Role of substance P nerves in longitudinal smooth muscle contractions of the esophagus.

Longitudinal muscle strips from different sites along the opossum esophagus were stimulated transmurally so as to produce neurally mediated contractions. Low-frequency transmural stimulation produced contractions after termination of the stimulus ("off" contractions), whereas high-frequency stimuli produced contractions beginning during the stimulus and extending beyond termination of the stimulus (extended-duration contractions). The intrastimulus portion of the extended-duration contraction was partially antagonized by atropine or substance P desensitization, whereas the poststimulus portion of the contraction was selectively and fully antagonized by desensitization with substance P. A combination of atropine and substance P desensitization abolished the extended-duration contraction. The amplitude of contraction was greater in the proximal than in the distal strips, irrespective of the mode of stimulation. The poststimulus portion of the extended-duration contraction was significantly longer in muscle strips taken from more distal than proximal portions of the esophagus. This gradient in duration of contractions was abolished by substance P desensitization but was not affected by atropine. Exogenously applied substance P (10 microM) produced equally sustained long-duration contractions at all sites along the esophagus. These observations suggest that a) both acetylcholine- and substance P-containing nerves are responsible for the extended-duration contraction of longitudinal muscle, and b) transmural stimulation causes an aborally directed increase in the duration of contractions; this gradient of increasing duration of contraction appears to be due to a more prolonged neural release of substance P at more distal sites.