Abstract
Regulation of the cytoplasmic calcium concentration ([Ca 2+] i) was studied in fura-2-loaded C-cells from two human medullary thyroid carcinomas (MTC). K + depolarization induced sustained rise of [Ca 2+] i reversed by verapamil. Elevation of external Ca 2+ from 0.5 to 3.0 mM triggered regular oscillations or steady-state increases of [Ca 2+] i. In Ca 2+-deficient medium Sr 2+ caused steady-state increase or oscillations of the 340/380 nm fluorescence ratio. The Ca 2+ and Sr 2+ actions were partially reversible by verapamil. La 3+ and Ce 3+ elicited transient [Ca 2+] i peaks independent of external Ca+ 2+, but no oscillations. The results indicate that human MTC cells express a parathyroid-like Ca 2+ sensor coupled to intracellular mobilization and influx of Ca 2+. A voltage-dependent Ca 2+ influx may be of importance for the oscillations of [Ca 2+] i.
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