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 Ca2+-activated chloride current ( ICl,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 μM ryanodine, 100 μM 4,4′-diisothiocyanostilbene-2,2′-disulfonic acid (DIDS), and 100 μM 4-acetamido-4′-isothiocyanostilbene-2,2′-disulfonic acid (SITS). The density of the Ca2+-activated chloride currents ( ICl,Ca) in left ventricular myocytes was significantly greater in the LVH rabbits than in the normal group. Our data indicate that abnormal intracellular Ca2+ fluctuation may exert a strong feedback on the membrane ICl,Ca, leading to a beat-to-beat change in the net repolarizing current that manifests as T-wave alternans on the ECG.
T-Wave Alternans, ST-Segment Depression, Left Ventricular Hypertrophy, and Mortality Risk in Patients with Ischemic Cardiomyopathy: Some Possible Confounding Factors