A pharmacological analysis of the peristaltic reflex in the isolated colon of the guinea-pig or cat.

1. The peristaltic reflex in the colon was elicited by a localized intraluminal stimulus. The contractile response of the longitudinal coat, which consists of two phases, begins before the start of propulsion. Although the contractions of the longitudinal and circular musculature are usually associated, they may be independent of each other. In particular, the longitudinal contraction does not seem to be necessary for propulsion.2. Both the longitudinal reflex contraction and the segmental responses of the circular muscle to distension, namely a contraction above and a relaxation below the bolus, are abolished by tetrodotoxin and ganglion blocking agents.3. In the guinea-pig, longitudinal and circular reflex contractions are usually resistant to antimuscarine, antihistamine and antitryptamine drugs but in the cat they are abolished by antimuscarine drugs. In both species, however, atropine and hyoscine can impair propulsion by blocking selectively the descending inhibition. In the cat, it is possible to find doses which abolish the descending inhibition without affecting the contractile responses of the longitudinal and circular muscle.4. Sympathetic denervation and pretreatment with reserpine do not affect the propulsive activity. The maintenance of the descending inhibition in denervated organs suggests that the inhibitory neurones to the circular muscle are not adrenergic.5. On the basis of the effects of drugs, the possible nervous mechanism subserving the polarity of propulsion has been examined. Such a mechanism seems to require an inhibitory pathway involving muscarinic receptors at some point.6. Pelvic nerve stimulation facilitates propulsive activity. The effect of transmural stimulation is different at low and at high frequencies of stimulation. The inhibitory effect of sympathetic stimulation on the reflex responses seems to be due mainly to an action on intrinsic nervous structures.