The effect of posture and saline loading on plasma renin activity and aldosterone concentration in pregnant, non-pregnant and estrogen-treated women.

The effect of contraceptive ingestion and pregnancy on components of the renin-angiotensin-aldosterone system was compared in 17 non-pregnant women, 8 non-pregnant women taking oral contraceptives, and 11 pregnant women. Plasma renin substrate concentrations and the dynamic responses of plasma renin activity and aldosterone concentrations to upright posture and iv saline infusion were evaluated. Renin substrate was significantly higher in those women taking oral contraceptives and among the pregnant subjects than the other non-pregnant group. No significant differences in plasma renin activity or aldosterone concentrations were seen before or after postural stimulation among the 2 non-pregnant groups. After saline loading renin was higher and aldosterone lower in the contraceptive-treated group. In contrast, the pregnant group had significantly higher values at every point. The response to posture and saline among the pregnant subjects was similar in direction and magnitude to those of the non-pregnant groups. The urinary excretion of sodium before and after saline infusion was significantly lower in the pregnant group than in the non-pregnant groups. These observations suggest that estrogen-induced increases in renin substrate do not alone account for the increases in the renin-angiotensin-aldosterone system observed during pregnancy, but rather such increases appear to represent a physiological response to increased sodium need during pregnancy.