Effective Accentuation of Voltage-Gated Sodium Current Caused by Apocynin (4′-Hydroxy-3′-methoxyacetophenone), a Known NADPH-Oxidase Inhibitor

Apocynin (aPO, 4′-Hydroxy-3′-methoxyacetophenone) is a cell-permeable, anti-inflammatory phenolic compound that acts as an inhibitor of NADPH-dependent oxidase (NOX). However, the mechanisms through which aPO can interact directly with plasmalemmal ionic channels to perturb the amplitude or gating of ionic currents in excitable cells remain incompletely understood. Herein, we aimed to investigate any modifications of aPO on ionic currents in pituitary GH(3) cells or murine HL-1 cardiomyocytes. In whole-cell current recordings, GH(3)-cell exposure to aPO effectively stimulated the peak and late components of voltage-gated Na+ current (I(Na)) with different potencies. The EC(50) value of aPO required for its differential increase in peak or late I(Na) in GH(3) cells was estimated to be 13.2 or 2.8 μM, respectively, whereas the K(D) value required for its retardation in the slow component of current inactivation was 3.4 μM. The current–voltage relation of I(Na) was shifted slightly to more negative potential during cell exposure to aPO (10 μM); however, the steady-state inactivation curve of the current was shifted in a rightward direction in its presence. Recovery of peak I(Na) inactivation was increased in the presence of 10 μM aPO. In continued presence of aPO, further application of rufinamide or ranolazine attenuated aPO-stimulated I(Na). In methylglyoxal- or superoxide dismutase-treated cells, the stimulatory effect of aPO on peak I(Na) remained effective. By using upright isosceles-triangular ramp pulse of varying duration, the amplitude of persistent I(Na) measured at low or high threshold was enhanced by the aPO presence, along with increased hysteretic strength appearing at low or high threshold. The addition of aPO (10 μM) mildly inhibited the amplitude of erg-mediated K+ current. Likewise, in HL-1 murine cardiomyocytes, the aPO presence increased the peak amplitude of I(Na) as well as decreased the inactivation or deactivation rate of the current, and further addition of ranolazine or esaxerenone attenuated aPO-accentuated I(Na). Altogether, this study provides a distinctive yet unidentified finding that, despite its effectiveness in suppressing NOX activity, aPO may directly and concertedly perturb the amplitude, gating and voltage-dependent hysteresis of I(Na) in electrically excitable cells. The interaction of aPO with ionic currents may, at least in part, contribute to the underlying mechanisms through which it affects neuroendocrine, endocrine or cardiac function.