1. Angiotonin does not exert its vasoconstrictor effect in the absence of a substance contained in red blood cells and serum which we have called "angiotonin-activator." A fraction has been separated from blood in which angiotonin-activator is concentrated. It contains little or no reninactivator.

2. Repeated intravenous injections of angiotonin into animals causes the pressor response gradually to lessen and finally to disappear (the phenomenon of tachyphylaxis), but much more slowly than when renin is injected. When the response to angiotonin is abolished, renin also fails to act. Large doses of renin reduce and finally abolish the responsiveness to angiotonin. Exhaustion of renin-activator in the blood abolishes the response to renin without abolishing the response to angiotonin.

3. Blood from animals made tachyphylactic by infusion of angiotonin contains greatly reduced amounts of angiotonin-activator. An inhibitor also appears in the blood.

4. Bilateral nephrectomy prolongs and greatly enhances the rise of arterial pressure following injection of angiotonin and renin. The enhancement reaches a maximum in from 24 to 30 hours after operation. Blood from these animals exhibits greatly increased ability to activate angiotonin and renin when tested in isolated perfused organs. Large amounts of angiotonin are required to reduce the amount of activator in their blood. Renin-activator is simultaneously but little affected.

5. Tranfusion of blood from an animal made tachyphylactic to angiotonin into a nephrectomized dog reduces the response of the latter to angiotonin. Angiotonin when added to the blood of the recipient of the transfusion and perfused through a rabbit's ear also exhibits greatly reduced vasoconstrictor action.

6. Transfusion of normal blood in large amounts into nephrectomized or hypertensive dogs reduces the recipient's response to renin. If renintachyphylaxis is established in the donor, transfusion abolishes the response to renin in the recipient. The blood from such animals exhibits greatly reduced vasoconstrictor properties when perfused through an isolated organ with renin or angiotonin.

7. Renin-tachyphylactic or normals dog's blood does not reduce arterial pressure elevated by a single injection of renin into nephrectomized dogs.

8. Nephrectomized dogs exhibit the greatest pressor response to infusion of angiotonin and renin, normal animals the least, and hypertensive animals about midway between.

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