Oxidative stress alters specific membrane currents in isolated cardiac myocytes

To evaluate the effects of oxidative stress on cardiac membrane currents, single cells from frog ventricle were exposed to tert-butyl hydroperoxide (t-BHP). Incubation of these cells with 2 mM t-BHP causes a rapid depletion of cellular glutathione, followed by a more gradual increase in the contents of malonaldialdehyde and conjugated dienes. Effects of this rapidly evolving oxidative stress were studied on sodium, calcium, and potassium currents of isolated ventricular cells. t-BHP caused a progressive decrease in the magnitude of sodium current obtained on depolarization from a holding potential of -85 mV, which was accompanied by a shift in the reversal potential toward more negative potentials. The voltage dependence of the steady-state parameters for activation and inactivation was shifted, such that in peroxide-exposed cells, there was a greater overlap of activation and inactivation parameters, which would be expected to result in an increased window current. In addition, in the presence of t-BHP, the time constant for activation was decreased at most depolarizing potentials, whereas the time constant for inactivation was increased. The resultant sodium current transients were, therefore, slower in the presence of the peroxide because of slower inactivation. Prolonged exposure of the cells to t-BHP led to a complete and selective inhibition of the Na⁺ current. However, even when all the Na⁺ current was inhibited, the K⁺ and Ca²⁺ currents remained essentially unaltered. Also, no large outward currents were observed at this stage, indicating that ATP concentration was not drastically decreased. The barrier properties of plasma membrane remained intact, as it was possible to form gigohm seals between the patch pipette and the plasma membrane of cells treated with 2-14 mM t-BHP for up to 30 minutes. These results account for the proarrhythmic effects of free radicals and oxidative stress on cardiac tissues.