Suppression of Calcium-Induced Repolarization Heterogeneity as a Mechanism of Nitroglycerin??s Antiarrhythmic Action

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Abstract
This study examined whether the antifibrillatory action of nitroglycerin (NTG) is attributable to reduction in calcium-induced heterogeneity of repolarization independent of autonomic and coronary vasodilatory influences. The effects of intrapericardial (IPC) NTG on coronary blood flow, contractility, repolarization, and arrhythmia susceptibility were measured in anesthetized pigs (N = 43). Autonomic influences were minimized by vagotomy and β-adrenergic blockade (metoprolol, 1.25 mg/kg, intravenous). Electrophysiological parameters were tested at 30 min, a time when coronary hemodynamics had returned to baseline. Intracoronary calcium chloride (CaCl2, 50-mg bolus) injection augmented contractility (dP/dtmax, 1760 ± 144 to 2769 ± 274 mmHg/s, and following NTG, 1531 ± 384 to 2138 ± 242 mmHg/s, P < 0.0002), reflecting increased myocardial intracellular calcium. Calcium increased repolarization heterogeneity (interlead precordial T-wave heterogeneity, 95 ± 15 to 264 ± 33 μV, P < 0.006; Tpeak−Tend, an index of transmural dispersion of repolarization, 37 ± 3 to 76 ± 6 ms, P < 0.05) and lowered repetitive extrasystole threshold (RET; 24 ± 2 to 13 ± 1 mA, and following NTG, 32 ± 4 to 18 ± 1 mA, P < 0.0001). IPC NTG raised the RET from baseline by 33% and blunted calcium-induced contractility (dP/dtmax by 23%, P < 0.05), repolarization changes (T-wave heterogeneity by 24%, P < 0.006; Tpeak−Tend by 18%, P = 0.04), and arrhythmia vulnerability (RET by 39%, P < 0.003). Thus, the capacity of NTG to suppress calcium-induced repolarization heterogeneity is an important mechanism of its antiarrhythmic action, which is independent of autonomic and vasodilatory actions.