The responses of the isolated, blood-perfused spleen of the dog to angiotensin, oxytocin and vasopressin.

1 The responses of the smooth muscle of the capsule and blood vessels of the isolated, blood-perfused spleen of the dog to angiotensin, oxytocin and vasopressin have been investigated and compared to the actions of the catecholamines, adrenaline and noradrenaline.2 Increasing doses of each of the three polypeptides cause graded increases in splenic vascular resistance and reductions in spleen volume.3 Doses of the polypeptides which evoked increases in splenic vascular resistance not significantly different from increases produced by chosen doses of each catecholamine caused significantly smaller reductions in spleen volume.4 The time-course of action of the polypeptides on the splenic vascular smooth muscle is different since the time to 50% recovery from vasopressin is highly significantly longer than that for equieffective doses of either angiotensin or oxytocin.5 Phenoxybenzamine, in a dose which almost blocked the actions of the catecholamines, increased the responses of the vascular and capsular smooth muscle to oxytocin, vasopressin and angiotensin. This increase was not observed with another alpha-adrenoceptor blocking agent, phentolamine.6 The significant species variation in the responses of the smooth muscle of the spleen to polypeptides and catecholamines are discussed and the results are considered in the context of the possible physiological roles of the polypeptides in haemorrhage.