Differential Regulation of Nuclear and Cytosolic Ca2+ in HeLa Cells*

The results reported in this study address the controversial issue that nuclear free Ca2+ ([Ca2+]n) may be regulated independently of cytosolic free Ca2+ ([Ca2+]c). We have measured [Ca2+]n and [Ca2+]c with recombinant aequorin targeted to the nucleus and cytosol in HeLa cells. We found that histamine, ATP, and ionomycin increased [Ca2+]c quantitatively more than [Ca2+]n, although the time course of these changes was similar. The difference between [Ca2+]c and [Ca2+]n depended on the stimulus, and the relative difference between [Ca2+]n and [Ca2+]c was less with ionomycin than with histamine or ATP. After depletion of the internal Ca2+ store, restoration of extracellular Ca2+ resulted in only increased [Ca2+]c without a significant increase in [Ca2+]n. Treatment with cyclopiazonic acid resulted in a delayed increases in [Ca2+]n compared to [Ca2+]c. These differences in both timing and magnitude of nuclear Ca2+ signals confirm that the cell can limit or delay increases in nuclear free Ca2+. Taken with the fact that an inositol phosphate signaling system resides in the nucleus and its envelope, our data support the hypothesis that [Ca2+]n may be independently regulated.