Figure 3. Model of inorganic phosphate transcellular transport in the proximal tubule. At the brush border a Na+/H+ exchanger and NaPi-II co-transporters operate. Nearly all proximal tubular reabsorption can be accounted for by the SLC34 (type II) family of sodium-dependent Pi transporters. The more abundant NaPi-IIa transporter is electrogenic with a 3:1 (Na: PO4) stoichiometry, preferentially transporting the divalent phosphate anion. The lesser abundant NaPi-IIc transporter is electroneutral with a 2:1 (Na: PO4) stoichiometry, but also prefers the divalent phosphate species. The HPO4- that enters the cell across the luminal surface mixes with the intracellular pool of Pi and is transported across the basolateral membrane. This process is poorly understood, but anion exchange mechanisms have been suggested. A Na+/K+ ATPase located on the basolateral membrane pumps Na+ out of the cell maintaining the inward downhill Na gradient, which serves as the driving force for luminal entry of Na+.