Differential effects of clonidine and dexmedetomidine on gastric emptying and gastrointestinal transit in the rat.

We have studied the effect of clonidine, dexmedetomidine and morphine on gastric emptying and gastrointestinal transit in the rat. In one group, each agonist was injected i.p. in 6-12 male Wistar rats. In another group of rats, yohimbine, naloxone or saline was injected with an agonist. At 30 min, radiolabelled saline 1 ml was infused into the stomach. At 1 h, gastric emptying and gastrointestinal transit were calculated by measuring the radioactivity in the gastrointestinal tract. We found that clonidine and dexmedetomidine strongly inhibited gastrointestinal transit (ED50 0.08 and 0.04 mg kg-1, respectively). They also significantly inhibited gastric emptying (P < 0.05), but the effect was weak (95% confidence intervals for difference from saline 8.2-34.9% with clonidine 1 mg kg-1 and 3.4-15.4% with dexmedetomidine 0.03 mg kg-1). Morphine strongly inhibited both gastric emptying and gastrointestinal transit (ED50 2.8 and 1.2 mg kg-1, respectively). Yohimbine significantly antagonized the inhibitory effects of clonidine and dexmedetomidine (P < 0.05), whereas naloxone, which significantly antagonized the effect of morphine (P < 0.01), did not antagonize the effect of either of the other agonists.