Importance of plasma angiotensin concentrations in a comparative study of responses to angiotensin in the maturing newborn lamb.

Plasma angiotensin concentrations were measured in a longitudinal study of the vascular, renal, and adrenal responses to infusions of angiotensin II (AII) in the maturing newborn lamb. Basal plasma concentrations of angiotensin increased with age and correlated with the rising arterial pressure that occurred with maturation. However, age was a stronger determinant of arterial pressure than was plasma angiotensin concentration. For any given dose of AII per kilogram of body weight, the actual plasma angiotensin concentration achieved increased as the lambs matured and gained weight. Therefore, a comparative study of biologic responses to AII in maturing animals must be based on actual plasma angiotensin concentrations achieved rather than on dose of AII infused per kilogram of body weight. When analyzed on the basis of actual plasma angiotensin concentration, the increase in arterial pressure and the suppression of plasma renin activity in response to increasing plasma angiotensin concentrations did not differ significantly as the lambs matured. However, the increment in plasma aldosterone concentrations in response to increasing plasma angiotensin concentrations was diminished in immature lambs (less than 18 days) when compared to the aldosterone responses in the same lambs at older ages.