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Long Vasodilator Reflexes Projecting Through the Myenteric Plexus in Guinea‐Pig Ileum

This study examined enteric neural reflexes activating submucosal cholinergic vasodilator motoneurons, which innervate the final resistance vessels regulating mucosal blood flow. Videomicroscopy was employed to monitor dilatation of submucosal arterioles in in vitro preparations from guinea‐pig ileum. Balloon distension of intact lumen evoked reflex vasodilatation and flat sheet preparations were employed to separate mucosal mechanical stimulation from intestinal distension. Mucosal stroking and balloon distension of the orad segment evoked vasodilatations > 1.5 cm from the stimulating site. Mucosal stimulation was blocked by combined 5‐HT3/5HT4 antagonists but distension‐evoked responses were unaffected. Distension‐evoked responses were also unaffected by nifedipine (5 μm) or nifedipine (1 μm) and wortmannin (300 nm), suggesting stretch activation rather than stretch‐activated contraction was involved. Mucosal and distension‐evoked responses were completely blocked when the myenteric plexus was surgically lesioned and were significantly inhibited by hexamethonium. The muscarinic antagonist 4‐DAMP, which inhibits vasodilatations evoked by submucosal cholinergic vasodilator neurons, blocked dilatations elicited by mucosal stimulation and balloon distension. Maximal dilatations evoked with either sensory modality could be further enhanced when stimulated with the second modality. Dilatations evoked by stimulation of the aborad segment were similar to those elicited in the orad segment. In conclusion, sensory mechanisms in the mucosa and muscularis propria activate vasodilator pathways in the myenteric plexus which project for significant distances in both ascending and descending directions before innervating submucosal arterioles. These reflexes could co‐ordinate mucosal blood flow during multiple motor events such as peristalsis and intestinal mixing between propulsive events.