Activation of The Apical Na/H Exchanger NHE3 By Formate : A Basis of Enhanced Fluid and Electrolyte Reabsorption By Formate in the Kidney Proximal Tubule

Formate stimulates sodium chloride and fluid reabsorption in kidney proximal tubule, however, the exact cellular mechanism of this effect remains unknown. We hypothesized that the primary target of formate is the apical Na/H exchanger. Here we demonstrate that formate directly enhances the apical Na/H exchanger (NHE3) activity in mouse kidney proximal tubule. In the absence of CO2/HCO3, addition of formate (500 μM) to the bath and lumen of microperfused mouse kidney proximal tubule caused significant intracellular alkalinization, with pHi increasing from baseline levels 7.17 ± 0.01 to 7.55 ± 0.01 (p<0.001, n=14), with a delta pH of 0.38 ± 0.02. Removal of the luminal chloride did not block the cell pH alkalinization by formate (baseline pH of 7.26 ± 0.01 to 7.53 ± 0.01 with formate, p<0.001, n=10), indicating that the apical Cl/OH exchanger was not the primary mediator of the effect of formate on cell pH. However, removal of sodium from the lumen or addition of EIPA completely prevented the cell pH alkalinization. Addition of formate to the lumen and bath in outer medullary collecting duct, which does not express any apical Na/H exchanger, did not cause any cell pH alkalinization. At lower concentrations (50 μM) formate caused significant intracellular pH alkalinization in proximal tubule cells, with pHi increasing from baseline levels 7.15 ± 0.02 to 7.36 ± 0.02 (p<0.02, n=11). Acetate, at 50μM, had no effect on intracellular pH. Formate effect was observed both in the absence and presence of CO2/HCO3 in the media. We conclude that formate stimulates the apical Na/H exchanger NHE3 in kidney proximal tubule. We propose that formate stimulation of chloride reabsorption in proximal tubule is indirect and is secondary to the activation of apical of Na/H exchanger NHE3, which then leads to the stimulation of apical chloride/base exchanger.