The plasma renin-angiotensin system in preeclampsia: effects of magnesium sulfate.

Two groups of women were studied in a prospective longitudinal fashion to determine the effects of a 2.5-hour infusion of magnesium sulfate upon the renin-angiotensin system. Serum magnesium concentration, angiotensin-converting enzyme concentration, and plasma renin activity were measured at uniform intervals in women with either preeclampsia or preterm labor. Plasma renin activity was significantly lower (3.9 +/- 2.2 versus 6.1 +/- 1.8 ng/mL/minute; P = .004) and angiotensin-converting enzyme significantly higher (47.1 +/- 14 versus 34.0 +/- 10 U/mL; P = .008) in women with preeclampsia than in those with preterm labor. Magnesium infusion was associated with a sustained decline in plasma renin activity in preeclamptic women (P = .003). A transient decline in angiotensin-converting enzyme (P = .009) was observed in women with preeclampsia, but not with preterm labor. In contrast to the sustained change in plasma renin activity, angiotensin-converting enzyme concentration returned to baseline activity levels by 2.5 hours. A nonsignificant negative relationship (P = .06) was noted between angiotensin-converting enzyme and gestational age in subjects with preeclampsia. We conclude that a short-term infusion of magnesium sulfate leads to a sustained decline in plasma renin activity in preeclamptic women, but exerts no sustained effect on angiotensin-converting enzyme in women with either preeclampsia or preterm labor.