Exertional heat stroke induces mesenteric glycocalyx damage regardless of estrous cycle phase in mice

Exertional heat stroke (EHS), the most severe manifestation of heat-related illness, induces organ injury and death. Besides being highly vascularized with microvessels, the intestines are among the main organs damaged by ischemia-reperfusion during EHS. In our murine model of EHS, female mice demonstrate greater exercise capacity in the heat compared to males. However, this advantage is lost in ovariectomized mice. Additionally, ovariectomized mice presented increased intestinal damage following EHS. Since ovarian hormones levels fluctuate during the estrous cycle (EC)—the rodent equivalent of the menstrual cycle—we hypothesized that the phase of the EC could influence exercise capacity in the heat and mesenteric microvascular health following EHS. Mesenteric microvascular function was assessed 3h, 14 days (14d) and 30 days (30d) after the EHS in adult C57BL/6 female mice implanted with telemetry sensors to measure core temperature (Tc). Following familiarization with wheel running, each mouse exercised in an individual forced wheel running inside an environmental chamber (37.5°C and 40% relative humidity) until reaching loss of consciousness (LOC), a measure of exercise capacity. On the morning of the EHS, vaginal cells were collected to determine the EC phase via Wright-Giemsa staining. Estrus was identified by the predominant presence of cornified epithelial cells, while diestrus by predominantly having leukocytes. In vivo mesenteric microvascular function was assessed via intravital microscopy using an automated capture and analysis system. Mice in diestrus (D, n=23) presented a lower time to LOC (D:170.5 ± 42.9 min E: 191.6 ± 34.4 min; p=0.04), and lower maximal speed (D:6.6 ± 1.2 m/min, E: 7.4 ± 1.0 m/min; p=0.02) compared to mice in the estrus (E, n=37). Although no differences between E and D were observed regarding mesenteric microvascular function, differences were observed between 3h (E: n=4, D: n=4) and the later-recovery time-points of 14d (E: n=6 D: n=2) and 30d (E: n=4, D: n=5) post EHS. Late-recovery animals showed higher total microvascular density (3h: 93.2 ± 13.4 10 -2 mm/mm 2 , 14d: 117.3 ± 17.0 10 -2 mm/mm 2 ; 30d: 130.3 ± 37.6 10 -2 mm/mm 2 ; p=0.02), with greater perfused boundary-region, an indication of glycocalyx damage (3h: 1.65 ± 0.15 μm, 14d: 1.84 ± 0.13 μm; 30d: 1.79 ± 0.12 μm; p=0.04). In conclusion the EC phase influenced the tolerance to EHS, but not the mesenteric microvascular function. Late recovery data suggest that EHS induces sustained injury in the mesenteric microvasculature, with increased red blood cells infiltration into the glycocalyx layer. DoD. USAMRAA. HT9425-23-PRMRP-FPA Grant log number BA220322 to OL This abstract was presented at the American Physiology Summit 2025 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.