Decreased vascular and increased adrenal and renal sensitivity to angiotensin II in the newborn lamb.

The effects of two doses of angiotensin II (0.025 micrograms/kg per min and 0.25 micrograms/kg per min) on plasma renin activity (PRA), aldosterone concentration, creatinine clearance, and sodium and potassium excretion were studied in two groups of 2- to 3-week-old lambs. The 0.025 micrograms/kg per min dose of angiotensin II also was studied in six adult ewes. In response to the angiotensin II infusions, PRA decreased in the newborns from baselines of 24.5 +/- 2.3 ng/ml per hr (mean +/- SE) and 22.9 +/- 3.5 to 13.6 +/- 0.8 and 13.0 +/- 2.2 at 30 minutes (P < 0.01), respectively, and returned to baseline at 60 minutes; PRA decreased from 1.7 +/- 0.1 to 0.95 +/- 0.3 ng/ml per hr at 30 minutes (P < 0.01) in the ewe and returned to baseline at 60 minutes. Plasma aldosterone increased in the newborns from baselines of 17.4 +/- 5.0 ng/dl and 14.7 +/0 3.9 to 33.1 +/- 6.9 and 32.5 +/- 6.3, respectively, at 15 minutes (P < 0.01) and returned to baseline at 60 minutes. Plasma aldosterone increased from 4.3 +/- 0.7 to 9.2 +/- 2.0 ng/dl in the ewe, and returned to baseline at 60 minutes. The change in the PRA and aldosterone responses from baseline to peak for the low and high angiotensin dose was similar in the newborn lambs and greater than in the ewe (P < 0.01). There was no change in the creatinine clearance, plasma sodium, or hematocrit. Urine sodium excretion increased from 0.16 +/- 0.04 total mEq/30 min to 0.87 +/- 0.27 (P < 0.05) in the newborns during the 0.25 micrograms/kg per min angiotensin II infusion. We conclude that, under basal conditions, the newborn lamb has high PRA and aldosterone levels with decreased pressor and increased aldosterone and renin release responsiveness to angiotensin II compared to the adult.