Placental and renal control of plasma osmolality in chronically cannulated ovine fetus.

This study investigated placental and renal control of plasma osmolality in mature (greater than 120 days, term 142-152 days) and immature (less than 120 days) chronically cannulated ovine fetuses. A transplacental osmotic gradient was generated by the rapid infusion of 20% mannitol (500 ml) into each of 20 pregnant cross-bred Merino ewes on at least two occasions. Maternal plasma osmolality increased by 29.2 +/- 1.0 (n = 29) mosmol/kg water (means +/- SE). Fetal plasma osmolalities increased to the same extent [14.8 +/- 0.9 (n = 29)] in immature and mature fetuses despite significantly higher plasma arginine vasopressin (AVP) concentrations [24.3 +/- 3.6 (n = 13) vs. 13.6 +/- 1.6 (n = 12) pg/ml] and significantly greater renal water retention in mature than immature fetuses (8.7 +/- 1.7 vs. 4.4 +/- 0.8 ml/h, P less than 0.05). Infusion of AVP intravenously to the fetus, at rates of 35-140 ng/h, for 1 h before the maternal mannitol did not alter fetal plasma osmolality. AVP, infused at 35-140 ng/h during the maternal mannitol infusions, did not alter fetal plasma osmolality increases. The conclusions are that 1) placental water transfer is the major determinant of fetal plasma osmolality and 2) the experiments suggest that placental water flux in the sheep is not regulated by endogenous or exogenous AVP.