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Abstract

The human body is equipped with an efficient protection system against hypocalcemia. This system is composed of parathyroid glands, bone, kidney, and intestine. By appropriate actions of parathyroid hormone (PTH) and active vitamin D (1,25-dihydroxyvitamin D), a small fall of extracellular calcium ion concentration is instantly corrected. Thus, a defect of any step in this system results in the development of hypocalcemia. Overloaded calcium either from bone or intestine is efficiently excreted into the urine. Thus, hypercalcemia develops almost exclusively when a greater amount of calcium than the kidney can excrete is loaded. In chronic dialysis patients, either hypocalcemia or hypercalcemia may develop because of defects in these two defense systems against calcium imbalance.

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Cloning and characterization of an extracellular Ca(2+)-sensing receptor from bovine parathyroid.

E M Brown, G Gamba, D. Riccardi … (1993)

Maintenance of a stable internal environment within complex organisms requires specialized cells that sense changes in the extracellular concentration of specific ions (such as Ca2+). Although the molecular nature of such ion sensors is unknown, parathyroid cells possess a cell surface Ca(2+)-sensing mechanism that also recognizes trivalent and polyvalent cations (such as neomycin) and couples by changes in phosphoinositide turnover and cytosolic Ca2+ to regulation of parathyroid hormone secretion. The latter restores normocalcaemia by acting on kidney and bone. We now report the cloning of complementary DNA encoding an extracellular Ca(2+)-sensing receptor from bovine parathyroid with pharmacological and functional properties nearly identical to those of the native receptor. The novel approximately 120K receptor shares limited similarity with the metabotropic glutamate receptors and features a large extracellular domain, containing clusters of acidic amino-acid residues possibly involved in calcium binding, coupled to a seven-membrane-spanning domain like those in the G-protein-coupled receptor superfamily.