A sensitive and specific assay for vasopressin in the circulating blood.

1. A search was made for an assay tissue with selective sensitivity to vasopressin. Of those smooth muscle preparations tested, the longitudinal muscle of the isolated rectum of the rabbit was the most satisfactory.2. The rabbit isolated rectum, bathed in Krebs solution, was contracted by acetylcholine, angiotensin II amide, bradykinin and 5-hydroxytryptamine. It was relaxed by vasopressin, oxytocin and the catecholamines.3. Vasopressin was active in concentrations of 4-100 muu./ml (0.01-0.25 ng/ml) and was 20-30 times more active than oxytocin. Bretylium had no effect on the relaxant action of vasopressin; nor did concentrations of alpha- and beta-adrenoceptor blocking agents sufficient to abolish the actions of catecholamines. Lignocaine reduced the sensitivity of the rabbit rectum to both vasopressin and oxytocin without altering the actions of adrenaline. High concentrations of either vasopressin or oxytocin desensitized the rabbit rectum to the actions of both hormones, without affecting the actions of adrenaline. It was concluded that vasopressin and oxytocin act on a common population of receptors different from those for catecholamines.4. Phentolamine, unlike other alpha-adrenoceptor antagonists, reduced the relaxant action of vasopressin on the rectum.5. When superfused with blood from an anaesthetized dog, the rabbit rectum maintained a higher tone than in Krebs solution; it retained its sensitivity to vasopressin. Pronethalol, administered intraluminally, reduced spontaneous movement and abolished the actions of low concentrations of catecholamines, thereby increasing the specificity of the assay. No other substance tested relaxed the rectum in concentrations likely to be found in blood.6. Vasopressin was stable in dog's blood; it survived passage through the pulmonary vascular bed; it had a half-life in the circulation of about 1 min.7. The half-life of vasopressin in the circulation may depend upon the duration of the infusion.