Rapid intramembranous absorption into the fetal circulation of arginine vasopressin injected intraamniotically

Recently an intramembranous pathway was reported in the ovine fetus as a route for the movement of a significant volume of water from the amniotic cavity directly into the fetal blood, which perfuses the fetal membranes and fetal surface of the placenta. To test whether this pathway could be an avenue for the movement of arginine vasopressin from the amniotic cavity into the fetal circulation, we injected 1 to 25 μg of arginine vasopressin into the amniotic cavity of two groups of chronically catheterized fetal sheep: a control group of seven animals and a group of seven animals with surgical ligation of the fetal esophagus. We found similar and highly significant increases of arginine vasopressin concentrations in both control and surgically ligated fetuses in amniotic fluid (p < 0.00001), fetal plasma (p < 0.0001), and fetal urine (p < 0.0001). Both groups had similar increases in arterial (p<0.0001) and venous (p < 0.003) pressures with simultaneous decreases in urine flow (p < 0.001 ) and heart rate (p < 0.0001 ) after the intraamniotic injection of arginine vasopressin. We conclude that amniotic arginine vasopressin can be rapidly absorbed in its biologically active form directly into the fetal circulation through the intramembranous pathway. Furthermore, the observation that esophageal ligation did not alter this absorption suggests that the intramembranous pathway may be important in the regulation of amniotic fluid volume and composition.