Effect of dopamine on the esophageal smooth muscle in vivo.

Intravenous administration of dopamine caused a dose-dependent reduction in the lower esophageal sphincter pressure and contractions in the lowermost part of the body of the esophagus in the opossum. A dose of 5 mug per kg produced maximal response. The onset of contractions in the body occurred around 20 sec after the onset of the sphincter response. The effect of dopamine was different from isoproterenol, which caused no contractions in the body and caused reduction in sphincter pressure, and from phenylephrine, which also caused no contractions in the body but caused contraction of the sphincter. Moreover, the effect of 5 mug per kg of dopamine was blocked by haloperidol but not by phentolamine, propranolol, bilateral cervical vagotomy, or tetrodotoxin. These studies suggested the presence of selective dopamine receptors which mediated contraction of the esophageal body and relaxation of the sphincter muscle. Since vagal stimulation also causes contraction in the body and relaxation of the sphincter, it was possible that vagal effect on the esophagus was mediated by dopamine receptors. However, haloperidol did not block the effect of electrical stimulation of vagus nerve on body of the esophagus or the lower esophageal sphincter.