Na⁺/HCO₃^- Cotransporter NBCn2 Mediates HCO₃^- Reclamation In The Apical Membrane Of Renal Proximal Tubules

It is of fundamental significance to maintain systemic acid-base homeostasis for virtually all biological activities in the body.The kidney plays an essential role for systemic acid-base balance regulation by reclaiming the alkaline substance HCO₃^-and excreting the non-volatile acid H⁺.In a nephron,proximal tubule is responsible for the reclamation of ⁸0% of filtered HCO₃^-.The conventional theory is that HCO₃^-reclamation by the proximal tubule entirely relies on the proton secretion mainly mediated by the apical Na⁺/H⁺ exchanger NHE3 and vacuolar proton pump.However,previous studies have shown that NHE3 and H⁺-ATPase cannot explain all the HCO₃^-reclamation by the proximal tubule.NBCn2 is an electroneutral Na⁺/HCO₃^-cotransporter encoded by Slc4a10.In a previous study,our lab has shown that the kidney expresses two groups of NBCn2 variants with different amino-termini starting with “MCDL” and “MEIK”,respectively.In the present study,we found that MCDL-NBCn2 and MEIK-NBCn2 have distinct tissue and cellular distribution in rat kidney.MCDL-NBCn2 is expressed at the apical membrane of the proximal tubule,whereas MEIK-NBCn2 is predominately at the basolateral membrane of the medullary thick ascending limbs and less in inner medullary collecting ducts.To study the role of MCDL-NBCn2 in HCO₃^-reabsorption,we examined the alteration of protein abundance of MCDL-NBCn2 and NHE3 in rat kidney in response to systemic acid-base disturbance.We found the regulation of MCDL-NBCn2 and NHE3 abundances elicit inverse relationship under different systemic acid-base models.Firstly,MCDLNBCn2 is significantly down-regulated,whereas NHE3 is up-regulated by NH4Cl-induced metabolic acidosis.Secondly,MCDL-NBCn2 was significantly up-regulated,whereas NHE3 was down-regualted by NaHCO₃^-induced metabolic alkalosis.Finally,MCDL-NBCn2 is substantially down-regulated,whereas NHE3 is substantially upregulated by hypokalaemic alkalosis.Based upon the aboves studies,we propose that:（1）the proximal tubule contains two distinct strategies for HCO₃^-reclamation,the conventional indirect pathway that depends on the proton secretion by NHE3 and vacuolar proton pump and the novel direct pathway that depends on the apical MCDL-NBCn2.（2）MCDL-NBCn2 and NHE3 each has different effeciencies for HCO₃^-reclamation and H⁺ secretion.The inverse regulation of MCDL-NBCn2 and NHE3 under systemic acid-base disturbances likely confers advantages for the animals under such conditions.So the working proportion of MCDL-NBCn2 and NHE3 varies with the change of the content of HCO₃^-and H⁺ in the blood and the tubular fluid to keep the acid-base homeostasis in the body.