Phosphate Transporters and Their Function

Plasma phosphate concentration is maintained within a relatively narrow range by control of renal reabsorption of filtered inorganic phosphate (P-i). P-i reabsorption is a transcellular process that occurs along the proximal tubule. P-i flux at the apical (luminal) brush border membrane represents the rate-limiting step and is mediated by three Na+-dependent P-i cotransporters (members of the SLC34 and SLC20 families). The putative proteins responsible for basolateral P-i flux have not been identified. The transport mechanism of the two kidney-specific SLC34 proteins (NaPi-IIa and NaPi-IIc) and of the ubiquitously expressed SLC20 protein (PiT-2) has been studied by heterologous expression to reveal important differences in kinetics, stoichiometry, and substrate specificity. Studies on the regulation of the abundance of the respective proteins highlight significant differences in the temporal responses to various hormonal and nonhormonal factors that can influence P-i homeostasis. The phenotypes of mice deficient in NaPi-IIa and NaPi-IIc indicate that NaPi-IIa is responsible for most P-i renal reabsorption. In contrast, in the human kidney, NaPi-IIc appears to have a relatively greater role. The physiological relevance of PiT-2 to P-i reabsorption remains to be elucidated.