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.