Regulation of ovarian ornithine decarboxylase by human chorionic gonadotrophin

Treatment of 29-day-old female Sprague-Dawley rats with human chorionic gonadotrophin (hCG) produced a large and rapid increase in the activity of ornithine decarboxylase. Measurements that use a specific radioimmunoassay showed that the increased activity could be accounted for by a parallel change in the amount of ornithine decarboxylase protein. The increased protein content was caused by an increased rate of synthesis, since the half-life of ornithine decarboxylase was not changed by the hormone treatment. The content of mRNA for ornithine decarboxylase was determined by hybridization with a cDNA probe, and it was found that the increased amount of protein was correlated with a change in the amount of mRNA. These results indicate that treatment with hCG induces ornithin decarboxylase in the rat ovary by increasing the production or the stability of the mRNA for this enzyme. The increased amount of ornithine decarboxylase led to an increase in putrescine in the ovary but did not increase the content of the polyamines spermidine and spermine. These findings show that, despite its rapid and large scale induction, ornithine decarboxylase is not the rate-limiting step that determines the content of these polyamines in this tissue. They also suggest that putrescine itself may play an important role in the ovary.