A three-dimensional model of the human extracellular Ca 2؉ -sensing receptor (CaSR) has been used to identify specific residues implicated in the recognition of two negative allosteric located in transmembranes (TM) 6 and TM7, in the binding pocket for both calcimimetics and calcilytics, despite important differences observed between each family of compounds. The TMs involved in the recognition of both calcilytics include residues located in TM3 (Arg-680 3.28 , Phe-684 3.32 , and Phe-688 3.36 ). However, our study indicates subtle differences between the binding of these two compounds. Importantly, the observation that some mutations that have no effect on calcimimetics recognition but which affect the binding of calcilytics in TM3 and TM5, suggests that the binding pocket of positive and negative allosteric modulators is partially overlapping but not identical. Our CaSR model should facilitate the development of novel drugs of this important therapeutic target and the identification of the molecular determinants involved in the binding of allosteric modulators of class 3 G-protein-coupled receptors.

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Abstract-Small increases in extracellular Ca2ϩ dilate isolated blood vessels. In the present study, the possibility that a vascular, extracellular Ca 2ϩ -sensing receptor (CaSR) could mediate these vasodilator actions was investigated. Novel ligands that interact with the CaSR were used in microelectrode recordings from rat isolated mesenteric and porcine coronary arteries. The major findings were that (1)