Modifications of sweating responses to thermal transients following heat acclimation

Eur J Appl Physiol (1983) 50:235-246 European Journal o f

Applied Physiology and Occupational Physiology �9 Springer-Verlag 1983

Modifications of Sweating Responses to Thermal Transients Following Heat Acclimation

J. P. Libert, V. Candas, and J. J. Vogt

Centre d'Etudes Bioclimatiques, 21 Rue Becquerel, F-67087 Strasbourg C6dex, France

Summary. The sweating response was studied before and after passive humid heat acclimation in four resting male subjects who were exposed to slow thermal transients increasing air and wall temperatures from 28 ~ C to 45 ~ C. The slopes of the ambient temperature increases were +0.19~ C �9 min-a; +0.16~ �9 rain -1 or +0.14~ �9 rain -t. Dew-point temperature and air velocity were kept constant (17.5 ~ C; 0.3 m �9 s-l). Continuous measurements were made of oesophageal temperature, mean skin temperature, whole-body sweat loss and of right upper limb sweating responses. The local sweating response was measured from an arm chamber under a local thermal clamp (Tsk,1 = 38 ~ C). The results confirmed the fact that heat acclimation to humid heat induces a shortening in the time lag of sweat onset and increases the local sweating rates while internal temperature changes are reduced. These modifications are interpreted as a non-linearity in the response of the central controller, involving both a change in the central gain and an upward resetting of the "local sweating rate-body temperature" curves, without any shifting of the hypothalamic set-point temperature as it is currently described. However, a modification of local sweat gland activity occurring with heat acclimation cannot be ruled out.

Key words: Heat acclimation - Transient thermal loads - Local sweating response

The relative importance of both skin and core temperatures in the modification of the sweating response with acclimation is well documented. It is well known that repeated exposures of non-acclimated men to hot environments lead to a greater and earlier sweat secretion at equivalent, or even lower, peripheral and central thermal drives (Ladell 1951; Belding and Hatch 1963; Fox et al. 1964; Wyndham 1967). The reasons for this phenomenon remain unclear. Kuno

Offprint requests to: Dr. J. P. Libert (address see above)

0301-5548/83/0050/0235/$ 02.40

236 J .P. Libert et al.

(1954), Wyndham (1967), Hrnane and Valatx (1973), and Nadel et al. (1974), reported that the sweating modifications occurring with heat acclimation were mainly due to an increase in the excitability or the sensitivity of the central nervous system to thermal stimuli. Hrnane and Valatx (1973) and Nadel et al. (1974) have suggested that this increase was due to a diminution of the hypothalamic set-point temperature. Fox et al. (1964), Collins et al. (1966), and later Chen and Elizondo (1974) suggested that the increased sweat output following heat acclimation is the result of local sweat-gland training. However, Fox et al. (1964) and Collins et al. (1966) did not rule out increased sensitivity of the temperature regulating center as an explanation for the earlier sweat onset and its occurrence at lower body temperature. Finally, Elizondo and Bullard (1971) presented evidence that sweating response modifications following short-term acclimation could be due to a potentiation of subliminal impulses at the sweat gland level.

These earlier studies were carried out mostly in hot steady-state conditions. In the present study, young male subjects were exposed to slow ambient temperature rises before and after humid heat acclimation. This procedure makes it possible to evaluate the influences of body temperature on the onset of sweating; the onset of sweating being easier to locate during thermal transients, and on the transient phase of sweating following short-term acclimation to heat.

Methods

Four healthy male subjects, whose physical characteristics are given in Table 1, volunteered to participate in experiments performed in a climatic chamber. Each exposure was carried out on a given subject at the same time of the day: in the morning between 08 : 00 and 11 : 00 or the afternoon between 14 : 00 and 17 : 00.

The subjects, wearing briefs, lay still on seven rubber straps (max. 50 ram-width) to permit free circulation of air around the body surface. The subject was randomly exposed to programmed slow air (Ta) and wall (Tw) temperature changes, rising linearly from 28 ~ C to 45 ~ C ( T a = Tw) at a rate of change depending on the exposure. The slopes of ambient temperature increases were: +0.14 ~ C . rain-l; +0.16 ~ C �9 min-1; +0.19 ~ C - rain -1 (the durations of the transient phases being respectively about 120, 105 and 90 rain).

During the entire period of an experiment, air velocity (Va) and dew-point temperature were kept constant (Va = 0.3 m �9 s-l; Tdp = 17.5 ~ C). Before each exposure, the subject lay still in the climatic chamber in a near neutral environment for 60 min ( T a = T w = 2 8 ~ C ; Tdp = 17.5 ~ C; V~ = 0.3 m - s-l). This long waiting period was necessary to obtain steady body temperatures.

The acclimation procedure consisted of a passive heat treatment period over 10 days in hot humid heat (Ta = Tw -- 48 ~ C; Tap = 27.8 ~ Va = 0.3 m - s - l ; daily exposure duration = 165 min),

Prior to acclimation the subject was exposed twice a week to experimental conditions, and, after the heat treatment, experiments were carried out every day.

The following physiological data were recorded every minute throughout each experi- ment: a) Oesophageal temperature by a thermistor introduced into the oesophagus via the nose. The thermistor was placed by using the electrocardiographic method of Brengelmann et al. (1979); b) 17 local skin temperatures by thermistors attached to forehead, left scapular region; right pectoral region; right and left arms, forearms, hands; left upper abdominal quadrant; right lumbar region; right and left upper legs, lower legs and foot. Mean skin temperature was calculated from the formula of Hardy and DuBois (1938);

Thermoregulatory Sweating and Heat Acclimation in Man

Table 1. Physical charactreristics of subjects and exposure periods

237

Subject Age Height Weight Body surface Exposure area period

(year) (m) (kg) (m 2)

1 23 1.84 74 1.96 A.M. 2 27 1.77 65 1.80 A.M. 3 27 1.71 66 1.77 P.M. 4 25 1.77 63 1.78 P.M.

c) Whole-body sweat loss by a continuous recording balance (1 g precision). This system has been previously described by Candas et al. (1979). d) The local sweating response of the right upper limb of the subject. This sweating response was measured from an arm chamber by a dew-point hygrometer, Schlumberger HCP 2 S Aqmel (Libert et al. 1979). The air temperature in the arm chamber was regulated by a servo system which maintained the average upper limb skin temperature at 38.0 + 0.1~ whatever the ambient temperature variations in the climatic chamber. This average local skin temperature was calculated from three thermistors attached to the right arm, forearm and hand. Thus the effect of local skin temperature on local sweating was kept constant. The time lag of this system due to delay of line transport was 5 s. The arm chamber was highly ventilated so that any sweat appearing on the upper limb skin surface was immediately evaporated into the airstream. The local sweat rate was calculated by integrating the recordings of dew-point differences between the upstream air (climatic chamber air) and the downstream air over a i min-period.

Environmental data (air temperature, wall temperatures, dew-point temperature and air velocity) were continuously recorded.

The accuracy of all temperature readings was 0.05 ~ C and variations + 0.01 ~ C.

Results

Typical local and whole-body sweat losses recorded for a subject exposed to slow air heating (+0.14~ C �9 min -1) before and after heat acclimation are plotted against time in Figs. 1 and 2.

The delay before the onset of local sweating is defined as being the time before the first burst of sweat output above the steady base line obtained during the neutral period. After the local sweat onset, the sweat-gland activity followed the air temperature increase with a periodic pattern of discharge (Fig. 1).

Similarly, the time lag before the onset of whole-body sweating recorded by the weighing balance is defined as being the time during which there is no increase of insensible body weight loss compared to the neutral period (Fig. 2). After sweat onset, total body sweating increases before reaching a steady state level during the last 30 min of heat exposure.

In the present work, for very slow air temperature changes, there was no relationship between the time lag in sweat onset and the transient duration of air temperature increase for both whole-body and local sweating responses. This observation disagrees with that obtained for steep rises in air temperatures (Libert et al. 1978; Libert 1980) where the initiation of whole-body or local arm sweating is linearly related to the durations (between 3 and 15 min) of the transient air temperature rises. No valid explanation for this discrepancy has yet been put forward.

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Tkermoregulatory Sweating and Heat Acclimation in Man 239

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local sweating rates are larger (P < 0.001), but the sweat rates of the rest of the body surface calculated by deducing the local sweating from the whole-body sweat rates as indicated by body weight losses are not modified. Thus in our experiments, acclimation to heat results only in an increase of the local arm sweating rate.

In Fig. 4, average patterns of body temperature changes from pre-heating levels are plotted against time. The responses of subjects are so similar that averaging the data does not alter any significant detail in the individual recordings.

Pre-heat mean skin (Tsko) and oesophageal (Teso) temperatures levels are not significantly modified by heat acclimation. At the beginning of the heat exposure, internal temperature was represented by oesophageal temperature falls below its pre-heating level. This is probably due to the return of cooled peripheral blood from skin layers to the core, when peripheral vasodilation occurs with skin warming. After this transient decrease, internal temperature in the unacclimated subjects slowly rises up to the end of the heating period.

At the end of the heating period, average oesophageal temperatures were lower in the acclimated state as compared with unacclimated. This is explained by the shorter time lag of whole-body onset of sweating, observed after heat acclimation, since the rates of body evaporative loss remain constant (Fig. 3).

240 J.P. Libert et al.

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Thermoregulatory Sweating and Heat Acclimation in Man 241

The data in Fig. 4 show that mean skin temperature changes are not modified by heat acclimation. This is a consequence of the fact that the evaporative skin cooling of the body surface (excluding the right upper limb where the average skin temperature is artificially kept constant) is the same before and after acclimation.

In this study, analysis of the relationship between the sweating response and body temperature was restricted to the local sweating response because of the lower inertia of the device measuring local sweating rate as compared to that of the continuous recording balance.

Initiation of Local Sweating

Individual time lags of sweat onset are plotted in Fig. 5 against the mean skin and oesophageal temperature changes observed at the onset of sweating. There is a linear relationship between the onset of local sweating and the mean skin temperature change in the unacclimated as well as in the acclimated state.

For acclimated subjects, the onset of sweating occurs at a 1 ~ C lower rise in mean skin temperature than for unacclimated subjects. This difference is statistically significant (two-tailed t-test; P < 0.001).

At the time of sweat onset, after heat acclimation, oesophageal temperature changes tend to be smaller than before heat acclimation but the differences are not statistically significant. The internal temperature level varies widely among individuals, the extremes range from 36.14 ~ C to 36.97 ~ C for unacclimated and from 36.11~ C to 36.91~ C for acclimated subjects.

Transient Phases of Sweating

In Fig. 6, for each subject, the local sweating rate observed before and after acclimation is plotted against oesophageal temperature change, and illustrates the effect of two levels of mean skin temperature change (1-2 ~ C and 2 - 3 ~ C). These two levels have been selected because it is within these ranges that the most numerous data are obtained whatever the experimental conditions.

The relationships between local sweating rates and oesophageal temperature changes are fitted by a least square method. For subjects 1 (curves A, B); 2 (curve B'); 3 (curve A') and 4 (curves A, B, B'), the relationships present a discontinuity, indicating that the thermoregulatory system is not a linear responding system. The gain values of the thermoregulatory system defined by the slopes of the lines describing the relation between local sweating rate and oesophageal temperature change are not constant. The gain of the thermo- regulatory system is self adjusted as a function of oesophageal temperature change. The gain values of the thermoregulatory system are large for smaller internal temperature changes, but decrease with increasing oesophageal temperatures. This confirms the results obtained in a previous study (Libert et al. 1982) on eight unacclimated subjects.

242 J .P . Libert et at.

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Fig. 6. Individual local sweating rates plotted as a function of oesophageal temperatures for two levels of mean skin tempera ture change: A = 1 - 2 ~ C ( t ) ; B = 2 - 3 ~ C (&) before heat acclimation; A ' = 1 - 2 ~ C (�9 B ' = 2 - 3 ~ C (ZX) after acclimation. The values in parenthesis represent the m e a n values of mean skin tempera ture changes calculated for each curve A, B and A ' , B ' . The extreme values of the point of zero central drive are indicated by a r r o w s on the x-axis

This interdependence of the gain adjustment and the internal temperature change explains why the non-linearity of the thermoregulatory system can only be described in experimental conditions where the oesophageal temperatures vary both below and above the initial levels observed at the end of the pre-heating period. So, in our experiment it is not possible to describe a curvilinear relation for other curves than those cited above.

If we considered only non-linear relationships, it appears that, for smaller oesophageal temperature changes, the mean skin temperature rise induces an increase of the gain value. Before acclimation, the point of zero central drive for sweating measured at the intersection between the two isotherms of variation in mean skin temperature (A and B) and the x-axis, varies between extremes of -0 .09~ and -0.16~ These values represent the central temperature references for which there is no sweating response whatever the mean skin temperature changes. This central temperature reference slightly decreased with heat acclimation (extremes -0.12 ~ C and -0.18 ~ C) but, due to the scattering of the data, this difference observed with heat acclimation is not statistically significant. Thus, the main modifications occurring with heat acclimation are

Thermoregulatory Sweating and Heat Acclimation in Man 243

both an increase in the gain value and an upward resetting of the "arm sweating rate-oesophageal temperature" curves with no significant shift of the central temperature reference.

Discussion

Initiation of Sweating

In this study, we made the assumption that, if the sweat gland activity depended solely on cutaneous receptors or on internal receptors, sweat production should have been elicited by a critical level of either the mean skin or internal temperature. But for unacclimated subjects as well as for acclimated ones, we found that the sweat onset was neither linked to a definite peripheral temperature level nor to an internal one. This disagrees with earlier results of Winslow et al. (1937) and of Benzinger (1969), suggesting that the onset of sweating is linked to a critical peripheral or internal temperature level.

Our results also present evidence that sweating initiation does not depend on two different types of regulation, depending on heat acclimation as suggested by Colin and Houdas (1965). Unlike the observations of these authors, the sweat output did not appear to be elicited by a rise in mean skin temperature for acclimated subjects when compared to the rise in internal temperature for unacclimated subjects. On the other hand, the fact that local skin temperature was kept constant at the same level (38.0 ~ C), independently of heat acclimation, ruled out any hypothesis ascribing the shortening of the time lag of sweat onset to a local skin temperature readjustment at skin receptor level. In disagreement with these previous studies, our experiments indicate that both peripheral and internal inputs play a role in sweat initiation. This confirms, for the most part, the results found in this field.

The smaller mean skin temperature changes at sweat onset following repeated heat exposures justify the conclusion that, after acclimation, the earlier onset of sweating can be attributed to an increased sensitivity of the thermoregulatory system. This confirms the conclusion of other studies (Kuno 1954; Wyndham 1967; Nadel et al. 1974). However, given the present evidence, it is not possible to choose between explanations calling for an increased sensitivity due to a local sweat gland modification effect (Fox et al. 1964; Collins et al. 1966; Elizondo and Bullard 1971; Chen and Elizondo 1974) and those calling for a lowering of the hypothalamic set-point temperature above which the sweating response occurs (Wyndham 1967; H6nane and Valatx 1973; Nadel et al. 1974).

Transient Phase of Sweating

Acclimation results in an enhancement of the local arm sweating response without any modification of the rest of body sweating (Fig. 3). This latter fact does not concur with a large number of previous studies performed in more

244 J.P. Libert et al.

severe heat conditions than in the present work. Belding and Hatch (1963) also reported this observation in dry moderate heat exposure as long as the evaporative sweating is adjusted to dissipate the total heat load. Increasing local sweat rates at the extremities during heat acclimation has been previously described, although local temperature was not kept at a high level (H6fler 1968). However, in our experimental conditions, the larger local arm sweating rates are undoubtedly due to the high level of local skin temperature imposed on the right upper limb, implying a potentiation of the central impulses at the sweat gland level and consequently an increase in local secretory capacity.

The local sweating rate capability is mainly achieved by an increase of the gain value during the first stage of heat exposure for smaller changes in internal temperature. For higher internal temperature changes, the sensitivity increase is linked to an upward shifting of the "local sweating rate-oesophageal temperature change" curves (Fig. 6). The thermoregulatory system is not a linear responding system but the gain is self adjusted as a function of the internal temperature change.

Since Nadel et al. (1971) demonstrated that the gain change results from an increase in peripheral local effect, the question can be raised whether the gain increase observed with acclimation in our experiments was of peripheral or of central origin.

The peripheral mechanism by which acclimation modifies the gain value of the thermoregulatory system may be due to the distribution of local skin temperatures (which are particularly different over the body surface during the first stage of heat exposure) and/or to the increase in number of active glands and/or to sweat-gland training by repeated heat exposures. Peter and Wyndham (1966) showed that heat acclimation did not modify the number of active sweat glands on either site of the body. In the present work, with local sweating being recorded under a local thermal damp, the gain change occurring with acclimation can be related to local sweat-gland training.

However, our experiments also present some evidence that the sweat rate increase occurring with acclimation may be explained in terms of a central gain increase. For unacclimated subjects, results plotted in Fig. 6 show that, in spite of the constant level of local skin temperature (Tsk,1 = 38 ~ C), the mean skin temperature change of the rest of the body surface induces an increase in the gain value. In this condition we can assume that the gain variation is of central origin. This conclusion is also compatible with the results of Hellon (1972) and of Murakami and Sakata (1980) who showed the existence of non-linear thermosensitive hypothalamic neurons responding to hypothalamic and spinal temperature variations. However, the difficulty of comparing the neuronal response characteristics with input-output relations when the pattern of transduction is not known implies that this last explanation must be considered as inconclusive in arguing for or against a linear temperature - effector response characteristic, as long as further investigations have not been made in this area.

These inferences are not in agreement with those of H6nane and Valatx (1973), who suggested that heat acclimation results only in a shift of hypothalamic set-point temperature without any change in the gain of the

Thermoregulatory Sweating and Heat Acclimation in Man 245

control system; a point of view supported by Nadel et al. (1974). It appears from data plotted on Fig. 6 that the central temperature reference is not appreciably modified by heat acclimation. The discrepancy between our results and those of other authors may be explained on the following basis: Thermal transients, contrary to steady state environments, induce (1) a greater peripheral sensitivity thus, the control of sweating may be related both to central and to local mechanisms; (2) internal temperature falls and consequently there is an enhancement of the sweating regulation by gain control.

In conclusion, the results of this study support the concept that the increased secretory capacity of sweat glands occurring with heat acclimation is due to a stronger sweating drive not resulting from a decrease in the central temperature reference but involving an increase of the central gain of the thermoregulatory system.

Acknowledgements. The authors are indebted to Professor K. J. Collins and Professor B. Metz for their interest and critical review of the manuscript.

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Accepted July 15, 1982