Total Metabolic Rate Research Articles

Incorporating spatial dependence into a multicellular tumor spheroid growth model

Recent models for organism and tumor growth yield simple scaling laws based on conservation of energy. Here, we extend such a model to include spatial dependence to model necrotic core formation. We adopt the allometric equation for tumor volume with a reaction-diffusion equation for nutrient concentration. In addition, we assume that the total metabolic energy and average cellular metabolic rate depend on nutrient concentration in a Michaelis-Menten-like manner. From experimental results, we relate the necrotic volume to nutrient consumption and estimate both the time and nutrient concentration at necrotic core formation. Based on experimental results, we demand that the necrotic core radius varies linearly with tumor radius after core formation and extend the equations for tumor volume and nutrient concentration to the postnecrotic core regime. In particular, we obtain excellent agreement with experimental data and the final steady-state viable rim thickness.

Modeling the Oxygen Consumption Rates in Pacific Salmon and Steelhead: Model Development

AbstractWe derived a series of models for estimating the standard metabolic rates, swimming costs, and total metabolic rates for sockeye salmon Oncorhynchus nerka and steelhead O. mykiss. The performance of these models was compared statistically and used to predict optimal cruising speeds. These predictions were tested with independent estimates of swimming speed obtained under field conditions. Standard metabolic rates were correlated with body mass and temperature. Swimming costs were correlated with body mass and swimming speed, whereas total metabolic rates were correlated with body mass, water temperature, and swimming speed. Swimming costs were also correlated with temperature and salinity in steelhead but not in sockeye salmon. Regression models accounted for 94–99% of the variance in standard metabolic rates, swimming costs, and total metabolic rates. The oxygen consumption rate models we derived for sockeye salmon were inadequate for describing oxygen consumption in other species of Pacific salmon, Oncorhynchus spp., indicating that the practice of borrowing parameters from closely related species can induce serious biases in model predictions. The models derived in this study also produced realistic estimates of swimming speed in sockeye salmon but not in steelhead. The models derived in this study appear to be useful in estimating swimming speed and total metabolic rates of sockeye salmon in the field but are not appropriate predictors for other species of Pacific salmon.

Air sacPO2 and oxygen depletion during dives of emperor penguins

In order to determine the rate and magnitude of respiratory O2 depletion during dives of emperor penguins (Aptenodytes forsteri), air sac O2 partial pressure (PO2) was recorded in 73 dives of four birds at an isolated dive hole. These results were evaluated with respect to hypoxic tolerance, the aerobic dive limit (ADL; dive duration beyond which there is post-dive lactate accumulation) and previously measured field metabolic rates (FMRs). 55% of dives were greater in duration than the previously measured 5.6-min ADL. PO2 and depth profiles revealed compression hyperoxia and gradual O2 depletion during dives. 42% of final PO2s during the dives (recorded during the last 15 s of ascent) were <20 mmHg (<2.7 kPa). Assuming that the measured air sac PO2 is representative of the entire respiratory system, this implies remarkable hypoxic tolerance in emperors. In dives of durations greater than the ADL, the calculated end-of-dive air sac O2 fraction was <4%. The respiratory O2 store depletion rate of an entire dive, based on the change in O2 fraction during a dive and previously measured diving respiratory volume, ranged from 1 to 5 ml O2 kg(-1) min(-1) and decreased exponentially with diving duration. The mean value, 2.1+/-0.8 ml O2 kg(-1) min(-1), was (1) 19-42% of previously measured respiratory O(2) depletion rates during forced submersions and simulated dives, (2) approximately one-third of the predicted total body resting metabolic rate and (3) approximately 10% of the measured FMR. These findings are consistent with a low total body metabolic rate during the dive.

Environmental levels of atrazine and its degradation products impair survival skills and growth of red drum larvae

Red drum larvae ( Sciaenops ocellatus) were exposed to environmentally realistic and sublethal levels of the herbicide atrazine (2-chloro-4-ethylamin-6-isopropylamino- S-triazine) to evaluate its effects on ecologically critical traits: growth, behavior, survival potential, and resting respiration rate. Settlement size larvae (7 mm total length) were given an acute exposure of atrazine at 0, 40, and 80 μg l −1 for 4 days. Tests of 96 h survival confirmed that these naturally occurring concentrations were sublethal for red drum larvae. Growth, routine swimming, antipredator responses to artificial and actual predators, and resting respiration rate were monitored 1 and 3 days after onset of exposure. Atrazine exposure significantly reduced growth rate. Atrazine-exposed larvae also exhibited significantly higher routine swimming speeds, swam in more convoluted paths, and were hyperactive. Responses to artificial and actual predators were not affected by atrazine exposure nor were resting respiration rates. The higher rate of travel (86% higher in atrazine-treated larvae) resulted in higher predicted encounter rates with prey (up to 71%) and slow moving predators (up to 63%). However, hyperactivity and faster active swimming speeds of exposed larvae indicated that naturally occurring sublethal levels of atrazine will result in an elevated rate of energy utilization (doubling the total metabolic rate), which is likely to increase the risk of death by starvation. Moreover, atrazine effects on growth will prolong the larval period, which could reduce the juvenile population by as much as 24%. We conclude that environmentally realistic levels of atrazine induce behavioral and physiological effects on fish larvae that would compromise their survival expectations.

Walking on inclines: energetics of locomotion in the antCamponotus

To assess energetic costs during rest and locomotion in a small insect, we measured metabolic rate in freely moving ants Camponotus sp. (average body mass 11.9 mg). The animals ran in a straight respirometric chamber in which locomotor speed and CO2 release were monitored simultaneously using flow-through respirometry and conventional video analysis. In resting intact ants, standard metabolic rate was on average 0.32 ml CO2 g(-1) body mass h(-1). During walking, the ants breathed continuously and metabolic rate increased between 4.3 times (level walking at 0-5 mm s(-1)) and 6.9 times (30 degrees ascent at 85-95 mm s(-1)) over resting rates. Metabolic rate increased linearly with increasing walking speed but superficially leveled off beyond speeds of about 70 mm s(-1). Walking on incline (uphill) or decline slopes (downhill) of up to 60 degrees had only a small effect on energy consumption compared to level walking. During slope walking, total metabolic rate averaged over all running speeds ranged from a minimum of 1.55+/-0.4 (horizontal running) to a maximum of 1.89+/-0.7 ml CO2 h(-1) g(-1) body mass (30 degrees downhill). The mean cost of transport in Camponotus was approximately 130 J g(-1) km(-1). The metabolic requirements in the comparatively small insect Camponotus for walking were mostly in the range expected from data obtained from other insects and small poikilotherms, and from allometric scaling laws.

Energy Expenditure and Adaptive Responses to an Acute Hypercaloric Fat Load in Humans with Lipodystrophy

Humans respond to an acute excess of ingested energy by storing the surplus energy as triglyceride in white adipose tissue. To study the energetic response to acute overfeeding in human subjects with limited adipose tissue capacity, we recruited seven subjects with lipodystrophy and seven lean healthy controls. Total fat mass was approximately 70% lower in lipodystrophic subjects (mean, 6.1 kg) than in body mass index-matched lean controls (mean, 22.0 kg). Energy expenditure and macronutrient oxidation rates were assessed in chamber calorimeters on two separate occasions for 40 h, during which time subjects consumed either an energy-balanced diet or a diet incorporating 30% excess energy as fat. On the energy-balanced diet, total daily energy expenditure and basal metabolic rate were linearly associated with lean mass in both groups (r(2) = 0.83) and were not significantly different between groups when corrected for lean mass. In response to the fat challenge, total energy expenditure did not increase significantly in healthy controls (9,472 +/- 1,069 to 9,724 +/- 1,114 kJ/d; P = 0.189). Substrate oxidation results confirm that excess fat was predominantly stored. In contrast, lipodystrophic subjects significantly increased total daily energy expenditure (11,081 +/- 1,226 to 11,730 +/- 1,374 kJ/d; P < 0.005). This was largely attributable to a 29% increase in fat oxidation. Thus, subjects with lipodystrophy uniquely respond to an acute hypercaloric load with a higher energy expenditure increment and by increasing fat oxidation. Insight into the molecular mechanisms responsible for this phenomenon may yield novel therapeutic approaches for obesity.

Direct calorimetric studies on the metabolic rate of Gammarus oceanicus from the brackish waters of the Baltic Sea

Heat production rates of the benthic amphipod Gammarus oceanicus from the brackish waters of the Gulf of Gdansk were measured by means of direct calorimetry at their usual ambient (“habitat”) salinity (7‰) and 10 °C. Animals exhibited locomotor activity during the measurements, so that the total metabolic rate was the sum of both resting and active metabolism. The mean specific metabolic rate amounted to 1.57 ± 0.61 mW g −1 wet weight (ww) ( n = 73, average wet weight 67.2 ± 34.0 mg). Smallest animals with a mean wet weight of 24.5 ± 3.4 mg exhibited the highest specific metabolic rates with 2.24 ± 0.57 mW g −1 ww ( n = 10), whereas the largest ones with wet weights of 117.7 ± 19.4 mg showed the lowest values with 1.10 ± 0.32 mW g −1 ww ( n = 15). There was a significant negative correlation ( R 2 = 0.49, P < 0.05) between the specific metabolic rate of G. oceanicus and its wet weight. The metabolic rate of females was higher by 29% ( P < 0.05) than that of males of the same length due to the differences in wet weight.

Priorities in perioperative geriatrics.

T he aging of the baby-boom population and the decreases in adult mortality seen in the last few decades will dramatically increase the age of Americans between 2010 and 2030. During that time, the population older than age 65 yr is expected to grow by 75%, whereas between 1995 and 2050, the cumulative growth of the population older than 85 yr is expected to exceed 400% (1). Furthermore, it has been reported that the increased demand for surgery in this population may exceed the rate of population growth (2). The implications of an aging population for the practice of anesthesiology are profound. Age-related changes in physiology and pharmacology can affect every aspect of perioperative care. The changes in surgical demographics will compel the anesthesiologist to become familiar with the physiology and clinical care of the aged. This review will serve as an introduction. First, some of the physiologic changes that occur with aging will be presented. Second, the preoperative assessment of the older surgical patient will be discussed. Third, some of the research related to intraoperative management of the geriatric surgical patient will be described. In the fourth section, we will discuss some geriatric-specific issues related to postoperative management.

Decreased food intake rather than zinc deficiency is associated with changes in plasma leptin, metabolic rate, and activity levels in zinc deficient rats

This study investigated the hypothesis that the reduced food intake and poor weight gain in zinc deficient rats is due to: increased plasma leptin concentration, increased physical activity and/or increased metabolic rate. Weanling rats were assigned to three groups: controls fed ad libitum (C), zinc deficient (ZD), and pair-fed controls (PF), and tested in a metabolic chamber and activity monitor at baseline and weekly for four weeks. At the end of the study, all groups were compared for differences in plasma leptin concentrations. ZD and PF animals had markedly reduced food intake and weight gain. ZD had reduced stereotypic and locomotor activity compared to PF animals and both groups demonstrated an abolished peri-nocturnal activity spike and were much less active than controls. This was associated with a reduced total metabolic rate by day 30: ZD (0.73 ± 0.07 kcal/hr, p = 0.0001) and PF (0.83 ± 0.06 kcal/hr, p = 0.0001) groups vs. controls (1.82 ± 0.09 kcal/hr). Plasma leptin concentrations in ZD (1.55 ± 0.06 μg/L) were lower than controls (2.01 ± 0.18 μg/L, p < 0.03), but neither ZD nor controls were statistically different from PF (1.68 ± 0.05 μg/L). Both low leptin concentrations and low metabolic rates in the ZD and PF rats were associated with decreased food intake rather than zinc deficiency. The reduced food intake and poor weight gain observed in zinc deficient rats could not be explained by elevated leptin concentrations, hypermetabolism, or increased activity. Low serum leptin concentrations, hypometabolism, and decreased activity are more likely the result of the anorexia of zinc deficiency.

Metabolic control of pulmonary ventilation in the developing chick embryo.

In birds, during the period from the breaking of the air cell by the beak (internal pipping) to hatching, pulmonary ventilation (VE) begins and gas exchange is jointly provided by the lungs and the chorioallantoic membrane (CAM). We asked to what extent, during this phase of two concurrent gas exchange organs, changes in the embryo's metabolic needs were accompanied by changes in VE. The carbon dioxide and oxygen exchange rates (VCO2, VO2) through lungs and CAM were separately, but simultaneously, measured in chicken embryos at 20-21 days of incubation, while VE was calculated from the measurements of pressure oscillations in the air cell during breathing. During the last 24 h of incubation, lung VO2 and VCO2 gradually progressed as the corresponding CAM values declined. An increase in egg temperature (T) from 33 to 39 degrees C increased the embryo's total metabolic rate, especially when the lungs were the predominant gas exchange route. Whether metabolism increased because of the embryo's development or because of the increase in T, VE was linearly proportional to lung VO2 and VCO2, and not to the embryo's total metabolic rate. Hence, in the developing chick embryo, VE control mechanisms sense the peripheral tissue requirements via the gaseous component of cellular metabolism.

Are the effects of T3 on resting metabolic rate in euthyroid rats entirely caused by T3 itself?

Because we previously reported that T3 and 3,5-diiodo-L-thyronine (3,5-T2) both increase resting metabolic rate (RMR), 3,5-T2 could be another thyroidal regulator of energy metabolism. This effect of 3,5-T2 is evident in rats made hypothyroid by propylthiouracil and iopanoic acid, not in normal euthyroid (N) rats. Possibly, under euthyroid conditions, active 3,5-T2 may need to be formed intracellularly from a precursor such as T3. We tested this hypothesis by giving a single injection of T3 to N rats and comparing the time course of the variations in RMR with those of the changes in the serum and hepatic levels of 3,5-T2. Acute injection had an evident effect on RMR, 25 h earlier, in N rats than in rats made hypothyroid by propylthiouracil and iopanoic acid, maximal values (+40%) being reached in the former at 24-26 h. In N rats, the simultaneous injection of actinomycin D with the T3 inhibited the late part of the effect (after 24 h) more strongly than the early part (14-24 h). In serum and liver, 3,5-T2 levels were increased significantly at 12-24 h after T3 injection into N rats, a time at which RMR was rising rapidly to peak. These results seem to indicate that when T3 is injected into N animals, not all the effects on RMR are attributable to T3 itself, the early effect presumably being largely because of its in vivo deiodination to 3,5-T2. Because the effects of T3 and 3,5-T2 are additive, in N rats, the two iodothyronines probably cooperate in vivo to determine the total metabolic rate.

Metabolic rates of juvenile scalloped hammerhead sharks (Sphyrna lewini)

Oxygen consumption of juvenile scalloped hammerhead sharks, Sphyrna lewini, was measured in a Brett-type flume (volume=635 l) to quantify metabolic rates over a range of aerobic swimming speeds and water temperatures. Oxygen consumption (log transformed) increased at a linear rate with increases in tailbeat frequency and swimming speed. Estimates of standard metabolic rate ranged between 161 mg O2 kg–1 h–1 at 21°C and 203 mg O2 kg–1 h–1 at 29°C (mean±SD: 189±15 mg O2 kg–1 h–1 at 26°C). Total metabolic rates ranged from 275 mg O2 kg–1 h–1 at swimming speeds of 0.5 body lengths per second (L s–1) to a maximum aerobic metabolic rate of 501 mg O2 kg–1 h–1 at 1.4 L s–1. Net cost of transport was highest at slower swimming speeds (0.5–0.6 L s–1) and was lowest between 0.75 and 0.9 L s–1. Therefore, these sharks are most energy efficient at swimming speeds between 0.75 and 0.9 L s–1. These data indicate that tailbeat frequency and swimming speed can be used as predictors of metabolic rate of free-swimming juvenile hammerhead sharks.

Differences between Humans and Mice in Efficacy of the Body Fat Lowering Effect of Conjugated Linoleic Acid: Role of Metabolic Rate

(1‐ 6) and that this effect is mediated by an enhanced energy expenditure (5‐7). The body fat-lowering effect of CLA has also been reported in humans (8 ‐12), but it seems to be less prominent than in mice. The dose administered or the length of the feeding period may partly explain this difference in magnitude of the effect between mice and humans, but differences in metabolic rate may play a major role. The relationship between basal metabolic rate or energy expenditure and body weight in different size mammals is described by the function Y 5 aX 0.75 , where Y is basal metabolic rate (kJ/d), X is body weight (kg) and a is basal metabolic rate per kg 0.75 per day, which is ;300 kJ/ (kg 0.75 z d) (13‐15). Thus, the basal metabolic rate in different size species is proportional to the body weight raised to the 0.75 power, the so called metabolic weight. If one assumes that the basal metabolic rate comprises ;75% of the total metabolic rate, then the total metabolic rate can be described by the function Y 5 400 X 0.75 . From this relationship, it follows that the total metabolic rate of a 70-kg human is 400 3 70 0.75 5 9680 kJ or 138 kJ per kg body weight. Similarly, the total metabolic rate of a 30 g mouse is 400 3 0.030 0.75 5 29 kJ or 961 kJ per kg body weight. A relative increase of 10% in energy expenditure per kg metabolic weight will also result in a 10% increase in the energy expenditure per kg body weight but the absolute increase in energy expenditure expressed in kJ per kg body weight will be greater in mice than in humans. Consequently, the reduction in body fat will also be greater in mice than in humans when a particular dose of CLA expressed in mg CLA per 1000 kJ energy intake or in mg CLA per kg metabolic weight [1 mg CLA per 1000 kJ energy intake 5 (400/1000) 5 0.4 mg CLA per kg metabolic weight] will result in a similar relative increase in energy expenditure in both mice and humans. For example, an increase of 10% in energy expenditure and no change in energy intake will theoretically result in mice during a period of 30 d in a reduction of 0.10 (energy expenditure increase) 3 961 (energy expenditure in kJ per kg body weight) 3 30 (d) 5 2883 kJ energy per kg body weight, which is equivalent to 2883/39.8 5 72.4 g of fat per kg body weight [the gross energy of body fat is 39.8 kJ per g (16)]. Similarly, it can be calculated that this 10% increase in energy expenditure will result in humans in a reduction of 414 kJ of energy per kg body weight, which is equivalent to 10.4 g of fat per kg body weight. Thus, theoretical calculations indicate that the reduction in body fat will be about seven times higher in mice than in humans because of the seven times higher metabolic rate per kg body weight. These calculations are based on the assumption that the increase in energy expenditure is exclusively at the expense of body fat, which is not always true (see footnote 2 of Table 1). The theoretical calculations are in line with the experimental data from various studies in mice and humans. The normalized results indicate that the body fat-lowering effect of CLA is considerably lower in humans than in mice (Table 1). There are, however, large variations in the normalized results among the various studies in both mice and humans. Nevertheless, the normalized body fat-lowering effect of CLA is on average approximately seven times higher in mice than in humans, similar to what the theoretical calculations would predict. The results in Table 1 also seem to support the above assumption that a particular dose of CLA will result in a similar relative increase in energy expenditure in both mice and humans. However, it is not clear whether this might also be true for other species. Thus, differences in metabolic rate may play a major role in explaining the differences between mice and humans in the body fat-lowering effect of CLA. The effects of CLA on energy metabolism observed in mice should be cautiously interpreted and extrapolation to humans should be done carefully. In contrast, the high metabolic rate of the mouse compared with humans may make the mouse an excellent model to study the effects of various dietary fats and other components on energy metabolism.

Convection requirement is established by total metabolic rate in the newborn tammar wallaby

Ventilation ( V ̇ e) and metabolic rate, determined from both pulmonary and cutaneous gas exchange, were measured in 39 newborn tammar wallabies, Macropus eugenii, aged between 0 and 3 days. In 1-day-old animals both total metabolic rate (skin+lung exchange) and ventilation were approximately 50% of the values predicted for eutherian newborns of equivalent body mass. Hence, the convection requirement ( V ̇ e/total metabolic rate) of the newborn tammar is close to predicted values for newborns and adult mammals in general. Metabolic rate in the newborn tammar is supported to some extent by cutaneous gas exchange, approximately 30% of the total in the 1-day-old animal. This ratio diminishes with increasing age as the lung takes on an increasingly more important role for respiratory exchange. The early establishment of the convection requirement in the newborn tammar, despite significant cutaneous gas exchange, provides supporting evidence that metabolic rate per se is important in establishing the level of ventilation.

Genetic and dietary factors affecting human metabolism of 1,3-butadiene

The objective of this project was to determine the factors associated with differences in butadiene (BD) inhalation uptake and the rate of metabolism for BD to epoxy butene by monitoring exhaled breath during and after a brief exposure to BD in human volunteers. A total of 133 subjects (equal males and females; four racial groups) provided final data. Volunteers gave informed consent and completed a questionnaire including diet and alcohol use. A venous blood sample was collected for genotyping CYP2E1. Subjects received a 20 min exposure to 2.0 ppm of BD, followed by a 40 min washout period. The total administered dose was 0.6 ppm*h, which is in the range of everyday exposures. Ten, 1 or 2 min exhaled breath samples (five during and five after exposure) were collected using an optimized strategy. BD was determined by GC-FID analysis. Breathing activity (minute ventilation, breath frequency and tidal volume) was measured to estimate alveolar ventilation. After the washout period, 250 mg of chlorzoxazone were administered and urine samples collected for 6 h to measure 2E1 phenotype. The total BD uptake during exposure (inhaled BD minus exhaled) was estimated. A three-compartment PBPK model was fitted to each subject's breath measurements to estimate personal and population model parameters, including in-vivo BD metabolic rate. A hierarchical Bayesian PBPK model was fit by Monte Carlo simulations to estimate model parameters. Regression and ANOVA analyses were performed. Earlier data analysis showed wide ranges for both total uptake BD and metabolic rate. Both varied significantly by sex and age, and showed suggestive differences by race, with Asians having the highest rates. The analyses reported here found no correlation between total BD uptake and metabolic rate. No significant differences were found for oxidation rates by 2E1 genotype or phenotype, but the rates showed trends consistent with reported differences by genotype and phenotype for chlorzoxazone metabolism. No effects on metabolic rate were observed for long-term alcohol consumption, or consumption in the past 24 h. Overall, neither dietary factors nor genetic differences explained much of the wide variability in metabolic rates. Population characteristics, age, sex, and race, were the most important explanatory variables, but a large fraction of the total variability in metabolism remains to be explained.

Effect of stimulation frequency on contraction-induced glucose transport in rat skeletal muscle.

Previous studies have indicated that frequency of stimulation is a major determinant of glucose transport in contracting muscle. We have now studied whether this is so also when total force development or metabolic rate is kept constant. Incubated soleus muscles were electrically stimulated to perform repeated tetanic contractions at four different frequencies (0.25, 0.5, 1, and 2 Hz) for 10 min. Resting length was adjusted to achieve identical total force development or metabolic rate (glycogen depletion and lactate accumulation). Overall, at constant total force development, glucose transport (2-deoxyglucose uptake) increased with stimulation frequency (P < 0.05; basal: 25 +/- 2, 0.25 Hz: 50 +/- 4, 0.5 Hz: 50 +/- 3, 1 Hz: 81 +/- 5, 2 Hz: 79 +/- 3 nmol. g(-1). 5 min(-1)). However, glucose transport was identical (P > 0.05) at the two lower (0.25 and 0.5 Hz) as well as at the two higher (1 and 2 Hz) frequencies. Glycogen decreased (P < 0.05; basal: 19 +/- 1, 0.25 Hz: 13 +/- 1, 0.5 Hz: 12 +/- 2, 1 Hz: 7 +/- 1, 2 Hz: 7 +/- 1 mmol/kg) and 5'-AMP-activated protein kinase (AMPK) activity increased (P < 0. 05; basal: 1.7 +/- 0.4, 0.25 Hz: 32.4 +/- 7.0, 0.5 Hz: 36.5 +/- 2.1, 1 Hz: 63.4 +/- 8.0, 2 Hz: 67.0 +/- 13.4 pmol. mg(-1). min(-1)) when glucose transport increased. Experiments with constant metabolic rate were carried out in soleus, flexor digitorum brevis, and epitrochlearis muscles. In all muscles, glucose transport was identical at 0.5 and 2 Hz (P > 0.05); also, AMPK activity did not increase with stimulation frequency. In conclusion, muscle glucose transport increases with stimulation frequency but only in the face of energy depletion and increase in AMPK activity. This indicates that contraction-induced glucose transport is elicited by metabolic demands rather than by events occurring early during the excitation-contraction coupling.

Gas exchange and ventilation during dormancy in the tegu lizard tupinambis merianae

The tegu lizard Tupinambis merianae exhibits an episodic ventilatory pattern when dormant at 17 degrees C but a uniform ventilatory pattern when dormant at 25 degrees C. At 17 degrees C, ventilatory episodes were composed of 1-22 breaths interspaced by non-ventilatory periods lasting 1.8-26 min. Dormancy at the higher body temperature was accompanied by higher rates of O(2) consumption and ventilation. The increase in ventilation was due only to increases in breathing frequency with no change observed in tidal volume. The air convection requirement for O(2) did not differ at the two body temperatures. The respiratory quotient was 0.8 at 17 degrees C and 1.0 at 25 degrees C. We found no consistent relationship between expired gas composition and the start/end of the ventilatory period during episodic breathing at 17 degrees C. However, following non-ventilatory periods of increasing duration, there was an increase in the pulmonary O(2) extraction that was not coupled to an equivalent increase in elimination of CO(2) from the lungs. None of the changes in the variables studied could alone explain the initiation/termination of episodic ventilation in the tegus, suggesting that breathing episodes are shaped by a complex interaction between many variables. The estimated oxidative cost of breathing in dormant tegus at 17 degrees C was equivalent to 52.3 % of the total metabolic rate, indicating that breathing is the most costly activity during dormancy.

Estimating power curves of flying vertebrates.

The power required for flight in any flying animal is a function of flight speed. The power curve that describes this function has become an icon of studies of flight mechanics and physiology because it encapsulates the accessible animal's flight performance. The mechanical or aerodynamic power curve, describing the increase in kinetic energy of the air due to the passage of the bird, is necessarily U-shaped, for aerodynamic reasons, and can be estimated adequately by lifting-line theory. Predictions from this and related models agree well with measured mechanical work in flight and with results from flow visualization experiments. The total or metabolic power curve also includes energy released by the animal as heat, and is more variable in shape. These curves may be J-shaped for smaller birds and bats, but are difficult to predict theoretically owing to uncertainty about internal physiological processes and the efficiency of the flight muscles. The limitations of some existing models aiming to predict metabolic power curves are considered. The metabolic power curve can be measured for birds or bats flying in wind tunnels at controlled speeds. Simultaneous determination in European starlings Sturnus vulgaris of oxygen uptake, total metabolic rate (using labelled isotopes), aerodynamic power output and heat released (using digital video thermography) enable power curves to be determined with confidence; flight muscle efficiency is surprisingly low (averaging 15-18 %) and increases moderately with flight speed, so that the metabolic power curve is shallower than predicted by models. Accurate knowledge of the power curve is essential since extensive predictions of flight behaviour have been based upon it. The hypothesis that the power curve may not in fact exist, in the sense that the cost of flight may not be perceived by a bird as a continuous smooth function of air speed, is advanced but has not yet formally been tested. This hypothesis is considered together with evidence from variation in flight behaviour, wingbeat kinematics and flight gait with speed. Possible constraints on flight behaviour can be modelled by the power curves: these include the effect of a maximum power output and a constraint on maximum speed determined by downstroke wingbeat geometry and the relationship between thrust and lift.

Metabolic costs of growth in free‐living Garter Snakes and the energy budgets of ectotherms

1. The metabolic or respiratory cost of growth (RG) is the increase in metabolic rate of a growing animal, and it represents chemical potential energy expended in support of net biosynthesis but not deposited as new tissue. 2. Two statistical methods (multiple non‐linear regression and analysis of regression residuals) were used to calculate RG from data (n = 68) from a doubly labelled water study of free‐ranging Garter Snakes (Thamnophis sirtalis fitchi) in northern California. 3. The sample‐wise (‘ecological’) cost of growth was 2·07 kJ per gram of net growth (equivalent to 8·63 kJ g–1 dry tissue); reanalysis of a subset of efficient growers yielded a more conservative ‘physiological’ estimate of 1·67 kJ g–1. 4. Our empirical estimate of RG, among the first reported for squamate reptiles and free‐living animals of any kind, compares closely with published, laboratory‐derived values for ectotherms. 5. The metabolic costs of growth accounted for an average of 30% of total field metabolic rates for these snakes, which were growing at a mean rate of 3% of body mass per day. However, our method probably underestimated the total ecological cost of growth for large animals, because potential growth costs that covary with body size were not included. 6. Distinction between conceptual and empirical energy budgets clarifies relationships among body size, metabolic rates, and the physiological and ecological costs of growth.

Metabolic adjustments during daily torpor in the Djungarian hamster.

Djungarian hamsters (Phodopus sungorus) acclimated to a short photoperiod (8:16-h light-dark cycle) display spontaneous daily torpor with ad libitum food availability. The time course of body temperature (Tb), metabolic rate, respiratory quotient (RQ), and substrate and enzyme changes was measured during entrance into torpor and in deep torpor. RQ, blood glucose, and serum lipids are high during the first hours of torpor but then gradually decline, suggesting that glucose is the primary fuel during the first hours of torpor, with a gradual change to lipid utilization. No major changes in enzyme activities were observed during torpor except for inactivation of the pyruvate dehydrogenase (PDH) complex in liver, brown adipose tissue, and heart muscle. PDH inactivation closely correlates with the reduction of total metabolic rate, whereas in brain, kidney, diaphragm, and skeletal muscle, PDH activity was maintained at the initial level. These findings suggest inhibition of carbohydrate oxidation in heart, brown adipose tissue, and liver during entrance into daily torpor.