Introduction: Amniotic fluid is essential for normal fetal growth and development. Under normal conditions, water flow progressively increases throughout gestation. Because of the osmotic pressure difference between, amniotic fluid (255mOsm/kg) and fetal blood (280 mOsm/kg), an osmotic gradient drives transport of fluid and solutes from the amniotic compartment into the fetal blood. Both phenomena support the hypothesis that aquaporins (AQPs) may be crucial in the regulation of fetal water flow. The expression of AQP1, 3, 8 and 9 has been described in fetal membranes. Emerging data correlates the abnormal expression of amnion AQPs with pathological alterations of amniotic fluid volume such as the oligohydramnios or polyhydramnios. Recently, we demonstrated that AQPs facilitate the water transport across the human amnion, being the AQP1 the most important contributor. However, the osmotic regulation of these proteins in human amnion was not studied yet. Objective: The aim of this work was to study the effect of hypo and hyperosmolar stress on the expression and function of the AQPs present in the human amnion. Methods: This study was approved by the ethics committee of the Hospital Nacional Dr. Prof. Alejandro Posadas, and written consent was obtained from the patients before the collection of samples. Human amnion explants were cultured in complete DMEM-F12 and different sucrose hyperosmolar solutions were added to generate the hyperosmolar conditions. To generate the hypoosmolar condition complete DMEM-F12 was diluted with sterile water. The expression of AQP1, 3, 8 and 9 was analyzed by the Western Blot and semiquantitative RT-PCR. Amnion explants were mounted in a modified Ussing chamber, net transepithelial water movement was recorded with and without an osmotic gradient generated by adding polyethyleneglycol (40 mOsm, Mr: 8000) and the osmotic permeability (pOsm) was calculated. Results: In amnion explants cultured in 400 mOsm (hyperosmolar condition), we observed that AQP1 and AQP9 expression significantly decreased (n=5; p<0,01), while AQP8 expression dramatically increased (n=5; p<0,05) compared to the isoosmolar condition. We did not observe significantly changes in AQP3 expression. In agreement with these results, pOsm also significantly decreased (n=5; p<0.001). In amnion explants cultured in 150 mOsm (hypoosmolar condition) we observed that AQP1 and 9 expressions increased significantly compared to the isoosmolar condition (n=5; p<0,01), and in agreement with these results the pOsm also significantly increased (n=5; p<0,001). Conclusions: Our findings showed for the first time, that the expression and function
of human amnion AQPs are regulated by changes in the osmolarity. Although the etiologies of the oligo and polyhydramnios are still unknown, our work provides new evidence that the altered expression of AQPs may be related to pathophysiology of these syndromes.