Histone H1 kinase activity in bovine oocytes following calcium stimulation

Abstract

The influence of number of Ca²⁺ stimulations on the profile of histone H1 kinase activity in bovine oocytes was investigated. A Ca²⁺ stimulation consisted of transferring oocytes directly from culture medium to mannitol containing 100 μM Ca²⁺ and pulsing oocytes with a 0.2 kVcm⁻¹, 20 μsec discharge. One, three, or six Ca²⁺ 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 Ca²⁺ in the pulsing medium did not inactivate H1 kinase. In the presence of Ca²⁺, 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 Ca²⁺ stimulations. A single Ca²⁺ stimulation led to H1 kinase inactivation, followed by reactivation of the kinase. Increasing the number of Ca²⁺ stimulations delayed the onset and reduced the extent of H1 kinase reactivation in the first parthenogenetic cell cycle.