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Mechanical properties and drug sensitivity of the circular smooth muscle obtained from the muscularis mucosa of the gastroesophageal junction of the guinea pig were studied. Spontaneity was observed in strips of the tissue. Electrical stimulation (0.1 msec) produced large, rapid contractions which differed from twitch-like contractions produced in the outer circular muscle layer of the gastroesophageal junction. Responses of the smooth muscle of the muscularis mucosa were blocked by atropine but not by d-tubocurarine. Repetitive stimulation induced an initial contraction followed by relaxation in a spontaneously active preparation. The initial contraction was blocked by atropine and tetrodotoxin and the relaxation was inhibited by propranolol, guanethidine and tetrodotoxin. Catecholamines, other than isoprenaline, raised the level of tone of preparations obtained from both the muscularis mucosa and the outer circular muscle layer. Responses of the circular smooth muscle were not influenced by catecholamines. Excitatory effects of catecholamines were completely blocked by alpha-blockers. Isoprenaline inhibited responses and the action of isoprenaline was antagonized by propranolol. Acetylcholine was excitatory and the effect was antagonized by atropine. These results suggest that there are two types of smooth muscle in the muscularis mucosa. In one type, adrenergic alpha-receptors are excitatory, beta-receptors inhibitory and cholinergic receptors are muscarinic. In the other type, adrenergic receptors are beta-inhibitory and cholinergic receptors are muscarinic. Circular smooth muscle of the muscularis mucosa seems to be innervated with excitatory cholinergic nerves and inhibitory adrenergic nerves. Responses of specific smooth muscle seem to be less sensitive to some of the drugs used.

Mechanical Activity of the Smooth Muscle of the Muscularis Mucosa of the Guinea Pig Esophagus and Drug Actions