Introduction

Inorganic phosphate (Pi) is a nutrient that is essential for cell function and skeletal mineralization. To accommodate these needs, transport systems have evolved to permit the efficient transfer of Pi anions across hydrophobic membrane barriers. Dietary Pi intake in North America is generally above the RDA and, except for the elderly where low Pi intakes are relatively common, Pi deficiency is unlikely to occur. The major proportion of ingested Pi is absorbed in the small intestine. Hormonal regulation of this process plays only a minor role in the maintenance of Pi homeostasis. Absorbed Pi is either incorporated into organic forms in cells, deposited into bone, or filtered by the kidney. Under normal conditions, only a small percentage of ingested Pi is retained and most of the absorbed Pi is excreted in the urine. The kidney is a major determinant of Pi homeostasis and tubular handling of Pi is subject to regulation by Pi intake and metabolic requirement. The molecular mechanisms involved in these processes are complex and are currently being investigated.

The present chapter will summarize our current knowledge of the cellular and molecular mechanisms involved in the transport of Pi by kidney, bone and intestine. Citations to review articles will be provided where possible.