Distension-evoked ascending and descending reflexes in the circular muscle of guinea-pig ileum: an intracellular study

Abstract

Reflex responses evoked by distension of the guinea-pig small intestine were recorded from the circular muscle with intracellular microelectrodes. For this purpose a mechanically stable preparation that allowed the intestinal wall to be distended within 9 mm of the recording site was developed. A segment of intestine was opened along the mesenteric border and pinned mucosa uppermost over a balloon set in the base of an organ bath, so that inflation of the balloon could distend the intestinal wall without simultaneously pushing against the mucosa. Compound excitatory junction potentials (EJPs) and compound inhibitory junction potentials (IJPs) were recorded at sites up to 40 mm oral and anal to the distending stimulus, respectively. The compound EJPs recorded orally had amplitudes of up to 24 mV and declined to baseline during distensions that exceeded 10–15 s. Distensions at intervals of less than 20 s evoked successively smaller oral compound EJPs; after four distensions in 30 s the amplitude of the compound EJP had fallen to less than 10%. The amplitude of the oral compound EJP was reduced by hyoscine (1 μM), but the extent of the reduction depended on the degree of distension; responses to mild stimuli were blocked, whereas those to strong stimuli were only slightly reduced. The amplitude of the hyoscine-resistant component of the compound EJP was markedly reduced by antagonists of substance P receptors in the muscle. In the presence of muscarinic and substance P receptor antagonists, a transient compound IJP could be detected on the oral side of the stimulus. The compound IJPs recorded anal to the distension had amplitudes up to 22 mV but the potential returned to baseline during prolonged distension. In the presence of hyoscine (1 μM) some inhibitory activity continued throughout prolonged stimuli. Compound IJP amplitudes were not significantly reduced by repeated distensions separated by more than 6 s. At anal sites a transient depolarization (off-response) was recorded immediately following the termination of a distension in some preparations. The off-response was unaffected by hyoscine and was more readily observed after the further addition of substance P antagonists. The compound IJPs were almost completely blocked by apamin (0.2 μM). The compound EJPs and IJPs recorded orally were blocked by hexamethonium (100 μM), but the amplitudes of compound IJPs recorded anally were significantly reduced by hexamethonium (100–200 μM) only at recording sites greater than 15 mm from the centre of the balloon. The off-response was reduced by hexamethonium at all sites. All responses to distension were blocked by tetrodotoxin (0.5 μM). Oral compound EJPs and anal compound IJPs of normal amplitude could still be evoked after removal of the mucosa and nearly all circular muscle from the site of distension. Such lesions did not prevent transmission of these reflexes along the intestine. Thus, both sensory neurons and the nervous pathways activated by stretch lie within the myenteric plexus.