Effects of 1-methyladenine on nuclear Ca²⁺ transients and meiosis resumption in starfish oocytes are mimicked by the nuclear injection of inositol 1,4,5-trisphosphate and cADP-ribose

The treatment of prophase-arrested starfish oocytes with the hormone 1-methyladenine (1-MA) induces the elevation of Ca²⁺ in both the cytoplasm and the germinal vesicle (nucleus), and is followed by the resumption of meiosis. The injection of the modulators of the intracellular Ca²⁺ channels inositol 1,4,5-trisphosphate (InsP₃) or cyclic adenosine diphosphate ribose (cADPr) into the germinal vesicle promoted meiosis resumption in the absence of 1-MA in about 50% of the oocytes. Caged InsP₃ or caged cADPr were injected into the nuclei of oocytes together with the Ca²⁺ indicator calcium green dextran; their photoreleasing elicited nuclear calcium spikes which, in the case of cADPr, had repetitive behaviour. The spikes were abolished by the nuclear injection of antagonists or antibodies to the InsP₃ or cADPr-sensitive Ca²⁺ channels. cADPr-modulated channels were localized on the membranes of the nuclear envelope using specific antibodies conjugated with IgG-gold complexes.