Use-dependent electrophysiologic effects of amiodarone in coronary artery disease and inducible ventricular tachycardia

Interaction of the antiarrhythmic drug Amiodarone with the sodium channel Nav1.5 depends on the extracellular pH

INTRODUCTION

Amiodarone (AMD) is a clinically used drug to treat arrhythmias with significant effect upon the cardiac sodium channel Nav1.5. AMD has a pKa of 6.56, and changes in extracellular pH (pHe) may alter its pharmacological properties. Here we explored how changes in pHe impacts the pharmacological properties of AMD upon human-Nav1.5-sodium-current (INa) and in ex vivo rat hearts.

METHODS

Embryonic-human-kidney-cells (HEK293) were used to transiently express the human alpha-subunit of NaV1.5 channels and the isolated heart of Wistar rats were used. Patch-Clamp technique was deployed to study INa and for electrocardiogram (ECG) evaluation the ex vivo heart preparation in the Langendorff system was applied.

RESULTS

The potency of AMD upon peak INa was ∼25x higher in pHe 7.0 when compared to pHe 7.4. Voltage dependence for activation did not differ among all groups. AMD shifted the steady-state inactivation curve to more hyperpolarized potentials, with similar magnitudes for both pHes. The recovery from INa inactivation was delayed in the presence of AMD with similar profile in both pHes. Interestingly, the use-dependent properties of AMD was distinct at pHe 7.0 and 7.4. Finally, AMD was able to change the ex vivo ECG profile, however at pHe 7.0+AMD a larger increase in the RR and QRS duration and in the QT interval when compared to pHe 7.4 was found.

CONCLUSIONS

The pharmacological properties of AMD upon NaV1.5 and isolated heart preparation depends on the pHe and its use in vivo during extracellular acidosis may cause a distinct biological response in the heart tissue.

Acute effects of amiodarone on membrane properties, refractoriness, and conduction in guinea pig papillary muscles

Amiodarone has potent and complex antiarrhythmic effects associated with a rare incidence of proarrhythmia. For a comprehensive understanding of its antiarrhythmic mechanisms in the same preparations, amiodarone (50 microM) was employed as it would be in the clinical setting and applied to guinea pig papillary muscles impaled by microelectrodes, paced at different rates, and superfused with various concentrations of potassium ([K]e). Amiodarone exerted complex actions, as follows: (1) The maximum rate of rise (Vmax) of the fast action potential (i.e., [K]e = 5.4-9.0 mM) as well as that of the slow action potential (i.e., [K]e = 15.0 mM in the presence of 1.0 microM isoproterenol) was suppressed in a rate-dependent manner. (2) Amiodarone exhibited a rate- and [K]e-dependent increase in the ratio of effective refractory period vs action potential duration at 90% repolarization (ERP/APD90), disclosing post-repolarization refractoriness. (3) Amiodarone had no effect on passive cable factors, such as threshold current and tissue resistance, during propagation. These versatile electrophysiological effects of amiodarone may contribute to its unique antiarrhythmic effects, as well as the low incidence of proarrhythmia with this drug.