INACTIVATION OF HISTONE H1 KINASE BY CA2+ IN RABBIT OOCYTES

The present study investigated the role of intracellular Ca2+ (Ca-i(2)) elevation on the inactivation of maturation promoting factor (MPF) in rabbit oocytes. The effects of the number of Ca2+ stimulations and of the amplitude of Ca-i(2+) elevation on the profile of histone H1 ki nase activity were determined. A Ca2+ stimulation consisted of transfe rring mature oocytes from culture medium to 0.3 M mannitol containing 0.1-1.0 mM CaCl2, and pulsing them at 1.25 kV/cm for 10 mu sec, or mic roinjecting 2-8 mM CaCl2 into the oocyte cytoplasm. The number of elec trically-induced Ca2+ stimulations was varied, and amplitude of the Ca -i(2+) rise was controlled by altering Ca2+ concentration in the pulsi ng medium or the injection pipette. Ca-i(2+) concentration was determi ned with fura-2 dextran; oocytes were snap-frozen at indicated time po ints and assayed for H1 kinase activity. The activity was quantified b y densitometry and expressed as a fraction of activity in nonstimulate d oocytes. Electrically-mediated Ca-i(2+) rises inactivated H1 kinase in a manner dependent on the number of Ca2+ stimulations. A single Ca2 + 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-i(2+) elevati on. Increasing the number of Ca2+ stimulations helped to maintain H1 k inase activity at basal (pronuclear) levels. The results show the nece ssity of a threshold of Ca-i(2+) concentration to trigger MPF inactiva tion, and suggest a role for the extended period of time over which Ca -i(2+) oscillates at fertilization. (C) 1995 Wiley Liss, Inc.