Induction of ornithine decarboxylase in rat ovary after administration of luteinizing hormone or human chorionic gonadotrophin

Abstract

Ornithine decarboxylase in rat ovary is stimulated 5- to 10-fold by a subcutaneous injection of luteinizing hormone (LH) or human chorionic gonadotrophin (HCG). A peak is observed between 3 and 5 hr after which the enzyme activity declines rapidly. The pattern of enzyme response to hormonal administration is similar to that observed during late proestrus. S-adenosylmethionine decarboxylase is not altered during this period or after administration of LH or HCG. Cycloheximide administered in vivo markedly reduced ovarian ornithine decarboxylase within 15 min, suggesting that the turnover of this enzyme in the ovary is very rapid. When injected immediately prior to HCG, cycloheximide also prevented the rise in ornithine decarboxylase activity. High doses of actinomycin d were also effective in blocking the response to HCG. Ornithine decarboxylase responds to a single injection of HCG at all stages of the estrus cycle, but repeated injections do not prevent the decline in enzyme activity occurring between 5 and 7 hr. Anti-HCG injected immediately before the primary hormone completely inhbited the rise in ornithine decarboxylase, but when given 1 hr after the primary injection, it was ineffective. It is suggested that these characteristics of ornithine decarboxylase, i.e. high rate of turnover, rapid induction and the inability of repeated hormone injections to prevent the eventual decline of enzyme activity, ensure that the increase in the formation of putrescine during proestrus is restricted to a relatively narrow time period, thus supporting the view that putrescine may have a specific role in the regulation of protein and RNA synthesis involved in the early phase of LH action on the ovary.