Thermoregulatory responses to temperature manipulation of the spinal cord in the marmot

Yellow-bellied marmots (M. flaviventris) acclimated to T_a = 20 °C were implanted with U-shaped polyethylene thermodes in the peridural space of the spinal cord. Decreasing the temperature of the cervical, thoracic, or lumbar areas of the cord increased heart rate, electromyographic activity, and oxygen consumption in the animals. These responses differed qualitatively from those elicited by heating the same cord areas, indicating specificity of the response to the temperature change.Increases in heat production were proportional to the amount of cooling of the cord. The thoracic area was found to be more thermosensitive than the lumbar area. No behavioral or physical thermoregulation was apparent when the spinal cord temperature was changed in these animals.In addition to the conclusion that regulation of spinal cord temperature may be important in the euthermic marmot, it was postulated that the temperature receptors located in the thoracic cord of the marmot may be important in maintaining shivering thermogenesis during arousal from hibernation.     

Human thermoregulatory responses during prolonged walking in water at 25, 30 and 35°C

Eight healthy and physically well-trained male students exercised on a treadmill for 60 min while being immersed in water to the middle of the chest in a laboratory flowmill. The water velocity was adjusted so that the intensity of exercise correspond to 50% maximal oxygen uptake of each subject, and experiments were performed once at each of three water temperatures: 25, 30, 35°C, following a 30-min control period in air at 25°C, and on a treadmill in air at an ambient temperature of 25°C. Thermal states during rest and exercise were determined by measuring rectal and skin temperatures at various points, and mean skin temperatures were calculated. The intensity of exercise was monitored by measuring oxygen consumption, and heart rate was monitored as an indicator for cardiovascular function. At each water temperature, identical oxygen consumption levels were attained during exercise, indicating that no extra heat was produced by shivering at the lowest water temperature. The slight rise in rectal temperature during exercise was not influenced by the water temperature. The temperatures of skin exposed to air rose slightly during exercise at 25°C and 30°C water temperature and markedly at 35°C. The loss of body mass increased with water temperature indicating that both skin blood flow and sweating during exercise increased with the rise in water temperature. The rise in body temperature provided the thermoregulatory drive for the loss of the heat generated during exercise. Heart rate increased most during exercise in water at 35°C, most likely due to enhanced requirements for skin blood flow. Although such requirements were certainly smallest at 25°C water temperature, heart rate at this temperature was slightly higher than at 30°C suggesting reflex activation of sympathetic control by cold signals from the skin. There was a significantly greater increase in mean skin and rectal temperatures in subjects exercising on the treadmill in air, compared to those exercising in water at 25°C. Accepted: 22 May 1998     

Heat increment of feeding and its thermoregulatory benefit in the muskrat (Ondatra zibethicus)

The calorigenic effect of feeding and its potential benefit in defraying thermoregulatory costs and attenuating immersion hypothermia of adult muskrats were investigated. A single session of feeding on aquatic vegetation was sufficient to raise the metabolic rate of muskrats for a period of at least 5 h. The peak postprandial rate of oxygen consumption averaged 1.42 times the level established for fasted animals, and the heat increment of feeding accounted for about 40% of the metabolizable energy intake of muskrats. There was no evidence of a postprandial rise in oxygen consumption of muskrats that entered water at 18–19°C after feeding. In aquatic trials, average and minimum steady-state oxygen consumption rates of fed muskrats were similar to, or even lower than values recorded from fasted animals, implying substitution of heat increment of feeding for thermoregulatory heat production. Our data did not support the hypothesis that heat increment of feeding retards body cooling in water. Net body temperature decline in water was actually higher in fed animals than in fasted controls. However, since previously fed muskrats also entered water at an elevated body temperature, the final body temperature (at 30 min immersion) was similar in all groups. These findings suggest that metabolic heat generated incidental to preimmersion feeding could provide a thermoregulatory benefit to muskrats by reducing the need for active thermogenesis in water.     

The role of prostaglandins and the hypothalamus in thermoregulation in the lizard, Phrynocephalus przewalskii (Agamidae)

Typically, small lizards rely heavily on behavioral thermoregulation rather than physiological mechanisms to control their rates of warming and cooling. We tested the hypothesis that prostaglandins participate in mediating the cardiovascular response to heating and cooling and temperature regulating neurons in the hypothalamus of the small lizard Phrynocephalus przewalskii. In vivo and in vitro treatments, heart rates (HRs) were all found to be higher during heating than during cooling, hysteresis was distinct below 30 and 26°C, respectively. In vivo, as administration of COX inhibitor, there were no differences in HR between heating and cooling at any body temperature and administration of agonist prostaglandins only produced a significant effect on HR below 25°C. Single-unit activity was recorded extracellularly in vitro with microelectrodes, found the firing rate of the continuous unit increased 23% when the temperature of the artificial cerebrospinal fluid dropped from 30–20°C. We conclude that prostaglandins appear to play only a limited role in modulating heart activity in Phrynocephalus przewalskii and suggest that cold-sensitive neurons in the preoptic and anterior hypothalamus (PO/AH) are involved in thermoregulatory control during heating or cooling.     

Age-related thermoregulatory differences during core cooling in humans

The current study assessed sympathetic neuronal and vasomotor responses, total body oxygen consumption, and sensory thermal perception to identify thermoregulatory differences in younger and older human subjects during core cooling. Cold fluid (40 ml/kg, 4 degrees C) was given intravenously over 30 min to decrease core temperature (Tc) in eight younger (age 18-23) and eight older (age 55-71) individuals. Compared with younger subjects, the older subjects had significantly lower Tc thresholds for vasoconstriction (35.5 +/- 0.3 vs. 36.2 +/- 0.2 degrees C, P = 0.03), heat production (35.2 +/- 0.4 vs. 35.9 +/- 0.1 degrees C, P = 0.04), and plasma norepinephrine (NE) responses (35.0 vs. 36.0 degrees C, P < 0.05). Despite a lower Tc nadir during cooling, the maximum intensities of the vasoconstriction (P = 0.03) and heat production (P = 0.006) responses were less in the older compared with the younger subjects, whereas subjective thermal comfort scores were similar. Plasma NE concentrations increased fourfold in the younger but only twofold in the older subjects at maximal Tc cooling. The vasomotor response for a given change in plasma NE concentration was decreased in the older group (P = 0.01). In summary, aging is associated with 1) a decreased Tc threshold and maximum response intensity for vasoconstriction, total body oxygen consumption, and NE release, 2) decreased vasomotor responsiveness to NE, and 3) decreased subjective sensory thermal perception.     

Thermoregulatory responses to thermal stimulation of the preoptic anterior hypothalamus in the red fox (Vulpes vulpes)

The preoptic anterior hypothalamus (POAH) thermoregulatory controller can be characterized by two types of control, an adjustable setpoint temperature and changing POAH thermal sensitivity. Setpoint temperatures for shivering (T_shiver) and panting (T_pant) both increased with decreasing ambient temperature (T_a), and decreased with increasing T_a. The POAH controller is twice as sensitive to heating as to cooling. Metabolic rate (MR) increased during both heating and cooling of the POAH. Resting temperature of the POAH was lower than internal body temperature (T_b) at all temperatures. This indicates the presence of some form of brain cooling mechanism. Decreased T_b during POAH heating was a result of increased heat dissipation, while higher T_b during POAH cooling was a result of increased heat production and reduced heat dissipation. The surface temperature responses indicated that foxes can actively control heat flow from body surface. Such control can be achieved by increased peripheral blood flow and vasodilation during POAH heating, and reduced peripheral blood flow and vasoconstriction during POAH cooling. The observed surface temperature changes indicated that the thermoregulatory vasomotor responses can occur within l min following POAH heating or cooling. Such a degree of regulation can be achieved only by central neural control. Only surface regions covered with relatively short fur are used for heat dissipation. These thermoregulatory effective surface areas account for approximately 33% of the total body surface area, and include the area of the face, dorsal head, nose, pinna, lower legs, and paws.     

Thermal responses of the fresh water turtle Mauremys caspica to step-function changes in the ambient temperature

1. 1.|The turtle Mauremys caspica cools significantly faster than it heats in air. The heating/cooling ratio is 0.49.

2. 2.|The variation of body temperature in relation to time-course in response to a step-function change of environmental temperature, fitted to a second-order system improves that of a first-order system.

3. 3.|The gradient between ambient temperature (T_a) and equilibrium body temperature (T_b) increases significantly and progressively when ambient temperature rises over 25°C.

4. 4.|At 40°C thermoregulatory hyperventilation was detected, implying an increase in air convection requirement (ventilation relative to O₂ consumption, ).

Thermoregulatory responses to preoptic-anterior hypothalamic heating and cooling in the bat,Eptesicus fuscus

Summary The central nervous control of temperature regulation in the bat, Eptesicus fuscus, was evaluated by heating the preoptic-anterior hypothalamus (PO/AH) of active, unanaesthetized bats. Because bats are metabolically very variable, change in body temperature was used as the criterion of change in heat balance in response to change in brain temperature and change in wing temperature as an indicator of vasomotor changes.Heating the preoptic-anterior hypothalamic area (PO/AH) of the bat Eptesicus fuscus caused an average increase in wing temperature due to vasodilation of 1.0° C and an average increase in body temperature of 0.4° C. Conversely, cooling the PO/AH led to an average decline in wing temperature due to vasoconstriction of 0.9° C and an average decline in body temperature of 0.4° C.Bats were heat-stressed to augment the responsiveness of the PO/AH. Heat-stress alone causes a rise in body temperature and wing temperature. Release from heat stress causes a fall in body temperature and a fall in wing temperature. When the PO/AH is heated following a period of high heat-stress, the body temperature continues to fall but wing temperature reverses its direction of change and rises. When bats are given a low heat-stress and simultaneous heating of the PO/AH, wing temperature rises in response to PO/AH temperature and the body temperature stabilizes. When the PO/AH is cooled in bats under high heat-stress, body temperature stabilizes and wing temperature falls. When bats are cold-stressed, body temperature and wing temperature fall regardless of heating of the PO/AH.These responses are related to the life habits of the bat.It is concluded that the PO/AH of the bat Eptesicus fuscus may be less thermally sensitive than the PO/AH in other vertebrates studied, and that other central nervous structures have acquired an increased thermoregulatory function.We thank Mrs. Ruth Chalmers for her excellent histological preparstions.This work was supported, in part, by National science Foundation grant GB 6303 and GB 13797.     

Role of the diencephalic preoptic region in coordination of thermoregulatory and nutrition-related reactions

In acute experiments on cats we studied convergence of the afferent temperature-related and glycemic signals on the neurons of the preoptic region (RPO1). Local heating and cooling (±7°C) of the skin on the contralateral forelimb pad were used for identification ofRPO neurons as thermosensitive units, while infusion of 0.1–0.4 ml of 5.5% glucose solution in the ipsilaterala. carotis revealed their sensitivity to shifts in the glucose concentration. More than half (57%) of glucosensitiveRPO neurons responded to the changes in peripheral temperature. The pattern of convergence and peculiarities of localization of the convergent neurons were studied. We propose the following hypothesis:RPO neurons receiving convergent thermal and glycemic inputs provide formation of integral controlling signals addressed to the ventromedial and lateral hypothalamic nuclei, which control the thermoregulatory food consumption.     

Thermoregulatory responses to acute heat loads in the FOK rat

FOK is an inbred rat strain with a genotypic adaptation to hot environments. The present study investigated the mechanism of the high heat tolerance of the FOK rat. Male FOK and WKAH rats were used. They were loosely restrained and placed individually in a direct calorimeter with an ambient temperature of 24°C. Their hypothalamic temperature, evaporative and nonevaporative heat loss and heat production were measured. After thermal equilibrium had been attained, the rats were warmed for 30 min with a chronically implanted intraperitoneal electric heater(internal heating). At least 90 min after the heating, the jacket water temperature surrounding the calorimeter chamber was gradually raised from 24°C to 36°C in 80 min (external warming). During the internal heating, changes in the thermoregulatory parameters did not differ between the groups. During the external warming, the evaporative heat loss of the FOK rat was significantly greater than that of the WKAH rat, while changes in nonevaporative heat loss and heat production did not differ between the groups. The results suggest that in the FOK rat, the improved heat tolerance is attributable to an enhanced evaporative heat loss response, but not to a facilitation of nonevaporative heat loss or of metabolic depression. Received: 8 March 1999 / Accepted: 14 July 1999     

Thermoregulation and daily activity patterns in a black desert grasshopper,Taeniopoda eques

Behavioural thermoregulation was studied in the western horse lubber grasshopper Taeniopoda eques (Burmeister), a native of the Chihuahuan Desert of North America. The grasshoppers regulated their temperature through a series of daily cyclical vertical movements between vegetation and the soil, and by the adoption of four thermoregulatory postures: flanking, crouching, stilting and stem-shading. At dawn, the grasshoppers moved from their nocturnal roost-plants to the ground, returned to bushes during the middle of the day, moved back to the open ground in the afternoon, then reascended vegetation at dusk. The occurrences of the four thermoregulatory postures were synchronized with these microhabitat shifts. During the cooler mornings and afternoons, the insects maximized heat gain by flanking and crouching, achieving thoracic temperatures of up to 16°C above ambient. Throughout the hot middle of the day the insects stilted and shaded, minimizing heat gain. These behaviours effectively kept the grasshoppers' body temperatures near the preferred temperature (36·2°C), but lower than the maximum voluntarily tolerated temperature (41·9°C), critical thermal maximum (45·2°C) and instantaneous lethal maximum (46·5°C). The body size of flanking insects influenced heating and cooling rates, wind effects and temperature excess at equilibrium. Both infrared and visible radiation appeared to elicit flanking. The need and ability to thermoregulate are influenced by this insect's reliance on chemical deterrents for defence.     

The combined effects of temperature,salinity, and declining oxygen tension on oxygen consumption in the marine pulmonate Amphibola crenata (Gmelin, 1791)

Oxygen consumption of Amphibola crenata (Gmelin) was measured in various salinity-temperature combinations (< 0.1‰ to 41‰ salinity and 5 to 30°C) in air, and following exposure to declining oxygen tensions. In all experimental conditions, respiration varied with the 0.44 power of the body weight (sd = 0.14). The aquatic rate was consistently higher than the aerial rate of oxygen consumption, although at 30 °C the two rates were similar. Oxygen consumption increased with temperature up to 25 °C in all salinities; the lowest values were recorded at temperatures below 10 °C and at 30 °C in the most dilute medium. At all exposure temperatures, the oxygen consumption of Amphibola decreased regularly with salinity down to 0.1 ‰, and following exposure to concentrated sea water (41‰). Salinity had the least effect at 15 °C which was the acclimation temperature. In general, all of the temperature coefficients (Q₁₀ values) were low, < 1.65. However, Q₁₀ values above 2.8 were recorded at a salinity of 17.8‰ between 10 and 15 °C. Oxygen consumption of all size classes of Amphibola was more temperature dependent in air than in water and small individuals show a greater difference between their aerial and aquatic rates than larger snails. The rates of oxygen consumption in declining oxygen tensions were expressed as fractions of the rates in air saturated sea water at each experimental salinity-temperature combination. The quadratic coefficient B₂ becomes increasingly more negative with both decreasing salinity and temperatures up to 20 °C. At higher temperatures (25 and 30 °C) the response is reversed such that O₂ uptake in snails becomes increasingly independent of declining oxygen tensions at higher salinities. On exposure to a salinity of 4‰, Amphibola showed no systematic response to declining oxygen tension with respect to temperature. The ability of Amphibola to maintain its rate of oxygen consumption in a wide range of environmental conditions is discussed in relation to its potential for invading terrestrial habitats and its widespread distribution on New Zealand's intertidal mudflats.     

Cortex senses environmental temperature earlier than the hypothalamus in awake rats

Simultaneous and direct recording of temperature from the body, hypothalamus, and cortex in animals exposed to acute thermal challenges lack evidence. This study was conducted to assess the usual concept, that brain temperature is rather stable when animals are exposed to different ambient temperatures. In this study, we report the characteristic changes in the body, hypothalamic, and cortical temperature, when the rats were acutely exposed to cold (6 °C) and hot (36 °C) ambient temperature. The results of our study show that the body temperature is robustly regulated while hypothalamic and cortical temperatures vary on challenges to ambient cold (6 °C) and warm (36 °C) exposure in awake rats. The onset of response was observed quickest in the cortex, indicating that the cortical thermal sensing may relay intracranial thermal input to the hypothalamus for the regulation of body temperature within narrow limits. The present findings contradict earlier evidence, which stated that the brain does not participate in thermal sensing.     

Regulation of body temperature and energy requirements of hibernating alpine marmots (Marmota marmota)

Body temperature and metabolic rate were recorded continuously in two groups of marmots either exposed to seasonally decreasing ambient temperature (15 to 0 degrees C) over the entire hibernation season or to short-duration temperature changes during midwinter. Hibernation bouts were characterized by an initial 95% reduction of metabolic rate facilitating the drop in body temperature and by rhythmic fluctuations during continued hibernation. During midwinter, we observed a constant minimal metabolic rate of 13.6 ml O(2) x kg(-1) x h(-1) between 5 and 15 degrees C ambient temperature, although body temperature increased from 7.8 to 17.6 degrees C, and a proportional increase of metabolic rate below 5 degrees C ambient temperature. This apparent lack of a Q(10) effect shows that energy expenditure is actively downregulated and controlled at a minimum level despite changes in body temperature. However, thermal conductance stayed minimal (7.65 +/- 1.95 ml O(2) x kg(-1) x h(-1) x degrees C(-1)) at all temperatures, thus slowing down cooling velocity when entering hibernation. Basal metabolic rate of summer-active marmots was double that of winter-fasting marmots (370 vs. 190 ml O(2) x kg(-1) x h(-1)). In summary, we provide strong evidence that hibernation is not only a voluntary but a well-regulated strategy to counter food shortage and increased energy demands during winter.     

Sows’ responses to increased heat load – A review

There is a comprehensive body of literature on how increased air temperature affects the physiology, production and behaviour of sows, while very few studies consider the thermal effects of air humidity and air velocity.This review summarises studies that have investigated effects of air temperature by reviewing published literature in which sows were exposed to at least two different levels of air temperature ranging from 15 °C to 39 °C. Increased rectal temperature was investigated in the majority of the studies (26) and on average, the rectal temperature increased by 0.099 °C per °C increased air temperature above 25 °C. The increase was smaller at lower air temperatures, and it was suggested that rectal temperature is practically unaffected by air temperatures in the range of 15 °C–21 °C. This review elucidates how air temperature also affects performance indicators such as respiration rate, vaginal temperature, skin temperature, feed intake, milk yield, body weight loss during lactation, mortality, litter daily weight gain during lactation and sow behaviour.One study reported how respiration rate, rectal temperature, vaginal temperature and skin temperature were affected by both air temperature and air humidity, and the results suggest that the relative significance of air temperature and humidity may be similar for sows and finishing pigs (e.g. an increase of 40% relative humidity at an air temperature of 30 °C has a similar effect as a 1.9 °C increase in temperature).Studies on mitigation methods against the effects of high temperature and humidity such as snout cooling, drip cooling and floor cooling were reviewed to extract knowledge related to the effects of air velocity, temperatures of surrounding surfaces and the opportunity for sows to moisten their skin.     

Effect of ambient temperature on thermal sensitivity of POAH area in the rabbit

The POAH area of five conscious, male rabbits was heated with an electric thermode. The hypothalamic temperature threshold (ΔT_hy) for thermoregulatory reactions was determined at ambient temperatures of 12, 16, 20 and 24°C. The ΔT_hy for cutaneous vasolidation decreased with increasing ambient temperature. By contrast, the ΔT_hy for respiratory reaction was not dependent on ambient but only on hypothalamic temperature. After the start of hypothalamic heating, thermoregulatory reactions developed in the following characteristic order: vasodilation in the nose area, ear vasodilation and respiratory reaction. It is concluded that in order to increase heat loss rabbits first utilize cutaneous vasodilation. When this becomes insufficient, as indicated by an increase in hypothalamic temperature.     

Thermal interaction between animal and microclimate: a comprehensive model

An equation based on heat transfer theory is developed to predict the rate of heat loss from a homeothermic vertebrate to the environment, specified by the air temperature, humidity, windspeed and radiation receipt. The analysis incorporates the animal's thermoregulatory responses--sweating ability, vasomotor action, and regulation of body-core temperature, metabolic and respiratory rate. The loss of heat and water vapour from cattle is used as an illustration, and particular attention is given to their heat balance in hot environments. The predicted rates of heat loss from cattle indoors at various air temperatures and humidities are consistent with experiments. Outdoors, intercepted solar radiation can reduce substantially heat loss through the body tissue when the air temperature is low. In contrast, at high air temperatures the heat dissipation may not be sensitive to the radiation load, although body-core temperature is. Increased rates of air movement can aggravate strain at low air temperatures, but mitigate strain in a hot environment.     

Changes of impulse activity recorded from the rat vagus nerve during short-lasting total cooling

Gradual cooling of anesthetized rats followed by a drop in rectal temperature (RT) increased the frequency of efferent impulses and decreased the frequency of afferent impulses in the vagus nerves. Preliminary short-lasting (5 h) moderate cooling of the animals in a thermochamber to +5°C (RT did not change), or intensive cooling to −20°C (RT dropped to 32°C) changed the response of efferent nerve fibers to cooling of the body. Under these conditions, a drop in RT to 29°C was followed by a significant increase in efferent discharges in the vagus nerve after additional cooling throughout the experiment, while an initial cooling phase (RT was equal to 35-30°C) was followed by some inhibitory effect. At the same time, the changes in the afferent link were different. As in the control, gradual cooling decreased frequency of afferent impulses, although the intensity of the effects was different. The involvement of the vagus nerve system in the maintenance of temperature homeostasis during body cooling has been discussed.     

Effect of TRPM8 ion channel activation on thermoregulatory response to cooling

In rats, the effect of activation of the cold- and menthol-sensitive TRPM8 ion channel on different thermoregulatory parameters: total oxygen consumption, carbon dioxide release, respiration coefficient, constriction response of skin blood vessels, muscle activity, was studied. Activation of TRPM8 with menthol even in thermoneutral conditions produces an increase in oxygen consumption and a decrease in respiratory coefficient, which may suggest enhanced non-shivering thermogenesis and lipolysis. Rapid cooling against the background of TRPM8 activation is characterized by a decrease in the temperature thresholds of all thermoregulatory responses without associated changes in sequences of their initiation as well as in enhancement of metabolic component of emergency thermogenesis which leads to improved maintenance of core temperature in conditions when external cold acts on the organism. The obtained data on the effect of TRPM8 activation on metabolic parameters in thermoneutral conditions and under cooling suggest acontinuous involvement of this receptor in regulation of total metabolism and, possibly, in determination of the type of organism's metabolism as well as in determination of organism's response to external cooling.     

Expired air temperature during prolonged exercise in cool- and hot-humid environments

The purpose of this investigation was to measure expired air temperature under cool- and hot-humid environmental conditions at rest and during prolonged exercise to: (1) establish if significant increases in body core temperature affected expired air temperature, and (2) to determine if the temperature setting for heating the pneumotachometer in an open-circuit system requires adjustment during prolonged exercise tests to account for changes in expired air temperature. Six male distance runners completed two tests in cool-humid [dry bulb temperature (T _db) 15.5 (SD 1.3)°C, wet bulb temperature (T _wb) 12.1 (SD 1.4)°C] and hot-humid [T _db 31.6 (SD 0.6)°C, T _wb 24.9 (SD 0.6)°C, black globe temperature (T _g) 34.3 (SD 0.3)°C] environments, running at a velocity corresponding to 65% [67.1 (SD 2.82)%] of their maximal oxygen uptake. Rectal temperature and expired air temperatures were compared at rest, and after 30 min and 60 min of exercise for each environment. The main finding of this investigation was a significant (P < 0.05) but small increase in expired air temperature between the 30-min and 60-min measures in the hot-humid environment. No significant differences in expired air temperature were found between the 30-min and 60-min measures in the cool-humid environment. These findings suggest that: (1) expired air temperature is influenced by elevations in body core temperature during prolonged exercise in hot-humid conditions, and (2) that the temperature setting for heating the head of the pneumotachometer (after determining the appropriate temperature through measuring expired air temperature for the set environmental condition) may require adjustment during prolonged exercise trials in hot-humid environmental conditions. Accepted: 27 February 1997