Activation of the renin-angiotensin system during ritodrine treatment in preterm labor.

Plasma renin activity, active renin concentrations, and total renin concentrations were measured serially in 15 women in preterm labor treated with ritodrine. All three parameters showed rapid increases during treatment that depended on ritodrine doses and paralleled changes in maternal pulse rate. The slope of the correlation between plasma renin activity and active renin concentrations suggested an increase in both active renin and renin substrate during ritodrine treatment. In contrast, levels of inactive renin remained unchanged for several hours during ritodrine infusion, showing only a late and much smaller response than that of either total or active renin concentrations. Thus ritodrine treatment results in further activation of an already activated renin-angiotensin system in pregnancy. This activation relates to an increase in both renin enzyme and its substrate. It may be responsible for the disturbance in water balance and the risk of pulmonary edema associated with beta-sympathomimetic drug treatment in preterm labor.