Figure 4

Hypothetical model of ovarian steroid mechanisms regulating the uterine redox environment. Smaller preovulatory follicles produce less estradiol at proestrus/estrus as well as less progesterone during early diestrus, compared to larger follicles. The endocrine milieu associated with smaller preovulatory follicles is characterized by reduced abundance of SOD1 and SOD2 transcripts, GPx activity, and CAT activity (left side). Such conditions are prone to increased oxidative stress, resulting in greater lipid peroxidation. Compensatory mechanisms, such as increased total SOD activity, maintain intra-uterine redox homeostasis, as suggested by the similar reactive species concentrations between the two groups. An alternative, integrative explanation (right side) for the results is that the high SOD activity in the presence of reduced CAT and GPx activity could lead to high levels of free hydroxyl radicals that are highly reactive, and could increase lipid peroxidation. We speculate that changes in the mechanisms controlling the redox status in the uterus of cows ovulating smaller follicles are associated with the lower fertility reported for this category of animals.