Active and inactive renin in pregnancy and in women on estrogen-containing oral contraceptives

Abstract

Total and active renin concentrations (TRC and ARC) were determined in pregnant women and in women on estrogen-containing oral contraceptives to study the variation of plasma renin forms in pregnancy. TRC was already elevated in the first trimester. After that TRC increased consistently reaching the maximum in the third trimester. The ratio of inactive renin concentrations (IRC) to TRC was between 20 and 30% throughout gestation. Therefore, it was supposed that the development of the placenta or the enlargement of the uterus do not affect the ratio of IRC to TRC too much. In women on oral contraceptives in whom plasma renin activity was increased due to elevation of renin substrate, the ratio of IRC to TRC was almost the same as that in normal controls. From these results, it was suggested that the development of the placenta and the enlargement of the uterus do not play an important role in the variation of plasma renin forms, although remarkable changes are observed in the renin substrate and total amounts of renin in pregnancy.

PIP: Changes in total and active renin concentrations (TRC and ARC) during pregnancy were studied; in addition, the changes in TRC and ARC in women taking oral contraceptives (OCs) were also investigated. 20 pregnant women aged 21-34 years were studied. TRC and ARC levels in normal subjects were 3.6 ng/ml/hour and 3.1 mg/ml/hour, respectively. Inactive renin concentration (IRC) was determined and its ratio was calculated from (TRC-ARC)/TRC. In normal subjects, IRC ratio was widely dispersed. TRC in women on OCs was slightly higher than normal, but ARC was not significantly different. Mean level of IRC/TRC (33%) in OC users was higher than that of normal subjects, but it was not significantly different. TRC and ARC showed an increasing tendency in term plasma. In contrast, the maximal value of the IRC ratio was in the second trymester. BY 3-4 weeks postdelivery, TRC and IRC were back to normal levels. When these measurements were related to cases of toxemia, 2 toxemic women showed values of TRC and IRC lower than those in normal pregnant women. It is concluded that development of the placenta or enlargement of the uterus does not affect the IRC ratio.