Aquaporins in ovine amnion: responses to altered amniotic fluid volumes and intramembranous absorption rates

Aquaporins ( AQP s) are transmembrane channel proteins that facilitate rapid water movement across cell membranes. In amniotic membrane, the AQP ‐facilitated transfer of water across amnion cells has been proposed as a mechanism for amniotic fluid volume ( AFV ) regulation. To investigate whether AQP s modulate AFV by altering intramembranous absorption ( IMA ) rate, we tested the hypothesis that AQP gene expression in the amnion is positively correlated with IMA rate during experimental conditions when IMA rate and AFV are modified over a wide range. The relative abundances of AQP 1, AQP 3, AQP 8, AQP 9, and AQP 11 mRNA and protein were determined in the amnion of 16 late‐gestation ovine fetuses subjected to 2 days of control conditions, urine drainage, urine replacement, or intraamniotic fluid infusion. AQP mRNA levels were determined by RT ‐ qPCR and proteins by western immunoblot. Under control conditions, mRNA levels among the five AQP s differed more than 20‐fold. During experimental treatments, mean IMA rate in the experimental groups ranged from 100 ± 120  mL /day to 1370 ± 270  mL /day. The mRNA levels of the five AQP s did not change from control and were not correlated with IMA rates. The protein levels of AQP 1 were positively correlated with IMA rates ( r 2  = 38%, P  = 0.01) while the remaining four AQP s were not. These findings demonstrate that five AQP s are differentially expressed in ovine amnion. Our study supports the hypothesis that AQP 1 may play a positive role in regulating the rate of fluid transfer across the amnion, thereby participating in the dynamic regulation of AFV.