Impact of the Estrous Cycle on Intestinal Injury in Exertional Heat Stroke in Mice

Abstract

Exertional heat stroke (EHS) is the most lethal manifestation of heat illnesses. EHS is characterized by loss of consciousness (LOC), central nervous system (CNS) dysfunction, and hyperthermia during physical exertion. It leads to multiorgan damage. EHS-induced intestinal injury is one of the severe consequences that can facilitate further systemic damage. Through the process of leaky gut, intestinal injury can lead to systemic inflammatory response syndrome (SIRS). To date, a significant gap remains in our understanding of female-specific physiological responses to extreme exercise-heat stress. One contributing factor to this gap is the inability to perform hypothesis-driven research in humans as EHS can be lethal. Therefore, the use of preclinical models in mammals represents a powerful tool to help fill this gap. In female mice, the variability in EHS susceptibility and its further consequences have not been explained. Mice undergo four phases of the estrous cycle (estrus, metestrus, diestrus, and proestrus). The distinct physiological characteristics of estrous phases (estrus and diestrus) may influence the EHS-induced intestinal damage. The overall objective of this project is to investigate phase-dependent injury severity, in both early recovery (30 min, 3 h, and 24 h) and late recovery stages (14 d and 30 d) post-EHS. Knowing that in our model estrus (E) and diestrus (D) showed no differences in hypothermic depth, 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, underwent an exertional heat stroke protocol. Post-EHS, we performed terminal experiments for early and late recovery timepoints. Following data collection and preparation, we evaluated tissue integrity using Chiu scale for early and late recovery timepoints, and intestinal morphometrics 3 h post-EHS. Our analyses of Chiu injury scores from early and late recovery stages demonstrated no differences. Similarly, the morphometric analyses showed no phase-specific differences. The fact that estrus animals showed similar levels of intestinal damage to diestrus, while running significantly longer, suggests that they were relatively more protected, as they tolerated greater thermal and exercise stress before reaching similar result.