The renin-angiotensin-aldosterone system in the newborn lamb: response to furosemide.

Nine newborn lambs between 24 and 48 hr of age were studied before and after infusion of furosemide (2 mg/kg) over 1-2 min. Plasma renin activity (PRA) increased within 8 min after furosemide from a baseline value of 12.6 +/- 3.5 ng/ml/hr (mean and SEM) to a level of 24.1 +/- 8.6 ng/ml/hr (P less than 0.05), and peaked 20 mins after the furosemide infusion at a level of 33.1 +/- 8.0 ng/ml/hr. Plasma aldosterone concentration increased from a baseline of 12.2 +/- 3.1 to 22.8 +/- 9.1 ng/dl 35 min after the furosemide infusion, P less than 0.05. There were no changes in plasma sodium or blood hemoatocrit and minimal changes in blood pressure and plasma protein concentrations during the first 35 min after the furosemide infusion. The results indicate that the renin-angiotensin-aldosterone system responds promptly to furosemide stimulation despite initially high PRA and aldosterone levels.