Electrical control activity of the lower esophageal sphincter in unanesthetized opossums

We recorded lower esophageal sphincter (LES) pressure and myoelectrical activity concurrently from the esophagus, LES, stomach, and proximal small intestine in unanesthetized opossums. LES electrical activity was characterized by almost continuous, spikelike oscillations at 15-40/min, which were accompanied by minicontractions 5-15 mmHg in amplitude. Basal LES pressure, however, did not depend on electrical oscillations. The LES exhibited cyclic changes in pressure and electrical activity synchronous with gastric contractions associated with the migrating myoelectric complex (MMC). During phase I, the LES pressure was stable and its electrical activity showed continuous low-amplitude (< 0.3 mV) oscillations at a rate of 20 ± 2 per min. During phases II and III of MMC-related gastric activity, the LES developed strong phasic contractions, while the electrical oscillations grouped into clusters with an increased rate (32 ± 1 per min) and amplitude (0.4-0.8 mV), each cluster corresponding to a phasic contraction. Feeding, pentobarbital, and cholinergic blockade abolished MMC-related LES activity and resulted in a steady rate of LES electrical oscillations. LES relaxation induced by swallows or esophageal balloon distention abolished the LES electrical oscillations. Increases in LES pressure induced by pharmacological and hormonal stimulation correlated with concurrent increases in the rate of electrical oscillations. We conclude that the opossum LES has a unique pattern of electrical activity that differs from the electrical activity recorded from smooth muscle elsewhere in the gastrointestinal tract. This electrical activity is a form of control wave associated with a minicontraction. Phasic LES contractions during the intestinal MMC cycle may result from an increase in the rate and amplitude of the LES electrical control waves and fusion of minicontractions.