Differences in the thermotropic behaviour of mitochondrial membrane respiratory enzymes from homeothermic and heterothermic endotherms

Metabolic rate and body temperature reduction during hibernation and daily torpor

Although it is well established that during periods of torpor heterothermic mammals and birds can reduce metabolic rates (MR) substantially, the mechanisms causing the reduction of MR remain a controversial subject. The comparative analysis provided here suggests that MR reduction depends on patterns of torpor used, the state of torpor, and body mass. Daily heterotherms, which are species that enter daily torpor exclusively, appear to rely mostly on the fall of body temperature (Tb) for MR reduction, perhaps with the exception of very small species and at high torpor Tb, where some metabolic inhibition may be used. In contrast, hibernators (species capable of prolonged torpor bouts) rely extensively on metabolic inhibition, in addition to Tb effects, to reduce MR to a fraction of that observed in daily heterotherms. In small hibernators, metabolic inhibition and the large fall of Tb are employed to maximize energy conservation, whereas in large hibernators, metabolic inhibition appears to be employed to facilitate MR and Tb reduction at torpor onset. Over the ambient temperature (Ta) range where torpid heterotherms are thermo-conforming, the Tb-Ta differential is more or less constant despite a decline of MR with Ta; however, in thermo-regulating torpid individuals, the Tb-Ta differential is maintained by a proportional increase of MR as during normothermia, albeit at a lower Tb. Thermal conductance in most torpid thermo-regulating individuals is similar to that in normothermic individuals despite the substantially lower MR in the former. However, conductance is low when deeply torpid animals are thermo-conforming probably because of peripheral vasoconstriction.

Thermal relations of metabolic rate reduction in a hibernating marsupial

We tested whether the reduction of metabolic rate (MR) in hibernating Cercartetus nanus (Marsupialia, 36 g) is better explained by the reduction of body temperature (Tb), the differential (delta T) between Tb and air temperature (Ta), or thermal conductance (C). Above the critical Ta during torpor (Ttc) of 4.8 +/- 0.7 degrees C where the Tb was not regulated, the steady-state MR was an exponential function of Tb (r2 = 0.92), and the overall Q10 was 3.3. However, larger Q10 values were observed at high Tb values during torpor, particularly within the thermoneutral zone (Q10 = 9.5), whereas low Q10 values were observed below Tb 20 degrees C (Q10 = 1.9). The delta T did not change over Ta 5-20 degrees C, although MR fell, and therefore the two variables were not correlated. Below the Ttc, Tb was regulated at 6.1 +/- 1.0 degrees C and MR increased proportionally to delta T. Our study suggests that MR in torpid C. nanus is largely determined by temperature effects and metabolic inhibition. In contrast, delta T explains MR only below the Ttc and C appears to affect MR only indirectly via changes of Tb, suggesting that delta T and C play only a secondary role in MR reduction during hibernation.

Reduction of metabolic rate and thermoregulation during daily torpor

Physiological mechanisms causing reduction of metabolic rate during torpor in heterothermic endotherms are controversial. The original view that metabolic rate is reduced below the basal metabolic rate because the lowered body temperature reduces tissue metabolism has been challenged by a recent hypothesis which claims that metabolic rate during torpor is actively downregulated and is a function of the differential between body temperature and ambient temperature, rather than body temperature per se. In the present study, both the steady-state metabolic rate and body temperature of torpid stripe-faced dunnarts, Sminthopsis macroura (Dasyuridae: Marsupialia), showed two clearly different phases in response to change of air temperature. At air temperatures between 14 and 30 degrees C, metabolic rate and body temperature decreased with air temperature, and metabolic rate showed an exponential relationship with body temperature (r2 = 0.74). The Q10 for metabolic rate was between 2 and 3 over the body temperature range of 16 to 32 degrees C. The difference between body temperature and air temperature over this temperature range did not change significantly, and the metabolic rate was not related to the difference between body temperature and air temperature (P = 0.35). However, the apparent conductance decreased with air temperature. At air temperatures below 14 degrees C, metabolic rate increased linearly with the decrease of air temperature (r2 = 0.58) and body temperature was maintained above 16 degrees C, largely independent of air temperature. Over this air temperature range, metabolic rate was positively correlated with the difference between body temperature and air temperature (r2 = 0.61). Nevertheless, the Q10 for metabolic rate between normothermic and torpid thermoregulating animals at the same air temperature was also in the range of 2-3.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of polyunsaturated and saturated dietary lipids on adipose tissue, brain and mitochondrial membrane fatty acid composition of a mammalian hibernator

Dietary lipid composition profoundly influences the hibernation pattern of the chipmunk Eutamias amoenus. The object of the present study was to investigate whether these physiological changes following feeding of saturated and unsaturated lipids were associated by compositional changes of fatty acids of tissues and membranes. Animals were fed with rodent chow (control diet), rodent chow with 10% sunflower seed oil (unsaturated diet) and rodent chow with 10% sheep fat (saturated diet). Diet-induced changes in the fatty acid composition of depot fat and brain total lipids and of mitochondrial phospholipids were determined. The fatty acid unsaturation index was lower in animals on saturated diet than in animals on unsaturated diet (depot fat 86.1 vs. 145.9; heart mitochondria 207.6 vs. 247.1; liver mitochondria 148.4 vs. 173.5). Pronounced differences between dietary groups were also observed in n-3 or n-6 fatty acids or their ratios of depot fat, brain and liver mitochondria. Generally, the diet-induced differences in tissue and membrane fatty acid composition in E. amoenus were more pronounced than those observed previously in non-hibernating species. Selective feeding and incorporation of high amounts of unsaturated fatty acids into tissues and cell membranes may be an important preparation for hibernation of E. amoenus which lowers its body temperature during torpor to about 0 degrees C.

Membrane function in mammalian hibernation

For homeotherms the maintenance of a high, uniform body temperature requires a constant energy supply and food intake. For many small mammals, the loss of heat in winter exceeds energy supply, particularly when food is scarce. To survive, some animals have developed a capacity for adaptive hypothermia in which they lower their body temperature to a new regulatory set-point, usually a few degrees above the ambient. This process, generally known as hibernation, reduces the temperature differential, metabolic activity, as well as the energy demand, and thus facilitates survival during winter. Successful hibernation in mammals requires that the enzymatic processes are regulated in such a manner that metabolic balance is maintained at both the high body temperature of the summer-active animal (37 degrees C) and the low body temperature of the winter-torpid animal (approx. 5 degrees C). This means that the cellular membranes have thermal properties capable of maintaining a balanced metabolism at these extreme physiological temperatures. The available evidence indicates that, for some tissues, preparation for hibernation involves an alteration in the lipid composition and thermal properties of cellular membranes. Marked differences in the thermal response of cellular membranes have been observed on a seasonal basis and, in some membranes, differences in lipid composition have been associated with the torpid state. However, to date, no consistent changes in lipid composition which would account for, or explain, the changes in membrane thermal response, have been detected. An important point to emphasize is that the process of 'homeoviscous adaptation', which occurs in procaryotes and some poikilotherms during acclimation to low temperatures, is not a characteristic feature of most membranes of mammalian hibernators.

The effect of temperature on isolated perfused hearts of heterothermic marsupials

1. The thermal response of isolated perfused hearts of four dasyurid marsupials was determined and compared with that of two rodents. 2. Heart beat rate was strongly temperature dependent in all species. 3. The temperature of cardiac arrest in the species investigated in the present study and of others collected from the literature occurred at a mean of about 13 degrees C in homeotherms, 7 degrees C in daily heterotherms, and 1 degrees C in hibernators. 4. For both marsupials and placentals the temperature of cardiac arrest in hibernators and daily heterotherms correlated with the minimum body temperature during torpor.

Reduction of metabolism during hibernation and daily torpor in mammals and birds: temperature effect or physiological inhibition?

The present study addresses the controversy of whether the reduction in energy metabolism during torpor in endotherms is strictly a physical effect of temperature (Q10) or whether it involves an additional metabolic inhibition. Basal metabolic rates (BMR; measured as oxygen consumption, VO2), metabolic rates during torpor, and the corresponding body temperatures (Tb) in 68 mammalian and avian species were assembled from the literature (n = 58) or determined in the present study (n = 10). The Q10 for change in VO2 between normothermia and torpor decreased from a mean of 4.1 to 2.8 with decreasing Tb from 30 to less than 10 degrees C in hibernators (species that show prolonged torpor). In daily heterotherms (species that show shallow, daily torpor) the Q10 remained at a constant value of 2.2 as Tb decreased. In hibernators with a Tb less than 10 degrees C, the Q10 was inversely related to body mass. The increase of mass-specific metabolic rate with decreasing body mass, observed during normothermia (BMR), was not observed during torpor in hibernators and the slope relating metabolic rate and mass was almost zero. In daily heterotherms, which had a smaller Q10 than the hibernators, no inverse relationship between the Q10 and body mass was observed, and consequently the metabolic rate during torpor at the same Tb was greater than that of hibernators. These findings show that the reduction in metabolism during torpor of daily heterotherms and large hibernators can be explained largely by temperature effects, whereas a metabolic inhibition in addition to temperature effects may be used by small hibernators to reduce energy expenditure during torpor.

Arrhenius parameters of mitochondrial membrane respiratory enzymes in relation to thermoregulation in endotherms

The relationship between the body temperature (Tb), the Arrhenius critical temperature (T*), and the apparent activation energy above T* (Ea1), of liver and heart mitochondrial respiratory enzymes from eleven homeothermic and eight heterothermic species was determined using a linear regression analysis. An inverse relation was observed between T* and Ea1 during torpor and hibernation. In all thermoregulatory states, T* decreased with Tb and T* was equal to or below Tb. During torpor Ea1 increased in a linear manner as Tb was lowered. It appears that the above Arrhenius parameters are closely linked to the thermoregulatory state of endotherms and thus may represent an adaptation for function at low Tb's.