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 kinase activity were determined. A Ca2+ stimulation consisted of transferring 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 microinjecting 2-8 mM CaCl2 into the oocyte cytoplasm. The number of electrically-induced Ca2+ stimulations was varied, and amplitude of the Ca-i(2+) rise was controlled by altering Ca2+ concentration in the pulsing medium or the injection pipette. Ca-i(2+) 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-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+) elevation. Increasing the number of Ca2+ stimulations helped to maintain H1 kinase activity at basal (pronuclear) levels. The results show the necessity of a threshold of Ca-i(2+) concentration to trigger MPF inactivation, and suggest a role for the extended period of time over which Ca-i(2+) oscillates at fertilization. (C) 1995 Wiley Liss, Inc.