BASIC STUDIES Role of Ca 21 -dependent potassium channels in in vitro anandamide-mediated mesenteric vasorelaxation in rats with biliary cirrhosis Ying-Ying Yang 1,2 , Han-Chieh Lin 3,4 , Yi-Tsau Huang 5 , Tzung-Yan Lee 6 , Ming-Chih Hou 3,4 , Ying-Wen Wang 3,4 , Fa-Yauh Lee 2,4 and Shou-Dong Lee 3,4 1 Institute of Clinical Medicine, National Yang-MingUniversity School of Medicine, Taipei, Taiwan 2 Department of Medicine, Division of General Medicine, Taipei Veterans General Hospital, Taipei, Taiwan 3 Department of Medicine, Division of Gastroenterology, Taipei Veterans General Hospital, Taipei,Taiwan 4 Department of Medicine, National Yang-MingUniversity School of Medicine, Taipei, Taiwan 5 Institute of Traditional Medicine, National Yang-Ming University School of Medicine, Taipei, Taiwan 6 Graduate Institute of Traditional Chinese Medicine, Chang Gung University, Taipei,Taiwan Keywords cirrhosis – endocannabinoids – potassium channels – vasodilatation Correspondence Han-Chieh Lin, MD, Department of Medicine, Division of Gastroenterology, Taipei Veterans General Hospital, No. 201, Sec. 2, Shih-Pai Road, Taipei 11217, Taiwan Tel: 1886 2 28757506 Fax: 1886 2 28739318 e-mail: hclin@vghtpe.gov.tw Received 30 December 2006 accepted 8 June 2007 DOI:10.1111/j.1478-3231.2007.01551.x Abstract Background/Aim: Anandamide can activate potassium (K 1 ) channels to induce an endothelium-dependent vasorelaxation in normal rat mesenteric arteries. Cannabinoids contribute partly to the splanchnic vasodilation in cirrhosis. This study investigated the roles of vascular K 1 channels in anandamide-induced mesenteric vasorelaxation in isolated rat cirrhotic vessels. Methods: The effects of the pretreatment of AM251, a specific CB 1 receptor antagonist, were assessed on the vascular reactivity to phenylephrine (PE), potassium chloride (KCl), acetylcho- line (ACh) and sodium nitroprusside (SNP). Additionally, cannabinoid (CB 1 and CB 2 ) receptors’ protein expression and the effects of different K 1 channel blockers on vascular reactivity to anandamide were also studied. Results: Cirrhotic mesen- teric arteries showed an overexpression of CB 1 receptor associated with hypor- eactivity to PE and KCl, and hyper-response to ACh, SNP and anandamide. Pretreatment with AM251 significantly improved the hyporeactivity to KCl and ameliorated the hyper-response to ACh in cirrhotic vessels. Increased relaxation response to anandamide was suppressed by combinations of vascular Ca 21 - dependent K 1 channel blockers (including apamin1charybdotoxin1iberiotoxin or apamin1TRAM-341iberiotoxin) (TRAM-34, 1-[(2-chlorophenyl)diphenyl- methyl]-1H-pyrazole). Conclusions: In cirrhotic mesenteric arteries, vascular CB 1 receptor and anandamide contribute to the in vitro hyporeactivity to KCl. In addition, hyper-response to ACh may probably act through the modulation of vascular Ca 21 -dependent K 1 channels. Splanchnic vasodilation is an essential factor in main- taining an elevated portal pressure in cirrhosis (1). The decreased vascular reactivity to vasoconstrictors, in- creased circulating vasodilators and increased endothe- lial-related vasodilatory activity have been suggested to contribute to the development of splanchnic hyperae- mia in cirrhosis (2, 3). The increased endothelial-related vasodilatory activity included the increased production of nitric oxide (NO) and prostacyclin (PGI 2 ) (2–6). However, certain studies showed that splanchnic vaso- dilation was resistant to blockade of NO and PGI 2 formation (4, 5). Actually, these studies suggested that potassium (K 1 ) channels play an important role in the splanchnic vasodilatation of cirrhotic portal hypertensive rats (4, 5, 7). It had been reported that K 1 channel-mediated vasodilation is initiated by an increase in intracellular Ca 21 of endothelial cells, with consequent activation of vascular Ca 21 -activated K 1 channels (K Ca channels) and smooth muscle cell (SMC) relaxation (8, 9). Endocannabinoids include anandamide, 2-arachi- donylglycerol, virodhamine, noladinether and N- arachidonoyl-dopamine (10). These substances can induce hypotension, via activation of the cannabinoid CB 1 receptor. Both circulating and endothelium- generated anandamide have been found to produce Liver International (2007) c  2007 The Authors. Journal compilation c  2007 Blackwell Munksgaard 1045 Liver International ISSN 1478-3223