Shear stress induced by acute heat exposure is not obligatory to protect against endothelial ischemia-reperfusion injury in humans

Home-based heat therapy lowers blood pressure and improves endothelial function in older adults

Advancing age is associated with vascular dysfunction and hypertension, both of which increase cardiovascular event risk. Heat therapy has emerged as a novel intervention to improve cardiovascular health in various populations. Therefore, we tested the hypothesis that home-based lower body heat therapy would reduce blood pressure and improve endothelium-dependent vasodilation in older adults. Ambulatory blood pressure monitoring was performed in 19 older adults (67 ± 7 yr) before and after 8 wk of a sham intervention or heat therapy. Endothelium-dependent vasodilation of the superficial femoral artery was assessed via flow-mediated dilation. Participants were provided with a pair of tube-lined pants connected to a portable water circulator to perform the home-based sessions. Water temperature was set to 31°C for sham and 51°C for heat therapy, resulting in target skin temperatures of ∼33°C and ∼40°C, respectively. Participants were instructed to wear the pants 4 days/wk for 60 min each session. Adherence was 100% for both groups. Heat therapy reduced ambulatory daytime systolic blood pressure by Δ -5 ± 8 mmHg, but was unchanged for the sham group (Δ 1 ± 6 mmHg; P = 0.04). Likewise, heat therapy increased flow-mediated dilation (P = 0.02), whereas there was no change across time for the sham group (P = 0.5). These results combined with a strong adherence rate suggest that home-based lower body heat therapy could be an alternative nonpharmacological intervention to reduce blood pressure and improve vascular function, ultimately reducing long-term cardiovascular event risk in older adults.NEW & NOTEWORTHY Advancing age is associated with vascular dysfunction and hypertension, both of which increase cardiovascular event risk. This study determined that 8 wk of home-based lower body heat therapy reduced ambulatory daytime systolic blood pressure and increased flow-mediated dilation of the superficial femoral artery, outcomes not observed in the sham group. These improvements, coupled with 100% adherence among participants, suggest that home-based heat therapy is a pragmatic and effective strategy for improving cardiovascular health in older adults.

Meta-analysis of heat-induced changes in cardiac function from over 400 laboratory-based heat exposure studies

Heat waves are associated with increased fatalities from adverse cardiovascular events attributed to the negative effects of heat on cardiac function. However, scientific understanding of acute cardiac adjustments to heat has come primarily from laboratory experiments employing insulated and encapsulated heating modalities, most commonly water-perfused suits. We evaluated whether findings from those studies reflect cardiac responses during more natural exposures to hot ambient conditions simulated in climate-controlled chambers by synthesizing the findings from over 400 laboratory-based heat exposure studies (6858 participant-exposures) published between 1961-2024. Among all included studies, median (interquartile range) elevations in core temperature and heart rate from baseline to end-exposure were 0.9 (0.5-1.3)°C and 27 (15-40) beats/min. Multilevel mixed-effects meta-analyses revealed exacerbated elevations in heart rate, cardiac output, and rate pressure product (estimate of cardiac workload) and blunted falls in systolic pressure in participants heated via encapsulated modalities. Leveraging the large dataset, we also provide empirical estimates of body temperature and cardiovascular responses to a wide range of conditions experienced during heat waves. With rising global temperatures, ecologically-minded physiological research is needed to improve understanding of the effects of heat stress on cardiac responses and further the development of robust climate health models and evidence-based heat-health guidance.

Shear Stress and Endothelial Mechanotransduction in Trauma Patients with Hemorrhagic Shock: Hidden Coagulopathy Pathways and Novel Therapeutic Strategies

Massive trauma remains a leading cause of death and a global public health burden. Post-traumatic coagulopathy may be present even before the onset of resuscitation, and correlates with severity of trauma. Several mechanisms have been proposed to explain the development of abnormal coagulation processes, but the heterogeneity in injuries and patient profiles makes it difficult to define a dominant mechanism. Regardless of the pattern of death, a significant role in the pathophysiology and pathogenesis of coagulopathy may be attributed to the exposure of endothelial cells to abnormal physical forces and mechanical stimuli in their local environment. In these conditions, the cellular responses are translated into biochemical signals that induce/aggravate oxidative stress, inflammation, and coagulopathy. Microvascular shear stress-induced alterations could be treated or prevented by the development and use of innovative pharmacologic strategies that effectively target shear-mediated endothelial dysfunction, including shear-responsive drug delivery systems and novel antioxidants, and by targeting the venous side of the circulation to exploit the beneficial antithrombogenic profile of venous endothelial cells.

Deficiency of neutrophil high-mobility group box-1 in liver transplant recipients exacerbates early allograft injury in mice

BACKGROUND AND AIMS

Early allograft dysfunction (EAD) is a severe event leading to graft failure after liver transplant (LT). Extracellular high-mobility group box-1 (HMGB1) is a damage-associated molecular pattern that contributes to hepatic ischemia-reperfusion injury (IRI). However, the contribution of intracellular HMGB1 to LT graft injury remains elusive. We hypothesized that intracellular neutrophil-derived HMGB1 from recipients protects from post-LT EAD.

APPROACH AND RESULTS

We generated mice with conditional ablation or overexpression of Hmgb1 in hepatocytes, myeloid cells, or both. We performed LTs and injected lipopolysaccharide (LPS) to evaluate the effect of intracellular HMGB1 in EAD. Ablation of Hmgb1 in hepatocytes and myeloid cells of donors and recipients exacerbated early allograft injury after LT. Ablation of Hmgb1 from liver grafts did not affect graft injury; however, lack of Hmgb1 from recipient myeloid cells increased reactive oxygen species (ROS) and inflammation in liver grafts and exacerbated injury. Neutrophils lacking HMGB1 were more activated, showed enhanced pro-oxidant and pro-inflammatory signatures, and reduced biosynthesis and metabolism of inositol polyphosphates (InsPs). On LT reperfusion or LPS treatment, there was significant neutrophil mobilization and infiltration into the liver and enhanced production of ROS and pro-inflammatory cytokines when intracellular Hmgb1 was absent. Depletion of neutrophils using anti-Ly6G antibody attenuated graft injury in recipients with myeloid cell Hmgb1 ablation.

CONCLUSIONS

Neutrophil HMGB1 derived from recipients is central to regulate their activation, limits the production of ROS and pro-inflammatory cytokines, and protects from early liver allograft injury.

Effect of acute heat exposure on the pressor response to a voluntary hypoxic apnea

The pressor response induced by a voluntary hypoxic apnea is mediated largely by increased sympathetic outflow. The neural control of blood pressure is altered in recovery from acute heat exposure, but its effect on the pressor response to a voluntary hypoxic apnea has never been explored. Therefore, we tested the hypothesis that prior heat exposure would attenuate the pressor response induced by a voluntary hypoxic apnea. Eleven healthy adults (five women) were exposed to whole body passive heating (water-perfused suit) sufficient to increase body core temperature by 1.2°C. Voluntary hypoxic apneas were performed at baseline and in recovery when body core temperature returned to ≤ 0.3°C of baseline. Participants breathed gas mixtures of varying [Formula: see text] (21%, 16%, and 12%; randomized) for 1 min followed by a 15-s end-expiratory apnea. The change in arterial oxygen saturation during each apnea did not differ from baseline to recovery (P = 0.6 for interaction), whereas the pressor response induced by a voluntary hypoxia apnea was reduced ([Formula: see text] 21%, baseline 17 ± 7 mmHg vs. recovery 14 ± 7 mmHg; [Formula: see text] 16%, baseline 24 ± 8 mmHg vs. recovery 18 ± 7 mmHg; [Formula: see text] 12%, baseline 28 ± 11 mmHg vs. recovery 24 ± 11 mmHg; P = 0.01 for main effect of time). These data suggest that prior heat exposure induces a cross-stressor effect such that the pressor response to a voluntary hypoxic apnea is attenuated.NEW & NOTEWORTHY The pressor response induced by a voluntary hypoxic apnea is mediated by increased sympathetic outflow. The neural control of blood pressure is altered in recovery from acute heat exposure, but its effect on the pressor response to a voluntary hypoxic apnea has never been explored. Our data suggest that prior heat exposure induces a cross-stressor effect such that the pressor response to a voluntary hypoxic apnea is attenuated.

Cardiovascular Adjustments After Acute Heat Exposure

In this review, we highlight recent studies from our group and others that have characterized the cardiovascular adjustments that occur after acute heat exposure. Special emphasis will be placed on underlying mechanisms and clinical implications. Finally, we postulate that these acute cardiovascular adjustments may predict the long-term adaptive response to chronic heat therapy.

Acute heat exposure protects against endothelial ischemia-reperfusion injury in aged humans

Nonpharmacological therapies that protect against endothelial ischemia-reperfusion injury (I/R) remain limited in aged adults. Acute heat exposure protects against endothelial I/R injury in young adults, but its efficacy has never been explored in aged adults. Therefore, we tested the hypothesis that acute heat exposure would prevent the attenuation of endothelium-dependent vasodilation after I/R injury in aged adults. Nine (2 men, 69 ± 8 yr) aged adults were exposed to a thermoneutral control condition or whole body passive heating (water-perfused suit) sufficient to increase body core temperature by 1.2°C. Experiments were separated by at least 7 days. Heat exposure was always performed first to time match the thermoneutral control condition. Endothelium-dependent vasodilation was assessed via flow-mediated dilation of the brachial artery before (pre-I/R) and after I/R injury (post-I/R), which was induced by 20 min of arm ischemia followed by 20 min of reperfusion. Flow-mediated dilation was reduced following I/R injury for the thermoneutral control condition (pre-I/R, 4.5 ± 2.9% vs. post-I/R, 0.9 ± 2.8%, P < 0.01), but was well maintained with prior heat exposure (pre-I/R, 4.4 ± 2.8% vs. post-I/R, 3.5 ± 2.8%, P = 0.5). Taken together, acute heat exposure protects against endothelial I/R injury in aged adults. These results highlight the therapeutic potential of heat therapy to prevent endothelial dysfunction associated with I/R injury in aged adults who are most at risk for an ischemic event.