Differential activation of parathyroid and renal Ca 2+ ‐sensing receptors underlies the renal phenotype in autosomal dominant hypocalcemia 1

Parathyroid Ca 2+ ‐sensing receptor (CaSR) activation inhibits parathyroid hormone (PTH) release, while activation of CaSRs in kidneys and intestine attenuates local transepithelial Ca 2+ transport. In patients with autosomal dominant hypocalcemia 1 (ADH1) due to activating CASR mutations, treatment of symptomatic hypocalcemia can be complicated by treatment‐induced hypercalciuria, resulting in nephrocalcinosis and renal insufficiency. Although CaSRs throughout the body are activated by increased extracellular Ca 2+ concentrations, it is not understood why some ADH1 patients have reduced PTH, but not hypercalciuria at presentation, despite CaSR expression in both kidney and parathyroid. Activation of the CaSR was studied in mouse models and a ADH1 patient. In vitro CaSR activation was studied in HEK293 cells. Mice with a gain‐of‐function mutation in Casr are hypocalcemic with reduced plasma PTH levels. However, renal CaSRs are not activated as indicated by normal urinary calcium handling and unaltered renal Cldn14 expression. Activation of renal CaSRs only occurred after increasing plasma Ca 2+ levels. Similarly, calcimimetic administration to wildtype mice induced hypocalcemia without activating renal CaSRs. Moreover, significant hypercalciuria was not observed in an ADH1 patient until blood Ca 2+ was normalized. In vitro experiments suggest that increased CaSR levels in the parathyroid relative to the kidney contribute tissue‐specific CaSR activation thresholds. Here we delineate tissue‐specific CaSR activation thresholds, where parathyroid CaSR overactivity can reduce plasma Ca 2+ to levels insufficient to activate renal CaSRs, even with overactivating mutations. These findings may aid in the management of ADH1 patients.