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Effect of Peroxynitrite on Passive K+ Transport in Human Red Blood Cells

Peroxynitrite is generated in vivo by the reaction between nitric oxide, from endothelial and other cells, and the superoxide anion. It is therefore pertinent to examine its effects on the membrane permeability of red blood cells. Treatment of human red blood cells with peroxynitrite (nominally 1 mM) markedly stimulated passive K<sup>+</sup> permeability. The main effect was on a Cl<sup>-</sup>-independent K<sup>+</sup> pathway, which remains unidentified. Although K<sup>+</sup>-Cl<sup>-</sup> cotransport (KCC) was stimulated, this was dependent on saline composition, being inhibited by physiological levels of glucose (IC<sub>50</sub> 4 mM), and also by sucrose and MOPS. Effects on the Cl<sup>-</sup>-independent K<sup>+</sup> pathway were less dependent on saline composition, and were not inhibited by amiloride, ethylisopropylamiloride, dimethylamiloride or gadolinium. Na<sup>+</sup>-K<sup>+</sup>-2Cl<sup>-</sup> cotransporter was inhibited whilst there was little effect on the Gardos channel (Ca<sup>2+</sup>-activated K<sup>+</sup> channel). Peroxynitrite was markedly more effective in oxygenated cells than deoxygenated ones. Treatment with peroxynitrite per se did not affect initial cell volume. Anisotonic swelling modestly increased the Cl<sub>-</sub>-independent K<sup>+</sup> influx, but did not affect peroxynitrite-stimulated KCC. Decreasing extracellular pH from 7.4 to 7.2 or 7.0 increased KCC stimulation, whilst the Cl<sup>-</sup>-independent component of K<sup>+</sup> transport was lowest at pH 7.2. Finally, protein phosphatase inhibition with calyculin A (100 nM) inhibited KCC, implying that, as with other KCC stimuli, peroxynitrite acts via decreased protein phosphorylation; pre-treatment with calyculin A also inhibited the Cl<sup>-</sup>-independent component of K<sup>+</sup> transport. These findings are relevant to the actions of peroxynitrite in vivo.