Fetal lung fluid response to maternal hyperosmolality.

Lung fluid production in utero, a significant source of amniotic fluid, may be regulated by the fetus in response to environmental stress. We monitored changes of maternal and fetal plasma osmolality, plasma arginine vasopressin level, and fetal lung fluid production in response to mannitol-induced change in maternal osmolality in five chronically catheterized ovine fetuses. Four additional ewes and their fetuses did not receive mannitol and served as controls. Maternal infusion of 20% mannitol resulted in maternal and subsequent fetal hyperosmolality (changing from 303 +/- 2 to 322 +/- 2 mOsm and 300 +/- 5 to 319 +/- 8 mOsm, respectively), Mean fetal lung fluid production significantly decreased (from 2.6 +/- 0.4 to 1.6 +/- 0.4 ml/10 min), whereas lung fluid osmolality significantly increased (from 298 +/- 2 to 303 +/- 3 mOsm) in the study animals. No changes in lung fluid sodium or potassium concentrations were observed in study or control fetuses. Fetal plasma arginine vasopressin level significantly increased in the study fetuses (2.2 +/- 0.3 to 5.3 +/- 1.6 microU/ml), though not in control animals. These results indicate that the fetal lamb responds to increased osmolality by decreasing lung fluid production, and that there is a concomitant increase in lung fluid osmolality. This response appears to be mediated via increased fetal plasma vasopressin levels.