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Characteristics of "on" and "off" contractions in esophageal circular muscle in vitro.

The prevalence, amplitude, and latency periods of "on," "off," and "intermediate" contractions in response to transmural stimulation were recorded in transverse rings of circular muscle from different levels of the opossum esophagus. Ten-second train stimuli consistently produced off contractions. On contractions were not seen at lower stimulus frequencies (2 and 5 Hz); however, their incidence approached 90% at higher frequencies (40 Hz). Intermediate contractions occurred only at stimulus frequencies of 10 Hz or greater and were less frequent than on contractions. In general, the on and intermediate contractions had significantly lower amplitudes than the off contraction. The on contraction occurred with a latency period from initiation of the stimulus. This latency was greater in the more distal sites and decreased with increasing stimulus frequency. The off contractions occurred with a latency period from termination of the stimulus. This latency was not dependent on either stimulus frequency or site along the esophagus. Atropine antagonized the on and intermediate contractions but had no such effect on the off contraction. Tetrodotoxin abolished the on, off, and intermediate contractions. This study suggests that an intramural mechanism exists that upon stimulation produces atropine-sensitive on contractions. These contractions may play a role in esophageal peristalsis.