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 ([Ca ²⁺]_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_2α 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 μM (n = 11, P < 0.01) and further decreased [Ca²⁺]_i, to 0.47 ± 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 [Ca ²⁺]_i. The K⁺-induced inhibition of EDR and increase in [Ca²⁺]_i was abolished by the Na ⁺-K⁺ pump inhibitor ouabain (10 μM). 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.