Schematic of solute transport pathways across a proximal tubule cell. Schematic of solute transport pathways across a proximal tubule cell. Luminal membrane transporters include the Na+-glucose cotransporters (SGLT1 and SGLT2), the Na+/H+ exchanger (NHE3), the Na+-HPO42– cotransporter (NaPi2), an unidentified Cl−/base exchanger (AE), and a K+ channel. Peritubular membrane transporters include the Na+-K+-ATPase, the K+-Cl− cotransporter (KCC), the Na+-3HCO3− cotransporter (NBC1), and the Na+-dependent Cl−/HCO3− exchanger (NDCBE), as well as K+ and Cl− channels. High water permeability is mediated by AQP1 channels, which after cGMP stimulation may also increase Na+entry. The tight junctions of proximal tubules allow substantial and relatively nonselective fluxes of cations and anions. Andrea J. Yool, and Alan M. Weinstein Physiology 2002;17:68-72 ©2002 by American Physiological Society