[Expression of aquaporin 2, a water channel, in human endometrium during the menstrual cycle]

To examine the expression of aquaporin 2 (AQP2) in human endometrium.

Specimens of human endometrium were collected from 87 women at different menstrual cycles, aged 30 +/- 3, 23 cases in the proliferative phase, 30 cases in the early secretory phase and 34 cases in the mid-secretory phase respectively. Immunohistochemistry and Western blotting were utilized to detect the localization and expression of AQP2 in the endometrium. Reverse transcription-polymerase chain reaction (RT-PCR) was employed to detect its messenger RNA. The PCR product was cloned and sequenced.

Positive immunoreactivity of AQP2 was found in the epithelia cells and glandular epithelial cells of all specimens of human endometrium at different phases of the menstrual cycle, and all stromal cells were not stained. The reactive substance was primarily distributed in the membrane and cytoplasm, but not in the nuclei of all positive cells. In Western blotting showed dominant bands with relative molecular weight between 35,000 Da and 50,000 Da, which was corresponded to the glycosylated form of AQP2 by the positive control from rat kidney. Semi-quantitative analysis showed that the relative expression of AQP2 in the mid-secretory phase was 1.63 +/- 0.15, significantly higher than those in the early secretory phase (1.33 +/- 0.14, P < 0.05) and that in the proliferative phase (1.03 +/- 0.10, P < 0.01). Message RNA was found out in all cases by RT-PCR and the PCR product was confirmed in nearly exact (99%) consistency with the GenBank by sequencing. AQP2 mRNA was expressed in all normal endometrium at different phases, and was weakly expressed in the endometrium at the proliferative phase. Sequencing showed that the AQP2 sequence was 99% homologous with that in the GenBank.

AQP2 expression in the human endometrium suggests that AQP2 may be involved in the regulation of uterine fluid homeostasis influenced by ovarian steroid hormones in the menstrual cycle.