http://www.paper.edu.cn Effect of Atropine on Denervated Rabbit Ear Blood Vessels *Shu-Qin Liu, *Wei-Jin Zang, *Zeng-Li Li, *Xiao-Jiang Yu, and †Bao-Ping Li similar to those of atropine and scopolamine. Henbane drugs Abstract: Surgical denervation of rabbit ear blood vessel beds was might exert multiple advantages in shock patients, but the va- combined with the isolated perfused rabbit ear technique to investi- sodilator effect is undoubtedly the main mechanism. Although gate the mechanism of atropine’s vasodilator action. Intramuscular many studies have been done throughout the world, the mecha- injection of atropine 0.2 mg/kg dilated the denervated blood vessels in the rabbit ear like innervated ones in vivo. Atropine at the maximal nisms of henbane drugs’ vasodilator action remain unclear. concentration (C )of3×10−6 to3×10−4 M did not increase ef- Atropine usually activates the central nervous system at max 8,9 fluent flow of the isolated perfused denervated rabbit ear under con- high doses. Scopolamine, in general, inhibits the central ner- −6 10–13 stant perfusion pressure, but chlorpromazine at a Cmax of 10 M and vous system at any dose. Anisodine slightly inhibits the acetylcholine (ACh) at 2.5 × 10−7 M significantly increased it and central nervous system compared with scopolamine.12,13 An- noradrenaline (NA) at 10−7 M significantly decreased it. Atropine at isodamine is so difficult to pass through the blood–brain bar- −7 −6 Cmax of3×10 M did not affect, but at3×10 M it abolished the rier that it might neither activate nor inhibit the central nervous −7 increase of the effluent flow induced by ACh 2.5 × 10 M. Atropine system.4,14 Therefore, the central nervous system is not the key at3×10−7 M did not affect it, but at 10−6,3×10−6, and 10−5 it signi- −7 pathway through which henbane drugs implement their vaso- ficantly alleviated the decrease of effluent flow induced by NA 10 dilator action. M. Because the increase of effluent flow of rabbit ear under constant Because atropine inhibits the activity of sweat glands perfusion pressure reflects vasodilation of the ear to some extent, the and usually increases body temperature, especially that of in- study suggests that atropine has no direct vasodilator action; its vaso- 15 dilator action is not attributed to blockade of M-cholinoreceptors lo- fants and small children, most investigators surmise that va- ␣ sodilation might be a compensatory reaction permitting the ra- cated on the vascular wall; however, the 1-adrenoceptor might be a 16,17 target site mediating atropine’s vasodilator action in vivo. diation of heat. However, animals that do not sweat, such as dogs, do not exhibit fever, but rather dilation of blood ves- Key Words: atropine, denervation, rabbit ear, vasodilation sels, after administration of atropine.16 Therefore, some inves- (J Cardiovasc Pharmacol™ 2004;43: 99–105) tigators also supposed that atropine has direct vasodilator ac- tion,16 but there is no experimental evidence. Vasodilation in response to atropine may be attributed to blockade of M- t has long been known that high doses of atropine can dilate cholinoceptors located on the vascular wall. Iblood vessels. In the 1960s, Chinese doctors repeated admin- Atropine reduces the vasocontraction induced by istration of high doses of atropine to salvage patients with bac- adrenaline (AD) or electrical stimulation of sympathetic nerve teremic shock, and mortalities were reduced as a result.1–3 in isolated perfused rabbit ear18 and increases the median le- Nowadays, atropine, scopolamine, and some other henbane thal dose (LD50) of AD for rabbits. Therefore, Luduena and drugs such as anisodamine and anisodine are important drugs Branin stated that atropine also blocked ␣-adrenoceptors in in the prevention of death in patients with bacteremic shock vivo.19 Therefore, we also investigated the question of whether and hemorrhagic shock.4–7 Anisodamine and anisodine are an- atropine-induced vasodilation in vivo be attributed to blockade ticholinergic (M-cholinoceptor antagonists) alkaloids origi- of ␣-adrenoceptors. We investigated the effects of atropine on nally extracted from Scopolia tangutica (a variety of datura) blood vessel in the rabbit ear after sympathetic and sensory and currently produced in China. Their chemical structures are denervation. Received February 4, 2003; accepted July 21, 2003. MATERIALS AND METHODS *From the Department of Pharmacology, School of Medicine; and †Experi- mental Center of Chemistry, School of Science, Xi’an Jiaotong University, Appliances Xi’an, China. We used a digital drop recorder (Model YSD-4; Second Supported by the National Natural Science Foundation of China (Nos. Radio Factory, Bengbu Medical College, China) and an elec- 39970273; 30270554) and the State Education Commission of China (Nos. 20010698034, 01161). trical stimulator (Model sen-7203; Nihon Kohden, Japan). The Reprints: Prof. Wei-Jin Zang (e-mail: zwj@mail xjtu.edu.cn). gravity-feed apparatus used was designed and made by our Copyright © 2003 by Lippincott Williams & Wilkins laboratory to meet the need for keeping constant perfusion J Cardiovasc Pharmacolீ • Volume 43, Number 1, January 2004 99 中国科技论文在线 http://www.paper.edu.cn Liu et al J Cardiovasc Pharmacolீ • Volume 43, Number 1, January 2004 pressure in the experiment such that the change of perfusion moved into the chlorpromazine group, adjusting each group to liquid height is no more than 1 mm H2O when 300 mL liquid eight rabbits. leaves the 3000-mL bottle. In Vivo Study Drugs and Reagents The similar positions of dorsal auricular arterial trunks in Atropine sulfate, acetylcholine chloride, chlorproma- each rabbit’s left and right ears were marked on the surface of zine hydrochloride, noradrenaline bitartrate, prazosin hydro- skin after the hair was shaved around them, and the diameters chloride, and sodium pentobarbital were purchased from of the dorsal auricular arterial trunks were measured with a pair Sigma Chemical. Other drugs and reagents are products of of compasses and the ruler in a dissecting microscope. Every pharmaceutical companies and chemical companies in China. rabbit was fixed in a rabbit box for approximately 40 minutes All reagents were of analytic grade or the highest purity avail- (the room temperature was approximately 25°C). Atropine able. (0.2 mg/kg), chlorpromazine (2.0 mg/kg), or saline solution (0.2 mL/kg) was administered intramuscularly to the rabbits in Animals their respective groups. Atropine was dissolved in saline solu- New Zealand white rabbits were supplied by the Medical tion (1%) and chlorpromazine was dissolved in 0.1 mM HCl Experimental Animal Center at Xi’an Jiaotong University. and diluted with saline solution to 1%. Therefore, the injection Thirty rabbits weighing 2.0–2.3 kg (15 male) were used in the volume of each drug was equal (0.2 mL/kg). All drugs were experiment. Among these animals, 10 (five male and five fe- incubated to 37°C before injection. The changes of blood ves- male) that were sensitive to atropine (intramuscular injection sels in rabbit ears were observed, and the diameters of the dor- of atropine sulfate caused blood vessels in ears to dilate) were sal auricular arterial trunks were measured. selected as the atropine group. Others were divided into two The change in diameter of dorsal auricular arterial trunks weight- and sex-matched groups, one to receive chlorproma- was analyzed with the paired t test, respectively. The differ- zine and one to receive normal saline solution. ence of diameter change between left and right ears was ana- lyzed with the group t test. Surgical Denervation Surgery was performed on all 30 rabbits in according to In Vitro Study the report of Morris and Bevan20 with slight revision. Every Model Confirmation rabbit was anesthetized by intraperitoneal injection of sodium This part of the experiment was performed with six pentobarbital (35 mg/kg). Hair was shaved bilaterally over model made animals (three male and three female) 1 day after identical areas before surgery. A medial incision was made in the in vivo study. The rabbits were anesthetized with sodium the neck and the right superior cervical ganglion and right cer- pentobarbital and fixed in rabbit boxes individually. After vical sympathetic preganglionic trunk were removed. The shaving, a 1-cm incision at the base of the medial border of the greater auricular nerve was exposed by a 1.5-cm incision on ear was made. Dorsal auricular artery and dorsal auricular the right side of the neck, and 1 cm was excised just proximal nerve were dissected from vein and other surrounding tissues, to the origin of the anterior and posterior branches. The inci- respectively. The artery was ligated at the base of the ear for sions were closed with four to six sutures. After a 1-cm inci- cannulation. The nerve also was ligated. A small incision was sion at the base of the medial border of right ear was made, the made on the dorsal auricular artery near the ligature with an dorsal auricular nerve, derived from the dorsal root of the sec- ophthalmologic scissors. An arterial cannula of 34°C Ringer ond cervical nerve, the auricular branch of the vagus, and the Locke solution containing heparin (10−4 U/L) was intubated facial nerve were exposed and excised. The anterior auricular into the artery. After the arterial cannula with the artery was artery was not touched. The incision was closed with four su- ligated, the ear was cut off rapidly and the base of the ear was tures. Sham operations were performed on the left side of the ligated with a cotton rope to prevent bleeding. After washing neck and the left ear, with resection of the auricular branches of the blood in the ear with Ringer Locke solution with use of a the vagus nerve and facial nerve.