Calcium-activated chloride current contributes to action potential alternations in left ventricular hypertrophy rabbit

T-wave alternans, characterized by a beat-to-beat change in T-wave morphology, amplitude, and/or polarity on the ECG, often heralds the development of lethal ventricular arrhythmias in patients with left ventricular hypertrophy (LVH). The aim of our study was to examine the ionic basis for a beat-to-beat change in ventricular repolarization in the setting of LVH. Transmembrane action potentials (APs) from epicardium and endocardium were recorded simultaneously, together with transmural ECG and contraction force, in arterially perfused rabbit left ventricular wedge preparation. APs and Ca(2+)-activated chloride current (I(Cl,Ca)) were recorded from left ventricular myocytes isolated from normal rabbits and those with renovascular LVH using the standard microelectrode and whole cell patch-clamping techniques, respectively. In the LVH rabbits, a significant beat-to-beat change in endocardial AP duration (APD) created beat-to-beat alteration in transmural voltage gradient that manifested as T-wave alternans on the ECG. Interestingly, contraction force alternated in an opposite phase ("out of phase") with APD. In the single myocytes of LVH rabbits, a significant beat-to-beat change in APD was also observed in both left ventricular endocardial and epicardial myocytes at various pacing rates. APD alternans was suppressed by adding 1 microM ryanodine, 100 microM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), and 100 microM 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS). The density of the Ca(2+)-activated chloride currents (I(Cl,Ca)) in left ventricular myocytes was significantly greater in the LVH rabbits than in the normal group. Our data indicate that abnormal intracellular Ca(2+) fluctuation may exert a strong feedback on the membrane I(Cl,Ca), leading to a beat-to-beat change in the net repolarizing current that manifests as T-wave alternans on the ECG.