Sodium Metabisulfite: Effects on Ionic Currents and Excitotoxicity

How sodium metabisulfite (SMB; Na₂S₂O₅), a popular food preservative and antioxidant, interacts with excitable membrane and induces excitotoxicity is incompletely understood. In this study, the patch-clamp technique was used to investigate and record the electrophysiological effect of SMB on electrically excitable HL-1 cardiomyocytes and NSC-34 neurons, as well as its relationship to pilocarpine-induced seizures and neuronal excitotoxicity in rats. We used Western blotting, to analyze sodium channel expression on hippocampi after chronic SMB treatment. It was found that voltage-gated Na⁺ current (I_Na) was stimulated, and current inactivation and deactivation were slowed in SMB-treated (30 μM) HL-1 cardiomyocytes. SMB-induced increases of I_Na were attenuated in cells treated with ranolazine (10 μM) or eugenol (30 μM). The current-voltage relationship of I_Na shifted to slightly more negative potentials in SMB-treated cells, the peak I_Na with an EC₅₀ value of 18 μM increased, and the steady-state inactivation curve of I_Na shifted to a more positive potential. However, the tail component of the rapidly activating delayed-rectifier K⁺ current (I_Kr) was dose-dependently inhibited. Cell-attached voltage-clamp recordings in SMB-treated cells showed that the frequency of action currents and prolonged action potential were higher. In SMB-treated NSC-34 neurons, the peak I_Na was higher; however, neither the time to peak nor the inactivation time constant (I_Na) changed. Pilocarpine-induced seizures were exacerbated, and acute neuronal damage and chronic mossy fiber sprouting increased in SMB-treated rats. Western blotting showed higher expression of the sodium channel in cells after chronic SMB treatment. We conclude that SMB contributes to the sodium channel-activating mechanism through which it alters cellular excitability and excitotoxicity in wide-spectrum excitable cells.