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Clonidine inhibits gastric motility in the rat

Asai, T.; Vickers, M. D.; Power, I.*

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European Journal of Anaesthesiology: May 1997 - Volume 14 - Issue 3 - p 316-319
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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 [6] and that of both solids and liquids in the dog [7]. In humans, oral clonidine did not delay gastric emptying of a radiolabelled meal [8], whereas, lidamidine which is also considered to be an alpha2 adrenoceptor agonist weakly delayed gastric emptying [9].

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 [12], 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.

Statistical analysis

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[13].


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.

Fig. 1.
Fig. 1.:
Effect of clonidine on gastric emptying of indigestible solids (steel balls). The steel balls were inserted into the stomach immediately after intraperitoneal injection of clonidine, and the number of balls which had passed into the intestine was counted at 3 h. A fitted logistic dose-response curve is shown together with the value obtained from each rat (○). Symbols are displaced slightly if there are the same values for each dose of clonidine.

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).

Table 1
Table 1:
Effect of either yohimbine or naloxone on the inhibitory effect of clonidine (ED75) on gastric emptying of steel balls (mean (range)). P-values calculated using the Mann–Whitney U-test and confidence limits of the difference between rats that received saline and those that received an antagonist are shown

Side effects

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 [18], 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.


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Gastrointestinal Tract, motility; Measurement, techniques, steel ball test; Complications, gastric stasis

© 1997 European Academy of Anaesthesiology