Vol. 286, Issue 2, 681-687, August 1998

Mechanism of Relaxant Effect of Clonidine in Isolated Bovine Tracheal Smooth Muscle¹

Masashi Arimitsu, Minori Mitsui-Saito, Koichi Sato, Hiroshi Ozaki, Yoshihisa Koga and Hideaki Karaki

Department of Anesthesiology (M.A., Y.K.), Kinki University School of Medicine, Osaka, Japan and Department of Veterinary Pharmacology (M.M.-S., K.S., H.O., H.K.), Graduate School of Agriculture and Life Sciences, The University of Tokyo, Tokyo, Japan

The relaxant effect of clonidine and the possible involvement of imidazoline I₁ receptors in bovine tracheal smooth muscle (BTSM) were examined. Clonidine caused concentration-dependent significant relaxation in BTSM precontracted with 0.1 or 1 µM carbachol (CCh) but not in 72.7 mM KCl-induced contraction. The relaxation in CCh-contracted BTSM was inhibited by yohimbine (1 µM) and idazoxan (10 and 30 µM) but not by tetrodotoxin, indomethacin and other adrenoceptor antagonists. Oxymetazoline (0.1-100 µM) and phentolamine (0.1-100 µM) caused concentration-dependent relaxation, which was attenuated by idazoxan (10 µM). Norepinephrine (0.1-100 µM) produced concentration-dependent relaxation, which was completely abolished by propranolol (10 µM) but not by yohimbine (1 µM). In fura-PE3/AM-loaded BTSM, CCh and 72.7 mM KCl increased intracellular calcium concentration ([Ca⁺⁺]_i) followed by contraction. The high K⁺-induced increase in [Ca⁺⁺]_i was not affected by clonidine. In CCh-stimulated BTSM, clonidine decreased [Ca⁺⁺]_i and muscle force in parallel, whereas verapamil decreased [Ca⁺⁺]_i more strongly than muscle force. Clonidine (100 µM) inhibited the transient increase in [Ca⁺⁺]_i induced by CCh but not by caffeine (20 mM) in Ca⁺⁺-free solution. Clonidine did not change the cAMP content in the presence of either 72.7 mM KCl or CCh. These results indicate that clonidine relaxes CCh-stimulated BTSM through the inhibition of CCh-induced increases in Ca⁺⁺-influx, Ca⁺⁺-release and intracellular signal transduction probably via imidazoline I₁ receptors.