Identification of therapeutic targets in a murine model of severe exertional heat stroke.

Exertional heat stroke (EHS) is a potentially lethal condition resulting from high core body temperatures (TC) in combination with a systemic inflammatory response syndrome (SIRS) with varying degrees of severity across victims, and limited understanding of the underlying mechanism(s). We established a mouse model of severe EHS to identify mechanisms of hyperthermia/inflammation that may be responsible for organ damage. Mice were forced to run on a motorized wheel in a 37.5°C chamber until loss of consciousness and were either removed immediately (exertional heat injury or EHI; TCMax = 42.4 ± 0.2°C) or remained in the chamber an additional 20 min (EHS; TCMax = 42.5 ± 0.4°C). Exercise control mice (ExC) experienced identical procedures to EHS at 25°C. At 3 h post-EHS, there was evidence for an immune/inflammatory response as elevated blood chemokine [interferon γ-induced protein 10 (IP-10), keratinocytes-derived chemokine (KC), macrophage inflammatory proteins (MIP-1α), MIP-1ß, MIP-2] and cytokine [granulocyte colony-stimulating factor (G-CSF), interleukins (IL-10), IL-6] levels peaked and were highest in EHS mice compared with EHI and ExC mice. Immunoblotting of organs susceptible to EHS damage indicated that several kinases were sensitive to stress associated with heat/inflammation and exercise; specifically, phosphorylation of liver c-Jun NH2-terminal kinase (JNK) at threonine 183/tyrosine 185 immediately (0 h) postheating related to heat illness severity. We have established a mouse EHS model, and JNK [or its downstream target(s)] could underlie EHS symptomatology, allowing the identification of molecular pathways or countermeasure targets to mitigate heat illness severity, enable complete recovery, and decrease overall EHS-related fatalities.

Vancomycin modestly attenuates symptom severity during onset of and recovery from exertional heat stroke in mice.

Increased intestinal permeability during exertion and subsequent leakage of bacteria into circulation is hypothesized to accelerate exertional heat stroke (EHS) onset and/or exacerbate EHS severity. To provide proof of concept for this theory, we targeted intestinal microbiota via antibiotic prophylaxis and determined whether vancomycin would delay EHS onset and/or mitigate EHS severity and mortality rates using a mouse model of EHS. Mice were 1) designated as EHS or Exercise Control (ExC) and 2) given 7 days of vancomycin (VEHS, VExC) or untreated water (EHS, ExC) before EHS/Exercise. Following EHS/ExC, mice were euthanized immediately (0 h) or returned to their home cage (25°C) and euthanized after 3 h or 24 h. VEHS mice exhibited reduced abundance and altered composition of fecal bacteria (with notable decreases in genera within orders Clostridiales and Bacteroidales); increased water consumption, lower core temperature (TC) before and during heating (TCMax), lower circulating markers of organ damage and inflammation at 24 h; and reduced hepatic activation of stress pathways at 0 and 3 h compared with EHS mice. Vancomycin-induced alterations to the intestinal microbiota likely influenced EHS outcomes, but it is unconfirmed whether this is due to attenuated bacterial leakage into circulation or other (in)direct effects on physiology and behavior (e.g., decreased TC, increased water consumption). To our knowledge, this is the first study quantitating antibiotic effects in conscious/unanesthetized, exertional HS animals.NEW & NOTEWORTHY Vancomycin prophylaxis lowered core temperature before and during EHS, mitigated EHS-associated rise of hepatic biomarkers and cytokines/chemokines in circulation (particularly at 24 h), and corresponded to inhibited phosphorylation of hepatic c-Jun NH2-terminal kinase on Threonine 183/Tyrosine 185 at 0 and 3 h in conscious, unanesthetized mice. However, vancomycin also induced cecal enlargement suggesting its off-target effects could limit its utility against EHS.

Amino acid solution mitigates hypothermia response and intestinal damage following exertional heat stroke in male mice.

Increased gut permeability is implicated in the initiation and extent of the cytokine inflammatory response associated with exertional heat stroke (EHS). The primary objective of this study was to determine if a five amino acid oral rehydration solution (5AAS), specifically designed for the protection of the gastrointestinal lining, would prolong time to EHS, maintain gut function and dampen the systemic inflammatory response (SIR) measured during EHS recovery. Male C57/BL6J mice instrumented with radiotelemetry were gavaged with 150 µL of 5AAS or H2 O, and ≈12 h later were either exposed to an EHS protocol where mice exercised in a 37.5°C environmental chamber to a self-limiting maximum core temperature (Tc,max) or performed the exercise control (EXC) protocol (25°C). 5AAS pretreatment attenuated hypothermia depth and length (p < 0.005), which are indicators of EHS severity during recovery, without any effect on physical performance or thermoregulatory responses in the heat as determined by percent body weight lost (≈9%), max speed (≈6 m/min), distance (≈700 m), time to Tc,max (≈160 min), thermal area (≈550°C∙min), and Tc,max (42.2°C). EHS groups treated with 5AAS showed a significant decrease in gut transepithelial conductance, decreased paracellular permeability, increased villus height, increased electrolyte absorption and changes in tight junction protein expression pattern suggestive of improved barrier integrity (p < 0.05). No differences were witnessed between EHS groups in acute phase response markers of liver, circulating SIR markers, or indicators of organ damage during recovery. These results suggest that a 5AAS improves Tc regulation during EHS recovery through maintaining mucosal function and integrity.

Impact of successive exertional heat injuries on thermoregulatory and systemic inflammatory responses in mice.

The purpose of the study was to determine if repeated exertional heat injuries (EHIs) worsen the inflammatory response. We assessed the impact of a single EHI bout (EHI0) or two separate EHI episodes separated by 1 (EHI1), 3 (EHI3), and 7 (EHI7) days in male C57BL/6J mice (n = 236). To induce EHI, mice underwent a forced running protocol until loss of consciousness or core temperature reached ≥ 42.7°C. Blood and tissue samples were obtained 30 min, 3 h, 1 day, or 7 days after the EHI. We observed that mice undergoing repeated EHI (EHI1, EHI3, and EHI7) had longer running distances before collapse (∼528 m), tolerated higher core temperatures (∼0.18°C higher) before collapse, and had higher minimum core temperature (indicative of injury severity) during recovery relative to EHI0 group (∼2.18°C higher; all P < 0.05). Heat resilience was most pronounced when latency was shortest between EHI episodes (i.e., thermal load and running duration highest in EHI1), suggesting the response diminishes with longer recoveries between EHI events. Furthermore, mice experiencing a second EHI exhibited increased serum and liver HSP70, and lower corticosterone, FABP2, MIP-1ß, MIP-2, and IP-10 relative to mice experiencing a single EHI typically at 30 min to 3 h after EHI. Our findings indicate that an EHI event may initiate some adaptive processes that provide acute heat resilience to subsequent EHI conditions. NEW & NOTEWORTHY Mice undergoing repeated exertional heat injuries, within 1 wk of an initial heat injury, appear to have some protective adaptations. During the second exertional heat injury, mice were able to run longer and sustain higher body temperatures before collapse. Despite this, the mice undergoing a second exertional heat injury were more resilient to the heat as evidenced by attenuated minimum body temperature, higher HPS70 (serum and liver), lower corticosterone, and lower FABP2.