Histone H1 kinase activity in bovine oocytes following calcium stimulation

Abstract The influence of number of Ca 2+ stimulations on the profile of histone H1 kinase activity in bovine oocytes was investigated. A Ca 2+ stimulation consisted of transferring oocytes directly from culture medium to mannitol containing 100 μM Ca 2+ and pulsing oocytes with a 0.2 kVcm −1 , 20 μsec discharge. One, three, or six Ca 2+ stimulations were given, each 22 min apart. Oocytes were frozen from 0 to 8 hr after the first stimulation at indicated time points and assayed for histone H1 kinase activity. H1 kinase activity was quantified using a densitometer and expressed as a percent of activity in nonpulsed metaphase II oocytes. Stimulating oocytes in the absence of Ca 2+ in the pulsing medium did not inactivate H1 kinase. In the presence of Ca 2+ , however, H1 kinase was rapidly inactivated after stimulation. A single stimulation decreased H1 kinase activity to 44% ± 11% of its initial level in 1 hr. However, H1 kinase was dramatically reactivated at 2 hr after the stimulation and reached 122% ± 22% of the initial activity at 6 hr. With three stimulations, basal H1 kinase activity was 21% ± 3% and was obtained in 30 min. H1 kinase reactivation started at 4 hr after the first stimulation and level of activity reached 38% ± 15% at 8 hr. Six stimulations also led to rapid H1 kinase inactivation and to a basal activity of 14% ± 0.4%. With six stimulations, however, basal H1 kinase activity was maintained over at least 8 hr, and no reactivation occurred during this period. Basal H1 kinase activity obtained after six stimulations was similar to that of fertilized oocytes. Immunoprecipitation of p34 cdc2 with an anti‐cdc2 antibody strongly suggested an identity between histone H1 kinase and maturation‐promoting factor. The data indicate that histone H1 kinase activity in oocytes could be regulated by the number of Ca 2+ stimulations. A single Ca 2+ stimulation led to H1 kinase inactivation, followed by reactivation of the kinase. Increasing the number of Ca 2+ stimulations delayed the onset and reduced the extent of H1 kinase reactivation in the first parthenogenetic cell cycle. © 1993 Wiley‐Liss, Inc.