Aging is associated with reduced thermoregulatory function in both men (9,10) and women (4,5). Recent studies show that age-related impairments in the rate of evaporative heat loss (and therefore whole-body sweating) result in greater body heat storage in older women (>40 yr) compared with young women (18–30 yr) during exercise in dry heat (17,26). However, less is known about the effect of aging on the body’s capacity to dissipate heat in women during humid heat exposure (4,8), even though these conditions are common in many parts of the world and in the workplace, and are often experienced by workers when wearing nonpermeable protective clothing. In such conditions, sweat rate often exceeds the maximal rate of evaporation possible in the environment (Emax), leading to greater nonevaporated (dripped) sweat relative to dry conditions (2). It is therefore possible that age-related impairments in whole-body evaporative heat loss capacity disappear in more humid environments, because a greater rate of sweating in young women compared with older women may not correspond to a greater rate of evaporative heat loss. Nevertheless, no study, to our knowledge, has been designed to evaluate this hypothesis during intermittent exercise or work which is inherent in many daily and work-related activities.
Several investigators have assessed heat loss responses (i.e., sweating and skin blood flow) in both dry and humid heat stress conditions in young and older men (12,16,19,25). However, because heat loss capacity differs markedly between men and women in dry heat stress conditions (7), it is important to examine women separately to evaluate the independent effect of aging. To date, only one group has assessed thermoregulatory function in young and older women in both dry and humid heat (8). In that study, older women with matched physical characteristics and aerobic power to their young counterparts displayed greater thermal strain during prolonged exercise after heat acclimation in both climate types, but age-related impairments in local sweating were observed in dry conditions only. Although these outcomes indicate that age-related impairments in local heat loss responses may be environment dependent, the effect of aging and climate type on whole-body heat loss capacity remains uncertain. Moreover, important age-related differences in heat loss capacity often occur during postexercise recovery (12,16–18), yet the exercise protocol used in that study did not permit the evaluation of this possibility. Therefore, our aim in this study was to use direct calorimetry to assess whole-body heat loss capacity in young and older women during short bouts of low- to moderate-intensity exercise separated by resting recovery periods in both hot-dry and hot-humid conditions. With this novel approach, the effects of aging and climate type on dry and evaporative heat exchanges occurring between the body and environment could be precisely quantified during non–steady‐state exercise conditions, which replicate the intensity and intermittent nature of many daily and occupational activities (1). This information may be particularly important for preventing heat-related illness during physically demanding work in older women, who now form a greater proportion of the work force than ever before (29). We hypothesized that older women would display impaired heat loss capacity relative to young women in dry heat stress, but heat loss responses would be similar between these groups in humid heat stress.
The experimental protocol was approved by the University of Ottawa Health Sciences and Science Research Ethics Board and is in agreement with the Declaration of Helsinki. All participants provided written, informed consent before their participation in the study.
Ten young (25 ± 4 yr) and 10 older (51 ± 7 yr) habitually active women participated in this study (Table 1). Participants were selected on the basis of their V˙O2peak and body surface area to minimize between-group differences in aerobic power and body morphology. A 3-month physical activity recall questionnaire (14) was used to determine that participants were habitually active, performing an average of 3 to 4 d of structured, aerobic exercise per week for a duration of 30 to 60 min per session. All participants were nonsmokers and did not report a history of cardiovascular, respiratory or metabolic disease. Five older women were postmenopausal. All young women performed these experiments ≤ 9 d after the onset of their self-reported menses. Participants were not taking any medication.
Participants completed one screening session and two randomly ordered experimental trials on different days separated by >48 h. During the screening visit, body height, mass, surface area and density, as well as V˙O2peak were determined. Body surface area was derived (6) from measures of standing height (model 2391; Detecto, Webb City, MO) and body mass (IND560; Mettler Toledo Inc., Mississauga, ON, Canada). Body density was measured using the hydrostatic weighing technique and was used to calculate body fat percentage (24). Indirect calorimetry was used to quantify V˙O2peak (MCD Medgraphics Ultima Series; MGC Diagnostics, St. Paul, MN) during a progressive incremental exercise protocol (3) on a semi-recumbent cycle ergometer (Corival, Lode B.V., Groningen, Netherlands) in thermoneutral conditions (~23°C). For the older women, a 12-lead ECG was monitored throughout the incremental exercise test by a qualified technician to detect any abnormalities in heart activity.
Before each experimental session, all participants refrained from alcohol or caffeine and nonsteroidal anti-inflammatory drugs for >24 h, strenuous physical activity for >12 h, and any food for >2 h. All subjects were instructed to consume approximately 200 to 500 mL of water approximately 2 h before arriving at the laboratory. Upon arrival, participants changed into shorts, sports-bra, socks and sandals and provided a urine sample to ensure euhydration. After instrumentation in a thermoneutral room (~25°C), participants entered the calorimeter where they rested (30 min) in an upright seated position to allow stabilization of all physiological variables. Participants then performed four bouts of semirecumbent cycling (15 min each; Ex1, Ex2, Ex3, Ex4) at a constant rate of metabolic heat production (300 W; equivalent to ~175 W·m−2), each followed by a resting recovery period (15 min each; R1, R2, R3, R4) for a total experimental duration of 2 h. In a balanced order, participants performed this protocol in a hot-dry (35°C, 20% relative humidity) and a hot-humid environment (35°C, 60% relative humidity) at the same time on different days.
The modified Snellen direct air calorimeter was used to directly measure the rates of whole-body evaporative and dry heat loss with an accuracy of ±2.3 W (22). The calorimeter inflow and outflow values of absolute humidity and air temperature were collected at 8-s intervals. Absolute humidity was measured using high precision dew point hygrometry (RH Systems model 373H, Albuquerque, NM), whereas air temperature was measured using high precision resistance temperature detectors (±0.002°C, Black Stack model 1560; Hart Electronics, UT). Air mass flow through the calorimeter was measured by differential thermometry over a known heat source placed in the effluent air stream. The real-time data for absolute humidity, air temperature, and air mass flow were displayed and recorded on a personal computer with LabVIEW software (Version 7.0, National Instruments, Austin, TX). The rate of evaporative heat loss was calculated using the calorimeter outflow − inflow difference in absolute humidity, multiplied by the air mass flow (kg air·s−1) and the latent heat of vaporization of sweat (2426 J·g sweat−1). The rate of dry heat loss was calculated using the calorimeter outflow − inflow difference in air temperature, multiplied by the air mass flow and specific heat capacity of air (1005 J·kg air−1·°C−1). In all instances, dry and evaporative heat losses were expressed as positive values, while dry heat gain from the environment (i.e., when ambient temperature exceeds that of the skin) was expressed as a negative value. Oxygen consumption, carbon dioxide production and minute ventilation were derived from continuous measures of expired gases and air flows (Moxus Modular Metabolic System; AEI Technologies, Pittsburgh, PA), and used to approximate metabolic energy expenditure (11). External work rate was controlled to maintain metabolic heat production (metabolic energy expenditure − external work) during exercise at 300 W (~175 W·m−2) for each participant. To account for respiratory heat exchange, expired air was recycled back into the calorimeter.
Core temperature was measured at the rectum by inserting a thermocouple probe (Mon-a-therm General Purpose Temperature Probe, Mallinckrodt Medical, St. Louis, MO) to a minimum of 12 cm past the anal sphincter. Skin temperature was measured continuously at four sites (upper back, chest, thigh, and calf) using T-type thermocouples (Concept Engineering, Old Saybrook, CT) attached to the skin with surgical tape. All temperature data were collected at 15-s intervals (HP Agilent data-acquisition module, model 2497A). HR was continuously recorded and saved using a Polar coded WearLink and transmitter, Polar RS400 interface, and Polar ProTrainer 5 software (Polar Electro Oy, Finland). A urine sample was collected before the experimental sessions for the measure of urine specific gravity (Reichert TS 400 total solids refractometer; Reichert, Depew, NY).
Metabolic heat production, dry and evaporative heat exchange, rectal and mean skin temperatures and HR were measured throughout and expressed as minute averages, with the final minute of each exercise and recovery period used for statistical analyses. Data for exercise and recovery were analysed separately. Baseline values were obtained by averaging the last 5 min of data recorded during the 30-min baseline period. The required rate of evaporative heat loss to attain heat balance (Ereq) was calculated as a sum of metabolic heat production and dry heat exchange. The net change in body heat content (storage) during each exercise and recovery period was measured as the temporal summation total heat loss (evaporative ± dry heat exchange) and metabolic heat production (11). Total body heat storage was calculated as the sum over the four exercise and recovery cycles. Mean skin temperature was calculated from a weighted average of the four local skin temperatures (upper back: 30%, chest: 30%, thigh: 20%, calf: 20%; 21). HR was expressed as a percentage of HR reserve (maximal HR − baseline HR), with maximal HR attained during the incremental maximal exercise test.
Metabolic heat production, total heat loss, dry and evaporative heat exchange, Ereq, body heat storage, rectal and mean skin temperatures and percentage HR reserve were analysed using a three-way, mixed model analysis of variance with the nonrepeated factor of group (young, older), and the repeated factors of condition (dry, humid) and exercise (Ex1, Ex2, Ex3, Ex4) or recovery time (R1, R2, R3, R4). Data collected before each trial (urine-specific gravity), at baseline (rectal and mean skin temperatures) or as a sum over the duration of the protocol (total body heat storage) were analyzed using a two-way, mixed-model analysis of variance with the nonrepeated factor of group (young, older), and the repeated factor of condition (dry, humid). When a significant interaction was detected, simple main effect tests were performed. After identifying any main effects, post hoc comparisons were carried out using paired (condition, time) or unpaired t tests (group). For comparisons between time points, the P value was adjusted with the Bonferroni procedure as a multiple comparison. Unpaired t tests were used for between-group comparisons of participant characteristics (Table 1). Alpha was set at the 0.05 level for all statistical comparisons. Data are reported as means ± 95% confidence intervals unless stated otherwise as SD. All analyses were performed using SPSS 24.0 (IBM, Armonk, NY, USA).
Physical characteristics and urine specific gravity
By design, age differed between each group (Table 1; P < 0.01). However, there were no significant between-group differences in height (P = 0.42), body mass (P = 0.10), body surface area (P = 0.21), body fat percentage (P = 0.09), or V˙O2peak (P = 0.15). Urine specific gravity before experimentation in the dry (young: 1.014 ± 0.004; older: 1.010 ± 0.004) and humid condition (young, 1.014 ± 0.005; older, 1.011 ± 0.004) did not differ between groups or between conditions (both P > 0.05).
Metabolic heat production and total heat loss are presented in Figure 1. Metabolic heat production during exercise and recovery were similar between-groups, conditions and across time (all P > 0.05). Over the four exercise bouts, the respective rates of metabolic heat production (mean ± SD) for the young and older groups were 298 ± 11 W and 302 ± 9 W in the dry condition, and 300 ± 13 W and 301 ± 12 W in the humid condition. This represented approximately 44% and 45% of V˙O2peak for the young and older groups, respectively. There was a significant group–time interaction for total heat loss during exercise (P = 0.02), with total heat loss being lower in the older group compared with young group during Ex1, Ex2 and Ex3 in the dry condition (all P < 0.05), and also during Ex1 and Ex2 in the humid condition (both P < 0.05). Total heat loss was also lower in the humid condition compared with the dry condition in all exercise bouts in both groups (all P < 0.05). During recovery, total heat loss was similar between groups (main effect of group, P = 0.95), but was lower in the humid condition compared with the dry condition during R4 in young women and during R2 and R4 in older women (all P < 0.05). Therefore, although total heat loss was reduced in humid conditions compared with dry conditions in both groups, total heat loss was lower in older relative to young women during exercise in both conditions.
Evaporative and dry heat losses are presented in Figure 2. Evaporative heat loss during exercise was similar between groups (main effect of group, P = 0.13), but was lower in the humid condition compared with the dry condition in all exercise bouts in both young and older women (all P < 0.01). There was a group–condition–time interaction for evaporative heat loss during recovery (P = 0.03), with evaporative heat loss being lower in the humid condition compared with dry condition during R1 and R4 in young women and during R2, R3 and R4 in older women (all P < 0.01). However, evaporative heat loss during recovery did not differ significantly between groups at any time point in either condition (all P > 0.05). Dry heat loss was also similar between groups during exercise (main effect of group, P = 0.06), but was lower (i.e., a more negative value denoting greater dry heat gain) in dry conditions compared with humid conditions in all exercise bouts in young women (all P < 0.05) and during Ex2, Ex3, and Ex4 in older women (all P < 0.05). During recovery, dry heat loss was lower in older relative to young women during R2 in the dry condition and during R1 in the humid condition (both P < 0.05), and was also lower in humid conditions compared with dry conditions in older women during R4 (P = 0.02). Taken together, these outcomes indicate that evaporative heat loss was similar between groups, but dry heat gain was greater in older relative to young women during recovery.
As a result of the greater dry heat gain in the dry condition (Fig. 2), Ereq during exercise was greater in the humid condition compared with the dry condition in older women during Ex3 and Ex4 (both P < 0.01), but did not differ between groups (main effect of group, P = 0.11) or over time (main effect of time, P = 0.34). Across groups and exercise bouts, Ereq averaged 312 ± 17 W and 304 ± 18 W in the dry and humid condition, respectively. There was a condition– time interaction for Ereq during recovery (P = 0.03), with Ereq being lower in humid conditions compared with dry conditions during R4 in older women (P = 0.01). However, Ereq during recovery was similar between groups (main effect of group, P = 0.77), and averaged 104 ± 15 W (R1), 105 ± 15 W (R2), 102 ± 14 W (R3) and 100 ± 18 W (R4) in dry and 98 ± 17 W (R1), 100 ± 18 W (R2), 99 ± 19 W (R3) and 89 ± 16 W (R4) in humid conditions across groups.
The changes in net body heat storage during exercise and recovery, and over the duration of the protocol are presented in Figure 3. Body heat storage was greater in the humid condition compared with the dry condition in both young and older women during all exercise and recovery bouts (all P < 0.05) with the exception of R2 in young women and Ex1 in older women (both P > 0.05). Body heat storage was also greater in older women compared with young women during Ex2 in the dry condition (P = 0.02) and also over the duration of the protocol in both dry and humid conditions (P < 0.01 and P = 0.04, respectively). Total body heat storage was also greater in the humid relative to the dry condition in both young and older women (both P < 0.01). These outcomes indicate that body heat storage over the 2-h protocol was greater in the humid condition compared with the dry condition in both groups, and also greater in older relative to young women in both conditions.
Baseline rectal temperature in the dry (young, 37.2°C ± 0.1°C; older, 37.2°C ± 0.1°C) and humid condition (young, 37.2°C ± 0.1°C; older, 37.3°C ± 0.2°C) were similar between groups (main effect of group, P = 0.45) and conditions (main effect of condition, P = 0.22). Mean skin temperature at baseline was also similar between conditions (main effect of condition, P = 0.83), but was greater in older women compared with young women in both the dry (young, 34.7°C ± 0.3°C; older, 34.2°C ± 0.2°C; P < 0.01) and humid condition (young, 34.7°C ± 0.2°C; older, 34.2°C ± 0.1°C; P < 0.01).
Rectal and mean skin temperature during exercise and recovery are presented in Table 2. There was a group–condition–time interaction for rectal temperature during exercise (P = 0.03). Nevertheless, rectal temperature during all exercise bouts did not differ significantly between groups in either condition (all P > 0.05) and was similar between conditions in both groups (all P > 0.05). During recovery, a group– time and group–condition interactions were detected for rectal temperature (both P < 0.05), with rectal temperature being higher in older women compared with young women in the dry condition during R3 and R4 (P = 0.04 and P < 0.01, respectively). However, rectal temperature during all recovery bouts did not differ significantly between conditions in either group (all P > 0.05). There were group–time and condition–time interactions for mean skin temperature during exercise (P < 0.01 and P = 0.03, respectively), with mean skin temperature being lower in older women compared with young women during Ex1 and Ex2 in both dry and humid conditions (all P < 0.05), and being greater in humid conditions compared with dry conditions during Ex2, Ex3 and Ex4 in older women (all P < 0.01). During recovery, mean skin temperature was similar between conditions (main effect of condition, P = 0.61), but was greater in older women compared with young women during R1 in both dry and humid conditions (P = 0.03 and P < 0.01, respectively).
To account for between-group differences in the maximal HR obtained during the incremental exercise test, HR data were analyzed as a percentage of HR reserve (Table 2). During both exercise and recovery, percentage HR reserve was similar between groups (main effect of group, both P > 0.05) and conditions (main effect of condition, both P > 0.05).
We assessed whole-body heat exchange and body heat storage in young and older women, with matched physical characteristics and V˙O2peak, during low- to moderate-intensity intermittent exercise in dry and humid heat. Total heat loss capacity was reduced in the humid condition compared with the dry condition in both groups, however in contrast to our working hypothesis, total heat loss in older women was impaired relative to their young counterparts during exercise in both dry and humid conditions. These age-related reductions in the body’s capacity to dissipate heat resulted in 26% and 16% greater total body heat storage in older women compared with young women in dry and humid conditions, respectively, and indicate that older women may be at greater risk of heat-related illness during light- to moderate-intensity activity in the heat, particularly in areas of high humidity. Despite this, core (rectal) temperature and percentage HR reserve were generally similar between groups in both conditions.
It is well known that evaporative heat loss is impaired in humid conditions compared with dry conditions (12,16,19,25), and this was realized during exercise and recovery in both young and older women (Fig. 2). Although these reductions in evaporative heat loss also resulted in an increase in skin temperature in older women (Table 2) and a subsequent reduction in dry heat gain relative to the dry condition in both groups (Fig. 2), total heat loss (evaporative ± dry heat exchange) during exercise and in the final recovery bout remained lower in the humid environment compared with the dry environment in both young and older women (Fig. 1). Consequently, body heat storage during both exercise and recovery and over the duration of the 2-h protocol was greater in the humid condition compared with the dry condition in both young and older women (Fig. 3). These findings are consistent with previous comparisons of whole-body heat loss in dry and humid heat in men (12,16) and indicate that exercise or work of low-to-moderate intensity in areas of high heat and humidity may more rapidly compromise safety than activity of the same intensity performed in dry heat stress conditions.
We hypothesized that older women would display impairments in whole-body heat loss relative to their young counterparts in dry, but not humid heat stress conditions. In contrast, age-related impairments in total heat loss were observed during exercise in both conditions (Fig. 1). As a result, body heat storage was greater in older women compared with young women during the second exercise bout in the dry condition and also over the duration of the protocol in both dry and humid conditions (Fig. 3). Although similar decrements in heat loss capacity have been reported in older men compared with young men in both dry and humid heat (16), a previous comparison of heat acclimated young and older women showed that aging impaired local sweating responses (ventilated capsules and sweat collection) in dry, but not humid heat stress conditions (8). However, it is possible that age-related differences in thermoregulatory function become less apparent after heat acclimation or perhaps the local heat loss responses measured in that study did not provide the sensitivity required to detect age-related differences in whole-body heat loss capacity. As such, our findings demonstrate perhaps for the first time that age-related impairments in heat loss occur during exercise in both dry and humid heat stress conditions in women.
Interestingly, these age-related impairments in total heat loss were not paralleled by comparable reductions in evaporative heat loss in either condition (Fig. 2). This may be explained by a reduction in Emax in the humid condition, which restricted evaporative heat loss to a similar extent in both groups. However, in the dry condition, this discrepancy is likely associated with the exercise intensity (300 W) and duration (15 min) used. Indeed, recent studies show that aging affects evaporative heat loss in a heat-load dependent manner (26,27), with age-related related reductions in heat loss capacity becoming more evident in older women (58 ± 5 yr) at exercise-induced heat loads ≥325 W in hot, dry environments (26). There is also evidence that age-related differences in evaporative heat loss become more apparent after 20 min of continuous exercise (27). Although the older women in that study were approximately 7 yr older than the older group in our study, and may have therefore possessed greater age-related decrements in heat loss capacity (15), it is possible that we would have observed impaired evaporative heat loss in older relative to young women if exercise was performed at a higher heat load and/or for a longer duration.
The impaired total heat loss in older relative to young women may have been associated with age-related reductions in skin temperature (Table 2), which caused subsequent increases in the thermal gradient for dry heat gain from the environment in older relative to young women during recovery in both dry and humid conditions (Fig. 2). Because these elevations in dry heat gain were not offset by concomitant increases in evaporative heat loss (Fig. 2), total heat loss was therefore lower in older adults compared with young adults, albeit only approaching significance during exercise (Fig. 1). While similar increases in dry heat gain and reductions in limb blood flow have been recently reported in older adults compared with young adults during passive heat exposure (13), the mechanism explaining these age-related increases in dry heat gain remains uncertain.
Despite the increased total body heat storage in older women compared with young women in both dry and humid heat (Fig. 3), rectal temperature was generally similar between each group in both conditions (Table 2). The only exception was during the third and fourth recovery periods in the dry condition, where rectal temperature was greater in the older women compared with young women. This discrepancy between thermometry and calorimetry has also been observed during intermittent exercise in young and older men (16,18), and may be explained by the phase delay associated with core temperature measurements from the rectum during non–steady-state conditions (28). In comparison to other regions of core temperature measurement (e.g., aural canal, esophagus), the rectum is a heavily insulated structure that receives limited blood flow during exercise (23), and therefore responds more slowly to changes in body heat storage. Indeed, our findings indicate that rectal temperature may not provide the sensitivity required to detect increases in body heat storage and thermal strain in older women relative to young women during intermittent activity.
Moreover, the observed elevations in total body heat storage in older women compared with young women were not coupled with similar increases in cardiovascular strain in older women in either dry or humid conditions (Table 2). Although this outcome is consistent with previous comparisons of young and older adults during exercise in both dry (17) and humid heat stress conditions (16), it is important to note that the young and older participants in these studies and in our study were selected on the basis of their training history and V˙O2peak to minimize between-group differences in aerobic fitness. In addition, the exercise protocol used in our study was of relatively short duration (2 h) when compared with the duration of many recreational (e.g., long-distance running, triathlon) and occupational activities (e.g., electrical utilities work, deep-mechanized mining). As such, we cannot discount the possibility that older women with lower aerobic fitness relative to their young counterparts might display greater cardiovascular strain during more prolonged activity in dry and humid heat.
These outcomes may have important implications for the design of strategies to protect older women from heat-related illness during daily and occupational activities performed in the heat. Recently, sex-related differences in local heat loss responses have been ascribed to morphological differences between men and women during low-to-moderate exercise in warm, dry conditions (20). However, during exercise in hotter environments, it is well established that women display sex-related reductions in heat loss capacity that are unrelated to morphology (7). As such, it is possible that older women may also be at greater risk of heat-related illness than older men during low- to moderate-intensity activity in the heat, particularly in areas of high humidity. Indeed, our findings indicate that alternative approaches need to be developed to prevent heat-related illness in older women in hot, humid conditions, and this represents an important area of future research. This is especially important, given the surge of women in the workforce over the past decades in occupations that often necessitate environmental heat exposure (i.e., mining, electrical utilities, forestry, agriculture, construction). Currently, heat exposure guidelines such as those associated with the American Conference of Governmental and Industrial Hygienists (ACGIH) Threshold Limit Values (TLV®; 1), are generalized to broad population groups and do not consider sex- or age-related differences in the body’s physiological capacity to dissipate heat in ascribing the work–rest allocations for different environmental conditions.
We showed that increasing ambient humidity reduces whole-body heat loss capacity in young and older women, but most importantly, we demonstrated that older women displayed reduced heat loss capacity relative to young women in both dry and humid heat stress. Our findings indicate that older women may be more vulnerable to heat-related illness than young women during protracted, low- to moderate-intensity exercise or work in areas of high heat and humidity, unless appropriate safety strategies considering the intensity of activity, age, and environmental conditions are used.
The authors thank all the participants who volunteered for the present study. The authors also thank Joanie Larose as well as all other members of the Human and Environmental Physiology Research Unit who assisted with data collection.
Grants: The study was conducted in the Human and Environmental Physiology Research Unit (HEPRU). This research was in part supported by the Ontario Ministry of Labour (14-R-001) and the Natural Sciences and Engineering Research Council of Canada (Discovery grant, RGPIN-06313-2014 and Discovery Grants Program-Accelerator Supplement, RGPAS-462252-2014) (all funds held by G. P. K.). G. P. K. is supported by a University of Ottawa Research Chair. S. R. N. is supported by a Postdoctoral Fellowship from the Human and Environmental Physiology Research Unit and M. P. P. is supported NSERC Canada Alexander Graham Bell Graduate Scholarship (CGS-D).
Disclosures: No conflict of interest, financial or otherwise, are declared by the author(s). The results of the present study do not constitute endorsement by ACSM. The results of the study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation.
M. P. P., S. G. H., A. D. F., P. B., R. J. S., and G. P. K. conceptualized and designed the research. M. P. P. performed experiments. S. R. N. and M. P. P. analyzed data. S. R. N., M. P. P., and G. P. K. interpreted results of experiments; S. R. N. prepared the figures. S. R. N. drafted the article. S. R. N., M. P. P., S. G. H., A. D. F., P. B., R. J. S., and G. P. K. edited and revised the article. S. R. N., M. P. P., S. G. H., A. D. F., P. B., R. J. S., and G. P. K. approved final version of the article.
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