The role of volume-regulated ion channels in atrial arrhythmopathy

Introduction
Atrial fibrillation (AF) is a complex and heterogeneous pathophysiological phenotype resulting from atrial cardiomyopathy secondary to various cardiovascular diseases. Myocardial ischaemia, for example, can lead to changes in intracellular osmolarity, creating an osmotic gradient that causes cell swelling. In this context, the ubiquitously expressed volume-regulated anion channel (VRAC), which plays a key role in cellular volume regulation is of particular interest. The protein LRRC8A (also known as Swell1), together with four other members of the LRRC8 family, appears to be a major VRAC in the heart. However, its role in arrhythmogenesis is still incompletely understood.

Methods

Atrial cardiomyocytes and fibroblasts were isolated from human tissue samples obtained from patients undergoing cardiac surgery with different rhythm states, including sinus rhythm (SR), paroxysmal AF (pAF) and chronic AF (cAF). Expression analysis at the mRNA and protein levels was performed by quantitative real-time PCR (qPCR) and immunoblotting. Functional relevance to cardiac electrophysiology was assessed using the patch-clamp technique. In addition, pharmacological characterisation of VRAC-mediated ionic currents was performed upon heterologous expression in Xenopus laevis oocytes, where cell swelling was induced by altering the extracellular osmolarity.

Results

Heterologous expression of human LRRC8 subunits in Xenopus laevis oocytes gave rise to osmosensitive currents that could be inhibited by classical VRAC blockers such as DCPIB and showed a distinct pharmacological profile. Expression analysis of cardiac tissue samples from patients with differing stages of atrial cardiomyopathy revealed differential regulation of volume-sensitive ion channels during atrial remodelling. qPCR demonstrated significant upregulation of LRRC8A mRNA in tissue samples from the cAF group. This observation was further reproduced at the protein level by immunoblotting. In addition, functional patch clamp measurements comparing VRAC current activation by hypotonic solution in human atrial cardiomyocytes showed a differential regulation of action potential duration (APD) between distinct rhythm stages: In atrial cardiomyocytes isolated from cAF patients, both APD₅₀ and APD₉₀ levels were significantly reduced upon induction of cell swelling.

Conclusion

The results of this study suggest that VRAC-mediated currents are actively involved in AF-related atrial remodelling and can be modulated by specific small molecule inhibitors. These findings extend our knowledge of the complex ionic circuitry underlying electrical atrial remodelling and may open new perspectives for therapeutic approaches specifically targeting atrial arrhythmopathy.