Transfer and dynamics of uric acid in the pregnant rhesus monkey. II. A mathematical model.

The objective of the present study was to develop a mathematical model of the dynamics of uric acid between fetal and maternal compartments in the term pregnant rhesus monkey. In 3 different animals 14C-labeled uric acid was injected into the fetal circulation, the amniotic fluid and the maternal circulation, respectively. In one experiment no uric acid was administered and the fetus was deliberately killed at the beginning of the experiment. Samples of fetal and maternal blood, maternal urine and amniotic fluid were collected at regular intervals. Semilogarithmic time-activity curves were constructed and time constants were determined. An open four-compartment model (fetal-placental plasma, fetal-placental interstitial space, amniotic fluid and maternal plasma) was applied to describe the intercompartmental dynamics of uric acid. Transplacental clearance was approx. 1 ml X min-1 in both directions, maternal renal clearance was about 17 ml X min-1. These results and the calculated values of the other intercompartmental clearances support earlier results, obtained with the steady infusion method. Uric acid concentrations in amniotic fluid and fetal plasma appeared to increase significantly during the experiments. The rise in amniotic fluid levels can only be explained by accepting a yet undefined compartment in which uric acid is produced and cleared directly into the amniotic cavity. It is speculated that this additional compartment could be the fetal lung.