Cardiovascular Adjustments After Acute Heat Exposure

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.

High temperature ameliorates high-fat diet-induced obesity by promoting ceramide breakdown in skeletal muscle tissue

Obesity is considered an epidemic often accompanied by insulin resistance (IR). Heat treatment (HT) has been shown to prevent high-fat diet-induced IR in skeletal muscle, but the underlying mechanisms are poorly understood. In this study, we discovered that high temperature alleviated the hallmarks of obesity by promoting glycogen synthesis and lowering blood glucose levels in skeletal muscle tissue (SMT). Additionally, HT maintained the decay phase of heat shock factor 1 (HSF1), leading to the activation of gene expression of heat shock proteins (HSPs), which contributed to the alleviation of IR in SMT of diet-induced obese (DIO) mice. Metabolomics and lipidomics analyses showed that HT promoted ceramide (Cer) breakdown, resulting in an elevation of both sphingomyelin (SM) and sphingosine, which further contributed to the amelioration of IR in SMT of DIO mice. Importantly, the increase in sphingosine was attributed to the heightened expression of the acid ceramidase N-acylsphingosine amidohydrolase 1 (ASAH1), and the inhibition of ASAH1 attenuated HT-relieved IR in SMT of DIO mice. Surprisingly, high temperature increased the composition of Cer and cholesteryl ester in lipid droplets of skeletal muscle cells. This not only helped alleviate IR but also prevented lipotoxicity in SMT of DIO mice. These findings revealed a previously unknown connection between a high-temperature environment and sphingolipid metabolism in obesity, suggesting that high temperature can improve IR by promoting Cer catabolism in SMT of obese mice.

Cyclooxygenase inhibition does not blunt thermal hyperemia in skeletal muscle of humans

Acute heat exposure increases skeletal muscle blood flow in humans. However, the mechanisms mediating this hyperemic response remain unknown. The cyclooxygenase pathway is active in skeletal muscle, is heat sensitive, and contributes to cutaneous thermal hyperemia in young healthy humans. Therefore, the purpose of this study was to test the hypothesis that cyclooxygenase inhibition would attenuate blood flow in the vastus lateralis muscle during localized heating. Twelve participants (6 women) were studied on two separate occasions: 1) time control (i.e., no ibuprofen); and 2) ingestion of 800 mg ibuprofen, a nonselective cyclooxygenase inhibitor. Experiments were randomized, counter-balanced, and separated by at least 10 days. Pulsed short-wave diathermy was used to induce unilateral deep heating of the vastus lateralis for 90 min, whereas the contralateral leg served as a thermoneutral control. Microdialysis was utilized to bypass the cutaneous circulation and directly measure local blood flow in the vastus lateralis muscle of each leg via the ethanol washout technique. Heat exposure increased muscle temperature and local blood flow (both P < 0.01 vs. baseline). However, the thermal hyperemic response did not differ between control and ibuprofen conditions (P ≥ 0.2). Muscle temperature slightly decreased for the thermoneutral leg (P < 0.01 vs. baseline), yet local blood flow remained relatively unchanged across time for control and ibuprofen conditions (both P ≥ 0.7). Taken together, our data suggest that inhibition of cyclooxygenase-derived vasodilator prostanoids does not blunt thermal hyperemia in skeletal muscle of young healthy humans.NEW & NOTEWORTHY Acute heat exposure increases skeletal muscle blood flow in humans. However, the mechanisms mediating this hyperemic response remain unknown. Using a pharmacological approach combined with microdialysis, we found that thermal hyperemia in the vastus lateralis muscle was well maintained despite the successful inhibition of cyclooxygenase. Our results suggest that cyclooxygenase-derived vasodilator prostanoids do not contribute to thermal hyperemia in skeletal muscle of young healthy humans.

The acute effect of whole-body heat therapy on peripheral and cerebral vascular reactivity in Black and White females

Among females in the U.S., Black females suffer the most from cardiovascular disease and stroke. While the reasons for this disparity are multifactorial, vascular dysfunction likely contributes. Chronic whole-body heat therapy (WBHT) improves vascular function, but few studies have examined its acute effect on peripheral or cerebral vascular function, which may help elucidate chronic adaptative mechanisms. Furthermore, no studies have investigated this effect in Black females. We hypothesized that Black females would have lower peripheral and cerebral vascular function relative to White females and that one session of WBHT would mitigate these differences. Eighteen young, healthy Black (n = 9; 21 ± 3 yr; BMI: 24.7 ± 4.5 kg/m2) and White (n = 9; 27 ± 3 yr; BMI: 24.8 ± 4.1 kg/m2) females underwent one 60 min session of WBHT (49 °C water via a tube-lined suit). Pre- and 45 min post-testing measures included post-occlusive forearm reactive hyperemia (peripheral microvascular function, RH), brachial artery flow-mediated dilation (peripheral macrovascular function, FMD), and cerebrovascular reactivity (CVR) to hypercapnia. Prior to WBHT, there were no differences in RH, FMD, or CVR (p > 0.05 for all). WBHT improved peak RH in both groups (main effect of WBHT: 79.6 ± 20.1 cm/s to 95.9 ± 30.0 cm/s; p = 0.004, g = 0.787) but not Δ blood velocity (p > 0.05 for both groups). WBHT improved FMD in both groups (6.2 ± 3.4 % to 8.8 ± 3.7 %; p = 0.016, g = 0.618) but had no effect on CVR in either group (p = 0.077). These data indicate that one session of WBHT acutely improves peripheral micro- and macrovascular but not cerebral vascular function in Black and White females.

The acute effect of heat exposure on forearm macro- and microvascular function: Impact of measurement timing, heating modality and biological sex

NEW FINDINGS

What is the central question of this study? Do measurement timing, heating modality and biological sex modulate the acute effect of heat exposure on brachial artery flow-mediated dilatation and postocclusion reactive hyperaemia? What is the main finding and its importance? The acute effect of heat exposure on brachial artery flow-mediated dilatation and postocclusion reactive hyperaemia is: (1) transient and short lasting; (2) different between forearm and whole-body heating; (3) unaffected by forearm heating during whole-body heating; and (4) not different but not always equivalent between males and females. These findings provide a useful basis for future studies to investigate the acute effect of heat exposure on vascular function.

ABSTRACT

The aim of this study was to gain a better understanding of the acute effect of heat exposure on brachial artery flow-mediated dilatation (FMD) and postocclusion reactive hyperaemia (PORH) by: characterizing the time course of changes post-heating; comparing forearm and whole-body heating; determining the impact of forearm heating during whole-body heating; and comparing males and females. Twenty adults (11 males and nine females; 28 ± 6 years of age) underwent two forearm [10 min electric blanket (EB) or 30 min hot water immersion (WI)] and two whole-body [60 min water-perfused suit with forearm covered (WBH-C) or uncovered (WBH-U)] heating modalities. The FMD and PORH were measured before and after (≤5, 30, 60, 90 and 120 min) heating. The FMD increased from baseline 30 min after EB, and 30 and 90 min after WI. In contrast, FMD decreased from baseline immediately after both WBH modalities. Peak PORH increased immediately after WI and both WBH modalities. Total PORH did not differ after WI, whereas it decreased immediately after both WBH modalities. Covering the forearm during WBH did not alter acute changes in FMD or PORH. Changes in FMD and PORH did not differ statistically between males and females during each heating modality, although the observed differences could not always be considered equivalent. These results demonstrate that the acute effect of heat exposure on brachial artery FMD and PORH is: (1) transient and short lasting; (2) different between forearm heating and WBH; (3) unaffected by direct forearm heating during WBH; and (4) not different but not always equivalent between males and females.