The renin-angiotensin system and sodium excretion during gestation.

The renin-angiotensin system shows a marked increase in activity in normal pregnancy, with the major changes occurring in the first trimester. Evidence is presented to show that there is a physiologically inactive prorenin present in the plasma during pregnancy that achieves highest levels in the first half of pregnancy. The significance of this "acid-activated" renin has yet to be determined. Plasma angiotensin II levels are elevated throughout normal pregnancy, and a significant positive relationship has been established between maternal venous and cord venous levels at delivery. Recent studies on plasma AII levels in pregnancy hypertension have shown a positive correlation between diastolic blood pressure and plasma AII, and there is evidence that the high circulating levels of angiotensin may result in a feed-back suppression of renal renin release. Studies on the fetal circulation at delivery have also shown high levels of AII in the cord venous blood of infants born to mothers with pregnancy hypertension. Despite the increased level of activity of the renin-angiotensin system in normal pregnancy, sodium homeostasis is maintained. The possible reasons for the increased activity are discussed briefly.