The rise of nuclear and cytosolic Ca2+ can be uncoupled in HeLa cells

It has long been assumed that ionized Ca2+ is transmitted from the cytosol to the cell nucleus through the nuclear pore complexes to trigger many nuclear functions such as gene transcription after agonist stimulation. However, this hypothesis has been challenged recently. In the present study, we have investigated the interplay of Ca2+ in the cytosol and nucleus of HeLa cells upon histamine stimulation by using confocal microscopy. In resting cells, addition of histamine (50 microM) produced synchronous Ca2+ signals in the cytosol and nucleus with a stronger fluo-3 emission in the nucleus. Our results also demonstrate that the rise of cytosolic and nuclear free Ca2+ can be uncoupled after histamine stimulation. This assertion is supported by the following observations: (1) when heparin was injected into the cytosol, the increase in fluo-3 fluorescence in the cytosol mediated by histamine was almost eliminated while that in the nucleus still occurred; (2) when heparin was in the nucleus, the increase in the free nucleoplasmic Ca2+ concentration ([Ca2+]n) elicited by histamine was abolished while the rise of cytosolic Ca2+ level ([Ca2+]c) was seen; (3) when we injected inositol 1,4,5-trisphosphate (IP3) directly into the nucleus, [Ca2+]n increased; and (4) in the cells given cytosolic injections of BAPTA-dextran (70 kDa), histamine evoked additional Ca2+ transients in the nucleus. These results suggest that the nucleus of the HeLa cell has its own IP3-sensitive Ca2+ store and that the Ca2+ signals in the nucleus and cytosol can be uncoupled.