Calcium sensing by human medullary thyroid carcinoma cells

Regulation of the cytoplasmic calcium concentration ([Ca2+]i) was studied in fura-2-loaded C-cells from two human medullary thyroid carcinomas (MTC). K+ depolarization induced sustained rise of [Ca2+]i reversed by verapamil. Elevation of external Ca2+ from 0.5 to 3.0 mM triggered regular oscillations or steady-state increases of [Ca2+]i. In Ca(2+)-deficient medium Sr2+ caused steady-state increase or oscillations of the 340/380 nm fluorescence ratio. The Ca2+ and Sr2+ actions were partially reversible by verapamil. La3+ and Ce3+ elicited transient [Ca2+]i peaks independent of external Ca2+, but no oscillations. The results indicate that human MTC cells express a parathyroid-like Ca2+ sensor coupled to intracellular mobilization and influx of Ca2+. A voltage-dependent Ca2+ influx may be of importance for the oscillations of [Ca2+]i.