Renal proximal tubular and small intestinal P_i-absorption/reabsorption are Na⁺ coupled and secondary active. Thereby, transport across the brush-border membrane is rate limiting and involves physiologically regulated Na⁺-P_i cotransporters. In the small intestine regulatory mechanisms require hours/days (e.g. P_i-diet, 1.25 (OH)₂ Vit D3); in the renal proximal tubule regulation occurs within minutes (parathyroid hormone, other peptide hormones; Pi-diet) as well as after hours (P_i-demand; P_i-diet). These regulatory phenomena are associated with altered brush border membrane expression of Na⁺-P_i cotransporters.

Brush border membrane P_i-flux is mediated by type II Na⁺-P_i cotransporters: type IIa (and to a smaller extent type IIc) in the kidney, type IIb in the small intestine. They mediate a 3 Na⁺ to 1 P_i cotransport: an interaction with 1 Na⁺ ion is followed by interaction with 1 P_i and then by 2 additional Na⁺ ions. In the absence of P_i transfer of Na⁺ can occur (slippage).

Type II cotransporters contain at least eight transmembrane domains (TM) and cytoplasmic oriented NH₂– and COOH-termini. Detailed structure/function relationships have been performed on the type IIa isoform: a glycosylated large extracellular loop separates the transporter into two domains (TM1-3 and TM4-8), both required for transport function. Predicted intracellular loop 1 (ICl-1) and extracellular loop 3 (ECL-3) contain intrasequence homologies. Chimera studies (IIa and IIb) led to the identification of sites contributing to Na⁺ affinity (TM-5) or pH sensitivity (ECL-3). Cysteine-scanning studies identified important (α-helical) structures in ICl-1 and ECL-3, suggesting that they are components of a permeation pore participating in slippage and cotransport. The type IIa cotransporter exerts its function as a monomer.

Altered brush-border membrane expression is achieved by specific membrane retrieval/insertion processes. Membrane retrieval of type IIa has been extensively characterized by taking parathyroid hormone (PTH) as an example. PTH activates protein kinase A and C regulatory cascades: luminal PTH preferentially activates the kinase C and basolateral PTH the kinase A regulatory pathway; thereby an activation of MAPK-kinase seems to play a central role in the internalization. At the level of the transporter, specificity for internalization is given by two basic amino acid residues in ICL-3. Reinsertion of type IIa Na⁺-P_i cotransporters depends on sequences contained in the COOH-terminus: a terminal TRL-sequence and an internal pair of basic amino acid residues; for the type IIb cotransporter a COOH-terminal L-residue determines apical expression. The terminal TRL-residues are probably required for apical scaffolding of the type IIa cotransporter involving PDZ-modules of NaP_i-Cap1 (4 modules) and NHE-RF1 (2 modules). These proteins interact at one of the PDZ-modules with the type IIa cotransporter and permit via an interaction at the remaining sites to build apical complexes containing other brush-border membrane proteins and/or elements of the cellular regulatory machinery (e.g. D-AKAP2; dual specific kinase A anchoring protein).