Abstract-The endothelium is a critical regulator of vascular tone, and dysfunction of the endothelium contributes to numerous cardiovascular pathologies. Recent studies suggest that apamin-sensitive, small-conductance, Ca 2ϩ -activated K ϩ channels may play an important role in active endothelium-dependent vasodilations, and expression of these channels may be altered in disease states characterized by vascular dysfunction. Here, we used a transgenic mouse (SK3 T/T ) in which SK3 expression levels can be manipulated with dietary doxycycline (DOX) to test the hypothesis that the level of expression of the SK subunit, SK3, in endothelial cells alters arterial function and blood pressure. SK3 protein was elevated in small mesenteric arteries from SK3 T/T mice compared with wild-type mice and was greatly suppressed by dietary DOX. SK3 was detected in the endothelium and not in the smooth muscle by immunohistochemistry. In whole-cell patch-clamp experiments, SK currents in endothelial cells from SK3 T/T mice were almost completely suppressed by dietary DOX. In intact arteries, SK3 channels contributed to sustained hyperpolarization of the endothelial membrane potential, which was communicated to the arterial smooth muscle. Pressure-and phenylephrine-induced constrictions of SK3 T/T arteries were substantially enhanced by treatment with apamin, suppression of SK3 expression with DOX, or removal of the endothelium. In addition, suppression of SK3 expression caused a pronounced and reversible elevation of blood pressure. These results indicate that endothelial SK3 channels exert a profound, tonic, hyperpolarizing influence in resistance arteries and suggest that the level of SK3 channel expression in endothelial cells is a fundamental determinant of vascular tone and blood pressure. Key Words: endothelium Ⅲ potassium channels Ⅲ vascular tone Ⅲ blood pressure B lood pressure and flow are regulated by the constriction and dilation of resistance arteries, generally with internal diameters Ͻ300 m. 1 Physiological stimulation through elevations in intravascular pressure or increased sympathetic activity promotes smooth muscle depolarization, intracellular Ca 2ϩ influx, and vasoconstriction. The resulting increase in total peripheral resistance within the vasculature increases blood pressure. 2 The endothelium exerts a dilating influence that opposes arterial constriction. Activation of K ϩ channels is thought to contribute to this influence through increased release of relaxing factors such as NO and prostacyclin (PGI 2 ) and through smooth muscle hyperpolarization. [3][4][5] The smallconductance Ca 2ϩ -activated K ϩ (SK) channel has received considerable attention as a potential mediator of these responses. SK channels are opened by intracellular Ca 2ϩ via an association with calmodulin 6 and are believed to play a role in the modulation of tissue excitability. 7 Of the three characterized SK channel isoforms (SK1, SK2, and SK3), 8 mRNA for SK2 and SK3 has been identified in endothelial cells. 9 Apamin, a toxin blocker of S...