Modifications of Human Cardiac Sodium Channel Gating by UVA Light

Abstract

Voltage-gated Na(+) channels are membrane proteins responsible for the generation of action potentials. In this report we demonstrate that UVA light elicits gating changes of human cardiac Na+ channels. First, UVA irradiation hampers the fast inactivation of cardiac Nav1.5 Na(+) channels expressed in HEK293t cells. A maintained current becomes conspicuous during depolarization and reaches its maximal quasi steady-state level within 5-7 min. Second, the activation time course is slowed by UVA light; modification of the activation gating by UVA irradiation continues for 20 min without reaching steady state. Third, along with the slowed activation time course, the peak current is reduced progressively. Most Na(+) currents are eliminated during 20 min of UVA irradiation. Fourth, UVA light increases the holding current nonlinearly; this phenomenon is slow at first but abruptly fast after 20 min. Other skeletal muscle Nav1.4 isoforms and native neuronal Na(+) channels in rat GH(3) cells are likewise sensitive to UVA irradiation. Interestingly, a reactive oxygen metabolite (hydrogen peroxide at 1.5%) and an oxidant (chloramine-T at 0.5 mM) affect Na(+) channel gating similarly, but not identically, to UVA. These results together suggest that UVA modification of Na(+) channel gating is likely mediated via multiple reactive oxygen metabolites. The potential link between oxidative stress and the impaired Na(+) channel gating may provide valuable clues for ischemia/reperfusion injury in heart and in CNS.

Oxidation of multiple methionine residues impairs rapid sodium channel inactivation

“…As previously shown by Wang and Wang [20], H 2 O 2 also removed inactivation (data not shown). Another means of generating reactive oxygen species in living cells is to load cells with the dye Lucifer Yellow (LY) via the patch pipette and to illuminate it with blue light (436 nm).…”

“…The time course of inactivation was also observed to be sensitive to oxidation in other Na V channel types (e.g., [19,20,26]). To compare the sensitivity quantitatively, we expressed Na V 1.2, Na V 1.5, and Na V 1.7 channels in HEK 293 cells and subjected them to ChT.…”

“…4b, top). As indicated by Wang and Wang [20], elimination of the Met from the IFM-motif fails to completely remove the oxidation sensitivity of the channel inactivation. However, the time course of ChT-induced loss of inactivation is much slower than in the wild type.…”

“…In fact, several studies using oxidants, such as chloramine-T (ChT) and H 2 O 2 , indicated that oxidation of Met may impair fast inactivation in both neuronal and muscle Na v channels [16][17][18][19]. Similar effects are evoked by irradiation of HEK 293 cells expressing the human isoforms of Na V 1.4 or Na V 1.5 with UV-A (320-380 nm wavelength) light, which triggers the production of intracellular ROS [20]. However, a mutant of the rat Na V 1.4 channel with the inactivating IFM motif mutated to IFI remained sensitive to both UV-A and H 2 O 2 exposure [20], thus, suggesting that the Met in the inactivation motif is not the only target.…”

Inflammation of Atrium After Cardiac Surgery Is Associated With Inhomogeneity of Atrial Conduction and Atrial Fibrillation

“…Both of these affect the sodium channels. 23 Interleukin 2 and free radicals decrease the upstroke velocity of the action potential by reducing sodium currents. 24 Tumor necrosis factor-␣ and oxidative stress also downregulate gap junction proteins and reduce cell coupling.…”