Heat induces interleukin-6 in skeletal muscle cells via TRPV1/PKC/CREB pathways

Endocrine Responses to Heated Resistance Exercise in Men and Women

ABSTRACT

Pryor, JL, Sweet, DK, Rosbrook, P, Qiao, J, Looney, DP, Mahmood, S, and Rideout, T. Endocrine responses to heated resistance exercise in men and women. J Strength Cond Res 38(7): 1248-1255, 2024-We examined the endocrine responses of 16 (female = 8) resistance trained volunteers to a single bout of whole-body high-volume load resistance exercise in hot (HOT; 40° C) and temperate (TEMP; 20° C) environmental conditions. Thermoregulatory and heart rate (HR) data were recorded, and venous blood was acquired before and after resistance exercise to assess serum anabolic and catabolic hormones. In men, testosterone increased after resistance exercise in HOT and TEMP ( p < 0.01), but postexercise testosterone was not different between condition ( p = 0.51). In women, human growth hormone was different between condition at pre-exercise ( p = 0.02) and postexercise ( p = 0.03). After controlling for pre-exercise values, the between-condition postexercise difference was abolished ( p = 0.16). There were no differences in insulin-like growth factor-1 for either sex ( p ≥ 0.06). In women, cortisol increased from pre-exercise to postexercise in HOT ( p = 0.04) but not TEMP ( p = 0.19), generating a between-condition difference at postexercise ( p < 0.01). In men, cortisol increased from pre-exercise to postexercise in HOT only ( p < 0.01). Rectal temperature increased to a greater extent in HOT compared with TEMP in both men ( p = 0.01) and women ( p = 0.02). Heart rate increased after exercise under both conditions in men and women ( p = 0.01), but only women experience greater postexercise HR in HOT vs. TEMP ( p = 0.04). The addition of heat stress to resistance exercise session did not overtly shift the endocrine response toward an anabolic or catabolic response. When acute program variables are prescribed to increase postresistance exercise anabolic hormones, adding heat stress is not synergistic but does increase physiologic strain (i.e., elevated HR and rectal temperature).

Resistance Training in the Heat: Mechanisms of Hypertrophy and Performance Enhancement

ABSTRACT

Pryor, JL, Sweet, D, Rosbrook, P, Qiao, J, Hess, HW, and Looney, DP. Resistance training in the heat: Mechanisms of hypertrophy and performance enhancement. J Strength Cond Res 38(7): 1350-1357, 2024-The addition of heat stress to resistance exercise or heated resistance exercise (HRE) is growing in popularity as emerging evidence indicates altered neuromuscular function and an amplification of several mechanistic targets of protein synthesis. Studies demonstrating increased protein synthesis activity have shown temperature-dependent mammalian target of rapamycin phosphorylation, supplemental calcium release, augmented heat shock protein expression, and altered immune and hormone activity. These intriguing observations have largely stemmed from myotube, isolated muscle fiber, or rodent models using passive heating alone or in combination with immobilization or injury models. A growing number of translational studies in humans show comparable results employing local tissue or whole-body heat with and without resistance exercise. While few, these translational studies are immensely valuable as they are most applicable to sport and exercise. As such, this brief narrative review aims to discuss evidence primarily from human HRE studies detailing the neuromuscular, hormonal, and molecular responses to HRE and subsequent strength and hypertrophy adaptations. Much remains unknown in this exciting new area of inquiry from both a mechanistic and functional perspective warranting continued research.

The mechanisms of exercise improving cardiovascular function by stimulating Piezo1 and TRP ion channels: a systemic review

Mechanosensitive ion channels are widely distributed in the heart, lung, bladder and other tissues, and plays an important role in exercise-induced cardiovascular function promotion. By reviewing the PubMed databases, the results were summarized using the terms "Exercise/Sport", "Piezo1", "Transient receptor potential (TRP)" and "Cardiovascular" as the keywords, 124-related papers screened were sorted and reviewed. The results showed that: (1) Piezo1 and TRP channels play an important role in regulating blood pressure and the development of cardiovascular diseases such as atherosclerosis, myocardial infarction, and cardiac fibrosis; (2) Exercise promotes cardiac health, inhibits the development of pathological heart to heart failure, regulating the changes in the characterization of Piezo1 and TRP channels; (3) Piezo1 activates downstream signaling pathways with very broad pathways, such as AKT/eNOS, NF-κB, p38MAPK and HIPPO-YAP signaling pathways. Piezo1 and Irisin regulate nuclear localization of YAP and are hypothesized to act synergistically to regulate tissue mechanical properties of the cardiovascular system and (4) The cardioprotective effects of exercise through the TRP family are mostly accomplished through Ca2+ and involve many signaling pathways. TRP channels exert their important cardioprotective effects by reducing the TRPC3-Nox2 complex and mediating Irisin-induced Ca2+ influx through TRPV4. It is proposed that exercise stimulates the mechanosensitive cation channel Piezo1 and TRP channels, which exerts cardioprotective effects. The activation of Piezo1 and TRP channels and their downstream targets to exert cardioprotective function by exercise may provide a theoretical basis for the prevention of cardiovascular diseases and the rehabilitation of clinical patients.

Docosahexaenoic and Eicosapentaenoic Acids Promote the Accumulation of Browning-Related Myokines via Calcium Signaling in Insulin-Resistant Mice

BACKGROUND

Myokines have a prominent effect on improving insulin resistance (IR) by inducing browning of white adipose tissue (WAT). Although docosahexaenoic acids (DHA) and eicosapentaenoic acids (EPA) play roles in improving IR and stimulating browning, whether they mediate myokines directly remains unknown.

OBJECTIVE

This study aims to investigate the effects of DHA and EPA on browning-related myokines under IR and clarify the mechanism via Ca2+ signaling.

METHODS

The expression and secretion levels of myokines in IR mice and IR myotubes were detected after DHA/EPA treatment. The crosstalk between myotubes and adipocytes was evaluated through a method in which IR adipocytes were treated with the culture medium supernatant of myotubes treated with DHA/EPA. The expression of browning markers in the WAT of IR mice and adipocytes was determined. A calcium chelator was used to determine whether DHA and EPA regulate myokine production through a calcium ion-dependent pathway.

RESULTS

In vivo experiments: 3:1 and 1:3 DHA/EPA promoted the mRNA levels of Irisin, IL-6, IL-15, and FGF21 in skeletal muscle, stimulated WAT browning, reduced lipid accumulation; 3:1 DHA/EPA upregulated the serum concentration of Irisin; 1:3 DHA/EPA upregulated the serum concentrations of Irisin, IL-6, and FGF21. In vitro experiments: the levels of Irisin and IL-6 in C2C12 myotubes and their medium supernatant were significantly elevated in the 3:1 and 1:3 groups and the upregulation of browning markers and reduction in fat accumulation were observed in adipocytes treated with the medium supernatant of C2C12 myotubes in the 3:1 and 1:3 groups. However, the above phenomena disappeared when Ca2+ signaling was inhibited.

CONCLUSIONS

Treatment with DHA and EPA at composition ratios of 3:1 and 1:3 induces browning of WAT in IR mice, which is likely related to the promotion of the accumulation of myokines, especially Irisin and IL-6, via Ca2+ signaling.

Potential role of passively increased muscle temperature on contractile function

Declines in muscle force, power, and contractile function can be observed in older adults, clinical populations, inactive individuals, and injured athletes. Passive heating exposure (e.g., hot baths, sauna, or heated garments) has been used for health purposes, including skeletal muscle treatment. An acute increase in muscle temperature by passive heating can increase the voluntary rate of force development and electrically evoked contraction properties (i.e., time to peak twitch torque, half-relation time, and electromechanical delay). The improvements in the rate of force development and evoked contraction assessments with increased muscle temperature after passive heating reveal peripheral mechanisms’ potential role in enhancing muscle contraction. This review aimed to summarise, discuss, and highlight the potential role of an acute passive heating stimulus on skeletal muscle cells to improve contractile function. These mechanisms include increased calcium kinetics (release/reuptake), calcium sensitivity, and increased intramuscular fluid.

Role of TRPV1 in High Temperature-Induced Mitochondrial Biogenesis in Skeletal Muscle: A Mini Review

Transient receptor potential vanilloid 1 (TRPV1) is a protein that is susceptible to cell environment temperature. High temperatures of 40–45°C can activate the TRPV1 channel. TRPV1 is highly expressed in skeletal muscle and located on the sarcoplasmic reticulum (SR). Therefore, TRPV1 activated by high-temperature stress releases Ca²⁺ from the SR to the cytoplasm. Cellular Ca²⁺ accumulation is a key event that enhances TRPV1 activity by directly binding to the N-terminus and C-terminus. Moreover, Ca²⁺ is the key messenger involved in regulating mitochondrial biogenesis in skeletal muscle. Long-term activation of TRPV1 may promote mitochondrial biogenesis in skeletal muscle through the Ca²⁺-CaMKII-p38 MAPK-PGC-1α signaling axis. The discovery of the TRPV1 channel highlights the potential mechanism for high-temperature stress improving muscle mitochondrial biogenesis. The appropriate hot stimulus in thermal environments might be beneficial to the muscular mitochondrial adaptation for aerobic capacity. However, the investigation of TRPV1 on mitochondrial biogenesis is at an early stage. Further investigations need to examine the role of TRPV1 in response to mitochondrial biogenesis in skeletal muscle induced by different thermal environments.

Sinomenine Attenuated Capsaicin-Induced Increase in Cough Sensitivity in Guinea Pigs by Inhibiting SOX5/TRPV1 Axis and Inflammatory Response

Background

Chronic cough is a common complaint which affects a large number of patients worldwide. Increased cough sensitivity is a very important cause of chronic persistent cough. However, there are limited clinical diagnosis and treatment for increased cough sensitivity. Transient receptor potential vanilloid-1 (TRPVl) is a member of the transient receptor potential (TRP) family of channels which is very closely associated with respiratory diseases. However, the mechanism through which TRPV1 that influences downstream events is still poorly understood.

Results

Capsaicin induced increase in cough sensitivity by upregulating the protein level of TRPV1, leading to the secretions of Substance P and neurokinin A which stimulated neurogenic inflammation. However, sinomenine, a component of traditional Chinese medicine, significantly attenuated the capsaicin-induced cough by inhibiting the expression of TRPV1 in guinea pigs. In addition, capsaicin increased the expression of SOX5 which mediated the transcriptional upregulation of TRPV1. However, pretreatment with sinomenine reduced the expression of SOX5.

Conclusion

These results indicate that capsaicin induced increase in cough sensitivity by activating neurogenic inflammation, while sinomenine attenuated the increase in cough sensitivity by inhibiting the expressions of SOX5 and TRPV1 in guinea pigs. This finding may provide a novel target for the treatment of aggravated cough sensitivity.

TRPV1 activation and internalization is part of the LPS-induced inflammation in human iPSC-derived cardiomyocytes

The non-selective cation channel transient receptor potential vanilloid 1 (TRPV1) is expressed throughout the cardiovascular system. Recent evidence shows a role for TRPV1 in inflammatory processes. The role of TRPV1 for myocardial inflammation has not been established yet. Human induced pluripotent stem cell (iPSC)-derived cardiomyocytes (hiPSC-CM) from 4 healthy donors were incubated with lipopolysaccharides (LPS, 6 h), TRPV1 agonist capsaicin (CAP, 20 min) or the antagonist capsazepine (CPZ, 20 min). TRPV1 expression was studied by PCR and western blotting. TRPV1 internalization was analyzed by immunofluorescence. Interleukin-6 (IL-6) secretion and phosphorylation of JNK, p38 and ERK were determined by ELISA. TRPV1-associated ion channel current was measured by patch clamp. TRPV1-mRNA and -protein were expressed in hiPSC-CM. TRPV1 was localized in the plasma membrane. LPS significantly increased secretion of IL-6 by 2.3-fold, which was prevented by pre-incubation with CPZ. LPS induced TRPV1 internalization. Phosphorylation levels of ERK, p38 or JNK were not altered by TRPV1 stimulation or inhibition. LPS and IL-6 significantly lowered TRPV1-mediated ion channel current. TRPV1 mediates the LPS-induced inflammation in cardiomyocytes, associated with changes of cellular electrophysiology. LPS-induced inflammation results in TRPV1 internalization. Further studies have to examine the underlying pathways and the clinical relevance of these findings.

Mice lacking endogenous TRPV1 express reduced levels of thermogenic proteins and are susceptible to diet-induced obesity and metabolic dysfunction

Transient receptor potential vanilloid subfamily 1 (TRPV1) is a non-selective cation channel protein expressed in neuronal and non-neuronal cells. Although TRPV1 is implicated in thermogenesis and diet-induced obesity (DIO), its precise role remains controversial. TRPV1-/- mice are protected from DIO, while TRPV1 activation enhances thermogenesis to prevent obesity. To reconcile this, we fed wild-type and TRPV1-/- mice for 32 weeks with normal chow or a high-fat diet and analyzed the weight gain, metabolic activities, and thermogenic protein expression in white and brown fats. TRPV1-/- mice became obese, exhibited reduced locomotor activity, reduced energy expenditure, enhanced hepatic steatosis, and decreased thermogenic protein expression in adipose tissues. Our data reveal that lack of TRPV1 does not prevent obesity, but rather enhances metabolic dysfunction.

Chronic capsiate supplementation increases fat-free mass and upper body strength but not the inflammatory response to resistance exercise in young untrained men: a randomized, placebo-controlled and double-blind study

BACKGROUND

Acute capsaicinoid and capsinoid supplementation has endurance and resistance exercise benefits; however, if these short-term performance benefits translate into chronic benefits when combined with resistance training is currently unknown. This study investigated changes of chronic Capsiate supplementation on muscular adaptations, inflammatory response and performance in untrained men.

METHODS

Twenty untrained men were randomized to ingest 12 mg Capsiate (CAP) or placebo in a parallel, double-blind design. Body composition and performance were measured at pre-training and after 6 weeks of resistance training. An acute resistance exercise session test was performed pre and post-intervention. Blood samples were collected at rest and post-resistance exercise to analyze Tumor necrosis factor- (TNF-), Soluble TNF- receptor (sTNF-r), Interleukin-6 (IL-6) and Interleukin-10 (IL-10).

RESULTS

Exercise and CAP supplementation increased fat-free mass in comparison to baseline by 1.5 kg (P < 0.001), however, the majority of the increase (1.0 kg) resulted from an increase in total body water. The CAP change scores for fat-free mass were significantly greater in comparison to the placebo (CAP ∆%= 2.1 ± 1.8 %, PLA ∆%= 0.7 ± 1.3 %, P = 0.043) and there was a significant difference between groups in the bench press exercise (P = 0.034) with greater upper body strength change score for CAP (∆%= 13.4 ± 9.1 %) compared to placebo (∆%= 5.8 ± 5.2 %), P = 0.041. CAP had no effect on lower body strength and no supplementation interactions were observed for all cytokines in response to acute resistance exercise (P > 0.05).

CONCLUSION

Chronic Capsiate supplementation combined with resistance training during short period (6 weeks) increased fat-free mass and upper body strength but not inflammatory response and performance in young untrained men.

Type I diabetes suppresses intracellular calcium ion increase normally evoked by heat stress in rat skeletal muscle

Heat stress, via its effects on muscle intracellular Ca2+ concentrations ([Ca2+]i), has been invoked as a putative therapeutic countermeasure to type 1 diabetes-induced muscle atrophy. Using a circulation- and neurally intact in vivo muscle preparation, we tested the hypothesis that impaired muscle Ca2+ homeostasis in type 1 diabetic rats is due to attenuated heat stress tolerance mediated via transient receptor potential vanilloid 1 (TRPV1). Male Wistar rats were randomly assigned to one of the following four groups: 1) healthy control 30°C (CONT 30°C); 2) CONT 40°C; 3) diabetes 30°C (DIA 30°C); and 4) DIA 40°C. The temperature of 40°C was selected because it exceeds the TRPV1 activation threshold. Spinotrapezius muscles of Wistar rats were exteriorized in vivo and loaded with the fluorescent Ca2+ probe Fura-2 AM. [Ca2+]i was estimated over 20 min using fluorescence microscopy (340/380 nm ratio) in quiescent muscle held at the required temperature, using a calibrated heat source applied to the ventral muscle surface. Western blotting was performed to determine the protein expression levels of TRPV1 in spinotrapezius muscle. After 20 min of heat stress, the CONT 40°C condition induced a 12.3 ± 5% [Ca2+]i (P < 0.05) elevation that was markedly absent in the DIA 40°C or other conditions. Thus, no significant differences were found among DIA 40°C, DIA 30°C, and CONT 30°C. TRPV1 protein expression was decreased by 42.0 ± 9% in DIA compared with CONT (P < 0.05) and, unlike CONT, heat stress did not increase TRPV1 phosphorylation. In conclusion, diabetes suppresses TRPV1 protein expression and function and inhibits the elevated myocyte [Ca2+]i evoked normally by heat stress. These results suggest that capsaicin or other therapeutic strategies to increase Ca2+ accumulation via TRPV1 might be more effective than hyperthermic therapy for type 1 diabetic patients.

Specialized, pro-resolving mediators as potential therapeutic agents for alleviating fibromyalgia symptomatology

OBJECTIVE

To present a hypothesis on a novel strategy in the treatment of fibromyalgia (FM).

DESIGN

A narrative review.

SETTING

FM as a disease remains a challenging concept for numerous reasons, including undefined etiopathogenesis, unclear triggers and unsuccessful treatment modalities. We hypothesize that the inflammatome, the entire set of molecules involved in inflammation, acting as a common pathophysiological instrument of gut dysbiosis, sarcopenia, and neuroinflammation, is one of the major mechanisms underlying FM pathogenesis. In this setup, dysbiosis is proposed as the primary trigger of the inflammatome, sarcopenia as the peripheral nociceptive source, and neuroinflammation as the central mechanism of pain sensitization, transmission and symptomatology of FM. Whereas neuroinflammation is highly-considered as a critical deleterious element in FM pathogenesis, the presumed pathogenic roles of sarcopenia and systemic inflammation remain controversial. Nevertheless, sarcopenia-associated processes and dysbiosis have been recently detected in FM individuals. The prevalence of pro-inflammatory factors in the cerebrospinal fluid and blood has been repeatedly observed in FM individuals, supporting an idea on the role of inflammatome in FM pathogenesis. As such, failed inflammation resolution might be one of the underlying pathogenic mechanisms. In accordance, the application of specialized, inflammation pro-resolving mediators (SPMs) seems most suitable for this goal.

CONCLUSIONS

The capability of various SPMs to prevent and attenuate pain has been repeatedly demonstrated in laboratory animal experiments. Since SPMs suppress inflammation in a manner that does not compromise host defense, they could be attractive and safe candidates for the alleviation of FM symptomatology, probably in combination with anti-dysbiotic medicine.

Skeletal muscle adaptations to heat therapy

The therapeutic effects of heat have been harnessed for centuries to treat skeletal muscle disorders and other pathologies. However, the fundamental mechanisms underlying the well-documented clinical benefits associated with heat therapy (HT) remain poorly defined. Foundational studies in cultured skeletal muscle and endothelial cells, as well as in rodents, revealed that episodic exposure to heat stress activates a number of intracellular signaling networks and promotes skeletal muscle remodeling. Renewed interest in the physiology of HT in recent years has provided greater understanding of the signals and molecular players involved in the skeletal muscle adaptations to episodic exposures to HT. It is increasingly clear that heat stress promotes signaling mechanisms involved in angiogenesis, muscle hypertrophy, mitochondrial biogenesis, and glucose metabolism through not only elevations in tissue temperature but also other perturbations, including increased intramyocellular calcium and enhanced energy turnover. The few available translational studies seem to indicate that the earlier observations in rodents also apply to human skeletal muscle. Indeed, recent findings revealed that both local and whole-body HT may promote capillary growth, enhance mitochondrial content and function, improve insulin sensitivity and attenuate disuse-induced muscle wasting. This accumulating body of work implies that HT may be a practical treatment to combat skeletal abnormalities in individuals with chronic disease who are unwilling or cannot participate in traditional exercise-training regimens.

Salivary Immunoglobulin A Secretion and Polymeric Ig Receptor Expression in the Submandibular Glands Are Enhanced in Heat-Acclimated Rats

Salivary immunoglobulin A (IgA) plays a critical role in mucosal immunity. Chronic exposure to moderate heat induces heat acclimation, which modifies salivary functions. However, the changes in salivary IgA secretion in heat-acclimated rats are unclear. In this study, we investigated salivary IgA secretion and the expression of polymeric Ig receptor (pIgR), a key mediator of mucosal IgA secretion, in the submandibular glands (SMGs) of heat-acclimated rats. Following maintenance at an ambient temperature (T_a) of 24 ± 0.1 °C for 10 days, male Wistar rats were subjected to T_a of 32 ± 0.2 °C for 5 days (HE group) for heat acclimation or maintained at T_a of 24 ± 0.1°C (CN group). The rats were then anesthetized, pilocarpine (0.5 mg/kg) was intraperitoneally injected, and saliva was collected. Afterward, the SMGs and plasma were sampled. The salivary IgA concentration and IgA flow rate were significantly higher in the HE group than in the CN group. Similarly, SMG pIgR expression was significantly higher in HE rats. The levels of plasma cytokines, including interleukin (IL)-5, IL-6, and interferon-γ, were significantly greater in HE rats than in CN rats. Heat acclimation may enhance oral immunity through salivary IgA secretion and pIgR upregulation in the SMGs.

TRPV1 Antagonists as Novel Anti-Diabetic Agents: Regulation of Oral Glucose Tolerance and Insulin Secretion Through Reduction of Low-Grade Inflammation?

With a global prevalence among adults over 18 years of age approaching 9%, Type 2 diabetes mellitus (T2DM) has reached pandemic proportions and represents a major unmet medical need. To date, no disease modifying treatment is available for T2DM patients. Accumulating evidence suggest that the sensory nervous system is involved in the progression of T2DM by maintaining low-grade inflammation via the vanilloid (capsaicin) receptor, Transient Receptor Potential Vanilloid-1 (TRPV1). In this study, we tested the hypothesis that TRPV1 is directly involved in glucose homeostasis in rodents. TRPV1 receptor knockout mice (Trpv1^−/−) and their wild-type littermates were kept on high-fat diet for 15 weeks. Moreover, Zucker obese rats were given the small molecule TRPV1 antagonist, N-(4-Tertiarybutylphenyl)-4-(3-cholorphyridin-2-yl)tetrahydropyrazine-1(2H)-carbox-amide (BCTC), per os twice-a-day or vehicle for eight days. Oral glucose tolerance and glucose-stimulated insulin secretion was improved by both genetic inactivation (Trpv1^−/− mice) and pharmacological blockade (BCTC) of TRPV1. In the obese rat, the improved glucose tolerance was accompanied by a reduction in inflammatory markers in the mesenteric fat, suggesting that blockade of low-grade inflammation contributes to the positive effect of TRPV1 antagonism on glucose metabolism. We propose that TRPV1 could be a promising therapeutic target in T2DM by improving glucose intolerance and correcting dysfunctional insulin secretion.

Curcuminoids and Novel Opportunities for the Treatment of Alzheimer's Disease: Which Molecules are Actually Effective?

BACKGROUND

Millions of people worldwide are suffering from Alzheimer's disease (AD), and there are only symptomatic treatments available for this disease. Thus, there is a great need to identify drugs capable of arresting or reversing AD. Constituents of the spice turmeric, in particular, curcuminoids, seem to be very promising, as evident from in vitro experiments and tests using animal models of AD. However, most of the clinical trials did not reveal any beneficial effects of curcuminoids in the treatment of AD. These controversies, including conflicting results of clinical trials, are thought to be related to bioavailability of curcuminoids, which is low unless it is enhanced by developing a special formulation. However, there is growing evidence suggesting that other reasons may be of even greater importance, but these avenues are less explored.

OBJECTIVE

Review relevant literature, and analyze potential reasons for the controversial results.

METHODOLOGY

Recent in vitro and preclinical studies; clinical trials (without a limiting period) were searched in PubMed and Google Scholar.

RESULTS

While recent in vitro and preclinical studies confirm the therapeutic potential of curcuminoids in the treatment of AD and cognitive dysfunctions, results of corresponding clinical trials remain rather controversial.

CONCLUSION

The controversial results obtained in the clinical trials may be in part due to particularities of the curcuminoid formulations other than bioavailability. Namely, it seems likely that the various formulations differ in terms of their minor turmeric constituent(s). We hypothesize that these distinctions may be of key importance for efficacy of the particular formulation in clinical trials. A testable approach addressing this hypothesis is suggested.

Accumulation of intramyocyte TRPV1-mediated calcium during heat stress is inhibited by concomitant muscle contractions

Heat stress promotes intramyocyte calcium concentration ([Ca²⁺]_i) accumulation via transient receptor potential vanilloid 1 (TRPV1) channels. We tested the hypothesis that muscle contractile activity concomitant with heat stress would accelerate the increase in [Ca²⁺]_i via TRPV1, further impairing [Ca²⁺]_i homeostasis. Spinotrapezius muscles of adult Wistar rats were exteriorized in vivo and loaded with the fluorescent Ca²⁺ probe fura 2-AM. Heat stress (muscle surface temperature 40°C) was used as TRPV1 activator. An isometric contraction (100 Hz, 5-10 V, 30 s) was induced electrically concomitant with heat stress. [Ca²⁺]_i was determined for 20 min using in vivo fluorescence microscopy, and the phosphorylation response of TRPV1 was determined by Western blotting. Heat stress induced a significant [Ca²⁺]_i increase of 18.5 ± 8.1% at 20 min and TRPV1 phosphorylation (+231%), which was inhibited by addition of the TRPV1 inhibitor (capsazepine). However, contrary to expectations, the heat stress and isometric contraction condition almost completely inhibited TRPV1 phosphorylation and the consequent [Ca²⁺]_i elevation (<2.8% accumulation during heat stress, P > 0.05). In conclusion, this in vivo physiological model demonstrated that isometric muscle contraction(s) can suppress the phosphorylation response of TRPV1 and maintain [Ca²⁺]_i homeostasis during heat stress. NEW & NOTEWORTHY This investigation is the first document the dynamics of intramyocyte calcium concentration ([Ca²⁺]_i) increase in the myoplasm of skeletal muscle fibers in response to heat stress where the muscle blood flow is preserved. Heat stress at 40°C drives a myoplasmic [Ca²⁺]_i accumulation in concert with transient receptor potential vanilloid 1 (TRPV1) phosphorylation. However, muscle contraction caused TRPV1 channel deactivation by dephosphorylation of TRPV1. TRPV1 inactivation via isometric contraction(s) permits maintenance of [Ca²⁺]_i homeostasis even under high imposed muscle temperature.

Heat induces myogenic transcription factors of myoblast cells via transient receptor potential vanilloid 1 (Trpv1)

Exercise generates heat, blood flow, and metabolic changes, thereby inducing hypertrophy of skeletal muscle cells. However, the mechanism by which heat incudes hypertrophy in response to heat is not well known. Here, we hypothesized that heat would induce differentiation of myoblast cells. We investigated the underlying mechanism by which myoblast cells respond to heat. When mouse myoblast cells were exposed to 42 °C for over 30 min, the phosphorylation level of protein kinase C (PKC) and heat shock factor 1 (Hsf1) increased, and the mRNA and protein expression level of heat shock protein 70 (Hsp70) increased. Inhibitors of transient receptor potential vanilloid 1 (Trpv1), calmodulin, PKC, and Hsf1, and the small interfering RNA‐mediated knockdown of Trpv1 diminished those heat responses. Heat exposure increased the phosphorylation levels of thymoma viral proto‐oncogene 1 (Akt), mammalian target of rapamycin (mTOR), eukaryotic translation initiation factor 4E binding protein 1 (Eif4ebp1), and ribosomal protein S6 kinase, polypeptide 1 (S6K1). The knockdown of Trpv1 decreased these heat‐induced responses. Antagonists of Hsp70 inhibited the phosphorylation level of Akt. Finally, heat increased the protein expression level of skeletal muscle markers such as myocyte enhancer factor 2D, myogenic factor 5, myogenic factor 6, and myogenic differentiation 1. Heat also increased myotube formation. Knockdown of Trpv1 diminished heat‐induced increases of those proteins and myotube formation. These results indicate that heat induces myogenic transcription factors of myoblast cells through the Trpv1, calmodulin, PKC, Hsf1, Hsp70, Akt, mTOR, Eif4ebp1, and S6K1 pathway. Moreover, heat increases myotube formation through Trpv1.

Acute and chronic effects of hot water immersion on inflammation and metabolism in sedentary, overweight adults

Regular exercise-induced acute inflammatory responses are suggested to improve the inflammatory profile and insulin sensitivity. As body temperature elevations partly mediate this response, passive heating might be a viable tool to improve the inflammatory profile. This study investigated the acute and chronic effects of hot water immersion on inflammatory and metabolic markers. Ten sedentary, overweight men [body mass index (BMI): 31.0 ± 4.2 kg/m2, mean ± SD] were immersed in water set at 39°C for 1 h (HWI) or rested for 1 h at ambient temperature (AMB). Venous blood was obtained before the session, immediately postsession, and 2 h postsession for assessment of monocyte intracellular heat shock protein-72 (iHsp72) and plasma concentrations of extracellular Hsp72 (eHsp72), interleukin-6 (IL-6), fasting glucose, insulin, and nitrite. Thereafter, participants underwent a 2-wk intervention period, consisting of 10 hot water immersion sessions (INT). Eight BMI-matched participants (BMI: 30.0 ± 2.5 kg/m2) were included as control (CON). Plasma IL-6 and nitrite concentrations were higher immediately following HWI compared with AMB (IL-6 P < 0.001, HWI: 1.37 ± 0.94 to 2.51 ± 1.49 pg/ml; nitrite P = 0.04, HWI: 271 ± 52 to 391 ± 72 nM), whereas iHsp72 expression was unchanged ( P = 0.57). In contrast to resting iHsp72 expression ( P = 0.59), fasting glucose ( P = 0.04; INT: 4.44 ± 0.93 to 3.98 ± 0.98 mmol/l), insulin ( P = 0.04; INT: 68.1 ± 44.6 to 55.0 ± 29.9 pmol/l), and eHsp72 ( P = 0.03; INT: 17 ± 41% reduction) concentrations were lowered after INT compared with CON. HWI induced an acute inflammatory response and increased nitric oxide bioavailability. The reductions in fasting glucose and insulin concentrations following the chronic intervention suggest that hot water immersion may serve as a tool to improve glucose metabolism.

NEW & NOTEWORTHY A single hot water immersion (HWI) session induces an acute increase in plasma interleukin-6 and nitrite concentrations but does not acutely elevate heat shock protein-72 expression in monocytes [intracellular Hsp72 (iHsp72)]. A chronic HWI intervention reduces fasting glucose and insulin concentrations in the absence of changes in resting iHsp72. Therefore, HWI shows potential as a strategy to combat chronic low-grade inflammation and improve glucose metabolism in individuals without the physical capacity to do so using exercise.

Differences in serum IL-6 response after 1 °C rise in core body temperature in individuals with spinal cord injury and cervical spinal cord injury during local heat stress

OBJECTIVES

Passive rise in core body temperature achieved by head-out hot water immersion (HHWI) results in acute increases in serum interleukin (IL)-6 but no change in plasma adrenaline in patients with cervical spinal cord injury (CSCI). The purpose of the present study was to determine the mechanism of heat stress-induced increase in serum IL-6.

SETTING

A cross-sectional study.

METHODS

The study subjects were nine with CSCI, ten with thoracic and lumbar spinal cord injury (TLSCI) and eight able-bodied (AB) subjects. Time since injury was 16.1 ± 3.4 years in TLSCI and 16.4 ± 4.1 years in CSCI. Subjects were subjected to lower-body heat stress (LBH) by wearing a hot water-perfused suit until 1 °C increase in core temperature. The levels of serum IL-6, plasma adrenaline, tumour necrosis factor (TNF)-α, C-reactive protein (CRP), and counts of blood cells were measured at normothermia and after LBH.

RESULTS

Serum IL-6 concentrations increased significantly immediately after LBH in all the three groups. ΔIL-6% was lower in CSCI subjects compared with AB subjects. Plasma adrenaline concentrations significantly increased after LBH in AB and TLSCI subjects, but did not change throughout the study in CSCI subjects. Cardiac output and heart rate increased at the end of LBH in all three groups.

CONCLUSIONS

Under a similar increase in core temperature, ΔIL-6% was lower in the CSCI group compared with the AB group. These findings suggest that the observed rise in IL-6 during hyperthermia is mediated, at least in part, by plasma adrenaline.

Effect of high frequency electromagnetic wave stimulation on muscle injury in a rat model

INTRODUCTION

The aim of this study was to investigate biological changes in tissues with muscle contusion after the application of high frequency (HF) electromagnetic wave.

METHODS

An acrylic pipe was placed on the right hind limb and a metallic ball was dropped inside the pipe, which resulted in a muscle contusion. After acquiring the optimal condition for muscle contusion, 20 Sprague-Dawley rats were allocated to the HF treatment (N = 10) and sham groups (N = 10), which then underwent muscle contusion injury at their right thigh. The thickness and circumference of the right thigh and the left thigh (negative control groups) were measured (day 0). HF electromagnetic wave stimulation for three days was performed on the contusion area in the HF group after one day. Thickness was measured at the thickest area of both hind limbs and the circumference was measured every day for three days. The sham group received no treatment, and the circumference and thickness were measured using the same method. After three days, Hematoxylin and eosin and immunohistochemical (IHC) staining for IL-1β were performed and TUNEL assay was conducted for apoptosis in the skin and muscle layers.

RESULTS

The thigh muscle thickness at day 1 was significantly different between groups (P = 0.018) and this difference was observed between both sham and control groups (corrected P = 0.007), and between sham and HF groups (corrected P = 0.043). Thigh circumference was significantly different at day 3 (P = 0.047) and this difference was found between sham and control groups (corrected P = 0.018), and between sham and HF groups (corrected P = 0.032). In the HF group, the inflammatory response was reduced to almost the same level as the control group. Evaluation of IL-1β level, the inflammatory cytokine, through IHC showed marked localization of IL-1β in muscle fibers of the sham group. However, significantly less IL-1β was observed in the muscle of the HF treatment group. There was neither injury nor apoptosis after HF stimulation.

CONCLUSIONS

Application of the HF showed therapeutic effect on muscle contusion by reducing muscle swelling. This effect might be caused by the anti-inflammatory action of the HF, which evoked energy into the injured muscle.