PHARMACOKINETICS OF OXYTOCIN IN THE HUMAN SUBJECT

Abstract

Oxytocin (OT) metabolic clearance rates (MCR) were measured during the constant infusion of 4 mU/min synthetic oxytocin (Pitocin) in 22 healthy adult subjects: 6 men, 6 nonpregnant women and 10 pregnant women. Pregnant subjects were at term and undergoing OT induction of labor. Mean (.+-. standard error of the mean) baseline plasma OT concentrations were similar for men, pregnant women and nonpregnant women. OT MCR was 27 .+-. 1.8 ml/kg per min in men; 20.6 .+-. 2.8 ml/kg per min in nonpregnant women; and 23.1 .+-. 2.6 ml/kg per min in pregnant women. These values were statistically similar. The OT MCR corrected to prepregnancy weight was 25.4 .+-. 2.0 ml/kg per min. This value is also statistically similar to the values in men and nonpregnant women. OT degradation was studied in vitro in pooled plasma of men, nonpregnant women, pregnant women and cord blood. No significant degradation was observed in men, nonpregnant women or cord plasma. There was an 85%/h decrease in OT concentration in plasma of pregnant women, confirming earlier reports. Apparently the cystine amino peptidase enzyme mediating OT degradation in pregnancy plasma is preferentially secreted by trophoblast cells into maternal plasma and does not seem to cross the placental barrier.