Fig. 2.

Major proximal tubule Na⁺ transporters. For simplicity, “pure” anion transporters, paracellular pathways, and distinction between S1, S2 and S3 proximal tubule segments have been omitted. The thickness of the arrows is intended to reflect the relative, quantitative transport of the indicated ion. NHE Na⁺–H⁺ exchanger, NaP_i IIa renal Na⁺-phosphate co-transporter, NBCe1-A electrogenic Na⁺/HCO₃⁻ co-transporter