Butters R R, Chattopadhyay N, Nielsen P, Smith C P, Mithal A, Kifor O, Bai M, Quinn S, Goldsmith P, Hurwitz S, Krapcho K, Busby J, Brown E M
Department of Medicine, Brigham and Women's Hospital, Boston, Massachusetts, USA.
J Bone Miner Res. 1997 Apr;12(4):568-79. doi: 10.1359/jbmr.1997.12.4.568.
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.
最近从哺乳动物的甲状旁腺、肾脏、大脑和甲状腺中克隆出的细胞外钙离子（Ca(0)2+）敏感受体（CaR）在维持Ca(0)2+近乎恒定方面起着核心作用。我们之前发现，新西兰白兔中正常存在的高钙血症与Ca(02+)调节甲状旁腺激素（PTH）释放的设定点升高有关（Ca(0)2+抑制激素分泌达到半数最大效应时的水平）。这一观察结果提示兔甲状旁腺中Ca(02+)传感机制发生了改变，我们现在通过分离和鉴定CaR的兔同源物来探究这种可能性。克隆出的兔肾脏CaR（RabCaR）与牛、人及大鼠的CaR在整体上具有高度同源性（氨基酸同一性>90%），尽管在细胞外结构域中可能参与结合配体的几个区域存在细微差异。通过Northern分析和/或免疫组织化学方法发现，甲状旁腺、甲状腺C细胞、小肠和大肠以及肾脏的髓袢升支粗段和集合管中也表达了相似或相同的受体。当在HEK293细胞中瞬时表达并通过CaR激动剂诱发细胞内钙离子（Ca(i)2+）增加来进行功能检测时，兔CaR对Ca(0)2+、Mg(0)2+和Gd(0)3+的表观亲和力与同时使用人CaR进行的研究所观察到的亲和力没有区别。因此，至少就其在HEK293细胞中表达时增加Ca(i)2+的能力而言，兔CaR的内在功能特性无法解释新西兰白兔体内观察到的高钙血症。
