A prodigious quantity of sodium and water are filtered by the glomerulus daily, 1.7 kg and 180L. The majority is absorbed, across the proximal tubular epithelium back into the circulation, preventing rapid dehydration and death. This occurs through a complicated mechanism involving ion transporters, water channels and enzymes. Fundamental to this life sustaining process is the activity of the epithelial sodium proton exchanger, NHE3. The simple exchange of luminal sodium for cytosolic protons, mediated by NHE3, permits the entrance of filtered sodium into the proximal tubular epithelial cytosol, which is then extruded back into the blood space by the basolateral sodium-potassium ATPase. While much is known about the regulation of this fundamental process, significant gaps in our understanding of the molecular details of NHE3 regulation remain.

This dissertation documents insights into the regulation of NHE3 activity. Firstly cell swelling is observed to activate NHE3 activity. The kinetics of this activation is dissected to reveal at least two inactive states of the exchanger. The mechanism of this swelling induced activation is probed and found to be dependent on alterations in the plasma membrane curvature. Secondly the live cell imaging technique, fluorescence recovery after photobleaching (FRAP), is employed to demonstrate that NHE3 is localized to its functional subcellular domain, the apical plasma membrane, through a retention mechanism. This mechanism is shown to be dependent on a physical attachment to the actin cytoskeleton. Finally, the molecular determinants of this attachment to the actin cytoskeleton are assessed. In contrast to the model currently present in the literature, ezrin is demonstrated not to make the functional link between NHE3 and the actin cytoskeleton. However, that ezrin participates in the regulation of NHE3 is confirmed.