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

Parathyroid cells express a plasma membrane calcium receptor (CaR), which i s stimulated by a rise in extracellular calcium concentration ([Ca2+]ext). A decreased sensitivity to [Ca2+]ext occurs in adenomatous parathyroid cell s in patients with primary hyperparathyroidism, but the underlying function al mechanism is not yet fully understood. This study explored whether CaR r esponsiveness is influenced by increasing the affinity of IP3 receptors - a major signalling component of other G-protein-coupled receptors. The sulphydryl reagent thimerosal was used to increase the responsiveness o f IP3-receptors. Quantitative fluorescence microscopy in Fura-2-loaded cell s was used to investigate the effects of thimerosal on the cytoplasmic calc ium concentrations ([Ca2+](i)) in human parathyroid cells and to compare it s effects in a rat medullary thyroid carcinoma cell line (rMTC6-23) also ex pressing CaR. During incubation in Ca2+-free medium, thimerosal 5 mu M induced a rapid su stained rise in [Ca2+](i) in human parathyroid cells and no further [Ca2+]( i) increase appeared in response to the CaR agonist Gd3+ (100 mu M). Thimer osal 1 mu M induced only slow and minimal changes of basal [Ca2+](i) and al lowed a rapid response to Gd3+ 20 nM (a concentration without effect in con trol cells). The slope of the thimerosal-induced [Ca2+](i) responses was st eeper following exposure to CaR agonists. In the presence of 1 mM [Ca2+]ext , thimerosal (0.5 mu M) induced a sharp increase in [Ca2+](i) to a peak (wi thin 60 s), followed either by return to basal [Ca2+](i) or by a plateau of slightly higher amplitude. Similar results were obtained using rMTC6-23 ce lls. Thimerosal increases the responsiveness to CaR agonists through modulation of the sensitivity of the IP3 receptor in both parathyroid and rMTC6-23 cel ls. (C) Harcourt Publishers Ltd 1999.