Mode of Action of Vasopressin on Isolated Proximal Colon of the Guinea‐pig

Vaughan Williams and Streeten (1952) reported that vasopressin, but not oxytocin, increased the activity of the colon of the dog recorded in situ.Botting (1965) found that the isolated proximal portion of the colon of another species, the guinea-pig, was sensitive to vasopressin but was comparatively insensitive to oxytocin.In addition Botting showed that hexamethonium and pentolinum partially inhibited the contractions of the tissue caused by vasopressin.This observation, together with the fact that the colon of the guinea-pig has a high count of ganglion cells (Tafuri, 1957) suggested that vasopressin might be capable of effecting a contraction of this tissue by stimulating nervous elements associated with the smooth muscle.This paper describes experiments which provide evidence in support of this suggestion. METHODSMale guinea-pigs (320 to 420 g body weight) were killed and the bulbous proximal portion of the colon was excised.The faeces were flushed out with a pipette and the tissue was suspended in a 5 ml.organ bath containing a modified Tyrode solution vigorously aerated (Botting, 1965).After allowing the tissue to rest for 30 min, regular contractions to vasopressin could be produced every 15 min.The responses were recorded by a frontal writing lever on a smoked drum. A ntagonist drugsThe effects of the following drugs on the contractions produced by vasopressin and reference drugs were investigated: hyoscine hydrobromide; hexamethonium bromide; pentolinium tartrate; tetraethylammonium bromide; mecamylamine hydrochloride; morphine hydrochloride; procaine hydrochloride; cocaine hydrochloride. Effect of lowered temperatureStandard responses of the tissue to vasopressin and appropriate reference drugs were obtained at 32' C, and were compared with those obtained on the same tissue at temperatures between 18 and 26' C. Effect of anoxiaAnoxia of the tissue was produced by substituting for the aerated bath solution a solution through which nitrogen gas had been bubbled for at least 20 min.Nitrogen was also bubbled through the solution in the organ bath during the period of anoxia.14765381a, 1966, 2,