More types than one: multiple muscarinic receptor coupled K+ currents undergo remodelling in an experimental model of atrial fibrillation

The common cardiac arrhythmia atrial fibrillation (AF) tends to show progression in its severity, which is associated with 'remodelling': structural and electrophysiological changes that facilitate arrhythmia induction and maintenance. In this issue of the BJP, Yeh and colleagues demonstrate for the first time, down-regulation of three types of muscarinic cholinergic receptor (mAChR) coupled K+ currents (IKM2, IKM3 and IKM4) and of M2, M3 and M4 mAChR subtype proteins, in a canine model of atrial tachycardia (AT) induced remodelling. The IKMs and their extent of AT-induced remodelling were similar in left-atrial and pulmonary vein (PV) myocytes, so remodelling of M2-M4 receptor-linked currents appears not to underlie the unique contribution of PVs to AF. Parasympathetic stimulation can increase susceptibility to AF; thus remodelling of M2-M4 receptors and K+ currents could be adaptive in AT. Further work is warranted to determine whether or not remodelling of multiple mAChRs and currents also contributes to human AF.

Atrial tachycardia induces remodelling of muscarinic receptors and their coupled potassium currents in canine left atrial and pulmonary vein cardiomyocytes

BACKGROUND AND PURPOSE

Both parasympathetic tone and atrial tachycardia (AT) remodelling of ion channels play important roles in atrial fibrillation (AF) pathophysiology. Different muscarinic cholinergic receptor (mAChR) subtypes (M2, M3, M4) in atrial cardiomyocytes are coupled to distinct K+-currents (called IKM2, IKM3, IKM4, respectively). Pulmonary veins (PVs) are important in AF and differential cholinergic current responses are a potential underlying mechanism. This study investigated AT-induced remodelling of mAChR subtypes and K+-currents in left-atrial (LA) and PV cardiomyocytes.

EXPERIMENTAL APPROACH

Receptor expression was assayed by western blot. IKM2, IKM3 and IKM4 were recorded with whole-cell patch-clamp in LA and PV cardiomyocytes of nonpaced control dogs and dogs after 7 days of AT-pacing (400 bpm).

KEY RESULTS

Current densities of IKM2, IKM3 and IKM4 were significantly reduced by AT-pacing in LA and PV cardiomyocytes. PV cardiomyocyte current-voltage relations were similar to LA for all three cholinergic currents, both in control and AT remodelling. Membrane-protein expression levels corresponding to M2, M3 and M4 subtypes were decreased significantly (by about 50%) after AT pacing. Agonist concentration-response relations for all three currents were unaffected by AT pacing.

CONCLUSIONS AND IMPLICATIONS

AT downregulated all three mAChR-coupled K+-current subtypes, along with corresponding mAChR protein expression. These changes in cholinergic receptor-coupled function may play a role in AF pathophysiology. Cholinergic receptor-coupled K+-currents in PV cardiomyocytes were similar to those in LA under control and AT-pacing conditions, suggesting that differential cholinergic current properties do not explain the role of PVs in AF.

Serum cardiac markers response to biphasic and monophasic electrical cardioversion for supraventricular tachyarrhythmia—a randomised study

BACKGROUND

Electrical cardioversion in patients with various types of supraventricular tachyarrhythmia (SVT) may induce serum cardiac markers elevation. Only a few studies have evaluated the impact of the type of shock waveform on electrical myocardial injury. The aim of our study was to compare the response of serum cardiac markers to biphasic and monophasic cardioversion for SVT.

METHODS

One hundred and forty one patients with various SVTs referred for electrical cardioversion were randomised to monophasic (MP) or biphasic (BP) cardioversion. Serum levels of creatine kinase (CK), MB fraction of CK (CK-MB), myoglobin and troponin I were analysed before cardioversion and 254+/-58 min after the procedure.

RESULTS

Average age of the patients was 67.9+/-11.3 years, 71 underwent BP and 70 MP cardioversion. In MP group, cumulative energy (CE)>150J was associated with significant elevation of CK and myoglobin levels after cardioversion (1.52+/-3.81 microkat/l and 187+/-433 microg/l), while CE<150J was not (-0.04+/-0.34 and 4+/-11, p<0.05). In BP group, CE>150J was associated with significant but smaller CK elevation (0.27+/-1.09 microkat/l, p<0.05) and comparable myoglobin elevation (80.7+/-21.4 microg/l, p<0.05). CE>150J was the only independent positive predictor for CK and myoglobin elevation in both groups. No significant changes in CK-MB and Troponin I levels after cardioversion were identified.

CONCLUSIONS

According to our study, electrical cardioversion for SVTs is not associated with biochemical signs of myocardial injury. Application of CE>150J can be followed by CK and myoglobin elevation most likely due to skeletal muscle damage. This reaction is more pronounced in MP than in BP cardioversion.

Magnesium for the Treatment and Prevention of Atrial Tachyarrhythmias

Atrial tachyarrhythmias, including atrial fibrillation and flutter, occur frequently. Magnesium has been studied in the early conversion and prevention of atrial tachyarrhythmias, as well as in prevention of atrial tachyarrhythmias after coronary artery bypass graft surgery. Early conversion of atrial tachyarrhythmias and control of heart rate may be greater with magnesium than with common antiarrhythmic agents. Magnesium appears to be less useful for preventing recurrent atrial tachyarrhythmias; however, discrepancies in study methodologies make interpretation of results difficult. The use of magnesium for prevention of postoperative atrial arrhythmias has produced conflicting results, likely due to differences in study design. From the limited data available, magnesium appears to have some inherent antiarrhythmic properties. Certain patient populations may derive benefit from magnesium for the treatment of atrial tachyarrhythmias. However, further study is necessary to define the role of magnesium clearly for the treatment or prevention of atrial tachyarrhythmias.