Intestinal integrity after exertional heat stroke in mice at different phases of the estrous cycle

Exertional heat stroke (EHS) is characterized by loss of consciousness (LOC), central nervous system (CNS) dysfunction in hyperthermic individuals during physical exertion. EHS leads to intestinal injury, which facilitates the process of leaky gut, leading to systemic inflammatory response syndrome (SIRS). To date, a significant gap remains in our understanding of female-specific responses to extreme exercise-heat stress. Female mice undergo four phases of the estrous cycle (estrus, metestrus, diestrus, and proestrus). We hypothesized that the distinct characteristics of the estrous phases (estrus and diestrus) may influence the EHS-induced intestinal damage. We examined whether phase-specific physiology estrus (E) vs. diestrus (D) alters the severity of intestinal injury during early and late recovery after EHS. (30 min, 3 h, and 24 h) and late recovery stages (14 d and 30 d) post-EHS. In our model, E animals ran longer than D while having a higher mortality rate. However, the surviving animals exhibited no differences in hypothermic depth, which led us to hypothesize that the level of intestinal damage would follow the same pattern. A total of 79 adult female (E: n = 42; D: n = 37) C57BL/6J mice, aged 16–17 weeks, ran in a forced wheel ergometer inside an environmental chamber set at 37.5°C and 40% RH until LOC. The estrous cycle phase was determined according to the cell type(s) present in vaginal smear before the EHS trial. Post-EHS, we performed terminal experiments for early and late recovery timepoints. A blinded grader to estrous cycle phase, was assigned to assess villi integrity for early and late recovery timepoints using Chiu scale for duodenum (DUO), jejunum (JEJ) and ileum (ILE) segments. Our analyses of Chiu injury scores showed no differences across all segments, at every timepoint: 30 min (DUO p = 0.4485; JEJ p = 0.4633; ILE p = 0.6856), 3 h (DUO p = 0.3456; JEJ p = 0.3394; ILE p = 0.6671), 24 h (DUO p = 0.5105; JEJ p = 0.2000; ILE p = 0.3007), 14 d (DUO p = 0.9681; JEJ p = 0.7988; ILE p = 0.8564), and 30 d (DUO p = 0.0962; JEJ p = 0.9009; ILE p = 0.8418). Additionally, non-survival animals (NS, 7%, all in E phase) presented lower villi integrity (DUO: E vs NS p = 0.0006, D vs NS p = 0.0006; JEJ: E vs NS p = 0.0029, D vs NS p = 0.0386; ILE: E vs NS p = 0.0059, D vs NS p = 0.0032). In conclusion, estrous cycle phase influences the tolerance to EHS without affecting intestinal damage for survival animals. However, animal in E presented a higher mortality rate, and the non-survivals presented a higher intestinal damage. Funding: DoD. USAMRAA. HT9425-23-PRMRP-FPA Grant log number BA220322 to OL. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.