Several lines of evidence indicate that increases in nuclear Ca(2+) have specific biological effects that differ from those of cytosolic Ca(2+), suggesting that they occur independently. The mechanisms involved in controlling nuclear Ca(2+) signaling are both controversial and still poorly understood. Using hypotonic shock combined with mechanical disruption, we obtained and characterized a fraction of purified nuclei from cultured rat skeletal myotubes. Both immunoblot studies and radiolabeled inositol 1,4,5-trisphosphate [IP(3)] binding revealed an important concentration of IP(3) receptors in the nuclear fraction. Immunofluorescence and immunoelectron microscopy studies localized type-1 and type-3 IP(3) receptors in the nucleus with type-1 receptors preferentially localized in the inner nuclear membrane. Type-2 IP(3) receptor was confined to the sarcoplasmic reticulum. Isolated nuclei responded to IP(3) with rapid and transient Ca(2+) concentration elevations, which were inhibited by known blockers of IP(3) signals. Similar results were obtained with isolated nuclei from the 1B5 cell line, which does not express ryanodine receptors but releases nuclear Ca(2+) in an IP(3)-dependent manner. Nuclear Ca(2+) increases triggered by IP(3) evoked phosphorylation of cAMP response element binding protein with kinetics compatible with sequential activation. These results support the idea that Ca(2+) signals, mediated by nuclear IP(3) receptors in muscle cells, are part of a distinct Ca(2+) release component that originates in the nucleus and probably participates in gene regulation mediated by cAMP response element binding protein.
