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

In rat small mesenteric arteries contracted with phenylephrine, 1‐ethyl‐2‐benzimidazolinone (1‐EBIO; 3 – 300 μ M ) evoked concentration‐dependent relaxation that, above 100 μ M , was associated with smooth muscle hyperpolarization. 1‐EBIO‐evoked hyperpolarization (maximum 22.1±3.6 mV with 300 μ M , n =4) was endothelium‐dependent and inhibited by charybdotoxin (ChTX 100 n M ; n =4) but not iberiotoxin (IbTX 100 n M ; n =4). In endothelium‐intact arteries, smooth muscle relaxation to 1‐EBIO was not altered by either of the potassium channel blockers ChTX (100 n M ; n =7), or IbTX (100 n M ; n =4), or raised extracellular K + (25 m M ). 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 μ M ) evoked a ChTX‐sensitive outward K‐current. In contrast, 1‐EBIO had no effect on smooth muscle cell conductance whereas NS 1619 (33 μ M ) stimulated an outward current while having no effect on the endothelial cells. These data show that with concentrations greater than 100 μ M , 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 IK Ca . However, 1‐EBIO can also relax the smooth muscle by an undefined mechanism, independent of any change in membrane potential.British Journal of Pharmacology (2001) 134 , 1548–1554; doi: 10.1038/sj.bjp.0704415