Anal sphincter pressure and the rectosphincteric reflex in the dog.

: Anal sphincter pressure (ASP) was measured in healthy conscious dogs, and the rectosphincteric reflex that causes inhibition of the internal sphincter was identified. Pressures were measured in 6 dogs by use of a perfused miniature 4-lumen catheter. To identify the rectosphincteric reflex, a balloon (fitted over the distal opening of the catheter) was insufflated with 25 ml of air. Mean (+/- SE) ASP was 74.7 +/- 4.2 mm of Hg. When the rectosphincteric reflex was stimulated by balloon distension, ASP decreased to 35.9 +/- 4.9 mm of Hg. The reduction in pressure persisted for 22 +/- 1.5 s, and ASP returned to predistension values, despite the maintenance of balloon distension. Atropine did not affect ASP or its reduction during balloon distension.