Myocardial oxidative stress and inflammation emerges during recovery from exertional heat stroke in female but not male mice
Authors: Garcia Christian K. , Robinson Gerard P. , Mattingly Alex J. , Iwaniec John D. , De Carvalho Lucas , Alzahrani Jamal , Murray Kevin O. , Laitano Orlando , Clanton Thomas L. 
Published: April 2020

INTRODUCTION

Epidemiological reports have shown an association between heat stroke and increased risk of cardiovascular disease later in life. Using a preclinical model of exertional heat stroke (EHS), we reported metabolic disturbances in the myocardium of female mice that emerge after 9–14 d of recovery. However, it is unknown whether these metabolic changes are accompanied by oxidative stress and inflammation in the myocardium.

PURPOSE

To determine whether EHS exposure induces oxidative stress and inflammation in the ventricular myocardium during recovery.

METHODS

Male and female C57BL6 mice underwent surgical implantation of telemetry devices to monitor core temperature, followed by 3 weeks of exercise wheel running, and an EHS trial, consisting of forced wheel running in 37.5°C, 40% humidity. Results were compared to matched exercise controls (EXC), that performed the same exercise at ~22°C and recovered for 4 d. Ventricular samples were collected at 30 min, 3 h, 24 h, 4 d, 9 d and 14 d post EHS for metabolomics testing (n=8/group). Portions of the ventricles from the 14 d time point were also preserved in formalin, sectioned and stained with H&E. The entire cross section of each ventricular section was divided into a grid and each section of the grid graded from 0–3 based on severity of inflammation.

RESULTS

Female mice exhibited greater heat tolerance but were exposed to much higher durations of exercise and heat load during the EHS protocol, as previously described. Metabolomic analyses determined that the myocardium of females exhibited multiple markers of oxidative stress that were not seen in males. Glutathione levels decreased by ~40% by 4 d and recovered at later timepoints. Oxidized glutathione increased by ~25% at 24 h post EHS and returned to baseline levels in later recovery periods. The oxidation product, 13‐HODE/9‐HODE continued to increase beyond 3 h post EHS, reaching ~400% of EXC (P<0.001) at 14 d. Other markers such as 4‐hydroxy‐nonenal glutathione, methionine sulfone and trimethylamine N‐oxide showed biphasic responses, characterized by a significant elevation at 30 min, a recovery at 3 h, and then large elevations, reaching 200%–400% of EXC. Histological markers of inflammation were also elevated in females in the 14 d EHS hearts compared to EXC (P<0.001). This was not seen in male hearts.

CONCLUSIONS

Our data suggest that EHS in females, but not males, results in oxidative stress and increased inflammation in the ventricular myocardium during recovery. We speculate that sex differences arose from either the greater heat load experienced by females or by unique sex‐dependent differences in metabolic control. Though the translation of these results to the human condition is highly speculative, it is possible that such EHS‐induced cardiovascular events may comprise an origin of the cardiovascular susceptibilities to disease later in life that have been previously observed in heat stroke victims.

Support or Funding Information

Supported by DoD Grant W81XWH‐15‐2‐0038 and the BK and Betty Stevens Professorship. Author’s views do not reflect US Army or DoD policy or endorsements.