Involvement of a cholinergic mechanism in the sustained depolarization and contraction of the lower oesophageal sphincter muscle cells in the cat

Abstract

Membrane potentials were recorded in vitro with intracellular electrodes from the circular muscle cells of the cat lower oesophageal sphincter and oesophageal body. In addition, the tension of lower oesophageal sphincter and oesophageal body strips was recorded isotonically. Under the experimental conditions, no spontaneous electrical activity or variation in the tension of the strips occurred. The resting membrane potential of the circular muscle cells was significantly lower in the lower oesophageal sphincter (−51.0 ± 0.3 mV) than in the circular muscle cells of the oesophageal body (−57.1 ± 0.4 mV). These values were not affected by infusion of tetrodotoxin 3.1 × 10 −6M. In the presence of atropine (3.5 × 10 −7M), the resting membrane potential of the circular muscle cells of the lower oesophageal sphincter increased significantly (−57.6 ± 0.4 mV), whereas the resting membrane potential of the circular muscle cells of the oesophageal body was not significantly affected (−57.8 ± 0.6 mV). In the presence of atropine, no significant difference in the values of the resting membrane potential of the circular muscle cells was observed between the lower oesophageal sphincter and the oesophageal body. Hyoscine (2.9 × 10 −7M) significantly increased the resting membrane potential of the circular muscle cells of the lower oesophageal sphincter, whereas eserine (3.6 × 10 −6M) significantly decreased it. Atropine induced a significant decrease in the membrane resistance of the circular muscle cells of the lower oesophageal sphincter. Atropine decreased the resting tension of lower oesophageal sphincter strips whereas eserine increased it, but no such effects were recorded on oesophageal body strips. These results suggest that, in the cat, the circular muscle cells of the lower oesophageal sphincter, but not those of the oesophageal body, are continuously influenced by cholinergic nerve terminals, which cause a low resting membrane potential, an increase in membrane resistance and persistent contraction. These effects of cholinergic neurons are not action potential dependent. The results also show that the sustained depolarization of the circular muscle cells of the lower oesophageal sphincter elicits persistent contraction of these cells. This mechanism may play a key role in the persistent closure of this sphincter.