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  • Title: Cloning and characterization of a calcium-sensing receptor from the hypercalcemic New Zealand white rabbit reveals unaltered responsiveness to extracellular calcium.
    Author: Butters RR, Chattopadhyay N, Nielsen P, Smith CP, Mithal A, Kifor O, Bai M, Quinn S, Goldsmith P, Hurwitz S, Krapcho K, Busby J, Brown EM.
    Journal: J Bone Miner Res; 1997 Apr; 12(4):568-79. PubMed ID: 9101368.
    Abstract:
    The extracellular Ca2+ (Ca(0)2+)-sensing receptor (CaR) recently cloned from mammalian parathyroid, kidney, brain, and thyroid plays a central role in maintaining near constancy of Ca(0)2+. We previously showed that the hypercalcemia normally present in New Zealand white rabbits is associated with an elevated set point for Ca(02+)-regulated PTH release (the level of Ca(0)2+ half-maximally inhibiting hormonal secretion). This observation suggested an alteration in the Ca(02+)-sensing mechanism in the rabbit parathyroid, a possibility we have now pursued by isolating and characterizing the rabbit homolog of the CaR. The cloned rabbit kidney CaR (RabCaR) shares a high degree of overall homology (> 90% amino acid identity) with the bovine, human, and rat CaRs, although it differs slightly in several regions of the extracellular domain potentially involved in binding ligands. By Northern analysis and/or immunohistochemistry, a similar or identical receptor is also expressed in parathyroid, thyroid C cells, small and large intestine, and in the thick ascending limb and collecting ducts of the kidney. When expressed transiently in HEK293 cells and assayed functionally through CaR agonist-evoked increases in Ca(i)2+, the rabbit CaR shows apparent affinities for Ca(0)2+, Mg(0)2+, and Gd(0)3+ that are indistinguishable from those observed in studies carried out concomitantly using the human CaR. Therefore, at least as assessed by its ability to increase Ca(i)2+ when expressed in HEK293 cells, the intrinsic functional properties of the rabbit CaR cannot explain the hypercalcemia observed in vivo in the New Zealand white rabbit.
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