1. Tachyphylaxis occurs when renin is repeatedly injected into dogs and cats regardless of whether they are normal, anesthetized, pithed, hepatectomized, suprarenalectomized, nephrectomized, or eviscerated.

2. The pressor response to renin in brief experiments is independent of the height of the arterial pressure or the presence of the suprarenals. Evisceration and large doses of ergotamine reduce the response. It is largely uninfluenced by pithing, intracisternal injection of renin, cocaine, strychnine, caffeine, and infusion of sodium bicarbonate or hydrochloric acid. It may be slightly increased by large blood transfusions or hepatectomy but the result is short lived.

3. There is no parallelism between the pressor responses to carotid sinus stimulation, adrenine, and tyramine on the one hand and renin on the other.

4. Section of the brain may be followed by depressor responses to renin.

5. Intracisternal injection of renin elicits no significant rise in blood pressure or other circulatory manifestations.

6. Continuous infusion of renin produces a prolonged rise of arterial pressure in normal and chronically suprarenalectomized dogs, but the pressure ultimately falls despite continued infusion.

7. Tachyphylaxis develops in the isolated rabbit's ear perfused with blood and small doses of renin. The same blood perfused through a second ear causes no vasoconstriction when renin is added. Addition of renin-activator restores the ability of renin to cause constriction.

8. Renin alone causes no vasoconstriction when perfused with Ringer's solution, but renin plus renin-activator restores activity. Tachyphylaxis does not develop when Ringer's solution is employed instead of recirculating blood.

9. Blood from animals made tachyphylactic by repeated injections of renin is lacking in activator and also fails to cause vasoconstriction in the rabbit's ear when renin and renin-activator are added.

10. Renin-activator is lost and tachyphylaxis develops more slowly during continuous infusion of renin. Blood pressure may fall after a period of renin infusion despite the pressure in the blood of excess renin. Injection of partially purified activator restores the activator content of the blood as demonstrated in the rabbit's ear, but no rise in arterial pressure occurs.

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