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Extracellular Ca2+ sensing in C-cells and parathyroid cells.

An essential function of C-cells and parathyroid cells is to monitor the extracellular Ca2+ concentration. The Ca(2+)-dependent secretion of calcitonin (CT) and parathyroid hormone is known to be mediated by corresponding changes in the intracellular Ca2+ concentration. To address the question of whether Ca2+ influx through voltage-dependent Ca2+ channels couples the extracellular to the intracellular Ca2+, we applied the patch clamp technique to C-cells of the rMTC 44-2 cell line and to parathyroid cells of the PT-r cell line. The rMTC cells displayed dihydropyridine-sensitive, voltage-dependent, high-threshold Ca2+ channels which allowed ion influx even at the resting potential of about -40 mV. Increases of the concentration of the extracellular divalent cation or adding the Ca2+ channel agonist Bay K 8644 stimulated the steady state ion influx. In contrast, PT-r cells exhibited only fast inactivating, low-threshold Ca2+ channel currents with no steady state conductivity for Ca2+ at the resting potential of around -40 mV. We conclude that dihydropyridine-sensitive Ca2+ channels allow steady state transmembranous Ca2+ influx in C-cells, thereby increasing the cytosolic Ca2+ and CT secretion. Parathyroid cells, however, lack long-lasting Ca2+ channel currents and obviously sense the extracellular Ca2+ concentration by other mechanisms.