Figure 6.

Model of phosphate reabsorption in the renal proximal tubule. Pi is reabsorbed via three sodium phosphate cotransporters: Npt2a, Npt2c and PiT-2. In humans, Npt2a and Npt2c are believed to play the most important role in phosphate reabsorption. The sodium phosphate cotransporters, which are positioned in the apical membrane of renal proximal tubule cells, use energy derived from the movement of sodium down its gradient to move Pi from the filtrate to the cell interior. The amount of phosphate reabsorbed is dependent on the abundance of the sodium phosphate cotransporters in the apical brush border membrane and hormones such as parathyroid hormone and fibroblast growth factor-23 decrease Pi reabsorption by decreasing the abundance of the sodium phosphate cotransporters in the brush border. Movement of phosphate from the interior of renal proximal tubular cells to the peritubular capillaries occurs via an unknown transporter.