Metabolic setpoint control mechanisms in different physiological systems at rest and during exercise

Using a number of different homeostatic control mechanisms in the brain and peripheral physiological systems, metabolic activity is continuously regulated at rest and during exercise to prevent catastrophic system failure. Essential for the function of these regulatory processes are baseline “setpoint” levels of metabolic function, which can be used to calculate the level of response required for the maintenance of system homeostasis after system perturbation, and to which the perturbed metabolic activity levels are returned to at the end of the regulatory process. How these setpoint levels of all the different metabolic variables in the different peripheral physiological systems are created and maintained, and why they are similar in different individuals, has not been well explained. In this article, putative system regulators of metabolic setpoint levels are described. These include that: (i) innate setpoint values are stored in a certain region of the central nervous system, such as the hypothalamus; (ii) setpoint values are created and maintained as a response to continuous external perturbations, such as gravity or “zeitgebers”, (iii) setpoint values are created and maintained by complex system dynamical activity in the different peripheral systems, where setpoint levels are regulated by the ongoing feedback control activity between different peripheral variables; (iv) human anatomical and biomechanical constraints contribute to the creation and maintenance of metabolic setpoints values; or (v) a combination of all these four different mechanisms occurs. Exercise training and disease processes can affect these metabolic setpoint values, but the setpoint values are returned to pre-training or pre-disease levels if the training stimulus is removed or if the disease process is cured. Further work is required to determine what the ultimate system regulator of metabolic setpoint values is, why some setpoint values are more stringently protected by homeostatic regulatory mechanisms than others, and the role of conscious decision making processes in determining the regulation of metabolic setpoint values.     

Quantitative assessment of regulation in metabolic systems

We show how metabolic regulation as commonly understood in biochemistry can be described in terms of metabolic control analysis. The steady-state values of the variables of metabolic systems (fluxes and concentrations) are determined by a set of parameters. Some of these parameters are concentrations that are set by the environment of the system; they can act as external regulators by communicating changes in the environment to the metabolic system. How effectively a system is regulated depends both on the degree to which the activity of the regulatory enzyme with which a regulator interacts directly can be altered by the regulator (its regulability) and on the ability of the regulatory enzyme to transmit the changes to the rest of the system (its regulatory capacity). The regulatory response of a system also depends on its internal organisation around key variable metabolites that act as internal regulators. The regulatory performance of the system can be judged in terms of how sensitivity the fluxes respond to the external stimulus and to what degree homeostasis in the concentrations of the internal regulators is maintained. We show how, on the level of both external and internal regulation, regulability can be quantified in terms of an elasticity coefficient and regulatory capacity in terms of a control coefficient. Metabolic regulation can therefore be described in terms of metabolic control analysis. The combined response relationship of control analysis relates regulability and regulatory capacity and allows quantification of the regulatory importance of the various interactions of regulators with enzymes in the system. On this basis we propose a quantitative terminology and analysis of metabolic regulation that shows what we should measure experimentally and how we should interpret the results. Analysis and numerical simulation of a simple model system serves to demonstrate our treatment.     

The responses of rat hepatocytes to glucagon and adrenaline. Application of quantified elasticity analysis.

The internal control of hepatocyte metabolism has been previously analysed using metabolic control analysis. The aim of this paper is to extend this analysis to include the responses of the cells to hormonal stimulus. Hepatocyte metabolism was divided into nine reaction blocks: glycogen breakdown, glucose release, glycolysis, lactate production, NADH oxidation, pyruvate oxidation, proton leak, mitochondrial phosphorylation and ATP consumption, linked by five intermediates: mitochondrial membrane potential, cytoplasmic NADH/NAD and total cellular ATP, glucose 6-phosphate and pyruvate. The kinetic responses of the reaction blocks to the intermediates were determined previously in the absence of added hormones. In this study, the changes in flux and intermediate levels that occurred upon addition of either glucagon or adrenaline were measured. From comparison of the fractional changes in fluxes and intermediate levels with the known kinetics of the system, it was possible to determine the primary sites of action of the hormones. The results show that the majority of processes in the cell are responsive to the hormones. The notable exception to this is the failure of adrenaline to have a direct effect on glycolysis. The activity change of each metabolic block observed in the presence of either hormone was quantified and compared to the indirect effects on each block caused by changes in metabolite levels. The second stage of the analysis was to use the calculated activity changes and the known control pattern of the system to give a semiquantitative analysis of the regulatory pathways employed by the hormones to achieve the changes in fluxes and metabolite levels. This was instructive in analysing, for example, how glucagon caused a decrease in flux through glycolysis and an increase in oxidative phosphorylation without large changes in metabolite levels (homeostasis). Conversely, it could be seen that the failure of adrenaline to maintain a constant glucose 6-phosphate concentration was due to the stimulation of glycogen breakdown and inhibition of glucose release.     

Use of in vivo kinetic techniques to evaluate metabolism of nutrients

Most nutrition research is related to rates of physiological processes. Information about those processes can be gained by in vivo kinetic techniques; however, many nutritionists are hesitant to use in vivo kinetics. The two basic in vivo kinetic techniques are single injection and continuous infusion of tracer into a pool of tracee. Either technique can form the basis for multiple-pool kinetics, or modeling. Solving a multiple-pool system can provide flow rates of substances between metabolic pools and is valuable for understanding a particular metabolic pathway or process. In vivo kinetic techniques can be valuable in understanding mechanisms whereby partitioning agents affect the distribution of nutrients, especially protein and fat, in food-producing animals. In vivo kinetics is a valuable tool for nutrition research and should be used more frequently.     

Modeling aerobic carbon oxidation and storage by integrating respirometric, titrimetric, and off-gas CO2 measurements

A method for detailed investigation of aerobic carbon degradation processes by microorganisms is presented. The method relies on an integrated use of the respirometric, titrimetric, and off-gas CO(2) measurements. The oxygen uptake rate (OUR), hydrogen ion production rate (HPR), and the carbon dioxide transfer rate (CTR) resulting from the biological as well as physicochemical processes, coupled with a metabolic model characterizing both the growth and carbon storage processes, enables the comprehensive study of the carbon degradation processes. The method allows the formation of carbon storage products and the biomass growth rates to be estimated without requiring any off-line biomass or liquid-phase measurements, although the practical identifiability of the system could be improved with additional measurements. Furthermore, the combined yield for biomass growth and carbon storage is identifiable, along with the affinity constant with respect to the carbon substrate. However, the individual yields for growth and carbon storage are not identifiable without further knowledge about the metabolic pathways employed by the microorganisms in the carbon conversion. This is true even when more process variables are measured. The method is applied to the aerobic carbon substrate degradation by a full-scale sludge using acetate as an example carbon source. The sludge was able to quickly take up the substrate and store it as poly-beta-hydroxybutyrate (PHB). The PHB formation rate was a few times faster than the biomass growth rate, which was confirmed by off-line liquid- and solid-phase analysis. The estimated combined yield for biomass growth and carbon storage compared closely to that determined from the theoretical yields reported in literature based on thermodynamics. This suggests that the theoretical yields may be used as default parameters for modeling purposes.     

Thermal sensation and thermophysiological responses to metabolic step-changes

This study investigated the effect on thermal perception and thermophysiological variables of controlled metabolic excursions of various intensities and durations. Twenty-four subjects were alternately seated on a chair or exercised by walking on a treadmill at a temperature predicted to be neutral at sedentary activity. In a second experimental series, subjects alternated between rest and exercise as well as between exercise at different intensities at two temperature levels. Measurements comprised skin and oesophageal temperatures, heart rate and subjective responses. Thermal sensation started to rise or decline immediately (within 1 min) after a change of activity, which means that even moderate activity changes of short duration affect thermal perceptions of humans. After approximately 15–20 min under constant activity, subjective thermal responses approximated the steady-state response. The sensitivity of thermal sensation to changes in core temperature was higher for activity down-steps than for up-steps. A model was proposed that estimates transient thermal sensation after metabolic step-changes. Based on predictions by the model, weighting factors were suggested to estimate a representative average metabolic rate with varying activity levels, e.g. for the prediction of thermal sensation by steady-state comfort models. The activity during the most recent 5 min should be weighted 65%, during the prior 10–5 min 25% and during the prior 20–10 min 10%.     

Limits of Adaptation: The Evolution of Selective Neutrality

Many enzymes in intermediary metabolism manifest saturation kinetics in which flux is a concave function of enzyme activity and often of the Michaelis-Menten form. The result is that, when natural selection favors increased enzyme activity so as to maximize flux, a point of diminishing returns will be attained in which any increase in flux results in a disproportionately small increase in fitness. Enzyme activity ultimately will reach a level at which the favorable effect of an increase in activity is of the order 1/(4Ne) or smaller, where Ne is the effective population number. At this point, many mutations that result in small changes in activity will result in negligible changes in fitness and will be selectively nearly neutral. We propose that this process is a mechanism whereby conditions for the occurrence of nearly neutral mutations and gene substitutions can be brought about by the long-continued action of natural selection. Evidence for the hypothesis derives from metabolic theory, direct studies of flux, studies of null and other types of alleles in Drosophila melanogaster and chemostat studies in Escherichia coli. Limitations and complications of the theory include changes in environment or genetic background, enzymes with sharply defined optima of activity, overdominance, pleiotropy, multifunctional enzymes and branched metabolic pathways. We conclude that the theory is a useful synthesis that unites many seemingly unrelated observations. The principal theoretical conclusion is that the conditions for the occurrence of neutral evolution can be brought about as an indirect result of the action of natural selection.     

Changes of insulin in plasma and receptor for insulin in various tissues after the exposure of rats to space flights and hypokinesia

The explanation of the mechanism of the response to gravity changes is of great importance for the determination of the capacity of human subjects to adapt to the load of gravitational stress. Therefore several studies were performed to investigate the activity of endocrine system, since the hormones are involved in the regulation of physiological functions and metabolic processes. However the studies of endocrine system activity during altered gravity conditions, especially during the weightlessness are influenced by the several interventions in biomedical observations due to operational program of astronauts, wide variability in individual response and tolerance, use of extensive countermeasures, differences in the type of space missions and in the studies after landing also a hypergravity effect at landing and variability in postflight readaptation process. The significant changes of plasma insulin and glucose levels were observed in astronauts during space flights and in the first days of recovery period. In the first inflight period plasma insulin levels were increased, unchanged or decreased however after 4-5 weeks of exposure to weightlessness a decrease of insulin plasma levels were noted. After space flights an increase of plasma insulin levels were demonstrated in experimental animals and in human subjects. Since plasma insulin level is considered as most important factor involved in the regulation for insulin receptors in target tissues, an investigation of insulin receptors in various tissues was performed in rats exposed to space flight or to hypokinesia (model used for simulation of some effects of microgravity).     

Between Metabolite Relationships: an essential aspect of metabolic change

Not only the levels of individual metabolites, but also the relations between the levels of different metabolites may indicate (experimentally induced) changes in a biological system. Component analysis methods in current 'standard' use for metabolomics, such as Principal Component Analysis (PCA), do not focus on changes in these relations. We therefore propose the concept of 'Between Metabolite Relationships' (BMRs): common changes in the covariance (or correlation) between all metabolites in an organism. Such structural changes may indicate metabolic change brought about by experimental manipulation but which are lost with standard data analysis methods. These BMRs can be analysed by the INdividual Differences SCALing (INDSCAL) method. First the BMR quantification is described and subsequently the INDSCAL method. Finally, two studies illustrate the power and the applicability of BMRs in metabolomics. The first study is about the induced plant response of cabbage to herbivory, of which BMRs are a considerable part. In the second study-a human nutritional intervention study of green tea extract-standard data analysis tools did not reveal any metabolic change, although the BMRs were considerably affected. The presented results show that BMRs can be easily implemented in a wide variety of metabolomic studies. They provide a new source of information to describe biological systems in a way that fits flawlessly into the next generation of systems biology questions, dealing with personalized responses. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s11306-011-0316-1) contains supplementary material, which is available to authorized users.     

Development of amino acid transport system B0,+ in mouse blastocysts

The capacity of preimplantation mouse blastocysts to express the novel amino acid transport activity provisionally designated system B0,+ increased approximately 3-fold 1 day after administration of estrogen to their progesterone-primed, ovariectomized mothers. Nevertheless, blastocysts obtained 22-25 h after estrogen administration (implanting blastocysts) had to be incubated in vitro for about 20 min before they fully expressed their B0,+ activity. No similar increase in B0,+ activity was observed upon incubation of blastocysts obtained before estrogen administration (diapausing blastocysts). Rapid metabolic changes can be induced in the uterus by massaging it with a blunt instrument while it is receptive to implantation, and this treatment was found to increase the apparent B0,+ activity in implanting but not diapausing blastocysts. In contrast, the activity of an incompletely characterized, Na+-independent system, which accepts L-lysine as a substrate, decreased more than 2-fold when implanting blastocysts were incubated in vitro. No change in Na+-independent lysine uptake was detected during incubation of diapausing blastocysts. It is suggested that both uteri and blastocysts develop the capacity to change rapidly some of their metabolic processes near the time of implantation, and one of the processes which may be subject to rapid change in blastocysts is amino acid transport. These developmental events appear to coincide with and could be required for the decidual cell response and implantation of blastocysts in the uterus.     

典型有机固废厌氧消化微生物研究现状与发展方向

经过人工富集和驯化的兼性和严格厌氧微生物是厌氧消化工艺的核心。不同厌氧消化体系中存在的问题大多可以通过改变微生物群落的代谢活性来得到有效改善。得益于微生物组学检测技术的快速发展,对厌氧消化系统中微生物多样性的认识获得了极大的拓展,同时在微生物类群间、微生物与环境的互作关系研究方面也取得了一系列新的进展。然而,有机固废厌氧消化系统中,各种微生物以及微生物和物质的相互作用构成了更为复杂的代谢网络,所以目前对这些互作关系的解析尚不完善。本文重点关注了厌氧消化过程中的典型菌群互作关系,阐述了典型有机固废厌氧消化系统中存在的问题及微生物在其中发挥的作用,最后,立足于现有组学技术推动的微生物组研究进展,对未来有机固废厌氧消化系统微生物组的研究提出展望。     

Specific Features of Metabolism in Humans Performing a Physical Exercise Test after 7-Day Dry Immersion

Healthy male volunteers (n = 5) aged 27–42 years participated in experimental studies (7-day immersion) designed to simulate the effects of microgravity. To study the metabolic changes caused by decreased weight bearing on the musculoskeletal system and a change in the position of the body relative to the gravity vector, a 15-min load test before and after immersion was used. A wide set of biochemical parameters characterizing the state of the energy metabolism, substrate levees, and enzyme activities, as well as the blood level of hormones, was measured in the blood plasma. Multifactor analysis was used in processing the experimental data. After immersion, a significant decrease in the blood plasma activity of isocitrate dehydrogenase, creatine phosphokinase, and lactate dehydrogenase was noted, whereas the growth hormone and insulin levels exceeded the baseline values. The physical exercise test increased the differences in the metabolic status before and after 7 days of immersion. The factor analysis allowed us to reveal the most significant biochemical variables for identifying a new metabolic state of the physiological systems after exposure to short-term simulated microgravity. Changes in the creatine phosphokinase activity and the human blood plasma levels of cortisol, triglycerides, insulin, and inorganic phosphate made the most significant contributions to these differences, and the direction of biochemical shifts in response to exercise was different before and after immersion. The results obtained are indicate that energy and substrate metabolism changes in response to a decrease in weight bearing and an altered body position relative to the gravity vector and that these changes are especially pronounced when an exercise test is used.     

Three-dimensional numerical approach to investigate the substrate transport and conversion in an immobilized enzyme reactor

This numerical study evaluates the momentum and mass transfer in an immobilized enzyme reactor. The simulation is based on the solution of the three-dimensional Navier-Stokes equation and a scalar transport equation with a sink term for the transport and the conversion of substrate to product. The reactor consists of a container filled with 20 spherical enzyme carriers. Each of these carriers is covered with an active enzyme layer where the conversion takes place. To account for the biochemical activity, the sink term in the scalar transport equation is represented by a standard Michaelis-Menten approach. The simulation gives detailed information of the local substrate and product concentrations with respect to external and internal transport limitations. A major focus is set on the influence of the substrate transport velocity on the catalytic process. For reactor performance analysis the overall and the local transport processes are described by a complete set of dimensionless variables. The interaction between substrate concentration, velocity, and efficiency of the process can be studied with the help of these variables. The effect of different substrate inflow concentrations on the process can be seen in relation to velocity variations. The flow field characterization of the system makes it possible to understand fluid mechanical properties and its importance to transport processes. The distribution of fluid motion through the void volume has different properties in different parts of the reactor. This phenomenon has strong effects on the arrangement of significantly different mass transport areas as well as on process effectiveness. With the given data it is also possible to detect zones of high, low, and latent enzymatic activity and to determine whether the conversion is limited due to mass transfer or reaction resistances.     

Presence and significance of lithium in the brown shrimp, Crangon crangon (L.)

1. In female (non-ovigerous and ovigerous) brown shrimps, Crangon crangon, whole animal lithium levels are about 20% higher than in males, which indicates that lithium has a special function in the female reproductive system. 2. Decreasing lithium levels in larger males suggest that with males a substantial part of the lithium present may be passively absorbed to the exoskeleton. 3. At low temperatures internal lithium concentrations are strongly increased. In response to external salinity the internal lithium concentrations show a typical regulation pattern. The effects of temperature and salinity clarify that lithium must be involved in metabolic processes. 4. Apart from a special function in the reproductive system, lithium can probably replace other ions in their function, e.g. in regulating the activity of enzymatic processes.     

Optimization of bioreactor using metabolic control analysis approach

A new methodology based on a metabolic control analysis (MCA) approach is developed for the optimization of continuous cascade bioreactor system. A general framework for representation of a cascade bioreactor system consisting of a large number of reactors as a single network is proposed. The kinetic and transport processes occurring in the system are represented as a reaction network with appropriate stoichiometry. Such representation of the bioreactor systems makes it amenable to the direct application of the MCA approach. The process sensitivity information is extracted using MCA methodology in the form of flux and concentration control coefficients. The process sensitivity information is shown to be a useful guide for determining the choice of decision variables for the purpose of optimization. A generalized problem of optimization of the bioreactor is formulated in which the decision variables are the operating conditions and kinetic parameters. The gradient of the objective function to be maximized with respect to all decision variables is obtained in the form of response coefficients. This gradient information can be used in any gradient-based optimization algorithm. The efficiency of the proposed technique is demonstrated with two examples taken from literature: biotransformation of crotonobetaine and alcohol fermentation in cascade bioreactor system.     

Leptin and metabolic control of reproduction

Leptin treatment prevents the effects of fasting on reproductive processes in a variety of species. The mechanisms that underlie these effects have not been elucidated. Progress in this area of research might be facilitated by viewing reproductive processes in relation to mechanisms that maintain fuel homeostasis. Reproduction, food intake, and fuel partitioning can be viewed as homeostatic responses controlled by a sensory system that monitors metabolic signals. These signals are generated by changes in intracellular metabolic fuel availability and oxidation rather than by changes in the amount of body fat or by changes in any aspect of body composition. Leptin might be viewed as either a mediator or as a modulator of the intracellular metabolic signal. Consistent with its purported action as a mediator of the metabolic signal, leptin synthesis and secretion are influenced acutely by changes in metabolic fuel availability, and these changes might lead to changes in reproductive function. The effects of leptin treatment on reproduction are blocked by treatments that inhibit intracellular fuel oxidation. Metabolic signals that inhibit reproduction in leptin-treated animals might act via neural pathways that are independent of leptin's action. Alternatively, both leptin and metabolic inhibitors might interact at the level of intracellular fuel oxidation. In keeping with the possibility that leptin modulates the metabolic signal, leptin treatment increases fuel availability, uptake, and oxidation in particular tissues. Leptin might affect reproduction indirectly by altering fuel oxidation or other peripheral processes such as gastric emptying. Reproductive processes are among the most energetically expensive in the female repertoire. Because leptin increases energy expenditure while simultaneously inhibiting energy intake, it may have limited use as a long-term treatment for infertility.     

Design of large metabolic responses. Constraints and sensitivity analysis

Metabolic control analysis (Kacser & Burns (1973). Symp. Soc. Exp. Biol.27, 65-104; Heinrich & Rapoport (1974). Eur. J. Biochem.42, 89-95) has been extensively used to describe the response of metabolic concentrations and fluxes to small (infinitesimal) changes in enzyme concentrations and effectors. Similarly, metabolic control design (Acerenza (1993). J. theor. Biol.165, 63-85) has been proposed to design small metabolic responses. These approaches have the limitation that they were not devised to deal with large (non-infinitesimal) responses. Here we develop a strategy to design large changes in the metabolic variables. The only assumption made is that, for all the parameter values under consideration, the system has a unique stable steady state. The procedure renders the kinetic parameters of the rate equations that when embedded in the metabolic network produce the pattern of large changes in the steady-state variables that we aim to design. Structural and kinetic constraints impose restrictions on the type of responses that could be designed. We show that these conditions can be transformed into the language of mean-sensitivity coefficients and, as a consequence, a sensitivity analysis of large metabolic responses can be performed after the system has been designed. The mean-sensitivity coefficients fulfil conservation and summation relationships that in the limit reduce to the well-known theorems for infinitesimal changes. Finally, it is shown that the same procedure that was used to design metabolic responses and analyse their sensitivity properties can also be used to determine the values of kinetic parameters of the rate laws operating "in situ".     

Time hierarchy, equilibrium and non-equilibrium in metabolic systems.

A metabolic system consists of cooperating biochemical reactions. The motion is described by differential equations in the metabolites. The right-hand sides of these equations are linear combinations of the velocities of the individual reactions. These velocities depend in a non-linear manner on the metabolite concentrations (according to the law of mass action). A characteristic "metabolic" time may be defined for the motion of the whole system. It scales the essential metabolic events whose evolution time is comparable to this metabolite time unit. The constituent reactions of the metabolic system have an individual characteristic time which need not coincide with the general metabolic time. The individual time characterises the approach to the individual equilibrium of the isolated undisturbed reaction. According to the ratio of these two time scales, a single reaction may be fast, or slow, or essential, as compared with the metabolic events. Characteristic time of a single reaction and its steady-state deviation from equilibrium are closely related. It can be shown that the relative deviation from equilibrium of a reaction within the metabolic network is of the same numerical order as the ratio between individual time to metabolic time. The interaction of many reactions with different characteristic times introduces a time hierarchy into the system. This can be made transparent by appropriate scaling and by linear transformation of the system. The subsystem of fast cooperating reactions (dehydrogenases, phosphotransferases) attains a state which is near to the individual equilibrium and reestablishes this state after perturbation. The equilibration is fast; an ultrarapid phase of cofactor equilibrium can be distinguished from the fast phase of substrate equilibrium (exchange of metabolic material between different pathways). During the slower metabolic phase these near-equilibria manifest themselves as stoichiometric linkage between unrelated metabolites. The latter cease to be independent variables and combine to metabolic pools. It can be strictly shown that the essential variables at the metabolic time scale are carrier pools and the degree of occupancy of these carriers by metabolic groups. Chemically different types of carrier pools may be functionally linked together by fast reactions. A consequence of such an arrangement of reactions are distance effects: Changes at one end of a metabolic map may be directly conveyed to other pathways via stoichiometric linkage brought about by fast equilibration of cofactor reactions.