Claus Jessen

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.

Effects of brain and trunk temperatures on exercise performance in goats

In 40 experiments on seven goats head and trunk temperatures were altered independently of each other and the effects on exercise performance on a treadmill (speed: 3 km/h, slope: 16%–20%) were observed. Brain temperature between 38.5°C and 42.0°C and trunk temperature between 39°C and 43.5°C did not reduce exercise performance or running time. Blood lactate concentration increased with rising brain and trunk temperatures, but did not exceed 13.1 mmol/l−1. Blood pressure and heart rate did not show any dependence on brain or trunk temperatures. Brain temperature between 42.0°C and 42.9°C shortened running time in 3 out of 12 experiments and reduced performance during shortlasting upward deviations of temperature. This suggests that in this species, the thermal safety limit to exercise is very close to that range of temperature which is likely to induce heat stroke.

Activity, blood temperature and brain temperature of free-ranging springbok

Abstract We used miniature data loggers to record temperature and activity in free-ranging springbok (Antidorcas marsupialis) naturally exposed to severe nocturnal cold and moderate diurnal heat. The animals were active throughout the day and night, with short rests; the intensity of activity increased during daylight. Arterial blood temperature, averaged over many days, exhibited a circadian rhythm with amplitude <1 °C, but with a wide range which resulted from sporadic rapid deviations of body temperature. Peak blood temperature occurred after sunset. Environmental thermal loads had no detectable effect on blood temperature, even though globe temperature varied by >10 °C from day to day and >20 °C within a day. Brain temperature increased approximately linearly with blood temperature but with a slope <1, so that selective brain cooling tended to be activated at high body temperature, but without a precise threshold for the onset of brain cooling. Low activity attenuated selective brain cooling and high activity abolished it, even at high brain temperature. Our results support the concept that selective brain cooling serves to modulate thermoregulation rather than to protect the brain against heat injury.

Threshold and slope of selective brain cooling

Experiments (n=50) in three conscious goats were performed in a thermoneutral environment to determine the threshold (i.e. the point at which the brain temperature is equal to the carotid blood temperature) and slope (i.e. the difference between brain and carotid blood temperatures as a function of carotid blood temperature) of selective brain cooling (SBC) and analyse the thermal inputs affecting them. Prior to the experiments the animals received carotid loops and an arteriovenous shunt to manipulate head and trunk temperatures independently of each other. The mean SBC threshold was 38.75° C Tcarotis and independent of Ttrunk. When body core temperature was increased from a hypo- to a moderately hyperthermic level, the SBC threshold was passed before metabolic rate had reached its minimum and before cutaneous vasodilation occurred. The mean SBC slope was 0.78 and rose with increasing Ttrunk. The degree of SBC was principally independent of respiratory heat loss: high levels of heat loss were found without SBC, and large degrees of SBC were observed at low levels of heat loss. The effect of SBC in and around normothermia is to smooth the onset of shivering or panting and to establish a range of internal temperature within which metabolic rate and respiratory heat loss are simultaneously at low levels.

Effects of dehydration and rehydration on body temperatures in the black Bedouin goat

Abstract The temperatures of the arterial blood and the brain in black Bedouin goats were measured continuously by miniature data loggers. The animals were either euhydrated or dehydrated to 75–80% of the initial body mass by withholding water for 3–4 days during exposure to intense solar radiation. The daily blood temperature means and maxima of were significantly higher in dehydration than in euhydration, but 40°C was rarely exceeded even during the hot hours of the day. Selective brain cooling occurred in euhydration, but its extent was small when blood temperature was below 39.5°C. In dehydration, however, selective brain cooling was frequent and the standard response when blood temperature exceeded 39°C. We believe that selective brain cooling contributes to the inhibition of evaporative heat loss, which is the primary cause of the higher blood temperature in dehydration. Rapid rehydration with cold water induced long-lasting depression of blood temperature. No evidence was found for mechanisms attenuating the subsequent decrease of brain temperature which occurred a few minutes after the uptake of cold water.

Evidence against brain stem cooling by face fanning in severely hyperthermic humans

To achieve a hyperthermic state 11 subjects exercised at 35° C air temperature in a water-impermeable outfit, until their oesophageal temperature (Tes) exceeded 39° C. Changes of brain stem temperature were assessed by the interspike intervals of auditory evoked potentials, which depend on brain stem temperature. These were recorded at rest before exercise (condition A), after exercise during a period when heat loss from the face was prevented by covering the head with a plastic hood (condition B), and again during face fanning (condition C). An increase in Tes from 37.14±0.25° C to 39.05±0.15° C (A to B) produced a significant reduction in interspike intervals, indicating an increase in brain stem temperature. Changing from conditions B to C, Tes and interspike intervals remained constant, indicating no change of brain stem temperature in spite of face fanning. Thus, even in severely heat stressed humans face fanning is not able to lower brain stem temperature significantly below that of the rest of the body core.

Intravascular heat exchanger for conscious goats.

SummaryPolyethylene tubings were chronically implanted into the vascular system of goats and served as heat exchagers to remove heat directly from the body core at a rate equalling several times resting heat production.

Seasonal variations of body temperature in goats living in an outdoor environment

Abstract 1. 1. Blood and brain temperatures were measured continuously in three animals on 319, 248, and 127 days. 2. 2. Over a 30°C range of the 24-h mean of air temperature, the 24-h mean of body core temperature changed 0.5°C or less. 3. 3. The 24-h mean of body core temperature was, in the range between 38°C and 39°C, mainly determined by non-thermal factors. 4. 4. The 24-h amplitude of body core temperature was closely correlated with the 24-h amplitude of air temperature, and was small in winter and large in summer. 5. 5. The relationship between blood and brain temperatures was highly variable and did not reveal a consistent temperature effect within the range of the most frequently occurring body core temperatures.

Central thermosensitivity in conscious goats: hypothalamus and spinal cord versus residual inner body.

Experiments were performed on conscious goats to confirm the suggestion that in this species the inner body contains more thermosensitive structures than those residing in the hypothalamus and spinal cord. For this purpose goats were chronically implanted with local thermodes and intravascular heat exchangers to allow independent temperature control of the hypothalamus, spinal cord and residual inner body. With the hypothalamus and spinal cord clamped simultaneously at different levels between 32°C and 40°C, residual internal temperature was lowered by subtracting heat via the intravascular heat exchanger. The residual internal temperature at which shivering and increased heat production occured due to heat extraction, was directly related to the value of the combined hypothalamic and spinal cord clamp temperature. The higher hypothalamic and spinal cord clamp temperatures were, the lower residual internal temperature fell before shivering occurred and heat production rose. Plosts relating residual internal temperature to hypothalamic and spinal cord temperature at different levels of heat production showed the signal input generated within the residual inner body to be of nearly the same order of magnitude as that from the hypothalamus and spinal cord.

Total body thermosensitivity and its spinal and supraspinal fractions in the conscious goose

Abstract1.Effects of general body cooling on heat production: an intravascular heat exchanger was used to alter total body temperature. Heat production increased with decreasing body temperature at an average rate of −12 W/kg·°C. The rate of rise was independent of air temperature. The threshold body temperature below which heat production rose was lower at higher air temperature.2.Effects of spinal cord cooling: heat production increased with decreasing spinal temperature at an average rate of −0.3 W/kg·°C. The rate of rise was not clearly affected by air temperature. The spinal threshold temperature was lower at warm ambient conditions. The results suggest that spinal thermosensitivity in the goose represents only a minor fraction of total body thermosensitivity.3.Effects of head cooling: heat exchangers enclosing the carotid arteries were used to alter the temperature of the blood supplied to the head. Cooling increased heat production. When the thermosensitivity of the area, which was affected by the heat exchanger, was calculated from the relationship between changes of heat production and brain temperature, values between −0.74 and −1.65 W/kg·°C were obtained. Measurements of brain, spinal cord and head skin temperatures suggest that the thermosensitive structures which mediated the responses, were predominantly situated in the brain.