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Department of Arctic Biology and Institute of Medical Biology, University of Tromsø, N-9037 Tromsø, Norway
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Two winter-insulated Norwegian reindeer (Rangifer tarandus tarandus) were exposed to air temperatures of 10, 20, 30, and 38°C while standing at rest in a climatic chamber. The direction of airflow through nose and mouth, and the total and the nasal minute volumes, respectively, were determined during both closed- and open-mouth panting. The animals alternated between closed- and open-mouth panting, but the proportion of open-mouth panting increased with increasing heat load. The shifts from closed- to open-mouth panting were abrupt and always associated with a rise in respiratory frequency and respiratory minute volume. During open-mouth panting, the direction of airflow was bidirectional in both nose and mouth, but only 2.4 ± (SD) 1.1% of the air was routed through the nose. Estimates suggest that the potential for selective brain cooling is markedly reduced during open-mouth panting in reindeer as a consequence of this airflow pattern.
heat stress; respiration; selective brain cooling; temperature regulation; thermal tachypnea
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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MANY MAMMALIAN SPECIES are able to cool their brain below body core temperature by a process termed selective brain cooling (SBC) (3, 7, 15, 17, 23, 24, 28). In artiodactyls, such as reindeer, SBC is caused by intracranial heat exchange between cool venous blood returning mainly from the nasal evaporative surfaces and warm arterial blood flowing to the brain (16). This exchange takes place in the cavernous sinus where the carotid artery forms the carotid rete (3). In most artiodactyls, the cooling of nasal venous blood for SBC is effectively enhanced through thermal panting (2). SBC has traditionally been ascribed a role as a mechanism for protection of the brain from overheating during hyperthermia (1, 6, 23), which thereby would allow extension of the range of body core temperatures over which an animal can function during dehydration and exercise (28). However, recently it has been suggested that SBC may act to modify thermoregulatory responses by changing the hypothalamic temperature (e.g., 12-15, 19, 21, 24).
In the resting winter-adapted reindeer, the rate of respiratory heat loss is enhanced through thermal panting at ambient temperatures above 0°C (4), whereas during running, thermal panting is initiated at considerably lower environmental temperatures (9). When exposed to mild heat loads, resting reindeer pant only through their nose (closed-mouth panting), whereas when exposed to more pronounced heat loads, the animals alternate between periods of closed- and open-mouth panting (17). Schmidt-Nielsen et al. (25) reported that respiratory cooling of the blood in the nasal mucosa of dogs is enhanced through increased panting both when the mouth is open and when it is closed. During open-mouth panting, this was reported to be achieved by inspiration through the nose and expiration through the mouth, which implies unidirectional airflow over the nasal mucosa. Accordingly, Johnsen et al. (17) reported a unidirectional airflow pattern similar to that reported for dogs by Schmidt-Nielsen et al. (25) in reindeer during open-mouth panting. Goldberg et al. (10), on the other hand, reported that shifts occur between unidirectional and bidirectional airflow through both the nose and mouth of resting and exercising open-mouth panting dogs.
Despite the fact that evaporative cooling of the nasal mucosa is generally assumed to be essential for efficient SBC (e.g. 3, 17), very little is known about the airflow distribution during open-mouth panting in mammals. The purpose of the present study, therefore, was to describe the variations in the airflow through the nose and the mouth in heat-stressed reindeer during shifts between closed- and open-mouth panting and to evaluate their possible implications for SBC.
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METHODS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Experimental animals. Two adult (3 yr old) female reindeer (Rangifer tarandus tarandus) were used. They were kept in outdoor pens at the Department of Arctic Biology, University of Tromsø, and were trained over a period of 1 yr before any experiments took place. Artificial reindeer feed [RF-80 (27)], mineral lick-stones (Slikko mineralstein, J. Østensjø, Haugesund, Norway), and water or snow were available ad libitum. The animals were weighed weekly (CWS-A1, Farmer Tronic AS, Give, Denmark), and their mean body mass during the experimental period was 60 ± 5 (SD) kg and 80 ± 9 kg for reindeer 1 and reindeer 2, respectively.
Experimental design. The direction of airflow in the nose and mouth, as well as the fractional nasal and total respiratory minute volumes and respiratory frequency, were determined during both closed- and open-mouth panting. All experiments were performed on animals while they were in their full winter pelts.
Experiments were performed in a climatic chamber (type 24/50 DU; Weiss Technik, Giessen, Germany) at air temperatures (Ta) of 10, 20, 30 and 38°C, which represent a mild to severe environmental heat load to resting, winter-insulated reindeer (4). The animals were always allowed to rest for
1 h at each Ta
before any measurements were made. The experimental periods lasted for
30 min during airflow direction measurements and for 30-70 min
during measurements of minute volumes. The temperature inside the
chamber was controlled within ±1°C in time and ±2°C in space at
Ta 10, 20, and 30°C. Additional heating by use of a
thermostatically controlled electrical heater (3 kW; VEAB El Master,
Lautom, Oslo, Norway) was required to raise Ta to 38°C.
During all experiments, Ta and the temperature 20 cm inside
the rectum (Trec) were continuously recorded by use of
copper-constantan thermocouples that were connected to a data
acquisition system (model PCA-48, Dianachart, Rockaway, NJ). The
thermocouples were regularly calibrated using a thermostatically
controlled calibration bath (model 6025, Hart Scientific, Pleasant
Grove, UT) and a 0°C ice point dry-well reference chamber (model
5115, Hart Scientific). During the measurements of minute volumes, the
temperature of the air that was collected in a 137-liter spirometer was
recorded using a copper-constantan thermocouple and a portable digital thermometer (type BAT-12, Physitemp, Clifton, NJ). The relative humidity of the chamber could not be controlled but varied between 29 and 58%, as determined by use of a humidity sensor (Vaisala HMI 32, Vaisala OY, Helsinki, Finland). The air pressure inside the chamber
equaled the outdoor barometric pressure, which was recorded at regular
intervals during experiments.
Directional airflow measurements. The direction of airflow through the nose and mouth was determined by recording air pressure changes inside the nasal and oral cavities. A 45-cm-long, 4-mm-OD, and 2-mm-ID polyethylene tube was inserted 2.5 cm into one of the nostrils and secured to the muzzle of the animal by use of tape. Another polyethylene tube was manually held inside the mouth during each period of open-mouth panting. The air-filled plastic tubes were connected to two pressure transducers (Transpac IV, ABBOT Ireland, Sligo, Ireland) that were connected via a Gould universal amplifier to a Gould thermal array recorder (model TA 4000, Gould Electronics, Cleveland, OH). The directional airflow measurements were also used for determination of respiratory frequency and for calculations of the fractions of time used for closed- and open-mouth panting.
Measurements of total minute volumes were made during episodes of both closed- and open-mouth panting, but only at those ambient temperatures at which episodes of open-mouth panting were observed. The animals were equipped with a mask (dead-space ~250 ml) covering both nose and mouth, which allowed the animal to freely open its mouth. The mask was equipped with one-way inlet and outlet valves. The expired air was collected into a 150-liter Douglas bag during timed periods of either closed- or open-mouth panting. After each sampling, the Douglas bag was emptied into a 137-liter spirometer for determination of the air volume. Measurements of nasal minute volumes during open-mouth panting were made by use of a mask (dead space ~35 ml) that covered only the nose of the animal. During periods of open-mouth panting, a valve unit with a one-way low-resistance valve for inspired air, and another for expired air, were easily fitted to the opening of the nose mask. The valve unit had a dead space of 20 ml. A 750-ml airtight plastic bag was connected to the inspiratory valve of the unit. Before each sampling, the bag was filled with an exact volume of ambient air by use of a 100-ml syringe with an airtight three-way stopcock. During sampling, the time required for the emptying of the bag was determined by use of a stopwatch. During measurements of minute volumes, nasal air pressure changes were simultaneously recorded. This was done to obtain data on respiratory frequency and also to allow comparison of breathing patterns with and without the mask systems, to make sure that the masks did not affect the respiratory patterns of the animals. All volumes were converted to standard temperature and pressure, dry, or STPD, values and were expressed as minute volumes per unit body mass (kg).Statistics.
Results from the two animals are presented separately as means ± SD of 2-5 experiments with each animal at each temperature. Statistical significance was tested by use of the nonparametric Wilcoxon's rank-sum test. Regression analyses were done by use of a
computer program (SigmaPlot, Jandel Scientific, San Rafael, CA).
P 
0.05 was considered significant.
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RESULTS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Directional airflow measurements.
Both animals always panted with their mouth either closed or open at
all ambient Ta values. The respiratory frequency
(fR) increased with increasing Ta, from
approximately 40 cycles/min during closed-mouth panting at
Ta = 10°C, to ~300 cycles/min during open-mouth
panting at Ta = 38°C (Fig.
1). Open-mouth panting coincided with a
fR of >200 cycles/min in both animals and was first
observed at Ta = 38°C in reindeer 1 and
at Ta = 20°C in reindeer 2. Shifts between closed- and open-mouth panting were abrupt without transitional stages (Fig. 2), and fR
during open-mouth panting was significantly higher than during
closed-mouth panting at the same Ta (P < 0.05, Fig. 1). The respired air was bidirectional through both nose and
mouth during all periods of open-mouth panting in both animals (Fig.
2).
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Minute volumes.
Shifts from closed- to open-mouth panting were always associated with a
significant increase in both fR (P < 0.05; Fig.
1) and respiratory minute volume (P < 0.05; Fig. 4),
whereas airflow through the nose fell markedly (Fig.
4). Thus, in reindeer 1, only
2.0% of the inspired air was routed through the nose during open-mouth
panting at Ta = 38°C, whereas in reindeer
2, 1.1, 2.8, and 3.6% of the inspired air were routed through the
nose during open-mouth panting at Ta 20, 30, and 38°C,
respectively (Fig. 4).
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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This study has shown that the flow of air in the open-mouth panting reindeer at rest is bidirectional both in the nose and in the mouth (Fig. 2). This is apparently in contrast to the unidirectional nasal airflow reported by Schmidt-Nielsen et al. (25) in open-mouth panting dogs and by Johnsen et al. (17) in reindeer. However, the study of Johnsen et al. was restricted to qualitative measurements of airflow direction in the nose alone, and inadequate bidirectional sensitivity in their system may have failed to detect the changes in airflow direction at the low nasal flow rates reported here during open-mouth panting. Goldberg et al. (10) confirmed the results of Schmidt-Nielsen et al. in dogs, but these authors observed that their dogs alternated between uni- and bidirectional nasal airflow during open-mouth panting. This suggests an important species difference in nasal airflow pattern between open-mouth panting dogs and reindeer.
This study has also shown that the nasal component of the respiratory minute volume in reindeer decreases with increasing heat load, despite an increase in respiratory frequency (Fig. 1) and total respiratory minute volume (Fig. 5). This decrease was due to a >92% reduction in nasal airflow as the animals shifted from closed- to open-mouth panting (Fig. 4), and overall nasal airflow consequently decreased as the proportion of open-mouth panting increased with increasing heat load (Fig. 3). As far as we know, such dramatic reductions in nasal airflow during shifts from closed- to open-mouth panting have not previously been reported for any panting mammal.
If one assumes that a supply of cool venous blood from the nasal mucosa is a prerequisite for SBC (e.g., 3, 17), a reduction of nasal airflow would imply a reduction in the capacity for SBC. Accordingly, Chesy et al. (7) observed a rise in brain temperature that coincided with the onset of open-mouth panting in the exercising ox, but they assumed that the rise in hypothalamic temperature caused the shift to open-mouth panting, and not the other way around. Data on brain temperature changes in relation to mode of panting in reindeer have not been reported. In fact, we are aware of only four publications that report brain temperature data in this species (17, 19, 20, 22), and of these, only one (17) is relevant for the problem at hand. Close scrutiny of the data presented in that report (e.g., Fig. 4 of Ref. 17) reveals that brain temperature was equal to, or even higher than, carotid arterial blood temperature under conditions that obviously imply open-mouth panting in a heat loaded resting reindeer. This suggests that SBC was indeed compromised in this situation. However, the animal still displayed some capacity for brain cooling, because brain temperature rose further, by ~0.5°C, upon clamping of the angular oculi vein.
To shed more light on this question, we calculated the effects of
a shift from closed- to open-mouth panting on the ability of the nasal
venous blood to cool the brain in one of our reindeer (reindeer
2). This was done first by estimating rates of respiratory heat
loss on the basis of recorded total and nasal minute volumes in various
experimental situations by use of the approach described by Folkow and
Blix (8). The results were used to estimate the theoretical maximum capacity for SBC, with the assumption that brain
temperature equals cerebral arterial temperature and that there is no
loss of cooling power between the nasal mucosa and the brain. The
estimates were made using the equation
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T is temperature change in carotid blood, equal to the
estimated maximum capacity for SBC (°C); RHLn is the
nasal respiratory heat loss rate (W/kg); m is mass flow of
cerebral arterial blood (0.048 g · s
1 · kg1); and c is the
specific heat of blood (3.650 J · g1 · °C1).
The mass flow of cerebral arterial blood (m) of 0.048 g · s1 · kg1 was estimated
from data on relative cerebral blood flow in resting sheep [2.05% of
cardiac output (11)], cardiac output in resting reindeer
[133 ml · min1 · kg1
(29)], and the density of human blood [1.060 g/ml
(30)]. The specific heat of reindeer blood (c) was
assumed to be the same as for human blood, i.e., 3.650 J · g1 · °C1
(30).
The estimates show that reindeer 2 was unable to maintain a high rate of respiratory heat loss at the highest Ta (Table 1). This was presumably a consequence of an inability to match the diminishing capacity for respiratory convective and evaporative heat loss, which was caused by Ta approaching body temperature and an increasing relative humidity at high Ta, with an increased respiratory minute volume. The resulting inability to maintain thermal balance is reflected in a significant increase in Trec at the highest Ta (Table 1). The estimates further show that nasal respiratory heat loss (RHLno) never contributed >0.05 W/kg, or 4% of total respiratory heat loss (RHLtot), during open-mouth panting (Table 1). Accordingly, the estimates of the theoretical maximum capacity for SBC show a decrease from 4.1-7.8°C (depending on heat load), during periods of closed-mouth panting, to only 0.2-0.3°C, during periods of open-mouth panting (Table 1). However, because our reindeer alternated between closed- and open-mouth panting at all the three Ta, the maximum capacity for SBC increased to ~0.9-7.3°C, depending on the heat load (Table 1).
These rough estimates suggest that, if SBC is dependent on a supply of cool venous blood from the nasal mucosa to the cavernous sinus, only minor SBC can be expected for resting reindeer during open-mouth panting at high heat loads. However, Khamas and Ghoshal (18) found that the major palatine veins of the sheep were continuous with the cavernous sinus via the pterygoid plexus, and these authors suggested that cooled blood from the palatine venous network may contribute to SBC. If the same is the case in reindeer, cool venous blood from the oral cavity could increase the theoretical maximum capacity for SBC during open-mouth panting, even in this species.
However, even so, why would a reindeer switch from a comfortable
closed-mouth to open-mouth panting when exposed to a severe heat load?
We suggest that it does so because of the resistance to airflow caused
by the conspicuous turbinates of the reindeer nose (5). A
steadily increasing heat load will impose a demand for enhanced
respiratory evaporation, which can only be met through increased
ventilation rates, because the temperature and water vapor saturation
of the exhaled air presumably are at or are close to their maximum
levels already at Ta of 10-20°C in winter-adapted reindeer (4). The resistance to nasal airflow may, in
fact, be particularly high at large heat loads, because the turbinate gap-width then presumably is particularly low because of increased nasal mucosal blood flow (16, 26). To further increase
ventilation (and thereby increase the respiratory heat loss rate), the
animal most likely must shift to open-mouth panting to allow the air to
follow an alternative low-resistance route. As a result, most of the
inspired (as well as the expired) air passes through the mouth during
open-mouth panting. In fact, our data suggest that the upper limit for
nasal airflow in reindeer is ~0.7-0.8
l · min1 · kg1, because
those were the highest values ever observed in both animals (Fig. 4).
Perspectives
This article reports that nasal airflow is drastically reduced in open-mouth panting reindeer under heat load. This may reduce the animal's capacity for selective brain cooling when it supposedly needs it the most, but the possible involvement of the oral surfaces in supplying cooled blood to the brain during open-mouth panting deserves attention. Even so, our observations may suggest that selective brain cooling in reindeer is more important when moderate heat loads prevail over long periods of time, rather than during short-lasting bouts of heavy exercise, as hitherto assumed.| |
FOOTNOTES |
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Address for reprint requests and other correspondence: L. P. Folkow, Dept. of Arctic Biology, Univ. of Tromsø, N-9037 Tromsø, Norway (E-mail: larsf{at}fagmed.uit.no).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 1 December 1999; accepted in final form 8 May 2000.
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REFERENCES |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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) and
open-mouth panting (
) in 2 adult female Norwegian
reindeer (reindeer 1,A; and reindeer
2,B) exposed to different ambient air temperatures
(Ta; °C). Nos. in parentheses are nos. of experiments.
* Significant differences (P
9.24 · Ta + 95.1 during closed-mouth panting
(r2 = 1). For reindeer 2,
fR = (0.190 · Ta)2 
R;
means ± SD;
l · min
;
l · min
) during periods of
alternating closed- and open-mouth panting at different Ta
(°C) in an adult female Norwegian reindeer (reindeer 2).
Data were derived by multiplying percentages for open- and closed-mouth
panting (Fig.