We compared the cardiovascular and hormonal responses to angiotensin converting enzyme inhibition and hemorrhage of 20% of blood volume in chronically instrumented unanesthetized newborn lambs and adult sheep. Administration of the nonsulfhydryl-containing converting-enzyme inhibitor enalapril reduced mean arterial pressure in the newborn but not in the adult animals. Blood pressure fell in both age groups after hemorrhage, and the hemorrhage-induced fall in blood pressure, integrated over the period of hypovolemia, was more pronounced when converting-enzyme inhibition was present in the lambs. This was not observed in the adults. Cardiac output fell following hemorrhage in both age groups, and the fall was greater when enalapril was present in the lambs, but this was not the case in the adults. Hemorrhage increased plasma renin activity in both groups, and enalapril augmented this increase. Plasma concentrations of vasopressin and catecholamines increased following hemorrhage within and between groups. Taken together these data suggest that the renin-angiotensin system plays a more important role in the maintenance of cardiovascular homeostasis in newborn lambs than it does in adult sheep, and catecholamine and vasopressin responses to volume loss can occur in the presence of blockade of the renin-angiotensin system.
