Inactivation of histone H1 kinase by Ca 2+ in rabbit oocytes

The present study investigated the role of intracellular Ca 2+ (Ca 2+ i ) elevation on the inactivation of maturation promoting factor (MPF) in rabbit oocytes. The effects of the number of Ca 2+ stimulations and of the amplitude of Ca 2+ i elevation on the profile of histone H1 kinase activity were determined. A Ca 2+ stimulation consisted of transferring mature oocytes from culture medium to 0.3 M mannitol containing 0.1–1.0 mM CaCl 2 , and pulsing them at 1.25 kV/cm for 10 μsec, or microinjecting 2–8 mM CaCl 2 into the oocyte cytoplasm. The number of electrically‐induced Ca 2+ stimulations was varied, and amplitude of the Ca 2+ i rise was controlled by altering Ca 2+ concentration in the pulsing medium or the injection pipette. Ca 2+ i concentration was determined with fura‐2 dextran; oocytes were snap‐frozen at indicated time points and assayed for H1 kinase activity. The activity was quantified by densitometry and expressed as a fraction of activity in nonstimulated oocytes. Electrically‐mediated Ca 2+ i rises inactivated H1 kinase in a manner dependent on the number of Ca 2+ stimulations. A single Ca 2+ stimulation inactivated H1 kinase to 30–40% of its initial activity. However, H1 kinase inactivation was only transient, regardless of the amplitude of the electrically‐ or injection‐mediated Ca 2+ i elevation. Increasing the number of Ca 2+ stimulations helped to maintain H1 kinase activity at basal (pronuclear) levels. The results show the necessity of a threshold of Ca 2+ i concentration to trigger MPF inactivation, and suggest a role for the extended period of time over which Ca 2+ i oscillates at fertilization. © 1995 Wiley‐Liss, Inc.