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

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

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