Transference of para-amino-hippurate from the mother to the amniotic fluid.

Abstract

There are few studies in which substances mainly cleared by the kidney are injected to the mother and the time course of their concentrations in amniotic fluid is analyzed. This type of studies may contribute to the knowledge of the transference of substances through the mother-fetus-amniotic fluid complex. Thirteen pregnant women were studied. Eleven with normal term pregnancies, and the remaining two mothers were at the 36th week of gestation and their fetuses were dead. A saturation dose of para-amino-hippurate (PAH) (8 mg/kg) was administered intravenously to all subjects. This dose was followed by a continuous infusion at a rate of 380 mcg/min/kg during 30 minutes in 11 mothers. In one of the remaining subjects, the infusion rate was 240 mcg/min/kg during 160 minutes and in the other, it was 50 mcg/min/kg for 450 minutes. Samples of maternal blood and amniotic fluid were simultaneously obtained, before PAH administration and at variable intervals thereafter. In mothers with live fetuses, PAH concentration in amniotic fluid increased not only during the infusion but also for at least two hours after its interruption (Fig. 2, Tab. I). When the infusion lasted enough and with stabilized PAH concentration in maternal plasma, amniotic fluid concentration of PAH attained higher values than in the mother (Fig. 1). Disappearance of PAH from amniotic fluid was very slow (45% in 5 hours) (Fig. 2). In cases with dead fetuses, the pattern of PAH concentration in amniotic fluid was completely different. The highest concentration was observed already in the first sample at the end of the infusion. From there on, no increment was recorded, and as soon as maternal blood concentration fell below the values of amniotic fluid, PAH began to disappear from this compartment at a much faster rate (45% in 1 hour) (Fig 3). The progressive rise of PAH in amniotic fluid may be explained as follows: (Fig. 5). PAH diffuses from maternal blood to fetal blood through the placenta. The fetal kidney removes PAH from fetal blood and concentrates it in urine. Periodic fetal micturition causes the rise of PAH in amniotic fluid. PAH may return from amniotic fluid to fetal blood mainly because of fetal swallowing; and the larger part of this PAH will be again excreted by the kidney into the amniotic compartment. This mechanism would explain the slow disappearance of the substance from amniotic fluid after the end of the infusion, and also the lack of correlation between concentrations in maternal plasma and amniotic fluid. Diffusion of PAH between blood and amniotic fluid may also exist through the walls of the vessels of the ovular membranes and the vessels of the umbilical cord; this mechanism would play a minor role in the concentration of PAH in amniotic fluid in subjects with live fetuses. Our results also show that mean PAH concentration in amniotic fluid increases as a linear function of time until 150 minutes after the onset of PAH administration to the mother (Fig. 4).