The steroid hormone aldosterone is a major regulatorof extracellular volume and blood pressure.Aldosterone effectors are for example the epithelialNa+ channel (ENaC), the Na+-K+-ATPase and theproximal tubule Na+/H+ exchanger isoform 3 (NHE3).The aim of this study was to investigate whetheraldosterone acts directly on proximal tubule cells tostimulate NHE3 and if so whether the EGF-receptor(EGFR) is involved. For this purpose, primary humanrenal proximal tubule cells were exposed toaldosterone. NHE3 activity was determined from Na+-dependent pH-recovery, NHE3 surface expressionwas determined by biotinylation and immunoblotting.EGFR-expression was assessed by ELISA. pHi-measurements revealed an aldosterone-inducedincrease in NHE3 activity, which was inhibited by themineralocorticoid receptor blocker spironolactone andby the EGFR-kinase inhibitor AG1478. Immunoprecipitationand immunoblot analysis showed analdosterone-induced increase in NHE3 surface expression, which was also inhibited byspironolactone and AG1478. Furthermore,aldosterone enhanced EGFR-expression.In conclusion, aldosterone stimulates NHE3 in humanproximal tubule cells. The underlying mechanismsinclude AG1478 inhibitable kinase and are paralleledby enhanced EGFR expression, which could becompatible with EGF-receptor-pathway-dependentsurface expression and activity of NHE3 in humanprimary renal proximal tubule epithelial cells.