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 F-2alpha 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 muM (n = 11, P < 0.01) and further decreased [Ca2+](i) to 0.47 &PLUSMN; 0.04 μM 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 muM). 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.