Calcium oscillations in Xenopus egg cycling extracts

Cell cycle in various types of eel Is and in early embryos is often accompa nied by transient changes in the concentration of free cytosolic calcium. I n the present study, using fluorescent indicator fura-2, we demonstrate tha t Ca2+ oscillates cyclically with an amplitude of about 100 nM and a period of mitotic cycle in cell-free Xenopus egg cycling extracts. It peaks in ea rly metaphase just preceding mitotic reactivation of Cdc2 kinase and MAPK a nd reaches a minimum in interphase. The source of Ca2+ in the extracts is a particulate fraction containing egg intracellular Ca2+ stores, since the a ddition of a calcium-mobilizing second messenger, inositol 1,4,5-trisphosph ate (IP3), induced a transient increase in Ca2+. The inclusion of heparin, an IP3 receptor antagonist, or ultrafiltration of the extracts prevented Ca 2+-releasing activity of IP3. The depletion of Ca2+ in the extracts by the calcium chelator BAPTA resulted in the blockade of cell cycle at different stages, depending on the time of drug administration. The addition of BAPTA late in interphase blocked cell cycle at mitotic entry in prophase, wherea s its application in anaphase or telophase blocked the extracts in early in terphase. BAPTA administration in metaphase before transition to anaphase b rought about a metaphase-like arrest in the cycling extracts. Inhibition of IP3-induced calcium release by heparin also arrested cell cycle progressio n in the cycling extracts.