Cloning and functional expression of a rat kidney extracellular calcium/polyvalent cation-sensing receptor

The maintenance of a stable extracellular concentration of ionized calcium depends on the integrated function of a number of specialized cells (e.g., parathyroid and certain kidney epithelial cells). We recently identified another G protein-coupled receptor (BoPCaR1) from bovine parathyroid that responds to changes in extracellular Ca²⁺ within the millimolar range and provides a key mechanism for regulating the secretion of parathyroid hormone. Using an homology-based strategy, we now report the isolation of a cDNA encoding an extracellular Ca²⁺/polyvalent cation-sensing receptor (RaKCaR) from rat kidney. The predicted RaKCaR protein shares 92% identity with BoPCaR1 receptor and features a seven membrane-spanning domain, characteristic of the G protein-coupled receptors, which is preceded by a large hydrophilic extracellular NH₂ terminus believed to be involved in cation binding. RaKCaR cRNA-injected Xenopus oocytes responded to extracellular Ca²⁺, Mg²⁺, Gd³⁺, and neomycin with characteristic activation of inositol phospholipid-dependent, intracellular Ca²⁺-induced Cl^- currents. In rat kidney, Northern analysis revealed RaKCaR transcripts of 4 and 7 kb, and in situ hybridization showed localization primarily in outer medulla and cortical medullary rays. Our results provide important insights into the molecular structure of an extracellular Ca²⁺/polyvalent cation-sensing receptor in rat kidney and provide another basis on which to understand the role of extracellular divalent cations in regulating kidney function in mineral metabolism.