Progressive Resetting of Sodium-Renin-Aldosterone Relationships in Human Pregnancy

Abstract

The responses of plasma renin activity (PRA), plasma aldosterone concentration (PAC), urine aldosterone excretion (UA) and plasma volume (PV) to dietary sodium manipulations were examined in 50 second and 40 third trimester primigravidae. PRA, PAC and UA fell significantly following one week on a high-salt (HS) diet and rose significantly following a low-salt (LS) diet at both stages. PRA values for pregnant subjects following intravenous saline loading at the completion of their HS diet were seven to eight-fold greater than for six non-pregnant subjects who had the same studies.The absolute values for PRA, PAC, UA and UNa. V, and the individual changes in each of these parameters following dietary manipulations were signficantly interrelated during both trimesters. The relationship between change in UNa.V (ΔNa) and change in PRA (ΔPA) was exponential., there was an increase in PRA independent of ΔNa during the second, but not the third trimester, and the sensitivity of ΔPA to ΔNa was greater in the third trimester.The plasma volume changed slightly, but significantly (p<0.001) in response to both diets in the second trimester but was more closely maintained during the third trimester. These results demonstrate that the renin-aldosterone system responds appropriately to changing sodium intake during pregnancy to maintain plasma volume constancy, that PRA is not suppressible to non-pregnant values despite the combination of dietary and intravenous sodium loading, and that significant differences in the dynamics of this system are present between trimesters