Separate activation of the cytoplasmic and nuclear calcium pools in maturing starfish oocytes

The dynamics of the cytoplasmic and nuclear Ca2+ pools in starfish oocytes arrested at the prophase of the first meiotic division or after induction of meiosis by 1-methyladenine (1-MA) have been studied by confocal microscopy. A 70 kDa fluorescent Ca2+ indicator has been injected in either the cytoplasm or the nucleus, and shown to remain restricted to the compartment of injection. 1-MA induced a first Ca2+ transient in the cytosol, followed by a nuclear transient, and eventually by a second cytosolic transient. The latter failed to occur if the nuclear peak was suppressed. This required the nuclear injection of antagonists of the inositol 1,4,5-trisphosphate (InsP3) and cyclic-ADPribose (cADPr) Ca2+ channels, showing that both channel types were active in the inner envelope membrane. The nuclear injection of the Ca2+ channel antagonists affected the process of meiosis reinitiation: in about one third of the injected oocytes no breakdown of the nuclear envelope (GVBD) was observed. In the others, even if GVBD eventually occurred, the intermixing of the nucleoplasm and cytoplasm was inhibited.