Alpha2 adrenoceptor agonists may have a role as adjuncts to anaesthetics [1,2]. They reduce anxiety, induce sedation and produce analgesia. They also decrease the amount of opioid and inhalation agent needed for induction and maintenance of anaesthesia while providing haemodynamic stability [1,2]. Alpha2 agonists produce less respiratory depression, nausea and vomiting or pruritis than opioids.
The effects of alpha2 adrenoceptor agonists on gastric emptying have not been well studied and the results are inconclusive. In some studies, peripherally injected alpha2 adrenoceptor agonists did not delay gastric emptying in rodents [3–5]. In other studies, alpha2 adrenoceptor agonists delayed gastric emptying of liquids in the rat  and that of both solids and liquids in the dog . In humans, oral clonidine did not delay gastric emptying of a radiolabelled meal , whereas, lidamidine which is also considered to be an alpha2 adrenoceptor agonist weakly delayed gastric emptying .
It has been shown that, compared with gastric emptying of liquid or digestible solids, the emptying of indigestible solids is more likely to be affected by pathological conditions, such as diabetes mellitus [10,11]. Therefore, it is possible that any inhibitory effect of a drug on gastric motility is more likely to be detected by a method using indigestible solids than using liquids. The aim of the current study was to examine whether clonidine inhibits gastric emptying of indigestible solids in the rat.
Materials and methods
We studied the effect of clonidine on gastric motility using 10 steel balls, as described by Sugahara and colleagues , but with some modifications. Male Wistar rats weighing between 200 and 250 g were used. The study was conducted under the Animal (Scientific Procedures) Act 1986. Before the experiment, rats were housed under standard controlled environmental conditions, with a 12 h light/dark cycle. The animals were fasted for 24 h to empty the stomach and small intestine before experiments; however, they were allowed free access to water until 20 to 30 min before the start of the experiment. Each animal was kept individually in a wire-mesh cage to prevent coprophagy during fasting. All experiments were started between 10.00 and 14.00 hours. Drugs were freshly prepared each day.
Six to 12 rats were used for each group. The animals were lightly anaesthetized by placing them in a closed transparent box containing 2% halothane in oxygen. As soon as the animals were unconscious, anaesthesia was discontinued and either saline or clonidine, at a dose ranging from 0.001 to 0.3 mg kg−1, in a volume of 1.0 mL kg−1 was injected intraperitoneally. Ten steel balls (1.0 mm in diameter; density: 8 g cm−3) were given through an orogastric cannula into the stomach immediately after the injection. The rat was allowed to recover from anaesthesia, which usually occurred within a few minutes. Three hours later, the animal was killed by an overdose of halothane, and the balls, which had passed into the intestine, were counted. Only rats with no corophagy and those in which all the ten balls were recovered from the gastrointestinal tract were used for analysis.
In another group of rats, either yohimbine, naloxone or saline was injected with the ED75 (see below) of clonidine to examine which receptor type was involved in the effects of clonidine.
The number of steel balls which had passed into the intestine was expressed as mean (range). The Mann–Whitney U-test was used to compare the number of balls that had passed into the intestine and to obtain confidence intervals for the difference between groups of rats, which were subjected to different treatments. The values for clonidine at doses greater than ED50 (see below) were pooled for this analysis. We used Minitab Release 8.2, which calculates confidence intervals as whole numbers for a confidence level close to 95%. These intervals are reported as, for example, [96.3% Cl: 3–7]. P-values were adjusted for ties. P<0.05 was considered significant.
The doses of clonidine which on average resulted in 50% and 75% of the balls being retained in the stomach (ED50 and ED75, respectively) were determined by fitting a logistic regression to the data for the test drug, using the statistical package GLIM (Generalized Linear Interactive Modelling), version 4, update 8, (The GLIM Co-ordinator, Numerical Algorithms Group Limited, Oxford, UK) running on a DEC mainframe computer under the Ultrix operating system (OSF/1 V2.OB). This fits the sigmoid logit curve to the observations of number of balls retained out of 10, against log dose, and assumes a binomial distribution of the residuals. The erratic nature of the data resulted in 'overdispersion' (variance of the residuals greater than in the binomial distribution) but this was corrected by scaling as explained in the GLIM manual. Fieller's equation was then used to calculate the 95% confidence interval (Cl) of the ED50.
Clonidine injected intraperitoneally significantly delayed gastric emptying of steel balls (P=0.002; 95.3% Cl for difference: 5–10) (Fig. 1). The ED50 for the inhibitory effect of clonidine was 0.0071 [95% Cl: 0.0033–0.013] mg kg−1.
Yohimbine 3.0 mg kg−1 significantly antagonized the inhibitory effect of clonidine 0.019 mg kg−1 (ED75) (Table 1). In contrast, naloxone 1.0 mg kg−1, did not significantly antagonize the effect of clonidine (Table 1).
Clonidine 1.0 mg kg−1 caused obvious sedation in all rats studied.
Clonidine delayed gastric emptying of indigestible solids, indicating that it has an inhibitory effect on gastric motility. The ED50 for the inhibitory effect of clonidine on gastric emptying of indigestible solids was 0.0071 mg kg−1. The ED50 for the antinociceptive effect of clonidine in the rat has been reported to be 0.1–0.8 mg kg−1 in thermal stimulus tests [14–16] and 0.08–0.15 mg kg−1 in chemical stimulus tests [15–17], and thus clonidine has an inhibitory effect on gastric motility at analgesic doses.
Yohimbine, an alpha2 adrenoceptor antagonist, but not naloxone, antagonized the inhibitory effect of clonidine. These indicate that the inhibitory effect of clonidine is mediated by the alpha2 adrenoceptor. It is possible that yohimbine itself accelerated the emptying and reduced the inhibitory effect of clonidine, rather than exerting a direct antagonism at the receptor. However, this is unlikely because yohimbine either has no effect or even inhibits gastric motility [6,18,19].
There are two main possible mechanisms for the inhibitory effect of clonidine on gastric emptying. One possibility is that clonidine affected pyloric tone and delayed gastric emptying. However, this is unlikely, because alpha2 adrenoceptor agonists either have no effect or relax the pylorus [20,21]. The other possibility is that clonidine delays gastric emptying by directly inhibiting gastric motility. Alpha2 adrenoceptor agonists inhibit the contraction of the stomach caused by stimulation of cholinergic neurones [18,22,23]. However, they may not lower the basal tone of the stomach , and do not inhibit contraction of the stomach induced by exogenous acetylcholine [22,23]. Nevertheless, in the current study, clonidine inhibited gastric emptying of indigestible solids, indicating that it has a direct inhibitory effect on the body of the stomach.
In conclusion, we have shown that, in the rat, clonidine inhibits gastric motility.
We thank Professor Emeritus W. W. Mapleson, Department of Anaesthetics and Intensive Care Medicine, University of Wales College of Medicine, for his helpful advice on statistical analysis, and Mr C. Juniper, HNC, for his skilful technical assistance.
1 Maze M, Tranquilli W. Alpha-2 adrenoceptor agonists: defining the role in clinical anesthesia. Anesthesiology
2 Aanta R, Scheinin M. Alpha2
-adrenergic agents in anaesthesia. Acta Anaesthesiol Scand
3 Ruwart MJ, Klepper MS, Rush BD. Clonidine delays small intestinal transit in the rat. J Pharmacol Exp Ther
4 Hsu WH. Xylazine-induced delay of small intestinal transit in mice. Eur J Pharmacol
5 Tanila H, Kauppila T, Taira T. Inhibition of intestinal motility and reversal of postlaparotomy ileus by selective α2-adrenergic drugs in the rat. Gastroenterology
6 Cooper SM, McRitchie B. Role of dopamine and α-adrenoreceptors in the control of gastric emptying in the rat: Possible involvement in the mechanism of action of metoclopramide. J Auton Pharmacol
7 Gullikson GW, Virina MA, Loeffler R, Erwin WD. α2
-Adrenergic model of gastroparesis: validation with renzapride, a stimulator of motility. Am J Physiol
8 Baxter AJ, Edwards CA, Holden S, Cunningham KM, Welch IMcL, Read NW. The effect of two α2
-adrenoceptor agonists and an antagonist on gastric emptying and mouth to caecum transmit time in humans. Aliment Pharm Therap
9 Sninsky CA, Davis RH, Clench MH, Thomas KD, Mathias JR. Effect of lidamidine hydrochloride and loperamide on gastric emptying and transit of the small intestine. Gastroenterology
10 Feldman MH, Smith HJ, Simon TR. Gastric emptying of solid radiopaque markers: studies in healthy subjects and diabetic patients. Gastroenterology
11 Bertrand J, Metman E-H, Danquechin Dorval E. Étude du temps d'évacuation gastrique de repas normaux au moyen de granules radio-opaques. Applications cliniques et validation. Gastroen Clin Biol
12 Sugahara S, Rosen M, Juniper CJ, Johnston KR, Davies RLL. Effect of intrathecal and intraperitoneal morphine on gastrointestinal motility in the rat. Eur J Anaesthesiol
13 Francis B, Green M, Payne C. The GLIM system—Release 4 Manual.
Clarendon Press: Oxford, 1993: 236–237, 470–475.
14 Spaulding TC, Venafro JJ, Ma MG, Fielding S. The dissociation of the antinociceptive effect of clonidine from supraspinal structures. Neuropharmacology
15 Tasker RAR, Melzack R. Different alpha-receptor subtypes are involved in clonidine-produced analgesia in different pain tests. Life Sci
16 Skingle M, Hayes AG, Tyers MB. Antinociceptive activity of clonidine in the mouse, rat and dog. Life Sci
17 Hayes AG, Skingle M, Tyers MB. Alpha-adrenoceptormediated antinociception and sedation in the rat and dog. Neuropharmacology
18 Fändriks L, Jönson C, Nylander O. Effects of splanchnic nerve stimulation and of clonidine on gastric and duodenal HCO3−
secretion in the anaesthetized cat. Acta Physiol Scand
19 Jonderko K, Kucio C. Effect of anti-obesity drugs promoting energy expenditure, yohimbine and ephedrine, on gastric emptying in obese patients. Aliment Pharm Therap
20 Allescher HD, Ahmad S, Kostolanska F, Kwan CY, Daniel EE. Modulation of pyloric motor activity via
adrenergic receptors. J Pharmacol Exp Ther
21 Merritt AM, Ruckebusch Y. Milk feeding and xylazine treatment induce increased antroduodenal motility in young cattle with opposite effects on duodenal digesta flow rate. Digest Dis Sci
22 Tack JF, Wood JD. Actions of noradrenaline on myenteric neurons in the guinea pig gastric antrum. J Autonom Nerv Syst
23 MacDonald A, Kelly J, Dettmer PW. Pre- and post-junctional α-adrenoceptor-mediated responses in the rat gastric fundus in-vitro. J Pharm Pharmacol