Molecular biology and clinical importance of the Ca(2+)-sensing receptor.

The Ca(2+)-sensing receptor (CaR) is a member of the seven-transmembrane domain, G protein-coupled receptor super-family. In the parathyroid gland, it mediates the inhibitory effects of extracellular Ca2+ on the secretion of parathyroid hormone. In the kidney, activation of the CaR causes decreased reabsorption of Ca2+ from the tubular lumen. Mutations in the CaR gene produce abnormalities of Ca2+ homeostasis. Heterozygous loss-of-function mutations cause familial hypocalciuric hypercalcemia. Homozygous loss-of-function mutations cause neonatal severe hyperparathyroidism. In contrast, gain-of-function CaR mutations result in autosomal dominant and sporadic hypoparathyroidism. The resulting hypoparathyroidism and hypocalcemia can range from asymptomatic to life-threatening. Patients with hypocalcemia due to CaR mutations also show disproportionate hypercalciuria that may increase the risk of nephrocalcinosis, nephrolithiasis, and renal insufficiency.