Pharmacologic identification of the lower esophageal sphincter

The distal 2/3 of the opossum esophagus contains only smooth muscle. Manometry shows that the most distal 1-2 cm is the lower esophageal sphincter. We used a variety of agonists to seek differences between circular muscle from the sphincteric segment and more rostral levels. Isometric contractions of strips from the distal 6 cm were recorded in vitro in response to acetylcholine, carbachol, methacholine, nicotine. DMPP, norepinephrine, norepinephrine with propranolol, barium, atropine, and potassium. Significant differences in threshold concentration occurred for all drugs except barium, atropine, and potassium, the more distal strips being more sensitive. The gradient of threshold was much steeper for norepinephrine than for the other drugs. Maximal responses did not differ among levels for the choline esters or ganglionic stimulants, but showed proximal diminution for norepinephrine. These differences in threshold concentration could represent differences in distribution density of drug receptor sites, differences in affinity of receptors for the agonists, differences in rates of uptake of agonists, or differences in rates of enzymatic hydrolysis; or they may have no common basis. The sphincter is defined, at least in part, in the esophageal wall rather than in the central nervous system. The greater magnitude of the difference in sensitivity to norepinephrine than for the other agents suggests that the adrenergic innervation is important in defining the lower esophageal sphincter.

Esophageal responses to distension and electrical stimulation

The opossum esophagus contains only smooth muscle in the distal two-thirds; it can be used to study autonomic control of esophageal smooth muscle. Three different preparations of opossum esophagus were used; the esophagus in vivo, the isolated whole esophagus, and isolated strips of the three layers of esophageal smooth muscle. Responses were examined to localized distension and to electrical stimulation. Distension of the esophagus in vivo produced three separate responses: inflation of a distending balloon caused a brief contraction rostral to the point of distension, the on response; maintenance of distension produced shortening of the esophagus, sustained for the duration of the distension, the duration response; and deflation of the balloon caused a single brief caudal circumferential contraction, apparently propagated caudad, the off response. In the isolated whole esophagus distension produced the same three responses. Electrical stimulation in this preparation produced apparently identical responses. Electrical stimulation of isolated strips of the three muscle layers showed that the muscularis mucosae and the longitudinal layer of the muscularis propria always respond with a duration response only. The circular layer of the muscularis propria responds with on and off responses only. These observations suggest that both types of stimuli excite the same afferent nerve fibers in local reflex pathways. Peristalsis can be maintained by these reflexes in the smooth muscle part of the esophagus independent of central nervous connections.

The human small intestinal slow wave

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