292 EFFECTS OF ACUTE SALT LOADING ON BLOOD PRESSURE REGULATION IN THE NEWBORN LAMB

Abstract

The normal newborn has an impaired conservation of sodium; high renin-angiotensin system (RAS) levels; and a decreased pressor responsiveness to angiotensin 11(A-11). The purpose of this study was to determine whether increasing the newborn salt intake would suppress the high RAS levels and alter blood pressure(BP) responsiveness. In Exper 1, 8 newborn lambs were infused with NaCl 10 mEq/kg in 20 ml/kg 5%D/W over 60 min, followed by serial doses of A-11 (0.025, 0.5, 0.1, and 0.25 ug/kg/min), each for 15 min. Mean aortic BP was monitored continuously. In Exper 2 (0.25 ug/kg/min) was infused into 5 newborn lambs after a similar NaCl loading. Plasma renin activity (PRA) was measured before and at 5, 15, and 30 min of the A-11 infusion. In Exper 1, A-11 (0.025 ug/kg/min) increased BP 30 mm Hg in the ewe, caused no change in the control newborn, and increased BP 15 mm Hg in the volume expanded newborn; (0.25 ug/kg/min) increased BP 30 mm Hg in the control newborn and 40 mm Hg in the volume expanded newborn. In Exper 2, PRA decreased from 22.1 ± 2.1 ng/ml/hr (M and SEM) to 10.8 ± 2.1 (p<.05) after the volume expansion alone, and to 5.2 ± 1.0 (p<.05) after 30 min of the A-11 infusion. BP decreased 10 mm Hg after the salt loading alone. In conclusion: Acute salt loading, 1) increases the pressor responsiveness to A-11; 2) suppresses the renin-angiotensin system; and 3) decreases basal BP in the newborn lamb. Therefore, the reninangiotensin system control of BP and A-11 responsiveness may be related to the newborn sodium state.