Bioenergetic scaling: metabolic design and body-size constraints in mammals

Heart Transplantation From Brain Dead Donors: A Systematic Review of Animal Models

Despite advances in mechanical circulatory devices and pharmacologic therapies, heart transplantation (HTx) is the definitive and most effective therapy for an important proportion of qualifying patients with end-stage heart failure. However, the demand for donor hearts significantly outweighs the supply. Hearts are sourced from donors following brain death, which exposes donor hearts to substantial pathophysiological perturbations that can influence heart transplant success and recipient survival. Although significant advances in recipient selection, donor and HTx recipient management, immunosuppression, and pretransplant mechanical circulatory support have been achieved, primary graft dysfunction after cardiac transplantation continues to be an important cause of morbidity and mortality. Animal models, when appropriate, can guide/inform medical practice, and fill gaps in knowledge that are unattainable in clinical settings. Consequently, we performed a systematic review of existing animal models that incorporate donor brain death and subsequent HTx and assessed studies for scientific rigor and clinical relevance. Following literature screening via the U.S National Library of Medicine bibliographic database (MEDLINE) and Embase, 29 studies were assessed. Analysis of included studies identified marked heterogeneity in animal models of donor brain death coupled to HTx, with few research groups worldwide identified as utilizing these models. General reporting of important determinants of heart transplant success was mixed, and assessment of posttransplant cardiac function was limited to an invasive technique (pressure-volume analysis), which is limitedly applied in clinical settings. This review highlights translational challenges between available animal models and clinical heart transplant settings that are potentially hindering advancement of this field of investigation.

Approaches for testing hypotheses for the hypometric scaling of aerobic metabolic rate in animals

Hypometric scaling of aerobic metabolism [larger organisms have lower mass-specific metabolic rates (MR/g)] is nearly universal for interspecific comparisons among animals, yet we lack an agreed upon explanation for this pattern. If physiological constraints on the function of larger animals occur and limit MR/g, these should be observable as direct constraints on animals of extant species and/or as evolved responses to compensate for the proposed constraint. There is evidence for direct constraints and compensatory responses to O2 supply constraint in skin-breathing animals, but not in vertebrates with gas-exchange organs. The duration of food retention in the gut is longer for larger birds and mammals, consistent with a direct constraint on nutrient uptake across the gut wall, but there is little evidence for evolving compensatory responses to gut transport constraints in larger animals. Larger placental mammals (but not marsupials or birds) show evidence of greater challenges with heat dissipation, but there is little evidence for compensatory adaptations to enhance heat loss in larger endotherms, suggesting that metabolic rate (MR) more generally balances heat loss for thermoregulation in endotherms. Size-dependent patterns in many molecular, physiological, and morphological properties are consistent with size-dependent natural selection, such as stronger selection for neurolocomotor performance and growth rate in smaller animals and stronger selection for safety and longevity in larger animals. Hypometric scaling of MR very likely arises from different mechanisms in different taxa and conditions, consistent with the diversity of scaling slopes for MR.

BODY SIZE AND SEXUAL SIZE DIMORPHISM IN MARINE IGUANAS FLUCTUATE AS A RESULT OF OPPOSING NATURAL AND SEXUAL SELECTION: AN ISLAND COMPARISON

Body size is often assumed to represent the outcome of conflicting selection pressures of natural and sexual selection. Marine iguana (Amblyrhynchus cristatus) populations in the Galápagos exhibit 10-fold differences in body mass between island populations. There is also strong sexual size dimorphism, with males being about twice as heavy as females. To understand the evolutionary processes shaping body size in marine iguanas, we analyzed the selection differentials on body size in two island populations (max. male mass 900 g in Genovesa, 3500 g in Santa Fé). Factors that usually confound any evolutionary analysis of body sizes-predation, interspecific food competition, reproductive role division-are ruled out for marine iguanas. We show that, above hatchlings, mortality rates increased with body size in both sexes to the same extent. This effect was independent of individual age. The largest animals (males) of each island were the first to die once environmental conditions deteriorated (e.g., during El Niños). This sex-biased mortality was the result of sexual size dimorphism, but at the same time caused sexual size dimorphism to fluctuate. Mortality differed between seasons (selection differentials as low as -1.4) and acted on different absolute body sizes between islands. Both males and females did not cease growth when an optimal body size for survival was reached, as demonstrated by the fact that individual adult body size phenotypically increased in each population under favorable environmental conditions beyond naturally selected limits. But why did marine iguanas grow "too large" for survival? Due to lek mating, sexual selection constantly favored large body size in males (selection differentials up to +0.77). Females only need to reach a body size sufficient to produce surviving offspring. Thereafter, large body size of females was less favored by fertility selection than large size in males. Resulting from these different selection pressures on male and female size, sexual size dimorphism was mechanistically caused by the fact that females matured at an earlier age and size than males, whereafter they constantly allocated resources into eggs, which slowed growth. The observed allometric increase in sexual size dimorphism is explained by the fact that the difference between these selective processes becomes larger as energy abundance in the environment increases. Because body size is generally highly heritable, these selective processes are expected to lead to genetic differences in body size between islands. We propose a common-garden experiment to determine the influence of genetic factors and phenotypic reaction norms of final body size.

Activity-induced manganese-dependent MRI (AIM-MRI) and functional MRI in awake rabbits during somatosensory stimulation

Activity-induced manganese-dependent MRI (AIM-MRI) is a powerful tool to track system-wide neural activity using high resolution, quantitative T1-weighted MRI in animal models and has significant advantages for investigating neural activity over other modalities including BOLD fMRI. With AIM-MRI, Mn(2+) ions enter neurons via voltage-gated calcium channels preferentially active during the time of experimental exposure. A broad range of AIM-MRI studies using different species studying different phenomena have been performed, but few of these studies provide a systematic evaluation of the factors influencing the detection of Mn(2+) such as dosage and the temporal characteristics of Mn(2+) uptake. We identified an optimal dose of Mn(2+) (25 mg/kg, s.c.) in order to characterize the time-course of Mn(2+) accumulation in active neural regions in the rabbit. T1-weighted MRI and functional MRI were collected 0-3, 6-9, and 24-27 h post-Mn(2+) injection while the vibrissae on the right side were vibrated. Significant BOLD activation in the left somatosensory (SS) cortex and left ventral posteromedial (VPM) thalamic nucleus was detected during whisker vibration. T1-weighted signal intensities were extracted from these regions, their corresponding contralateral regions and the visual cortex (to serve as controls). A significant elevation in T1-weighted signal intensity in the left SS cortex (relative to right) was evident 6-9 and 24-27 h post-Mn(2+) injection while the left VPM thalamus showed a significant enhancement (relative to the right) only during the 24-27 h session. Visual cortex showed no hemispheric difference at any timepoint. Our results suggest that studies employing AIM-MRI would benefit by conducting experimental manipulations 6-24 h after subcutaneous MnCl2 injections to optimize the concentration of contrast agent in the regions active during the exposure.

Metabolic Field (Schrodinger); an explanatory platform for biology: Based on lecture at Trinity College, Dublin, Ireland, July 18, 2012

Metabolism represents the nexus of fundamental physical forces, which while present in all structure and function require new explanatory emergent principles, which, so far, cannot be predicted or derived solely from description of chemistry and physics. Metabolism is essentially concerned with the transduction of energy flows with respect to time, space, and matter. Language models and metaphors contribute to construction of scientific explanation within biology. The concept of a metabolic field yields a deeper, broader, more quantitative integrated theoretical framework leading to novel predictive models of systems biology.

Are mutagenic non D-loop direct repeat motifs in mitochondrial DNA under a negative selection pressure?

Non D-loop direct repeats (DRs) in mitochondrial DNA (mtDNA) have been commonly implicated in the mutagenesis of mtDNA deletions associated with neuromuscular disease and ageing. Further, these DRs have been hypothesized to put a constraint on the lifespan of mammals and are under a negative selection pressure. Using a compendium of 294 mammalian mtDNA, we re-examined the relationship between species lifespan and the mutagenicity of such DRs. Contradicting the prevailing hypotheses, we found no significant evidence that long-lived mammals possess fewer mutagenic DRs than short-lived mammals. By comparing DR counts in human mtDNA with those in selectively randomized sequences, we also showed that the number of DRs in human mtDNA is primarily determined by global mtDNA properties, such as the bias in synonymous codon usage (SCU) and nucleotide composition. We found that SCU bias in mtDNA positively correlates with DR counts, where repeated usage of a subset of codons leads to more frequent DR occurrences. While bias in SCU and nucleotide composition has been attributed to nucleotide mutational bias, mammalian mtDNA still exhibit higher SCU bias and DR counts than expected from such mutational bias, suggesting a lack of negative selection against non D-loop DRs.

High fatty acid oxidation capacity and phosphorylation control despite elevated leak and reduced respiratory capacity in northern elephant seal muscle mitochondria

Northern elephant seals (Mirounga angustirostris) are extreme, hypoxia-adapted endotherms that rely largely on aerobic metabolism during extended breath-hold dives in near freezing water temperatures. While many aspects of their physiology have been characterized to account for these remarkable feats, the contribution of adaptations in the aerobic powerhouses of muscle cells, the mitochondria, are unknown. In the present study, the ontogeny and comparative physiology of elephant seal muscle mitochondrial respiratory function was investigated under a variety of substrate conditions and respiratory states. Intact mitochondrial networks were studied by high-resolution respirometry in saponin-permeabilized fiber bundles obtained from primary swimming muscles of pup, juvenile, and adult seals, and compared to fibers from adult human vastus laterais. Results indicate that seal muscle maintains a high capacity for fatty acid oxidation despite a progressive decrease in total respiratory capacity as animals mature from pups to adults. This is explained by a progressive increase in phosphorylation control and fatty acid utilization over pyruvate in adult seals compared to humans and seal pups. Interestingly, despite higher indices of oxidative phosphorylation efficiency, juvenile and adult seals also exhibit a ~50% greater capacity for respiratory leak compared to humans and pups. The ontogeny of this phenotype suggests it is an adaptation of muscle to the prolonged breath-hold exercise and highly variable ambient temperatures experienced by mature elephant seals. These studies highlight the remarkable plasticity of mammalian mitochondria to meet the demands for both efficient ATP production and endothermy in a cold, oxygen-limited environment.

Effects of isoflurane anesthesia on the cardiovascular function of the C57BL/6 mouse

Isoflurane (ISO) is the most commonly used inhalational anesthetic for experimental interventions in mice and is preferred for imaging technologies that require the mouse to remain anesthetized for relatively long time periods. This study compares the stability of mean arterial pressure (MAP), heart rate (HR), and body temperature under ISO concentrations of 1%, 1.5%, and 2% (volume-to-volume, v/v) for up to 90 minutes postinduction. At all three levels of anesthesia, we examined evoked physiological responses to fractional inspiratory ratio variations of oxygen (FiO2) and nitrous oxide (N2O). In addition, we determined the hemodynamic effects of anesthesia on pH, glucose, insulin, glucocorticoids, and partial pressure of oxygen and of carbon dioxide in the blood (paO2, paCO2). The results indicate that the most appropriate ISO dose level was 1.5% v/v, yielding stable MAP and HR values comparable to those observed in the animal's conscious state, with a minute-to-minute variability in MAP and HR of .11%. Based on such recordings, the optimal FiO2 appeared to be 50%. The additional use of N2O was associated with higher and more stable values of MAP and HR. Arterial pH values were within the physiological range and varied between 7.20 and 7.43. ISO anesthesia at 1.5% v/v was also associated with mild hyperglycemia (+47%), whereas insulin levels and corticosteroids remained unaltered. We conclude that the application of isoflurane as an inhalational anesthetic in the mouse can be optimized to attain stable hemodynamics by administering it at 1.5% v/v and by supplementing it with N2O.

Efficiency of cross-bridges and mitochondria in mouse cardiac muscle

The aim of this study was to make cellular-level measurements of the mechanical efficiency of mouse cardiac muscle and to use these measurements to determine (1) the work performed by a cross-bridge in one ATP-splitting cycle and (2) the fraction of the free energy available in metabolic substrates that is transferred by oxidative phosphorylation to free energy in ATP (i.e. mitochondrial thermodynamic efficiency). Experiments were performed using isolated left ventricular mouse papillary muscles (n = 9; studied at 27°C) and the myothermic technique. The production of work and heat was measured during and after 40 contractions at a contraction frequency of 2 Hz. Each contraction consisted of a brief isometric period followed by isovelocity shortening. Work output, heat output and enthalpy output were all independent of shortening velocity. Maximum initial mechanical efficiency (mean ± SEM) was 31.1 ± 1.3% and maximum net mechanical efficiency 16.9 ± 1.5%. It was calculated that the maximum work per cross-bridge cycle was 20 zJ, comparable to values for mouse skeletal muscle, and that mitochondrial thermodynamic efficiency was 72%. Analysis of data in the literature suggests that mitochondrial efficiency of cardiac muscle from other species is also likely to be between 70 and 80%.

Feeding muscles scale differently from swimming muscles in sunfish (Centrarchidae)

The physiological properties of vertebrate skeletal muscle typically show a scaling pattern of slower contractile properties with size. In fishes, the myotomal or swimming muscle reportedly follows this pattern, showing slower muscle activation, relaxation and maximum shortening velocity (V(max)) with an increase in body size. We asked if the muscles involved in suction feeding by fishes would follow the same pattern. We hypothesized that feeding muscles in fishes that feed on evasive prey are under selection to maintain high power output and therefore would not show slower contractile properties with size. To test this, we compared contractile properties in feeding muscles (epaxial and sternohyoideus) and swimming muscle (myotomal) for two members of the family Centrarchidae (sunfish): the bluegill (Lepomis macrochirus) and the largemouth bass (Micropterus salmoides). Consistent with our predictions, the V(max) of myotomal muscle in both species slowed with size, while the epaxials showed no significant change in V(max) with size. In the sternohyoideus, V(max) slowed with size in the bluegill but increased with size in the bass. The results indicate that scaling patterns of contractile properties appear to be more closely tied to muscle function (i.e. locomotion versus feeding) than overall patterns of size.

Biodiversity and body size are linked across metazoans

Body size variation across the Metazoa is immense, encompassing 17 orders of magnitude in biovolume. Factors driving this extreme diversification in size and the consequences of size variation for biological processes remain poorly resolved. Species diversity is invoked as both a predictor and a result of size variation, and theory predicts a strong correlation between the two. However, evidence has been presented both supporting and contradicting such a relationship. Here, we use a new comprehensive dataset for maximum and minimum body sizes across all metazoan phyla to show that species diversity is strongly correlated with minimum size, maximum size and consequently intra-phylum variation. Similar patterns are also observed within birds and mammals. The observations point to several fundamental linkages between species diversification and body size variation through the evolution of animal life.

Biology and technology for photochemical fuel production

Sunlight is the ultimate energy source for the vast majority of life on Earth, and organisms have evolved elegant machinery for energy capture and utilization. Solar energy, whether converted to wind, rain, biomass or fossil fuels, is also the primary energy source for human-engineered energy transduction systems. This tutorial review draws parallels between biological and technological energy systems. Aspects of biology that might be advantageously incorporated into emerging technologies are highlighted, as well as ways in which technology might improve upon the principles found in biological systems. Emphasis is placed upon artificial photosynthesis, as well as the use of protonmotive force in biology.

The allometry of host-pathogen interactions

Background

Understanding the mechanisms that control rates of disease progression in humans and other species is an important area of research relevant to epidemiology and to translating studies in small laboratory animals to humans. Body size and metabolic rate influence a great number of biological rates and times. We hypothesize that body size and metabolic rate affect rates of pathogenesis, specifically the times between infection and first symptoms or death.

Methods and Principal Findings

We conducted a literature search to find estimates of the time from infection to first symptoms (t_S) and to death (t_D) for five pathogens infecting a variety of bird and mammal hosts. A broad sampling of diseases (1 bacterial, 1 prion, 3 viruses) indicates that pathogenesis is controlled by the scaling of host metabolism. We find that the time for symptoms to appear is a constant fraction of time to death in all but one disease. Our findings also predict that many population-level attributes of disease dynamics are likely to be expressed as dimensionless quantities that are independent of host body size.

Conclusions and Significance

Our results show that much variability in host pathogenesis can be described by simple power functions consistent with the scaling of host metabolic rate. Assessing how disease progression is controlled by geometric relationships will be important for future research. To our knowledge this is the first study to report the allometric scaling of host/pathogen interactions.

Energy expenditure in Crocidurinae shrews (Insectivora): Is metabolism a key component of the insular syndrome?

A cascade of morphological, ecological, demographical and behavioural changes operates within island communities compared to mainland. We tested whether metabolic rates change on islands. Using a closed circuit respirometer, we investigated resting metabolic rate (RMR) of three species of Crocidurinae shrews: Suncus etruscus, Crocidura russula, and C. suaveolens. For the latter, we compared energy expenditure of mainland and island populations. Our measurements agree with those previously reported for others Crocidurinae: the interspecific comparison (ANCOVA) demonstrated an allometric relation between energy requirements and body mass. Energy expenditure also scaled with temperature. Island populations (Corsica and Porquerolles) of C. suaveolens differed in size from mainland (gigantism). A GLM showed a significant relationship between energy expenditure, temperature, body mass and locality. Mass specific RMR allometrically scales body mass, but total RMR does not significantly differ between mainland and island, although island shrews are giant. Our results are consistent with other studies: that demonstrated that the evolution of mammalian metabolism on islands is partially independent of body mass. In relation to the insular syndrome, we discuss how island selective forces (changes in resource availability, decrease in competition and predation pressures) can operate in size and physiological adjustments.

Roles of hierarchical and metabolic regulation in the allometric scaling of metabolism in Panamanian orchid bees

Assessment of the relative importance of variation in enzyme concentration[E] and metabolic regulation in accounting for interspecific variation in metabolic rates is an unrealized area of research. Towards this end, we used metabolic flux rates during hovering and enzymatic flux capacities(Vmax values, equal to [E]×kcat,where kcat is catalytic efficiency) in flight muscles measured in vitro from 14 orchid bee species ranging in body mass from 47 to 1065 mg. Previous studies revealed that, across orchid bee species,wingbeat frequencies and metabolic rates decline in parallel with increasing body mass. Vmax values at some enzymatic steps in pathways of energy metabolism decline with increasing mass while, at most other steps, Vmax values are mass-independent. We quantified the relative importance of `hierarchical regulation' (alteration in Vmax, indicative of alteration in [E]) and `metabolic regulation' (resulting from variation in substrate, product or modulator concentrations) in accounting for interspecific variation in flux across species. In addition, we applied the method of phylogenetically independent contrasts to remove the potentially confounding effects of phylogenetic relationships among species. In the evolution of orchid bees, hierarchical regulation completely accounts for allometric variation in flux rates at the hexokinase step while, at other reactions, variation in flux is completely accounted for by metabolic regulation. The predominant role played by metabolic regulation is examined at the phosphoglucoisomerase step using the Haldane relationship. We find that extremely small variation in the concentration ratio of [product]/[substrate] is enough to cause the observed interspecific variation in net flux at this reaction in glycolysis.

Maximal enzyme activities, and myoglobin and glutathione concentrations in heart, liver and skeletal muscle of the Northern Short-tailed shrew (Blarina brevicauda; Insectivora: Soricidae)

We measured the enzymes of glycolysis, Krebs Cycle, beta-oxidation and electron transport in the heart, liver and skeletal muscle of the Northern Short-tailed Shrew, Blarina brevicauda. Additionally, we measured the amount of myoglobin in skeletal and heart muscle as well as the concentration of glutathione in heart. The picture that emerges is of an aerobically well-endowed animal with constrained anaerobic capacity as indicated by small activities of glycolytic enzymes and creatine kinase. Lipid metabolism and amino acid transamination, as well as gluconeogenesis, are predominant in processing carbon resources and probably reflect the large contribution lipid and protein make to the diet of this carnivore. The citrate synthase activity is the largest of any reported value for vertebrate heart (250 U/g). The additional, very active cytochrome c oxidase activity (220 U/g) and large myoglobin concentrations (8 mg/g) in heart are clearly the underpinnings of the rapid metabolic rates reported for small insectivores. The potential for generation of reactive oxygen species must be great since the total glutathione concentration (165 mumol/g) is 300-fold greater in shrew hearts than in hearts of rats.

Multi-level regulation and metabolic scaling

Metabolic control analysis has revealed that flux through pathways is the consequence of system properties, i.e. shared control by multiple steps, as well as the kinetic effects of various pathways and processes over each other. This implies that the allometric scaling of flux rates must be understood in terms of properties that pertain to the regulation of flux rates. In contrast,proponents of models considering the scaling of branching or fractal-like systems suggest that supply rates determine metabolic rates. Therefore, the allometric scaling of supply alone provides a sufficient explanation for the allometric scaling of metabolism. Examination of empirical data from the literature of comparative physiology reveals that basal metabolic rates (BMR)are driven by rates of energy expenditure within internal organs and that the allometric scaling of BMR can be understood in terms of the scaling of the masses and metabolic rates of internal organs. Organ metabolic rates represent the sum of tissue metabolic rates while, within tissues, cellular metabolic rates are the outcome of shared regulation by multiple processes. Maximal metabolic rates (MMR, measured as maximum rates of O2 consumption, V̇O2max) during exercise also scale allometrically, are also subject to control by multiple processes, but are due mainly to O2 consumption by locomotory muscles. Thus, analyses of the scaling of MMR must consider the scaling of both muscle mass and muscle energy expenditure. Consistent with the principle of symmorphosis, allometry in capacities for supply (the outcome of physical design constraints) is observed to be roughly matched by allometry in capacities for demand (i.e. for energy expenditure). However, physiological rates most often fall far below maximum capacities and are subject to multi-step regulation. Thus, mechanistic explanations for the scaling of BMR and MMR must consider the manner in which capacities are matched and how rates are regulated at multiple levels of biological organization.

Metabolic scaling: a many-splendoured thing

Animals at rest and during exercise display rates of aerobic metabolism, VO2, that represent mainly the sum of mitochondrial respiration rates in various organs. The relative contributions of these organs change with physiological state such that internal organs such as liver, kidney and brain account for most of the whole-body VO2 at rest, while locomotory muscles account for >90% of the maximum rate, VO2max, during maximal aerobic exercise. Mechanisms that regulate VO2 are complex and the relative importance of each step in a series, estimated by metabolic control analysis, depends upon the level of biological organization under consideration as well as physiological state. Despite this complexity, prominent single-cause models propose that metabolic rates are supply-limited and that the scaling of supply systems provides a sufficient explanation for the allometric scaling of metabolism. We argue that some assumptions, as well as current interpretations of the meaning (or consequences) of these constraints are flawed, i.e., elephants do not have lower mass-specific basal or maximal rates of aerobic metabolism because their mitochondria are more supply-limited than those of shrews. Animals do not violate the laws of physics, and the allometric scaling of supply systems would be expected, to some extent, to be matched by capacities for (and rates of) energy expenditure. But life is not so simple. Animals are so diverse that to do justice to metabolic scaling, it is also necessary to consider the scaling of energy expenditure. It is by doing so that models of metabolic scaling can be consistent with current paradigms in metabolic regulation and accommodate the range of inter- and intraspecific exponents found in nature. The "allometric cascade," a first attempt at such an accounting, was a source of great satisfaction to Peter Hochachka. It was the last door that he helped open to comparative physiologists before he said goodbye.

Endosymbiosis and the design of eukaryotic electron transport

The bioenergetic organelles of eukaryotic cells, mitochondria and chloroplasts, are derived from endosymbiotic bacteria. Their electron transport chains (ETCs) resemble those of free-living bacteria, but were tailored for energy transformation within the host cell. Parallel evolutionary processes in mitochondria and chloroplasts include reductive as well as expansive events: On one hand, bacterial complexes were lost in eukaryotes with a concomitant loss of metabolic flexibility. On the other hand, new subunits have been added to the remaining bacterial complexes, new complexes have been introduced, and elaborate folding patterns of the thylakoid and mitochondrial inner membranes have emerged. Some bacterial pathways were reinvented independently by eukaryotes, such as parallel routes for quinol oxidation or the use of various anaerobic electron acceptors. Multicellular organization and ontogenetic cycles in eukaryotes gave rise to further modifications of the bioenergetic organelles. Besides mitochondria and chloroplasts, eukaryotes have ETCs in other membranes, such as the plasma membrane (PM) redox system, or the cytochrome P450 (CYP) system. These systems have fewer complexes and simpler branching patterns than those in energy-transforming organelles, and they are often adapted to non-bioenergetic functions such as detoxification or cellular defense.

On being the right size: heart design, mitochondrial efficiency and lifespan potential

1. From the smallest shrew or bumble-bee bat to the largest blue whale, heart size varies by over seven orders of magnitude (from 12 mg to 600 kg). This study reviews the scaling relationships between heart design, cellular bioenergetics and mitochondrial efficiencies in mammals of different body sizes. 2. The [31P]-nuclear magnetic resonance-derived [phosphocreatine]/[ATP] ratio in hearts of smaller mammals is significantly higher (2.7 +/- 0.3 for mouse; n = 22) than in larger mammals (1.6 +/- 0.3 for humans; n = 13). 3. The inverse of the free myocardial cytosolic [ADP] concentration and the cytosolic phosphorylation ratio ([ATP]/[ADP][Pi]) scales with heart size and with absolute mitochondrial and myofibrillar volumes, close to a quarter-power (from -0.22 to -0.28; r = 0.99). 4. Assuming a similar mitochondrial P/O ratio and the same maximal amount of work required to convert 1 mol NADH to 0.5 mol O2 (i.e. 212.25 kJ/mol), the higher [ATP]/[ADP][Pi] ratios or cellular driving forces (DeltaG'ATP) in hearts of smaller mammals imply greater mitochondrial efficiencies in coupling ATP production to electron transport as body size decreases. For a P/O ratio of 2.5, the mitochondrial efficiency in the heart of a shrew, mouse, human and whale is 84, 82, 71 and 65%, respectively. 5. Higher cytosolic ATP]/[ADP][Pi] ratios and DeltaG'ATP values imply that the hearts of smaller mammals operate further from equilibrium than hearts of larger mammals. 6. As a consequence of scaling relationships, a number of remarkable invariants emerge when comparing heart function from the smallest shrew to the largest whale; the total volume of blood pumped by each heart in a lifetime is approximately 200 million L/kg heart and the total number of heart beats is approximately 1.1 billion per lifetime. 7. Similarly, the metabolic potential (total O2 consumed during adult lifespan per g bodyweight) for a 2 g shrew or a 100000 kg blue whale is approximately 38 L O2 consumed or 8.5 mol ATP/g body mass per lifetime. 8. The importance of quarter-power scaling relationships linking structural, metabolic and bioenergetic design to the natural ageing process and maximum lifespan potential is discussed.

Changes in substrate metabolism in isolated mouse hearts following ischemia-reperfusion

Several genetic and transgenic mouse models are currently being used for studying the regulation of myocardial contractility under normal conditions and in disease states. Little information has been provided, however, about myocardial energy metabolism in mouse hearts. We measured glycolysis, glucose oxidation and palmitate oxidation (using 3H-glucose, 14C-glucose and 3H-palmitate) in isolated working mouse hearts during normoxic conditions (control group) and following a 15 min global no-flow ischemic period (reperfusion group). Fifty min following reperfusion (10 min Langendorff perfusion + 40 min working heart perfusion) aortic flow, coronary flow, cardiac output, peak systolic pressure and heart rate were 44 +/- 4, 88 +/- 4, 57 +/- 4, 94 +/- 2 and 81 +/- 4% of pre-ischemic values). Rates of glycolysis and glucose oxidation in the reperfusion group (13.6 +/- 0.8 and 2.8 +/- 0.2 micromol/min/g dry wt) were not different from the control group (12.3 +/- 0.6 and 2.5 +/- 0.2 micromol/min/g dry wt). Palmitate oxidation, however, was markedly elevated in the reperfusion group as compared to the control group (576 +/- 37 vs. 357 +/- 21 nmol/min/g dry wt, p < 0.05). This change in myocardial substrate utilization was accompanied by a marked fall in cardiac efficiency measured as cardiac output/oxidative ATP production (136 +/- 10 vs. 54 +/- 5 ml/micromol ATP, p < 0.05, control and reperfusion group, respectively). We conclude that ischemia-reperfusion in isolated working mouse hearts is associated with a shift in myocardial substrate utilization in favour of fatty acids, in line with previous observations in rat.

Size-abundance relationships in an Amazonian bird community: implications for the energetic equivalence rule

We studied size-abundance relationships in a species-rich Amazonian bird community and found that the slope of the logarithmic relationship between population density and bodymass (b = -0.22) is significantly shallower than expected under Damuth's energetic equivalence rule (EER), which states that population energy use (PEU) is independent of species body mass. We used estimates of avian field metabolic rates to examine the logarithmic relationship between PEU and body mass and its variation among ecological guilds. The relationship for all species had a significantly positive slope (b = 0.46), indicating that PEU of larger species was greater than that of smaller species. Analyses of guilds revealed significant variation. The slopes of the frugivore-omnivore, insectivore, and granivore guilds were all significantly positive, with that of the frugivore-omnivore guild being the steepest. In contrast, PEU did not vary significantly with species body mass among raptors. These results were confirmed, in analyses using both species values and phylogenetically independent contrasts, and the results do not support the EER in this community. The spatial distribution of resources and mechanisms of interference competition within guilds may explain why most patterns differed from the predictions of the EER. Other sources of variation, including the effects of scale, are also discussed.

Energy consumption by phospholipid metabolism in mammalian brain

Until recently, brain phospholipid metabolism was thought to consume only 2% of the ATP consumed by the mammalian brain as a whole. In this paper, however, we calculate that 1.4% of total brain ATP consumption is consumed for the de novo synthesis of ether phospholipids and that another 5% is allocated to the phosphatidylinositide cycle. When added to previous estimates that fatty acid recycling within brain phospholipids and maintenance of membrane lipid asymmetries of acidic phospholipids consume, respectively, 5% and 8% of net brain ATP consumption, it appears that phospholipid metabolism can consume up to 20% of net brain ATP consumption. This new estimate is consistent with recent evidence that phospholipids actively participate in brain signaling and membrane remodeling, among other processes.

Heart design: free ADP scales with absolute mitochondrial and myofibrillar volumes from mouse to human

Our aim was to estimate a number of bioenergetic parameters in the beating mouse, rat and guinea pig heart in situ and compare the values to those in hearts of mammals over a 2000-fold range in body mass. For the mouse, rat and guinea pig heart, we report a phosphorylation ratio of 1005+/-50 (n=16), 460+/-32 (n=10) and 330+/-22 (n=5) mM(-1) and a free cytosolic [ADP] concentration of 13, 18 and 22 microM, respectively. When each parameter was plotted against body mass, they scaled closely to the quarter power (-0.28, r=0.99 and -0.23, r=0.97). A similar regression slope was found when the inverse of free [ADP] was plotted against absolute mitochondrial (slope=-0.26, r=0.99) and myofibrillar volumes (slope=-0.24, r=0.99). The similar slopes indicate that the ratio of absolute mitochondria and myofibrillar volumes in the healthy mammalian heart is a constant, and independent of body size. In conclusion, our study supports the hypothesis that the mammalian heart has a number of highly conserved thermodynamic and kinetic parameters that obey quarter-power laws linking the phosphorylation ratio, ATP turnover rates, free [ADP] and absolute mitochondrial volumes to body size. The results are discussed in terms of possible mechanisms and potential deviations from these laws in some disease states.

5'-Adenosine monophosphate and adenosine metabolism, and adenosine responses in mouse, rat and guinea pig heart

We examined myocardial 5'-adenosine monophosphate (5'-AMP) catabolism, adenosine salvage and adenosine responses in perfused guinea pig, rat and mouse heart. MVO(2) increased from 71+/-8 microl O(2)/min per g in guinea pig to 138+/-17 and 221+/-15 microl O(2)/min per g in rat and mouse. VO(2)/beat was 0.42+/-0.03, 0.50+/-0.03 and 0.55+/-0.04 microl O(2)/g in guinea pig, rat and mouse, respectively. Resting and peak coronary flows were highest in mouse vs. rat and guinea pig, and peak ventricular pressures and Ca(2+) sensitivity declined as heart mass increased. Net myocardial 5'-AMP dephosphorylation increased significantly as mass declined (3.8+/-0.5, 9.0+/-1.4 and 11.0+/-1.6 nmol/min per g in guinea pig, rat and mouse, respectively). Despite increased 5'-AMP catabolism, coronary venous [adenosine] was similar in guinea pig, rat and mouse (45+/-8, 69+/-10 and 57+/-14 nM, respectively). Comparable venous [adenosine] was achieved by increased salvage vs. deamination: 64%, 41% and 39% of adenosine formed was rephosphorylated while 23%, 46%, and 50% was deaminated in mouse, rat and guinea pig, respectively. Moreover, only 35-45% of inosine and its catabolites derive from 5'-AMP (vs. IMP) dephosphorylation in all species. Although post-ischemic purine loss was low in mouse (due to these adaptations), functional tolerance to ischemia decreased with heart mass. Cardiovascular sensitivity to adenosine also differed between species, with A(1) receptor sensitivity being greatest in mouse while A(2) sensitivity was greatest in guinea pig. In summary: (i) cardiac 5'-AMP dephosphorylation, VO(2), contractility and Ca(2+) sensitivity all increase as heart mass falls; (ii) adaptations in adenosine salvage vs. deamination limit purine loss and yield similar adenosine levels across species; (iii) ischemic tolerance declines with heart mass; and (iv) cardiovascular sensitivity to adenosine varies, with increasing A(2) sensitivity relative to A(1) sensitivity in larger hearts.

31P NMR quantitation of phosphorus metabolites in rat heart and skeletal muscle in vivo

The aim of this study was to examine two methods of 31P NMR quantitation of phosphocreatine (PCr), ATP, and P(i) in rat heart and skeletal muscle in vivo. The first method employed an external standard of phenylphosphonic acid (PPA; 10 mM), and the second method used an enzymatic measurement of tissue ATP equated to the area under the betaATP peak. With the use of the external standard, the concentrations of ATP, PCr, and P(i) in the rat heart were 4.48 +/- 0.33, 9.21 +/- 0.65, and 2.25 +/- 0.16 micromol/g wet wt, respectively. With the use of the internal ATP standard, measured on the same tissue, the contents (means +/- SE) were 4.78 +/- 0.19, 9.83 +/- 0.18, and 2.51 +/- 0.33 micromol/g wet wt, respectively (n = 7). In skeletal muscle, ATP, PCr, and P(i) were 6.09 +/- 0.19, 23.44 +/- 0.88, and 1.81 +/- 0.18 micromol/g wet wt using the PPA standard and 6.03 +/- 0.19, 23.30 +/- 1.30, and 1.82 +/- 0.19 micromol/g wet wt using the internal ATP standard (n = 6). There was no significant difference for each metabolite as measured by the two methods of quantification in heart or skeletal muscle. The results validate the use of an external reference positioned symmetrically above the coil and imply that each has similar NMR sensitivities (similar signal amplitude per mole of 31P between PPA and tissue phosphorus compounds). We conclude that PCr, ATP, and P(i) are nearly 100% visible in the normoxic heart and nonworking skeletal muscle given the errors of measurement.

Relationship of hepatocyte ploidy levels with body size and growth rate in mammals

To elucidate possible causes of the elevation of genome number in somatic cells, hepatocyte ploidy levels were measured cytofluorimetrically and related to the organismal parameters (body size, postnatal growth rate, and postnatal development type) in 53 mammalian species. Metabolic scope (ratio of maximal metabolic rate to basal metabolic rate) was also included in 23 species. Body masses ranged 10(5) times, and growth rate more than 30 times. Postnatal growth rate was found to have the strongest effect on the hepatocyte ploidy. At a fixed body mass the growth rate closely correlates (partial correlation analysis) with the cell ploidy level (r = 0.85, P < 10(-6)), whereas at a fixed growth rate body mass correlates poorly with ploidy level (r = -0.38, P < 0.01). The mature young (precocial mammals) of the species have, on average, a higher cell ploidy level than the immature-born (altricial) animals. However, the relationship between precocity of young and cell ploidy levels disappears when the influences of growth rate and body mass are removed. Interspecies variability of the hepatocyte ploidy levels may be explained by different levels of competition between the processes of proliferation and differentiation in cells. In turn, the animal differences in the levels of this competition are due to differences in growth rate. A high negative correlation between the hepatocyte ploidy level and the metabolic scope indicates a low safety margin of organs with a high number of polyploid cells. This fact allows us to challenge a common opinion that increasing ploidy enhances the functional capability of cells or is necessary for cell differentiation. Somatic polyploidy can be considered a "cheap" solution of growth problems that appear when an organ is working at the limit of its capabilities.

Decreasing ratios of phosphocreatine to beta-ATP correlates to progressive acute rejection in a concordant mouse heart to rat xenotransplantation model

Biopsies are difficult to perform in rodent heart transplant models without compromising the graft function and therefore other means to evaluate the grafts repeatedly and noninvasively are warranted. The goal of the present study was to measure changes in ratios of high energy phosphorus containing metabolites detected with in vivo 31Phosphorous Magnetic Resonance Spectroscopy ((31)P MRS) in a xenotransplantation model and to investigate if these ratios correlated to histological signs of acute xenograft rejection. Thirty-five heart transplantations were performed (NMRI-mice to Lewis (RT1(1)) rats). Thirteen heart transplants underwent repeated daily in vivo (31)P MRS measurements and 22 grafts were measured on any of 4 postoperative days and thereafter sacrificed for histology. A modified scoring system based on Billingham's criteria was used to stage the rejection process. The median graft survival was 3.0 +/- 0.44 (median +/- SD) days (n = 17). Significant differences, both overall and interday, could be calculated for the phosphocreatine (PCr)/beta-adenosine triphosphate (beta-ATP) ratios and for the rejection score. The decreases in PCr/beta-ATP ratios correlated significantly to the progressive acute rejection process in the sacrificed grafts (P = 0.01). Further studies are indicated to establish the potential of (31)P MRS in immunosuppressed recipients of vascularized xenotransplants with prolonged graft survival.

Detection and quantification of free cytosolic inorganic phosphate and other phosphorus metabolites in the beating mouse heart muscle in situ

The aim of this study was the quantification of inorganic phosphate (Pi) and other phosphorus metabolites by (31)P NMR spectroscopy in the mouse heart muscle in situ, beating at around 600 min(-1). Male adult Quacker-bush mice (mean weight 32 +/- 7 g) were anaesthetized, ventilated and placed in a temperature-controlled animal holder. A purpose-built (31)P NMR surface coil was positioned against the exposed left ventricular myocardium. Partial signal overlap of Pi with 2,3-DPG from chamber blood was minimized using a DEPTH pulse sequence (180 degrees -90 degrees -180 degrees -180 degrees -acq.). Quantification of phosphorus metabolites was performed using an external standard positioned directly above the surface coil. We report for the mouse myocardium in situ an intracellular free [Pi] of <0.4 mM, pH of 7.32 +/- 0.1, free [Mg2+] of 0.41 +/- 0.1 mM, free [ADP] of 13 +/- 1.5 microM, [ATP] of 5 +/- 0.5 mM and [PCr] of 14 +/- 1.5 mM. The phosphorylation ratio (ATP/ADP Pi) was 1005 +/- 200 mM (-1) for a PCr/ATP ratio of 2.7 +/- 0.3. It was concluded that the detection of free [Pi] in the mouse myocardium in situ can be greatly enhanced using a DEPTH pulse sequence. Quantification of compounds using an external standard positioned directly above the surface coil gave comparable results to estimations using internal ATP that was quantified enzymatically. The close agreement between the external and internal methods indicates that ATP is 100% NMR visible in the mouse heart in situ.

MRI/MRS assessment of in vivo murine cardiac metabolism, morphology, and function at physiological heart rates

Transgenic mice are increasingly used to probe genetic aspects of cardiovascular pathophysiology. However, the small size and rapid rates of murine hearts make noninvasive, physiological in vivo studies of cardiac bioenergetics and contractility difficult. The aim of this report was to develop an integrated, noninvasive means of studying in vivo murine cardiac metabolism, morphology, and function under physiological conditions by adapting and modifying noninvasive cardiac magnetic resonance imaging (MRI) with image-guided (31)P magnetic resonance spectroscopy techniques used in humans to mice. Using spatially localized, noninvasive (31)P nuclear magnetic resonance spectroscopy and MRI at 4.7 T, we observe mean murine in vivo myocardial phosphocreatine-to-ATP ratios of 2.0 +/- 0.2 and left ventricular ejection fractions of 65 +/- 7% at physiological heart rates ( approximately 600 beats/min). These values in the smallest species studied to date are similar to those reported in normal humans. Although these observations do not confirm a degree of metabolic scaling with body size proposed by prior predictions, they do suggest that mice can serve, at least at this level, as a model for human cardiovascular physiology. Thus it is now possible to noninvasively study in vivo myocardial bioenergetics, morphology, and contractile function in mice under physiological conditions.

Methodology for measuring in vitro/ex vivo cardiac energy metabolism

The high energy demands of the heart are met primarily by the metabolism of fatty acids and carbohydrates. These energy substrates are efficiently and rapidly metabolized in order to produce the high levels of adenosine triphosphate (ATP) necessary to sustain both contractile activity and other cellular functions. Alterations in energy metabolism contribute to abnormal heart function in many cardiac diseases. As a result, a number of techniques have been developed to directly measure energy metabolism in the heart in order to study energy metabolism. Two important variables that must be considered when making these measurements are energy substrate supply to the heart and the metabolic demand of the heart (i.e. contractile function). The use of the in vitro/ex vivo heart, perfused with relevant energy substrates, is a useful experimental approach that accounts for these variables. This paper overviews a number of the techniques that are used to measure energy substrate metabolism in the isolated perfused heart. Recently developed technology that allows for the direct measurement of energy metabolism in an isolated working mouse heart preparation are also described.

Chronotropic and vasodilatory responses to adenosine and isoproterenol in mouse heart: effects of adenosine A1 receptor overexpression

1. Chronotropic and vasodilatory effects of adenosine receptor activation with 2-chloroadenosine (2-ClAdo) and beta-adrenoceptor activation with isoproterenol were studied in wild-type murine hearts and transgenic hearts overexpressing the A1 adenosine receptor. 2. Treatment of wild-type hearts with 2-ClAdo induced bradycardia (pEC50 6.4+/-0.2) and vasodilatation (pEC50 7.9+/-0.1; minimal resistance 2.2+/-0.2 mmHg/mL per min per g). The A1 receptor-mediated bradycardia was 20-fold more sensitive in transgenic hearts (pEC50 7.7+/-0.2), whereas coronary vasoactivity of 2-ClAdo was unaltered (pEC50 7.6+/-0.1). 3. beta-Adrenoceptor stimulation with isoproterenol increased heart rate (pEC50 8.5+/-0.2; maximal rate 594+/-23 b.p.m.) and produced vasodilation (pEC50 8.7+/-0.1; minimal resistance 1.7 +/-0.2 mmHg/ml, per min per g) in wild-type hearts. Treatment with 10 IU/mL adenosine deaminase increased the magnitude of the tachycardia (maximal rate 653+/-27 b.p.m.) without altering potency (pEC50 8.5+/-0.1). Antagonism of A1 receptors with 10nmol/L 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) produced a comparable increase in the magnitude of the chronotropic response (maximal rate 695+/-26b.p.m.) without altering potency (pEC50 8.3+/-0.1). 4. Isoproterenol-mediated vasodilatation was unaltered by transgenic A1 receptor overexpression. Overexpression of A1 receptors significantly reduced the maximal heart rate during beta-adrenoceptor stimulation by 35% (to 381 +/-28 b.p.m.) without altering potency (pEC50 8.4+/-0.2). At 10nmol/L, DPCPX increased the magnitude of the chronotropic response to isoproterenol in transgenic hearts (maximal heart rate 484+/-36 b.p.m.) without altering potency (pECs50 8.3+/-0.2). 5. The data show that transgenic A1 receptor overexpression selectively sensitizes the cardiovascular A1 receptor response and that A1 receptor activation by endogenous adenosine depresses the magnitude, but not potency, of the beta-adrenoceptor-mediated chronotropic response in mouse heart. The A1 receptor-mediated depression of beta-adrenoceptor responsiveness is non-competitive (reduced response magnitude with no change in sensitivity). This indicates that A1 receptor activation non-competitively inhibits effector mechanisms activated by beta-adrenoceptors (e.g. adenylate cyclase) and/or A1 receptors activate unrelated but opposing mechanisms. This inhibitory response may have physiological importance during periods of sympathetic stimulation of cardiac work.

Glucose and fatty acid metabolism in the isolated working mouse heart

Although isolated perfused mouse heart models have been developed to study mechanical function, energy substrate metabolism has not been examined despite the expectation that the metabolic rate for a heart from a small mammal should be increased. Consequently, glucose utilization (glycolysis, oxidation) and fatty acid oxidation were measured in isolated working mouse hearts perfused with radiolabeled substrates, 11 mM glucose, and either 0.4 or 1.2 mM palmitate. Heart rate, coronary flow, cardiac output, and cardiac power did not differ significantly between hearts perfused at 0.4 or 1.2 mM palmitate. Although the absolute values obtained for glycolysis and glucose oxidation and fatty acid oxidation are significantly higher than those reported for rat hearts, the pattern of substrate metabolism in mouse hearts is similar to that observed in hearts from larger mammals. The metabolism of mouse hearts can be altered by fatty acid concentration in a manner similar to that observed in larger animals; increasing palmitate concentration altered the balance of substrate metabolism to increase overall energy derived from fatty acids from 64 to 92%.

Contribution of mitochondria and peroxisomes to palmitate oxidation in rat and bovine tissues

Total and peroxisomal palmitate oxidation capacities and mitochondrial enzyme activities were compared in tissues from growing rats, preruminant calves and 15-month-old bulls. Total palmitate oxidation rates were 1.9-5.2-fold higher in rat than in bovine tissues and 1.7-fold higher in the heart and muscles from calves than from growing bulls. The peroxisomal contribution to palmitate oxidation was similar between rats and bovines (i.e. calves and bulls) in liver (35-51%), heart (26%) but not in muscles (14 +/- 3% in rats vs 33 +/- 4.5% in bovines, P < 0.05). Mitochondrial enzyme activities were 1.8-4.8-fold higher in rat than in bovine tissues but the citrate synthase to cytochrome-c oxidase ratio was the highest in the liver (17-38), intermediate in the heart and muscles from calves and rats (6-10) and the lowest in heart and muscles from bulls (2-3, P < 0.05). In all tissues and animal groups, palmitate oxidation rates were similar per unit cytochrome-c oxidase activity, but not always per unit citrate synthase activity. Therefore, differences in mitochondrial contents (as between rats and bovines) or in mitochondrial characteristics (as between liver and muscles) relate to the differences in palmitate oxidation capacity.

Theoretical considerations on the role of membrane potential in the regulation of endosomal pH

Na+,K(+)-ATPase has been observed to partially inhibit acidification of early endosomes by increasing membrane potential, whereas chloride channels have been observed to enhance acidification in endosomes and lysosomes. However, little theoretical analysis of the ways in which different pumps and channels may interact has been carried out. We therefore developed quantitative models of endosomal pH regulation based on thermodynamic considerations. We conclude that 1) both size and shape of endosomes will influence steady-state endosomal pH whenever membrane potential due to the pH gradient limits proton pumping, 2) steady-state pH values similar to those observed in early endosomes of living cells can occur in endosomes containing just H(+)-ATPases and Na+,K(+)-ATPases when low endosomal buffering capacities are present, and 3) inclusion of active chloride channels results in predicted pH values well below those observed in vivo. The results support the separation of endocytic compartments into two classes, those (such as early endosomes) whose acidification is limited by attainment of a certain membrane potential, and those (such as lysosomes) whose acidification is limited by the attainment of a certain pH. The theoretical framework and conclusions described are potentially applicable to other membrane-enclosed compartments that are acidified, such as elements of the Golgi apparatus.

ADP-regulation of mitochondrial free radical production is different with complex I- or complex II-linked substrates: implications for the exercise paradox and brain hypermetabolism

In agreement with classic studies, succinate-supplemented rat and pigeon heart and nonsynaptic brain mitochondrial free radical production is stopped by ADP additions causing the stimulation of respiration from State 4 to State 3. Nevertheless, with Complex I-linked substrates, mitochondria produce free radicals in State 3 at rates similar or somewhat higher than during resting respiration. The absence of sharp increases in free radical production during intense respiration is possible due to strong decreases of free radical leak in State 3. The results indicate that Complex I is the main mitochondrial free radical generator in State 3, adding to its already known important generation of active oxygen species in State 4. The observed rate of mitochondrial free radical production with Complex I-linked substrates in the active State 3 can help to explain two paradoxes: (a) the lack of massive muscle oxidative damage and shortening of life span due to exercise, in spite of up to 23-fold increases of oxygen consumption together with the very low levels of antioxidants present in heart, skeletal muscle, and brain; (b) the presence of some degree of oxidative stress during exercise and hyperactivity in spite of the stop of mitochondrial free radical production by ADP with succinate as substrate.