Nuclear calcium signaling by inositol trisphosphate in GH3 pituitary cells

It has been proposed that nuclear and cytosolic Ca(2+) ([Ca(2+)](N) and [Ca(2+)](C)) may be regulated independently. We address here the issue of whether inositol trisphosphate (IP(3)) can, bypassing changes of [Ca(2+)](C), produce direct release of Ca(2+) into the nucleoplasm. We have used targeted aequorins to selectively measure and compare the changes in [Ca(2+)](C) and [Ca(2+)](N) induced by IP(3) in GH(3) pituitary cells. Heparin, an IP(3) inhibitor that does not permeate the nuclear pores, abolished the [Ca(2+)](C) peaks but inhibited only partly the [Ca(2+)](N) peaks. The permeant inhibitor 2-aminoethoxy-diphenyl-borate (2-APB) blocked both responses. Removal of ATP also inhibited more strongly the [Ca(2+)](C) than [Ca(2+)](N) peak. The [Ca(2+)](N) and [Ca(2+)](C) responses differed also in their sensitivity to IP(3), the nuclear response showing higher affinity. Among IP(3) receptors, type 2 (IP(3)R2) has a higher affinity for IP(3) and is not inactivated by ATP removal. We find that IP(3)R2 immunoreactivity is present inside the nucleus whereas the other IP(3)R subtypes are detected only in the cytoplasm. The nuclear envelope (NE) of GH(3) cells showed deep invaginations into the nucleoplasm, with cytosol and cytoplasmic organella inside. These results indicate that GH(3) pituitary cells possess mechanisms able to produce selective increases of [Ca(2+)](N).