The continuous estimation of angiotensin formed in the circulation of the dog.

1. The use of the rat colon as a blood-bathed organ is described for detecting changes in angiotensin concentration in the circulating blood of dogs.2. Partial occlusion of the aorta by a balloon inflated above the renal arteries leads to a contraction of the blood-bathed rat colon.3. From the experimental evidence, it is concluded that this contraction is due to an increased concentration of circulating angiotensin, brought about by the liberation of renin from the kidneys.4. The characteristics of renin release have been studied. It occurs within seconds of reducing the blood pressure to the kidneys and is proportional to the degree of reduction of blood pressure.5. With a prolonged reduction of renal blood pressure the concentration of angiotensin increases over the first 10-12 min and then reaches a stable level.6. After a small haemorrhage angiotensin often appears in the circulation without a concomitant release of catechol amines.7. Greater haemorrhages induce the secretion of catechol amines as well as of renin. The catechol amine secretion is inhibited by ganglion block, but the renin secretion is not.8. It is concluded that the secretion of renin by the kidneys in response to a fall of renal blood pressure is a physiological response, probably of importance in homoeostasis.