Interactive effect of morphine and dexmedetomidine on gastric emptying and gastrointestinal transit in the rat

We studied the interactive effect of dexmedetomidine and morphine on gastric emptying and gastrointestinal transit in the rat. In one group of rats, to examine the interactive effect on gastrointestinal transit, the two drugs were injected i.p. in a fixed ratio (30: 1)--that of their ED50 values--in six doses (0.062-1.075 mg kg-1), each to a different group of 5-8 male rats for each dose. In another group, to examine the interactive effect on gastric emptying, either dexmedetomidine or saline was injected with saline or with the ED25 or ED50 of morphine (0.22 and 2.8 mg kg-1, respectively). In both groups, at 30 min, radiolabelled saline 1 ml was infused into the stomach; at 1 h, gastric emptying or gastrointestinal transit was calculated by measuring the radioactivity in the gastrointestinal tract. Morphine and dexmedetomidine produced a supra-additive inhibitory effect on gastrointestinal transit (P = 0.02). Dexmedetomidine 0.01 mg kg-1, which itself significantly inhibited gastric emptying (P < 0.01), did not significantly alter the inhibitory effect of morphine on gastric emptying. Therefore, in rats, dexmedetomidine markedly enhanced the inhibitory effect of morphine on gastrointestinal transit, but it did not significantly alter the effect on gastric emptying.