Animal model for the study of internal anal sphincter activity.

This paper describes a simple and reliable model for studying the internal anal sphincter (IAS) in the rat. The activity of the sphincter was recorded by a two-balloon system. The compliance of the balloon allowed measurements of pressure changes lower than 0.1 cm H2O. The anatomical and physiological characteristics of the rat IAS were found to be in many aspects similar to those found in the human. The rectoanal reflex in the rat consists of relaxation followed by contraction. The threshold for this response is 0.4-0.6 ml of air. The response of the IAS to prolonged rectal distension showed adaptation in 50% of the animals. Intraarterial application of drugs produced rapid, dose-dependent and reproducible effects on the IAS. The results described in this work show that the rat is a useful and convenient model for studying anal function. It may be used to investigate both the physiology and pharmacology of the IAS activity.