Figure 8.

The proposed mechanism underlying uterine fluid pH regulation in the endometrial epithelial cell under P influence and at Ds. CAII may be involved in intracellular generation of H⁺ and HCO₃ ⁻. H⁺ will then be extruded into the lumen via the apically located NHE1. NHE2 and NHE4, which are expressed at the apical membrane at a lower level than NHE1 may also participate in luminal H⁺ secretion in-exchange with Na⁺. Meanwhile, HCO₃ ⁻ is expelled into the plasma via the basolaterally located SLC26A6 or other HCO₃ ⁻ transporters including SLC24A4 (although their expression at the basolateral membrane under P influence is unknown) to maintain the intracellular pH homeostasis. Na⁺ that accumulates in the cell will be extruded into the plasma via basolateral Na⁺/K⁺-ATPase or SLC24A4. The mechanism underlying uterine fluid pH regulation under these conditions is not well understood as compared to the mechanisms involved under E dominance. CAII: carbonic anhydrase II, SLC26A6: Cl⁻/HCO₃ ⁻ exchanger, SLC24A4: sodium bicarbonate cotransporter, NHE: sodium-proton exchanger, Na⁺/K⁺-ATPase: Na⁺-K⁺-ATPase pump.