The Balance Of HCO₃^- Secretion Versus Reabsorption In The Endometrial Epithelium Regulates Uterine Fluid PH

As one of the most important acid-base homeostasis regulating factors,HCO₃^-participates in a variety of mammalian reproductive events,like sperm capacitation,fertilization,embryo's early development before implantation.The uterine fluid has a very high concentration of HCO₃^-which varies regularly with estrous cycle.The estrous cycle can be divided into four stages,proestrus estrus metestrus and diestrus.It reported that in the female reproductive tract of rhesus monkeys,the HCO₃^-concentration is about35 mM in diestrus and can reach approximately 90 mM in estrus which is almost 4 times the concentration of blood.High concentration of HCO₃^-sustains an alkaline pH which is essential for sperm capacitation and fertilization.The mechanism of HCO₃^-secretion through the endometrium epithelium has been studied for more than a decade.Those proteins involved in HCO₃^-secretion include NCBT,NHE on the basolateral membrane and SLC26A6,CFTR on the apical membrane.It's believed that this cyclical change is the result of both secretion and absorption,while no paper about the absorption mode has been reported.In this research,the functional role of NBCn1 in regulating HCO₃^-in uterine fluid was explored.The expression of NBCn1 was detected by cDNA cloning and western blotting.And it was located mainly at the apical memebrane of endometrial epithelia by immunofluorescence.NBCn1 is an electroneutral Na⁺/HCO₃^-cotransporter which mediates the HCO₃^-absorption when expressed at the apical side of epithelia.Taken together,we speculated that there must be a HCO₃^-absorptive mechanism and NBCn1may transport HCO₃^-from uterine lumen into epithelia.Then the expression pattern of 3 candidate proteins with the estrous cycle was analysed.NBCn1 SLC26A4 and SLC26A6 both have remarkable changes.SLC26A4and SLC26A6 reach their highest level at proestrus and lowest at metestrus which correlates well with the HCO₃^-concentration changing pattern.While NBCn1 reaches its highest level at diestrus and lowest at estrus which implies that NBCn1 may function in the revrese direction.We also examined the effect of exogenous hormones by using ovariectomized mice.We found that SLC26A4 and SLC26A6 were both significantly up-regulated by estradiol but trend to be down-regulated by progesterone while NBCn1 has the opposite regulation pattern.It was significantly up-regulated by progesterone but down-regulated by estrodial.These also correlate well with the expression pattern in estrous cycle.In vivo perfusion of rat uterus showed that the HCO₃^-secretion ability of uterus in estrus was significantly higher than uterus in diestrus while the HCO₃^-absorption ability of uterus in estrus was significantly lower than uterus in diestrus.This result indicates that there must be an independent HCO₃^-absorption pathway apart from the secretion one.Taken together,we concluded that it's the relative change of HCO₃^-secretion and absorption which leaded to the cyclical change of the concentration of uterine HCO₃^-.Both pathways are regulated by ovarian hormones.And the NBCn1 located at the apical membrane of endometrial epithelia may participate in this HCO₃^-absorption.