Small-intestinal myoelectric activity in sheep: rebound excitation versus phase-3-like activity revealed by hexamethonium and atropine administration

Postinhibitory or rebound excitation (RE) is a known phenomenon in gastrointestinal motility, but its precise character and triggering mechanisms have not been defined so far. This study was thus devoted to analyzing its occurrence following hexamethonium (Hx) and atropine (At) administration during various phases of the migrating myoelectric or motor complex (MMC) in fasted, non-fasted, and fed sheep and to determine the nature of RE in comparison with phase 3 of the small-intestinal MMC. In the course of chronic experiments, various doses of Hx and At evoked RE alternating with phase-3-like activity (not the organized phase 3 of the MMC or its fragments) with periods and intensities related to the drug dose. In non-fasted sheep these changes were less pronounced and more delayed, while after feeding no excitatory response was observed. When the drug was given during phase 1 of the MMC, RE did not occur or was greatly reduced and delayed. Hx triggered RE mostly in the duodenum and At mostly in the jejunum. Rather, no RE was observed in the ileum. It is concluded that Hx and At inhibit small-intestinal motility and evoke RE and phase-3-like activity as a secondary stimulatory response in conscious sheep.