Nuclear calcium signaling by inositol trisphosphate in GH3 pituitary cells

It has been proposed that nuclear and cytosolic Ca2+ ([Ca2+](N) and [Ca2+](C)) may be regulated independently. We address here the issue of whether inositol trisphosphate (IP3) can, bypassing changes of [Ca2+](C), produce direct release of Ca2+ into the nucleoplasm. We have used targeted aequorins to selectively measure and compare the changes in [Ca2+](C) and [Ca2+](N) induced by IP3 in GH(3) pituitary cells. Heparin, an IP3 inhibitor that does not permeate the nuclear pores, abolished the [Ca2+](C) peaks but inhibited only partly the [Ca2+](N) peaks. The permeant inhibitor 2-aminoethoxy-diphenyl-borate (2-APB) blocked both responses. Removal of ATP also inhibited more strongly the [Ca2+](C) than [Ca2+](N) peak. The [Ca2+](N) and [Ca2+](C) responses differed also in their sensitivity to IP3, the nuclear response showing higher affinity. Among IP3 receptors, type 2 (IP(3)R2) has a higher affinity for IP3 and is not inactivated by ATP removal. We find that IP(3)R2 immunoreactivity is present inside the nucleus whereas the other IP3R subtypes are detected only in the cytoplasm. The unclear 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 [Ca2+](N). (c) 2007 Elsevier Ltd. All rights reserved.