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Thursday, May 17, 2012

Ethanol Enhances Human Hyperpolarization-Activated Cyclic Nucleotide-Gated Currents

There is a clear association between excessive ethanol (EtOH) consumption and the risk of sudden cardiac death. The hyperpolarization-activated cyclic nucleotide-gated (HCN) current, If, is known to contribute to spontaneous pacemaker activity of sinoatrial (SA) node cells. However, the exact mechanisms of EtOH on arrhythmia induction are not well understood.

The preparations of SA node were excised from rabbit heart, transmembrane potentials were recorded by standard glass microelectrode technique, and a whole-cell patch clamp technique was used to record If in enzymatically isolated rabbit SA node pacemaker cells. Human HCN2 (hHCN2) and HCN4 channels were heterologously expressed in Xenopus oocytes and studied using 2-electrode voltage clamp technique.

Superfusion of EtOH increased the spontaneous firing frequency of SA node cells in a reversible fashion. Treatment with ivabradine irreversibly depressed basal firing frequency and markedly attenuated the enhancement effect of EtOH on firing. The stimulatory effects of EtOH on If were concentration-dependent in the range of 1 to 100 mM, with an average EC50 value of 20.81 ± 6.71 mM and Hill coefficient of 1.19 ± 0.10. Furthermore, EtOH reversibly enhanced the HCN currents in a concentration-dependent fashion with an EC50 value of 18.41 ± 2.75 mM for the HCN2 channel and 21.98 ± 3.54 mM for the HCN4 channel, which was accompanied by the acceleration of activation and deactivation kinetics. In addition, EtOH, at both moderate and high doses, caused a shift in the voltage dependence of hHCN4 channel activation to more depolarizing potentials. However, superfusion of high, not moderate, concentration of EtOH caused a shift in the voltage dependence of hHCN2 channel activation to more hyperpolarizing potentials.

This study provides insight into the molecular interaction of EtOH and the hHCN channels, which may shed light on elucidating the potentially proarrhythmic mechanism of EtOH.

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