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Journal of Applied Physiology logoLink to Journal of Applied Physiology
. 2021 Dec 23;132(2):367–374. doi: 10.1152/japplphysiol.00740.2021

The effect of local passive heating on skeletal muscle histamine concentration: implications for exercise-induced histamine release

Joshua E Mangum 1, Karen Wiedenfeld Needham 1, Dylan C Sieck 1, Matthew R Ely 1, Emily A Larson 1, Mairin C Peck 1, Christopher T Minson 1, John R Halliwill 1,✉
PMCID: PMC8799384  PMID: 34941436

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Keywords: antihistamines, endurance exercise, mast cells, passive heating, postexercise hypotension

Abstract

Aerobic exercise induces mast cell degranulation and increases histamine formation by histidine decarboxylase, resulting in an ∼150% increase in intramuscular histamine. The purpose of this study was to determine if the increase in skeletal muscle temperature associated with exercise is sufficient to explain this histamine response. Specifically, we hypothesized that local passive heating that mimics the magnitude and time course of changes in skeletal muscle temperature observed during exercise would result in increased intramuscular histamine concentrations comparable to exercising values. Seven subjects participated in the main study in which pulsed short-wave diathermy was used to passively raise the temperature of the vastus lateralis over 60 min. Heating increased intramuscular temperature from 32.6°C [95% confidence interval (CI) 32.0°C to 33.2°C] to 38.9°C (38.7°C to 39.2°C) (P < 0.05) and increased intramuscular histamine concentration from 2.14 ng/mL (1.92 to 2.36 ng/mL) to 2.97 ng/mL (2.57 to 3.36 ng/mL) (P < 0.05), an increase of 41%. In a follow-up in vitro experiment using human-derived cultured mast cells, heating to comparable temperatures did not activate mast cell degranulation. Therefore, it appears that exercise-associated changes in skeletal muscle temperature are sufficient to generate elevations in intramuscular histamine concentration. However, this thermal effect is most likely due to changes in de novo histamine formation via histidine decarboxylase and not due to degranulation of mast cells. In conclusion, physiologically relevant increases in skeletal muscle temperature explain part, but not all, of the histamine response to aerobic exercise. This thermal effect may be important in generating positive adaptations to exercise training.

NEW & NOTEWORTHY The “exercise signal” that triggers histamine release within active skeletal muscle during aerobic exercise is unknown. By mimicking the magnitude and time course of increasing skeletal muscle temperature observed during aerobic exercise, we demonstrate that part of the exercise-induced rise in histamine is explained by a thermal effect, with in vitro experiments suggesting this is most likely via de novo histamine formation. This thermal effect may be important in generating positive adaptations to exercise training.

INTRODUCTION

Blood flow to previously exercised skeletal muscle remains elevated for several hours following the cessation of exercise, a phenomenon termed sustained postexercise vasodilation (1, 2). This is associated with systemic reductions in arterial pressure to below preexercise levels following moderate-intensity aerobic exercise, known as postexercise hypotension (2). Although the hypotension results from both central neural and local vascular changes in response to whole body exercise (3), the local vascular component of sustained postexercise vasodilation can be isolated from the central component by using the model of unilateral dynamic knee-extension exercise (2, 4, 5). Within this model, sustained postexercise vasodilation is solely the result of histamine receptor activation, as histamine H1 and H2 receptor antagonism abolishes the response (4).

In addition to being the essential mediator of sustained postexercise vasodilation, we have demonstrated that intramuscular histamine plays an important role in the transcriptome response to exercise (6, 7). Along these lines, Van der Stede et al. (8) have recently shown that histamine H1 and H2 receptor antagonism diminishes the training adaptations to high-intensity interval training in men, providing further support for the concept of histamine as an important molecular transducer of exercise adaptation.

We have demonstrated that histamine concentrations within exercising skeletal muscle increase in response to exercise (9). The source of intramuscular histamine appears to be a combination of release from mast cells via degranulation and de novo formation via increased activity of the enzyme histidine decarboxylase (9). To date, the exercise “trigger” that causes the increase in histamine levels within exercising skeletal muscle has not been identified. There are multiple factors associated with exercise that could potentially signal the local release of histamine or increase its formation, including the physical contraction of skeletal muscle fibers (i.e., mechanical stresses), an increase in skeletal muscle temperature (10), as well as a multitude of chemical signals such as the decrease in pH (11), an increase in reactive oxygen species (12), and release of myokines. Of these, oxidative stress does not appear to contribute (13), but the other potential exercise triggers of local histamine release remain untested.

Metabolic heat production by exercising skeletal muscle results in an increase in skeletal muscle temperature that is well appreciated (10, 14, 15). This local elevation in temperature could potentially raise histamine concentrations by an increase in mast cell degranulation or by an increase in the activity of histidine decarboxylase (16, 17). Therefore, the overall purpose of this study was to determine if the increase in skeletal muscle temperature during exercise is a stimulus to elicit an increase in intramuscular histamine concentration. Our study was informed by the observation that unilateral dynamic knee-extension exercise represents a sufficient stimulus for increasing intramuscular histamine concentration (4). Therefore, it stands to reason that increasing muscle temperature to a comparable extent, in the absence of exercise, might replicate the rise in intramuscular histamine observed during exercise. In other words, we attempted to isolate temperature as a variable by using a passive heating modality that would not involve muscle contraction but would generate intramuscular temperatures that are comparable to this well-established exercise model.

This concept was advanced in three sequential studies. Study 1 was operationally driven, aimed to quantify the rise in skeletal muscle temperature using unilateral dynamic knee-extension exercise to provide a target temperature for Study 2. Study 2 tested the hypothesis that local passive heating of skeletal muscle using pulsed short-wave diathermy would result in increased intramuscular histamine concentrations that are closely matched to exercising values. Study 3 tested the hypothesis that heating to physiological temperatures would activate degranulation of mast cells in vitro.

METHODS

All studies were approved by the Institutional Review Board at the University of Oregon and were performed in accordance with the principles outlined by the Declaration of Helsinki. Written informed consent was obtained from all the subjects after a verbal and written briefing of all experimental procedures.

Subjects

Five (1 woman, 4 men) individuals participated in study 1 and seven (2 women, 5 men) individuals participated in study 2. All subjects were young, healthy, nonsmokers, and moderately active. There was no overlap between subjects in the two studies. All subjects reported to the laboratory in the morning after an overnight fast and were required to abstain from caffeine, alcohol, and exercise for 24 h before studies. No subjects were using over-the-counter or prescription medications at the time of study, apart from oral contraceptives. Female participants were studied during the early follicular phase of their menstrual cycle or the placebo phase of oral contraceptive use.

Study 1 Experimental Approach

The goal of this study was to define the magnitude and time course of changes in skeletal muscle temperature during aerobic exercise of a modality, intensity, and duration that are known to generate elevations in intramuscular histamine (9). This was operationally driven research, to determine the target change of skeletal muscle temperature that would be used in study 2. An intramuscular thermocouple was placed into the right vastus lateralis before exercise to measure intramuscular temperature. Subjects then performed unilateral dynamic knee-extension exercise at 60% of their peak power for 60 min, having been familiarized with the exercise modality and undergone a peak test on a screening visit as described previously (4, 5, 9).

Study 2 Experimental Approach

The goal of this experiment was to passively raise local skeletal muscle temperature to values observed during dynamic unilateral knee-extension exercise and measure intramuscular histamine concentrations via skeletal muscle microdialysis. We aimed to raise skeletal muscle temperature to the level that was observed in study 1, replicating the time course of skeletal muscle temperature during our standardized protocol of 60 min of unilateral knee-extension exercise at 60% of peak power. An intramuscular thermocouple and microdialysis probe were placed into the right vastus lateralis before heating to measure intramuscular temperature and sample interstitial fluid for histamine concentrations.

Study 3 Experimental Approach

The goal of this experiment was to assess the impact of in vitro heating on degranulation of mast cells across the range of physiological temperatures observed in studies 1 and 2.

Measurements

Dynamic unilateral knee-extension exercise.

All subjects in study 1 underwent a peak dynamic unilateral knee-extension exercise test to volitional fatigue to determine the work rate (power output) for the experiment. Exercise was performed using a custom-built knee-extension ergometer based on a computer-controlled step-motor that generated resistance against the subject’s lower leg (4, 5, 9). Subjects were seated in an upright position and they were instructed to perform knee extensions with their right leg over a 45° range of motion, starting with the leg hanging at ∼90° of flexion and maintaining a cadence of 45 extensions per minute, following audio and visual feedback. Workload was increased 3 W every minute until the subject could not keep up the required cadence or range of motion. During the subsequent experiment, the subject performed dynamic unilateral knee-extension exercise with the right leg for 60 min at 60% of peak power at a cadence of 45 extensions per minute. Power was ramped upward at the onset of exercise over the first 5 min to 60% peak power. Power output was recorded continuously throughout the 60-min unilateral dynamic knee-extension exercise.

Intramuscular temperature.

For studies 1 and 2, an intramuscular thermocouple was placed into the right vastus lateralis using sterile technique. The skin was initially anesthetized using prilocaine hydrochloride (4%; Septodont, Lancaster, PA) followed by a lidocaine (2%) and epinephrine (1:100,000) solution (Septodont, Lancaster, PA) applied down to the underlying fascia. Care was taken to ensure that prilocaine and lidocaine were not injected into the skeletal muscle. An 18-gauge Tuohy introducer needle (Integra LifeSciences, Princeton, NJ) was inserted into the right vastus lateralis in a direction parallel with muscle fiber orientation as determined by ultrasonography. The inner cannula was removed from the introducer needle and the thermocouple wire was threaded through the inner portion of the introducer needle. Once the thermocouple wire was in place within the skeletal muscle, the introducer needle was removed leaving the thermocouple wire in place. After insertion, the thermocouple was held in place by covering the entry site with a sterile transparent medical dressing (Tegaderm, 3 M, St. Paul, MN). Temperature was recorded throughout the experiment with a commercial analog-to-digital data acquisition system (Windaq, Dataq Instruments, Akron, OH).

Intramuscular microdialysis.

For study 2, an intramuscular microdialysis probe was placed in the right vastus lateralis using sterile technique (9) analogous to placement of the intramuscular thermocouple. The skin and underlying fascia were anesthetized, and probes were inserted in the vastus lateralis in a direction parallel with muscle fiber orientation using a splitable introducer. After insertion, probes were held in place by covering the entry site with a sterile transparent medical dressing (Tegaderm). The probe had a 20-kDa molecular mass cutoff with a 30-mm polyarylethersulphone membrane (63 MD Catheter, MDialysis, Stockholm, Sweden). The microdialysis probe was perfused continuously at a rate of 5 µL/min (CMA 400 Microdialysis pump, CMA, North Chelmsford, MA) with a 0.9% saline solution. Dialysate was collected during 60 min of supine rest before heating and then every 20 min during heating. Dialysate histamine concentration was measured using an enzyme-linked immunosorbent assay (Rocky Mountain Diagnostics, Colorado Springs, CO), performed in accordance with the manufacturer’s instructions.

Heart rate.

Heart rate was monitored using a three-lead electrocardiograph (Tango+; SunTech Medical, Raleigh, NC) during exercise (study 1) and passive heating (study 2).

Pulsed short-wave diathermy.

Pulsed short-wave diathermy (Megapulse II, Accelerated Care Plus-LLC, Topeka, KS) was used to passively increase the local temperature of skeletal muscle without inducing systemic thermoregulatory responses during study 2. The pulsed electromagnetic energy was applied at 27.12 MHz (800 bursts/s; 400-ps burst duration) averaging 48 W for 60 min with a 200 cm2 induction drum positioned 3.5 cm above the tissue so that there was no direct contact (2.5-cm spacer and 1-cm gap between spacer and skin surface). The drum was positioned so the tip of the microdialysis probe and thermocouple were centered under the drum. Although diathermy has the potential to heat metal devices, it does not appear to adversely affect the measurement of temperature with an intramuscular thermocouple (18). Further, we independently compared thermocouple and thermometer measurements in a water bath warmed by diathermy and did not observe any artifact of local heating of thermocouple probes.

In vitro mast cell degranulation assay.

Degranulation of mast cells was quantified by the release of the granule marker β-hexosaminidase in a standard bioassay (19) during study 3. An immortalized human mast cell line (LUVA) (20) from a male donor (EG1701-FP, Kerafast, Boston, MA) was cultured and plated following the vendor’s instructions and standard methods (19, 21) on a 96-well plate (5 × 105 cells/well) in 50-µL imaging buffer (142 mM NaCl, 5 mM NaHCO3, 10 mM HEPES, 16 mM glucose, 2 mM KCl, 2 mM CaCl2, 1 mM MgCl2, 0.1% BSA, pH to 7.3 with NaOH). Imaging buffer (100 µL) was added to blank wells. As a positive control, 2 mg/mL compound 48/80 (Sigma-Aldrich, St. Louis, MO) in 50-µL imaging buffer was added to a subset of plated cells such that the final well concentration was 1 mg/mL to activate mast cell degranulation. An additional 50 µL of imaging buffer (vehicle) was added to the remaining wells. Cells were then incubated at 34, 37, and 40°C for 20 min before centrifugation [400 RCF, 5 min, 22°C (room temperature)], and collection of supernatants was used to assess spontaneous mast cell degranulation at each temperature (22). To determine total β-hexosaminidase activity, 0.06% Triton X-100 (Sigma-Aldrich, St. Louis, MO) was added to pelleted cells to lyse them. Supernatants, cell lysates, and blank wells were reacted with 2 mM 4-nitrophenyl N-acetyl-β-d-glucosamide (Sigma-Aldrich, St. Louis, MO), the substrate for β-hexosaminidase, at 37°C for 2 h. Then, 0.5 M Tris(hydroxymethyl)aminomethane (Sigma-Aldrich, St. Louis, MO), pH 9.0, was used to stop the reaction, and the absorbance was read at 405 nm. Degranulation was expressed as a percentage of β-hexosaminidase activity in the supernatant divided by the total (supernatant plus pellet) activity after correcting for background readings of cells containing only imaging buffer and substrate. Temperature conditions were assessed in duplicate in a single experiment and each experiment was repeated in triplicate.

Statistical Analysis

Our primary outcome variables in studies 1 and 2 were analyzed using a one-way repeated-measures ANOVA with a priori contrasts for differences in skeletal muscle temperature and histamine concentrations over time from baseline (Prism 9.1 for Windows, GraphPad Software, San Diego, CA). We also performed an inflection point analysis using a piecewise linear regression between the change in histamine concentration and muscle temperature to explore a threshold for the effect of muscle temperature on histamine concentration. In study 3, our primary outcome variable was analyzed using a one-way ANOVA to test for differences in mast cell degranulation across different physiological temperatures. Significance was set at P < 0.05. Data are reported as means and SD for subject characteristics or means and 95% confidence intervals (95% CI) for when comparing means across conditions.

RESULTS

Subject physical characteristics for studies 1 and 2 are provided in Table 1, which are typical of young healthy recreationally active individuals. No subject participated in both studies.

Table 1.

Subject characteristics

Characteristic Study 1: Exercise Study 2: Heating
n 5 (1 F, 4 M) 7 (2 F, 5 M)
Age, yr 23 ± 3 26 ± 4
Height, cm 160 ± 8 168 ± 11
Weight, kg 73.2 ± 5.1 75.4 ± 13.4
Body mass index, kg/m2 23.3 ± 2.4 23.9 ± 2.7
Peak power, W 33.6 ± 5.3
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Values are expressed as means ± SD. F, female; M, male.

Heart Rate Response to Exercise

Subjects performed 60 min of unilateral dynamic knee-extension exercise at 20.2 W (95% CI 17.4 to 23.0). Heart rate increased in response to exercise from a baseline heart rate of 66 beats/min (95% CI 61 to 71) to 96 beats/min (92 to 101) at the end of exercise (P < 0.05).

Intramuscular Temperature Response to Exercise

Intramuscular temperature during 60 min of unilateral dynamic knee-extension exercise is shown in Fig. 1. Before exercise, the temperature of skeletal muscle was 34.8°C (95% CI 34.0°C to 35.5°C) and increased to 38.9°C (38.4°C to 39.4°C) by the end of exercise (P < 0.01).

Figure 1.

Figure 1.

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Individual data for intramuscular temperature during 60 min of unilateral dynamic knee-extension exercise (n = 5 subjects). Lines represent means and 95% confidence intervals. *P < 0.05 vs. baseline.

Heart Rate Response to Local Heating

Heart rate did not change from a baseline of 59 beats/min (95% CI 57 to 61) to the end of passive local heating [58 (55 to 62) beats/min, P = 0.70].

Intramuscular Temperature Response to Local Heating

Intramuscular temperature during 60 min of local passive heating is shown in Fig. 2. Before heating, the temperature of skeletal muscle was 32.6°C (95% CI 32.0 to 33.2) and was increased to 38.9°C (38.7 to 39.2) by the end of heating (P < 0.05).

Figure 2.

Figure 2.

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Individual data for intramuscular temperature during 60 min of heating by pulsed short-wave diathermy (n = 7 subjects). Lines represent means and 95% confidence intervals. *P < 0.05 vs. baseline.

Intramuscular Histamine Response to Local Heating

Intramuscular histamine concentration during 60 min of local passive heating is shown in Fig. 3. Before heating, intramuscular histamine was 2.14 ng/mL (1.92 to 2.36) and increased to 2.97 ng/mL (2.57 to 3.36) by the end of heating (P < 0.05), an increase of 41%. Each subject showed an increase in histamine concentration between baseline and 60 min (Fig. 3, right).

Figure 3.

Figure 3.

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Individual data for histamine concentrations in skeletal muscle dialysate during 60 min of heating by pulsed short-wave diathermy (n = 7 subjects). Left: lines represent means and 95% confidence intervals. Right: lines show individual patterns of response from baseline to 60 min of heating. *P < 0.05 vs. baseline.

As it appeared, there might be a threshold temperature for the rise in histamine concentration, we performed an inflection point analysis as shown in Fig. 4. The intersection between piecewise linear regressions was at 38.4°C.

Figure 4.

Figure 4.

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Inflection point analysis of response to heating (n = 7 subjects). Lines show best fit for piecewise linear regression to identify the inflection point for the rise in histamine concentration versus intramuscular temperature.

Mast Cell Degranulation Response to In Vitro Heating

Mast cell spontaneous degranulation (measured as β-hexosaminidase release) during in vitro heating is shown in Fig. 5. Mast cell degranulation did not differ across the physiological temperatures that were studied (P = 0.84). In contrast, compound 48/80, which directly promotes mast cell degranulation, induced robust degranulation in this bioassay.

Figure 5.

Figure 5.

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Mast cell degranulation, expressed as % of content, at different physiological temperatures (n = 3 replicates per condition). Compound 48/80 was used as a positive control for induced degranulation. Bars represent means and 95% confidence intervals.

DISCUSSION

The purpose of these studies was to determine if an increase in skeletal muscle temperature, mimicking what is observed during exercise, is a sufficient stimulus to provoke intramuscular histamine release. If this is true, it would suggest that the increase in skeletal muscle temperature that occurs during exercise is a contributing stimulus to the exercise-induced release of histamine and that this pathway may also be activated in heat stress, heat therapy, and in other settings of elevated muscle temperature. Experimentally, we attempted to isolate temperature as a variable by using a passive heating modality that would not involve muscle contraction but would generate intramuscular temperatures that are comparable to those observed with a well-established exercise model, which elicits a sustained but localized release of histamine within the active skeletal muscle (2, 4, 5).

Based on our observations of muscle temperature during an hour of unilateral dynamic knee-extension exercise at 60% of peak power output, we determined that an intramuscular temperature of 38.9°C would be an ecologically valid end point for testing this concept. This temperature is slightly higher than the 38.2°C reported by Kenny et al. (15) in response to 15-min bilateral knee extension but slightly lower than the 40.2°C reported by Saltin et al. (23) following cycling to exhaustion. The rise in skeletal muscle temperature varies depending on the type, duration, and intensity of exercise, the exercise environment, and training status (10, 14).

Using pulsed short-wave diathermy, we were able to passively generate a physiologically and ecologically valid intramuscular temperature of 38.9°C. Although this rise in temperature with local passive heating was slower than what we observed during exercise, it did result in an increase in intramuscular histamine concentration as hypothesized and this was seen in all subjects.

This heating-induced rise in intramuscular histamine concentration could represent one of two processes: 1) mast cell degranulation and release of stored histamine or 2) de novo formation of histamine by histidine decarboxylase. Both processes are involved in exercise-induced release of intramuscular histamine (9). Thus, as an initial survey of these possibilities, we tested whether heating would activate degranulation of mast cells in vitro. Briefly, it did not. In contrast, it has previously been documented that the activity of histidine decarboxylase increases with temperature (16). Thus, it appears that heating-induced elevations in histamine concentration within skeletal muscle are mediated by de novo histamine formation and not by mast cell degranulation. As such, the rise in skeletal muscle temperature during exercise is likely to account for some portion of the de novo formation of histamine that occurs during exercise, but it is not the trigger for exercise-induced mast cell degranulation. That trigger remains elusive.

Studying the Effect of Exercise on Muscle Temperature

It is well established that there is a rise in skeletal muscle temperature during exercise due to metabolic heat production from muscular contractions (10, 14, 15). This increase in skeletal muscle temperature can have a beneficial effect on enzyme activity and may play a role in matching substrate utilization to exercise intensity/duration (24). Temperature can also affect contractile properties of skeletal muscle including cross-bridge turnover, maximum power output, neuromuscular function, calcium sequestration, and overall metabolism during recovery (25). During exercise, there are changes in other factors such as oxygen and carbon dioxide tension, pH, and metabolic substrate and product concentrations in addition to temperature changes, making exercising skeletal muscle tissue a complicated signaling milieu to investigate. The difficulty lies in being able to isolate any of these factors from the others in an attempt to determine direct cause-effect relations. Therefore, the goal of this study was to passively increase skeletal muscle temperature to “exercise levels” without contractions to determine the cause-effect relation between muscle temperature and intramuscular histamine concentration. Ideally, this would be done without generating a systemic thermal strain that would result in whole body thermoregulatory responses that could confound interpretation.

Passive Heating by Pulsed Short-Wave Diathermy

Several interventions or modalities have been previously used to raise (or lower) skeletal muscle temperature, such as whole body and limb water immersion (26), but these interventions are accompanied by elevated skin temperature and (depending on the thermal load) initiate thermoregulatory reflex responses. We were also cognizant of the potential for water immersion to have hydrostatic influences on skeletal muscle tissue and were wary of technical challenges associated with maintaining a sterile field for both the intramuscular temperature and dialysate probes with water immersion. Pulsed short-wave diathermy has been used extensively within the athletic training community to warm skeletal muscle without significant increases in skin temperature/discomfort (18, 27, 28). Therefore, we elected to use pulsed short-wave diathermy in this experiment as it has been shown to effectively raise skeletal muscle temperature without any significant changes in central hemodynamics that would be suggestive of thermoregulatory reflex responses (29). In the present study, pulsed short-wave diathermy generated a steady rise in skeletal muscle temperature that, while not identical to what is observed during our representative exercise bout, shares a general pattern and magnitude of response to what we observed with unilateral dynamic knee-extension exercise at 60% of peak power output, but without the accompanying heart rate response.

Heating and Skeletal Muscle Histamine

Our laboratory has previously shown that intramuscular histamine concentrations increase in response to unilateral dynamic knee-extension exercise at 60% of peak power output (9). Specifically, compared with baseline levels, histamine concentrations were elevated 149% during exercise and 37% during recovery from exercise in our prior study. We have previously shown that intramuscular histamine concentrations are stable in nonexercising muscle over a period of hours (9). In our prior work, when de novo histamine formation was blocked (by inhibition of histidine decarboxylase) before exercise, histamine only increased 87% during exercise and was no longer elevated after exercise (9). In comparison, in the present study, we found that passive heating increased histamine concentrations by 41%, quantitatively similar to what is seen during recovery from exercise but less than what is observed during exercise. Taken together, this suggests that the rise in skeletal muscle temperature during exercise contributes to part, but not all, of the rise in histamine concentration that occurs with exercise.

Temperature and Human Mast Cell Degranulation

It is known that temperature is linearly related to histidine decarboxylase activity, at least in vitro (16, 17). This could explain our observations with passive heating (although we noted what appears to be a threshold phenomenon for histamine concentrations). But we also wanted to explore thermal effects on the mast cell contribution to histamine levels. Mast cells, when activated, can degranulate and release histamine from storage vesicles (30). These vesicles also contain tryptase and β-hexosaminidase. In our prior work, we used tryptase levels in skeletal muscle to demonstrate the contribution of mast cell degranulation to the exercise-induced histamine elevations (9). In the present study, we used a mast cell degranulation assay based on β-hexosaminidase release to assess, in vitro, whether physiologically and ecologically valid changes in temperature would trigger degranulation in isolated human mast cells.

As to be noted from Fig. 5, there is some degree of spontaneous degranulation in isolated mast cell experiments, consistent with what others have reported (19, 21). However, we observed that varying temperatures from 34°C to 40°C had no discernable influence on mast cell degranulation in isolated human mast cells. In contrast, we observed an expected and robust mast cell response to compound 48/80, which served as a positive control to ensure that the mast cells were functional.

Sources of Histamine

The observations in the present study and prior studies suggest that elevations in skeletal muscle temperature cause histamine release via de novo formation by histidine decarboxylase and that this thermal effect would contribute to exercise-induced histamine elevations within active skeletal muscle. Our in vitro experiments suggest that elevations in muscle temperature do not cause the mast cell degranulation that also contributes to exercise-induced histamine elevations. The signal that contributes to mast cell degranulation with exercise remains to be elucidated.

We want to acknowledge that responses of isolated human mast cells and histidine decarboxylase in vitro would not capture potential cross talk between other cell types that would surround mast cells imbedded in human skeletal muscle tissue or other factors that could influence mast cell activation and/or histidine decarboxylase activity. Likewise, our current study does not distinguish direct thermal effects on enzyme kinetics from any effects that might be secondary to changes in expression of histidine decarboxylase. However, the speed of the response to passive heating and exercise does not seem compatible with the time course for transcription/translation events (6) that can induce histidine decarboxylase as seen in rodent models (31–33). Endo and colleagues have postulated that the induction of histidine decarboxylase by exercise is important to replenish the pool of mast cell histamine lost with degranulation (31–33), but the present study would suggest there is more to the story. It seems probable that a second pool of histidine decarboxylase, not limited to mast cells, but perhaps localized to other cells such as vascular smooth muscle or endothelial cells may be important to both heating-induced and exercise-induced histamine elevations (34). It would also appear that this pool of histidine decarboxylase has a threshold for activation that is around 38.4°C, a threshold readily surpassed during sustained (>20 min) moderate-intensity exercise.

Perspectives

Both whole body and single-leg aerobic exercise initiate a sustained postexercise vasodilatory response that is dependent upon histaminergic signaling (4, 35–37). We have previously shown that histamine is released in skeletal muscle during exercise (9) and that activation of histamine H1 and H2 receptors have a profound influence on the transcriptome response to exercise, crossing multiple domains including inflammation, vascular function, metabolism, and cellular regulation (6, 7). A recent tour de force study by Van der Stede et al. (8) found that histamine H1 and H2 receptor antagonism diminished the training adaptations to high-intensity interval training in men, providing further support for the concept that histamine is an important molecular transducer of exercise adaptation. The current study suggests that some of these responses are temperature-dependent and/or temperature-sensitive. This provides an interesting bridge to research over the past decade suggesting that various cooling modalities, when applied during recovery from exercise, often blunt the adaptations to exercise training (38), whereas a growing body of evidence suggests that passive heating (via diathermy, hot water immersion, or traditional sauna), also referred to as heat therapy, generates many of the same positive health benefits as routine aerobic exercise (39, 40). For example, 8 wk of passive heating has been shown to promote angiogenesis and enhance muscle strength (41) and passive heating can reduce the vascular dysfunction associated with immobilization (42). This may not be coincidental; rather, it may be due to the broad impact of histamine within skeletal muscle tissue in response to exercise and heating.

Summary

The findings from this study demonstrate that local passive heating can elicit an increase in histamine concentrations within skeletal muscle tissue. Therefore, the increase in temperature in skeletal muscle during exercise is one of the causes of exercise-induced histamine release. In vitro experiments with isolated human mast cells suggest that this may be due to an increase in histidine decarboxylase activity, rather than a cause-effect relation between temperature and mast cell degranulation. In conclusion, physiologically relevant increases in skeletal muscle temperature explain part, but not all, of the histamine response to aerobic exercise. This thermal effect may be important in generating the positive adaptations to exercise training and in response to heat therapy.

GRANTS

This research was funded, in part, by the National Institutes of Health Grants AG072805, HL115027, and HL144128, American Heart Association Grant-in-Aid AHA 17GRNT33660656555632Z, and the Eugene and Clarissa Evonuk Memorial Graduate Fellowship.

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

J.E.M., K.W.N., C.T.M., and J.R.H. conceived and designed research; J.E.M., K.W.N., D.C.S., M.R.E., E.A.L., and M.C.P. performed experiments; J.E.M., K.W.N., and J.R.H. analyzed data; J.E.M., C.T.M., and J.R.H. interpreted results of experiments; J.E.M. and J.R.H. prepared figures; J.E.M. and J.R.H. drafted manuscript; J.E.M., K.W.N., D.C.S., M.R.E., E.A.L., M.C.P., C.T.M., and J.R.H. edited and revised manuscript; J.E.M., K.W.N., D.C.S., M.R.E., E.A.L., M.C.P., C.T.M., and J.R.H. approved final version of manuscript.

ACKNOWLEDGMENTS

This study was conducted by Joshua E. Mangum in partial fulfillment of the requirements for the doctoral degree at the University of Oregon. We thank the subjects who cheerfully participated in this research study.

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