Experimental and clinical studies suggest that class I and class III antiarrhythmic drugs may be subject to pharmacological tolerance during long term treatment, leading to loss of therapeutic effectiveness. 2 The aim of this study was to ascertain whether prolonged in vivo treatment with the Class Ia agent quinidine can modify cardiac (electrical and mechanical) responses to the drug. 3 A group of guinea-pigs (n = 7) was treated intraperitoneally (q.d.) for 6 days with 75 mg kg-1 quinidine sulphate. Preliminary pharmacokinetic experiments indicated that this dose could attain Plasma concentrations similar to those that are therapeutic in man (2-5 mg l-1). A control group (n = 7) received a saline solution for the same period. 4 Twenty-four hours after the last administration hearts were removed and retrogradely perfused at constant flow (stimulation frequency: 2.5 Hz). The following parameters were measured: maximal derivative of intraventricular pressure (dP/dtmax); coronary perfusion pressure (Cp); PR, QRS and JT intervals, on surface ECG. The effects of quinidine on these parameters were measured at different concentrations (2, 4, 8, 12, 16, 20 microns) and compared in the two experimental groups. 5 In the group quinidine decreased in a dose-dependent manner dP/dt and increased PR and QRS intervals. JT interval was increased at the lowest concentrations and decreased at the highest (biphasic effect). Cp did not change significantly. 6 In the pretreated group quinidine qualitatively produced the same effects on dP/dt and ECG intervals as in control group. Also the magnitude of these effects was not significantly different between the two groups. In contrast with findings in control experiments. Cp was significantly decreased by increasing quinidine concentration. Mean baseline Cp was higher in pretreated than in the control group (though not significantly, P = 0.072) and quinidine addition abolished this difference. Thus, it is suggested that quinidine withdrawal induced a rebound increase in coronary tone, due to the unmasking of vasoconstrictor homeostatic mechanisms elicited by the in vivo vasodilating effect of the drug. 7 In conclusion, our data do not support the possibility that tolerance ensues during long term quinidine treatment, at least as far as electrophysiological and contractility effects are concerned. Further experimental work is needed to explain the appearance of a coronary vasodilating effect in pretreated hearts.

Calcium-antagonist effects of norbormide on isolated perfused heart and cardiac myocytes of guinea-pig: a comparison with verapamil

1. Cardiac effects on norbormide and verapamil were compared in single ventricular myocytes, right atria, and Langendorff perfused hearts isolated from guinea-pigs. 2. In ventricular myocytes, norbormide 50 microM inhibited the peak calcium current (ICa) by 49.6 +/- 3.9% without altering the shape of the current-voltage relationship; verapamil 1 microM inhibited ICa by 83.2 +/- 3.3%. Neither norbormide nor verapamil affected ICa at the first beat after a 3 min quiescence period; during repeated depolarizations, both drugs cumulatively blocked ICa (use-dependence), with time constants of 23.0 +/- 7.0 s for norbormide and 91.3 +/- 8.4 s for verapamil. 3. In constant-flow perfused hearts electrically driven at 2.5 Hz or 3.3 Hz, both norbormide and verapamil concentration-dependently decreased ventricular contractility (dP/dtmax), atrio-ventricular (AV) conduction velocity and coronary pressure. Intraventricular conduction velocity was slightly decreased by norbormide but not by verapamil. At an equivalent change in AV conduction, norbormide depressed heart contractility less than verapamil. The effects of norbormide on AV conduction, intraventricular conduction, and contractility were frequency-dependent. Furthermore, the curves correlating the mechanical and electrical effects of norbormide at the two frequencies used were apparently coincident, while those of verapamil were clearly separated. 4. In spontaneously beating right atria, norbormide and verapamil decreased the frequency of sinus node (SA) in a concentration-dependent way. At an equivalent effect on the AV conduction, norbormide exerted a greater effect on sinus frequency than verapamil. 5. These results indicate that in guinea-pig heart norbormide has the pharmacological profile of a Ca-antagonist with strong electrophysiological properties. In comparison with verapamil, norbormide is more selective on SA and AV node tissues and exerts a weaker negative inotropic effect on ventricles. In principle, this pattern of effects may be an advantage in treating supraventricular tachyarrhythmias in patients with heart failure. The effect of norbormide on intraventricular conduction may represent an additional antiarrhythmic mechanism.