Electrophysiologic alterations in the rabbit nodal cells induced by membrane lipid peroxidation

To investigate cellular electrophysiologic alterations due to lipid peroxidation of the cell membrane by free radicals as a possible cause of coronary reperfusion arrhythmias, we studied the effects of t-butyl hydroperoxide on the spontaneous action potential and membrane currents of the rabbit sinoatrial and atrioventricular node preparations (0.2 × 0.2 × 0.1 mm). 1–5 min of superfusion with t-butyl hydroperoxide (100–500 μM) caused a transient increase in the spontaneous firing frequency by 9%, accompanied by a 4% increase in the action potential amplitude and a 33% increase in the maximal rate of depolarization (P<0.05, n = 6). t-Butyl hydroperoxide then gradually suppressed physiological automaticity, but induced abnormal repetitive firing due to early and delayed after-depolarizations. 15 min of superfusion with t-butyl hydroperoxide caused a complete standstill of nodal cells at a resting potential of −46 ± 3 mV (n = 12). Such effects of t-butyl hydroperoxide on the spontaneous action potential were attenuated by pretreating the cells with butylated hydroxytoluene, a lipid peroxidation inhibitor. Voltage clamp experiments using double microelectrode methods revealed that t-butyl hydroperoxide transiently increased the Ca²⁺ current by 22% after 5 min of superfusion but subsequently reduced it to 46% of the control value after 15 min (P<0.05, n = 6). Similar biphasic changes were observed in the delayed rectifying K⁺ current and hyperpolarization-activated inward current (n = 6). Background current was progressively increased without any change in its reversal potential (n = 6). These results suggest that membrane lipid peroxidation may accelerate or suppress physiological automaticity and induce abnormal automaticity by both impairing cellula metabolic function and damaging the lipid membrane structure as well as ionic channel protein.