Threshold and slope of selective brain cooling

Body water conservation through selective brain cooling by the carotid rete: a physiological feature for surviving climate change?

Some mammals have the ability to lower their hypothalamic temperature below that of carotid arterial blood temperature, a process termed selective brain cooling. Although the requisite anatomical structure that facilitates this physiological process, the carotid rete, is present in members of the Cetartiodactyla, Felidae and Canidae, the carotid rete is particularly well developed in the artiodactyls, e.g. antelopes, cattle, sheep and goats. First described in the domestic cat, the seemingly obvious function initially attributed to selective brain cooling was that of protecting the brain from thermal damage. However, hyperthermia is not a prerequisite for selective brain cooling, and selective brain cooling can be exhibited at all times of the day, even when carotid arterial blood temperature is relatively low. More recently, it has been shown that selective brain cooling functions primarily as a water-conservation mechanism, allowing artiodactyls to save more than half of their daily water requirements. Here, we argue that the evolutionary success of the artiodactyls may, in part, be attributed to the evolution of the carotid rete and the resulting ability to conserve body water during past environmental conditions, and we suggest that this group of mammals may therefore have a selective advantage in the hotter and drier conditions associated with current anthropogenic climate change. A better understanding of how selective brain cooling provides physiological plasticity to mammals in changing environments will improve our ability to predict their responses and to implement appropriate conservation measures.

Adaptation to Heat and Water Shortage in Large, Arid-Zone Mammals

Although laboratory studies of large mammals have revealed valuable information on thermoregulation, such studies cannot predict accurately how animals respond in their natural habitats. Through insights obtained on thermoregulatory behavior, body temperature variability, and selective brain cooling in free-living mammals, we show here how we can better understand the physiological capacity of large mammals to cope with hotter and drier arid-zone habitats likely with climate change.

Coordination of breathing with nonrespiratory activities

Many articles in this section of Comprehensive Physiology are concerned with the development and function of a central pattern generator (CPG) for the control of breathing in vertebrate animals. The action of the respiratory CPG is extensively modified by cortical and other descending influences as well as by feedback from peripheral sensory systems. The central nervous system also incorporates other CPGs, which orchestrate a wide variety of discrete and repetitive, voluntary and involuntary movements. The coordination of breathing with these other activities requires interaction and coordination between the respiratory CPG and those governing the nonrespiratory activities. Most of these interactions are complex and poorly understood. They seem to involve both conventional synaptic crosstalk between groups of neurons and fluid identity of neurons as belonging to one CPG or another: neurons that normally participate in breathing may be temporarily borrowed or hijacked by a competing or interrupting activity. This review explores the control of breathing as it is influenced by many activities that are generally considered to be nonrespiratory. The mechanistic detail varies greatly among topics, reflecting the wide variety of pertinent experiments.

Selective brain cooling in Arabian oryx (Oryx leucoryx): a physiological mechanism for coping with aridity?

Selective brain cooling is a thermoregulatory effector proposed to conserve body water and, as such, may help artiodactyls cope with aridity. We measured brain and carotid blood temperature, using implanted data loggers, in five Arabian oryx (Oryx leucoryx) in the desert of Saudi Arabia. On average, brain temperature was 0.24±0.05°C lower than carotid blood temperature for four oryx in April. Selective brain cooling was enhanced in our Arabian oryx compared with another species from the same genus (gemsbok Oryx gazella gazella) exposed to similar ambient temperatures but less aridity. Arabian oryx displayed a lower threshold (37.8±0.1°C vs 39.8±0.4°C), a higher frequency (87±6% vs 15±15%) and a higher maximum magnitude (1.2±0.2°C vs 0.5±0.3°C) of selective brain cooling than did gemsbok. The dominant male oryx displayed less selective brain cooling than did any of the other oryx, but selective brain cooling was enhanced in this oryx as conditions became hotter and drier. Enhanced selective brain cooling in Arabian oryx supports the hypothesis that selective brain cooling would bestow survival advantages for artiodactyl species inhabiting hot hyper-arid environments.

Thermoregulation in pronghorn antelope (Antilocapra americana, Ord) in winter

Conservation of energy is a prerequisite thermoregulatory strategy for survival in northern hemisphere winters. We have used thermistor/data logger assemblies to measure temperatures in the brain, carotid artery, jugular vein and abdominal cavity, in pronghorn antelope to determine their winter body temperature and to investigate whether the carotid rete has a survival role. Over the study period mean black globe and air temperature were -0.5+/-3.2 degrees C and -2.0+/-3.4 degrees C, respectively, and mean daytime solar radiation was approximately 186 W m(-2). Brain temperature (T(brain), 39.3+/-0.3 degrees C) was higher than carotid blood temperature (T(carotid), 38.5+/-0.4 degrees C), and higher than jugular temperature (T(jugular), 37.9+/-0.7 degrees C). Minimum T(brain) (38.5+/-0.4 degrees C) and T(carotid) (37.8+/-0.2 degrees C) in winter were higher than the minimum T(brain) (37.7+/-0.5 degrees C) and T(carotid) (36.4+/-0.8 degrees C) in summer that we have reported previously. Compared with summer, winter body temperature patterns were characterized by an absence of selective brain cooling (SBC), a higher range of T(brain), a range of T(carotid) that was significantly narrower (1.8 degrees C) than in summer (3.1 degrees C), and changes in T(carotid) and T(brain) that were more highly correlated (r=0.99 in winter vs r=0.83 in summer). These findings suggest that in winter the effects of the carotid rete are reduced, which eliminates SBC and prevents independent regulation of T(brain), thus coupling T(brain) to T(carotid). The net effect is that T(carotid) varies little. A possible consequence is depression of metabolism, with the survival advantage of conservation of energy. These findings also suggest that the carotid rete has wider thermoregulatory effects than its traditional SBC function.

Fever and sickness behavior during an opportunistic infection in a free-living antelope, the greater kudu (Tragelaphus strepsiceros)

To study their thermal responses to climatic stress, we implanted seven greater kudu ( Tragelaphus strepsiceros) with intra-abdominal, brain, carotid, and subcutaneous temperature data loggers, as well as an activity logger. Each animal was also equipped with a collar holding a miniature black globe thermometer, which we used to assess thermoregulatory behavior. The kudu ranged freely within succulent thicket vegetation of the Eastern Cape Province, South Africa. The kudu spontaneously developed a bacterial pneumonia and consequent fever that lasted between 6 and 10 days. The fever was characterized by a significant increase in mean 24-h abdominal temperature from 38.9 ± 0.2°C to 40.2 ± 0.4°C (means ± SD, t6 = 11.01, P < 0.0001), although the amplitude of body temperature rhythm remained unchanged ( t6 = 1.18, P = 0.28). Six of the kudu chose warmer microclimates during the fever than when afebrile ( P < 0.0001). Despite the selection of a warmer environment, on the first day of fever, the abdominal-subcutaneous temperature difference was significantly higher than on afebrile days ( t5 = 3.06, P = 0.028), indicating vasoconstriction. Some kudu displayed increased frequency of selective brain cooling during the fever, which would have inhibited evaporative heat loss and increased febrile body temperatures, without increasing the metabolic maintenance costs of high body temperatures. Average daily activity during the fever decreased to 60% of afebrile activity ( t6 = 3.46, P = 0.014). We therefore have recorded quantitative evidence for autonomic and behavioral fever, as well as sickness behavior, in the form of decreased activity, in a free-living ungulate species.

Dehydration increases the magnitude of selective brain cooling independently of core temperature in sheep

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 degrees C, were exposed for nine days to a cyclic protocol with daytime heat (40 degrees 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 degrees C) than in euhydration (-0.05 +/- 0.14 degrees C, P = 0.0008). The threshold temperature for selective brain cooling was not significantly different during euhydration (39.27 degrees C) and dehydration (39.14 degrees 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 degrees 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.

The contribution of carotid rete variability to brain temperature variability in sheep in a thermoneutral environment

The degree of variability in the temperature difference between the brain and carotid arterial blood is greater than expected from the presumed tight coupling between brain heat production and brain blood flow. In animals with a carotid rete, some of that variability arises in the rete. Using thermometric data loggers in five sheep, we have measured the temperature of arterial blood before it enters the carotid rete and after it has perfused the carotid rete, as well as hypothalamic temperature, every 2 min for between 6 and 12 days. The sheep were conscious, unrestrained, and maintained at an ambient temperature of 20-22 degrees C. On average, carotid arterial blood and brain temperatures were the same, with a decrease in blood temperature of 0.35 degrees C across the rete and then an increase in temperature of the same magnitude between blood leaving the rete and the brain. Rete cooling of arterial blood took place at temperatures below the threshold for selective brain cooling. All of the variability in the temperature difference between carotid artery and brain was attributable statistically to variability in the temperature difference across the rete. The temperature difference between arterial blood leaving the rete and the brain varied from -0.1 to 0.9 degrees C. Some of this variability was related to a thermal inertia of the brain, but the majority we attribute to instability in the relationship between brain blood flow and brain heat production.

Mechanisms for the control of respiratory evaporative heat loss in panting animals

Panting is a controlled increase in respiratory frequency accompanied by a decrease in tidal volume, the purpose of which is to increase ventilation of the upper respiratory tract, preserve alveolar ventilation, and thereby elevate evaporative heat loss. The increased energy cost of panting is offset by reducing the metabolism of nonrespiratory muscles. The panting mechanism tends to be important in smaller mammalian species and in larger species is supplemented by sweating. At elevated respiratory frequencies and body temperatures alveolar hyperventilation begins to develop but is accompanied by a decline in the control of carbon dioxide partial pressure in arterial blood, probably through central chemoreceptors. Most heat exchange takes place at the nasal epithelial lining, and venous drainage can be directed to a special network of arteries at the base of the brain whereby countercurrent heat transfer can occur, which results in selective brain cooling. Such a phenomenon has also been suggested in nonpanting species, including humans, and although originally thought to be a mechanism for protecting the thermally vulnerable brain is now considered to be one of the thermoregulatory reflexes whereby respiratory evaporation can be closely controlled in the interests of thermal homeostasis.

Selective brain cooling seems to be a mechanism leading to human craniofacial diversity observed in different geographical regions

Selective brain cooling (SBC) can occur in hyperthermic humans despite the fact that humans have no carotid rete, a vascular structure that facilitates countercurrent heat exchange located at the base of the skull in some mammals. Emissary and angular veins, upper respiratory tract, tympanic cavity and cerebrospinal fluid are major components of SBC system in humans. The efficiency of SBC is increased by evaporation of sweat on the head and by ventilation through the nose, but it is surprising to find out that mammals do not display SBC during exercise hyperthermia. What is the explanation then for the SBC at high body temperatures? Our hypothesis is that selective brain cooling protects the brain from thermal damage in a long-standing manner by allowing adaptive mechanisms to change the craniofacial morphology appropriate for different environmental conditions. Since the brain can only be as big that can cool, it is not surprising to find a lower (below 1300 cm(3)) cranial volume in Australian Aborigines with respect to the one (over 1450 cm(3)) in Eskimos. In addition to lower brain volume, other craniofacial features such as thick everted lips, broader nasal cavity and bigger paranasal sinuses that provide more evaporating surfaces seem to be anatomical variations developed in time for an effective SBC in hot climates. It was reported previously that these biological adaptations result from the tissues of neural crest origin. Among the crest derivatives, leptomeninges (pia and arachnoid mater), skeletal and connective tissues of the face and much of the skull seem to be structures upon which environment operates to produce more convenient craniofacial morphology for an effective SBC. In conclusion, selective brain cooling seems to be a mechanism leading to adaptive craniofacial diversity observed in different geographical regions. Thus, SBC is necessary for long-term biological adaptation, not for protecting the brain from acute thermal damage.

Scrotal heating stimulates panting and reduces body temperature similarly in febrile and non-febrile rams (Ovis aries)

It is known that heating the ram scrotum stimulates heat loss resulting in a decrease in body temperature and that during fever core temperature increases, but local scrotal thermoeffectors operate to maintain normal scrotal temperature. We have investigated whether scrotal warming influences core body temperature and the panting effector during fever generation. We measured rectal temperature, intrascrotal temperature, scrotal skin temperature and respiratory frequency in four adult Merino rams following intravascular injection of saline or lipopolysaccharide at an ambient temperature of 18-20 degrees C while scrotal skin temperature was maintained at 33 degrees C or elevated to 41 degrees C. Compared to maintaining normal scrotal temperature, heating the scrotum increased respiratory frequency and reduced rectal temperature by a similar amount following LPS as following saline. Fever was associated with decreased respiratory frequency compared to saline at both 33 and 41 degrees C scrotal temperature, suggesting that the fever was generated mainly by decreasing respiratory heat loss. We conclude that scrotal thermal afferent stimulation resulted in an offset for the set-point of body temperature regulation in both normothermic and febrile rams.

Variability in brain and arterial blood temperatures in free-ranging ostriches in their natural habitat

We used implanted miniature data loggers to measure brain (in or near the hypothalamus) and carotid arterial blood temperatures at 5 min intervals in six free-ranging ostriches Struthio camelus in their natural habitat, for a period of up to 14 days. Carotid blood temperature exhibited a large amplitude (3.0-4.6 degrees C) circadian rhythm, and was positively correlated with air temperature. During the day, brain temperature exceeded carotid blood temperature by approx. 0.4 degrees C, but there were episodes when brain temperature was lowered below blood temperature. Selective brain cooling, however, was not present in all ostriches, and was not tightly coupled to the prevailing body temperature. Brain temperature was maintained within narrow daily limits of approx. 2 degrees C, and varied significantly less than blood temperature at short time scales of 5 to 20 min. At night, brain temperature exceeded blood temperature by as much as 3 degrees C. We attribute the elevated brain temperatures to warming of cerebral arterial blood, by reduced heat exchange in the ophthalmic rete or possibly heat gain from cranial structures, before supplying the hypothalamus. Further studies are necessary to elucidate the significance of such variations in brain temperature and the importance of selective brain cooling in free-living birds.

Adaptive heterothermy and selective brain cooling in arid-zone mammals

Adaptive heterothermy and selective brain cooling are regarded as important thermal adaptations of large arid-zone mammals. Adaptive heterothermy, a process which reduces evaporation by storing body heat, ought to be enhanced by ambient heat load and by water deficit, but most mammals studied fail to show at least one of those attributes. Selective brain cooling, the reduction of brain temperature below arterial blood temperature, is most evident in artiodactyls, which possess a carotid rete, and traditionally has been considered to protect the brain during hyperthermia. The development of miniature ambulatory data loggers for recording body temperature allows the temperatures of free-living wild mammals to be measured in their natural habitats. All the African ungulates studied so far, in their natural habitats, do not exhibit adaptive heterothermy. They have low-amplitude nychthemeral rhythms of temperature, with mean body temperature over the night exceeding that over the day. Those with carotid retes (black wildebeest, springbok, eland) employ selective brain cooling but zebra, without a rete, do not. None of the rete ungulates, however, seems to employ selective brain cooling to prevent the brain overheating during exertional hyperthermia. Rather, they use it at rest, under moderate heat load, we believe in order to switch body heat loss from evaporative to non-evaporative routes.

Rectal temperature measurement results in artifactual evidence of selective brain cooling

Selective brain cooling (SBC) is defined as a brain temperature cooler than the temperature of arterial blood from the trunk. Surrogate measures of arterial blood temperature have been used in many published studies on SBC. The use of a surrogate for arterial blood temperature has the potential to confound proper identification of SBC. We have measured brain, carotid blood, and rectal temperatures in conscious sheep exposed to 40, 22, and 5 degrees C. Rectal temperature was consistently higher than arterial blood temperature. Brain temperature was consistently cooler than rectal temperature during all exposures. Brain temperature only fell below carotid blood temperature during the final few hours of 40 degrees C exposure and not at all during the 5 degrees C exposure. Consequently, using rectal temperature as a surrogate for arterial blood temperature does not provide a reliable indication of the status of the SBC effector. We also show that rapid suppression of SBC can result if the animals are disturbed.

Selective brain cooling in mammals and birds

Artiodactyls and felids have a carotid rete that can cool the blood destined for the brain and consequently the brain itself if the cavernous sinus receives cool blood returning from the nose. This condition is usually fulfilled in resting and moderately hyperthermic animals. During severe exercise hyperthermia, however, the venous return from the nose bypasses the cavernous sinus so that brain cooling is suppressed. This is irreconcilable with the assumption that the purpose of selective brain cooling (SBC) is to protect the brain from thermal damage. Alternatively, SBC is seen as a mechanism engaging the thermoregulatory system in a water-saving economy mode in which evaporative heat loss is inhibited by the effects of SBC on brain temperature sensors. In nonhuman mammals that do not have a carotid rete, no evidence exists of whole-brain cooling. However, the surface of the cavernous sinus is in close contact with the base of the brain and is the likely source of unregulated regional cooling of the rostral brain stem in some species. In humans, the cortical regions next to the inner surface of the cranium are very likely to receive some regional cooling via the scalp-sinus pathway, and the rostral base of the brain can be cooled by conduction to the nearby respiratory tract; mechanisms capable of cooling the brain as a whole have not been found. Studies using conventional laboratory techniques suggest that SBC exists in birds and is determined by the physical conditions of heat transfer from the head to the environment instead of physiological control mechanisms. Thus except for species possessing a carotid rete, neither a coherent pattern of SBC nor a unifying concept of its biological significance in mammals and birds has evolved.

Controlled brain hypothermia by extracorporeal carotid blood cooling at normothermic trunk temperatures in pigs

Cerebral hypothermia improves outcomes after brain injury. A technique is presented for isolated brain cooling in pigs by cooling the natural blood supply of the brain. Under general anesthesia both common carotid arteries were exteriorized. One proximal carotid artery was connected to both distal carotid arteries and a heat exchanger in this line controlled brain temperature. The second proximal carotid artery was connected to an external jugular vein and a heat exchanger in this arteriovenous shunt was used to clamp trunk temperature. Thalamic brain temperatures of anesthetized juvenile pigs (N = 8) were clamped at 38, 25, and 30 degrees C while trunk core temperature was clamped at 38 degrees C. Approximately 7 min were needed to decrease brain temperature from 38 to 25 degrees C, reducing brain electric activity by 76% and increasing the temperature differences between different brain sites. Mean arterial blood pressure, heart rate, and cardiac output showed no significant change. Re-establishment of normothermic brain temperature led to a virtually complete recovery of brain electric activity. The technique is suitable for investigations of ischemic and traumatic injuries.

Brain Cooling: An Economy Mode of Temperature Regulation in Artiodactyls

Artiodactyls employ selective brain cooling (SBC) regularly during experimental hyperthermia. In free-ranging antelopes, however, SBC often was present when body temperature was low but absent when brain temperature was near 42 degrees C. The primary effect of SBC is to adjust the activity of the heat loss mechanisms to the magnitude of the heat stress rather than to the protection of the brain from thermal damage.

Selective brain cooling reduces respiratory water loss during heat stress

Terrestrial mammals developed several mechanisms to reduce water loss to counteract water shortage. One avenue of water loss is the evaporative heat loss by sweating and panting, which increases with body temperature. Sweating and panting are activated by temperature signals of the body, whereby the brain is the most important site in generating temperature signals. Goats, like other artiodactyls, can cool their brains selectively below the temperature of the trunk core. The aim of the present study is to determine whether an inhibition of the selective brain cooling (SBC) mechanism will increase substantially the respiratory evaporative water loss during heat stress due to the higher brain temperature. The inhibition of SBC was performed by increasing brain temperature experimentally at the same rate as trunk temperature by means of extracorporeal heat exchangers. These experiments without SBC resulted in higher respiratory evaporative water loss compared to experiments with normal SBC. Eighteen experiments in two conscious goats had shown that at a trunk temperature of 40 degrees C the respiratory water loss was reduced on average by 29 g/hr (0.7 l/day) due to the effect of SBC. This amount of water corresponds to about one third of the general water requirements. In conclusion, SBC substantially reduces the water loss in goats during heat stress and consequently improves survival chances during water shortage.

A difference of 5 degrees C between ear and rectal temperatures in a febrile patient

A 4-year-old boy with a history of seizures triggered by fever presented at an emergency department (ED) with tachycardia, skin vasoconstriction, and a rectal temperature of 42.2 degrees C. However, his ear temperature (as repeatedly measured in two ears, by two experienced nurses, and with two infrared thermometers) was between 36.4 degrees C and 37.6 degrees C. Antipyretic therapy resulted in skin vasodilation, a rapid decrease of rectal temperature, restoration of heart rate, and disappearance of the difference between the two temperatures. Seizures did not occur. This case shows that infrared ear thermometry cannot be recommended in EDs as the procedure of choice for detecting fever in small children, especially when they are vasoconstricted.

Unilateral selective brain cooling

In species with a carotid rete the arterial blood flowing to the brain can be cooled by passing the carotid rete. The mechanism is termed selective brain cooling (SBC). The aim of the study was to evaluate whether SBC could be induced unilaterally. 27 experiments were performed in 2 conscious goats which were prepared with carotid loops to manipulate the blood temperature of the left and right carotid artery independently of each other. The temperature of the left and right hemisphere of the brain was controlled by means of extracorporeal heat exchangers acting on the carotid blood while trunk temperature was clamped at 39.5 degrees C by a heat exchanger in an arteriovenous shunt. Unilateral warming of the brain induced ipsilateral SBC only, and was accompanied by a bilateral increase of the ear skin temperature. The results demonstrate the precise control of brain temperature by SBC since even unilateral temperature deviations of the brain can be reduced by SBC. In conclusion SBC regulates the temperature of single hemispheres rather than the mean brain temperature.

Selective brain cooling in goats: effects of exercise and dehydration

1. Measurements of brain and central blood temperature (Tbr and Tbl), metabolic rate (MR) and respiratory evaporative heat loss (REHL) were made in trained goats walking on a treadmill at 4.8 km h-1 at treadmill inclines of 0, 5, 10, 15 and 20% when they were fully hydrated and at 0% when they had been deprived of water for 72 h. 2. In hydrated goats, exercise MR increased progressively with increasing treadmill incline. Both Tbl and Tbr rose during exercise, but Tbl always rose more than Tbr, and selective brain cooling (SBC = Tbl - Tbr) increased linearly with Tbl. Significant linear relationships were also present between REHL and Tbl and between SBC and REHL. Neither the slope of the regression relating SBC to Tbl nor the threshold Tbl for onset of SBC was affected by exercise intensity. Manual occlusion of the angularis oculi veins decreased SBC in a walking goat, while occlusion of the facial veins increased SBC. 3. Dehydrated goats had higher levels of Tbl, Tbr and SBC during exercise, but the relationship between SBC and Tbl was the same in hydrated and dehydrated animals. In dehydrated animals, REHL at a given Tbl was lower and SBC was thus maintained at reduced rates of REHL. 4. It is concluded that SBC is a linear function of body core temperature in exercising goats and REHL appears to be a major factor underlying SBC in exercise. The maintenance of SBC in spite of reduced REHL in dehydrated animals could be a consequence of increased vascular resistance in the facial vein and increased flow of cool nasal venous blood into the cranial cavity.

Selective brain cooling in resting and exercising Norwegian reindeer (Rangifer tarandus tarandus)

The threshold body core temperature for selective brain cooling (SBC) as well as the slope of brain cooling were determined in three Norwegian reindeer (Rangifer tarandus tarandus) during rest and during exercise. Brain temperature was measured in the hypothalamus (Thypo) and blood temperature (Tblood) was measured either in the right carotid artery or in a few cases in the right atrium of the heart. During rest the animals were subjected to ramp-like increases of Tblood by means of a thermostatically controlled water circulated heat exchanger (HE) introduced into the rumen via a chronically implanted rumen cannula. During exercise the animals ran on a treadmill at a speed of between 5.5-8.0 km hr-1 and a slope of 13.5 degrees for periods of 30-60 min. The elevation of Tblood during both rest and exercise resulted in significant amounts of SBC. The mean threshold for SBC (Thypo = Tblood) during rest was 38.7 degrees C. The threshold for SBC was elevated significantly to 39.5 degrees C during exercise. The mean slope of SBC (increase of SBC per degree increase of Tblood) was 0.82 both during rest and exercise.

Effects of selective brain cooling on mechanisms of respiratory heat loss

Experiments (n = 36) in three conscious goats were performed at 35 degrees C air temperature and low (LH) or high (HH) humidity. Prior to the experiments the animals received carotid loops and an arteriovenous shunt, which made it possible to increase the temperature of the blood flowing to head and trunk (series A), or to increase the temperature of the trunk at constant carotid blood and hypothalamic temperature (Thyp), respectively (series B). Owing to the smaller cooling power of the inspired air in HH, the slope of respiratory evaporative heat loss versus aorta blood temperature (Taor) was reduced in series A and B. In series A the slopes of respiratory minute volume (VE) and respiratory frequency (RF) versus Taor were larger in HH than in LH. The effects were caused by a reduction of selective brain cooling in HH, which resulted in higher levels of Thyp. This is concluded from the results of series B, in which Thyp was equal in LH and HH, and the slopes of VE and RF over Taor showed no differences. Thus, selective brain cooling contributes to counteract the deterioration of the gain of the respiratory heat loss mechanism, which occurs during exposure to humid air.