Single-channel recording of inositol trisphosphate receptor in the isolated nucleus of a muscle cell line.

Nuclear calcium appears to have an important role in the regulation of gene expression in many cells, but the mechanisms involved in controlling nuclear Ca2+ signaling are controversial and still poorly understood. We have described the presence of inositol 1,4,5 trisphosphate (IP3) receptors in the nuclei of skeletal muscle cells. Now, we have characterized the properties of the IP3 receptors channels present in the nuclei of the 1B5 cell line, which do not express any isoforms of the ryanodine receptor. Immunocytochemistry of isolated nuclei confirmed the presence of IP3R in the nuclear envelope and fluorescence measurements in nuclei suspensions allowed us to document ATP-dependent calcium loading by the nucleus and its release upon IP3 addition. Patch clamp of nuclear membranes was performed, and single-channel activity recorded was dependent on the presence of IP3 in the pipette; single-channel conductance was in the range reported in the literature for these channels, and the open probability was shown to be dependent on IP3 concentration. The presence of functional IP3 receptors in the nuclear envelope membrane is likely to have an important function in the regulation of nucleoplasmic calcium concentration and consequently in the regulation of transcription in muscle cells.