A New Model of Exertional Heat Stroke in Male Mice Reveals Sustained Metabolic Dysfunction in the Myocardium

Abstract

Exertional heat stroke (EHS) is a life threating condition that has grown in prevalence with rising global temperatures. Development and use of new preclinical models are necessary to understand the pathophysiology. In our previous model of EHS, a metabolic disorder occurred in the ventricular myocardium (VM) after 14 days of recovery only in female mice. However, females were able to achieve much higher levels of exercise than males. In this study we developed a new model, specific for male mice, that prolonged exercise time and intensity but at a lower environmental temperature (Te). We then tested whether this approach would induce similar metabolic disorders of the VM in male hearts that was seen previously in females.MethodsFourteen C57BL\6 male mice were exposed to EHS by running on a forced running wheel in an environmental chamber until symptom limitation (loss of consciousness). The new model reduced Te to 34.5°C compared to 37.5°C in the original EHS model. Another group of mice was exercise‐matched to the EHS mice, but at a Te of 22.5°C. After symptom limitation was achieved, Te was kept at 37.5°C for 2 hours, post EHS. After 2 weeks of recovery from EHS, the mice were sacrificed and ventricles rapidly removed and stored at ‐80°C. The tissues were later submitted for untargeted metabolomic analysis (Metabolome, Inc. Raleigh, NC).ResultsWe found that the average max temp. achieved was 41.4±0.4°C, which is ~1°C cooler than the original EHS model. The mice lost an average of 6.7±2.1%BW during EHS. The ascending thermal load they achieved was only 54.7±41.2 °C•min, approximately ~1/3 of the original model. Within 804 metabolites identified from the VM, 113 were significantly (P < 0.05) elevated; whereas, 15 were significantly reduced. Pathway categories of prominently and significantly changed metabolites included the: 1) pentose phosphate:­ribose 1‐P, ­ribonate; 2) glycolysis:­glucose, ­pyruvate, ­fructose, ­fructose‐6‐phosphate, decreased 3‐phosphoglycerate; 3) TCA cycle: ­isocitrate, ­itaconate, ­succylcarnitine, ­succinate; 4) methionine metabolism:­S‐methylmethionine, ­N‐formylmethionine, ­S‐adenosylmethionine, ­4 MeTHF; 5) antioxidant defense: ­glutathione, ­cysteine, ­cystathionine, ­ascorbate;6) fat metabolism: ­variety of long and med chain fatty acids, ­carnitine, ­deoxycarnitine, decreases in a variety of acylcarnitines, and 6) injury, inflammation and stress:­corticosterone, ­kynurenine, ­triethylamine N‐oxide, ­N‐methylproline, and ­AICA.ConclusionsBy increasing the time and intensity of exercise in the heat, even at a much lower Te, male mice exhibited strikingly similar metabolic disorders in the myocardium, after 2 weeks of recovery, that were previously reported in female mice only. These results suggest that it is not the actual thermal load or the max temperature that is critical, but rather the “exertional” load or time of exercise exposure in a warm environment that is the trigger for inducing profound metabolic disorders in the VM.