Modulation of lower esophageal sphincter relaxation in the opossum

The relationship of lower esophageal sphincter (LES) relaxation and sphincter pressure was studied in the basal state and after neurohumoral stimulation in the anesthetized opossum. LES relaxation was induced by electrical vagal stimulation, balloon distension, and swallowing, whereas LES pressure was increased by infusions of gastrin I, histamine, phenylephrine, or bethanechol. Each agent was selected to give an approximately twofold increase in LES pressure, at the highest dosage. The lower dosage gave a submaximal increase in LES pressure. Neither gastrin nor histamine modified sphincter relaxation as induced by vagal stimulation, esophageal distension, or swallowing. Phenylephrine or bethanechol made the LES less sensitive to vagal stimulation or esophageal distension without reducing maximal LES relaxation. Only bethanechol significantly decreased sphincter relaxation in response to swallowing (P less than 0.05). The results of this study suggest that LES relaxation is not a fixed response simply dependent on the inhibitory stimulus or the resting tonic level of LES pressure, but may be modulated by both alpha-adrenergic and cholinergic drugs.