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

Abstract

1. The role of α2-adrenoceptors in the response of aorta smooth muscle rings to the α2-adrenoceptors agonists UK 14,304 and clonidine was studied. 2. Stimulation by 1 - 10 nM UK 14,304 caused dose-dependent relaxant responses in BaCl2-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 nM-10 μM) 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 α2-adrenoceptors at lower concentrations and on both α1 and α2-adrenoceptors above 10 nM. 3. 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 α1-, whereas adrenaline has similar affinities for α1- and α2-adrenoceptors. 4. 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. 5. 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. 6. Our results indicate that, in rat aorta, α2-adrenoceptors are also present in the smooth muscle, and that these receptors act through small-conductance ATP-sensitive K+ channels.