Role of vasopressin in fetal homeostasis

The role of vasopressin (VP) and the kidney in the maintenance of solute and volume homeostasis was studied in chronically instrumented fetal lambs during the third trimester. Plasma VP, urine and plasma osmolality, and urine output were measured in 74 simultaneous samples. The results show a strong positive nonlinear correlation between plasma VP and osmolality when a latter is higher than 290 mosmol/kg (r = 0.803, P = 0.03) and between plasma VP and urine osmolality (r = 0.806, P = 0.05). No correlation was found between fetal and maternal plasma VP. However, linear correlations were found in plasma osmolality and sodium concentrations between mother and fetus; mean maternal-fetal gradients were 4 mosmol/kg and 3.1 meq/liter, respectively. The highest values for plasma VP and osmolality found in our study were 7.0 pg/ml and 317 mosmol/kg, respectively. There values corresponded to urine output of 0.02 ml.kg-1.min-1 and osmolality of 517 mosmol/kg. In conclusion, there studies demonstrate that the relationship among plasma osmolality, plasma vasopressin concentration, and urine osmolality in the lamb fetus are qualitatively similar to those of the adult. The results suggest that the fetal neurohypophysis and kidney may participate in the maintenance of fetal osmolar and volume homeostasis.