Nicola Lai

Modeling oxygenation in venous blood and skeletal muscle in response to exercise using near-infrared spectroscopy

Noninvasive, continuous measurements in vivo are commonly used to make inferences about mechanisms controlling internal and external respiration during exercise. In particular, the dynamic response of muscle oxygenation (Sm(O(2))) measured by near-infrared spectroscopy (NIRS) is assumed to be correlated to that of venous oxygen saturation (Sv(O(2))) measured invasively. However, there are situations where the dynamics of Sm(O(2)) and Sv(O(2)) do not follow the same pattern. A quantitative analysis of venous and muscle oxygenation dynamics during exercise is necessary to explain the links between different patterns observed experimentally. For this purpose, a mathematical model of oxygen transport and utilization that accounts for the relative contribution of hemoglobin (Hb) and myoglobin (Mb) to the NIRS signal was developed. This model includes changes in microvascular composition within skeletal muscle during exercise and integrates experimental data in a consistent and mechanistic manner. Three subjects (age 25.6 +/- 0.6 yr) performed square-wave moderate exercise on a cycle ergometer under normoxic and hypoxic conditions while muscle oxygenation (C(oxy)) and deoxygenation (C(deoxy)) were measured by NIRS. Under normoxia, the oxygenated Hb/Mb concentration (C(oxy)) drops rapidly at the onset of exercise and then increases monotonically. Under hypoxia, C(oxy) decreases exponentially to a steady state within approximately 2 min. In contrast, model simulations of venous oxygen concentration show an exponential decrease under both conditions due to the imbalance between oxygen delivery and consumption at the onset of exercise. Also, model simulations that distinguish the dynamic responses of oxy-and deoxygenated Hb (HbO(2), HHb) and Mb (MbO(2), HMb) concentrations (C(oxy) = HbO(2) + MbO(2); C(deoxy) = HHb + HMb) show that Hb and Mb contributions to the NIRS signal are comparable. Analysis of NIRS signal components during exercise with a mechanistic model of oxygen transport and metabolism indicates that changes in oxygenated Hb and Mb are responsible for different patterns of Sm(O(2)) and Sv(O(2)) dynamics observed under normoxia and hypoxia.

Exercise training decreases activation of the mitochondrial fission protein dynamin-related protein-1 in insulin-resistant human skeletal muscle

Defects in mitochondrial dynamics, the processes of fission, fusion, and mitochondrial autophagy, may contribute to metabolic disease including type 2 diabetes. Dynamin-related protein-1 (Drp1) is a GTPase protein that plays a central role in mitochondrial fission. We hypothesized that aerobic exercise training would decrease Drp1 Ser(616) phosphorylation and increase fat oxidation and insulin sensitivity in obese (body mass index: 34.6 ± 0.8 kg/m(2)) insulin-resistant adults. Seventeen subjects performed supervised exercise for 60 min/day, 5 days/wk at 80-85% of maximal heart rate for 12 wk. Insulin sensitivity was measured by hyperinsulinemic-euglycemic clamp, and fat oxidation was determined by indirect calorimetry. Skeletal muscle biopsies were obtained from the vastus lateralis muscle before and after the 12-wk program. The exercise intervention increased insulin sensitivity 2.1 ± 0.2-fold (P < 0.01) and fat oxidation 1.3 ± 0.3-fold (P < 0.01). Phosphorylation of Drp1 at Ser(616) was decreased (pre vs. post: 0.81 ± 0.15 vs. 0.58 ± 0.14 arbitrary units; P < 0.05) following the intervention. Furthermore, reductions in Drp1 Ser(616) phosphorylation were negatively correlated with increases in fat oxidation (r = -0.58; P < 0.05) and insulin sensitivity (rho = -0.52; P < 0.05). We also examined expression of genes related to mitochondrial dynamics. Dynamin1-like protein (DNM1L; P < 0.01), the gene that codes for Drp1, and Optic atrophy 1 (OPA1; P = 0.05) were significantly upregulated following the intervention, while there was a trend towards an increase in expression of both mitofusin protein MFN1 (P = 0.08) and MFN2 (P = 0.07). These are the first data to suggest that lifestyle-mediated improvements in substrate metabolism and insulin sensitivity in obese insulin-resistant adults may be regulated through decreased activation of the mitochondrial fission protein Drp1.

Functional abdominal wall reconstruction improves core physiology and quality-of-life

INTRODUCTION One of the goals of modern ventral hernia repair (VHR) is restoring the linea alba by returning the rectus muscles to the midline. Although this practice presumably restores native abdominal wall function, improvement of abdominal wall function has never been measured in a scientific fashion. We hypothesized that a dynamometer could be used to demonstrate an improvement in rectus muscle function after open VHR with restoration of the midline, and that this improvement would be associated with a better quality-of-life. METHODS Thirteen patients agreed to dynamometric analysis before and 6 months after an open posterior component separation (Rives-Stoppa technique complimented with a transversus abdominis muscle release) and mesh sublay. Analysis done using a dynamometer (Biodex 3, Corp, Shirley, NY) included measurement of peak torque (PT; N*m) and PT per bodyweight (BW; %) generated during abdominal flexion in 5 settings: Isokinetic analysis at 45°/s and 60°/s as well as isometric analysis at 0°, -15°, and +15°. Power (W) was calculated during isokinetic settings. Quality-of-life was measured using our validated HerQles survey at the time of each dynamometric analysis. RESULTS Thirteen patients (mean age, 54 ± 9 years; mean body mass index, 31 ± 7 kg/m(2)) underwent repair with restoration of the midline using the aforementioned technique. Mean hernia width was 12.5 cm (range, 5-19). Improvements in PT and PT/BW were significant in all 5 settings (P < .05). Improvement in power during isokinetic analyses at 45°/s and 60°/s was also significant (P < .05). All patients reported an improvement in quality-of-life, which was associated positively with each dynamometric parameter. CONCLUSION Restoration of the linea alba during VHR is associated with improved abdominal wall functionality. Analysis of rectus muscle function using a dynamometer showed statistical improvement by isokinetic and isometric measurements, all of which were associated with an improvement in quality-of-life.

A prior bout of contractions speeds V̇o2 and blood flow on-kinetics and reduces the V̇o2 slow-component amplitude in canine skeletal muscle contracting in situ

It was the purpose of this study to examine the effect of a priming contractile bout on oxygen uptake (VO2) on-kinetics in highly oxidative skeletal muscle. Canine gastrocnemii (n=12) were stimulated via their sciatic nerves (8 V, 0.2-ms duration, 50 Hz, 200-ms train) at a rate of 2 contractions/3 s (approximately 70% peak VO2) for two 2-min bouts, separated by 2 min of recovery. Blood flow was recorded with an ultrasonic flowmeter, and muscle oxygenation monitored via near-infrared spectroscopy. Compared with the first bout (bout 2 vs. bout 1), the VO2 primary time constant (mean+/-SD, 9.4+/-2.3 vs. 12.0+/-3.9 s) and slow-component amplitude (5.9+/-6.3 vs. 12.1+/-9.0 ml O2.kg wet wt(-1).min(-1)) were significantly reduced (P<0.05) during the second bout. Blood flow on-kinetics were significantly speeded during the second bout (time constant=7.7+/-2.6 vs. 14.8+/-5.8 s), and O2 extraction was greater at the onset of contractions (0.050+/-0.030 vs. 0.020+/-0.010 ml O2/ml blood). Kinetics of muscle deoxygenation were significantly slower at the onset of the second bout (7.2+/-2.2 vs. 4.4+/-1.2 s), while relative oxyhemoglobin concentration was elevated throughout the second bout. These results suggest that better matching of O2 delivery to VO2 speeds Vo(2) on-kinetics at this metabolic rate, but do not eliminate a potential role for enhanced metabolic activation. Additionally, altered motor unit recruitment at the onset of a second bout is not a prerequisite for reductions in the VO2 slow-component amplitude after a priming contractile bout in canine muscle in situ.

Linking pulmonary oxygen uptake, muscle oxygen utilization and cellular metabolism during exercise

The energy demand imposed by physical exercise on the components of the oxygen transport and utilization system requires a close link between cellular and external respiration in order to maintain ATP homeostasis. Invasive and non-invasive experimental approaches have been used to elucidate mechanisms regulating the balance between oxygen supply and consumption during exercise. Such approaches suggest that the mechanism controlling the various subsystems coupling internal to external respiration are part of a highly redundant and hierarchical multi-scale system. In this work, we present a “systems biology” framework that integrates experimental and theoretical approaches able to provide simultaneously reliable information on the oxygen transport and utilization processes occurring at the various steps in the pathway of oxygen from air to mitochondria, particularly at the onset of exercise. This multi-disciplinary framework provides insights into the relationship between cellular oxygen consumption derived from measurements of muscle oxygenation during exercise and pulmonary oxygen uptake by indirect calorimetry. With a validated model, muscle oxygen dynamic responses is simulated and quantitatively related to cellular metabolism under a variety of conditions.

Relating tissue/organ energy expenditure to metabolic fluxes in mouse and human: experimental data integrated with mathematical modeling

Mouse models of human diseases are used to study the metabolic and physiological processes leading to altered whole‐body energy expenditure (EE), which is the sum of EE of all body organs and tissues. Isotopic techniques, arterio‐venous difference of substrates, oxygen, and blood flow measurements can provide essential information to quantify tissue/organ EE and substrate oxidation. To complement and integrate experimental data, quantitative mathematical model analyses have been applied in the design of experiments and evaluation of metabolic fluxes. In this study, a method is presented to quantify the energy expenditure of the main mouse organs using metabolic flux measurements. The metabolic fluxes and substrate utilization of the main metabolic pathways of energy metabolism in the mouse tissue/organ systems and the whole body are quantified using a mathematical model based on mass and energy balances. The model is composed of six organ/tissue compartments: brain, heart, liver, gastrointestinal tract, muscle, and adipose tissue. Each tissue/organ is described with a distinct system of metabolic reactions. This model quantifies metabolic and energetic characteristics of mice under overnight fasting conditions. The steady‐state mass balances of metabolites and energy balances of carbohydrate and fat are integrated with available experimental data to calculate metabolic fluxes, substrate utilization, and oxygen consumption in each tissue/organ. The model serves as a paradigm for designing experiments with the minimal reliable measurements necessary to quantify tissue/organs fluxes and to quantify the contributions of tissue/organ EE to whole‐body EE that cannot be easily determined currently.

Hemoglobin and Myoglobin Contributions to Skeletal Muscle Oxygenation in Response to Exercise

The quantitative contributions of hemoglobin and myoglobin oxygenation in skeletal muscle depend on physiological factors, especially muscle blood flow (Q( m )) and capillary permeability-surface area (PS). Near-infrared spectroscopy (NIRS) can be used to quantify total heme oxidation, but it is unable to distinguish between hemoglobin and myoglobin. Therefore, a mechanistic computational model has been developed to distinguish the contributions of oxygenated hemoglobin and myoglobin to the total NIRS signal. Model simulations predict how Q( m ) and PS can affect oxygenated hemoglobin and myoglobin.Although both hemoglobin and myoglobin oxygenation decrease with impaired Q( m ), simulations show that myoglobin provides a greater contribution to the overall NIRS signal. A decrease of PS primarily affects myoglobin oxygenation. Based on model simulations, the contribution of myoglobin oxygenation to the total NIRS signal can be significantly different under pathophysiological conditions, such as diabetes and peripheral arterial disorder.

Contraction-by-contraction VO2 and computer-controlled pump perfusion as novel techniques to study skeletal muscle metabolism in situ.

The purpose of this research was to develop new techniques to 1) rapidly sample venous O(2) saturation to determine contraction-by-contraction oxygen uptake (Vo(2)), and 2) precisely control the rate and pattern of blood flow adjustment from one chosen steady state to another. An indwelling inline oximeter probe connected to an Oximetrix 3 meter was used to sample venous oxygen concentration ([O(2)]) (via fractional saturation of Hb with O(2)). Data from the Oximetrix 3 were filtered, deconvolved, and processed by a moving average second by second. Computer software and a program written in-house were used to control blood flow with a peristaltic pump. The isolated canine gastrocnemius muscle complex (GS) in situ was utilized to test these techniques. A step change in metabolic rate was elicited by stimulating GS muscles via their sciatic nerves (supramaximal voltage, 8 V; 50 Hz, 0.2-ms pulse width; train duration 200 ms) at a rate of either 1 contraction/2 s, or 2 contractions/3 s. With arterial [O(2)] maintained constant, blood flow and calculated venous [O(2)] were averaged over each contraction cycle and used in the Fick equation to calculate contraction-by-contraction Vo(2). About 5-8 times more data points were obtained with this method compared with traditional manual sampling. Software-controlled pump perfusion enabled the ability to mimic spontaneous blood flow on-kinetics (tau: 14.3 s) as well as dramatically speed (tau: 2.0 s) and slow (tau: 63.3 s) on-kinetics. These new techniques significantly improve on existing methods for mechanistically altering blood flow kinetics as well as accurately measuring muscle oxygen consumption kinetics during transitions between metabolic rates.

Multi-scale model of O2 transport and metabolism: response to exercise.

Regulation of pulmonary oxygen uptake (VO2p) during exercise depends on cellular energy demand, blood flow, ventilation, oxygen exchange across membranes, and oxygen utilization in the contracting skeletal muscle. In human and animal studies of metabolic processes that control cellular respiration in working skeletal muscle, pulmonary VO2 dynamics is measured at the mouth using indirect calorimetry. To provide information on the dynamic balance between oxygen delivery and oxygen consumption at the microvascular level, muscle oxygenation is measured using near‐infrared spectroscopy. A multi‐scale computational model that links O2 transport and cellular metabolism in the skeletal muscle was developed to relate the measurements and gain quantitative understanding of the regulation of VO2 at the cellular, tissue, and whole‐body level. The model incorporates mechanisms of oxygen transport from the airway openings to the cell, as well as the phosphagenic and oxidative pathways of ATP synthesis in the muscle cells.

PathCase-SB architecture and database design

BackgroundIntegration of metabolic pathways resources and regulatory metabolic network models, and deploying new tools on the integrated platform can help perform more effective and more efficient systems biology research on understanding the regulation in metabolic networks. Therefore, the tasks of (a) integrating under a single database environment regulatory metabolic networks and existing models, and (b) building tools to help with modeling and analysis are desirable and intellectually challenging computational tasks.DescriptionPathCase Systems Biology (PathCase-SB) is built and released. The PathCase-SB database provides data and API for multiple user interfaces and software tools. The current PathCase-SB system provides a database-enabled framework and web-based computational tools towards facilitating the development of kinetic models for biological systems. PathCase-SB aims to integrate data of selected biological data sources on the web (currently, BioModels database and KEGG), and to provide more powerful and/or new capabilities via the new web-based integrative framework. This paper describes architecture and database design issues encountered in PathCase-SBs design and implementation, and presents the current design of PathCase-SBs architecture and database.ConclusionsPathCase-SB architecture and database provide a highly extensible and scalable environment with easy and fast (real-time) access to the data in the database. PathCase-SB itself is already being used by researchers across the world.