Oral-nasal membranes are not the major route for fetal absorption of amniotic fluid arginine vasopressin

Intraamniotically injected substances such as arginine vasopressin and digoxin have been found to rapidly appear in the ovine fetal circulation, irrespective of whether the fetal esophagus has been ligated or occluded. To determine if the ovine fetal oral-nasal membrane plays a significant role in this fetal absorption of amniotic substances, we used two groups of chronically catheterized fetal sheep with a surgical glove sewed over the fetal head to prevent access of the head to amniotic fluid. In the first group 22.5 μg of arginine vasopressin was injected into the amniotic cavity: in the second group 22.5 μg was injected into the glove over the fetal head. We found that, after injection into the amniotic cavity, there were rapid and highly significant increases in amniotic fluid arginine vasopressin concentrations, from 6.1 ± 1.3 to 51,249 ± 18,182 pg/ml (mean ± SE) (p < 0.00001). Concurrently there was a rapid increase in fetal plasma arginine vasopressin concentrations from 4.5 ± 1.3 to 93.8 ± 18.9 pg/ml (p < 0.00001). The increase was significant within 15 minutes and reached a maximum at 60 minutes after the injection. Fetal arterial pressure increased by 10 ± 2 mm Hg, whereas heart rate decreased by 30 ± 5 beats/min (p < 0.00001). In contrast, after the injection into the glove covering the fetal head, there were no significant changes in any of the measured parameters. This suggests that the ovine fetal oral-nasal membrane is not a significant route of absorption of amniotic fluid arginine vasopressin and that the most likely route of absorption is the vascularized fetal surface of the placenta and vascularized fetal membranes, i.e., the intramembranous pathway.