Influence of successive vagal stimulations on contractions in esophageal smooth muscle of opossum.

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Abstract
Studies were performed in anesthetized opossums to investigate the influence of successive vagal stimuli on esophageal contractions. Mechanical activity was recorded manometrically 5 cm above the lower esophageal sphincter. Contractions in the esophagus were evoked by electrical stimuli of 2.5 mA, and 1-ms pulse duration applied to the cervical vagi, at various train durations and frequencies. Paired or multiple stimuli of 1-s train length were also tested at different interstimulus intervals (ISI). Paired stimuli at an ISI of less than or equal to 3 s and at a frequency of less than or equal to 10 Hz showed refractoriness, i.e., the contractions to the first stimulus inhibited the contraction to the second stimulus. A frequency of 50 Hz showed initial inhibition, i.e., the second stimulus inhibited the contraction to the first stimulus. Repetitive stimuli applied at a rate of 8/min (ISI 6.5 s) evoked contractions to each stimulus. At 15/min, every second or third contraction was inhibited. With stimuli applied at 30/min, contractions occurred only in response to the first and/or the last stimulus; depending upon the frequency of vagal stimulation. The intervening stimuli did not evoke any contractions. A long train stimulus produced an initial, a terminal, or both contractions depending on the stimulation value. These studies show that (a) vagal efferent stimulation causes initial inhibition and refractoriness in the esophageal smooth muscle; (b) the degree of initial inhibition increases with increasing frequency of stimulation; (c) the occurrence of contractions only at the onset and the end of a long train stimulus may be due to the influence of initial inhibition and refractoriness.