Modular regulation analysis of heart contraction: application to in situ demonstration of a direct mitochondrial activation by calcium in beating heart

Integrative Methods for Studying Cardiac Energetics

The more recent studies of human pathologies have essentially revealed the complexity of the interactions involved at the different levels of integration in organ physiology. Integrated organ thus reveals functional properties not predictable by underlying molecular events. It is therefore obvious that current fine molecular analyses of pathologies should be fruitfully combined with integrative approaches of whole organ function. It follows that an important issue in the comprehension of the link between molecular events in pathologies and whole organ function/dysfunction is the development of new experimental strategies aimed at the study of the integrated organ physiology. Cardiovascular diseases are a good example as heart submitted to ischemic conditions has to cope both with a decreased supply of nutrients and oxygen, and the necessary increased activity required to sustain whole body-including the heart itself-oxygenation.By combining the principles of control analysis with noninvasive ³¹P NMR measurement of the energetic intermediates and simultaneous measurement of heart contractile activity, we developed MoCA (for Modular Control and regulation Analysis), an integrative approach designed to study in situ control and regulation of cardiac energetics during contraction in intact beating perfused isolated heart (Diolez et al., Am J Physiol Regul Integr Comp Physiol 293(1):R13-R19, 2007). Because it gives real access to integrated organ function, MoCA brings out a new type of information-the "elasticities," referring to integrated internal responses to metabolic changes-that may be a key to the understanding of the processes involved in pathologies. MoCA can potentially be used not only to detect the origin of the defects associated with the pathology, but also to provide the quantitative description of the routes by which these defects-or also drugs-modulate global heart function, therefore opening therapeutic perspectives. This review presents selected examples of the applications to isolated intact beating heart that evidence different modes of energetic regulation of cardiac contraction. We also discuss the clinical application by using noninvasive ³¹P cardiac energetics examination under clinical conditions for detection of heart pathologies.

A simple model of cardiac mitochondrial respiration with experimental validation

Cardiac mitochondria are intracellular organelles that play an important role in energy metabolism and cellular calcium regulation. In particular, they influence the excitation-contraction cycle of the heart cell. A large number of mathematical models have been proposed to better understand the mitochondrial dynamics, but they generally show a high level of complexity, and their parameters are very hard to fit to experimental data. We derived a model based on historical free energy-transduction principles, and results from the literature. We proposed simple expressions that allow to reduce the number of parameters to a minimum with respect to the mitochondrial behavior of interest for us. The resulting model has thirty-two parameters, which are reduced to twenty-three after a global sensitivity analysis of its expressions based on Sobol indices. We calibrated our model to experimental data that consists of measurements of mitochondrial respiration rates controlled by external ADP additions. A sensitivity analysis of the respiration rates showed that only seven parameters can be identified using these observations. We calibrated them using a genetic algorithm, with five experimental data sets. At last, we used the calibration results to verify the ability of the model to accurately predict the values of a sixth dataset. Results show that our model is able to reproduce both respiration rates of mitochondria and transitions between those states, with very low variability of the parameters between each experiment. The same methodology may apply to recover all the parameters of the model, if corresponding experimental data were available.

Energy Deregulation Precedes Alteration in Heart Energy Balance in Young Spontaneously Hypertensive Rats: A Non Invasive In Vivo31P-MR Spectroscopy Follow-Up Study

Introduction

Gradual alterations in cardiac energy balance, as assessed by the myocardial PCr/ATP-ratio, are frequently associated with the development of cardiac disease. Despite great interest for the follow-up of myocardial PCr and ATP content, cardiac MR-spectroscopy in rat models in vivo is challenged by sensitivity issues and cross-contamination from other organs.

Methods

Here we combined MR-Imaging and MR-Spectroscopy (Bruker BioSpec 9.4T) to follow-up for the first time in vivo the cardiac energy balance in the SHR, a genetic rat model of cardiac hypertrophy known to develop early disturbances in cytosolic calcium dynamics.

Results

We obtained consistent ³¹P-spectra with high signal/noise ratio from the left ventricle in vivo by using a double-tuned (³¹P/¹H) surface coil. Reasonable acquisition time (<3.2min) allowed assessing the PCr/ATP-ratio comparatively in SHR and age-matched control rats (WKY): i) weekly from 12 to 21 weeks of age; ii) in response to a bolus injection of the ß-adrenoreceptor agonist isoproterenol at age 21 weeks.

Discussion

Along weeks, the cardiac PCr/ATP-ratio was highly reproducible, steady and similar (2.35±0.06) in SHR and WKY, in spite of detectable ventricular hypertrophy in SHR.

At the age 21 weeks, PCr/ATP dropped more markedly (-17.1%±0.8% vs. -3,5%±1.4%, P<0.001) after isoproterenol injection in SHR and recovered slowly thereafter (time constant 21.2min vs. 6.6min, P<0.05) despite similar profiles of tachycardia among rats.

Conclusion

The exacerbated PCr/ATP drop under ß-adrenergic stimulation indicates a defect in cardiac energy regulation possibly due to calcium-mediated abnormalities in the SHR heart. Of note, defects in energy regulation were present before detectable abnormalities in cardiac energy balance at rest.

Evaluation of in vivo mitochondrial bioenergetics in skeletal muscle using NMR and optical methods

It is now clear that mitochondria are involved as either a cause or consequence of many chronic diseases. This central role of the mitochondria is due to their position in the cell as important integrators of cellular energetics and signaling. Mitochondrial function affects many aspects of the cellular environment such as redox homeostasis and calcium signaling, which then also exert control over mitochondrial function. This complex dynamic between mitochondrial function and the cellular environment highlights the value of examining mitochondria in vivo in the intact physiological environment. This review discusses NMR and optical approaches used to measure mitochondria ATP and oxygen fluxes that provide in vivo measures of mitochondrial capacity and quality in animal and human models. Combining these in vivo measurements with more traditional ex vivo analyses can lead to new insights into the importance of the cellular environment in controlling mitochondrial function under pathological conditions. Interpretation and underlying assumptions for each technique are discussed with the goal of providing an overview of some of the most common approaches used to measure in vivo mitochondrial function encountered in the literature.

Magnetic resonance-compatible model of isolated working heart from large animal for multimodal assessment of cardiac function, electrophysiology, and metabolism

To provide a model close to the human heart, and to study intrinsic cardiac function at the same time as electromechanical coupling, we developed a magnetic resonance (MR)-compatible setup of isolated working perfused pig hearts. Hearts from pigs (40 kg, n = 20) and sheep (n = 1) were blood perfused ex vivo in the working mode with and without loaded right ventricle (RV), for 80 min. Cardiac function was assessed by measuring left intraventricular pressure and left ventricular (LV) ejection fraction (LVEF), aortic and mitral valve dynamics, and native T1 mapping with MR imaging (1.5 Tesla). Potential myocardial alterations were assessed at the end of ex vivo perfusion from late-Gadolinium enhancement T1 mapping. The ex vivo cardiac function was stable across the 80 min of perfusion. Aortic flow and LV-dP/dtmin were significantly higher (P < 0.05) in hearts perfused with loaded RV, without differences for heart rate, maximal and minimal LV pressure, LV-dP/dtmax, LVEF, and kinetics of aortic and mitral valves. T1 mapping analysis showed a spatially homogeneous distribution over the LV. Simultaneous recording of hemodynamics, LVEF, and local cardiac electrophysiological signals were then successfully performed at baseline and during electrical pacing protocols without inducing alteration of MR images. Finally, (31)P nuclear MR spectroscopy (9.4 T) was also performed in two pig hearts, showing phosphocreatine-to-ATP ratio in accordance with data previously reported in vivo. We demonstrate the feasibility to perfuse isolated pig hearts in the working mode, inside an MR environment, allowing simultaneous assessment of cardiac structure, mechanics, and electrophysiology, illustrating examples of potential applications.

Hypothesis on Skeletal Muscle Aging: Mitochondrial Adenine Nucleotide Translocator Decreases Reactive Oxygen Species Production While Preserving Coupling Efficiency

Mitochondrial membrane potential is the major regulator of mitochondrial functions, including coupling efficiency and production of reactive oxygen species (ROS). Both functions are crucial for cell bioenergetics. We previously presented evidences for a specific modulation of adenine nucleotide translocase (ANT) appearing during aging that results in a decrease in membrane potential - and therefore ROS production-but surprisingly increases coupling efficiency under conditions of low ATP turnover. Careful study of the bioenergetic parameters (oxidation and phosphorylation rates, membrane potential) of isolated mitochondria from skeletal muscles (gastrocnemius) of aged and young rats revealed a remodeling at the level of the phosphorylation system, in the absence of alteration of the inner mitochondrial membrane (uncoupling) or respiratory chain complexes regulation. We further observed a decrease in mitochondrial affinity for ADP in aged isolated mitochondria, and higher sensitivity of ANT to its specific inhibitor atractyloside. This age-induced modification of ANT results in an increase in the ADP concentration required to sustain the same ATP turnover as compared to young muscle, and therefore in a lower membrane potential under phosphorylating-in vivo-conditions. Thus, for equivalent ATP turnover (cellular ATP demand), coupling efficiency is even higher in aged muscle mitochondria, due to the down-regulation of inner membrane proton leak caused by the decrease in membrane potential. In the framework of the radical theory of aging, these modifications in ANT function may be the result of oxidative damage caused by intra mitochondrial ROS and may appear like a virtuous circle where ROS induce a mechanism that reduces their production, without causing uncoupling, and even leading in improved efficiency. Because of the importance of ROS as therapeutic targets, this new mechanism deserves further studies.

Integrative methods for studying cardiac energetics

The more recent studies of human pathologies have essentially revealed the complexity of the interactions involved at the different levels of integration in organ physiology. Integrated organ thus reveals functional properties not predictable by underlying molecular events. It is therefore obvious that current fine molecular analyses of pathologies should be fruitfully combined with integrative approaches of whole organ function. It follows an important issue in the comprehension of the link between molecular events in pathologies, and whole organ function/dysfunction is the development of new experimental strategies aimed at the study of the integrated organ physiology. Cardiovascular diseases are a good example as heart submitted to ischemic conditions has to cope both with a decreased supply of nutrients and oxygen, and the necessary increased activity required to sustain whole body-including the heart itself-oxygenation.By combining the principles of control analysis with noninvasive (31)P NMR measurement of the energetic intermediates and simultaneous measurement of heart contractile activity, we developed MoCA (for Modular Control and Regulation Analysis), an integrative approach designed to study in situ control and regulation of cardiac energetics during contraction in intact beating perfused isolated heart (Diolez et al., Am J Physiol Regul Integr Comp Physiol 293(1):R13-R19, 2007). Because it gives real access to integrated organ function, MoCA brings out a new type of information-the "elasticities," referring to internal responses to metabolic changes-that may be a key to the understanding of the processes involved in pathologies. MoCA can potentially be used not only to detect the origin of the defects associated with the pathology, but also to provide the quantitative description of the routes by which these defects-or also drugs-modulate global heart function, therefore opening therapeutic perspectives. This review presents selected examples of the applications to isolated intact beating heart and a wider application to cardiac energetics under clinical conditions with the direct study of heart pathologies.

Effects of pyruvate on the energetics of rat ventricles stunned by ischemia–reperfusion

Pyruvate (Pyr) was proposed as an additive to cold high-K+–low-Ca2+ cardioplegia (CPG) to protect the heart during surgery. We explored whether Pyr and CPG would work synergistically to protect rat hearts from stunning during ischemia–reperfusion (I/R). We measured the heat release and contractility of perfused ventricles during I/R, and the cytosolic and mitochondrial [Ca2+] in cardiomyocytes by confocal microscopy. We found that under cold-CPG (30 °C), 10 mmol·L−1 Pyr reduced the post-ischemic contractile recovery (PICR) as well as muscle economy, when added either before ischemia or during I/R, which was reversed by blockade of UCam. In noncardioplegic hearts, Pyr was cardioprotective when it was present during I/R, more so at 37 °C than at 30 °C, with improved economy. In cardiomyocytes, the addition of Pyr to CPG slightly increased the mitochondrial [Ca2+] but decreased cytosolic [Ca2+]. The results suggest that Pyr only protects hearts from stunning when present before ischemia and during reperfusion, and that it dampens the cardioprotective properties of CPG. The mechanisms underlying such different behavior depend on the dynamic balance between Pyr stimulation of the energetic state and mitochondrial Ca2+ uptake. Our results support the use of Pyr in stunned hearts, but not in cold high-K+ cardioplegia.

Mitochondrial energetics is impaired in vivo in aged skeletal muscle

With aging, most skeletal muscles undergo a progressive loss of mass and strength, a process termed sarcopenia. Aging-related defects in mitochondrial energetics have been proposed to be causally involved in sarcopenia. However, changes in muscle mitochondrial oxidative phosphorylation with aging remain a highly controversial issue, creating a pressing need for integrative approaches to determine whether mitochondrial bioenergetics are impaired in aged skeletal muscle. To address this issue, mitochondrial bioenergetics was first investigated in vivo in the gastrocnemius muscle of adult (6 months) and aged (21 months) male Wistar rats by combining a modular control analysis approach with ³¹P magnetic resonance spectroscopy measurements of energetic metabolites. Using this innovative approach, we revealed that the in vivo responsiveness (‘elasticity’) of mitochondrial oxidative phosphorylation to contraction-induced increase in ATP demand is significantly reduced in aged skeletal muscle, a reduction especially pronounced under low contractile activities. In line with this in vivo aging-related defect in mitochondrial energetics, we found that the mitochondrial affinity for ADP is significantly decreased in mitochondria isolated from aged skeletal muscle. Collectively, the results of this study demonstrate that mitochondrial bioenergetics are effectively altered in vivo in aged skeletal muscle and provide a novel cellular basis for this phenomenon.

Mitochondrial network energetics in the heart

At the core of eukaryotic aerobic life, mitochondrial function like 'hubs' in the web of energetic and redox processes in cells. In the heart, these networks-extending beyond the complex connectivity of biochemical circuit diagrams and apparent morphology-exhibit collective dynamics spanning several spatiotemporal levels of organization, from the cell, to the tissue, and the organ. The network function of mitochondria, i.e., mitochondrial network energetics, represents an advantageous behavior. Its coordinated action, under normal physiology, provides robustness despite failure in a few nodes, and improves energy supply toward a swiftly changing demand. Extensive diffuse loops, encompassing mitochondrial-cytoplasmic reaction/transport networks, control and regulate energy supply and demand in the heart. Under severe energy crises, the network behavior of mitochondria and associated glycolytic and other metabolic networks collapse, thereby triggering fatal arrhythmias.

Non-invasive integrative analysis of contraction energetics in intact beating heart

The comprehensive study of human pathologies has revealed the complexity of the interactions involved in cardiovascular physiology. The recent validation of system's biology approaches - like our Modular Control and Regulation Analysis (MoCA) - motivates the current interest for new integrative and non-invasive analyses that could be used for medical study of human heart contraction energetics. By considering heart energetics as a supply-demand system, MoCA gives access to integrated organ function and brings out a new type of information, the "elasticities", which describe in situ the regulation of both energy demand and supply by cellular energetic status. These regulations determine the internal control of contraction energetics and may therefore be a key to the understanding of the links between molecular events in pathologies and whole organ function/dysfunction. A wider application to the effects of cardiac drugs in conjunction with the direct study of heart pathologies may be considered in the near future. MoCA can potentially be used not only to detect the origin of the defects associated with the pathology (elasticity analyses), but also to provide a quantitative description of how these defects influence global heart function (regulation analysis) and therefore open new therapeutic perspectives. Several key examples of current applications to intact isolated beating heart are presented in this paper. The future application to human pathologies will require the use of non-invasive NMR techniques for the simultaneous measurement of energy status ((31)P NMR) and heart contractile activity (3D MRI). This article is part of a Directed Issue entitled: Bioenergetic dysfunction, adaptation and therapy.

Acute and chronic effects of bupivacaine on muscle energetics during contraction in vivo: a modular metabolic control analysis

Bupivacaine is a widely used anaesthetic injected locally in clinical practice for short-term neurotransmission blockade. However, persistent side effects on mitochondrial integrity have been demonstrated in muscle parts surrounding the injection site. We use the precise language of metabolic control analysis in the present study to describe in vivo consequences of bupivacaine injection on muscle energetics during contraction. We define a model system of muscle energy metabolism in rats with a sciatic nerve catheter that consists of two modules of reactions, ATP/PCr (phosphocreatine) supply and ATP/PCr demand, linked by the common intermediate PCr detected in vivo by 31P-MRS (magnetic resonance spectroscopy). Measured system variables were [PCr] (intermediate) and contraction (flux). We first applied regulation analysis to quantify acute effects of bupivacaine. After bupivacaine injection, contraction decreased by 15.7% and, concomitantly, [PCr] increased by 11.2%. The regulation analysis quantified that demand was in fact directly inhibited by bupivacaine (−21.3%), causing an increase in PCr. This increase in PCr indirectly reduced mitochondrial activity (−22.4%). Globally, the decrease in contractions was almost fully explained by inhibition of demand (−17.0%) without significant effect through energy supply. Finally we applied elasticity analysis to quantify chronic effects of bupivacaine iterative injections. The absence of a difference in elasticities obtained in treated rats when compared with healthy control rats clearly shows the absence of dysfunction in energetic control of muscle contraction energetics. The present study constitutes the first and direct evidence that bupivacaine myotoxicity is compromised by other factors during contraction in vivo, and illustrates the interest of modular approaches to appreciate simple rules governing bioenergetic systems when affected by drugs.

A fast black-blood sequence for four-dimensional cardiac manganese-enhanced MRI in mouse

The increasing number of mouse models of cardiac diseases requires improvements in the current MRI tools. Anatomic and functional cardiac phenotyping by MRI calls for both time and space resolution in three dimensions. Black-blood contrast is often needed for the accurate delineation of myocardium and chambers, and is consistent with manganese contrast enhancement. In this article, we propose a fast, three-dimensional, time-resolved (four-dimensional), black-blood MRI sequence that allows mouse heart imaging at 10 periods of the cardiac cycle within 30  min at an isotropic resolution of 200  µm. Two-dimensional imaging was possible within 80  s. Blood cancellation was achieved by employing bipolar gradients without the use of a double inversion recovery preparation scheme. Saturation slices were added in two-dimensional experiments for better blood nulling. The rapidity of the two-dimensional acquisition protocol allowed the measurement of the time course of contrast enhancement on manganese infusion. Owing to the very high contrast-to-noise ratio, manganese-enhanced MRI in four dimensions made possible the accurate assessment of regional cardiac volumes in healthy animals. In experimentally infarcted mice, the size of the ischemic zone could be measured easily with this method. The technique might be valuable in evaluating mouse heart diseases and their follow-up in longitudinal studies.

System analysis of the effect of various drugs on cardiac contraction energetics

We used MoCA (Modular Control and Regulation Analysis) to demonstrate in intact beating rat heart that physiological activation of contraction by adrenaline involves the almost perfect parallel activation of both mitochondria and myofibrils by intracellular Ca(2+). This explains the perfect homoeostasis of the energetic intermediate PCr (phosphocreatine) in heart. When using drugs specifically stimulating either supply or demand activities, MoCA helped reveal the very specific mode of regulation of heart contraction energetics. Only activation of myofibrils activity (demand), either by increasing intracellular Ca(2+) concentration or myofibrils sensitivity to Ca(2+), triggers activation of contractile activity. In contrast, the activation of mitochondrial activity (supply) has strictly no effect on contraction, either directly or through PCr changes (intermediate).

Absence of mitochondrial activation during levosimendan inotropic action in perfused paced guinea pig hearts as demonstrated by modular control analysis

Levosimendan is a calcium sensitizer developed for the treatment of heart failure. It increases contractile force by enhancing the sensitivity of myofilaments to calcium. Besides this sensitizing effect, the drug has also been reported to show some inhibitory action on phosphodiesterase 3 (PDE3). The inotropic effects of levosimendan have been studied on guinea pig paced perfused hearts by using modular control analysis (MoCA) (Diolez P, Deschodt-Arsac V, Raffard G, Simon C, Santos PD, Thiaudiere E, Arsac L, Franconi JM. Am J Physiol Regul Integr Comp Physiol 293: R13-R19, 2007.), an integrative approach of heart energetics using noninvasive (31)P NMR. The aim was to evaluate quantitatively the respective effects of this drug on energy supply and demand modules. Under our experimental conditions, 0.7 muM levosimendan induced a 45% increase in paced heart output associated with a 7% decrease in phosphocreatine and a negligible increase in oxygen consumption. Because MoCA allows in situ study of the internal regulations in intact beating heart energetics, it was applied to describe quantitatively by which routes levosimendan exerts its inotropic action. MoCA demonstrated the absence of any significant effect of the drug on the supply module, which is responsible for the lower increase in oxygen consumption, compared with epinephrine, which increases the ratio between myocardial oxygen consumption and cardiac contraction. This result evidences that, under our conditions, a possible effect of levosimendan on PDE3 activity and/or intracellular calcium remains very low on mitochondrial activity and insignificant on integrated cardiac energetics. Thus, levosimendan inotropic effect on guinea pig heart depends almost entirely on the calcium-sensitizing properties leading to myofilament activation and the concomitant activation of energy supply by the decrease in PCr, therefore improving energetic efficiency of contraction.

Tissue Doppler image-derived measurements during isovolumic contraction predict exercise capacity in patients with reduced left ventricular ejection fraction

OBJECTIVES

We explored the incremental value of quantification of tissue Doppler (TD) velocity during the brief isovolumic contraction (IVC) phase of the cardiac cycle for the prediction of exercise performance in patients referred for cardiopulmonary exercise testing (CPET).

BACKGROUND

Experimental studies have shown that rapid left ventricular (LV) shape change during IVC is essential for optimal onset of LV ejection. However, the incremental value of measuring IVC velocities in clinical settings remains unclear.

METHODS

A total of 82 subjects (age 53+/-14 years, 56 men) were studied with echocardiography and CPET. Reduced LV ejection fraction (EF) (EF<50%) was present in 38 (46%) subjects. Pulsed-wave annular TD velocities were averaged from the LV lateral and septal annulus during isovolumic contraction (IVCa), ejection, isovolumic relaxation, and early and late diastole (Aa) and compared with peak oxygen consumption (VO2) and percentage of the predicted peak VO2 (% predicted peak VO2) obtained from CPET.

RESULTS

Patients with reduced EF had lower IVCa (6.3 vs. 4.5 cm/s, p=0.04), ejection (7.7 vs. 5.5 cm/s, p<0.001), and Aa velocities (7.9 vs. 6.6 cm/s, p=0.04). Similarly, % predicted peak VO2 was lower in patients with reduced EF (52.9% vs. 73.1%, p<0.001) and correlated with the variations in IVCa (r=0.7, p=0.001). Multivariate analysis of 2-dimensional and Doppler variables in the presence of reduced LV EF revealed only IVCa and Aa as independent predictors of % predicted peak VO2 (r2=0.612, p=0.02 for IVCa and p=0.009 for Aa). The overall performance of IVCa in the prediction of exercise capacity was good (area under the curve=0.86, p<0.001).

CONCLUSIONS

Assessment of TD-derived IVC and atrial stretch velocities provide independent prediction of exercise capacity in patients with reduced LV EF. Assessment of LV pre-ejectional stretch and shortening mechanics at rest may be useful for determining the myocardial functional reserve of patients with reduced EF.

Improved energy supply regulation in chronic hypoxic mouse counteracts hypoxia-induced altered cardiac energetics

BACKGROUND

Hypoxic states of the cardiovacular system are undoubtedly associated with the most frequent diseases of modern time. Therefore, understanding hypoxic resistance encountered after physiological adaptation such as chronic hypoxia, is crucial to better deal with hypoxic insult. In this study, we examine the role of energetic modifications induced by chronic hypoxia (CH) in the higher tolerance to oxygen deprivation.

METHODOLOGY/PRINCIPAL FINDINGS

Swiss mice were exposed to a simulated altitude of 5500 m in a barochamber for 21 days. Isolated perfused hearts were used to study the effects of a decreased oxygen concentration in the perfusate on contractile performance (RPP) and phosphocreatine (PCr) concentration (assessed by (31)P-NMR), and to describe the integrated changes in cardiac energetics regulation by using Modular Control Analysis (MoCA). Oxygen reduction induced a concomitant decrease in RPP (-46%) and in [PCr] (-23%) in Control hearts while CH hearts energetics was unchanged. MoCA demonstrated that this adaptation to hypoxia is the direct consequence of the higher responsiveness (elasticity) of ATP production of CH hearts compared with Controls (-1.88+/-0.38 vs -0.89+/-0.41, p<0.01) measured under low oxygen perfusion. This higher elasticity induces an improved response of energy supply to cellular energy demand. The result is the conservation of a healthy control pattern of contraction in CH hearts, whereas Control hearts are severely controlled by energy supply.

CONCLUSIONS/SIGNIFICANCE

As suggested by the present study, the mechanisms responsible for this increase in elasticity and the consequent improved ability of CH heart metabolism to respond to oxygen deprivation could participate to limit the damages induced by hypoxia.

Modular regulation analysis of integrative effects of hypoxia on the energetics of contracting skeletal muscle in vivo

In the exercising muscle, acute reduction in ambient oxygen impairs muscle contraction because of the effects of hypoxia on mitochondrial ATP supply. The less marked impairment reported after long-term exposure to hypoxia points to changes in the regulation of the energetic system of contraction in HC (hypoxic conditioned) animals. This energetic system is conceptually defined here as two modules: the ATP/PCr (phosphocreatine)-producer and the ATP/PCr-consumer connected by energetic intermediates. Modular control analysis that combines top-down control analysis with non-invasive 31P-NMR spectroscopy was used to describe the effects of hypoxia on each module and their adaptation. Modulations of steady levels of ATP turnover (indirectly assessed as force output) and muscle PCr were obtained in HC rats (6 weeks at 10.5% O2) compared with N (normoxic) rats. Modular control and regulation analyses quantified the elasticity to PCr of each module in N and HC rats as well as the direct effect of acute hypoxia on the ATP/PCr-producer module. Similar elasticities in N and HC rats indicate the absence of response to long-term hypoxia in internal regulations of the ATP supply and demand pathways. The less marked impairment of contraction by acute hypoxia in HC rats (-9+/-6% versus -17+/-14% in N rats, P<0.05) was therefore fully explained by a lower direct effect (HC -31+/-13% versus N -44+/-23%, P<0.05) of acute hypoxia on mitochondrial ATP supply. This points to a positive adaptation to chronic hypoxia. Modular control analysis in vivo may provide powerful tools to find out improved function (alternatively dysfunction) at the system level in conditioned animals.

In vivo modular control analysis of energy metabolism in contracting skeletal muscle

We used (31)P MRS (magnetic resonance spectroscopy) measurements of energetic intermediates [ATP, P(i) and PCr (phosphocreatine)] in combination with the analytical tools of metabolic control analysis to study in vivo energy metabolism in the contracting skeletal muscle of anaesthetized rats over a broad range of workload. According to our recent MoCA (modular control analysis) used to describe regulatory mechanisms in beating heart, we defined the energetic system of muscle contraction as two modules (PCr-Producer and PCr-Consumer) connected by the energetic intermediates. Hypoxia and electrical stimulation were used in this in vivo study as reasonably selective modulations of Producer and Consumer respectively. As quantified by elasticity coefficients, the sensitivities of each module to PCr determine the control of steady-state contractile activity and metabolite concentrations. The magnitude of the elasticity of the producer was high (4.3+/-0.6) at low workloads and decreased 5-fold (to 0.9+/-0.2) at high workloads. By contrast, the elasticity of the consumer remained low (0.5-1.2) over the range of metabolic rates studied. The control exerted by each module over contraction was calculated from these elasticities. The control of contraction was found on the consumer at low workloads and then swung to the producer, due to the workload-dependent decrease in the elasticity of producer. The workload-dependent elasticity and control pattern of energy production in muscle is a major difference from heart. Since module rate and elasticity depend on the concentrations of substrates and products, the absence of homoeostasis of the energetic intermediates in muscle, by contrast with heart, is probably the origin of the workload-dependent elasticity of the producer module.

Modular control analysis of effects of chronic hypoxia on mouse heart

Modular control analysis (MoCA; Diolez P, Deschodt-Arsac V, Raffard G, Simon C, Santos PD, Thiaudiere E, Arsac L, Franconi JM. Am J Physiol Regul Integr Comp Physiol 293: R13-R19, 2007) was applied here on perfused hearts to describe the modifications of the regulation of heart energetics induced in mice exposed to 3-wk chronic hypoxia. MoCA combines 31P-NMR spectroscopy and modular (top down) control analysis to describe the integrative regulation of energy metabolism in the intact beating heart, on the basis of two modules [ATP/phosphocreatine (PCr) production and ATP/PCr consumption] connected by the energetic intermediates. In contrast with previous results in rat heart, in which all control of contraction was on ATP demand, mouse heart energetics presented a shared control of contraction between ATP/PCr-producing and -consuming modules. In chronic hypoxic mice, the decrease in heart contractile activity and PCr-to-ATP ratio was surprisingly associated with an important and significant higher response of ATP/PCr production (elasticity) to PCr changes compared with control hearts (-10.4 vs. -2.46). By contrast, no changes were observed in ATP/PCr consumption since comparable elasticities were observed. Since elasticities determine the regulation of energetics of heart contraction, the present results show that this new parameter may be used to uncover the origin of the observed dysfunctions under chronic hypoxia conditions. Considering the decrease in mitochondrial content reported after exposure to chronic hypoxia, it appears that the improvement of ATP/PCr production response to ATP demand may be viewed as a positive adaptative mechanism. It now appears crucial to understand the very processes responsible for ATP/PCr producer elasticity toward the energetic intermediates, as well as their regulation.

Physiological heart activation by adrenaline involves parallel activation of ATP usage and supply

During low-to-high work transition in adult mammalian heart in vivo the concentrations of free ADP, ATP, PCr (phosphocreatine), P(i) and NADH are essentially constant, in striking contrast with skeletal muscle. The direct activation by calcium ions of ATP usage and feedback activation of ATP production by ADP (and P(i)) alone cannot explain this perfect homoeostasis. A comparison of the response to adrenaline (increase in rate-pressure product and [PCr]) of the intact beating perfused rat heart with the elasticities of the PCr producer and consumer to PCr concentration demonstrated that both the ATP/PCr-producing block and ATP/PCr-consuming block are directly activated to a similar extent during physiological heart activation. Our finding constitutes a direct evidence for the parallel-activation mechanism of the regulation of oxidative phosphorylation in heart postulated previously in a theoretical way.