The distribution and regulation of temperature in the rat

Hypoxia alters the thermogenic response to cold in adult homeothermic and heterothermic rodents

KEY POINTS

For small mammals living in a cold, hypoxic environment, supplying enough O₂ to sustain thermogenesis can be challenging. While heterothermic mammals are generally more tolerant of cold and hypoxia than homeothermic mammals, how they regulate O₂ supply and demand during progressive cooling in hypoxia is largely unknown. We show that as ambient temperature is reduced in hypoxia, body temperature falls in both homeotherms and heterotherms. In the homeothermic rat, a decrease in O₂ consumption rate and lung O₂ extraction accompany this fall in body temperature, despite a relative hyperventilation. Paradoxically, in heterothermic mice, hamsters and ground squirrels, body temperature decreases more than in the homeothermic rat, even though they maintain ventilation, increase lung O₂ extraction and maintain or elevate their O₂ consumption rates. Variation in cold and hypoxia tolerance among homeotherms and heterotherms reflects divergent strategies in how O₂ supply and demand are regulated under thermal and hypoxic challenges.

ABSTRACT

Compared to homeothermic mammals, heterothermic mammals are reported to be exceptionally tolerant of cold and hypoxia. We hypothesised that this variation in tolerance stems from divergent strategies in how homeotherms and heterotherms regulate O₂ supply versus O₂ demand when exposed to hypoxia during progressive cooling. To test this hypothesis, we exposed adult rodents ranging in their degree of heterothermic expression (homeotherm: rats, facultative heterotherms: mice and hamsters, and obligate heterotherm: ground squirrels) to either normoxia (21% O₂ ) or environmental hypoxia (7% O₂ ), while reducing ambient temperature from 38 to 5°C. We found that when ambient temperature was reduced in normoxia, all species increased their O₂ consumption rate and ventilation in parallel, maintaining a constant ventilatory equivalent and level of lung O₂ extraction. Surprisingly, body temperature fell in all species, significantly so in the heterotherms. When ambient temperature was reduced in hypoxia, however, the homeothermic rat decreased their O₂ consumption rate and lung O₂ extraction despite an increase in their ventilatory equivalent, indicative of a relative hyperventilation. Heterotherms (mice, hamsters and ground squirrels), on the other hand, decreased their ventilatory equivalent, but increased lung O₂ extraction and maintained or elevated their O₂ consumption rates, yet their body temperature fell even more than in the rat. These results are consistent with the idea that homeotherms and heterotherms diverge in the strategies they use to match O₂ supply and O₂ demand, and that enhanced cold and hypoxia tolerance in heterotherms may stem from an improved ability to extract O₂ from the inspired air.

Measurement of temperature in the rat by rectal probe and telemetry yields compatible results

The change in body temperature of the rat is commonly measured using biotelemetry or the rectal probe. The authors report that the two methods yield qualitatively similar but quantitatively different results in two experiments. In Experiment 1, both methods detected a salicylate-affected reduction in handling-induced hyperthermia. In Experiment 2, both methods were useful in detecting the hypothermia induced by the muscarinic agonist oxotremorine. In both Experiments 1 and 2, measurements of baseline temperature were higher when measured with the rectal probe. Baseline temperature is measured with biotelemetry prior to handling animals, whereas the act of measuring baseline temperature with the rectal probe necessitates handling. The investigators hypothesized that a rise in baseline temperature produced by handling at least partially accounts for the greater hypothermic response obtained in Experiment 2 using measurements obtained with the rectal probe. In Experiment 3, baseline temperature was measured with biotelemetry after animals were handled. Handling produced an increase in baseline temperature. The hypothermic response to oxotremorine was increased when the higher posthandling baseline temperature was used to calculate the hypothermic response of animals. The authors conclude that differences in baseline temperature and hypothermic response obtained with the two methods are related to an effect of handling.

The circadian rhythm of body temperature

This paper reviews the literature on the circadian rhythm of body temperature (CRT). The review starts with a brief discussion of methodological procedures followed by the description of known patterns of oscillation in body temperature, including ultradian and infradian rhythms. Special sections are devoted to issues of species differences, development and aging, and the relationships between the CRT and the circadian rhythm of locomotor activity, between the CRT and the thermoregulatory system, and between the CRT and states of disease. A section on the nervous control of the CRT is followed by summary and conclusions.

Thermal biology of the laboratory rat

The purpose of this paper is to thoroughly review the literature and present a data base of the basic thermoregulatory parameters of the laboratory rat. This review surveys the pertinent papers dealing with various aspects of the thermal biology of the laboratory rat, including: metabolism, thermoneutrality, core and brain temperature, thermal tolerance, thermal conductance and insulation, thermoregulatory effectors (i.e., thermogenesis, peripheral vasomotor tone, evaporation, and behavior), thermal acclimation, growth and reproduction, ontogeny, aging, motor activity and exercise, circadian rhythm and sleep, gender differences, and other parameters. It is shown that many facets of the thermoregulatory system of the laboratory rat are typical to that of most homeothermic species. However, is several instances the rat exhibits unique thermoregulatory responses which are not comparable to other species.

The role of the liver in non-shivering thermogenesis in the rat

1. The temperatures of the liver and afferent aortic and portal blood were measured in 20 degrees C acclimated rats at different environmental temperatures (T(a)) between 20 and 37 degrees C. The heated-thermocouple technique was used to measure the metabolic heat production of the liver, its blood flow, and to correct the temperature difference between it and the reference junction for blood flow.2. The temperature of the liver was higher than that of the afferent blood. On raising T(a) from 20 to 30 degrees C the liver temperature fell and the temperature difference between the liver and the aortic blood was reduced despite the decrease in the thermal gradient between the liver and the exterior and a small reduction in hepatic blood flow. A further rise in T(a) to 37 degrees C led to an increase in the liver and blood temperatures. The same pattern was seen when the temperature differences were corrected for blood flow.3. Metabolic heat production in the liver decreased when T(a) was raised from 20 to 30 degrees C.4. In a 20 degrees C environment inhibition of non-shivering thermogenesis with propranolol HCl (10 mg/kg body wt. I.V.) led to a fall in the temperature of the liver and its metabolic heat production towards levels found in untreated rats at T(a) = 30 degrees C. Consequently, in treated rats the change in metabolic heat production on raising T(a) to 30 degrees C was less than in untreated ones.5. These results are interpreted as evidence for the participation of the liver in thermoregulatory non-shivering thermogenesis.