Angiotensin II-like activity in circulating arterial blood in immature and adult rabbits.

1. Angiotensin II-like activity was estimated from the contraction of the rat ascending colon superfused with arterial blood in an extracorporeal circuit. The colon was calibrated by infusion of synthetic angiotensin II amide (Hypertensin, CIBA Ltd).2. Resting levels of angiotensin II-like activity were obtained by comparison of the length of a rat colon preparation superfused alternately with blood from an intact and from a nephrectomized rabbit.3. The resting levels of angiotensin II-like activity reached a maximum value of about 1.6 ng/ml. at 10-14 days of age, and thereafter declined to the adult value of 0.23 ng/ml. Rabbits of less than 100 g body weight had resting levels in the same range as adults.4. Removal of 25% of the calculated blood volume induced five times greater increments in arterial angiotensin II-like activity in immature than in adult rabbits (1.68 +/- 0.26 S.E. of mean ng/ml. compared with 0.34 +/- 0.08 S.E. of mean ng/ml.).5. No evidence was obtained that the high levels of angiotensin II-like activity encountered in immature rabbits were attributable to a slower rate of inactivation of angiotensin by tissues.6. The pronounced activity of the renin-angiotensin system demonstrated by these experiments in immature rabbits especially during haemorrhage is consistent with the suggestion that this system is of particular importance in cardiovascular control in early life.