Plasma renin activity and adrenal angiotensin II receptors in fetal, newborn, adult and pregnant rabbits.

Plasma renin activity (PRA) and adrenal angiotensin II receptors have been studied simultaneously in the rabbit at various stages of development. In the fetus both parameters show very low values but increase rapidly during the last 4 days of gestation. PRA reaches a maximal level in the early post-natal period but the concentration of adrenal angiotensin II receptors continues to increase further up to the adult state. In adult females, pregnancy results in an initial rise in PRA and adrenocortical angiotensin II receptors. However, PRA remains at a high level throughout the gestation period whereas the number of adrenocortical angiotensin II receptors decreases progressively as pregnancy progresses. The role of circulating angiotensin in the regulation of the concentration and affinity of its receptor sites is discussed.