Myogenic oscillatory mechanism for opossum esophageal smooth muscle contractions

We evaluated the control of phasic contractions in opossum esophageal circular smooth muscle by determining the contractile response in vitro to agents that cause membrane depolarization and excitation by different mechanisms. Transverse muscle strips taken from different sites along the length of the smooth muscle esophagus were exposed to progressively increasing concentrations of tetraethylammonium (1-30 mM), K⁺ (4.6-30 mM), or bethanechol (10^-6 to 10^-2 M). In normally inactive esophageal circular smooth muscle, tetraethylammonium and high K⁺ concentration elicited phasic contractions that were not blocked by atropine and tetrodotoxin. Bethanechol, an M₂ muscarinic receptor agonist that acts selectively on smooth muscle, elicited phasic contractions that were not blocked by tetrodotoxin. We conclude that a latent myogenic oscillatory mechanism for control of phasic contractions exists in esophageal circular smooth muscle and that it may be activated by nonspecific excitation of the smooth muscle membrane. We suggest that this myogenic oscillatory mechanism is likely excited and modulated by nerves.