Allometric cascade as a unifying principle of body mass effects on metabolism

Circulation and metabolic rates in a natural hibernator: an integrative physiological model

Small hibernating mammals show regular oscillations in their heart rate and body temperature throughout the winter. Long periods of torpor are abruptly interrupted by arousals with heart rates that rapidly increase from 5 beats/min to over 400 beats/min and body temperatures that increase by ∼30°C only to drop back into the hypothermic torpid state within hours. Surgically implanted transmitters were used to obtain high-resolution electrocardiogram and body temperature data from hibernating thirteen-lined ground squirrels (Spermophilus tridecemlineatus). These data were used to construct a model of the circulatory system to gain greater understanding of these rapid and extreme changes in physiology. Our model provides estimates of metabolic rates during the torpor-arousal cycles in different model compartments that would be difficult to measure directly. In the compartment that models the more metabolically active tissues and organs (heart, brain, liver, and brown adipose tissue) the peak metabolic rate occurs at a core body temperature of 19°C approximately midway through an arousal. The peak metabolic rate of the active tissues is nine times the normothermic rate after the arousal is complete. For the overall metabolic rate in all tissues, the peak-to-resting ratio is five. This value is high for a rodent, which provides evidence for the hypothesis that the arousal from torpor is limited by the capabilities of the cardiovascular system.

Functional linkages for the pace of life, life-history, and environment in birds

For vertebrates, body mass underlies much of the variation in metabolism, but among animals of the same body mass, metabolism varies six-fold. Understanding how natural selection can influence variation in metabolism remains a central focus of Physiological Ecologists. Life-history theory postulates that many physiological traits, such as metabolism, may be understood in terms of key maturational and reproductive characteristics over an organism's life-span. Although it is widely acknowledged that physiological processes serve as a foundation for life-history trade-offs, the physiological mechanisms that underlie the diversification of life-histories remain elusive. Data show that tropical birds have a reduced basal metabolism (BMR), field metabolic rate, and peak metabolic rate compared with temperate counterparts, results consistent with the idea that a low mortality, and therefore increased longevity, and low productivity is associated with low mass-specific metabolic rate. Mass-adjusted BMR of tropical and temperate birds was associated with survival rate, in accordance with the view that animals with a slow pace of life tend to have increased life spans. To understand the mechanisms responsible for a reduced rate of metabolism in tropical birds compared with temperate species, we summarized an unpublished study, based on data from the literature, on organ masses for both groups. Tropical birds had smaller hearts, kidneys, livers, and pectoral muscles than did temperate species of the same body size, but they had a relatively larger skeletal mass. Direct measurements of organ masses for tropical and temperate birds showed that the heart, kidneys, and lungs were significantly smaller in tropical birds, although sample sizes were small. Also from an ongoing study, we summarized results to date on connections between whole-organism metabolism in tropical and temperate birds and attributes of their dermal fibroblasts grown in cell culture. Cells derived from tropical birds had a slower rate of growth, consistent with the hypothesis that these cells have a slower metabolism. We found that dermal fibroblasts from tropical birds resisted chemical agents that induce oxidative and non-oxidative stress better than do cells from temperate species, consistent with the hypothesis that birds that live longer invest more in self-maintenance such as antioxidant properties of cells.

Causes and significance of variation in mammalian basal metabolism

Mammalian basal metabolic rates (BMR) increase with body mass, whichs explains approximately 95% of the variation in BMR. However, at a given mass, there remains a large amount of variation in BMR. While many researchers suggest that the overall scaling of BMR with body mass is due to physiological constraints, variation at a given body mass may provide clues as to how selection acts on BMR. Here, we examine this variation in BMR in a broad sample of mammals and we test the hypothesis that, across mammals, body composition explains differences in BMR at a given body mass. Variation in BMR is strongly correlated with variation in muscle mass, and both of these variables are correlated with latitude and ambient temperature. These results suggest that selection alters BMR in response to thermoregulatory pressures, and that selection uses muscle mass as a means to generate this variation.

Adaptation versus allometry: population and body mass effects on hypoxic metabolism in Fundulus grandis

Hypoxia has significant effects on organisms, from metabolic reduction to death, and could be an important evolutionary force affecting the variation among populations within a species. To determine intraspecific variation in hypoxic metabolism and the effect of body mass, we examine rates of oxygen consumption (M(O2)) at seven oxygen concentrations among seven populations of Fundulus grandis that inhabit a mosaic of habitats with different frequencies and intensities of hypoxia. For M(O2), there is a significant interaction (P< 0.05) between body mass and oxygen concentrations: log(10) body mass: log(10) M(O2) slopes were steeper at intermediate oxygen partial pressures (Po(2)) than either normoxic or lowest Po(2) (ANCOVA, P<0.001). Additionally, the PO(2crit) (Po(2) where M(O2) can no longer be maintained) was a negative function of body mass (P < 0.04). At the lowest Po(2) (1.8 kPa), there was a significant difference in M(O2) among populations: one of the populations from environments more frequently stressed by hypoxia has greater M(O2) at the lowest oxygen concentrations. With few differences among populations, the most important effects were how body mass affected M(O2) at intermediate Po(2) and the negative relationship between body mass and PO(2crit). These findings suggest that an increase in body size is a useful strategy to minimize the effect of hypoxia.

Scaling of muscle metabolic enzymes: an historical perspective

In this paper, we take an historical approach to reviewing research into the patterns of metabolic enzymes in muscle in relation to body size, focusing on mitochondrial enzymes. One of the first studies on allometric scaling of muscle enzymes was published in an early issue of this journal (George and Talesara, 1961 Comp. Biochem. Physiol. 3: 267-273). These researchers studied a number of locally available birds and a bat, measuring the activity of the mitochondrial enzyme succinate dehydrogenase in relation to body mass and muscle structure. Though the phenomenon of allometric scaling of metabolism was well recognized even 50 years earlier, this study was one of the first to explore the enzymatic underpinnings of the metabolic patterns in different animals. In this review, we begin by considering the George and Talesara study in the context of this early era in metabolic biochemistry and comparative physiology. We review subsequent studies in the last 50 years that continued the comparative analysis of enzyme patterns in relation to body size in diverse experimental models. This body of work identified a recurrent (though not ubiquitous) reciprocal relationship between oxidative and glycolytic enzymes. In the last 10 years, studies have focused on identifying the molecular mechanisms that determine the muscle metabolic enzyme phenotype.

Cold acclimation in Peromyscus: individual variation and sex effects in maximum and daily metabolism, organ mass and body composition

We studied metabolic and organ mass responses to thermal acclimation (7 weeks at 5 degrees C or 23 degrees C) in deer mice, Peromyscus maniculatus. Cold acclimation resulted in significantly higher maximal oxygen consumption in thermogenesis (V(O(2)max)) and daily mean oxygen consumption (V(O(2)mean)), an increase in the mass of most visceral organs, a lower absolute body fat and a marginally significant increase in hematocrit. The mass of digestive organs and body fat content differed significantly between sexes. Acclimation effects on fat content were more pronounced in females. Variation in heart and lung mass was positively correlated with V(O(2)max) and V(O(2)mean), while body fat content was negatively correlated with both traits. Nonetheless, a large fraction of the metabolic difference between cold- and warm-acclimated groups remained unexplained. Associations between traits at lower levels of biological organization measured here and whole-organism energetics remained consistent across acclimation temperatures, except for the correlation between kidney mass and V(O(2)mean), which was positive and significant in cold acclimation and negligible following warm acclimation. We conclude that: (1) V(O(2)max) and V(O(2)mean) share a common physiological basis that remains overall the same across acclimation regimes; (2) changes in these traits are associated primarily with changes in heart mass; and (3) male and female deer mice respond differently to thermal acclimation, possibly due to differences in reproductive allocation.

Cell size but not genome size affects scaling of metabolic rate in eyelid geckos

The metabolic theory of ecology (MTE) predicts the ubiquity of the of 3/4 scaling exponent relating metabolic rate (MR) to body mass, as well as cell-size invariance coupled with body-size dependence of cellular MR in quickly dividing cells. An alternative prediction is that MR scales interspecifically with a coefficient that is between 2/3 and 1, depending on the cell size and cell MR, which is mostly driven by the cell surface-to-volume ratio. We tested (1) the contribution of cell size to interspecific differences in MR and (2) whether the cell size-MR relationship is mediated by genome size (GS), which usually correlates positively with cell size. We tested (1) and (2) using erythrocyte area as a proxy for cell size in 14 eyelid geckos, which belong to a monophyletic group exhibiting large body-size variation. The scaling of standard MR (SMR) was significantly lower than 3/4, whereas mass-specific SMR correlated with erythrocyte area in both phylogenetically adjusted and conventional analyses. This points to cell-size variation as the factor governing metabolic rate scaling, which questions predictions of the MTE. However, the nonsignificance of the correlation between mass-specific SMR and GS undermines the strength of the relation between GS and cell size, at least in these species.

Can greater muscularity in larger individuals resolve the 3/4 power-law controversy when modelling maximum oxygen uptake?

BACKGROUND

The power function relationship, MR = a.m(b), between metabolic rate (MR) and body mass m has been the source of much controversy amongst biologists for many years. Various studies have reported mass exponents (b) greater than the anticipated 'surface-area' exponent 0.67, often closer to 0.75 originally identified by Kleiber.

AIM

The study aimed to provide a biological explanation for these 'inflated' exponents when modelling maximum oxygen uptake (max), based on the observations from this and previous studies that larger individuals develop disproportionately more muscle mass in the arms and legs.

RESEARCH DESIGN AND SUBJECTS

A cross-sectional study of 119 professional soccer players from Croatia aged 18-34 was carried out.

RESULTS

Here we confirm that the power function relationship between max and body mass of the professional soccer players results in an 'inflated' mass exponent of 0.75 (95% confidence interval from 0.56 to 0.93), but also the larger soccer players have disproportionately greater leg muscle girths. When the analysis was repeated incorporating the calf and thigh muscle girths rather than body mass as predictor variables, the analysis not only explained significantly more of the variance in max, but the sum of the exponents confirmed a surface-area law.

CONCLUSIONS

These findings confirm the pitfalls of fitting body-mass power laws and suggest using muscle-girth methodology as a more appropriate way to scale or normalize metabolic variables such as max for individuals of different body sizes.

Trans‐sodium crocetinate does not affect oxygen uptake in rats during treadmill running

Trans-sodium crocetinate (TSC), the isomer of the carotenoid compound crocetin, is found markedly to increase survival in hemorrhagic shock subsequent to 50-60% blood loss, mainly via restored resting oxygen consumption (VO(2)), blood pressure and heart rate. The proposed mechanism is that TSC increases oxygen diffusivity, and thus availability, in plasma. If this were found to be a prominent feature in the oxygen transfer from blood to skeletal muscle fiber mitochondria, increased VO(2) during exercise would be expected because of reduced partial pressure of venous oxygen (increased utilization), which we aimed to elucidate in this study. Male Sprague-Dawley rats were intravenously injected with 0.3 mL kg(-1) TSC (40 microg mL(-1)) or placebo and immediately thereafter tested on a ramped treadmill test protocol. Rats were introduced to the experimental protocols beforehand. Administration of TSC had a neutral effect on submaximal and maximal VO(2) (VO(2max)) as well as running performance measured as maximal running time and maximal aerobic running velocity. Thus, in this study we cannot report any effects of TSC on steady-state submaximal VO(2) or VO(2max) at exhaustive exercise.

Allometric metabolic scaling and fetal and placental weight

BACKGROUND

We tested the hypothesis that the fetal-placental relationship scales allometrically and identified modifying factors of that relationship.

MATERIALS AND METHODS

Among women delivering after 34 weeks but prior to 43 weeks' gestation, 24,601 participants in the Collaborative Perinatal Project (CPP) had complete data for placental gross proportion measures, specifically, placental weight (PW), disk shape, larger and smaller disk diameters and thickness, and umbilical cord length. The allometric metabolic equation was solved for alpha and beta by rewriting PW = alpha(BW)beta as ln(PW) = ln alpha + beta[ln(BW)]. alpha(iota) was then the dependent variable in regressions with p < 0.05 significant.

RESULTS

Mean beta was 0.78 + 0.02 (range 0.66, 0.89), which is consistent with the scaling exponent 0.75 predicted by Kleiber's Law. Gestational age, maternal age, maternal BMI, parity, smoking, socioeconomic status, infant sex, and changes in placental proportions each had independent and significant effects on alpha.

CONCLUSIONS

We find an allometric scaling relation between the placental weight and the birthweight in the CPP cohort with an exponent approximately equal to 0.75, as predicted by Kleiber's Law. This implies that: (1) placental weight is a justifiable proxy for fetal metabolic rate when other measures of fetal metabolic rate are not available; and (2) the allometric relationship between placental and birthweight is consistent with the hypothesis that the fetal-placental unit functions as a fractal supply limited system. Furthermore, our data suggest that the maternal and fetal variables we examined have at least part of their effects on the normal balance between placental weight and birth weight via effects on gross placental growth dimensions.

The influence of oxygen and high-energy phosphate diffusion on metabolic scaling in three species of tail-flipping crustaceans

We examined the influence of intracellular diffusion of O(2) and high-energy phosphate (HEP) molecules on the scaling with body mass of the post-exercise whole-animal rate of O(2) consumption (V(O(2))) and muscle arginine phosphate (AP) resynthesis rate, as well as muscle citrate synthase (CS) activity, in three groups of tail-flipping crustaceans. Two size classes in each of three taxa (Palaemonetes pugio, Penaeus spp. and Panulirus argus) were examined that together encompassed a 27,000-fold range in mean body mass. In all species, muscle fiber size increased with body mass and ranged in diameter from 70+/-1.5 to 210+/-8.8 microm. Thus, intracellular diffusive path lengths for O(2) and HEP molecules were greater in larger animals. The body mass scaling exponent, b, for post-tail flipping V(O(2)) (b=-0.21) was not similar to that for the initial rate of AP resynthesis (b=-0.12), which in turn was different from that of CS activity (b=0.09). We developed a mathematical reaction-diffusion model that allowed an examination of the influence of O(2) and HEP diffusion on the observed rate of aerobic flux in muscle. These analyses revealed that diffusion limitation was minimal under most conditions, suggesting that diffusion might act on the evolution of fiber design but usually does not directly limit aerobic flux. However, both within and between species, fibers were more diffusion limited as they grew larger, particularly when hemolymph P(O(2)) was low, which might explain some of the divergence in the scaling exponents of muscle aerobic capacity and muscle aerobic flux.

Traditional allometric analysis fails to provide a valid predictive model for mammalian metabolic rates

The field of biological allometry was energized by the publication in 1997 of a theoretical model purporting to explain 3/4-power scaling of metabolic rate with body mass in mammals. This 3/4-power scaling exponent, which was first reported by Max Kleiber in 1932, has been derived repeatedly in empirical research by independent investigators and has come to be known as`Kleiber's Law'. The exponent was estimated in virtually every instance,however, by fitting a straight line to logarithmic transformations of data and by then re-expressing the resulting equation in the arithmetic scale. Because this traditional method may yield inaccurate and misleading estimates for parameters in the allometric equation, we re-examined the comprehensive data set that led Savage and colleagues to reaffirm the view that the metabolic rate of mammals scales to the 3/4-power of body mass. We found that a straight line fitted to logged data for the basal metabolic rate (BMR) of mammals ranging in size from a 2.4 g shrew to a 3672 kg elephant does not satisfy assumptions underlying the analysis and that the allometric equation obtained by back-transformation underestimates BMR for the largest species in the sample. Thus, the concept of 3/4-power scaling of metabolic rate to body mass is not well supported because the underlying statistical model does not apply to mammalian species spanning the full range in body size. Our findings have important implications with respect to methods and results of other studies that used the traditional approach to allometric analysis.

Mean mass-specific metabolic rates are strikingly similar across life's major domains: Evidence for life's metabolic optimum

A fundamental but unanswered biological question asks how much energy, on average, Earth's different life forms spend per unit mass per unit time to remain alive. Here, using the largest database to date, for 3,006 species that includes most of the range of biological diversity on the planet-from bacteria to elephants, and algae to sapling trees-we show that metabolism displays a striking degree of homeostasis across all of life. We demonstrate that, despite the enormous biochemical, physiological, and ecological differences between the surveyed species that vary over 10(20)-fold in body mass, mean metabolic rates of major taxonomic groups displayed at physiological rest converge on a narrow range from 0.3 to 9 W kg(-1). This 30-fold variation among life's disparate forms represents a remarkably small range compared with the 4,000- to 65,000-fold difference between the mean metabolic rates of the smallest and largest organisms that would be observed if life as a whole conformed to universal quarter-power or third-power allometric scaling laws. The observed broad convergence on a narrow range of basal metabolic rates suggests that organismal designs that fit in this physiological window have been favored by natural selection across all of life's major kingdoms, and that this range might therefore be considered as optimal for living matter as a whole.

Environment, migratory tendency, phylogeny and basal metabolic rate in birds

Basal metabolic rate (BMR) represents the minimum maintenance energy requirement of an endotherm and has far-reaching consequences for interactions between animals and their environments. Avian BMR exhibits considerable variation that is independent of body mass. Some long-distance migrants have been found to exhibit particularly high BMR, traditionally interpreted as being related to the energetic demands of long-distance migration. Here we use a global dataset to evaluate differences in BMR between migrants and non-migrants, and to examine the effects of environmental variables. The BMR of migrant species is significantly higher than that of non-migrants. Intriguingly, while the elevated BMR of migrants on their breeding grounds may reflect the metabolic machinery required for long-distance movements, an alternative (and statistically stronger) explanation is their occupation of predominantly cold high-latitude breeding areas. Among several environmental predictors, average annual temperature has the strongest effect on BMR, with a 50% reduction associated with a 20 degrees C gradient. The negative effects of temperature variables on BMR hold separately for migrants and non-migrants and are not due their different climatic associations. BMR in migrants shows a much lower degree of phylogenetic inertia. Our findings indicate that migratory tendency need not necessarily be invoked to explain the higher BMR of migrants. A weaker phylogenetic signal observed in migrants supports the notion of strong phenotypic flexibility in this group which facilitates migration-related BMR adjustments that occur above and beyond environmental conditions. In contrast to the findings of previous analyses of mammalian BMR, primary productivity, aridity or precipitation variability do not appear to be important environmental correlates of avian BMR. The strong effects of temperature-related variables and varying phylogenetic effects reiterate the importance of addressing both broad-scale and individual-scale variation for understanding the determinants of BMR.

Sizing up allometric scaling theory

Metabolic rate, heart rate, lifespan, and many other physiological properties vary with body mass in systematic and interrelated ways. Present empirical data suggest that these scaling relationships take the form of power laws with exponents that are simple multiples of one quarter. A compelling explanation of this observation was put forward a decade ago by West, Brown, and Enquist (WBE). Their framework elucidates the link between metabolic rate and body mass by focusing on the dynamics and structure of resource distribution networks—the cardiovascular system in the case of mammals. Within this framework the WBE model is based on eight assumptions from which it derives the well-known observed scaling exponent of 3/4. In this paper we clarify that this result only holds in the limit of infinite network size (body mass) and that the actual exponent predicted by the model depends on the sizes of the organisms being studied. Failure to clarify and to explore the nature of this approximation has led to debates about the WBE model that were at cross purposes. We compute analytical expressions for the finite-size corrections to the 3/4 exponent, resulting in a spectrum of scaling exponents as a function of absolute network size. When accounting for these corrections over a size range spanning the eight orders of magnitude observed in mammals, the WBE model predicts a scaling exponent of 0.81, seemingly at odds with data. We then proceed to study the sensitivity of the scaling exponent with respect to variations in several assumptions that underlie the WBE model, always in the context of finite-size corrections. Here too, the trends we derive from the model seem at odds with trends detectable in empirical data. Our work illustrates the utility of the WBE framework in reasoning about allometric scaling, while at the same time suggesting that the current canonical model may need amendments to bring its predictions fully in line with available datasets.

The rate at which an organism produces energy to live increases with body mass to the 3/4 power. Ten years ago West, Brown, and Enquist posited that this empirical relationship arises from the structure and dynamics of resource distribution networks such as the cardiovascular system. Using assumptions that capture physical and biological constraints, they defined a vascular network model that predicts a 3/4 scaling exponent. In our paper we clarify that this model generates the 3/4 exponent only in the limit of infinitely large organisms. Our calculations indicate that in the finite-size version of the model metabolic rate and body mass are not related by a pure power law, which we show is consistent with available data. We also show that this causes the model to produce scaling exponents significantly larger than the observed 3/4. We investigate how changes in certain assumptions about network structure affect the scaling exponent, leading us to identify discrepancies between available data and the predictions of the finite-size model. This suggests that the model, the data, or both, need reassessment. The challenge lies in pinpointing the physiological and evolutionary factors that constrain the shape of networks driving metabolic scaling.

The energetics and scaling of search strategies in bacteria

The influence of body size on the energetic cost of movement is well studied in animals but has been rarely investigated in bacteria. Here, I calculate the cost of four chemotactic strategies for different-sized bacteria by adding the costs of their locomotion and reorientation. Size differences of 0.1 microm result in 100,000-fold changes in the energetic cost of chemotaxis. The exact cost for any given size is a nonlinear function of flagella length, the minimum speed necessary to detect and respond to a signal, and the gradient of the signal. These parameters are interlinked in such a way that body size and strategy are tightly coupled to particular environmental gradients, offering avenues for explaining and exploring diversity and competition. The analysis here has implications beyond bacteria. Power-law regression through the minimum costs of transport for different kinds of chemotaxis has the same slope as that for swimming animals, suggesting a universal allometric equation for all swimming organisms.

Oral amino acid supplementation counteracts age-induced sarcopenia in elderly rats

We investigated the effects of a specific mixture of amino acid (AA) supplements on the adaptation changes induced by aging in the soleus muscle of rats. Male Wistar rats were divided into 3 groups (n = 5 each): young control (YO), 3 months of age; elderly control (EL), 18 months of age; and elderly orally supplemented with an AA mixture (EL-AA), 18 months of age, given as 0.1 g/kg per day in drinking water for 8 weeks. Myosin heavy chain (MHC) composition was analyzed in all muscles. The total fiber number and fiber cross-sectional area of types 1 and 2A fibers were also measured in immunostained sections of the soleus muscle. The ratios between the sarcomere volume (Vsar) and the total volume (Vtot) and single muscle fibers were studied by electron microscopy. The expression of total and phosphorylated serine/threonine protein kinase mammalian target of rapamycin (mTOR), a potent regulator of messenger RNA translation initiation, was also determined in all groups. Aging was associated with an overall shift toward the expression of a slower MHC phenotype, atrophy of fast and slow fibers, a significant decrease in Vtot/Vsar, and no changes in total fiber number. AA supplementation antagonized the effects of aging. A shift toward the expression of faster MHC isoforms was observed. Fiber atrophy appeared to be partly counteracted by the AA supplements; we noted an increase in cross-sectional area fibers and Vtot/Vsar in EL-AAs. Total and phosphorylated mTOR expression appeared to decrease in EL and was restored by the AA supplements. Collectively, these results suggest that aging-induced muscle adaptations can be partly restored by AA supplementation. An mTOR signal pathway may mediate the effects on fiber trophism.

Does size matter for hypoxia tolerance in fish?

Fish cover a large size range, from milligrams to tonnes, and many of them are regularly exposed to large variations in ambient oxygen levels. For more than half a century, there have been various, often divergent, claims regarding the effect of body size on hypoxia tolerance in fish. Here, we attempt to link old and new empirical data with the current understanding of the physiological mechanisms behind hypoxia tolerance. Three main conclusions are drawn: (1) body size per se has little or no impact on the ability to take up oxygen during hypoxic conditions, primarily because the respiratory surface area matches metabolic rate over a wide size range. If size-related differences are seen in the ability for oxygen uptake in a species, these are likely to reflect adaptation to different life-styles or habitat choice. (2) During severe hypoxia and anoxia, where fish have to rely on anaerobic ATP production (glycolysis) for survival, large individuals have a clear advantage over smaller ones, because small fish will run out of glycogen or reach lethal levels of anaerobic end-products (lactate and H(+)) much faster due to their higher mass-specific metabolic rate. (3) Those fish species that have evolved extreme adaptations to hypoxia, including haemoglobins with exceptionally high oxygen affinities and an alternative anaerobic end-product (ethanol), reveal that natural selection can be a much more powerful determinant of hypoxia tolerance than scaling of physiological functions.

Clinical potential of Spirulina as a source of phycocyanobilin

Recent research reveals that free bilirubin functions physiologically as a potent inhibitor of NADPH oxidase activity. The chromophore phycocyanobilin (PCB), found in blue-green algae and cyanobacteria such as Spirulina, also has been found to be a potent inhibitor of this enzyme complex, likely because in mammalian cells it is rapidly reduced to phycocyanorubin, a close homolog of bilirubin. In light of the protean roles of NADPH oxidase activation in pathology, it thus appears likely that PCB supplementation may have versatile potential in prevention and therapy -- particularly in light of rodent studies demonstrating that orally administered Spirulina or phycocyanin (the Spirulina holoprotein that contains PCB) can exert a wide range of anti-inflammatory effects. Until PCB-enriched Spirulina extracts or synthetically produced PCB are commercially available, the most feasible and least expensive way to administer PCB is by ingestion of whole Spirulina. A heaping tablespoon (about 15 g) of Spirulina can be expected to provide about 100 mg of PCB. By extrapolating from rodent studies, it can be concluded that an intake of 2 heaping tablespoons daily would be likely to have important antioxidant activity in humans -- assuming that humans and rodents digest and absorb Spirulina-bound PCB in a comparable manner. An intake of this magnitude can be clinically feasible if Spirulina is incorporated into "smoothies" featuring such ingredients as soy milk, fruit juices, and whole fruits. Such a regimen should be evaluated in clinical syndromes characterized and in part mediated by NADPH oxidase overactivity in affected tissues.

Running speed and maximal oxygen uptake in rats and mice: practical implications for exercise training

Valid and reliable experimental models are essential to gain insight into the cellular and molecular mechanisms underlying the beneficial effects of exercise in prevention, treatment, and rehabilitation of lifestyle-related diseases. Studies with large changes, low variation, and reproducible training outcome require individualized training intensity, controlled by direct measurements of maximal oxygen uptake or heart rate. As this approach is expensive and time consuming, we discuss whether maximal treadmill running speed in a gradually increasing ramp protocol might be sufficient to control intensity without losing accuracy. Combined data from six studies of rats and mice from our lab demonstrated a close correlation between running speed and oxygen uptake. This relationship changed towards a steeper linear slope after endurance training, indicating improved work economy, that is, less oxygen was consumed at fixed submaximal running speeds. Maximal oxygen uptake increased 40-70% after high-intensity aerobic interval training in mice and rats. The speed at which oxygen uptake reached a plateau, increased in parallel with the change in maximal oxygen uptake during the training period. Although this suggests that running speed can be used to assess training intensity throughout a training program, the problem is to determine the exact relative intensity related to maximal oxygen uptake from running speed alone. We therefore suggest that directly measured oxygen uptake should be used to assess exercise intensity and optimize endurance training in rats and mice. Running speed may serve as a supplement to ensure this intensity.

The adaptive evolution of the mammalian mitochondrial genome

Background

The mitochondria produce up to 95% of a eukaryotic cell's energy through oxidative phosphorylation. The proteins involved in this vital process are under high functional constraints. However, metabolic requirements vary across species, potentially modifying selective pressures. We evaluate the adaptive evolution of 12 protein-coding mitochondrial genes in 41 placental mammalian species by assessing amino acid sequence variation and exploring the functional implications of observed variation in secondary and tertiary protein structures.

Results

Wide variation in the properties of amino acids were observed at functionally important regions of cytochrome b in species with more-specialized metabolic requirements (such as adaptation to low energy diet or large body size, such as in elephant, dugong, sloth, and pangolin, and adaptation to unusual oxygen requirements, for example diving in cetaceans, flying in bats, and living at high altitudes in alpacas). Signatures of adaptive variation in the NADH dehydrogenase complex were restricted to the loop regions of the transmembrane units which likely function as protons pumps. Evidence of adaptive variation in the cytochrome c oxidase complex was observed mostly at the interface between the mitochondrial and nuclear-encoded subunits, perhaps evidence of co-evolution. The ATP8 subunit, which has an important role in the assembly of F₀, exhibited the highest signal of adaptive variation. ATP6, which has an essential role in rotor performance, showed a high adaptive variation in predicted loop areas.

Conclusion

Our study provides insight into the adaptive evolution of the mtDNA genome in mammals and its implications for the molecular mechanism of oxidative phosphorylation. We present a framework for future experimental characterization of the impact of specific mutations in the function, physiology, and interactions of the mtDNA encoded proteins involved in oxidative phosphorylation.

Moving towards acceleration for estimates of activity-specific metabolic rate in free-living animals: the case of the cormorant

1. Time and energy are key currencies in animal ecology, and judicious management of these is a primary focus for natural selection. At present, however, there are only two main methods for estimation of rate of energy expenditure in the field, heart rate and doubly labelled water, both of which have been used with success; but both also have their limitations. 2. The deployment of data loggers that measure acceleration is emerging as a powerful tool for quantifying the behaviour of free-living animals. Given that animal movement requires the use of energy, the accelerometry technique potentially has application in the quantification of rate of energy expenditure during activity. 3. In the present study, we test the hypothesis that acceleration can serve as a proxy for rate of energy expenditure in free-living animals. We measured rate of energy expenditure as rates of O2 consumption (VO2) and CO2 production (VCO2) in great cormorants (Phalacrocorax carbo) at rest and during pedestrian exercise. VO2 and VCO2 were then related to overall dynamic body acceleration (ODBA) measured with an externally attached three-axis accelerometer. 4. Both VO2 and VCO2 were significantly positively associated with ODBA in great cormorants. This suggests that accelerometric measurements of ODBA can be used to estimate VO2 and VCO2 and, with some additional assumptions regarding metabolic substrate use and the energy equivalence of O2 and CO2, that ODBA can be used to estimate the activity specific rate of energy expenditure of free-living cormorants. 5. To verify that the approach identifies expected trends in from situations with variable power requirements, we measured ODBA in free-living imperial cormorants (Phalacrocorax atriceps) during foraging trips. We compared ODBA during return and outward foraging flights, when birds are expected to be laden and not laden with captured fish, respectively. We also examined changes in ODBA during the descent phase of diving, when power requirements are predicted to decrease with depth due to changes in buoyancy associated with compression of plumage and respiratory air. 6. In free-living imperial cormorants, ODBA, and hence estimated VO2, was higher during the return flight of a foraging bout, and decreased with depth during the descent phase of a dive, supporting the use of accelerometry for the determination of activity-specific rate of energy expenditure.

Allometric scaling in centrarchid fish: origins of intra- and inter-specific variation in oxidative and glycolytic enzyme levels in muscle

The influence of body size on metabolic rate, muscle enzyme activities and the underlying patterns of mRNA for these enzymes were explored in an effort to explain the genetic basis of allometric variation in metabolic enzymes. We studied two pairs of sister species of centrarchid fish: black bass (largemouth bass Micropterus salmoides and smallmouth bass Micropterus dolomieui) and sunfish (pumpkinseed Lepomis gibbosus and bluegill Lepomis macrochirus). Our goal was to assess the regulatory basis of both intraspecific and interspecific variation relative to body size, as well as to gain insights into the evolutionary constraints within lineages. Whole animal routine metabolic rate showed scaling coefficients not significantly different from 1, ranging from (+0.87 to +0.96). However, there were significant effects of body size on the specific activities of oxidative and glycolytic enzymes. Mass-specific activity of the oxidative enzyme citrate synthase (CS) scaled negatively with body size in each species, with scaling coefficients ranging from -0.15 to -0.19, whereas the glycolytic enzyme pyruvate kinase (PK) showed positive scaling, with scaling coefficients ranging from +0.08 to +0.23. The ratio of mass-specific enzyme activity in PK to CS increased with body size, whereas the ratio of mRNA transcripts of PK to CS was unaffected, suggesting the enzyme relationships were not due simply to transcriptional regulation of both genes. The mass-dependent differences in PK activities were best explained by transcriptional regulation of the muscle PK gene; PK mRNA was a good predictor of PK specific enzyme activity within species and between species. Conversely, CS mRNA did not correlate with CS specific enzyme activities, suggesting post-transcriptional mechanisms may explain the observed inter-specific and intraspecific differences in oxidative enzymes.

[Why 37 degrees C? Evolutionary fundamentals of thermoregulation]

Homeothermy is the result of an evolutionary process during which every increase in oxygen supply led to a consecutive increase in metabolic rate and, thus, to a new dependence on favorable ambient conditions. In response to the food scarcity of winter months, some inhabitants of temperate zones developed an ability to hibernate which is characterized by a fully thermocontrolled reduction in body temperature down to near zero values. Hibernation thus illustrates that in homeotherms, not only the body shell is poikilothermic, but also the core temperature is more variable than often assumed. However, in contrast to clinical hypothermia, natural torpidity does not consist of a cold-induced reduction in metabolic rate, but of an endogenous metabolic reduction with subsequent lowering of body temperature. As a factor of metabolic suppression, the pH has been suspected which, in hibernators, is kept constant at 7.4 by relative hypoventilation (pH-stat) which differs from its passive shift in the poikilothermic body shell (alpha-stat). In clinical hypothermia, temperature governs the metabolic rate in that, depending on the state of thermoregulation, either a cold defense reaction with an increased metabolic rate (accidental hypothermia) or a cold-induced reduction in metabolic rate (induced hypothermia) occurs. However, as can be learned from hibernators, the lower limit of hypothermia tolerance seems to be due to a uniform minimal metabolic rate rather than to the species-specific body temperature at which this metabolic limit is reached, depending on body size and basal metabolic rate. Accordingly, in judging the sequelae of hypothermia, the degree of cooling should be given less emphasis than the resulting effects on metabolic rate.