THE NUCLEUS IS INSULATED FROM LARGE CYTOSOLIC CALCIUM-ION CHANGES

EXTRACELLULAR events regulate functions in the cell nucleus by means o f calcium ions acting through effector enzymes1-5. Recently, the tradi tional view of the nuclear pore as freely permeable to small ions6,7 h as been questioned as a result of reports that nuclear calcium can be regulated independently of cytosolic calcium8-12. We have used confoca l microscopy of fluorescent Ca2+ indicators to investigate the Ca2+ dy namics between cytosol and nucleus in neurons. We find that a previous ly reported amplification of Ca2+ changes in the nucleus13-16 is a mea surement artefact. Small changes of cytosolic Ca2+ cause equally rapid changes in nuclear Ca2+, consistent with the free diffusion of Ca2+ t hrough nuclear pores. In contrast, large cytosolic Ca2+ increases (abo ve 300 nM) are attempted in the nucleus. Our results show the nuclear envelope shapes but does not block the passage of Ca2+ signals from cy tosol to nucleus.