Heat acclimation and cross-tolerance against novel stressors: genomic–physiological linkage

Cross-adaptation from heat stress to hypoxia: A systematic review and exploratory meta-analysis

Cross-adaptation (CA) refers to the successful induction of physiological adaptation under one environmental stressor (e.g., heat), to enable subsequent benefit in another (e.g., hypoxia). This systematic review and exploratory meta-analysis investigated the effect of heat acclimation (HA) on physiological, perceptual and physical performance outcome measures during rest, and submaximal and maximal intensity exercise in hypoxia. Database searches in Scopus and MEDLINE were performed. Studies were included when they met the Population, Intervention, Comparison, and Outcome criteria, were of English-language, peer-reviewed, full-text original articles, using human participants. Risk of bias and study quality were assessed using the COnsensus based Standards for the selection of health status Measurement INstruments checklist. Nine studies were included, totalling 79 participants (100 % recreationally trained males). The most common method of HA included fixed-intensity exercise comprising 9 ± 3 sessions, 89 ± 24-min in duration and occurred within 39 ± 2 °C and 32 ± 13 % relative humidity. CA induced a moderate, beneficial effect on physiological measures at rest (oxygen saturation: g = 0.60) and during submaximal exercise (heart rate: g = -0.65, core temperature: g = -0.68 and skin temperature: g = -0.72). A small effect was found for ventilation (g = 0.24) and performance measures (peak power: g = 0.32 and time trial time: g = -0.43) during maximal intensity exercise. No effect was observed for perceptual outcome measures. CA may be appropriate for individuals, such as occupational or military workers, whose access to altitude exposure prior to undertaking submaximal activity in hypoxic conditions is restricted. Methodological variances exist within the current literature, and females and well-trained individuals have yet to be investigated. Future research should focus on these cohorts and explore the mechanistic underpinnings of CA.

A century of exercise physiology: concepts that ignited the study of human thermoregulation. Part 4: evolution, thermal adaptation and unsupported theories of thermoregulation

This review is the final contribution to a four-part, historical series on human exercise physiology in thermally stressful conditions. The series opened with reminders of the principles governing heat exchange and an overview of our contemporary understanding of thermoregulation (Part 1). We then reviewed the development of physiological measurements (Part 2) used to reveal the autonomic processes at work during heat and cold stresses. Next, we re-examined thermal-stress tolerance and intolerance, and critiqued the indices of thermal stress and strain (Part 3). Herein, we describe the evolutionary steps that endowed humans with a unique potential to tolerate endurance activity in the heat, and we examine how those attributes can be enhanced during thermal adaptation. The first of our ancestors to qualify as an athlete was Homo erectus, who were hairless, sweating specialists with eccrine sweat glands covering almost their entire body surface. Homo sapiens were skilful behavioural thermoregulators, which preserved their resource-wasteful, autonomic thermoeffectors (shivering and sweating) for more stressful encounters. Following emigration, they regularly experienced heat and cold stress, to which they acclimatised and developed less powerful (habituated) effector responses when those stresses were re-encountered. We critique hypotheses that linked thermoregulatory differences to ancestry. By exploring short-term heat and cold acclimation, we reveal sweat hypersecretion and powerful shivering to be protective, transitional stages en route to more complete thermal adaptation (habituation). To conclude this historical series, we examine some of the concepts and hypotheses of thermoregulation during exercise that did not withstand the tests of time.

Characteristics of menstrual cycle disorder and saliva metabolomics of young women in a high-temperature environment

Objective: Menstrual disorders induced by high-temperature environments, can seriously damage women's reproductive health and workability. The regulation mechanism underlying it is not yet to be elucidated. Saliva is an information-rich biological fluid that can reflect systemic diseases. Here, we investigated the characteristics of menstrual cycle disorders and saliva metabolomics to provide a deeper insight of the regulation mechanism of young women in high-temperature environments. Methods: Women from high and normal temperature areas of China were selected and divided into two groups-high-temperature (H group) and control (C group). A questionnaire survey was conducted in summer (July) to investigate the incidence rate of menstrual disorders, characteristics of the disorders, and factors influencing the risk of these disorders in different regions. Metabolomics was applied to analyze the characteristics of the salivary metabolites and neurotransmitters in the two groups of women with menstrual disorders. Results: The incidence rate of menstrual disorders was significantly higher in the H group than that in the C group (p < 0.05). High-temperature environment, stress, and sleep quality were identified as critical factors associated with menstrual disorders. Non-targeted saliva metabolomics identified 64 significantly different metabolites between two groups, which mainly enriched in metabolic pathways such as carbohydrate metabolism, membrane transport, digestive system, and nucleotide metabolism (p < 0.05). N-acetylneuraminic acid, MYO, and tyramine may be candidate markers for early diagnosis of menstrual disorders in high temperature environments. Metabolites may be involving in the acute-phase response during an inflammatory process, to affecting the reproductive system by influencing the HPA axis loop. Regulations about oocyte membrane production and the luteal functions would be exerted in menstrual disorders. Targeted metabolomics of neurotransmitters revealed increased expression of histamine (HA) and glutamine and decreased expression of 5-hydroxyindole acetic acid (5-HIAA) (p < 0.05). Conclusion: Menstrual disorder characteristics induced by high temperature environments were specific. Anxiety, sleep quality and temperature feeling were the key factors to the menstrual disorder. endocrine regulation mechanism and inflammatory reactions might contribute to the development of menstrual disorders through influencing the formation of the follicular cell membrane.

Classic and exertional heatstroke

In the past two decades, record-breaking heatwaves have caused an increasing number of heat-related deaths, including heatstroke, globally. Heatstroke is a heat illness characterized by the rapid rise of core body temperature above 40 °C and central nervous system dysfunction. It is categorized as classic when it results from passive exposure to extreme environmental heat and as exertional when it develops during strenuous exercise. Classic heatstroke occurs in epidemic form and contributes to 9-37% of heat-related fatalities during heatwaves. Exertional heatstroke sporadically affects predominantly young and healthy individuals. Under intensive care, mortality reaches 26.5% and 63.2% in exertional and classic heatstroke, respectively. Pathological studies disclose endothelial cell injury, inflammation, widespread thrombosis and bleeding in most organs. Survivors of heatstroke may experience long-term neurological and cardiovascular complications with a persistent risk of death. No specific therapy other than rapid cooling is available. Physiological and morphological factors contribute to the susceptibility to heatstroke. Future research should identify genetic factors that further describe individual heat illness risk and form the basis of precision-based public health response. Prioritizing research towards fundamental mechanism and diagnostic biomarker discovery is crucial for the design of specific management approaches.

Heat Reacclimation Using Exercise or Hot Water Immersion

INTRODUCTION

The aim of this study was to compare the effectiveness of exercise versus hot water immersion heat reacclimation (HRA) protocols.

METHODS

Twenty-four participants completed a heat stress test (HST; 33°C, 65% RH), which involved cycling at a power output equivalent to 1.5 W·kg-1 for 35 min whereby thermophysiological variables were measured. This was followed by a graded exercise test until exhaustion. HST1 was before a 10-d controlled hyperthermia (CH) heat acclimation (HA) protocol and HST2 immediately after. Participants completed HST3 after a 28-d decay period without heat exposure and were then separated into three groups to complete a 5-d HRA protocol: a control group (CH-CON, n = 8); a hot water immersion group (CH-HWI, n = 8), and a controlled hyperthermia group (CH-CH, n = 8). This was followed by HST4.

RESULTS

Compared with HST1, time to exhaustion and thermal comfort improved; resting rectal temperature (Tre), end of exercise Tre, and mean skin temperature (Tsk) were lower; and whole body sweat rate (WBSR) was greater in HST2 for all groups (P < 0.05). After a 28-d decay, only WBSR, time to exhaustion, and mean Tsk returned to pre-HA values. Of these decayed variables, only WBSR was reinstated after HRA; the improvement was observed in both the CH-CH and the CH-HWI groups (P < 0.05).

CONCLUSION

The data suggest that HRA protocol may not be necessary for cardiovascular and thermal adaptations within a 28-d decay period, as long as a 10-d CH-HA protocol has successfully induced these physiological adaptations. For sweat adaptations, a 5-d CH or HWI-HRA protocol can reinstate the lost adaptations.

Heat therapy: mechanistic underpinnings and applications to cardiovascular health

Cardiovascular diseases (CVD) are the leading cause of death worldwide, and novel therapies are drastically needed to prevent or delay the onset of CVD to reduce the societal and healthcare burdens associated with these chronic diseases. One such therapy is "heat therapy," or chronic, repeated use of hot baths or saunas. Although using heat exposure to improve health is not a new concept, it has received renewed attention in recent years as a growing number of studies have demonstrated robust and widespread beneficial effects of heat therapy on cardiovascular health. Here, we review the existing literature, with particular focus on the molecular mechanisms that underscore the cardiovascular benefits of this practice.

Alterations in the gut microbiome and metabolic profile in rats acclimated to high environmental temperature

Heat acclimation (HA) is the best strategy to improve heat stress tolerance by inducing positive physiological adaptations. Evidence indicates that the gut microbiome plays a fundamental role in the development of HA, and modulation of gut microbiota can improve tolerance to heat exposure and decrease the risks of heat illness. In this study, for the first time, we applied 16S rRNA gene sequencing and untargeted liquid chromatography–mass spectrometry (LC‐MS) metabolomics to explore variations in the gut microbiome and faecal metabolic profiles in rats after HA. The gut microbiota of HA subjects exhibited higher diversity and richer microbes. HA altered the gut microbiota composition with significant increases in the genera Lactobacillus (a major probiotic) and Oscillospira alongside significant decreases in the genera Blautia and Allobaculum. The faecal metabolome was also significantly changed after HA, and among the 13 perturbed metabolites, (S)‐AL 8810 and celastrol were increased. Moreover, the two increased genera were positively correlated with the two upregulated metabolites and negatively correlated with the other 11 downregulated metabolites, while the correlations between the two decreased genera and the upregulated/downregulated metabolites were completely contrary. In summary, both the structure of the gut microbiome community and the faecal metabolome were improved after 28 days of HA. These findings provide novel insights regarding the improvement of the gut microbiome and its functions as a potential mechanism by which HA confers protection against heat stress.

Early life thermal stress: impacts on future temperature preference in weaned pigs (3 to 15 kg)

Thermal stress can result in productivity losses, morbidity, and mortality if proper management practices are not employed. A basic understanding of the relationship between animals and the thermal environment is crucial to assess the environment’s impact on livestock performance. Therefore, the study objective was to evaluate whether different early life thermal stressors (ELTS) altered the temperature preference of pigs later in life. Twelve sows and their litters were randomly exposed to 1 of 3 ELTS treatments from 7 to 9 d of age: early life heat stress (ELHS; cycling 32 to 38 °C; n = 4), early life cold stress (ELCS; 25.4±1.1 °C without heating lamp; n = 4), or early life thermoneutral (ELTN; 25.4±1.1 °C with a heating lamp; n = 4) conditions. From 10 to 20 d, (weaning) all piglets were exposed to ELTN conditions. At weaning, pigs were randomly assigned to groups of 4 of the same sex and ELTS treatment. Temperature preference, where pigs freely choose a temperature, was assessed in 21 groups (n = 7 groups per ELTS treatment) using 1 of 3 thermal gradient apparatuses (22 to 40 °C). Testing began at 26 ± 1.3 d of age to give pigs time to acclimate to solid food after weaning and 1 group per ELTS treatment were tested simultaneously in each apparatus. Pigs were given 24 h to acclimate followed by a 24-h testing period. Behavior (active and inactive), posture (upright, sternal, and lateral lying), and location were documented every 20 min using instantaneous scan samples. Preferred feeding temperature was determined by the latency to empty a feeder in each location. Data were analyzed using PROC MIXED in SAS 9.4. A cubic regression model was used to calculate the peak temperature preference of pigs based on the temperature pigs spent most of their time. The preference range was calculated using peak temperature preference ±SE for each ELTS treatment group. Early life thermal stress altered where pigs spent most of their time within the thermal gradient (P = 0.03) with ELTN pigs preferring cooler temperatures (peak preference of 23.8 °C) compared with their ELCS exposed counterparts (peak preference of 26.0 °C; P < 0.01). However, ELHS exposed pigs (peak preference of 25.6 °C) did not differ in their temperature preference compared with ELTN or ELCS exposed counterparts (P > 0.05). In summary, ELCS exposure altered pig temperature preference later in life indicating that ELTS can alter temperature preference in pigs.

Extracellular vesicles shuttle protective messages against heat stress in bovine granulosa cells

Elevated summer temperature is reported to be the leading cause of stress in dairy and beef cows, which negatively affects various reproductive functions. Follicular cells respond to heat stress (HS) by activating the expression of heat shock family proteins (HSPs) and other antioxidants. HS is reported to negatively affect the bi-directional communication between the follicular cells and the oocyte, which is partly mediated by follicular fluid extracellular vesicles (EVs) released from surrounding cells. As carriers of bioactive molecules (DNA, RNA, protein, and lipids), the involvement of EVs in mediating the stress response in follicular cells is not fully understood. Here we used an in vitro model to decipher the cellular and EV-coupled miRNAs of bovine granulosa cells in response to HS. Moreover, the protective role of stress-related EVs against subsequent HS was assessed. For this, bovine granulosa cells from smaller follicles were cultured in vitro and after sub-confluency, cells were either kept at 37 °C or subjected to HS (42 °C). Results showed that granulosa cells exposed to HS increased the accumulation of ROS, total oxidized protein, apoptosis, and the expression of HSPs and antioxidants, while the viability of cells was reduced. Moreover, 14 and 6 miRNAs were differentially expressed in heat-stressed granulosa cells and the corresponding EVs, respectively. Supplementation of stress-related EVs in cultured granulosa cells has induced adaptive response to subsequent HS. However, this potential was not pronounced when the cells were kept under 37 °C. Taking together, EVs generated from granulosa cells exposed to HS has the potential to shuttle bioactive molecules to recipient cells and make them robust to subsequent HS.

Hypobaric hypoxia preconditioning protects against hypothalamic neuron apoptosis in heat-exposed rats by reversing hypothalamic overexpression of matrix metalloproteinase-9 and ischemia

Background: Hypoxia-inducible factor-1α (HIF-1α), heat shock protein-72 (HSP-72), hemeoxygenase-1 (HO-1), and matrix metalloproteinase-9 (MMP-9) have been identified as potential therapeutic targets in the brain for cerebral ischemia. To elucidate their underlying mechanisms, we first aimed to ascertain whether these proteins participate in the pathogenesis of heat-induced ischemic damage to the hypothalamus of rats. Second, we investigated whether hypobaric hypoxia preconditioning (HHP) attenuates heat-induced hypothalamic ischemic/hypoxic injury by modulating these proteins in situ. Methods: Anesthetized rats treated with or without HHP were subjected to heat stress. Hypothalamic ischemic/hypoxic damage was evaluated by measuring hypothalamic levels of cerebral blood flow (CBF), partial oxygen pressure (PO2), and hypothalamic temperature via an implanted probe. Hypothalamic apoptotic neurons were counted by measuring the number of NeuN/caspase-3/DAPI triple-stained cells. Hypothalamic protein expression of HIF-1α, HSP-72, HO-1, and MMP-9 was determined biochemically. Results: Before the start of the thermal experiments, rats were subjected to 5 hours of HHP (0.66 ATA or 18.3% O2) daily for 5 consecutive days per week for 2 weeks, which led to significant loss of body weight, reduced brown adipose tissue (BAT) wet weight and decreased body temperature. The animals were then subjected to thermal studies. Twenty minutes after heat stress, heat-exposed rats not treated with HHP displayed significantly higher core and hypothalamic temperatures, hypothalamic MMP-9 levels, and numbers of hypothalamic apoptotic neurons but significantly lower mean blood pressure, hypothalamic blood flow, and PO2 values than control rats not exposed to heat. In heat-exposed rats, HHP significantly increased the hypothalamic levels of HIF-1α, HSP-72, and HO-1 but significantly alleviated body and hypothalamic hyperthermia, hypotension, hypothalamic ischemia, hypoxia, neuronal apoptosis and degeneration. Conclusions: HHP may protect against hypothalamic ischemic/hypoxic injury and overexpression of MMP-9 by upregulating the hypothalamic expression of HIF-1α, HSP-72, and HO-1 in rats subjected to heatstroke.

Effects of heat stress on ovarian development and the expression of HSP genes in mice

Heat stress reduces oocyte competence, thereby causing lower fertility in animals. Chronic and acute heat stresses cause extensive morphological damage in animals, but few reports have focused on the effects of chronic and acute heat stresses on ovarian function and heat shock protein (HSP) gene expression during ovarian injury. In this study, we subjected female mice to chronic and acute heat stresses; we then calculated the ovary index, examined ovary microstructure, and measured the expression of multiple HSP family genes. Chronic heat stress reduced whole-body and ovarian growth but had little effect on the ovarian index; acute heat stress did not alter whole-body or ovarian weight. Both chronic and acute heat stresses impaired ovary function by causing the dysfunction of granular cells. Small HSP genes increased rapidly after heat treatment, and members of the HSP40, HSP70, and HSP90 families were co-expressed to function in the regulation of the heat stress response. We suggest that the HSP chaperone machinery may regulate the response to heat stress in the mouse ovary.

Alpha lipoic acid supplementation ameliorates the wrath of simulated tropical heat and humidity stress in male Murrah buffaloes

A supplement which ameliorates temperature-humidity menace in food producing livestock is a prerequisite to develop climate smart agricultural packages. A study was conducted to investigate the heat stress ameliorative efficacy of alpha lipoic acid (ALA) in male Murrah water buffaloes (Bubalus bubalis). Eighteen animals (293.61 ± 4.66Kg Bwt) were randomly allocated into three groups (n = 6); NHSC (non-heat-stressed control), HS (heat-stressed) and HSLA (heat-stressed-supplemented with ALA@32 mg/kg Bwt orally) based on the temperature humidity index (THI) and ALA supplementation. HS and HSLA were exposed to simulated heat challenge in a climatically controlled chamber (40 °C) for 21 consecutive days, 6 h daily. Physiological responses viz. Respiration rate (RR), Pulse rate (PR) and Rectal temperature (RT) were recorded daily before and after heat exposure. Blood samples were collected at the end of heat exposure on days 1, 6, 11, 16, and 21 and on day 28 (7th day post exposure which is considered as recovery) for peripheral blood mononuclear cells (PBMCs) separation, followed by RNA and Protein extraction for Real time quantitative PCR and Western blot analysis respectively, of heat shock proteins (HSPs). Two-way repeated measure ANOVA was performed between groups at different experimental periods. RR (post exposure) in HS and HSLA was significantly higher (P < 0.05) than NHSC from day 1 onwards but HSLA varied significantly from the HS 8th day onwards. Post exposure RT and PR in both HS and HSLA varied (P < 0.05) from NHSC throughout the study; but between HS and HSLA, RT significantly varied on initial 2 days and last 6 days (from days 16 to 21). HSP70 mRNA expression significantly up regulated in high THI groups with respect to the low THI group throughout the experimental period. During chronic stress (days 16 and 21) HSP70 significantly (P < 0.05) increased in HS but not in HSLA (P > 0.05) with respect to NHSC. ALA supplementation up-regulates and sustains (P < 0.05) the expression of HSP90 in HSLA in comparison to the HS and NHSC. HSP105 expression was significantly up-regulated (P < 0.05) in HS on days 16 and 21 (during long-term exposure) but only on day 21 (P < 0.05) in HSLA. HSP70, HSP90, and HSP105 protein expression dynamics were akin to the mRNA transcript data between the study groups. In conclusion, supplementing ALA ameliorates the deleterious effect of heat stress as reflected by improved physiological and cellular responses. ALA supplementation improved cellular antioxidant status and sustained otherwise easily decaying heat shock responses which concertedly hasten the baton change from a limited window of thermo tolerance to long run acclimatization.

Circular RNA Expression Profiling and Selection of Key Circular RNAs in the Hypothalamus of Heat-Acclimated Rats

Circular RNAs (circRNAs) have vital roles in great variety of biological processes. However, expression levels and functions of circRNAs related to heat acclimation (HA) are poorly understood. This study is the first time an in-depth circRNA expression profiling were used to investigate circRNA–miRNA interactions in HA rats in order to further comprehend the mechanisms underlying HA. CircRNA expression profile was performed in rats’ hypothalamus of HA and control group with microarray assays and their functions were predicted by using Bioinformatics analysis. Differential circRNAs and their regulated downstream miRNAs and mRNAs were quantitatively validated by means of quantitative polymerase chain reaction in real-time (RT-qPCR). Enzyme-linked immunosorbent assay (ELISA) was then applied to predict the expression of proteins. In total, 53 circRNAs were expressed distinctively between the HA and Control; up- and down-regulation of circRNAs were 28 and 25, respectively, in HA (fold change > 1.5, P < 0.05). Three circRNAs and two miRNAs and three predicted mRNAs were obviously regulated after validated by RT-qPCR in HA rats. Two proteins expression were proportional to their mRNA changes. Further analysis demonstrates that circRNAs closest to HA can be categorized into three signal pathways: including rno_circRNA_014301-vs-rno-miR-3575-vs-Hif-1α, rno_circRNA_014301-vs-rno-miR-3575-vs-Lppr4, and rno_circRNA_010393-vs-rno-miR-20b-3p-vs-Mfap4 in hypoxia response pathways, substance/energy metabolism, and inflammatory response pathways. Our findings implicate that many circRNAs regulate expressions of genes that interact with each other to exert their functions during HA.

Effect of single and repeated heat stress on chemical signals of heat shock response cascade in the rat's heart

Exposure to sublethal heat stress activates a complex cascade of signaling events, such as activators (NO), signal molecules (PKCε), and mediators (HSP70 and COX-2), leading to implementation of heat preconditioning, an adaptive mechanism which makes the organism more tolerant to additional stress. We investigated the time frame in which these chemical signals are triggered after heat stress (41 ± 0.5°С/45 min), single or repeated (24 or 72 h after the first one) in heart tissue of male Wistar rats. The animals were allowed to recover 24, 48 or 72 h at room temperature. Single heat stress caused a significant increase of the concentration of HSP70, NO, and PKC level and decrease of COX-2 level 24 h after the heat stress, which in the next course of recovery gradually normalized. The second heat stress, 24 h after the first one, caused a significant reduction of the HSP70 levels, concentration of NO and PKCɛ, and significant increase of COX-2 concentration. The second exposure, 72 h after the first heat stress, caused more expressive changes of HSP70 and NO in the 24 h-recovery groups. The level of PKCɛ was not significantly changed, but there was significantly increased COX-2 concentration during recovery. Serum activity of AST, ALT, and CK was reduced after single exposure and increased after repeated exposure to heat stress, in both time intervals. In conclusion, a longer period of recovery (72 h) between two consecutive sessions of heat stress is necessary to achieve more expressive changes in mediators (HSP70) and triggers (NO) of heat preconditioning.

Effects of heat and cold on health, with special reference to Finnish sauna bathing

Environmental stress such as extremely warm or cold temperature is often considered a challenge to human health and body homeostasis. However, the human body can adapt relatively well to heat and cold environments, and recent studies have also elucidated that particularly heat stress might be even highly beneficial for human health. Consequently, the aim of the present brief review is first to discuss general cardiovascular and other responses to acute heat stress, followed by a review of beneficial effects of Finnish sauna bathing on general and cardiovascular health and mortality as well as dementia and Alzheimer's disease risk. Plausible mechanisms included are improved endothelial and microvascular function, reduced blood pressure and arterial stiffness, and possibly increased angiogenesis in humans, which are likely to mediate the health benefits of sauna bathing. In addition to heat exposure with physiological adaptations, cold stress-induced physiological responses and brown fat activation on health are also discussed. This is important to take into consideration, as sauna bathing is frequently associated with cooling periods in cold(er) environments, but their combination remains poorly investigated. We finally propose, therefore, that possible additive effects of heat- and cold-stress-induced adaptations and effects on health would be worthy of further investigation.

Heat and Hypoxic Acclimation Increase Monocyte Heat Shock Protein 72 but Do Not Attenuate Inflammation following Hypoxic Exercise

Acclimation to heat or hypoxic stress activates the heat shock response and accumulation of cytoprotective heat shock proteins (HSPs). By inhibiting the NF-κB pathway HSP72 can preserve epithelial function and reduce systemic inflammation. The aim of this study was to determine the time course of mHSP72 accumulation during acclimation, and to assess intestinal barrier damage and systemic inflammation following hypoxic exercise. Three groups completed 10 × 60-min acclimation sessions (50% normoxic VO₂peak) in control (n = 7; 18°C, 35% RH), hypoxic (n = 7; F_iO₂ = 0.14, 18°C, 35% RH), or hot (n = 7; 40°C, 25% RH) conditions. Tumor necrosis factor-α (TNF-α), interleukin 6 (IL-6), interleukin 10 (IL-10), and intestinal fatty acid binding protein (I-FABP) were determined at rest and following a cycling normoxic stress test (NST; ~2 weeks before acclimation), pre-acclimation hypoxic stress test (HST1; F_iO₂ = 0.14, both at 50% normoxic VO₂peak; ~1 week before acclimation) and post-acclimation HST (48 h; HST2). Monocyte HSP72 (mHSP72) was determined before and after exercise on day 1, 3, 5, 6, and 10 of acclimation. Accumulation of basal mHSP72 was evident from day 5 (p < 0.05) of heat acclimation and increased further on day 6 (p < 0.01), and day 10 (p < 0.01). In contrast, basal mHSP72 was elevated on the final day of hypoxic acclimation (p < 0.05). Following the NST, plasma TNF-α (–0.11 ± 0.27 ng^.mL⁻¹), IL-6 (+0.62 ± 0.67 ng^.mL⁻¹) IL-10 (+1.09 ± 9.06 ng^.mL⁻¹) and I-FABP (+37.6 ± 112.8 pg^.mL⁻¹) exhibited minimal change. After HST1, IL-6 (+3.87 ± 2.56 ng^.mL⁻¹), IL-10 (+26.15 ± 26.06 ng^.mL⁻¹) and I-FABP (+183.7 ± 182.1 pg^.mL⁻¹) were elevated (p < 0.01), whereas TNF-α was unaltered (+0.08 ± 1.27; p > 0.05). A similar trend was observed after HST2, with IL-6 (+3.09 ± 1.30 ng^.mL⁻¹), IL-10 (+23.22 ± 21.67 ng^.mL⁻¹) and I-FABP (+145.9 ±123.2 pg^.mL⁻¹) increased from rest. Heat acclimation induces mHSP72 accumulation earlier and at a greater magnitude compared to matched work hypoxic acclimation, however neither acclimation regime attenuated the systemic cytokine response or intestinal damage following acute exercise in hypoxia.

Dicer protein levels elevated by mild hyperthermia promote a pro-survival phenotype

Cellular exposure to mild stress (39.5°C - 41.5°C) induces thermotolerance, rendering cells resistant to a subsequent heat shock (>42°C) insult. We found that mild hyperthermia at 39.5°C leads to elevations in dicer, a protein well-known for its role in microRNA processing and for its role in cellular stress responses. However, whether elevated dicer protein levels play a role in sustaining a thermotolerant phenotype has, to our knowledge, not been reported. Here we demonstrate that elevated dicer protein is linked to a thermotolerant phenotype in the cervical carcinoma cell line HeLa and in murine embryonic fibroblasts (MEF), and demonstrate that dicer plays a role in mediating PKR and eIF2α phosphorylation. These findings suggest that dicer's role in thermotolerance may be to relay signals to key ER stress pathway components. Moreover, utilizing a MEF cell line defective in microRNA processing, we suggest that dicer's influence on PKR and eIF2α phosphorylation is likely distinct from its microRNA processing role. ATF4 and CHOP are well characterized stress response factors proximal to eIF2α. Evidence is presented that elevated dicer protein in thermotolerant cells differentially modulates ATF4 and CHOP levels to promote a pro-survival phenotype. This work contributes new information on dicer's role in cellular stress responses by defining a pro-survival phenotype in heat stress resistant cells which is sustained, at least in part, by elevated dicer protein levels. Our results suggest an ancillary role for dicer in the cellular stress pathways activated by mild hyperthermia that is likely distinct from its role in microRNA processing.

Long-term HIF-1α transcriptional activation is essential for heat-acclimation-mediated cross tolerance: mitochondrial target genes

An important adaptive feature of heat acclimation (HA) is the induction of cross tolerance against novel stressors (HACT) Reprogramming of gene expression leading to enhanced innate cytoprotective features by attenuating damage and/or enhancing the response of "help" signals plays a pivotal role. Hypoxia-inducible factor-1α (HIF-1α), constitutively upregulated by HA (1 mo, 34°C), is a crucial transcription factor in this program, although its specific role is as yet unknown. By using a rat HA model, we studied the impact of disrupting HIF-1α transcriptional activation [HIF-1α:HIF-1β dimerization blockade by intraperitoneal acriflavine (4 mg/kg)] on its mitochondrial gene targets [phosphoinositide-dependent kinase-1 (PDK1), LON, and cyclooxygenase 4 (COX4) isoforms] in the HA rat heart. Physiological measures of cardiac HACT were infarct size after ischemia-reperfusion and time to rigor contracture during hypoxia in cardiomyocytes. We show that HACT requires transcriptional activation of HIF-1α throughout the course of HA and that this activation is accompanied by two metabolic switches: 1) profound upregulation of PDK1, which reduces pyruvate entry into the mitochondria, consequently increasing glycolytic lactate production; 2) remodeling of the COX4 isoform ratio, inducing hypoxic-tolerant COX4.2 dominance, and optimizing electron transfer and possibly ATP production during the ischemic and hypoxic insults. LON and COX4.2 transcript upregulation accompanied this shift. Loss of HACT despite elevated expression of the cytoprotective protein heat shock protein-72 concomitantly with disrupted HIF-1α dimerization suggests that HIF-1α is essential for HACT. The role of a PDK1 metabolic switch is well known in hypoxia acclimation but not in the HA model and its ischemic setting. Remodeling of COX4 isoforms by environmental acclimation is a novel finding.

Passive heat therapy improves endothelial function, arterial stiffness and blood pressure in sedentary humans

KEY POINTS

A recent 30 year prospective study showed that lifelong sauna use reduces cardiovascular-related and all-cause mortality; however, the specific cardiovascular adaptations that cause this chronic protection are currently unknown. We investigated the effects of 8 weeks of repeated hot water immersion ('heat therapy') on various biomarkers of cardiovascular health in young, sedentary humans. We showed that, relative to a sham group which participated in thermoneutral water immersion, heat therapy increased flow-mediated dilatation, reduced arterial stiffness, reduced mean arterial and diastolic blood pressure, and reduced carotid intima media thickness, with changes all on par or greater than what is typically observed in sedentary subjects with exercise training. Our results show for the first time that heat therapy has widespread and robust effects on vascular function, and as such, could be a viable treatment option for improving cardiovascular health in a variety of patient populations, particularly those with limited exercise tolerance and/or capabilities.

ABSTRACT

The majority of cardiovascular diseases are characterized by disorders of the arteries, predominantly caused by endothelial dysfunction and arterial stiffening. Intermittent hot water immersion ('heat therapy') results in elevations in core temperature and changes in cardiovascular haemodynamics, such as cardiac output and vascular shear stress, that are similar to exercise, and thus may provide an alternative means of improving health which could be utilized by patients with low exercise tolerance and/or capabilities. We sought to comprehensively assess the effects of 8 weeks of heat therapy on biomarkers of vascular function in young, sedentary subjects. Twenty young, sedentary subjects were assigned to participate in 8 weeks (4-5 times per week) of heat therapy (n = 10; immersion in a 40.5°C bath sufficient to maintain rectal temperature ≥ 38.5°C for 60 min per session) or thermoneutral water immersion (n = 10; sham). Eight weeks of heat therapy increased flow-mediated dilatation from 5.6 ± 0.3 to 10.9 ± 1.0% (P < 0.01) and superficial femoral dynamic arterial compliance from 0.06 ± 0.01 to 0.09 ±0.01 mm(2)  mmHg(-1) (P = 0.03), and reduced (i.e. improved) aortic pulse wave velocity from 7.1 ± 0.3 to 6.1 ± 0.3 m s(-1) (P = 0.03), carotid intima media thickness from 0.43 ± 0.01 to 0.37 ± 0.01 mm (P < 0.001), and mean arterial blood pressure from 83 ± 1 to 78 ± 2 mmHg (P = 0.02). No changes were observed in the sham group or for carotid arterial compliance, superficial femoral intima media thickness or endothelium-independent dilatation. Heat therapy improved endothelium-dependent dilatation, arterial stiffness, intima media thickness and blood pressure, indicating improved cardiovascular health. These data suggest heat therapy may provide a simple and effective tool for improving cardiovascular health in various populations.

Genomic analysis of Ugandan and Rwandan chicken ecotypes using a 600 k genotyping array

Background

Indigenous populations of animals have developed unique adaptations to their local environments, which may include factors such as response to thermal stress, drought, pathogens and suboptimal nutrition. The survival and subsequent evolution within these local environments can be the result of both natural and artificial selection driving the acquisition of favorable traits, which over time leave genomic signatures in a population. This study’s goals are to characterize genomic diversity and identify selection signatures in chickens from equatorial Africa to identify genomic regions that may confer adaptive advantages of these ecotypes to their environments.

Results

Indigenous chickens from Uganda (n = 72) and Rwanda (n = 100), plus Kuroilers (n = 24, an Indian breed imported to Africa), were genotyped using the Axiom® 600 k Chicken Genotyping Array. Indigenous ecotypes were defined based upon location of sampling within Africa. The results revealed the presence of admixture among the Ugandan, Rwandan, and Kuroiler populations. Genes within runs of homozygosity consensus regions are linked to gene ontology (GO) terms related to lipid metabolism, immune functions and stress-mediated responses (FDR < 0.15). The genes within regions of signatures of selection are enriched for GO terms related to health and oxidative stress processes. Key genes in these regions had anti-oxidant, apoptosis, and inflammation functions.

Conclusions

The study suggests that these populations have alleles under selective pressure from their environment, which may aid in adaptation to harsh environments. The correspondence in gene ontology terms connected to stress-mediated processes across the populations could be related to the similarity of environments or an artifact of the detected admixture.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2711-5) contains supplementary material, which is available to authorized users.

Proteomic profiling of heat acclimation in cerebrospinal fluid of rabbit

Heat acclimation (AC) is a phenotypic adaptation to the high ambient temperatures. So far, the physiological effects of AC have been well studied, but the molecular mechanisms underlying it, especially the proteomic studies have been rarely reported. Conducting a protein profile of cerebrospinal fluid (CSF) can facilitate the understanding of molecular pathways involved in AC and identifying stress-specific proteins as a laboratory biomarker. In this study we carried out proteomic profiling of the AC in CSF of rabbit, which would allow a deep insight into molecular signals underlying the AC. For this purpose, rabbits were subjected to AC (dry bulb temperature of (36±1)°C, wet bulb temperature of (29±0.5)°C, black-bulb temperature of (40±1.0)°C, 100min per day for 21days, untreated rabbits were used as controls. We adopted a gel-free proteomic approach (iTRAQ) method to identify protein composition in CSF of rabbits with AC. In total, 1310 proteins were identified. Among which 127 were significant up-regulated and 77 were down-regulated. According to the functions, all AC-induced proteins were classified into 8 categories, including plasma protein factors, metabolism-related proteins, energy metabolism-related proteins, cell surface/intercellular matrix proteins, stress related proteins, tumor-related proteins, as well as housekeeping proteins and putative proteins. Meanwhile, a total of 21 pathways were found involved in the developing of AC. Further analysis indicated that proteins mostly close to AC were grouped into two signal pathways, the immune-related signal pathways and the carbohydrate/lipoprotein metabolism-related signal pathways. Our study was first to carry out the whole proteomic picture of AC, and screen out the critical signaling pathways involved in this physical procedure.

BIOLOGICAL SIGNIFICANCE

This study reported the comparative proteomic analysis of cerebrospinal fluid of rabbits between heat acclimation and normal conditions using the gel-free proteomic mass-spectrometry approach with isotope-labeled samples (iTRAQ) techniques. Mass spectrometry analysis of the proteins from heat acclimated rabbits resulted in the identification of a total of 1310 proteins, among these, 204 proteins were related to the formation of heat acclimation. These proteins were assigned to 8 categories according to their functions. Additionally, 21 pathways involved in infectious diseases, metabolism, immunology, blood circulation, transcriptional regulation and renin-angiotensin were identified by pathway analysis in heat acclimation. This study was the first to use rabbits as a model for unraveling the molecular pathways underlying the establishment of integrative heat acclimation.

Heat shock protein 72 may improve hypotension by increasing cardiac mechanical efficiency and arterial elastance in heatstroke rats

OBJECTIVE

We attempted to test the hypothesis that preinduction of heat shock protein (HSP) 72 in the heart would improve left ventricular performance in rat heatstroke.

METHODS

Cardiac expression of HSP 72 was quantitatively evaluated by western blot analysis in rats 0h, 12h, or 72h after mild heat preconditioning (MHP; 43°C for 30min). They were subjected to severe heat stress (SHS; 43°C for 70min) to induce heatstroke. A 1.2F catheter-tip pressure transducer was inserted into the left ventricle of these group rats under general anesthesia to record hemodynamic in the closed chest with a pressure-volume loop module data recording and analysis system.

RESULTS

At the time point of heatstroke onset, compared with normothermic controls, group rats with 12h post-MHP had significantly increased cardiac HSP 72, decreased hyperthermia, decreased hypotension, decreased bradycardia, increased end-systolic pressure, increased end-diastolic pressure, increased stroke volume, decreased end-systolic volume, decreased end-diastolic pressure, increased cardiac output, increased ejection fraction, increased stroke work, increased arterial elastance, and decreased time constant of fall in ventricular pressure by Glantz-methods. With the loss of cardiac HSP 72 expression observed at 72h in post-MHP group rats, an insignificant protection against left ventricular performance was observed.

CONCLUSION

Preinduction of cardiac HSP 72 may improve hypotension in heatstroke rats by increasing both cardiac mechanical efficiency and arterial elastance.

Epigenetics and cytoprotection with heat acclimation

Studying “phenotypic plasticity” involves comparison of traits expressed in response to environmental fluctuations and aims to understand tolerance and survival in new settings. Reversible phenotypic changes that enable individuals to match their phenotype to environmental demands throughout life can be artificially induced, i.e., acclimation or occur naturally, i.e., acclimatization. The onset and achievement of acclimatory homeostasis are determined by molecular programs that induce the acclimated transcriptome. In heat acclimation, much evidence suggests that epigenetic mechanisms are powerful players in these processes. Epigenetic mechanisms affect the accessibility of the DNA to transcription factors, thereby regulating gene expression and controlling the phenotype. The heat-acclimated phenotype confers cytoprotection against novel stressors via cross-tolerance mechanisms, by attenuation of the initial damage and/or by accelerating spontaneous recovery through the release of help signals. This indispensable acclimatory feature has a memory and can be rapidly reestablished after the loss of acclimation and the return to the physiological preacclimated phenotype. The transcriptional landscape of the deacclimated phenotype includes constitutive transcriptional activation of epigenetic bookmarks. Heat shock protein (HSP) 70/HSP90/heat shock factor 1 memory protocol demonstrated constitutive histone H4 acetylation on hsp70 and hsp90 promotors. Novel players in the heat acclimation setup are poly(ADP-ribose)ribose polymerase 1 affecting chromatin condensation, DNA linker histones from the histone H1 cluster, and transcription factors associated with the P38 pathway. We suggest that these orchestrated responses maintain euchromatin and proteostasis during deacclimation and predispose to rapid reacclimation and cytoprotection. These mechanisms represent within-life epigenetic adaptations and cytoprotective memory.

Cross Acclimation between Heat and Hypoxia: Heat Acclimation Improves Cellular Tolerance and Exercise Performance in Acute Normobaric Hypoxia

BACKGROUND

The potential for cross acclimation between environmental stressors is not well understood. Thus, the aim of this investigation was to determine the effect of fixed-workload heat or hypoxic acclimation on cellular, physiological, and performance responses during post acclimation hypoxic exercise in humans.

METHOD

Twenty-one males (age 22 ± 5 years; stature 1.76 ± 0.07 m; mass 71.8 ± 7.9 kg; [Formula: see text]O2 peak 51 ± 7 mL(.)kg(-1.)min(-1)) completed a cycling hypoxic stress test (HST) and self-paced 16.1 km time trial (TT) before (HST1, TT1), and after (HST2, TT2) a series of 10 daily 60 min training sessions (50% N [Formula: see text]O2 peak) in control (CON, n = 7; 18°C, 35% RH), hypoxic (HYP, n = 7; fraction of inspired oxygen = 0.14, 18°C, 35% RH), or hot (HOT, n = 7; 40°C, 25% RH) conditions.

RESULTS

TT performance in hypoxia was improved following both acclimation treatments, HYP (-3:16 ± 3:10 min:s; p = 0.0006) and HOT (-2:02 ± 1:02 min:s; p = 0.005), but unchanged after CON (+0:31 ± 1:42 min:s). Resting monocyte heat shock protein 72 (mHSP72) increased prior to HST2 in HOT (62 ± 46%) and HYP (58 ± 52%), but was unchanged after CON (9 ± 46%), leading to an attenuated mHSP72 response to hypoxic exercise in HOT and HYP HST2 compared to HST1 (p < 0.01). Changes in extracellular hypoxia-inducible factor 1-α followed a similar pattern to those of mHSP72. Physiological strain index (PSI) was attenuated in HOT (HST1 = 4.12 ± 0.58, HST2 = 3.60 ± 0.42; p = 0.007) as a result of a reduced HR (HST1 = 140 ± 14 b.min(-1); HST2 131 ± 9 b.min(-1) p = 0.0006) and Trectal (HST1 = 37.55 ± 0.18°C; HST2 37.45 ± 0.14°C; p = 0.018) during exercise. Whereas PSI did not change in HYP (HST1 = 4.82 ± 0.64, HST2 4.83 ± 0.63).

CONCLUSION

Heat acclimation improved cellular and systemic physiological tolerance to steady state exercise in moderate hypoxia. Additionally we show, for the first time, that heat acclimation improved cycling time trial performance to a magnitude similar to that achieved by hypoxic acclimation.

A comparison of two commercially available ELISA methods for the quantification of human plasma heat shock protein 70 during rest and exercise stress

This study compared resting and exercise heat/hypoxic stress-induced levels of plasma extracellular heat shock protein 70 (eHSP70) in humans using two commercially available enzyme-linked immunosorbent assay (ELIS)A kits. EDTA plasma samples were collected from 21 males during two separate investigations. Participants in part A completed a 60-min treadmill run in the heat (HOT70; 33.0 ± 0.1 °C, 28.7 ± 0.8 %, n = 6) at 70 % V̇O2max. Participants in part B completed 60 min of cycling exercise at 50 % V̇O2max in either hot (HOT50; 40.5 °C, 25.4 relative humidity (RH)%, n = 7) or hypoxic (HYP50; fraction of inspired oxygen (FIO2) = 0.14, 21 °C, 35 % RH, n = 8) conditions. Samples were collected prior to and immediately upon termination of exercise and analysed for eHSP70 using EKS-715 high-sensitivity HSP70 ELISA and new ENZ-KIT-101 Amp'd(™) HSP70 high-sensitivity ELISA. ENZ-KIT was superior in detecting resting eHSP70 (1.54 ± 3.27 ng · mL(-1); range 0.08 to 14.01 ng · mL(-1)), with concentrations obtained from 100 % of samples compared to 19 % with EKS-715 assay. The ENZ-KIT requires optimisation prior to running samples in order to ensure participants fall within the standard curve, a step not required with EKS-715. Using ENZ-KIT, a 1:4 dilution allowed for quantification of resting HSP70 in 26/32 samples, with a 1:8 (n = 3) and 1:16 (n = 3) dilution required to determine the remaining samples. After exercise, eHSP70 was detected in 6/21 and 21/21 samples using EKS-715 and ENZ-KIT, respectively. eHSP70 was increased from rest after HOT70 (p < 0.05), but not HOT50 (p > 0.05) or HYP50 (p > 0.05) when analysed using ENZ-KIT. It is recommended that future studies requiring the precise determination of resting plasma eHSP70 use the ENZ-KIT (i.e. HSP70 Amp'd(®) ELISA) instead of the EKS-715 assay, despite additional assay development time and cost required.

Long-term heat exposure prevents hypoxia-induced apoptosis in mouse fibroblast cells

Long-term continuous exposure to high ambient temperatures induces complete heat acclimation in humans and animals. However, to date, the effects of long-term exposure to heat stress on cells have not been fully evaluated. In this study, we investigated an adaptive physiological process induced in culture cells by continuous exposure to mild heat stress for 60 days. The results of this investigation provide evidence that after long-term heat acclimation in cells, (1) heat shock protein levels are increased, (2) hypoxia inducible factor-1α (HIF-1α) expression is upregulated, and (3) heat shock-induced and hypoxia-induced apoptoses are attenuated. These results suggest that the hypoxia response pathway is an intrinsic part of the heat acclimation repertoire and that the HIF-1 pathway following long-term heat acclimation induces cells with cross tolerance against hypoxia.

Quantitative proteomics of heat-treated human cells show an across-the-board mild depletion of housekeeping proteins to massively accumulate few HSPs

Classic semiquantitative proteomic methods have shown that all organisms respond to a mild heat shock by an apparent massive accumulation of a small set of proteins, named heat-shock proteins (HSPs) and a concomitant slowing down in the synthesis of the other proteins. Yet unexplained, the increased levels of HSP messenger RNAs (mRNAs) may exceed 100 times the ensuing relative levels of HSP proteins. We used here high-throughput quantitative proteomics and targeted mRNA quantification to estimate in human cell cultures the mass and copy numbers of the most abundant proteins that become significantly accumulated, depleted, or unchanged during and following 4 h at 41 °C, which we define as mild heat shock. This treatment caused a minor across-the-board mass loss in many housekeeping proteins, which was matched by a mass gain in a few HSPs, predominantly cytosolic HSPCs (HSP90s) and HSPA8 (HSC70). As the mRNAs of the heat-depleted proteins were not significantly degraded and less ribosomes were recruited by excess new HSP mRNAs, the mild depletion of the many housekeeping proteins during heat shock was attributed to their slower replenishment. This differential protein expression pattern was reproduced by isothermal treatments with Hsp90 inhibitors. Unexpectedly, heat-treated cells accumulated 55 times more new molecules of HSPA8 (HSC70) than of the acknowledged heat-inducible isoform HSPA1A (HSP70), implying that when expressed as net copy number differences, rather than as mere "fold change" ratios, new biologically relevant information can be extracted from quantitative proteomic data. Raw data are available via ProteomeXchange with identifier PXD001666.

Human monocyte heat shock protein 72 responses to acute hypoxic exercise after 3 days of exercise heat acclimation

The aim of this study was to determine whether short-term heat acclimation (STHA) could confer increased cellular tolerance to acute hypoxic exercise in humans as determined via monocyte HSP72 (mHSP72) expression. Sixteen males were separated into two matched groups. The STHA group completed 3 days of exercise heat acclimation; 60 minutes cycling at 50% V̇O2peak in 40°C 20% relative humidity (RH). The control group (CON) completed 3 days of exercise training in 20°C, 40% RH. Each group completed a hypoxic stress test (HST) one week before and 48 hours following the final day of CON or STHA. Percentage changes in HSP72 concentrations were similar between STHA and CON following HST1 (P = 0.97). STHA induced an increase in basal HSP72 (P = 0.03) with no change observed in CON (P = 0.218). Basal mHSP72 remained elevated before HST2 for the STHA group (P < 0.05) and was unchanged from HST1 in CON (P > 0.05). Percent change in mHSP72 was lower after HST2 in STHA compared to CON (P = 0.02). The mHSP72 response to hypoxic exercise was attenuated following 3 days of heat acclimation. This is indicative of improved tolerance and ability to cope with the hypoxic insult, potentially mediated in part by increased basal reserves of HSP72.

Patterns of gene expression associated with recovery and injury in heat-stressed rats

Background

The in vivo gene response associated with hyperthermia is poorly understood. Here, we perform a global, multiorgan characterization of the gene response to heat stress using an in vivo conscious rat model.

Results

We heated rats until implanted thermal probes indicated a maximal core temperature of 41.8°C (T_c,Max). We then compared transcriptomic profiles of liver, lung, kidney, and heart tissues harvested from groups of experimental animals at T_c,Max, 24 hours, and 48 hours after heat stress to time-matched controls kept at an ambient temperature. Cardiac histopathology at 48 hours supported persistent cardiac injury in three out of six animals. Microarray analysis identified 78 differentially expressed genes common to all four organs at T_c,Max. Self-organizing maps identified gene-specific signatures corresponding to protein-folding disorders in heat-stressed rats with histopathological evidence of cardiac injury at 48 hours. Quantitative proteomics analysis by iTRAQ (isobaric tag for relative and absolute quantitation) demonstrated that differential protein expression most closely matched the transcriptomic profile in heat-injured animals at 48 hours. Calculation of protein supersaturation scores supported an increased propensity of proteins to aggregate for proteins that were found to be changing in abundance at 24 hours and in animals with cardiac injury at 48 hours, suggesting a mechanistic association between protein misfolding and the heat-stress response.

Conclusions

Pathway analyses at both the transcript and protein levels supported catastrophic deficits in energetics and cellular metabolism and activation of the unfolded protein response in heat-stressed rats with histopathological evidence of persistent heat injury, providing the basis for a systems-level physiological model of heat illness and recovery.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-1058) contains supplementary material, which is available to authorized users.

Heat acclimation memory: do the kinetics of the deacclimated transcriptome predispose to rapid reacclimation and cytoprotection?

Faster reinduction of heat acclimation (AC) after its decline indicates "AC memory." Our previous results revealed involvement of epigenetic mechanisms of transcriptional regulation. We hypothesized that the decline of AC (DeAC) is a period of "dormant memory" during which many processes are alerted to enable rapid reacclimation (ReAC). Using a genomewide approach we studied the AC, DeAC, and ReAC transcriptomes, to uncover hallmark pathways linked to "molecular memory" in the cardioacclimatome. Fifty rats subjected to heat acclimation [34°C for 2d (AC2d) or 30d (AC30)], DeAC (24°C, 30 days), ReAC (34°C, 2 days), and untreated controls were used. The GeneChip Rat Gene 1.0 ST Array was employed for left ventricular (cardiac) mRNA hybridization. Three independent bioinformatic analyses showed that 1) during AC2d enrichment of DNA impair/repair-linked genes is seen, and this is the molecular on-switch of acclimation; 2) genes activated in AC30 underlie the qualitative physiological adaptations of cardiac performance; 3) particular molecular programs encompassing constitutive upregulation of p38 MAPK, Jak/Stat, and Akt pathways and targets are specifically activated during DeAC and ReAC; and 4) epigenetic markers such as linker histones (histones H1 cluster), associated with nucleosome spacing, transcriptional chromatin modifiers, poly-(ADP-ribose) polymerase-1 (PARP1) linked to chromatin compaction, and microRNAs are only altered during DeAC/ReAC. The latter are newcomers to the AC/DeAC puzzle. We suggest that these transcriptional responses maintain euchromatin and proteostasis and enable faster physiological recovery upon ReAC by rapidly reestablishing the protected acclimated cardiophenotype. We propose that the cardiac AC model can be applied to acclimation processes in general.

Effects of cadmium exposure on the gill proteome of Cottus gobio: modulatory effects of prior thermal acclimation

Temperature and trace metals are common environmental stressors, and their importance is increasing due to global climate change and anthropogenic pollution. The aim of the present study was to investigate whether acclimation to elevated temperature affects the response of the European bullhead (Cottus gobio) to subsequent cadmium (Cd) exposure by using enzymatic and proteomic approaches. Fish acclimated to 15 (standard temperature), 18 or 21 °C for 28 days were exposed to 1mg Cd/L for 4 days at the respective acclimation temperature. First, exposure to Cd significantly decreased the activity of the lactate dehydrogenase (LDH) in gills of fish acclimated to 15 or 18 °C. However, an acclimation to 21 °C suppressed the inhibitory effect of Cd. Second, using a proteomic analysis by 2D-DIGE, we observed that thermal acclimation was the first parameter affecting the protein expression profile in gills of C. gobio, while subsequent Cd exposure seemed to attenuate this temperature effect. Moreover, our results showed opposite effects of these two environmental stressors at protein expression level. From the 52 protein spots displaying significant interaction effects of temperature and Cd exposure, a total of 28 different proteins were identified using nano LC-MS/MS and the Peptide and Protein Prophet algorithms of Scaffold software. The identified differentially expressed proteins can be categorized into diverse functional classes, related to protein turnover, folding and chaperoning, metabolic process, ion transport, cell signaling and cytoskeleton. Within a same functional class, we further reported that several proteins displayed reverse responses following sequential exposure to heat and Cd. This work provides insights into the molecular pathways potentially involved in heat acclimation process and the interactive effects of temperature and Cd stress in ectothermic vertebrates.

Differential expression of heat shock transcription factors and heat shock proteins after acute and chronic heat stress in laying chickens (Gallus gallus)

Heat stress due to high environmental temperature negatively influences animal performances. To better understand the biological impact of heat stress, laying broiler breeder chickens were subjected either to acute (step-wisely increasing temperature from 21 to 35°C within 24 hours) or chronic (32°C for 8 weeks) high temperature exposure. High temperature challenges significantly elevated body temperature of experimental birds (P<0.05). However, oxidation status of lipid and protein and expression of heat shock transcription factors (HSFs) and heat shock proteins (HSPs) 70 and 90 were differently affected by acute and chronic treatment. Tissue-specific responses to thermal challenge were also found among heart, liver and muscle. In the heart, acute heat challenge affected lipid oxidation (P = 0.05) and gene expression of all 4 HSF gene expression was upregulated (P<0.05). During chronic heat treatment, the HSP 70 mRNA level was increased (P<0.05) and HSP 90 mRNA (P<0.05) was decreased. In the liver, oxidation of protein was alleviated during acute heat challenge (P<0.05), however, gene expression HSF2, 3 and 4 and HSP 70 were highly induced (P<0.05). HSP90 expression was increased by chronic thermal treatment (P<0.05). In the muscle, both types of heat stress increased protein oxidation, but HSFs and HSPs gene expression remained unaltered. Only tendencies to increase were observed in HSP 70 (P = 0.052) and 90 (P = 0.054) gene expression after acute heat stress. The differential expressions of HSF and HSP genes in different tissues of laying broiler breeder chickens suggested that anti-heat stress mechanisms might be provoked more profoundly in the heart, by which the muscle was least protected during heat stress. In addition to HSP, HSFs gene expression could be used as a marker during acute heat stress.

Heat acclimation and cross tolerance to hypoxia: Bridging the gap between cellular and systemic responses

Recent research has suggested a potential for some of the physiological and cellular responses to heat acclimation to carry over to improved tolerance of the novel stresses of another environment. This cross-tolerance is evident in heat-acclimated animals that exhibit enhanced tolerance to either hypoxic or ischemic stress, and is primarily attributed to shared cellular stress response pathways. These pathways include Hypoxia-Inducible Factor-1 (HIF-1) and Heat Shock Proteins (HSP). Whether these shared cellular stress response pathways translate to systemic cross-tolerance (improved exercise tolerance, reduced risk of environment-associated illness) has not been clearly shown, particularly in humans. This review highlights the HIF-1 and HSP pathways and their relationship with systemic acclimation responses, and further examines the potential cellular and systemic adaptations that may result in cross-tolerance between hot and hypoxic environments.

The protective effect of heat acclimation from hypoxic damage in the brain involves changes in the expression of glutamate receptors

Long-term heat acclimation (34 °C, 30d) alters the physiological responses and the metabolic state of organisms. It also improves ability to cope with hypoxic stress via a cross-tolerance mechanism. Within the brain, the hippocampal and frontal cortex neurons are the most sensitive to hypoxia and cell death is mainly caused by calcium influx via glutamate-gated ion channels, specifically NMDA and AMPA receptors. GluN1 subunit levels of NMDA-R correspond to NMDA-R levels. GluN2B/GluN2A subunit ratio is a qualitative index of channel activity; a higher ratio implies lower calcium permeability. The GluA2 subunit of AMPA-R controls channel permeability by inhibiting calcium penetration. Here, in rats model we (i)used behavioral-assessment tests to evaluate heat acclimation mediated hypoxic (15' 4.5 ± 0.5% O2) neuroprotection, (ii) measured protein and transcript levels of NMDA-R and AMPA-R subunits before and after hypoxia in the hippocampus and the frontal cortex, to evaluate the role of Ca(2+) in neuro-protection/cross-tolerance. Behavioral tests confirmed hypoxic tolerance in long-term (30d) but not in short-term (2d) heat acclimated rats. Hypoxic tolerance in the long-term acclimated phenotype was accompanied by a significant decrease in basal NMDA receptor GluN1 protein and an increase in its mRNA. The long-term acclimated rats also showed post ischemic increases in the GluN2B/GluN2A subunit ratio and GluA2 subunit of the AMPA receptor, supporting the hypothesis that reduced calcium permeability contributes to heat acclimation mediated hypoxia cross-tolerance. Abrupt post ischemic change in GluN2B/GluN2A subunit ratio with no change in NMDA-R subunits transcript levels implies that post-translational processes are inseparable acclimatory cross-tolerance mechanism.

Neuroprotective effect of ischemic preconditioning in focal cerebral infarction: relationship with upregulation of vascular endothelial growth factor

Neuroprotection by ischemic preconditioning has been confirmed by many studies, but the precise mechanism remains unclear. In the present study, we performed cerebral ischemic preconditioning in rats by simulating a transient ischemic attack twice (each a 20-minute occlusion of the middle cerebral artery) before inducing focal cerebral infarction (2 hour occlusion-reperfusion in the same artery). We also explored the mechanism underlying the neuroprotective effect of ischemic preconditioning. Seven days after occlusion-reperfusion, tetrazolium chloride staining and immunohistochemistry revealed that the infarct volume was significantly smaller in the group that underwent preconditioning than in the model group. Furthermore, vascular endothelial growth factor immunoreactivity was considerably greater in the hippocampal CA3 region of preconditioned rats than model rats. Our results suggest that the protective effects of ischemic preconditioning on focal cerebral infarction are associated with upregulation of vascular endothelial growth factor.

Thermal sensation and cell adaptability

Whole person adaptive comfort is discussed with reference to recent findings in molecular scale systems biology. The observations are upscaled to hypotheses relating to less traditional interpretations of thermal processes, which have new implications for indoor climate management and design. Arguments are presented for a revision of current focus, model and paradigm. The issue is seen as a problem of integrating theoretical development, conceptual modeling and as an investigation of the extent to which environments and acclimatization can be used to achieve individual fitness and health, not only at the subjective comfort level, as hitherto promoted. It is argued that there are many questions yet to be asked about adaptability before celebrating a particular adaptive state.

The role and dynamics of β-catenin in precondition induced neuroprotection after traumatic brain injury

Preconditioning via heat acclimation (34°C 30 d) results in neuroprotection from traumatic brain injury due to constitutive as well as dynamic changes triggered by the trauma. Among these changes is Akt phosphorylation, which decreases apoptosis and induces HIF1α. In the present study we investigated the Akt downstream GSK3β/β -catenin pathway and focused on post injury alternations of β catenin and its impact on the cellular response in preconditioned heat acclimated mice. We found that the reduction in motor disability is accompanied with attenuation of depressive like behavior in heat acclimated mice that correlates with the GSK3β phosphorylation state. Concomitantly, a robust β catenin phosphorylation is not followed by its degradation, or by reduced nuclear accumulation. Enhanced tyrosine phosphorylation of β catenin in the injured area weakens the β catenin-N cadherin complex. Membrane β catenin is transiently reduced in heat acclimated mice and its recovery 7 days post TBI is accompanied by induction of the synaptic marker synaptophysin. We suggest a set of cellular events following traumatic brain injury in heat acclimated mice that causes β catenin to participate in cell-cell adhesion alternations rather than in Wnt signaling. These events may contribute to synaptogenesis and the improved motor and cognitive abilities seen heat acclimated mice after traumatic brain injury.

Heat-related illness: a hot topic in critical care

With current predictions of climate change, the incidence of heat-related illnesses is projected to increase. Heat-related illnesses occur on a continuum from mild symptoms to fatalities. To prevent heat-related illnesses, nurses should have comprehension of persons at risk. Primary treatment of heat-related illness centers on cooling, but not overcooling, the patient. Heatstroke involves coagulopathies and cytokines, and can result in systemic inflammatory response syndrome and multiple organ dysfunction. Critical care nursing intervention requires more than effective cooling to support bodily processes that have been damaged or destroyed by the pathophysiology of heatstroke.

Mitochondrial performance in heat acclimation--a lesson from ischemia/reperfusion and calcium overload insults in the heart

Long-term heat acclimation (LTHA; 30 days, 34°C) causes phenotypic adaptations that render protection against ischemic/reperfusion insult (I/R, 30 min global ischemia and 40 min reperfusion) via heat acclimation-mediated cross-tolerance (HACT) mechanisms. Short-term acclimation (STHA, 2 days, 34 °C), in contrast, is characterized by cellular perturbations, leading to increased susceptibility to insults. Here, we tested the hypothesis that enhanced mitochondrial respiratory function is part of the acclimatory repertoire and that the 30-day regimen is required for protection via HACT. We subjected isolated hearts and mitochondria from controls (C), STHA, or LTHA rats to I/R, hypoxia/reoxygenation, or Ca2+ overload insults. Mitochondrial function was assessed by measuring O2 consumption, membrane potential (ΔΨm), mitochondrial Ca2+ ([Ca2+]m), ATP production, respiratory chain complex activities, and molecular markers of mitochondrial biogenesis. Our results, combining physiological and biochemical parameters, confirmed that mitochondria from LTHA rats subjected to insults, in contrast to C, preserve respiratory functions (e.g., upon I/R, C mitochondria fueled by glutamate-malate, demonstrated decreases of 81%, 13%, 25%, and 50% in O2/P ratio, ATP production, ΔΨm, and complex I activity, respectively, whereas the corresponding LTHA parameters remained unchanged). STHA mitochondria maintained ΔΨm but did not preserve ATP production. LTHA [Ca2+]m was significantly higher than that of C and STHA and was not affected by the hypoxia/reoxygenation protocol compared with C. Enhanced mitochondrial biogenesis markers, switched-on during STHA coincidentally with enhanced membrane integrity (ΔΨm), were insufficient to confer intact respiratory function upon insult. LTHA was required for respiratory complex I adaptation and HACT. Stabilized higher basal [Ca2+]m and attenuated Ca2+ overload are likely connected to this adaptation.

Parental hypoxic exposure confers offspring hypoxia resistance in zebrafish (Danio rerio)

Maternal influences are a potentially important component of transgenerational transfer of phenotype in vertebrates. This study on zebrafish (Danio rerio) examined how chronic hypoxic exposure on adults affected the phenotype of their offspring. Separate adult populations were exposed to hypoxia (13.1 kPa O2) or normoxia (21.1 kPa O2) for periods ranging from 1 to 12 weeks. Adults were then returned to normoxia and bred within experimental groups. Adult fecundity and egg characteristics (volume of egg, yolk and perivitelline fluid) were assessed. Subsequently, larval body length, time to loss of equilibrium in severe hypoxia (~4 kPa O2), and critical thermal minima (CTMin) and maxima (CTMax) were measured at 6, 9, 12, 15, 18, 21 and 60 days post fertilization (dpf). Adult fecundity was depressed by hypoxic exposure. Egg component volumes were also depressed in adults exposed to 1-2 weeks of hypoxia, but returned to control levels following longer hypoxic exposure. Adult hypoxic exposures of &gt;1 week resulted in longer body lengths in their larval offspring. Time to loss of equilibrium in severe hypoxia (i.e. hypoxic resistance) in control larvae decreased from 6 to 12 dpf, remaining constant thereafter. Notably, hypoxic resistance from 6-18 dpf was ~15% lower in larvae whose parents were exposed to just 1 week of chronic hypoxia, but resistance was significantly increased by ~24-30% in 6-18 dpf in larvae from adults exposed to 2, 3 or 4 weeks of hypoxia. CTMin (~39.5°C) and CTMax (~10-12 °C) were unchanged by parental hypoxic exposure. This study demonstrates that parental hypoxic exposure in adult zebrafish has profound epigenetic effects on the morphological and physiological phenotype of their offspring.

Molecular signals that shape the integrative responses of the heat-acclimated phenotype

The introduction of molecular biology to thermoregulation was delayed compared with its application in other research fields pertinent to human health and disease. Using principles from molecular biology, we revisited fundamental problems in integrative and environmental physiology and were able to explore new research horizons. Global genomic responses in tandem with an appropriate physiological experimental model are a good experimental design strategy that can unravel the molecular mechanisms underlying integrative thermoregulatory responses. In this way, dynamic adaptation models, with accentuated or diminished regulatory circuits, triggered by superimposition of novel stressors sharing similar protective pathways, have significant benefits. On the basis of this approach, we will discuss the molecular physiological linkage of heat acclimation alone or combined with exercise training and decipher stress-specific genes in the thermoregulatory circuits in the heart and skeletal muscles. Opposing/competing adaptive features are required for each of the above-mentioned physiological conditions. Aerobic training increases the capacity to store/use ATP. In contrast, the acclimated phenotype attempts to counteract excessive heat production. Nevertheless, both treatments augment muscle force generation. These changes are tissue-specific; in the exercise-trained rat heart, there is up-regulation of Ca2+-induced Ca2+ release mechanism genes, whereas in the skeletal muscle (soleus), the enrichment is found in genes involved in metabolism. The final issue discussed in this review is the possibility that heat shock proteins serve as consensus markers of heat stress. The role of the autonomic nervous system in their induction during heat stress and how they affect integrative body systems are described.

Thermotolerance and heat acclimation may share a common mechanism in humans

Thermotolerance and heat acclimation are key adaptation processes that have been hitherto viewed as separate phenomena. Here, we provide evidence that these processes may share a common basis, as both may potentially be governed by the heat shock response. We evaluated the effects of a heat shock response-inhibitor (quercetin; 2,000 mg/day) on established markers of thermotolerance [gastrointestinal barrier permeability, plasma TNF-α, IL-6, and IL-10 concentrations, and leukocyte heat shock protein 70 (HSP70) content]. Heat acclimation reduced body temperatures, heart rate, and physiological strain during exercise/heat stress) in male subjects (n = 8) completing a 7-day heat acclimation protocol. These same subjects completed an identical protocol under placebo supplementation (placebo). Gastrointestinal barrier permeability and TNF-α were increased on the 1st day of exercise/heat stress in quercetin; no differences in these variables were reported in placebo. Exercise HSP70 responses were increased, and plasma cytokines (IL-6, IL-10) were decreased on the 7th day of heat acclimation in placebo; with concomitant reductions in exercise body temperatures, heart rate, and physiological strain. In contrast, gastrointestinal barrier permeability remained elevated, HSP70 was not increased, and IL-6, IL-10, and exercise body temperatures were not reduced on the 7th day of heat acclimation in quercetin. While exercise heart rate and physiological strain were reduced in quercetin, this occurred later in exercise than with placebo. Consistent with the concept that thermotolerance and heat acclimation are related through the heat shock response, repeated exercise/heat stress increases cytoprotective HSP70 and reduces circulating cytokines, contributing to reductions in cellular and systemic markers of heat strain. Exercising under a heat shock response-inhibitor prevents both cellular and systemic heat adaptations.

The potential for clinical use of cannabinoids in treatment of cardiovascular diseases

Cannabinoids, the constituents of the marijuana plant and their analogs, have not only neurobehavioral but also cardiovascular effects. Great advances in the last couple of decades have led to better understanding of the physiological effects of the cannabinoids and of their role in various cardiovascular pathologies. The potential therapeutic use of cannabinoids in various cardiac diseases, such as ischemic injury, heart failure, and cardiac arrhythmias, has been studied in animal models. The purpose of this article is to review the physiological cardiovascular properties of cannabinoids and to summarize the knowledge related to their potential therapeutic use.