An important function of renal epithelial cells is the reabsorption of volume, Na+ and bicarbonate (HCO3) from the filtrate. About 85% of the filtrate’s HCO3−− and 70% of the Na+ are being reabsorbed in the proximal tubule segment of the nephron. As a result, these cells are being continuously subject to large fluxes of Na+ and base through the cell. Renal medullary cells, for instance, are being subject to large fluctuations in osmolality of the extracellular fluid. Transient imbalances in fluid, base and Na+ flow through the cell can cause substantial alterations in cellular volume, pH and electrolytes.

The regulation of transepithelial transport on one hand and of cellular volume-, pH- and electrolyte-homeostasis on the other hand, is of a major physiological interest. Angiotensin II (Ang II) is a powerful regulator of Na+and HCO3− transport in the proximal tubule. The involvement of ion channels and electrogenic transporters in cellular homeostasis, has been studied by many laboratories using conventional elecrophysiological techniques to measure the resulting changes in membrane potential and ionic currents.