Slowly inactivating sodium currents are reduced by exposure to oxidative stress

Exposure of cardiac myocytes to oxidant stress has been implicated in the development of reperfusion arrhythmias. Studies on the effects of free radical generating systems on the fast sodium current have suggested an increase in a "window" current. The resulting increase in sodium influx has been hypothesized to cause an intracellular sodium load that stimulates Na+, Ca2+ exchange and promotes a Ca2+ overload. To test this proposal, the time course for effects of oxidative stress on a sodium current elicited with voltage ramps was investigated in feline ventricular myocytes. No window current was observed; instead, a slowly inactivating sodium current was generated at negative voltages near the sodium threshold potential. At room temperature there were no effects of a 30-min exposure to 1 mm H2O2 on this slowly inactivating sodium current. Likewise, there were no effects of either 1 mm H2O2 or 1.5 mm t-butyl hydroperoxide on fast sodium currents recorded at cool temperatures (12-15 degrees C). Experiments were repeated with t-butyl hydroperoxide at warm temperatures (30-33 degrees C), and the fast sodium current was reduced in magnitude and the reversal potential shifted to more negative voltages. These results demonstrate a temperature dependence for the loss of the fast sodium current during exposure to t-butyl hydroperoxide. Two exponentials were fit to the decaying phase of the fast sodium current and the slow time constant of inactivation was prolonged, suggesting delayed inactivation of the sodium current. Currents elicited with a steady-state inactivation protocol suggested development of a non-inactivating component during exposure to t-butyl hydroperoxide at warm temperatures. Direct evaluation of the slowly inactivating sodium current elicited by voltage ramps at warm temperatures (33-35 degrees C), and analysed as subtraction currents to remove background leak currents, showed a gradual reduction. It is concluded that the non-inactivating component identified during analysis of the fast sodium current was not the result of enhancement of either a slowly inactivating sodium current or a window current. Thus, an increase in sodium influx through voltage-dependent sodium channels does not occur during exposure to oxidative stress, and therefore, cannot induce an intracellular sodium load.