Expression Of Na⁺/HCO₃^– Cotransporter NBCn1 And NBCn2 In Small Intestine And Kidney And Their Roles In Regulation Of Systemic Electrolyte And Fluid Balance

It is of fundamental significance to maintain systemic electrolyte and fluid balance in the body.The small intestine and the kidneys play important roles in these processes.The small intestine is responsible for the absorption of water and NaCl,whereas the kidney is responsible for the reabsorption and excretion of water and NaCl.In the small intestine,NaCl absorption is mainly coordinated by the sodium-hydrogen exchanger NHE3(SLC9A3)and the anion exchanger family SLC26 in the apical membrane,and the Na⁺-K⁺pump in the basolateral membrane.In the kidney,approximately 70%of NaCl reabsorption is responsible for the proximal tubule,the medullary thick ascending limb(m TAL)is responsible for about 20%,and the distal convoluted tubule and collection tube are responsible for 10%.In m TAL,NaCl reabsorption mainly depends on the cooperation of the NKCC2 and NHE3 of the apical membrane,the Na⁺-K⁺pump of the base membrane,and the chloride channel Cl C-2.The electroneutral Na⁺/HCO₃^-cotransporters NBCn1(SLC4A7)and NBCn2(SLC4A10)belong to the SLC4 family and operate with a Na⁺:HCO₃^–stoichiometry of1:1.In the present paper,we investigated the expression and physiological roles of NBCn1and NBCn2 in the NaCl transport of small intestine and kidney epithelial cells.Firstly,c DNA cloning showed that Slc4a10 gene is expressed in the small intestine.The expression and localization of NBCn2 protein in the small intestine tissue were determined by Western blotting and immunofluorescence staining.Our data showed that NBCn2 is expressed in the apical membrane of jejunal epithelial cells.In order to understand the role of NBCn2 in the small intestine,we induced high-salt model in rats by adding 1.5%NaCl to drinking water for one week.Western blotting showed that,compared with the control,the expression of NBCn2(reduced by 56%)and NHE3(reduced by 40%)in the small intestine tissue was significantly decreased under high salt conditions.By changing the NaCl level in the feed,the low NaCl group(0.025%),the normal NaCl group(0.25%),and the high NaCl group(8%)were set for two weeks.Western blotting showed that there was no significant difference in NBCn2 expression levels in the small intestine of the three groups.Finally,we examined the effect of treatment for two weeks by altering just the sodium level in the chow with other inorganic salt ions and Cl^-unchanged.We found that,no significant difference in the expression level of NBCn2 in the small intestine for three groups of low sodium(0.02%),normal(0.1%),and high sodium(3%).In the kidney,compared to the control,1.5%NaCl in drinking water for one week significantly increased the expression NBCn1(by 83%)and NBCn2(by 59%).After two weeks of treatment by changing the NaCl level in the feed,compared with the normal NaCl group(0.25%),the low NaCl group(0.025%)had no significant change in NBCn1and NBCn2;the high NaCl group(8%)increased NBCn1 by 132%.NBCn2 increased by66%.Finally,we examined the effect of treatment for two weeks by altering just the sodium level in the chow with other inorganic salt ions unchanged.The results showed that,compared with the control(0.1%Na⁺),no significant change in NBCn1 and NBCn2was observed in the low sodium(0.02%).However,compared to control,high sodium(3%Na⁺)increased NBCn1 by 244%and NBCn2 by 163%.Based on the above research results,we propose:(1)NBCn2 is involved in NaCl absorption in the small intestine;(2)NBCn1 and NBCn2 inhibit NaCl reabsorption and promote NaCl excretion in kidney m TAL under high salt condition;(3)Under short-term(e.g.,1 week)of high salt conditions,systemic homeostasis of NaCl is presumably maintained by decreasing NBCn2 in the small intestine and increase NBCn1 and NBCn2in the kidney.Under long-term(e.g.,2 weeks)of high salt or high sodium conditions,systemic NaCl homeostasis mainly maintained mainly by increasing NBCn1 and NBCn2in the kidney.