Inhibition of endothelium-dependent vasorelaxation by extracellular K(+): a novel controlling signal for vascular contractility

The effects of extracellular K+ on endothelium-dependent relaxation (EDR) and on intracellular Ca2+ concentration (Ca2+]i) were examined in mouse aorta, mouse aorta endothelial cells (MAEC), and human umbilical vein endothelial cells (HUVEC). In mouse aortic rings precontracted with prostaglandin F2alpha or norepinephrine, an increase in extracellular K+ concentration (K+]o) from 6 to 12 mM inhibited EDR concentration dependently. In endothelial cells, an increase in K+]o inhibited the agonist-induced Ca2+]i increase concentration dependently. Similar to K+, Cs+ also inhibited EDR and the increase in Ca2+]i concentration dependently. In current-clamped HUVEC, increasing K+]o from 6 to 12 mM depolarized membrane potential from -32.8 +/- 2.7 to -8.6 +/- 4.9 mV (n = 8). In voltage-clamped HUVEC, depolarizing the holding potential from -50 to -25 mV decreased Ca2+]i significantly from 0.95 +/- 0.03 to 0.88 +/- 0.03 microM (n = 11, P < 0.01) and further decreased Ca2+]i to 0.47 +/- 0.04 microM by depolarizing the holding potential from -25 to 0 mV (n = 11, P < 0.001). Tetraethylammonium (1 mM) inhibited EDR and the ATP-induced Ca2+]i increase in voltage-clamped MAEC. The intermediate-conductance Ca2+-activated K+ channel openers 1-ethyl-2-benzimidazolinone, chlorozoxazone, and zoxazolamine reversed the K+-induced inhibition of EDR and increase in Ca2+]i. The K+-induced inhibition of EDR and increase in Ca2+]i was abolished by the Na+-K+ pump inhibitor ouabain (10 microM). These results indicate that an increase of K+]o in the physiological range (6-12 mM) inhibits Ca2+]i increase in endothelial cells and diminishes EDR by depolarizing the membrane potential, decreasing K+ efflux, and activating the Na+-K+ pump, thereby modulating the release of endothelium-derived vasoactive factors from endothelial cells and vasomotor tone.