Electrical activity of longitudinal and circular muscle during peristalsis

Action potentials were recorded simultaneously from the longitudinal and circular muscle layers of the guinea pig isolated small intestine. Both the graded reflex of the longitudinal muscle and the peristaltic reflex proper could be evoked by raising the intraluminal pressure. At low intraluminal pressures, intervals between spike bursts of the circular muscle were longer than those of the longitudinal muscle. The higher the intraluminal pressure, the shorter became the intervals between spike bursts in the circular muscle, until both muscle layers showed synchronous discharge of action potentials. Tetrodotoxin (100 nM) abolished the excitation of both circular and longitudinal muscles produced by raising intraluminal pressure. Hexamethonium (280 microM) abolished excitation of the circular muscle but not that of the longitudinal muscle. Atropine (100 nM) reduced the excitatory effects of raising pressure on both muscle layers but did not abolish them. The atropine-resistant excitation of the circular, but not the longitudinal, muscle was reversibly blocked by exposure to substance P (100-500 nM). Chymotrypsin (200 micrograms/ml) reversibly abolished the atropine-resistant excitation of the circular muscle. It was concluded that during peristalsis both longitudinal and circular muscle layers are activated synchronously; muscle activation during peristalsis is not entirely cholinergic but may involve in addition a substance P-like peptide.