ATP-DEPENDENT ACCUMULATION AND INOSITOL TRISPHOSPHATE-MEDIATED OR CYCLIC ADP-RIBOSE-MEDIATED RELEASE OF CA2-ENVELOPE( FROM THE NUCLEAR)

Uptake and release of Ca2+ from isolated liver nuclei were studied wit h fluorescent probes. We show with the help of digital imaging and con focal microscopy that the Ca2+-sensitive fluorescent probe Fura 2 is c oncentrated in or around the nuclear envelope and that the distributio n of Fura 2 fluorescence is similar to that of an endoplasmic reticulu m marker. The previously demonstrated ATP-dependent uptake of Ca2+ int o isolated nuclei and release of the accumulated Ca2+ by inositol 1,4, 5-trisphosphate (IP3) are therefore due to transport of Ca2+ into and out of the nuclear envelope and not the nucleoplasm. Dextrans labeled with fluorescent Ca2+ indicators (calcium-Green 1 and Fura 2) are dist ributed uniformly in the nucleoplasm and can be used to show that chan ges in the external Ca2+ concentration produce rapid changes in the nu cleoplasmic Ca2+ concentration. Nevertheless, IP3 and cyclic ADP-ribos e evoke transient intranuclear Ca2+ elevations. The release from the C a2+ stores in or around the nuclear envelope appears to be directed in to the nucleoplasm from where it can diffuse out through the permeable nuclear pore complexes.