Relationships between enzymatic flux capacities and metabolic flux rates: nonequilibrium reactions in muscle glycolysis

In situ microwave fixation provides an instantaneous snapshot of the brain metabolome

Brain glucose metabolism is highly heterogeneous among brain regions and continues postmortem. In particular, we demonstrate exhaustion of glycogen and glucose and an increase in lactate production during conventional rapid brain resection and preservation by liquid nitrogen. In contrast, we show that these postmortem changes are not observed with simultaneous animal sacrifice and in situ fixation with focused, high-power microwave. We further employ microwave fixation to define brain glucose metabolism in the mouse model of streptozotocin-induced type 1 diabetes. Using both total pool and isotope tracing analyses, we identified global glucose hypometabolism in multiple brain regions, evidenced by reduced 13C enrichment into glycogen, glycolysis, and the tricarboxylic acid (TCA) cycle. Reduced glucose metabolism correlated with a marked decrease in GLUT2 expression and several metabolic enzymes in unique brain regions. In conclusion, our study supports the incorporation of microwave fixation for more accurate studies of brain metabolism in rodent models.

Safety factors as a 'design' principle of animal form and function: an historical perspective

For well over 150 years, factors of safety (also known as safety factors) have been a fundamental engineering concept that expresses how much stronger a system is compared with the intended load. The pioneering work of Robert McNeill Alexander in the early 1980s applied this engineering concept to biomechanics. Over the next decade, evidence from comparative biomechanics supported the idea that safety factors are a fundamental principle of animal form and function. In terms of physiology, Jared Diamond related the maximal capacity of a physiological process to normal functional demands and incorporated evolutionary thinking into the concept of safety factors. It was proposed that evolutionary reasoning is required to understand the magnitudes of biological reserve capacities, an idea called 'quantitative evolutionary design'. However, the general idea of safety factors as related to organismal form and function is much older. In 1906, Samuel James Meltzer, a physiologist and physician, presented the 5th Harvey Lecture to the New York Academy of Medicine; a lecture entitled 'The Factors of Safety in Animal Structure and Animal Economy', which was later published in Science in 1907. The 1907 paper is rarely cited and has never been cited within comparative biomechanics or comparative physiology. The purpose of this Commentary is to highlight Meltzer's historical contribution to the concept of safety factors as a general principle of organismal 'design'.

Energy metabolism design of the striated muscle cell

The design of the energy metabolism system in striated muscle remains a major area of investigation. Here, we review our current understanding and emerging hypotheses regarding the metabolic support of muscle contraction. Maintenance of ATP free energy, so called energy homeostasis, via mitochondrial oxidative phosphorylation is critical to sustained contractile activity, and this major design criterion is the focus of this review. Cell volume invested in mitochondria reduces the space available for generating contractile force, and this spatial balance between mitochondria acontractile elements to meet the varying sustained power demands across muscle types is another important design criterion. This is accomplished with remarkably similar mass-specific mitochondrial protein composition across muscle types, implying that it is the organization of mitochondria within the muscle cell that is critical to supporting sustained muscle function. Beyond the production of ATP, ubiquitous distribution of ATPases throughout the muscle requires rapid distribution of potential energy across these large cells. Distribution of potential energy has long been thought to occur primarily through facilitated metabolite diffusion, but recent analysis has questioned the importance of this process under normal physiological conditions. Recent structural and functional studies have supported the hypothesis that the mitochondrial reticulum provides a rapid energy distribution system via the conduction of the mitochondrial membrane potential to maintain metabolic homeostasis during contractile activity. We extensively review this aspect of the energy metabolism design contrasting it with metabolite diffusion models and how mitochondrial structure can play a role in the delivery of energy in the striated muscle.

From OCR and ECAR to energy: Perspectives on the design and interpretation of bioenergetics studies

Biological energy transduction underlies all physiological phenomena in cells. The metabolic systems that support energy transduction have been of great interest due to their association with numerous pathologies including diabetes, cancer, rare genetic diseases, and aberrant cell death. Commercially available bioenergetics technologies (e.g., extracellular flux analysis, high-resolution respirometry, fluorescent dye kits, etc.) have made practical assessment of metabolic parameters widely accessible. This has facilitated an explosion in the number of studies exploring, in particular, the biological implications of oxygen consumption rate (OCR) and substrate level phosphorylation via glycolysis (i.e., via extracellular acidification rate (ECAR)). Though these technologies have demonstrated substantial utility and broad applicability to cell biology research, they are also susceptible to historical assumptions, experimental limitations, and other caveats that have led to premature and/or erroneous interpretations. This review enumerates various important considerations for designing and interpreting cellular and mitochondrial bioenergetics experiments, some common challenges and pitfalls in data interpretation, and some potential "next steps" to be taken that can address these highlighted challenges.

A thermodynamic function of glycogen in brain and muscle

Brain and muscle glycogen are generally thought to function as local glucose reserves, for use during transient mismatches between glucose supply and demand. However, quantitative measures show that glucose supply is likely never rate-limiting for energy metabolism in either brain or muscle under physiological conditions. These tissues nevertheless do utilize glycogen during increased energy demand, despite the availability of free glucose, and despite the ATP cost of cycling glucose through glycogen polymer. This seemingly wasteful process can be explained by considering the effect of glycogenolysis on the amount of energy obtained from ATP (ΔG'_ATP). The amount of energy obtained from ATP is reduced by elevations in inorganic phosphate (Pi). Glycogen utilization sequesters Pi in the glycogen phosphorylase reaction and in downstream phosphorylated glycolytic intermediates, thereby buffering Pi elevations and maximizing energy yield at sites of rapid ATP consumption. This thermodynamic effect of glycogen may be particularly important in the narrow, spatially constrained astrocyte processes that ensheath neuronal synapses and in cells such as astrocytes and myocytes that release Pi from phosphocreatine during energy demand. The thermodynamic effect may also explain glycolytic super-compensation in brain when glycogen is not available, and aspects of exercise physiology in muscle glycogen phosphorylase deficiency (McArdle disease).

Limits to load-lifting performance in a passerine bird: the effects of intraspecific variation in morphological and kinematic parameters

Although more massive flight muscles along with larger wings, higher wingbeat frequencies and greater stroke amplitudes enhance force and power production in flapping flight, the extent to which these parameters may be correlated with other morphological features relevant to flight physiology and biomechanics remains unclear. Intraspecifically, we hypothesized that greater vertical load-lifting capacity would correlate with higher wingbeat frequencies and relatively more massive flight muscles, along with relatively bigger hearts, lungs, and stomachs to enhance metabolic capacity and energy supply, but also with smaller body size given the overall negative allometric dependence of maximum flight performance in volant taxa. To explore intraspecific correlates of flight performance, we assembled a large dataset that included 13 morphological and kinematic variables for a non-migratory passerine, the Eurasian tree sparrow (Passer montanus). We found that heavier flight muscles and larger wings, heavier stomachs and shorter bills were the most important correlates of maximum load-lifting capacity. Surprisingly, wingbeat frequency, wing stroke amplitude and masses of the heart, lungs and digestive organs (except for the stomach) were non-significant predictor variables relative to lifting capacity. The best-fit structural equation model (SEM) indicated that load-lifting capacity was positively correlated with flight muscle mass, wing area and stomach mass, but was negatively correlated with bill length. Characterization of individual variability in flight performance in a free-ranging passerine indicates the subtlety of interaction effects among morphological features, some of which differ from those that have been identified interspecifically for maximum flight performance in birds.

A Bacterial Growth Law out of Steady State

Bacterial growth follows simple laws in constant conditions. However, bacteria in nature often face fluctuating environments. We therefore ask whether there are growth laws that apply to changing environments. We derive a law for upshifts using an optimal resource-allocation model: the post-shift growth rate equals the geometrical mean of the pre-shift growth rate and the growth rate on saturating carbon. We test this using chemostat and batch culture experiments, as well as previous data from several species. The increase in growth rate after an upshift indicates that ribosomes have spare capacity (SC). We demonstrate theoretically that SC has the cost of slow steady-state growth but is beneficial after an upshift because it prevents large overshoots in intracellular metabolites and allows rapid response to change. We also provide predictions for downshifts. The present study quantifies the optimal degree of SC, which rises the slower the growth rate, and suggests that SC can be precisely regulated.

Expression and activity of lipid and oxidative metabolism enzymes following elevated temperature exposure and thyroid hormone manipulation in juvenile lake whitefish (Coregonus clupeaformis)

Temperature has unequivocal effects on several aspects of fish physiology, but the full extent of its interaction with key endocrine signaling systems to influence metabolic function remains unknown. The aim of the current study was to assess the individual and combined effects of elevated temperature and hyperthyroidism on hepatic metabolism in juvenile lake whitefish by quantifying mRNA abundance and activity of key metabolic enzymes. Fish were exposed to 13 (control), 17 or 21 °C for 0, 4, 8 or 24 days in the presence or absence of low-T₄ (1 µg × g body weight^-1) or high-T₄ (10 µg × g body weight^-1) treatment. Our results demonstrate moderate sensitivity to elevated temperature in this species, characterized by short-term changes in mRNA abundance of several metabolic enzymes and long-term declines in citrate synthase (CS) and cytochrome c oxidase (COX) activities. T₄-induced hyperthyroidism also had several short-term effects on mRNA abundance of metabolic transcripts, including depressions in acetyl-coA carboxylase β (accβ) and carnitine palmitoyltransferase 1β (cpt1β), and stabilization of cs mRNA levels; however, these effects were primarily limited to elevated temperature groups, indicating temperature-dependent effects of exogenous T₄ treatment in this species. In contrast, maximal CS and COX activities were not altered by hyperthyroidism at any temperature. Collectively, our data suggest that temperature has the potential to manipulate thyroid hormone physiology in juvenile lake whitefish and, under warm-conditions, hyperthyroidism may suppress certain elements of the β-oxidation pathway without substantial impacts on overall cellular oxidative capacity.

Sugar Metabolism in Hummingbirds and Nectar Bats

Hummingbirds and nectar bats coevolved with the plants they visit to feed on floral nectars rich in sugars. The extremely high metabolic costs imposed by small size and hovering flight in combination with reliance upon sugars as their main source of dietary calories resulted in convergent evolution of a suite of structural and functional traits. These allow high rates of aerobic energy metabolism in the flight muscles, fueled almost entirely by the oxidation of dietary sugars, during flight. High intestinal sucrase activities enable high rates of sucrose hydrolysis. Intestinal absorption of glucose and fructose occurs mainly through a paracellular pathway. In the fasted state, energy metabolism during flight relies on the oxidation of fat synthesized from previously-ingested sugar. During repeated bouts of hover-feeding, the enhanced digestive capacities, in combination with high capacities for sugar transport and oxidation in the flight muscles, allow the operation of the “sugar oxidation cascade”, the pathway by which dietary sugars are directly oxidized by flight muscles during exercise. It is suggested that the potentially harmful effects of nectar diets are prevented by locomotory exercise, just as in human hunter-gatherers who consume large quantities of honey.

Design principles of autocatalytic cycles constrain enzyme kinetics and force low substrate saturation at flux branch points

A set of chemical reactions that require a metabolite to synthesize more of that metabolite is an autocatalytic cycle. Here, we show that most of the reactions in the core of central carbon metabolism are part of compact autocatalytic cycles. Such metabolic designs must meet specific conditions to support stable fluxes, hence avoiding depletion of intermediate metabolites. As such, they are subjected to constraints that may seem counter-intuitive: the enzymes of branch reactions out of the cycle must be overexpressed and the affinity of these enzymes to their substrates must be relatively weak. We use recent quantitative proteomics and fluxomics measurements to show that the above conditions hold for functioning cycles in central carbon metabolism of E. coli. This work demonstrates that the topology of a metabolic network can shape kinetic parameters of enzymes and lead to seemingly wasteful enzyme usage.

DOI:http://dx.doi.org/10.7554/eLife.20667.001

Many bacteria are able to produce all the molecules they need to survive from a limited supply of nutrients. This allows the bacteria to thrive even in harsh environments where other organisms struggle to live. The bacteria act as miniature chemical factories to convert nutrients into the desired molecules via a series of chemical reactions. Some molecules are made in sets of reactions termed autocatalytic cycles. These reaction sets require a molecule to be present in the cell in order to produce more of that molecule; like how a savings account needs to contain some money before it can generate more via interest.

Bacteria have many different enzymes that each drive specific chemical reactions. In order for an autocatalytic cycle to work properly, the cell needs to maintain adequate supplies of the molecule it is trying to make and all of the “intermediate” molecules in the cycle. If less of an intermediate molecule is produced, for example, the cell needs to reduce the demand for that molecule by controlling later chemical reactions in the cycle. Bacteria control chemical reactions by regulating the activities of the enzymes involved, but it is not clear exactly how they regulate the enzymes that drive autocatalytic cycles.

Barenholz et al. combined two approaches called proteomics and fluxomics to study autocatalytic cycles in a bacterium known as E. coli. The experiments suggest several core principles allow autocatalytic cycles to work smoothly in the bacteria. The next step is to apply these principles to different kinds of molecules produced in bacterial cells. A future challenge is to search for other structures that regulate chemical reactions in E. coli and other bacteria. Extending our understanding of autocatalytic cycles and other pathways of chemical reactions is essential for designing and engineering new reactions in bacteria. Such knowledge can be used to modify bacteria to produce valuable chemicals in environmentally friendly ways.

DOI:http://dx.doi.org/10.7554/eLife.20667.002

Modeling the flux of metabolites in the juvenile hormone biosynthesis pathway using generalized additive models and ordinary differential equations

Juvenile hormone (JH) regulates development and reproductive maturation in insects. The corpora allata (CA) from female adult mosquitoes synthesize fluctuating levels of JH, which have been linked to the ovarian development and are influenced by nutritional signals. The rate of JH biosynthesis is controlled by the rate of flux of isoprenoids in the pathway, which is the outcome of a complex interplay of changes in precursor pools and enzyme levels. A comprehensive study of the changes in enzymatic activities and precursor pool sizes have been previously reported for the mosquito Aedes aegypti JH biosynthesis pathway. In the present studies, we used two different quantitative approaches to describe and predict how changes in the individual metabolic reactions in the pathway affect JH synthesis. First, we constructed generalized additive models (GAMs) that described the association between changes in specific metabolite concentrations with changes in enzymatic activities and substrate concentrations. Changes in substrate concentrations explained 50% or more of the model deviances in 7 of the 13 metabolic steps analyzed. Addition of information on enzymatic activities almost always improved the fitness of GAMs built solely based on substrate concentrations. GAMs were validated using experimental data that were not included when the model was built. In addition, a system of ordinary differential equations (ODE) was developed to describe the instantaneous changes in metabolites as a function of the levels of enzymatic catalytic activities. The results demonstrated the ability of the models to predict changes in the flux of metabolites in the JH pathway, and can be used in the future to design and validate experimental manipulations of JH synthesis.

Thermal sensitivity and phenotypic plasticity of cardiac mitochondrial metabolism in European perch, Perca fluviatilis

Cellular and mitochondrial metabolic capacity of the heart has been suggested to limit performance of fish at warm temperatures. We investigated this hypothesis by studying the effects of acute temperature increases (16, 23, 30, 32.5 and 36°C) on the thermal sensitivity of 10 key enzymes governing cardiac oxidative and glycolytic metabolism in two populations of European perch (Perca fluviatilis) field-acclimated to 15.5 and 22.5°C, as well as the effects of acclimation on cardiac lipid composition. In both populations of perch, the activity of glycolytic (pyruvate kinase and lactate dehydrogenase) and tricarboxylic acid cycle (pyruvate dehydrogenase and citrate synthase) enzymes increased with acute warming. However, at temperatures exceeding 30°C, a drastic thermally induced decline in citrate synthase activity was observed in the cold- and warm-acclimated populations, respectively, indicating a bottleneck for producing the reducing equivalents required for oxidative phosphorylation. Yet, the increase in aspartate aminotransferase and malate dehydrogenase activities occurring in both populations at temperatures exceeding 30°C suggests that the malate-aspartate shuttle may help to maintain cardiac oxidative capacities at high temperatures. Warm acclimation resulted in a reorganization of the lipid profile, a general depression of enzymatic activity and an increased fatty acid metabolism and oxidative capacity. Although these compensatory mechanisms may help to maintain cardiac energy production at high temperatures, the activity of the electron transport system enzymes, such as complexes I and IV, declined at 36°C in both populations, indicating a thermal limit of oxidative phosphorylation capacity in the heart of European perch.

Proteomics Unveils Fibroblast-Cardiomyocyte Lactate Shuttle and Hexokinase Paradox in Mouse Muscles

Quantitative mapping, given in biochemically interpretable units such as mol per mg of total protein, of tissue-specific proteomes is prerequisite for the analysis of any process in cells. We applied label- and standard-free proteomics to characterize three types of striated muscles: white, red, and cardiac muscle. The analysis presented here uncovers several unexpected and novel features of striated muscles. In addition to differences in protein expression levels, the three muscle types substantially differ in their patterns of basic metabolic pathways and isoforms of regulatory proteins. Importantly, some of the conclusions drawn on the basis of our results, such as the potential existence of a "fibroblast-cardiomyocyte lactate shuttle" and the "hexokinase paradox" point to the necessity of reinterpretation of some basic aspects of striated muscle metabolism. The data presented here constitute a powerful database and a resource for future studies of muscle physiology and for the design of pharmaceutics for the treatment of muscular disorders.

Differences in metabolic rate and evaporative water loss associated with sexual dimorphism in thynnine wasps

Species with sexual dimorphism provide powerful study systems for understanding adaptation to different lifestyles as it removes the potentially confounding effects of phylogeny. Thynnine wasps have a stark sexual dimorphism where males fly patrols in search of the flightless, predominantly fossorial females with which to mate. Using flow-through respirometry, we tested the prediction that the highly active males of the thynnine wasp Zaspilothynnus nigripes would have high metabolic rates (VCO2) relative to females. Further, the females, which spend more time underground, were predicted to exhibit lower evaporative water loss (EWL) than males. Metabolic rate of both sexes increased exponentially between 12 and 28 °C. As predicted, males had higher mass-corrected VCO2 at identical temperatures than females. Alternatively, there were no differences in the EWL at identical temperatures between sexes, suggesting that experiencing the same environmental conditions during mating may favour similar EWL. Interestingly, Z. nigripes were estimated to undergo a decrease in metabolism at approximately 30 °C. It is proposed that Z. nigripes persist despite sensitivity to high temperatures using a combination of behavioural strategies and emergence during a period of relatively benign climate that ameliorates the impacts of high temperatures.

Model-assisted analysis of sugar metabolism throughout tomato fruit development reveals enzyme and carrier properties in relation to vacuole expansion

A kinetic model combining enzyme activity measurements and subcellular compartmentation was parameterized to fit the sucrose, hexose, and glucose-6-P contents of pericarp throughout tomato (Solanum lycopersicum) fruit development. The model was further validated using independent data obtained from domesticated and wild tomato species and on transgenic lines. A hierarchical clustering analysis of the calculated fluxes and enzyme capacities together revealed stage-dependent features. Cell division was characterized by a high sucrolytic activity of the vacuole, whereas sucrose cleavage during expansion was sustained by both sucrose synthase and neutral invertase, associated with minimal futile cycling. Most importantly, a tight correlation between flux rate and enzyme capacity was found for fructokinase and PPi-dependent phosphofructokinase during cell division and for sucrose synthase, UDP-glucopyrophosphorylase, and phosphoglucomutase during expansion, thus suggesting an adaptation of enzyme abundance to metabolic needs. In contrast, for most enzymes, flux rates varied irrespectively of enzyme capacities, and most enzymes functioned at <5% of their maximal catalytic capacity. One of the major findings with the model was the high accumulation of soluble sugars within the vacuole together with organic acids, thus enabling the osmotic-driven vacuole expansion that was found during cell division.

Energy and metabolism

Although firmly grounded in metabolic biochemistry, the study of energy metabolism has gone well beyond this discipline and become integrative and comparative as well as ecological and evolutionary in scope. At the cellular level, ATP is hydrolyzed by energy-expending processes and resynthesized by pathways in bioenergetics. A significant development in the study of bioenergetics is the realization that fluxes through pathways as well as metabolic rates in cells, tissues, organs, and whole organisms are "system properties." Therefore, studies of energy metabolism have become, increasingly, experiments in systems biology. A significant challenge continues to be the integration of phenomena over multiple levels of organization. Body mass and temperature are said to account for most of the variation in metabolic rates found in nature. A mechanistic foundation for the understanding of these patterns is outlined. It is emphasized that evolution, leading to adaptation to diverse lifestyles and environments, has resulted in a tremendous amount of deviation from popularly accepted scaling "rules." This is especially so in the deep sea which constitutes most of the biosphere.

A quantitative measure of electrostatic perturbation in holo and apo enzymes induced by structural changes

Biological pathways are subject to subtle manipulations that achieve a wide range of functional variation in differing physiological niches. In many instances, changes in the structure of an enzyme on ligand binding germinate electrostatic perturbations that form the basis of its changed catalytic or transcriptional efficiency. Computational methods that seek to gain insights into the electrostatic changes in enzymes require expertise to setup and computing prowess. In the current work, we present a fast, easy and reliable methodology to compute electrostatic perturbations induced by ligand binding (MEPP). The theoretical foundation of MEPP is the conserved electrostatic potential difference (EPD) in cognate pairs of active site residues in proteins with the same functionality. Previously, this invariance has been used to unravel promiscuous serine protease and metallo-β-lactamase scaffolds in alkaline phosphatases. Given that a similarity in EPD is significant, we expect differences in the EPD to be significant too. MEPP identifies residues or domains that undergo significant electrostatic perturbations, and also enumerates residue pairs that undergo significant polarity change. The gain in a certain polarity of a residue with respect to neighboring residues, or the reversal of polarity between two residues might indicate a change in the preferred ligand. The methodology of MEPP has been demonstrated on several enzymes that employ varying mechanisms to perform their roles. For example, we have attributed the change in polarity in residue pairs to be responsible for the loss of metal ion binding in fructose 1,6-bisphosphatases, and corroborated the pre-organized state of the active site of the enzyme with respect to functionally relevant changes in electric fields in ketosteroid isomerases.

Up-regulation of 2-oxoglutarate dehydrogenase as a stress response

2-Oxoglutarate dehydrogenase multienzyme complex (OGDHC) operates at a metabolic cross-road, mediating Ca(2+)- and ADP-dependent signals in mitochondria. Here, we test our hypothesis that OGDHC plays a major role in the neurotransmitter metabolism and associated stress response. This possibility was assessed using succinyl phosphonate (SP), a highly specific and efficient in vivo inhibitor of OGDHC. Animals exposed to toxicants (SP, ethanol or MnCl(2)), trauma or acute hypoxia showed intrinsic up-regulation of OGDHC in brain and heart. The known mechanism of the SP action as OGDHC inhibitor pointed to the up-regulation triggered by the enzyme impairment. The animal behavior and skeletal muscle or heart performance were tested to correlate physiology with the OGDHC regulation and associated changes in the glutamate and cellular energy status. The SP-treated animals exhibited interdependent changes in the brain OGDHC activity, glutamate level and cardiac autonomic balance, suggesting the neurotransmitter role of glutamate to be involved in the changed heart performance. Energy insufficiency after OGDHC inhibition was detectable neither in animals up to 25 mg/kg SP, nor in cell culture during 24 h incubation with 0.1 mM SP. However, in animals subjected to acute ethanol intoxication SP did evoke energy deficit, decreasing muscular strength and locomotion and increasing the narcotic sleep duration. This correlated with the SP-induced decrease in NAD(P)H levels of the ethanol-exposed neurons. Thus, we show the existence of natural mechanisms to up-regulate mammalian OGDHC in response to stress, with both the glutamate neurotransmission and energy production potentially involved in the OGDHC impact on physiological performance. This article is part of a Directed Issue entitled: Bioenergetic dysfunction, adaptation and therapy.

Metabolic adaptation of MDCK cells to different growth conditions: effects on catalytic activities of central metabolic enzymes

Lactate and ammonia are the most important waste products of central carbon metabolism in mammalian cell cultures. In particular during batch and fed-batch cultivations these toxic by-products are excreted into the medium in large amounts, and not only affect cell viability and productivity but often also prevent growth to high cell densities. The most promising approach to overcome such a metabolic imbalance is the replacement of one or several components in the culture medium. It has been previously shown that pyruvate can be substituted for glutamine in cultures of adherent Madin-Darby canine kidney (MDCK) cells. As a consequence, the cells not only released no ammonia but glucose consumption and lactate production were also reduced significantly. In this work, the impact of media changes on glucose and glutamine metabolism was further elucidated by using a high-throughput platform for enzyme activity measurements of mammalian cells. Adherent MDCK cells were grown to stationary and exponential phase in six-well plates in serum-containing GMEM supplemented with glutamine or pyruvate. A total number of 28 key metabolic enzyme activities of cell extracts were analyzed. The overall activity of the pentose phosphate pathway was up-regulated during exponential cell growth in pyruvate-containing medium suggesting that more glucose-6-phosphate was channeled into the oxidative branch. Furthermore, the anaplerotic enzymes pyruvate carboxylase and pyruvate dehydrogenase showed higher cell specific activities with pyruvate. An increase in cell specific activity was also found for NAD(+)-dependent isocitrate dehydrogenase, glutamate dehydrogenase, and glutamine synthetase in MDCK cells grown with pyruvate. It can be assumed that the increase in enzyme activities was required to compensate for the energy demand and to replenish the glutamine pool. On the other hand, the activities of glutaminolytic enzymes (e.g., alanine and aspartate transaminase) were decreased in cells grown with pyruvate, which seems to be related to a decreased glutamine metabolism.

LGH, Welch KC. The sugar oxidation cascade: aerial refueling in hummingbirds and nectar bats

Most hummingbirds and some species of nectar bats hover while feeding on floral nectar. While doing so, they achieve some of the highest mass-specific values among vertebrates. This is made possible by enhanced functional capacities of various elements of the ‘O2 transport cascade’, the pathway of O2 from the external environment to muscle mitochondria. Fasted hummingbirds and nectar bats fly with respiratory quotients (RQs; ) of ∼0.7, indicating that fat fuels flight in the fasted state. During repeated hover-feeding on dietary sugar, RQ values progressively climb to ∼1.0, indicating a shift from fat to carbohydrate oxidation. Stable carbon isotope experiments reveal that recently ingested sugar directly fuels ∼80 and 95% of energy metabolism in hover-feeding nectar bats and hummingbirds, respectively. We name the pathway of carbon flux from flowers, through digestive and cardiovascular systems, muscle membranes and into mitochondria the ‘sugar oxidation cascade’. O2 and sugar oxidation cascades operate in parallel and converge in muscle mitochondria. Foraging behavior that favours the oxidation of dietary sugar avoids the inefficiency of synthesizing fat from sugar and breaking down fat to fuel foraging. Sugar oxidation yields a higher P/O ratio (ATP made per O atom consumed) than fat oxidation, thus requiring lower hovering per unit mass. We propose that dietary sugar is a premium fuel for flight in nectarivorous, flying animals.

Evolution of allosteric citrate binding sites on 6-phosphofructo-1-kinase

As an important part of metabolism, metabolic flux through the glycolytic pathway is tightly regulated. The most complex control is exerted on 6-phosphofructo-1-kinase (PFK1) level; this control overrules the regulatory role of other allosteric enzymes. Among other effectors, citrate has been reported to play a vital role in the suppression of this enzyme's activity. In eukaryotes, amino acid residues forming the allosteric binding site for citrate are found both on the N- and the C-terminal region of the enzyme. These site has evolved from the phosphoenolpyruvate/ADP binding site of bacterial PFK1 due to the processes of duplication and tandem fusion of prokaryotic ancestor gene followed by the divergence of the catalytic and effector binding sites. Stricter inhibition of the PFK1 enzyme was needed during the evolution of multi-cellular organisms, and the most stringent control of PFK1 by citrate occurs in vertebrates. By substituting a single amino acid (K557R or K617A) as a component of the allosteric binding site in the C-terminal region of human muscle type PFK-M with a residue found in the corresponding site of a fungal enzyme, the inhibitory effect of citrate was attenuated. Moreover, the proteins carrying these single mutations enabled growth of E. coli transformants encoding mutated human PFK-M in a glucose-containing medium that did not support the growth of E. coli transformed with native human PFK-M. Substitution of another residue at the citrate-binding site (D591V) of human PFK-M resulted in the complete loss of activity. Detailed analyses revealed that the mutated PFK-M subunits formed dimers but were unable to associate into the active tetrameric holoenzyme. These results suggest that stricter control over glycolytic flux developed in metazoans, whose somatic cells are largely characterized by slow proliferation.

Quantifying global tolerance of biochemical systems: design implications for moiety-transfer cycles

Robustness of organisms is widely observed although difficult to precisely characterize. Performance can remain nearly constant within some neighborhood of the normal operating regime, leading to homeostasis, but then abruptly break down with pathological consequences beyond this neighborhood. Currently, there is no generic approach to identifying boundaries where local performance deteriorates abruptly, and this has hampered understanding of the molecular basis of biological robustness. Here we introduce a generic approach for characterizing boundaries between operational regimes based on the piecewise power-law representation of the system's components. This conceptual framework allows us to define “global tolerance” as the ratio between the normal value of a parameter and the value at such a boundary. We illustrate the utility of this concept for a class of moiety-transfer cycles, which is a widespread module in biology. Our results show a region of “best” local performance surrounded by “poor” regions; also, selection for improved local performance often pushes the operating values away from regime boundaries, thus increasing global tolerance. These predictions agree with experimental data from the reduced nicotinamide adenine dinucleotide phosphate (NADPH) redox cycle of human erythrocytes.

The ability of organisms to survive under a multitude of conditions is readily apparent. This robustness in performance is difficult to precisely characterize and quantify. At a biochemical level, it leads to physiological behavior when the parameters of the system remain within some neighborhood of their normal values. However, this behavior can change abruptly, often becoming pathological, as the boundary of the neighborhood is crossed. Currently, there is no generic approach to identifying and characterizing such boundaries. In this paper, we address the problem by introducing a method that involves quantitative concepts for boundaries between regions and “global tolerance”. To illustrate the power of these concepts, we analyzed a large class of biological modules called moiety-transfer cycles and characterized the specific case of the NADPH redox cycle in human erythrocytes, which is involved in conferring resistance to malaria. Our results show that the wild-type system operates well within a region of “best” local performance that is surrounded by “poor” regions.

Lifetime performance in foraging honeybees: behaviour and physiology

Honeybees, Apis mellifera, gradually increase their rate of forage uptake as they gain foraging experience. This increase in foraging performance has been proposed to occur as a result of learning; however, factors affecting flight ability such as changes in physiological components of flight metabolism could also contribute to this pattern. Thus, the purpose of this study was to assess the contribution of physiological changes to the increase in honeybee foraging performance. We investigated aspects of honeybee flight muscle biochemistry throughout the adult life, from non-foraging hive bees, through young and mature foragers, to old foragers near the end of their lifespan. Two-dimensional gel proteomic analysis on honeybee thorax muscle revealed an increase in several proteins from hive bees to mature foragers including troponin T 10a, aldolase and superoxide dismutase. By contrast, the activities (V(max)) of enzymes involved in aerobic performance, phosphofructokinase, hexokinase, pyruvate kinase and cytochrome c oxidase, did not increase in the flight muscles of hive bees, young foragers, mature foragers and old foragers. However, citrate synthase activity was found to increase with foraging experience. Hence, our results suggest plasticity in both structural and metabolic components of flight muscles with foraging experience.

Flux control and excess capacity in the enzymes of glycolysis and their relationship to flight metabolism in Drosophila melanogaster

An important question in evolutionary and physiological genetics is how the control of flux-base phenotypes is distributed across the enzymes in a pathway. This control is often related to enzyme-specific levels of activity that are reported to be in excess of that required for demand. In glycolysis, metabolic control is frequently considered vested in classical regulatory enzymes, each strongly displaced from equilibrium. Yet the contribution of individual steps to control is unclear. To assess enzyme-specific control in the glycolytic pathway, we used P-element excision-derived mutagenesis in Drosophila melanogaster to generate full and partial knockouts of seven metabolic genes and to measure tethered flight performance. For most enzymes, we find that reduction to half of the normal activity has no measurable impact on wing beat frequency. The enzymes catalyzing near-equilibrium reactions, phosphoglucose isomerase, phosphoglucomutase, and triosephosphate isomerase fail to show any decline in flight performance even when activity levels are reduced to 17% or less. At reduced activities, the classic regulatory enzymes, hexokinase and glycogen phosphorylase, show significant drops in flight performance and are nearer to saturation. Our results show that flight performance is canalized or robust to the activity variation found in natural populations. Furthermore, enzymes catalyzing near-equilibrium reactions show strong genetic dominance down to low levels of activity. This implies considerable excess enzyme capacity for these enzymes.

To DGC or not to DGC: oxygen guarding in the termite Zootermopsis nevadensis (Isoptera: Termopsidae)

The ability of some insects to engage in complex orchestrations of tracheal gas exchange has been well demonstrated, but its evolutionary origin remains obscure. According to a recently proposed hypothesis, insects may employ spiracular control of gas exchange to guard tissues against long-term oxidative damage by using the discontinuous gas-exchange cycle (DGC) to limit internal oxygen partial pressure (PO2). This manuscript describes a different approach to oxygen guarding in the lower termite Zootermopsis nevadensis. These insects do not display a DGC but respond to elevated oxygen concentrations by restricting spiracular area, resulting in a transient decline in CO2 emission. High internal CO2 concentrations are then maintained; restoring normoxia results in a transient reciprocal increase in CO2 emission caused by release of excess endotracheal CO2. These changes in spiracular area reflect active guarding of low internal O2 concentrations and demonstrate that regulation of endotracheal hypoxia takes physiological priority over prevention of CO2 build-up. This adaptation may reflect the need to protect oxygen-sensitive symbionts (or, gut bug guarding). Termites may eschew the DGC because periodic flushing of the tracheal system with air may harm the obligate anaerobes upon which the lower termites depend for survival on their native diet of chewed wood.

Evolution of enzymes in a series is driven by dissimilar functional demands

That distinct enzyme activities in an unbranched metabolic pathway are evolutionarily tuned to a single functional requirement is a pervasive assumption. Here we test this assumption by examining the activities of two consecutively acting enzymes in human erythrocytes with an approach to quantitative evolutionary design that avoids the above-mentioned assumption. We previously found that avoidance of NADPH depletion during the pulses of oxidative load to which erythrocytes are normally exposed is the main functional requirement mediating selection for high glucose-6-phosphate dehydrogenase activity. In the present study, we find that, in contrast, the maintenance of oxidized glutathione at low concentrations is the main functional requirement mediating selection for high glutathione reductase activity. The results in this case show that, contrary to the assumption of a single functional requirement, natural selection for the normal activities of the distinct enzymes in the pathway is mediated by different requirements. On the other hand, the results agree with the more general principles that underlie our approach. Namely, that (i) the values of biochemical parameters evolve so as to fulfill the various performance requirements that are relevant to achieve high fitness, and (ii) these performance requirements can be inferred from quantitative systems theory considerations, informed by knowledge of specific aspects of the biochemistry, physiology, genetics, and ecology of the organism.

Steady-state effects of temperature acclimation on the transcriptome of the rainbow trout heart

Cold-acclimated (CA) phenotype of trout heart was induced by 4-wk acclimation at 4°C and was characterized by 32.7% increase in relative heart mass and 49.8% increase in ventricular myocyte size compared with warm-acclimated (WA; 18°C) fish ( P < 0.001). Effect of temperature acclimation on transcriptome of the rainbow trout heart was examined using species-specific microarray chips containing 1,380 genes. After 4 wk of temperature acclimation, 8.8% (122) of the genes were differently expressed in CA and WA hearts, and most of them (82%) were upregulated in the cold ( P < 0.01). Transcripts of genes engaged in protein synthesis and intermediary metabolism were most strongly upregulated, whereas genes contributing to the connective tissue matrix were clearly repressed. Extensive upregulation of the genes coding for ribosomal proteins and translation elongation and initiation factors suggest that the protein synthesis machinery of the trout heart is enhanced in the cold and is an essential part of the compensatory mechanism causing and maintaining the hypertrophy of cardiac myocytes. The prominent depression of collagen genes may be indicative of a reduced contribution of extracellular matrix to the remodeling of the CA fish heart. Temperature-related changes in transcripts of metabolic enzymes suggest that at mRNA level, glycolytic energy production from carbohydrates is compensated in the heart of CA rainbow trout, while metabolic compensation is absent in mitochondria. In addition, the analysis revealed three candidate genes: muscle LIM protein, atrial natriuretic peptide B, and myosin light chain 2, which might be central for induction and maintenance of the hypertrophic phenotype of the CA trout heart. These findings indicate that extensive modification of gene expression is needed to maintain the temperature-specific phenotype of the fish heart.

Allometric scaling of flight energetics in orchid bees: evolution of flux capacities and flux rates

The evolution of metabolic pathways involved in energy production was studied in the flight muscles of 28 species of orchid bees. Previous work revealed that wingbeat frequencies and mass-specific metabolic rates decline in parallel by threefold as body mass increases interspecifically over a 20-fold range. We investigated the correlated evolution of metabolic rates during hovering flight and the flux capacities, i.e. Vmaxvalues, of flight muscle enzymes involved in substrate catabolism, the Krebs cycle and the electron transport chain. Vmax at the hexokinase (HK) step scales allometrically with an exponent almost identical to those obtained for wingbeat frequency and mass-specific metabolic rate. Analysis of this relationship using phylogenetically independent contrasts supports the hypothesis of correlated evolution between HK activity and mass-specific metabolic rate. Although other enzymes scale allometrically with respect to body mass, e.g. trehalase, glycogen phosphorylase and citrate synthase, no other enzyme activities were correlated with metabolic rate after controlling for phylogenetic relatedness. Pathway flux rates were used with enzyme Vmax values to estimate fractional velocities(fraction of Vmax at which enzymes operate) for various reactions to gain insights into enzyme function and how this varies with body mass. Fractional velocity is highly conserved across species at the HK step,but varied at all other steps examined. These results are discussed in the context of the regulation and evolution of pathways of energy metabolism.

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.

Energy substrate metabolism in cardiac hypertrophy

Cardiac hypertrophy is a response to long-term pathologic (eg, hypertension) or physiologic (eg, exercise) hemodynamic overload accompanied by changes in energy substrate utilization. The pattern of substrate utilization (or metabolic phenotype) differs dramatically between pathologic and physiologic cardiac hypertrophy with directionally opposite changes in oxidation of fatty acids and glucose and glycolysis. These findings indicate that the metabolic response to long-term alterations in hemodynamic workload is not stereotypical, but is influenced by the nature of the stimulus leading to cardiac hypertrophy. Although the changes in substrate utilization are adaptive, in the case of pathologic stimuli, the changes in metabolism interfere with functional resiliency of the heart to metabolic stress, as occurs during ischemia-reperfusion. The distinct metabolic phenotypes of hearts hypertrophied in response to pathologic or physiologic stimuli are due not only to alteration in expression of metabolic enzymes and proteins, but also to post-translational modulation of metabolic enzymes and proteins.

Control of maximum metabolic rate in humans: dependence on performance phenotypes

Borrowing from metabolic control analysis the concept of control coefficients or ci values, defined as fractional change in MMR/fractional change in the capacity of any given step in ATP turnover, we used four performance phenotypes to compare mechanisms of control of aerobic maximum metabolic rate (MMR): (i) untrained sedentary (US) subjects, as a reference group against which to compare (ii) power trained (PT), (iii) endurance trained (ET), and (iv) high altitude adapted native (HA) subject groups. Sprinters represented the PT group; long distance runners illustrated the ET group; and Andean natives represented the HA group. Numerous recent studies have identified contributors to control on both the adenosine triphosphate (ATP) supply side and the ATP demand side of ATP turnover. From the best available evidence it appears that at MMR all five of the major steps in energy delivery (namely, ventilation, pulmonary diffusion, cardiac output, tissue capillary--mitochondrial O2 transfer, and aerobic cell metabolism per se) approach an upper functional ceiling, with control strength being distributed amongst the various O2 flux steps. On the energy demand side, the situation is somewhat simplified since at MMR approximately 90% of O2-based ATP synthesis is used for actomyosin (AM) and Ca2+ ATPases; at MMR these two ATP demand rates also appear to be near an upper functional ceiling. In consequence, at MMR the control contributions or ci values are distributed amongst all seven major steps in ATP supply and ATP demand pathways right to the point of fatigue. Relative to US (the reference group), in PT subjects at MMR control strength shifts towards O2 delivery steps (ventilation, pulmonary diffusion, and cardiac output); here physiological regulation clearly dominates MMR control. In contrast in ET and HA subjects at MMR control shifts towards the energy demand steps (AM and Ca2+ ATPases), and more control strength is focussed on tissue level ATP supply and ATP demand. One obvious advantage of the ET and HA biochemical-level control is improved metabolite homeostasis. Additionally, with some reserve capacity in the O2 delivery steps, the focussing of control on ATP turnover at the tissue level has allowed nature to improve on an 'endurance machine' design.

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.

'Futile cycle' enzymes in the flight muscles of North American bumblebees

In the flight muscles of European bumblebees, high activities of fructose-1,6-bisphosphatase (FbPase) relative to phosphofructokinase (PFK) have suggested a thermogenic 'futile cycle' important for regional endothermy. We find generally low activities of FbPase (0.7-19.7 units g(-1) thorax) in North American Bombus species, with the exception of Bombus rufocinctus, where activity (43.1 units g(-1) thorax) is comparable with that of European congeners. These data, taken with estimates of maximal rates of heat production by cycling, do not support a significant thermogenic role for the PFK/FbPase cycle. In agreement with earlier studies, both PFK and FbPase activities were found to scale allometrically with body size (allometric exponents -0.18 and -1.33, respectively). The cycle may serve to supplement thermogenesis or amplify glycolytic flux in rest-to-flight transitions, especially in smaller bees.

Seasonal temperature compensation in the horse mussel, Modiolus modiolus: metabolic enzymes, oxidative stress and heat shock proteins

Seasonal collections of the subtidal horse mussel, Modiolus modiolus, from a depth of 10 m were made at the Isles of Shoals, New Hampshire to assess changes in overall energetic demand, measured as respiration, the maximal activities of rate-limiting enzymes of intermediate metabolism, level of oxidative stress, and the expression of heat shock proteins (HSP). Weighted respiration rates of mussels from winter collections were significantly lower than summer rates but decreased by less than 20%. Specific activities of several rate-limiting enzymes were measured in mussels from the summer and winter collections at the temperature of collection and the reciprocal seasonal temperature (15 and 5 degrees C). Comparisons of these enzyme activities and the protein concentrations of hexokinase and citrate synthase show that a quantitative strategy is used to acclimatize to winter temperatures by these rate-limiting enzymes of intermediate metabolism. The activities and protein concentrations of the antioxidant enzyme, Cu/Zn superoxide dismutase (SOD) is seasonally indistinguishable while the concentration of HSP 70 was greater in winter than in summer samples. These results show that mussels seasonally compensate for decreases in temperature by increasing the concentration of rate-limiting metabolic enzymes while maintaining the same level of antioxidant protection in summer and winter consistent with high aerobic metabolism in both winter and summer. Lastly, the significantly greater concentrations of HSP70 in winter samples suggests that protein chaperone functions must be maintained while other seasonal adjustments to cold temperatures are occurring.

Mapping Determinants of Variation in Energy Metabolism, Respiration and Flight in Drosophila

We employed quantitative trait locus (QTL) mapping to dissect the genetic architecture of a hierarchy of functionally related physiological traits, including metabolic enzyme activity, metabolite storage, metabolic rate, and free-flight performance in recombinant inbred lines of Drosophila melanogaster. We identified QTL underlying variation in glycogen synthase, hexokinase, phosphoglucomutase, and trehalase activity. In each case variation mapped away from the enzyme-encoding loci, indicating that trans-acting regions of the genome are important sources of variation within the metabolic network. Individual QTL associated with variation in metabolic rate and flight performance explained between 9 and 35% of the phenotypic variance. Bayesian QTL analysis identified epistatic effects underlying variation in flight velocity, metabolic rate, glycogen content, and several metabolic enzyme activities. A region on the third chromosome was associated with expression of the glucose-6-phosphate branchpoint enzymes and with metabolic rate and flight performance. These genomic regions are of special interest as they may coordinately regulate components of energy metabolism with effects on whole-organism physiological performance. The complex biochemical network is encoded by an equally complex network of interacting genetic elements with potentially pleiotropic effects. This has important consequences for the evolution of performance traits that depend upon these metabolic networks.

Acidosis inhibits oxidative phosphorylation in contracting human skeletal muscle in vivo

This study tested the hypothesis that acidic pH inhibits oxidative ATP supply during exercise in hand (first dorsal interosseus, FDI) and lower limb (leg anterior compartment, LEG) muscles. We measured oxidative flux and estimated mitochondrial capacity using the changes in creatine phosphate concentration ([PCr]) and pH as detected by 31P magnetic resonance (MR) spectroscopy during isometric exercise and recovery. The highest oxidative ATP flux in sustained exercise was about half the estimated mitochondrial capacity in the LEG (0.38 +/- 0.06 vs. 0.90 +/- 0.14 mM ATP s(-1), respectively), but at the estimated capacity in the FDI (0.61 +/- 0.05 vs. 0.61 +/- 0.09 mM ATP s(-1), respectively). During sustained exercise at a higher contraction rate, intracellular acidosis (pH < 6.88) prevented a rise in oxidative flux in the LEG and FDI despite significantly increased [ADP]. We tested whether oxidative flux could increase above that achieved in sustained exercise by raising [ADP] (> 0.24 mM) and avoiding acidosis using burst exercise. This exercise raised oxidative flux (0.69 +/- 0.05 mM ATP s(-1)) to nearly twice that found with sustained exercise in the LEG and matched (0.65 +/- 0.11 mM ATP s(-1)) the near maximal flux seen during sustained exercise in the FDI. Thus both muscles reached their highest oxidative fluxes in the absence of acidosis. These results show that acidosis inhibits oxidative phosphorylation in vivo and can limit ATP supply in exercising muscle to below the mitochondrial capacity.

Patterns of control of maximum metabolic rate in humans

In this analysis, four performance phenotypes were used to compare mechanisms of control of aerobic maximum metabolic rate (MMR): (i) untrained sedentary (US) subjects, as a reference group against which to compare (ii) power trained (PT), (iii) endurance trained (ET) and (iv) high altitude adapted native (HA) subject groups. Sprinters represented the PT group; long distance runners illustrated the ET group; and Quechuas represented the HA group. Numerous recent studies have identified contributors to control on both the adenosine triphosphate (ATP) supply side and the ATP demand side of ATP turnover. Control coefficients or c(i) values were defined as fractional change in MMR/fractional change in the capacity of any given step in ATP turnover. From the best available evidence it appears that at MMR all five of the major steps in energy delivery (namely, ventilation, pulmonary diffusion, cardiac output, tissue capillary - mitochondrial O(2) transfer, and aerobic cell metabolism per se) approach an upper functional ceiling, with control strength being distributed amongst the various O(2) flux steps. On the energy demand side, the situation is somewhat simplified since at MMR approximately 90% of O(2)-based ATP synthesis is used for actomyosin (AM) and Ca(2+) ATPases; at MMR these two ATP demand rates also appear to be near an upper functional ceiling. In consequence, at MMR the control contributions or c(i) values are rather evenly divided amongst all seven major steps in ATP supply and ATP demand pathways right to the point of fatigue. Relative to US (the reference group), in PT subjects at MMR control strength shifts towards O(2) delivery steps (ventilation, pulmonary diffusion and cardiac output). In contrast in ET and HA subjects at MMR control shifts towards the energy demand steps (AM and Ca(2+) ATPases), and more control strength is focussed on tissue level ATP supply and ATP demand. One obvious advantage of the ET and HA control pattern is improved metabolite homeostasis. Another possibility is that, with some reserve capacity in the O(2) delivery steps and control focussed on ATP turnover at the tissue level, nature has designed the ideal 'endurance machine'.

Key metabolic enzymes and muscle structure in triplefin fishes (Tripterygiidae): a phylogenetic comparison

Metabolic potential and muscle development were investigated relative to habitat and phylogeny in seven species of New Zealand triplefin fishes. Activity was measured in three principal glycolytic enzymes (lactate dehydrogenase, pyruvate kinase and phosphofructokinase) and two oxidative enzymes (citrate synthase and L3-hydroxyacyl CoA:NAD(+) oxidoreductase). The non-bicarbonate buffering capacity of caudal muscle was also estimated. Phylogenetic independent contrast analyses were used to reduce the effects of phylogenetic history in analyses. A positive relationship between metabolic potential and the effective water velocity at respective habitat depths was found only after the exclusion from analyses of the semi-pelagic species Obliquichthys maryannae. O. maryannae showed high glycolytic enzyme activities, and displayed double the activity of both oxidative enzymes relative to the six benthic species. Histochemically stained sections taken immediately posterior to the vent showed that adult O. maryannae and larval Forsterygion lapillum had significantly more red muscle, and smaller cross-sectional areas of white and red muscle fibres, than adults of benthic species. The distribution of red muscle in adult O. maryannae resembled that of larval F. lapillum, and differed from the typical teleost pattern seen in adults of the six benthic species. Both adult O. maryannae and larval F. lapillum have an expansive lateralis superficialis muscle, typical of larval fish, which encompasses much of the caudal trunk. Results suggest that anaerobic potential in New Zealand triplefins: (a) increases with the locomotory requirements of different habitats, and (b) displays a negative relationship with depth-dependent water velocities in benthic species. O. maryannae appears to have increased aerobic potential for sustained swimming by paedomorphic retention of larval muscle architecture.

Allometric scaling and proportion regulation in the freshwater planarian Schmidtea mediterranea

The regulation of scale and proportion in living organisms is an intriguing and enduring problem of biology. Regulatory mechanisms for controlling body size and proportion are clearly illustrated by the regeneration of missing body parts after amputation, in which the newly regenerated tissues ultimately attain a size that is anatomically congruent with the size of the rest of the organism. Understanding the molecular processes underpinning scaling would have deep consequences for our comprehension of tissue regeneration, developmental ontogeny, growth, and evolution. Although many theories have been put forward to explain this process, it is interesting that no satisfactory mechanistic explanation is currently available to explain scalar relationships. We chose to investigate the freshwater planarian, a commonly used model system for the study of metazoan regeneration, to delineate a strategy for the molecular dissection of scale and proportion mechanisms in metazoans. Here, we report on the cloning and discrete expression pattern of a novel planarian gene, which shares homology with the DEG/ENaC super-family of sodium channels. We have named H.112.3c cintillo ("head ribbon" in Spanish) and present a strategy for using the expression of this gene to monitor scale and proportion regulation during regeneration, growth and degrowth in the freshwater planarian Schmidtea mediterranea.

Artificial selection for high activity favors mighty mini-muscles in house mice

After 14 generations of selection for voluntary wheel running, mice from the four replicate selected lines ran, on average, twice as many revolutions per day as those from the four unselected control lines. To examine whether the selected lines followed distinct strategies in the correlated responses of the size and metabolic capacities of the hindlimb muscles, we examined mice from selected lines, housed for 8 wk in cages with access to running wheels that were either free to rotate ("wheel access" group) or locked ("sedentary"). Thirteen of twenty individuals in one selected line (line 6) and two of twenty in another (line 3) showed a marked reduction ( approximately 50%) in total hindlimb muscle mass, consistent with the previously described expression of a small-muscle phenotype. Individuals with these "mini-muscles" were not significantly smaller in total body mass compared with line-mates with normal-sized muscles. Access to free wheels did not affect the relative mass of the mini-muscles, but did result in typical mammalian training effects for mitochondrial enzyme activities. Individuals with mini-muscles showed a higher mass-specific muscle aerobic capacity as revealed by the maximal in vitro rates of citrate synthase and cytochrome c oxidase. Moreover, these mice showed the highest activities of hexokinase and carnitine palmitoyl transferase. Females with mini-muscles showed the highest levels of phosphofructokinase, and males with mini-muscles the highest levels of pyruvate dehydrogenase. As shown by total muscle enzyme contents, the increase in mass-specific aerobic capacity almost completely compensated for the reduction caused by the "loss" of muscle mass. Moreover, the mini-muscle mice exhibited the lowest contents of lactate dehydrogenase and glycogen phosphorylase. Interestingly, metabolic capacities of mini-muscled mice resemble those of muscles after endurance training. Overall, our results demonstrate that during selection for voluntary wheel running, distinct adaptive paths that differentially exploit the genetic variation in morphological and physiological traits have been followed.

Hummingbird foraging and the relation between bioenergetics and behaviour

Because of their small size and expensive mode of flight, hummingbirds display some of the highest known mass-specific rates of aerobic metabolism among vertebrates. High enzymatic flux capacities through pathways of carbohydrate and long-chain fatty acid oxidation indicate that either substrate can fuel flight. Although hummingbirds are known to rely on fat to fuel migratory flight, short foraging bouts are fueled by the oxidation of carbohydrate, not fat. This allows birds refueling at meadows during migration to deposit fat at higher rates and avoids the energetic inefficiency that results from synthesizing fat from dietary sugar, and then breaking down the fat to fuel foraging flight. On cold mornings in subalpine meadows, refueling hummingbirds achieve net energy gain despite the high energetic costs of thermoregulation and flight. In doing so, they sustain the highest known time-averaged metabolic rates among vertebrates. However, low sucrose concentrations, provided in volumes large enough to allow the maintenance of energy balance at low temperature, result in energy deficit and mass loss. The problem of disposing of dietary water at low ambient temperature when intake rates are elevated suggests that the kidneys may be involved in establishing the upper limit to intake rates and, therefore, maximum sustained metabolic rates. It is suggested that hummingbird behaviour and metabolism have coevolved to maximize net energy gain. Further, the energetics of hummingbird thermoregulation and flight may have influenced the evolution of sucrose content in floral nectar.

Going with the flow or life in the fast lane: contrasting mitochondrial responses to thermal change

Temperature is one of the most important environmental factors affecting the physiology of animals. Seasonal fluctuations in temperature are of particular importance in aquatic ectotherms since their body temperature is in equilibrium with their environment. When an organism faces adverse environmental conditions, it can either remain active or enter into metabolic depression, adopting the strategy that maximises its fitness. Physiological responses to environmental stress occur at many different levels of organisation in an animal. Here, we focus on mitochondria, given their central importance in cellular energy metabolism. We contrast the thermal biology of skeletal muscle mitochondria from cold-active species with that of species that spend their winters in a metabolically depressed state. Specifically, we examine the modifications of mitochondrial properties during thermal/seasonal acclimation and examine mechanisms by which these modifications can arise. While compensatory responses to cold acclimation include increases in mitochondrial abundance, in the oxidative capacities of individual mitochondria and adjustments of ADP affinities, metabolic depression can reduce tissue levels of mitochondrial enzymes and mitochondrial proton leak rates.

Quantitative evolutionary design

The field of quantitative evolutionary design uses evolutionary reasoning (in terms of natural selection and ultimate causation) to understand the magnitudes of biological reserve capacities, i.e. excesses of capacities over natural loads. Ratios of capacities to loads, defined as safety factors, fall in the range 1.2-10 for most engineered and biological components, even though engineered safety factors are specified intentionally by humans while biological safety factors arise through natural selection. Familiar examples of engineered safety factors include those of buildings, bridges and elevators (lifts), while biological examples include factors of bones and other structural elements, of enzymes and transporters, and of organ metabolic performances. Safety factors serve to minimize the overlap zone (resulting in performance failure) between the low tail of capacity distributions and the high tail of load distributions. Safety factors increase with coefficients of variation of load and capacity, with capacity deterioration with time, and with cost of failure, and decrease with costs of initial construction, maintenance, operation, and opportunity. Adaptive regulation of many biological systems involves capacity increases with increasing load; several quantitative examples suggest sublinear increases, such that safety factors decrease towards 1.0. Unsolved questions include safety factors of series systems, parallel or branched pathways, elements with multiple functions, enzyme reaction chains, and equilibrium enzymes. The modest sizes of safety factors imply the existence of costs that penalize excess capacities. Those costs are likely to involve wasted energy or space for large or expensive components, but opportunity costs of wasted space at the molecular level for minor components.

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

The power function of basal metabolic rate scaling is expressed as aM(b), where a corresponds to a scaling constant (intercept), M is body mass, and b is the scaling exponent. The 3/4 power law (the best-fit b value for mammals) was developed from Kleiber's original analysis and, since then, most workers have searched for a single cause to explain the observed allometry. Here we present a multiple-causes model of allometry, where the exponent b is the sum of the influences of multiple contributors to metabolism and control. The relative strength of each contributor, with its own characteristic exponent value, is determined by the control contribution. To illustrate its use, we apply this model to maximum versus basal metabolic rates to explain the differing scaling behaviour of these two biological states in mammals. The main difference in scaling is that, for the basal metabolic rate, the O(2) delivery steps contribute almost nothing to the global b scaling exponent, whereas for the maximum metabolic rate, the O(2) delivery steps significantly increase the global b value.

Partly ordered synthesis and degradation of glycogen in cultured rat myotubes

The following questions concerning glycogen synthesis and degradation were examined in cultured rat myotubes. 1) Is synthesis and degradation of the individual glycogen molecule a strictly ordered process, with the last glucosyl unit incorporated into the molecule being the first to be released (the last-in-first-out principle), or is it a random process? 2) Are all glycogen molecules in skeletal muscle synthesized and degraded in phase (simultaneous order) or out of phase (sequential order)? Basal glycogen stores were minimized by fasting and were subsequently replenished in two intervals, the first (0-0.5 h) with tritium-labeled and the second (0.5-3 h) with carbon-labeled glucose as precursor. Glycogen degradation was initiated by addition of forskolin. The kinetics of glycogen accumulation as well as degradation could be approximated by monoexponential equations with rate constants of 0.81 and 1.39 h(-1), respectively. The degradation of glycogen largely followed the last-in-first-out principle, particularly in the initial period. Analysis of the size of the glycogen molecules and the beta-dextrin limit during glycogen accumulation and degradation showed that both synthesis and degradation of glycogen molecules are largely sequential and the small deviation from this order is most pronounced at the beginning of the accumulation and at the end of the degradation period. This pattern may reflect the number of synthase and phosphorylase molecules and fits well with the role of glycogen in skeletal muscle as a readily available energy store and with the known structure of the glycogen molecule. It is emphasized that the observed nonlinear relation between the change in glycogen concentration and release of label during glycogen degradation may have important practical consequences for interpretation of experimental data.

Fuel use in hawkmoth (Amphion floridensis) flight muscle: enzyme activities and flux rates

The fuels used by the hawkmoth Amphion floridensis to power flight are determined by nectar-feeding, with fed moths using primarily carbohydrate and unfed moths using primarily fat. To investigate the metabolic pathways underlying fuel-use flexibility in this species, we measured the maximal activities of several key metabolic enzymes in the flight muscle of fed and unfed individuals, for which metabolic rates and fuel utilization had been previously determined. Hexokinase (HK) and phosphofructokinase (PFK) occur at high activities and, during carbohydrate-fueled flight, are estimated to operate at fractional velocities comparable to those of exclusively carbohydrate-utilizing insects. Females exhibited higher glycolytic enzyme activities than did males, and males regulated PFK activity according to nectar feeding. Although beta-hydroxyacyl-CoA dehydrogenase (HOAD) was found at high activities, carnitine palmitoyl transferase (CPT) was not detectable, suggesting that fatty acids may be utilized via a carnitine-independent pathway during flight. Principal component analysis revealed a tendency for the activities of citrate synthase, HK, PFK, and HOAD to be positively correlated among individuals, as well as a lesser tendency for the activities of glycolytic vs. mitochondrial enzymes to be negatively correlated with each other. However, the principal components did not correlate with variation in either oxygen consumption rate or fuel use in vivo, suggesting that variation in enzyme concentration did not determine differences among individuals in metabolic performance during flight. J. Exp. Zool. 290:108-114, 2001.