Activation of endothelial cell IKCa with 1-ethyl-2-benzimidazolinone evokes smooth muscle hyperpolarization in rat isolated mesenteric artery

1 In rat small mesenteric arteries contracted with phenylephrine, 1-ethyl-2 -benzimidazolinone (1-EBIO; 3-300 muM) evoked concentration-dependent relax ation that, above 100 muM, was associated with smooth muscle hyperpolarizat ion. 2 1-EBIO-evoked hyperpolarization (maximum 22.1+/-3.6 mV with 300 muM, n=4) was endothelium-dependent and inhibited by charybdotoxin (ChTX 100 nM; n=4 ) but not iberiotoxin (IbTX 100 nm; n=4). 3 In endothelium-intact arteries, smooth muscle relaxation to 1-EBIO was no t altered by either of the potassium channel blockers ChTX (100 nM; n=7), o r IbTX (100 nM; n=4), or raised extracellular K- (25 mM). Removal of the en dothelium shifted the relaxation curve to the right but did not reduce the maximum relaxation. 4 In freshly isolated mesenteric endothelial cells, 1-EBIO (600 muM) evoked a ChTX-sensitive outward K-current. In contrast, 1-EBIO had no effect on s mooth muscle cell conductance whereas NS 1619 (33 muM) stimulated an outwar d current while having no effect on the endothelial cells. 5 These data show that with concentrations greater than 100 muM, 1-EBIO sel ectively activates outward current in endothelial cells, which presumably u nderlies the smooth muscle hyperpolarization and a component of the relaxat ion. Sensitivity to block with charybdotoxin but not iberiotoxin indicates this current is due to activation of IKCa. However, 1-EBIO can also relax t he smooth muscle by an undefined mechanism, independent of any change in me mbrane potential.