Figure 9.

Proposed model for reabsorption and secretion of ${\text{HCO}}_3^{-}$ in endometrial epithelium. (A) Secretion of ${\text{HCO}}_3^{-}$ by the endometrial epithelium. (B) Reabsorption of ${\text{HCO}}_3^{-}$ by the endometrial epithelium. In the reabsorption mode, the endometrial epithelium would perform net fluid reabsorption, whereas in the secretion mode, the endometrial epithelium would perform net fluid secretion, via members of aquaporins, such as AQP2, AQP3, AQP5 expressed at the apical and/or basolateral membranes of the endometrial epithelium (Mobasheri et al., 2005; Hildenbrand et al., 2006; Skowronski, 2010; for review, see Zhu et al., 2015). Our expression data and in vivo perfusion data indicate that the activities for ${\text{HCO}}_3^{-}$ reabsorption and secretion are present at the same time in the endometrial epithelium. We propose that the activity for secretion is dominant during estrus stage when the uterine fluid volume is large and [${\text{HCO}}_3^{-}$] is high, whereas the activity for reabsorption is dominant during diestrus stage when the uterine fluid volume is small and [${\text{HCO}}_3^{-}$] is low.