Abstract
The presence of inhibitory nonadrenergic noncholinergic (NANC) intrinsic innervation of the circular muscle of the gastrointestinal sphincters of the South American (SA) opossum was investigated in vitro. Isolated circular muscle strips from the esophagogastric and ileocolonic junctions but not from the gastroduodenal(pylorus)region developed spontaneous tension. Tetrodotoxin (TTX, 1 microM) augmented the spontaneous tension only in the ileocolonic junction strips. Electrical field stimulation of esophagogastric and ileocolonic junction strips caused frequency-dependent responses consisting of a relaxation at lower frequencies (< 1 Hz) and a biphasic response or contraction field stimulation abolished the spontaneous activity at lower frequencies and induced contractions at higher frequencies. The responses elicited by electrical field stimulation in the three sphincters were abolished by TTX (1 microM). Electrical field-induced contractions were reduced while relaxations were enhanced by atropine (1 microM). In the presence of atropine (1 microM) and guanethidine (3 microM), electrical field stimulation, nicotine and ATP induced frequency- or concentration-dependent relaxations of the three sphincters that were abolished by TTX (1 microM). Isoproterenol and sodium nitroprusside caused concentration-dependent relaxations which were TTX-resistant. These findings indicate that the sphincteric circular muscle of the SA opossum gastrointestinal tract is relaxed by the activation of intrinsic NANC nerves and therefore can be used as a model for the study of the mechanisms involved in these responses.
Highlights
Nerve-induced relaxation of gastrointestinal smooth muscle is involved in important physiological processes of the digestive tract such as lower esophageal sphincter relaxation following swallowing, accommodation to distention of the proximal stomach and descending inhibition [1]
Isoproterenol and sodium nitroprusside caused concentration-dependent relaxations which were TTX-resistant. These findings indicate that the sphincteric circular muscle of the South American (SA) opossum gastrointestinal tract is relaxed by the activation of intrinsic nonadrenergic noncholinergic (NANC) nerves and can be used as a model for the study of the mechanisms involved in these responses
Following the description that the smooth muscle composition of the North American (NA) opossum lower esophageal sphincter, in contrast to that of more common laboratory animals, resembles more closely the human lower esophageal sphincter [13] most of the current concepts regarding the neural control of gastrointestinal sphincters were derived from in vivo and in vitro studies performed on this animal model [3,4,5,6,7,8,14,15]
Summary
Nerve-induced relaxation of gastrointestinal smooth muscle is involved in important physiological processes of the digestive tract such as lower esophageal sphincter relaxation following swallowing, accommodation to distention of the proximal stomach and descending inhibition [1]. Experiments designed to elucidate the mediator and mechanisms involved in NANC nerve-induced relaxations of sphincteric muscle in the gastrointestinal tract have been performed mainly in North American (NA) opossums (Didelphis virginiana) [3,4,5,6,7,8] or in dogs [912]. It became of interest to characterize in vitro the neurally induced responses of gastrointestinal sphincters from Didelphis albiventris, an opossum species readily available in South America (SA), in order to determine whether it could be an alternative model to investigate the inhibitory innervation of sphincteric muscle from the gastrointestinal tract
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