Calcitonin increases cytosolic free calcium concentration via capacitative calcium influx

The calcitonin receptor has been proposed to function as an extracellular Ca2+ concentration ([Ca2+]o) sensor (Stroop, S. D., Thompson, D. L., Kuestner, R. E., and Moore, E. E. (1993) J. Biol. Chem. 268, 19927-19930). To test this hypothesis we studied the LLC-PK1 renal tubular cells and the PC1 cells, a cell line stably transfected with the cloned porcine calcitonin receptor. [Ca2+]i was measured by fura-2 single cell microfluorometry. Addition to the cells equilibrated in 1.25 mM Ca(2+)-containing media of 1-10 mM extracellular Ca2+ did not result in a significant increase of [Ca2+]i. Treatment with 10(-7) M salmon calcitonin (sCT) elicited a rapid, persistent elevation of [Ca2+]i. Addition of 1-10 mM extracellular Ca2+ in the presence of sCT induced a significant [Ca2+]i elevation, about 10-fold that observed in the absence of the hormone. Ca2+ influx was inhibited by lanthanum. The rise of [Ca2+]i at elevated [Ca2+]o was not due to a Ca2+ sensing mechanism with release of Ca2+ from intracellular stores, since it was prolonged, and was not abolished by prior depletion of Ca2+ stores with 10(-6)M thapsigargin. On the contrary, this agent potentiated Ca2+ influx after addition of 1-10 mM Ca2+ by 13-fold versus control. Prior stimulation of [Ca2+]i with 10(-7) M arginine-vasopressin had similar effects, enhancing the subsequent Ca2+ influx. Enhancement of Ca2+ influx by sCT was confirmed by increased Mn2+ quenching of fura-2 fluorescence. In conclusion, arginine-vasopressin or calcitonin enhance Ca2+ influx in LLC-PK1 cells via a Ca2+ release-activated conductance, probably dependent upon capacitative Ca2+ entry. Thus, these effects are not unique to the calcitonin receptor and argue against the receptor functioning as a [Ca2+]o sensor.