By cooling the hypothalamus during hyperthermia, selective brain cooling reduces the drive on evaporative heat loss effectors, in so doing saving body water. To investigate whether selective brain cooling was increased in dehydrated sheep, we measured brain and carotid arterial blood temperatures at 5-min intervals in nine female Dorper sheep (41 ± 3 kg, means ± SD). The animals, housed in a climatic chamber at 23°C, were exposed for nine days to a cyclic protocol with daytime heat (40°C for 6 h). Drinking water was removed on the 3rd day and returned 5 days later. After 4 days of water deprivation, sheep had lost 16 ± 4% of body mass, and plasma osmolality had increased from 290 ± 8 to 323 ± 9 mmol/kg (P < 0.0001). Although carotid blood temperature increased during heat exposure to similar levels during euhydration and dehydration, selective brain cooling was significantly greater in dehydration (0.38 ± 0.18°C) than in euhydration (−0.05 ± 0.14°C, P = 0.0008). The threshold temperature for selective brain cooling was not significantly different during euhydration (39.27°C) and dehydration (39.14°C, P = 0.62). However, the mean slope of lines of regression of brain temperature on carotid blood temperature above the threshold was significantly lower in dehydrated animals (0.40 ± 0.31) than in euhydrated animals (0.87 ± 0.11, P = 0.003). Return of drinking water at 39°C led to rapid cessation of selective brain cooling, and brain temperature exceeded carotid blood temperature throughout heat exposure on the following day. We conclude that for any given carotid blood temperature, dehydrated sheep exposed to heat exhibit selective brain cooling up to threefold greater than that when euhydrated.
- body temperature
- evaporative heat loss
several species of mammal, particularly members of the order Artiodactyla, which includes sheep, goats, pigs, and antelope, employ the carotid rete to lower brain temperature below arterial blood temperature, a process termed selective brain cooling (for reviews, see Refs. 15 and 30). In artiodactyls the carotid rete, a bilateral network of arteries in the main arterial supply to the brain, lies at the base of the brain within the cavernous sinus, which receives cool venous blood draining from the nasal mucosa and other areas of the head. The thin walls and large surface area of carotid rete vessels allow warmer arterial blood in the rete to exchange heat rapidly with the cool venous blood in the sinus, facilitating delivery of cooler arterial blood to the brain and thus selective brain cooling. In resting, normothermic mammals with a carotid rete, brain temperature usually exceeds carotid arterial blood temperature by 0.2 to 0.5°C. However, during the hyperthermia invoked by moderate exercise or heat exposure, brain temperature rises more slowly than blood temperature does and selective brain cooling is exhibited. Usually, the more hyperthermic an animal becomes, the greater the magnitude of selective brain cooling.
Traditionally, selective brain cooling was thought to protect the brain from thermal damage during heat stress (for a review, see Ref. 28). However, recent studies, mainly from free-living animals, have yielded data that are inconsistent with that concept (15, 30). Rather than being a process that favors protection of the brain, selective brain cooling appears to play a role in balancing thermoregulatory and osmoregulatory needs (15, 30). By cooling the hypothalamus and hence temperature sensors that drive evaporative heat loss (16), selective brain cooling inhibits evaporative heat loss and therefore conserves body water. In hyperthermic goats, Kuhnen (20) has shown that selective brain cooling substantially reduces respiratory evaporative water loss.
If selective brain cooling does serve primarily to reduce water loss, then it is likely to be particularly valuable to an animal that is under concurrent heat stress and dehydration. Jessen and his colleagues (17) have shown that selective brain cooling occurred rarely in three euhydrated black Bedouin goats kept in outdoor enclosures but frequently and to a larger extent when the goats were dehydrated. However, the dehydrated goats had carotid arterial blood and brain temperatures higher than those when they were euhydrated, so the increased selective brain cooling may have arisen only as a product of the higher core temperatures. In goats, at least under controlled laboratory conditions, there is a precise relationship between brain temperature and arterial blood temperature. At a threshold brain temperature of ∼38.8°C, brain temperature uncouples from arterial blood temperature and selective brain cooling is invoked (21, 22). Jessen et al. (17) were not able to determine whether the threshold for selective brain cooling was lowered with dehydration, an adjustment that might contribute to inhibiting evaporative heat loss, because the Bedouin goats rarely used selective brain cooling when euhydrated. Baker and Nijland (5) found that the threshold for selective brain cooling was unchanged by hydration status in goats exercising on a treadmill, and that, when expressed as a function of arterial blood temperature, selective brain cooling was not influenced by dehydration. Selective brain cooling, however, is not exclusively under thermal control, but also is influenced by nonthermal inputs, in particular, sympathetic nervous system activity (12, 30). Increased sympathetic activity during exercise increases the threshold for selective brain cooling (7, 23), so it may not be possible to distinguish the relative importance of hydration status on the implementation of selective brain cooling in exercising animals.
To investigate whether the threshold for selective brain cooling is lowered by dehydration and whether selective brain cooling is increased independently of core temperature, we measured brain and carotid arterial blood temperatures of sheep, a species in which selective brain cooling is conspicuous and in which evaporative heat loss occurs via panting and sweating. We did not wish to elicit severe hypohydration hyperthermia, so the nine sheep were exposed to only moderate heat stress (40°C dry heat for 6 h each day) and moderate dehydration (5 days of water deprivation). By obtaining the first continuous recordings of brain and carotid arterial blood temperature in laboratory animals during euhydration, dehydration, and rehydration, we have shown that dehydration did not alter the threshold temperature for selective brain cooling.
MATERIALS AND METHODS
Experiments were performed on nine adult female (age 2–3 years, mass 41 ± 3 kg, means ± SD) Dorper sheep (Dorset × Persian, Ovis aries). Dorpers are a mutton breed and carry a light wool coat. The sheep were obtained from the South African Agricultural Research Council, Irene, where they had been exposed to temperate summer heat in outdoor pens (indoors at night) before being transported to our laboratory. During the study, sheep were housed in either an indoor animal facility or a climatic chamber (see below) in two groups (sheep 1–5, sheep 6–9). Lucerne chaff and commercial sheep pellets (Epol, Johannesburg, South Africa) were provided each morning, and water was provided ad libitum (except during dehydration trials). All procedures were approved by the Animal Ethics Screening Committee of the University of the Witwatersrand (protocol number 2004/94/5).
Using general anesthesia and in sterile conditions, we implanted miniature data loggers with thermistor sensors for temperature measurement. Anesthesia was induced by intramuscular injection of ketamine (2.5 mg/kg, Anaket-V, Centaur Labs, Johannesburg, South Africa) and medetomidine hydrochloride (0.04 mg/kg, Domitor, Novartis, Johannesburg, South Africa), and maintained with isoflurane (2–3% in oxygen, Isofor, Safe Line Pharmaceuticals, Johannesburg, South Africa). Sheep received prophylactic long-acting penicillin (4–7 ml im, Peni LA, Phenix, South Africa) and an analgesic and anti-inflammatory medication (3.0–4.5 ml sc, Dexa-Tomanol, Centaur Labs, Johannesburg, South Africa). Lignocaine (0.1 g, Bayer Animal Health, Johannesburg, South Africa) mixed with epinephrine (Kyron Labs, Johannesburg, South Africa) was injected under the scalp at the site where the brain probe would be inserted, primarily to anesthetize the periosteum. Respiratory rate (visual observation), heart rate, and percentage hemoglobin oxygen saturation (Nonin Handheld Pulse Oximeter, Plymouth, MN) were monitored throughout surgery.
A thermistor in a blind-ended and thin-walled polytetrafluoroethylene tube (OD 0.9 mm; segment of a Straight Aortic Flush 4F Catheter, Cordis, The Netherlands) was inserted into the left common carotid artery at a position midway along the length of the neck, and advanced 80 mm, toward the heart. The base of the thermistor probe was secured by purse-string sutures to the vessel wall, while the remainder of the probe lay free in the arterial lumen. Outside the vessel, the thermistor was connected via PTFE-covered leads to a data logger (see below). The logger, covered with an inert wax (Sasol, EXP987, Johannesburg, South Africa), was placed in a subcutaneous pouch dorsal to the vessel. For brain temperature measurement, a second data logger was positioned subcutaneously in the neck, behind the left ear. Its lead was advanced subcutaneously to the skull, where it was connected to a head plate and guide tube. The guide tube, constructed from cellulose acetate butyrate tubing (length 44 mm, OD 3.2 mm, ID 1.6 mm; World Precision Instruments, Sarasota, FL) and sealed at the tip by a stainless-steel cap, was inserted via a hole drilled through the skull. Anatomic markers, verified in our previous study (25), were used to guide the probe tip near to the hypothalamus. The guide tube was connected to a small polyvinyl chloride headplate (10 × 10 × 3 mm), which was secured to the skull by two bone screws. A third temperature logger, with an on-board temperature sensor, was inserted into the abdominal cavity, via a small incision in the left paralumbar fossa. Wounds were treated with a topical antiseptic spray (Necrospray, Centaur Labs, Johannesburg, South Africa), and a stent bandage was sutured over the neck wound (the bandage was removed 1 wk after surgery). After surgery, sheep were returned to pens in the animal facility (see below).
At the end of all experiments, using the same anesthetic and surgical procedures as before, we removed data loggers, headplates, and thermistors. All loggers were in perfect order, wounds had healed, and there were no signs of infection. Examination of the carotid arteries revealed no occlusion, that is, the thermistor probes had measured the temperature of free-flowing blood. After recovery from surgery, sheep were returned to stock at the Agricultural Research Council.
Body temperature measurement.
The miniature data loggers (StowAway, Onset Computer, Pocasset, MA) had dimensions of ∼50 × 50 × 25 mm and a mass of ∼ 50 g when covered in wax. They had a storage capacity of 32 kb, a measurement range from +34 to +46°C, and a resolution of 0.04°C. The loggers were set to record temperatures averaged over 5-min epochs. The wax-coated loggers were calibrated against a high-accuracy thermometer (Quat 100, Heraeus, Hanau, Germany) in an insulated water bath, and all had a calibrated accuracy equal to or better than the resolution of the loggers (0.04–0.05°C).
For 2 wk before and for 12 days (sheep 1–5) or 22 days (sheep 6–9) after surgery, sheep were housed in an indoor animal facility (ambient temperature 22–25°C) with a 12:12-h light-dark cycle (lights on at ∼0700). At the start of experimental trials, sheep were transported to a temperature-controlled climatic chamber (7.5 m2) with a 12:12-h light-dark cycle (lights on at 0715), where they were housed for 11 days in a group of five animals (there was one nonexperimental animal with sheep 6–9). Sheep were fed and given water, and the climatic chamber was cleaned, once per day, but otherwise they were disturbed as little as possible. They were inspected regularly through a peephole by researchers and veterinary staff. On the first two days dry-bulb temperature was maintained at 23°C (relative humidity 40%). Thereafter, animals were exposed for 8 days to a cyclic regimen of thermal stress; between 0900 and 1500 dry-bulb temperature was set to 40°C (relative humidity 57%) and between 1500 and 0900 at 23°C (relative humidity 40%). On the final day, dry-bulb temperature was returned to 23°C. The schedule is summarized in Fig. 1 (bottom).
At 1200 on the 3rd day of heat exposure (day 5), drinking water was removed from the climatic chamber. Drinking water at a temperature of 39°C was returned 120 h later at the end of day 9 at 1200 to each sheep individually. The volume of water consumed by each sheep within 5-min after water was returned was recorded by an observer (the difference in mass of the container before and after drinking), and remaining water was removed from the room. Communal drinking water, at a temperature of ∼18°C, was returned 1 h later.
At ∼1510, after air temperature had been reduced from 40 to 23°C, on day 4 (the day before water removal) and day 8 (the 4th day of water deprivation; 99 h without drinking water), each sheep was weighed, and 10 ml blood was taken from the jugular vein. Hematocrit (Heraeus hematocrit centrifuge, Hanau, Germany) and osmolality (Wescor 5500 vapor pressure osmometer; Wescor, Logan, UT) were determined for each sample. We did not obtain weight or blood at the end of water deprivation (after 120 h) because stress induced by the interventions would have altered thermoregulatory responses of the sheep to rehydration (see below). After day 11, animals were returned to their pen in the indoor animal facility.
Because water was removed and returned at 1200, analyses were made with a day defined as 1200–1155. Daily minimum and maximum body temperatures were the lowest and highest, respectively, of the 288 5-min readings obtained within the 24 h. Comparisons of body temperatures and of selective brain cooling between euhydration and dehydration were made by comparing temperatures in the 48-h period before water removal (euhydration; days 3 and 4) and the final 48-h period of water deprivation (dehydration; days 8 and 9). Selective brain cooling was calculated as “carotid blood temperature − brain temperature,” with positive values reflecting positive selective brain cooling. Temperatures were compared statistically by repeated-measures ANOVA or paired t-tests, with Bonferroni correction, where appropriate.
To examine temperatures during hyperthermia, we averaged the temperatures recorded every 5-min between 1000 and 1200; we did not include data between 1200 and 1500, when body temperatures often were higher than those between 1000 and 1200, because of the confounding effects on temperature of returning water at 1200 after 5 days of water deprivation. Analyses of temperatures over the entire period when animals were hyperthermic (∼1000–1500), not available for the 5th day of water deprivation and so not reported here, yielded the same statistical results as analyses for 1000–1200.
Original 5-min recordings of body temperatures were used to find the daily mean, SD, minimum, maximum, and amplitude of carotid blood temperature and brain temperature for each animal. Following Jessen et al. (18), the relationship between brain temperature and blood temperature in each animal was analyzed by sorting all 5-min measurements of arterial blood temperature into 0.1-°C classes, and determining the mean, standard deviation, maxima, and minima of brain temperature at each class of blood temperature. The frequencies at which each of the 0.1°C classes of blood temperature occurred also were determined.
Sample size was nine, unless otherwise stated. Values of P < 0.05 are considered significant. All data are reported as means ± SD.
Effect of water deprivation on body mass and osmolality.
Measurements made 99 h after the start of the 120-h (5-day) water deprivation period revealed that body mass had decreased, on average, by 16% (from 41.3 ± 2.6 to 34.5 ± 1.8 kg, t = 10.2, P < 0.0001) and plasma osmolality had increased by 11% from 290 ± 8 to 323 ± 9 mmol/kg (t = 7.8, P < 0.0001). The percentage change in body mass was linearly correlated to the percentage change in osmolality (r2 = 0.5, P = 0.03). Hematocrit did not differ in euhydrated (33 ± 3%) and dehydrated (35 ± 3%) sheep (t = 1.6, P = 0.15), probably because of splenic contraction during blood sampling. Sheep continued to feed on lucerne and pellets throughout the period of water deprivation.
Effect of water deprivation on core temperature.
Mean 24-h carotid arterial blood temperature did not differ significantly between euhydration (39.29 ± 0.29°C; 48 h before water deprivation) and dehydration (39.42 ± 0.29°C, t = 1.2, P = 0.27; final 48 h of water deprivation). However, over 2 h on each day (1000–1200) when sheep were hyperthermic, mean carotid blood temperature was significantly higher in dehydration (39.83 ± 0.22°C) than in euhydration (39.55 ± 0.26°C, t = 2.5, P = 0.038), although only by ∼0.3°C. In contrast to what we would expect if the dehydrated animals displayed adaptive heterothermy (30), the minimum carotid blood temperature was significantly higher in the dehydrated state (38.90 ± 0.34°C) than in euhydration (38.54 ± 0.26°C, t=4.0, P = 0.004). That pattern can be seen clearly in Fig. 1 (middle), which shows all 5-min measurements of carotid blood temperature over the full experimental period for one sheep.
Patterns of abdominal temperature were similar to those of carotid arterial blood temperature. Mean 24-h abdominal temperature did not differ in euhydration (39.52 ± 0.29°C) and in dehydration (39.70 ± 0.31°C, t = 1.5, P = 0.16), but over the 2 h of heat exposure (1000–1200) on each day, abdominal temperature was slightly, but significantly, higher in dehydration (40.08 ± 0.29°C) than in euhydration (39.76 ± 0.24°C, t = 3.1, P = 0.015). On average, over the experimental period, abdominal temperature significantly exceeded carotid blood temperature, but only by ∼0.2°C (abdominal 39.48 ± 0.24°C, carotid 39.29 ± 0.20°C, t = 6.0, P = 0.0003). The amount by which abdominal temperature exceeded carotid blood temperature increased slightly at higher body temperatures; the average slope of straight lines regressing carotid blood temperature on abdominal temperature (n = 9) was 1.06 ± 0.07 (significantly different to 1, t = 2.4, P = 0.04).
Relationship between water deprivation, carotid blood temperature, and selective brain cooling.
The mean magnitude (averaged over nine sheep) of selective brain cooling did not differ in euhydration (0.05 ± 0.17°C) and in dehydration (0.16 ± 0.13°C, t = 1.8, P = 0.10) over the full 48 h that we analyzed. However, between 1000 and 1200 when the sheep were hyperthermic, selective brain cooling was significantly greater in dehydration (0.38 ± 0.18°C) than in euhydration, when, on average, it was absent (−0.05 ± 0.14°C, t = 5.2, P = 0.0008). There was considerable interindividual variability, with some sheep only occasionally exhibiting selective brain cooling and others using selective brain cooling routinely. Over the last 2 days of water deprivation, for example, the sheep shown in Fig. 1 spent 90% of the time employing selective brain cooling (calculated as the percentage of 5-min brain temperature readings at least 0.05°C cooler than carotid blood temperature), while another sheep used selective brain cooling for only 33% of the 48-h period. On average, the proportion of each period spent using selective brain cooling was the same in dehydrated sheep (64 ± 22%) and euhydrated sheep (48 ± 24%, t = 1.5, P = 0.17). Nevertheless, in all sheep there was a progressive increase in the magnitude of selective brain cooling between 1000 and 1200, when the animals were hyperthermic, over the period of water deprivation. Figure 2 (top) shows the mean difference between carotid blood and brain temperature (positive values = selective brain cooling) during the 2 h (1000–1200) of heat exposure (40°C), on the two days when sheep had access to water and over 5 days without water. Selective brain cooling was significantly greater on days 7, 8, and 9 (which correspond to days 3, 4, and 5 of dehydration; P < 0.05) than that during euhydration (F = 20.9, P < 0.0001). Mean carotid blood temperature (bottom), measured over the same period, was significantly higher (P < 0.01) than that in euhydration only on day 9 (F = 18.1, P < 0.0001).
Figure 3 (top) shows the average 24-h pattern of selective brain cooling when sheep were euhydrated and dehydrated. As a group, the sheep exhibited selective brain cooling throughout the night, and the magnitude did not differ between dehydration (0.13 ± 0.16°C) and euhydration (0.12 ± 0.20°C, t = 0.08, P = 0.94). Attenuation of selective brain cooling near to 0700 and 1900 was associated with climatic chamber lights switching on and off, respectively, and between 0800 and 0900 with cage cleaning and feeding. Dehydration did not alter the 24-h rhythm of carotid blood temperature; the area between the 24-h curves of carotid blood temperature (Fig. 3, bottom) did not differ significantly from 0 (t = 1.2, P = 0.28). Despite only small differences in carotid temperature during heat exposure in the euhydrated and dehydrated state, selective brain cooling, on average, was 2–3 times greater in dehydration than in euhydration.
To further investigate how progressive water deprivation altered selective brain cooling, we plotted brain temperature as a function of carotid blood temperature on each day, as shown typically in Fig. 4 for one sheep. In animals with access to drinking water (for example, Fig. 4, day 3 and day 4), brain temperature typically exceeded carotid blood temperature at carotid temperatures less than 39°C, while selective brain cooling of up to ∼0.5°C was evident at higher body temperatures, as shown by the slopes of linear regression lines fitted to the data. With continued water deprivation, the slopes of regression lines fitted to the data decreased (Fig. 4, days 5 to 9), reflecting a progressive increase in the magnitude of selective brain cooling and an increase in selective brain cooling independent of changes in carotid blood or brain temperatures.
The conventional analysis for summarizing the overall relationship between carotid blood and brain temperature (18) is shown in Fig. 5, for the sheep in Fig. 4, during euhydration (days 3 and 4) and dehydration (days 8 and 9). The frequency distribution of carotid blood temperature (top) shows that the mode of carotid blood temperature in this animal was similar in euhydration (39.9°C) and in dehydration (39.7°C) but that the animal did not exhibit carotid blood temperatures lower than 39.2°C when dehydrated. At the modes of carotid blood temperature, brain temperature, on average, was slightly lower than carotid blood temperature in both the euhydrated and dehydrated state (Fig. 5, bottom). Above the modes, the animal exhibited greater selective brain cooling when dehydrated than when euhydrated, despite experiencing a similar frequency of carotid blood temperatures.
The point at which mean brain temperature is equal to the mean carotid blood temperature, as shown in Fig. 5, is known as the threshold of selective brain cooling. For the nine sheep, the mean threshold temperature for selective brain cooling was not significantly different during euhydration (39.27 ± 0.78°C) and dehydration (39.14 ± 0.39°C, t = 0.51, P = 0.62). However, the mean slope of lines of regression of brain temperature on carotid blood temperature above the threshold was significantly lower in dehydrated (slope = 0.40 ± 0.31) than in euhydrated (slope = 0.87 ± 0.11, t = 4.1, P = 0.003) sheep. Thus the mean body temperature at which selective brain cooling was initiated did not change, but there was increased sensitivity at carotid blood temperatures above the threshold, leading to selective brain cooling up to threefold greater in dehydrated than in euhydrated sheep.
Effect of rehydration on carotid blood temperature and selective brain cooling.
Before the return of drinking water at 1200 on the 5th day of water deprivation, all sheep were exhibiting substantial selective brain cooling (0.62 ± 0.18°C, range 0.40 to 1.04°C), as shown for two sheep in Fig. 6. Return of drinking water (long arrow), heated to a temperature of 39°C, led to rapid rise in brain temperature (0.13°C/min, on average over 5 min), and the abolition of selective brain cooling. The response of brain and carotid blood temperatures of the sheep shown in the Fig. 6, top, was typical for eight of the nine sheep, which drank between 4.1 and 6.1 liters of the warm water. After the abolition of selective brain cooling, there was a gradual fall in carotid blood and brain temperature, at a time when body temperatures continued to rise slowly on the other days of heat exposure, and selective brain cooling was conspicuous (see Fig. 3, for example). Carotid blood and brain temperature fell further at 1300 (small arrow), when communal drinking water (at ∼18°C) was returned. We could not measure the volume consumed by each sheep at this time.
One sheep (Fig. 6, bottom) was reluctant to drink at 1200 and consumed only 0.3 liters of water. Unlike the other sheep, following the initial abolition of selective brain cooling when water was offered, this sheep reinstituted selective brain cooling about 15 min later. When communal drinking water was returned at 1300, the sheep exhibited a second steep rise of brain temperature and thereafter did not exhibit significant selective brain cooling until 2100 that night. Following drinking of water by the sheep at 1300 (which we observed through the peephole), there was a pronounced fall in carotid blood temperature.
As shown in Figs. 2 and 4, over the 24 h following the return of drinking water (day 10), there was a reduction in selective brain cooling during heat stress. On average, brain temperature exceeded carotid blood temperature by 0.18 ± 0.13°C between 1000 and 1200, during heat exposure on the day following water return (Fig. 2), significantly more than that when the sheep were euhydrated (t = 3.5, P = 0.008). Indeed, on the day following rehydration, the sheep employed selective brain cooling only between 2000 and 0620, that is, only at night, and at a time when ambient temperature was 23°C and body temperature was below 39.5°C. The absence of selective brain cooling during heat exposure was associated with a carotid blood temperature 0.51°C lower than that over the initial 48 h of euhydration (t = 4.0, P = 0.004; Fig. 2).
We have shown for the first time that the threshold temperature for selective brain cooling in sheep is not shifted by dehydration. Thus, dehydrated sheep do not further reduce evaporative water loss during heat stress by employing selective brain cooling at lower core temperatures. However, at carotid artery blood temperatures above the threshold for selective brain cooling (∼39.2°C), the sheep increased the magnitude of selective brain cooling when they were dehydrated. That increase was not a consequence of higher core temperatures, but rather appeared to reflect increased sensitivity for selective brain cooling at carotid blood temperatures above the threshold. Despite similar carotid blood temperatures during 6 h of heat exposure (40°C), selective brain cooling was up to threefold greater in dehydration than in euhydration. Unlike Jessen et al. (17) in their study on goats, we were able to determine unequivocally the threshold of selective brain cooling in our sheep in both hydration states because the sheep exhibited selective brain cooling frequently when they were hydrated and normothermic. Indeed, the typical response of sheep at night, when their carotid temperatures were decreasing slowly, was to implement low-amplitude (mean ∼0.13°C) selective brain cooling (Fig. 3). That pattern has been observed before in sheep in the laboratory (see Fig. 2 and Ref. 26) and in sheep housed outdoors (A. Fuller, unpublished observations), and may explain why we did not detect a significant increase in the percentage of time spent using selective brain cooling in dehydrated animals. So if selective brain cooling indeed is a means for reducing evaporative heat loss, the dehydrated sheep increased their water saving by lowering brain temperature further below carotid blood temperature during heat stress, presumably by increasing flow of cool venous blood to the cavernous sinus (24), rather than by employing selective brain cooling for longer periods.
If lowering of brain temperature reduced the drive on evaporative heat loss effectors, and so attenuated panting and sweating, we would have expected the sheep to exhibit higher core temperatures when selective brain cooling was greatest. We found that mean carotid arterial blood temperature was higher during heat stress in dehydrated sheep than in euhydrated sheep, but only by about 0.3°C. Intermittent measurements of rectal temperature obtained, most famously, from camels (34), but also from many other species possessing a carotid rete (e.g., see Ref. 35), indicate that dehydration elicits a hyperthermia of up to 3°C at rest in hot environments. There is no evidence that dehydration leads to elevated core temperature in animals not exposed to heat (3). The weak hypohydration hyperthermia that we observed in our heat-exposed sheep, a tenth of that observed previously in other species, was not the consequence of us measuring carotid arterial blood temperature, rather than rectal temperature, as a measure of the animal's thermal status. We made concurrent measurements of carotid arterial blood temperature and temperature within the abdomen, and abdominal temperature also was elevated by only 0.3°C during heat exposure in dehydrated sheep.
Like our sheep, Bedouin goats housed outdoors demonstrated moderate hypohydration hyperthermia, with maximum daily carotid blood temperature elevated by only about 0.6°C (17). The researchers argued that the hyperthermia expected from reduced evaporative water loss in dehydrated animals was countered by a decrease in metabolic heat production. Metabolic rate may be up to 40% lower in dehydrated than in euhydrated Bedouin goats, even if the animals continue to feed (9, 32). Reduced metabolism also has been observed in dehydrated cats (10) and camels (33). If a similar decrease in metabolic rate occurred in our sheep during dehydration, the modest increase in core temperature of our heat-exposed and dehydrated sheep is not at odds with the inhibition of evaporative heat loss by selective brain cooling.
We did not measure whether selective brain cooling was associated with changes in respiratory or cutaneous water loss or whether the animals demonstrated changes in heat production, because making those measurements in our laboratory would have required human interference, or the isolation of animals and attachment of tethered equipment. Our protocol required that the animals be housed communally, with as little interference from human observers as possible. That requirement was imposed because of the well-described effects of sympathetic stress on selective brain cooling in sheep (4, 25, 31) and other ungulates (30). Sympathetic stimulation inhibits selective brain cooling, and, in sheep, appears to do so either by constricting the angularis oculi veins, which supply the cavernous sinus with cool venous blood returning from the nasal mucosa, or by constricting arteriovenous anastomotic shunts in the nasal mucosa (4, 24). Our sheep exhibited transient increases in brain temperature and abolition of selective brain cooling in response to people in the climatic chamber, and even when the lights in the chamber were switched on or off (Fig. 3). Return of drinking water after 5 days of water deprivation also led to a rapid rise in brain temperature and abolition of selective brain cooling. We believe that this rise in brain temperature was not related to water intake itself but reflected excitement and sympathetic activation. In the one sheep that did not drink appreciably when water was offered initially, brain temperature rose rapidly like that of the other animals. In that sheep, selective brain cooling again was suppressed only after communal drinking water was restored (Fig. 6, bottom).
The finding that selective brain cooling did not return in the other eight sheep after they had consumed water at 1200 (Fig. 6, top), even though they still were exposed to heat and that their carotid blood temperatures were above the threshold for selective brain cooling, further supports our view that selective brain cooling serves to reduce evaporative heat loss in dehydrated sheep. After 120 h of water deprivation, all of the sheep were using substantial selective brain cooling during the heat-exposure period (mean ∼0.6°C). But after the sheep drank and their body fluid status presumably was restored, selective brain cooling was inhibited until ∼2000 and was absent during heat exposure on the following day. In parallel with the inhibition of selective brain cooling in the 24-h period following the return of drinking water, carotid blood temperature was lower in that period than on any other day of the study (Fig. 2), raising the possibility that rehydration after dehydration initiates rebound suppression of selective brain cooling and enhanced evaporative cooling.
Our study is the first to describe brain and carotid blood temperatures in the 24 h following rehydration. Jessen and colleagues (17) and Robertshaw and Dmi'el (32) reported changes in brain and arterial blood temperatures following rehydration in goats, but for only 90 min. Both of those studies showed a rapid fall in blood temperature, associated with inhibition of selective brain cooling. In hyperthermic animals, part of the drive to increase heat loss is provided by brain temperature sensors (14), so the rapid rise in brain temperature at drinking may be responsible for enhancing evaporative heat loss. Indeed, Baker (2) has shown that hypohydrated goats increased sweating 3 min after starting to drink, when plasma volume and osmolality had not changed, but at a time when we predict brain temperature would have been rising at its most rapid rate. However, there are likely to be additional mechanisms, other than a rise in brain temperature, that contribute to the initial fall in carotid blood temperature with drinking. Baboons, which do not possess a carotid rete and are incapable of regulating brain temperature independently of arterial blood temperature (27), also demonstrated a rapid fall in blood temperature (and a fall rather than rise in brain temperature) after drinking body-temperature water after three days of water deprivation (D. Mitchell, unpublished observations). Dehydration-induced suppression of cutaneous vasodilatation was released immediately after drinking in humans, presumably by the stimulation of oropharyngeal reflexes (19).
Identification of the stimulus that enhances selective brain cooling during dehydration, at carotid blood temperatures above the threshold for selective brain cooling, requires further study. We have shown a progressive increase in the magnitude of selective brain cooling with continued dehydration (Figs. 2 and 4), the interpretation being that selective brain cooling changes in response to progressive changes of another variable. Typically, water deprivation leads to decreased body water and an increase in osmolality. It is widely held that changes in fluid status and osmotic stimuli are responsible for modifying temperature regulation mechanisms in dehydration (1), possibly by increasing ANG II and AVP release (8). In sheep and other ruminants, the effects of water deprivation are buffered because water is drawn from the rumen in the early stages of dehydration (13). Nevertheless, plasma osmolality and sodium concentration increase in sheep even after 1 day of water deprivation (28), so it is possible that hyperosmolality may have played a role in mediating the increased selective brain cooling in our animals.
A major effect of dehydration on the thermoregulation of heat-exposed mammals is to reduce and delay the onset of evaporative heat loss (1). Our sheep had a light wool coat and were acclimatized to heat, so cutaneous evaporative heat loss may have presented a significant route for heat dissipation. Nevertheless, panting usually is the major avenue of evaporative heat loss in sheep during heat exposure (6). Heat exchange with inspired air leads to cooling of venous blood in the nasal mucosa, so it may seem counterintuitive that selective brain cooling was increased during dehydration. However, Kuhnen and Jessen (21) showed, in an elegant series of experiments using extracorporeal heat exchangers in goats, that selective brain cooling may be absent at high levels of respiratory evaporative heat loss and present even when respiratory evaporative heat loss is negligible. Indeed, our sheep exhibited selective brain cooling at night, when their core temperature was decreasing and respiratory evaporative heat loss was likely to have been low. It is possible, though, that respiratory evaporative heat loss of our sheep was not altered by dehydration. Mammals that have the capacity for sweating and panting may suppress only sweating, while increasing the rate of panting, when they are dehydrated (11, 32). We obtained sporadic measurements of respiratory rate from five of the sheep and found that it did not differ between euhydration and dehydration (data not shown).
In summary, we have shown that an elevated core temperature, typically associated with hypohydration, was not responsible for enhanced selective brain cooling in dehydrated sheep. Rather, our study supports the idea (17) that dehydration increases selective brain cooling, which, in turn, contributes to the inhibition of evaporative heat loss, and promotes the development of hyperthermia. Dehydration did not alter the threshold temperature for selective brain cooling, but, above that threshold, at any given carotid blood temperature, dehydrated animals exhibited selective brain cooling up to threefold greater in magnitude than that when euhydrated. Rehydration suppressed selective brain cooling, even when the sheep were exposed to heat. How dehydration mediates an increase in the sensitivity of selective brain cooling is not clear.
This work was funded by the National Research Foundation, South Africa, the Iris Ellen Hodges Trust, and the Friedel Sellschop Award from the University of the Witwatersrand.
We thank the staff of the Central Animal Service at the University of the Witwatersrand for their care of the animals, and Brenda de Witt, Linda Fick, Robyn Hetem, Peter Kamerman, and Lennox Nqobo for help with experiments.
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- Copyright © 2007 the American Physiological Society