Differential responses of proximal tubule Na+/H+ exchanger NHE3 to low pH: comparison between metabolic and respiratory acidosis

Abstract

The Na+/H+ exchanger isoform 3 (NHE3) is essential for HCO3 − reabsorption in renal proximal tubules. The goal of this work was to evaluate whether there are differences between metabolic and respiratory acidosis with regard to NHE3 modulation and to identify which are the relevant parameters that may trigger these distinct adaptative responses. Metabolic acidosis was achieved by lowering [HCO3 −] in the cell culture medium and respiratory acidosis by increasing pCO2 in the incubator chamber. We found that cell‐surface NHE3 expression increased in response to metabolic and respiratory acidosis. Mild (pH 7.21±0.02) and severe (6.95±0.07) metabolic acidosis increased Nhe3 mRNA levels, while mild (7.11±0.03) and severe (6.86±0.01) respiratory acidosis did not up‐regulate Nhe3 expression. Analyses of the Nhe3 promoter suggested that the regulatory elements sensitive to metabolic acidosis are located in the region spanning −466 to −153 pb. Two SP1 consensus binding sites were localized in this region and metabolic acidosis caused an increment in the SP1 protein levels. We conclude that low pH might be related to NHE3 displacement to the apical membrane, however extracellular pH does not seem to be the parameter that up‐regulates NHE3 expression. Moreover, SP1 was identified as a potential transcription factor that mediates the response to metabolic acidosis in proximal tubule cells. Financial support: CNPq and FAPESP.