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Allantoic Fluid Compositional Changes During Acute Urine Drainage in Fetal Sheep

Division of Maternal-Fetal Medicine, Departments of Obstetrics and Gynecology and Reproductive Medicine, University of California at Davis and San Diego. Davis and San Diego, California; wmgilbert{at}ucdavis.edu

Objective: This study examines the effects of ovine fetal urine drainage, which prevents urine inflow into the allantoic and amniotic cavities, on allantoic and amniotic fluid osmolalities and electrolyte concentrations in an attempt to study solute movement out of the allantoic cavity.

Methods: Nine chronically catheterized fetal sheep at 132 ± 2 (mean ± standard error) [SE] days' gestation, with allantoic and amniotic cavity catheters, were studied. Seven animals were studied for 9 hours, with maternal and fetal blood drawn hourly, and amniotic and allantoic fluid sampled for osmolality and electrolyte concentrations. On the second day, seven animals (five from the above group) were studied in the same fashion, except that fetal urine was drained after the first hour.

Results: Allantoic and amniotic fluid osmolality increased significantly (13.2 ± 1.1 (SE) mOsm/kg, P < .0001, 7.5 ± 1.5, P < .03, respectively) after fetal urine drainage but not sufficiently enough to account for the large amount of fetal urine diverted (217 mL per 8 hours). Fetal (3.1 ± 0.6, P < .04) and maternal plasma (2.4 ± 0.5, P < .03) osmolalities increased significantly, and this increase was consistent with the amount of fetal urine drained over the 8-hour drainage period. Allantoic (4.0 ± 1.7 mEq/L, P < .003) and amniotic sodium (4.2 ± 1.5 mEq/L, P < .03) and amniotic chloride (3.1 ± 1.6 mEq/L, P < .04) increased significantly as compared with control animals.

Conclusion: During the fetal urine drainage period, the increases in allantoic and amniotic fluid osmolalities and electrolyte concentrations confirmed that water left these compartments via the intramembranous pathway. If only water (and not solutes) had left these compartments, then the increase in osmolalities would have been too small to account for the large volume of diverted fetal urine. Therefore, slutes, in addition to water, must leave the allantoic cavity through the intramembranous pathway as no other pathway exists. Furthermore, solute movement through the intramembranous pathway may explain the ability of the fetus to maintain molecular and solute gradients between fluid compartments. Finally, the intramembranous pathway may play an important role in amniotic and allantoic fluid volume regulation and composition.

Key Words: Allantoic cavity • amniotic fluid • intramembranous pathway • fetus

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