Arginine vasopressin and arginine vasotocin inhibit ovine fetal/maternal water transfer.

Abstract

Summary: After the infusion of 500 ml 20% mannitol to 12 pregnant ewes, we studied maternal and fetal plasma osmolality and maternal hematocrit responses and monitored blood pressure, heart rate, blood gas, and pH in chronically catheterized fetal lambs at 129–140 days gestation. To examine the effect of arginine vasotocin (AVT) and arginine vasopressin (AVP) on fetal/maternal water transfer, the mannitol infusion was repeated 1 day before or after the baseline mannitol study during a 2-h fetal infusion of AVT (7 μU · kg−1 · min−1) or AVP (20–400 μU · kg−1 · min−1) in eight and five animals, respectively. Mannitol was administered to the ewe after 1 h of the 2-h AVT or AVP infusion. The fetal and maternal osmolality and hematocrit responses were compared to the baseline mannitol responses in the same sheep. Maternal and fetal blood gases and pH were measured throughout the study periods during the AVT studies. In four of the ewes, an identical (sham) study was performed substituting 500 ml normal saline for the mannitol infusion to the ewe. Fetal AVP infusion alone produced a significant decrease in fetal plasma osmolality (P < 0.001; two-way analysis of variance) and a slight increase in maternal osmolality (P < 0.05). Fetal AVP or AVT infusion significantly obtunded the fetal osmolality increment induced by maternal mannitol (each P < 0.001); moreover, there was a greater decrease in maternal hematocrit after mannitol alone than after mannitol with AVT (P < 0.05). Fetal hematocrit was maintained unchanged by transfusion with heparinized maternal blood. Maternal and fetal heart rates and maternal blood pressure remained unchanged during the studies. But during the fetal AVT infusion, fetal blood pressure increased from 64 ± 3 to 75 ± 2 mniHg (P < 0.01); fetal and maternal Po2 and pH remain unchanged; and fetal Pco2 increased from 30 ± 2 to 37 ± 4 (P < 0.05). Normal saline (sham protocol) produced no change in any parameter. The data indicate that fetal blood levels of AVP and AVT can influence transplacental water flow. Fetal AVP infusion produces a net gain in fetal water by inhibiting baseline fetal to maternal water flow. Both AVP and AVT infusion inhibit fetal to maternal water transfer after an osmotic stimulus to the ewe.