SeungSoo Chung, SeongChun Kwon, YunSuk Kim, DuckSun Ahn, YoungHo Lee and TaickSang Nam*
Department of Physiology, College of Medicine, Yonsei University, Seoul 120-752, Korea
*Corresponding author. FAX: +82-2-393-0203, E-mail: tsnam@yumc.yonsei.ac.kr
Abstract: Effects of clonidine, an imidazoline derivative as well as a2-adrenoceptor agonist, on carbachol (CCh)-evoked contraction in guinea pig ileal smooth muscle were studied using isometric tension recording. To investigate the cellular mechanisms of the inhibitory effect of clonidine, its effects on CCh-evoked nonselective cationic current (ICCh), voltage-dependent Ca2+ current (ICa) and voltage-dependent K+ current (IK) was also studied using patch-clamp recording techniques in single ileal cells. Clonidine inhibited the contraction evoked by CCh (1 mM) in a concentration-dependent manner with an IC50 value of 61.7 ± 2.5 mM. High K+ (40 mM)-evoked contraction was only slightly inhibited even when clonidine was used at 300 mM. Externally applied clonidine inhibited ICCh dose-dependently with an IC50 of 42.0 ± 2.6 mM. When applied internally via patch pipettes, clonidine was without effect. An ICCh-like current induced by GTPgS was also inhibited by bath application of clonidine. None of KU14R and BU224, both imidazoline receptor blockers, and yohimbine, an a2-adrenergic blocker, significantly affects the inhibitory effect of clonidine on ICCh. Clonidine (300 mM) only slightly decreased membrane currents flowing through voltage-gated Ca2+ channels or K+ channels. These data indicate that clonidine relaxes smooth muscle contraction produced by muscarinic receptor activation and suggest that the effect of clonidine seems due mainly to inhibition of ICCh via acting directly on the involved cationic channel.
Keywords: Gastrointestinal smooth muscle, Carbachol, Cationic current, Clonidine, Imidazoline
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