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The effects induced by 5‐hydroxytryptamine (5‐HT) on gastrointestinal myoelectric activity in conscious sheep were recorded through electrodes chronically implanted and analysed by computer. The 5‐HT receptors and the cholinergic neuronal pathways involved in these actions were investigated.The intravenous (i.v.) administration of 5‐HT (2, 4 and 8 μg kg−1min−1, 5 min) induced an antral inhibition concomitant with a duodenal activity front that migrated to the jejunum, followed by a period of intestinal inactivity. This myoelectric pattern closely resembled that observed in the phases III and I of the migrating myoelectric complex (MMC) in sheep. The 0.5 μg kg−1min−1dose evoked the same pattern in only two out of the six animals used. Likewise, the 1 μg kg−1min−1dose similarly affected four of the six animals. In addition, a transient stimulation was observed in the antrum and jejunum when the two highest doses were used.The 5‐HT1antagonist, methiothepin (0.1 mg kg−1), the 5‐HT2antagonists, ritanserin (0.1 mg kg−1) and ketanserin (0.3 mg kg−1), the 5‐HT3antagonists, granisetron (0.2 mg kg−1) and ondansetron (0.5 mg kg−1), as well as the 5‐HT4antagonist, GR113808 (0.2 mg kg−1), did not modify the spontaneous gastrointestinal myoelectric activity. However, the cholinoceptor antagonists, atropine (0.2 mg kg−1) and hexamethonium (2 mg kg−1), inhibited gastrointestinal activity.When these antagonists were injected i.v. 10 min before 5‐HT (2 or 4 μg kg−1min−1, 5 min), only GR113808, atropine and hexamethonium were able to modify the 5‐HT‐induced actions, all of them being completely blocked by the three antagonists.Our data show that 5‐HT initiates a MMC‐like pattern in the gastrointestinal area in sheep through 5‐HT4receptors. Furthermore, these actions are mediated by cholinergic neural pathways involving muscarinic and nicotinic receptors. However, our results do not indicate a role for either 5‐HT1, 5‐HT2or 5‐HT3receptors in the 5‐HT‐induced effects.

Longitudinal smooth muscle of the sheep duodenum showed a rhythmic spontaneous activity with an average frequency of 5·6 ± 0·55 phasic movements min−1and a mean value of the amplitude of phasic contractions of 0·956 ± 0·1 g. When the strips were incubated in Ca2+-free medium, the spontaneous motility amplitude (SMA) was reduced to 37 ± 8·2% of control values. In Ca2+-free medium plus EDTA (1 or 2 Mm), the SMA was strongly reduced to 21·9 ± 8·3 and 1·8 ± 1·8%, respectively. Verapamil, nifedipine and diltiazem diminished the SMA. The EC50 value for verapamil was 10−9M., whereas that for diltiazem was 2 × 10−9M and for nifedipine was 3 × 10−14M. Trifluoperazine and TMB-8 reduced the SMA with EC50 values of 7 × 10−6and 3 × 10−5m, respectively. The spontaneous activity in the sheep duodenum seemed to be mediated by influx extracellular Ca2+, which enters through potential-dependent channels and intracellular Ca2+release.