Ovarian carbonyl reductase activity in rats at persistent oestrus

Age-related changes and effects of gonadotrophins on ovarian carbonyl reductase (CR) of virgin female Wistar-Imamichi rats were investigated. In mature rats ovarian CR is an LH-dependent oxidoreductase and is closely involved in the periovulatory process. After 8 months of age, all of the rats exhibited persistent oestrus and the ovarian mass was significantly decreased, whereas 4- and 5-month-old rats showed the regular 4 day oestrous cycles and ovulation occurred. The ovarian CR activity towards four substrates declined steeply consistent with persistent oestrus and the enzyme activity for each substrate at 15 months of age was 1.4-53.7% lower than at 4 months of age. The ovarian CR concentration measured by western blot analysis was also lower at 8 months of age. Equine chorionic gonadotrophin (eCG) and hCG were administered to 10- and 15-month-old rats at persistent oestrus to examine whether the ovarian CR in rats at persistent oestrus is influenced by gonadotrophins. Treatment with hCG not only increased ovarian CR activity and its concentration but also caused ovulation at both ages. The immunoreactivity to anti-ovarian CR antibody in the theca cells of the hCG-treated ovary was greater than that of the control ovary. Equine(e) CG did not stimulate ovarian CR activity in rats at persistent oestrus, although ovarian mass of 10-month-old rats was increased after administration of 50 iu of eCG. These results indicate that changes in ovarian CR with ageing are related to the transition of regular oestrous cyclicity to acyclicity and that the expression of ovarian CR markedly decreased in aged rats is stimulated by hCG.