Abstract Cumulative addition of atropine to the organ bath containing endothelium-intact (+E) rat aorta, which was precontracted with phenylephrine (PE, 1 μM) and subse- quently relaxed with carbachol (1μM), caused biphasic changes in the vascular contractility of +E rat aortic rings. Low concentrations of atropine (10 nM–1.0 μM) caused progressive restoration of contraction to PE; whereas at higher concentrations (1–100 μM), atropine caused pro- gressive relaxation. Atropine-induced aortic relaxation was significantly inhibited upon endothelium removal by ei- ther rubbing or saponin treatment, but considerable relax- ation still persisted in the range of 30–100 μM atropine. Similar findings were also obtained when the nitric oxide (NO) generation was inhibited with 300 μM NO synthase inhibitor, L-NAME. Atropine-induced relaxation was also observed when 5-hydroxytryptamine (5-HT) was used as the agonist and the atropine-relaxation was more potent at lower concentrations of PE and 5-HT. However, atropine had no effect on the contraction elicited by KCl or prosta- glandin F 2α . Also, atropine-induced relaxation was not af- fected by indomethacin (1–10 μM), nicotine (10–100 μM) or hexamethonium (30 μM). Pretreatment of +E aorta with tetraethylammonia (TEA, 3–10 mM) or 4-aminopyri- dine (4-AP, 1–3mM) showed prominent inhibitory effect on atropine-induced relaxation; on the other hand, prein- cubation with glibenclamide (1–10 μM), BaCl 2 (1–30 μM) or 2 μM charybdotoxin and apamin, had little effect on the relaxation induced by atropine. When added to tissues af- ter relaxation to atropine, TEA and 4-AP concentration- dependently reversed the relaxation in -E aorta, whereas in +E aorta, TEA up to 30 mM and 4-AP up to 10 mM only partially affected atropine-induced relaxation. Al- though TEA and 4-AP potentiated the PE-contraction, such potentiation is unlikely to contribute to the change in sen- sitivity to atropine-induced relaxation, since in the pres- ence of 15 mM KCl, which also potentiated PE-contrac- tion to a comparable extent, the atropine-relaxation re- mains unchanged. Scopolamine also acts like atropine, except that the effect of scopolamine was smaller than that of atropine and is primarily endothelium-dependent. Atropine-induced relaxation also occurs in medium artery (renal artery) and small muscular artery (mesenteric artery). In conclusion, atropine-relaxation is mediated in part via voltage-dependent K + channels in both smooth muscle and endothelium and forms the mechanistic basis for the observed vasodilation, reduced blood pressure and facial flushing following atropine overdose. Keywords Atropine · Scopolamine · Anticholinergic alkaloids · Vascular relaxation · Tetraethylammonium · Potassium channels · Rat aortic smooth muscle · Endothelium Abbreviations 5-HT 5-hydroxytryptamine · NO Nitric oxide · PE Phenylephrine · PSS Physiological saline solution · TEA Tetraethylammonium · 4-AP 4-aminopyridine Introduction Atropine, a (±)-hyoscyamine extracted from plants of the potato family such as belladonna hyocymus or stramonium, is a widely used non-selective antagonist against the acti- vation by acetylcholine or carbachol of the cholinergic muscarinic receptors. For effective in vitro pharmacologi- cal blockade of muscarinic receptors, atropine is usually used within the range of 10 nM to 10 μM. It is also used Chiu-Yin Kwan · Wen-Bo Zhang · Tony K. Kwan · Yasushi Sakai In vitro relaxation of vascular smooth muscle by atropine: involvement of K + channels and endothelium Naunyn-Schmiedeberg’s Arch Pharmacol (2003) 368 : 1–9 DOI 10.1007/s00210-003-0759-7 Received: 13 January 2003 / Accepted: 6 April 2003 / Published online: 11 June 2003 ORIGINAL ARTICLE This work was supported by a seeding grant (to CYK) from McMaster University, a grant support from Showa University (to YS) as well as a summer studentship (to TKK) C.-Y. Kwan (✉) · W.-B. Zhang · T. K. Kwan Department of Medicine, Faculty of Health Sciences, McMaster University, Hamilton, Ontario, Canada, Fax: +1-905-5223114, e-mail: kwancy@mcmaster.ca Y. Sakai Department of Occupational Therapy, College of Biomedical Sciences, Showa University, Yokohama, Japan © Springer-Verlag 2003