Alpha‐2 adrenoceptors are present in rat aorta smooth muscle cells, and their action is mediated by ATP‐sensitive K + channels

The role of α 2 ‐adrenoceptors in the response of aorta smooth muscle rings to the α 2 ‐adrenoceptors agonists UK 14,304 and clonidine was studied. Stimulation by 1–10 n M UK 14,304 caused dose‐dependent relaxant responses in BaCl 2 ‐contracted endothelium‐denuded aorta rings, and hyperpolarization in rings with or without endothelium, which were inhibited by yohimbine and glibenclamide, but not affected by prazosin, propranolol, apamin or iberiotoxin. At higher concentrations (10 n M –10 μ M ) UK 14,304 also induced a depolarizing effect which was potentiated by yohimbine and inhibited by prazosin. These results indicate that UK 14,304 acts on α 2 ‐adrenoceptors at lower concentrations and on both α 1 ‐ and α 2 ‐adrenoceptors above 10 n M . In rings, with or without endothelium, noradrenaline had a depolarizing effect which was inhibited by prazosin. Adrenaline did not affect the membrane potential but in the presence of prazosin caused hyperpolarization, which was inhibited by yohimbine and glibenclamide. These results indicate that noradrenaline is more selective for α 1 ‐, whereas adrenaline has similar affinities for α 1 ‐ and α 2 ‐adrenoceptors. In aortae with endothelium, L ‐NNA caused a small depolarization but did not affect the hyperpolarization induced by UK 14,304, indicating that NO is not involved in that response. Glibenclamide induced a small depolarization in aortae, with or without endothelium, indicating that ATP‐sensitive K + channels may play a role in maintaining the smooth muscle's membrane potential. Our results indicate that, in rat aorta, α 2 ‐adrenoceptors are also present in the smooth muscle, and that these receptors act through small‐conductance ATP‐sensitive K + channels.British Journal of Pharmacology (2000) 131 , 788–794; doi: 10.1038/sj.bjp.0703630