Pathological impact of hyperpolarization-activated chloride current peculiar to rat pulmonary vein cardiomyocytes

Pulmonary veins (PVs) are believed to be a crucial origin of atrial fibrillation. We recently reported that rat PV cardiomyocytes exhibit arrhythmogenic automaticity in response to norepinephrine. Herein, we further characterized the electrophysiological properties underlying the potential arrhythmogenicity of PV cardiomyocytes. Patch clamping studies revealed a time dependent hyperpolarization-activated inward current in rat PV cardiomyocytes, but not in left atrial (LA) myocytes. The current was Cs⁺ resistant, and was not affected by removal of external Na⁺ or K⁺. The current was inhibited with Cd²⁺, and the reversal potential was sensitive to changes in [Cl⁻] on either side of the membrane in a manner consistent with a Cl⁻ selective channel. Cl⁻ channel blockers attenuated the current, and slowed or completely inhibited the norepinephrine-induced automaticity. The biophysical properties of the hyperpolarization-activated Cl⁻ current in rat PVs were different from those of ClC-2 currents previously reported: (i) the voltage-dependent activation of the Cl⁻ current in rat PVs was shifted to negative potentials as [Cl⁻]_i increased, (ii) the Cl⁻ current was enhanced by extracellular acidification, and (iii) extracellular hyper-osmotic stress increased the current, whereas hypo-osmotic cell swelling suppressed the current. qPCR analysis revealed negligible ClC-2 mRNA expression in the rat PV. These findings suggest that rat PV cardiomyocytes possess a peculiar voltage-dependent Cl⁻ channel, and that the channel may play a functional role in norepinephrine-induced automaticity.