Bulk flow of amniotic fluid water in response to maternal osmotic challenge

Amniotic fluid volume is regulated by a complex system of fluid exchanges with fetal and maternal fluid compartments. To assess possible hormonal control of amniotic fluid water exchange with the maternal vascular compartment, we studied the effect of intra-amniotic injections of prolactin, vasopressin, or vasotocin on the amniotic to maternal water flux induced by the acute intravenous infusion of mannitol to the pregnant ewe. This mannitol stimulus increased amniotic fluid osmolality secondary to a shift of free water to the maternal vascular/extracellular compartments. Prior intra-amniotic injection of prolactin but not vasopressin or vasotocin blunted the amniotic fluid osmolar response to maternal mannitol infusion. These results suggest that resorption of amniotic fluid water may occur at the chorioamnion and that amniotic fluid prolactin may have a regulatory function in amniotic fluid volume and osmolar homeostasis.