Ovine fetal adaptations to chronically reduced urine flow: preservation of amniotic fluid volume.

Adequate amniotic fluid (AF) volume is maintained by a balance of fetal fluid production (lung liquid and urine) and resorption (swallowing and intramembranous flow). Because fetal urine is the principle source of AF, alterations in urine flow and composition directly impact AF dynamics. Intra-amniotic 1-desamino-8-D-arginine vasopressin (DDAVP) is rapidly absorbed into fetal plasma and induces a marked fetal urinary antidiuresis. To examine the effect of intra-amniotic- DDAVP-induced fetal urinary responses on AF volume and composition, six chronically prepared ewes with singleton fetuses (gestation 128 +/- 2 days) were studied for 72 h after a single intra-amniotic DDAVP (50-microgram) injection. After DDAVP, fetal urine osmolality significantly increased at 2 h (157 +/- 13 to 253 +/- 21 mosmol/kg) and remained elevated at 72 h (400 +/- 13 mosmol/kg). Urinary sodium (33.0 +/- 4.5 to 117.2 +/- 9.7 meq/l) and chloride (26.0 +/- 2.8 to 92.4 +/- 8.1 meq/l) concentrations similarly increased. AF osmolality increased (285 +/- 3 to 299 +/- 4 mosmol/kg H2O), although there was no change in fetal plasma osmolality (294 +/- 2 mosmol/kg). Despite a 50% reduction in fetal urine flow (0.12 +/- 0.03 to 0.05 +/- 0.02 ml.kg-1.min-1 at 2 h and 0.06 +/- 0.01 ml.kg-1.min-1 after 72 h), AF volume did not change (693 +/- 226 to 679 +/- 214 ml). There were no changes in fetal arterial blood pressures, pH, PCO2, or PO2 after DDAVP. We conclude the following. 1) Intra-amniotic DDAVP injection induces a prolonged decrease in fetal urine flow and increases in urine and AF osmolalities. 2) Despite decreased urine flow, AF volume does not change. We speculate that, in response to DDAVP-induced fetal oliguria, reversed intramembranous flow (from isotonic fetal plasma to hypertonic AF) preserves AF volume.