Thimerosal increases the responsiveness of the calcium receptor in human parathyroid and rMTC6–23 cells

Abstract

Parathyroid cells express a plasma membrane calcium receptor (CaR), which is stimulated by a rise in extracellular calcium concentration ([Ca2+]ext). A decreased sensitivity to [Ca2+]ext occurs in adenomatous parathyroid cells in patients with primary hyperparathyroidism, but the underlying functional mechanism is not yet fully understood. This study explored whether CaR responsiveness is influenced by increasing the affinity of IP3receptors – a major signalling component of other G-protein-coupled receptors.The sulphydryl reagent thimerosal was used to increase the responsiveness of IP3-receptors. Quantitative fluorescence microscopy in Fura-2-loaded cells was used to investigate the effects of thimerosal on the cytoplasmic calcium concentrations ([Ca2+]i) in human parathyroid cells and to compare its effects in a rat medullary thyroid carcinoma cell line (rMTC6-23) also expressing CaR.During incubation in Ca2+-free medium, thimerosal 5 μM induced a rapid sustained rise in [Ca2+]iin human parathyroid cells and no further [Ca2+]iincrease appeared in response to the CaR agonist Gd3+(100 μM). Thimerosal 1 μM induced only slow and minimal changes of basal [Ca2+]iand allowed a rapid response to Gd3+20 nM (a concentration without effect in control cells). The slope of the thimerosal-induced [Ca2+]iresponses was steeper following exposure to CaR agonists. In the presence of 1 mM [Ca2+]ext, thimerosal (0.5 μM) induced a sharp increase in [Ca2+]ito a peak (within 60 s), followed either by return to basal [Ca2+]ior by a plateau of slightly higher amplitude. Similar results were obtained using rMTC6–23 cells.Thimerosal increases the responsiveness to CaR agonists through modulation of the sensitivity of the IP3receptor in both parathyroid and rMTC6–23 cells.