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Golub AS, Song BK, Nugent WH, Pittman RN. Dynamics of PO 2 and VO 2 in resting and contracting rat spinotrapezius muscle. Front Physiol 2023; 14:1172834. [PMID: 37538372 PMCID: PMC10396398 DOI: 10.3389/fphys.2023.1172834] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/23/2023] [Accepted: 07/03/2023] [Indexed: 08/05/2023] Open
Abstract
This study examined changes in interstitial PO2, which allowed calculation of VO2 during periods of rest, muscle contraction and recovery using an in situ rat spinotrapezius muscle preparation. The PO2 was measured using phosphorescence quenching microscopy and the muscle VO2 was calculated as the rate of O2 disappearance during brief periods of muscle compression to stop blood flow with a supra-systolic pressure. The PO2 and VO2 measurements were made during "5 s compression and 15 s recovery" (CR) cycles. With all three stimulation frequencies, 1, 2 and 4 Hz, the fall in interstitial PO2 and rise in VO2 from resting values occurred within the first 20 s of contraction. The PO2 during contraction became lower as stimulation frequency increased from 1 to 4 Hz. VO2 was higher at 2 Hz than at 1 Hz contraction. With cessation of stimulation, PO2 began increasing exponentially towards baseline values. After 1 and 2 Hz contraction, the fall in muscle VO2 was delayed by one CR cycle and then exponentially decreased towards resting values. After 4 Hz stimulation, VO2 increased for 2 cycles and then decreased. The post-contraction transients of PO2 and VO2 were not synchronous and had different time constants. With further analysis two distinct functional responses were identified across all stimulation frequencies having PO2 during contraction above or below 30 mmHg. The corresponding VO2 responses were different - for "high" PO2, muscle VO2 reached high levels, while for the "low" PO2 data set muscle VO2 remained low. Recovery patterns were similar to those described above. In summary, local microscopic PO2 and VO2 were measured in resting and contracting muscle in situ and the post-contraction transients of PO2 and VO2 were all much slower than the onset transients.
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Affiliation(s)
- Aleksander S. Golub
- Department of Physiology and Biophysics, Medical College of Virginia Campus, Virginia Commonwealth University, Richmond, VA, United States
- Song Biotechnologies LLC, Cockeysville, MD, United States
| | - Bjorn K. Song
- Department of Physiology and Biophysics, Medical College of Virginia Campus, Virginia Commonwealth University, Richmond, VA, United States
- Song Biotechnologies LLC, Cockeysville, MD, United States
| | - William H. Nugent
- Department of Physiology and Biophysics, Medical College of Virginia Campus, Virginia Commonwealth University, Richmond, VA, United States
- Song Biotechnologies LLC, Cockeysville, MD, United States
| | - Roland N. Pittman
- Department of Physiology and Biophysics, Medical College of Virginia Campus, Virginia Commonwealth University, Richmond, VA, United States
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Increasing Oxygen Uptake in Cross-Country Skiers by Speed Variation in Work Intervals. Int J Sports Physiol Perform 2021; 17:384-390. [PMID: 34814113 DOI: 10.1123/ijspp.2021-0226] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/07/2021] [Revised: 06/21/2021] [Accepted: 07/04/2021] [Indexed: 11/18/2022]
Abstract
PURPOSE Accumulated time at a high percentage of peak oxygen consumption (VO2peak) is important for improving performance in endurance athletes. The present study compared the acute physiological and perceived effects of performing high-intensity intervals with roller ski double poling containing work intervals with (1) fast start followed by decreasing speed (DEC), (2) systematic variation in exercise intensity (VAR), and (3) constant speed (CON). METHODS Ten well-trained cross-country skiers (double-poling VO2peak 69.6 [3.5] mL·min-1·kg-1) performed speed- and duration-matched DEC, VAR, and CON on 3 separate days in a randomized order (5 × 5-min work intervals and 3-min recovery). RESULTS DEC and VAR led to longer time ≥90% VO2peak (P = .016 and P = .033, respectively) and higher mean %VO2peak (P = .036, and P = .009) compared with CON, with no differences between DEC and VAR (P = .930 and P = .759, respectively). VAR, DEC, and CON led to similar time ≥90% of peak heart rate (HRpeak), mean HR, mean breathing frequency, mean ventilation, and mean blood lactate concentration ([La-]). Furthermore, no differences between sessions were observed for perceptual responses, such as mean rate of perceived exertion, session rate of perceived exertion or pain score (all Ps > .147). CONCLUSIONS In well-trained XC skiers, DEC and VAR led to longer time ≥90% of VO2peak compared with CON, without excessive perceptual effort, indicating that these intervals can be a good alternative for accumulating more time at a high percentage of VO2peak and at the same time mimicking the pronounced variation in exercise intensities experienced during XC-skiing competitions.
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Wilson DF, Matschinsky FM. Metabolic Homeostasis in Life as We Know It: Its Origin and Thermodynamic Basis. Front Physiol 2021; 12:658997. [PMID: 33967829 PMCID: PMC8104125 DOI: 10.3389/fphys.2021.658997] [Citation(s) in RCA: 17] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/26/2021] [Accepted: 03/19/2021] [Indexed: 12/25/2022] Open
Abstract
Living organisms require continuous input of energy for their existence. As a result, life as we know it is based on metabolic processes that extract energy from the environment and make it available to support life (energy metabolism). This metabolism is based on, and regulated by, the underlying thermodynamics. This is important because thermodynamic parameters are stable whereas kinetic parameters are highly variable. Thermodynamic control of metabolism is exerted through near equilibrium reactions that determine. (1) the concentrations of metabolic substrates for enzymes that catalyze irreversible steps and (2) the concentrations of small molecules (AMP, ADP, etc.) that regulate the activity of irreversible reactions in metabolic pathways. The result is a robust homeostatic set point (−ΔGATP) with long term (virtually unlimited) stability. The rest of metabolism and its regulation is constrained to maintain this set point. Thermodynamic control is illustrated using the ATP producing part of glycolysis, glyceraldehyde-3-phosphate oxidation to pyruvate. Flux through the irreversible reaction, pyruvate kinase (PK), is primarily determined by the rate of ATP consumption. Change in the rate of ATP consumption causes mismatch between use and production of ATP. The resulting change in [ATP]/[ADP][Pi], through near equilibrium of the reactions preceding PK, alters the concentrations of ADP and phosphoenolpyruvate (PEP), the substrates for PK. The changes in ADP and PEP alter flux through PK appropriately for restoring equality of ATP production and consumption. These reactions appeared in the very earliest lifeforms and are hypothesized to have established the set point for energy metabolism. As evolution included more metabolic functions, additional layers of control were needed to integrate new functions into existing metabolism without changing the homeostatic set point. Addition of gluconeogenesis, for example, resulted in added regulation to PK activity to prevent futile cycling; PK needs to be turned off during gluconeogenesis because flux through the enzyme would waste energy (ATP), subtracting from net glucose synthesis and decreasing overall efficiency.
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Affiliation(s)
- David F Wilson
- Department of Biochemistry and Biophysics, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA, United States
| | - Franz M Matschinsky
- Department of Biochemistry and Biophysics, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA, United States
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Bossi AH, Mesquida C, Passfield L, Rønnestad BR, Hopker JG. Optimizing Interval Training Through Power-Output Variation Within the Work Intervals. Int J Sports Physiol Perform 2020; 15:982-989. [PMID: 32244222 DOI: 10.1123/ijspp.2019-0260] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/01/2019] [Revised: 10/02/2019] [Accepted: 10/17/2019] [Indexed: 11/18/2022]
Abstract
PURPOSE Maximal oxygen uptake (V˙O2max) is a key determinant of endurance performance. Therefore, devising high-intensity interval training (HIIT) that maximizes stress of the oxygen-transport and -utilization systems may be important to stimulate further adaptation in athletes. The authors compared physiological and perceptual responses elicited by work intervals matched for duration and mean power output but differing in power-output distribution. METHODS Fourteen cyclists (V˙O2max 69.2 [6.6] mL·kg-1·min-1) completed 3 laboratory visits for a performance assessment and 2 HIIT sessions using either varied-intensity or constant-intensity work intervals. RESULTS Cyclists spent more time at >90%V˙O2max during HIIT with varied-intensity work intervals (410 [207] vs 286 [162] s, P = .02), but there were no differences between sessions in heart-rate- or perceptual-based training-load metrics (all P ≥ .1). When considering individual work intervals, minute ventilation (V˙E) was higher in the varied-intensity mode (F = 8.42, P = .01), but not respiratory frequency, tidal volume, blood lactate concentration [La], ratings of perceived exertion, or cadence (all F ≤ 3.50, ≥ .08). Absolute changes (Δ) between HIIT sessions were calculated per work interval, and Δ total oxygen uptake was moderately associated with ΔV˙E (r = .36, P = .002). CONCLUSIONS In comparison with an HIIT session with constant-intensity work intervals, well-trained cyclists sustain higher fractions of V˙O2max when work intervals involved power-output variations. This effect is partially mediated by an increased oxygen cost of hyperpnea and not associated with a higher [La], perceived exertion, or training-load metrics.
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Wilson DF, Matschinsky FM. Hyperbaric oxygen toxicity in brain: A case of hyperoxia induced hypoglycemic brain syndrome. Med Hypotheses 2019; 132:109375. [PMID: 31454640 DOI: 10.1016/j.mehy.2019.109375] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/06/2019] [Revised: 08/09/2019] [Accepted: 08/18/2019] [Indexed: 12/25/2022]
Abstract
Hyperbaric oxygen exposure is a recent hazzard for higher animals that originated as humans began underwater construction, exploration, and sports. Exposure can lead to abnormal brain EEG, convulsions, and death, the time to onset of each stage of pathology decreasing with increase in oxygen pressure. We provide evidence that hyperoxia, through oxidative phosphorylation, increases the energy state ([ATP]/[ADP][Pi]) of cells critical to providing glucose to cells behind the blood brain barrier (BBB). Brain cells without an absolute dependence on glucose metabolism; i.e. those having sufficient ATP synthesis using lactate and glutamate as oxidizable substrates, are not themselves very adversely affected by hyperoxia. The increased energy state and decrease in free [AMP], however, suppress glucose transport through the blood brain barrier (BBB) and into cells behind the BBB. Glucose has to pass in sequence through three steps of transport by facilitated diffusion and transporter activity for each step is regulated in part by AMP dependent protein kinase. The physiological role of this regulation is to increase glucose transport in response to hypoxia and/or systemic hypoglycemia. Hyperoxia, however, through unphysiological decrease in free [AMP] suppresses 1) glucose transport through the BBB (endothelial GLUT1 transporters) into cerebrospinal fluid (CSF); 2) glucose transport from CSF into cells behind the BBB (GLUT3 transporters) and (GLUT4 transporters). Cumulative suppression of glucose transport results in local regions of hypoglycemia and induces hypoglycemic failure. It is suggested that failure is initiated at axons and synapses with insufficient mitochondria to meet their energy requirements.
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Affiliation(s)
- David F Wilson
- Department of Biochemistry and Biophysics, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA 19104, USA.
| | - Franz M Matschinsky
- Department of Biochemistry and Biophysics, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA 19104, USA.
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Wilson DF, Matschinsky FM. Oxygen dependence of glucose sensing: role in glucose homeostasis and related pathology. J Appl Physiol (1985) 2019; 126:1746-1755. [PMID: 30991014 DOI: 10.1152/japplphysiol.00047.2019] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/08/2023] Open
Abstract
In glucose homeostasis, glucose concentration is sensed by its metabolism through glucokinase (GCK) and oxidative phosphorylation. Because oxidative phosphorylation is an integral part of the sensory system, glucose sensing is necessarily dependent on oxygen pressure. Much of the dependence on oxygen is suppressed by location of glucose sensing cells in tissues with well-regulated blood flow. In healthy individuals the oxygen dependence is primarily observed in response to transient global hypoxia events such as during birth or transition to high altitude. The GCK sensing system is, however, used to control release of both insulin and glucagon, the preeminant hormonal regulators of blood glucose, as well as glucose sensitive neuronal activity. Suppression of oxygen delivery to glucose-sensing cells or interference with regulation of tissue blood flow by either local or systemic causes, stresses the glucose regulatory system. This is true whether the stress is imposed locally, such as by altered oxygen delivery to the pancreas, or globally, as in pulmonary insufficiency or exposure to high altitude. It may be expected that chronic application of this stress predisposes individuals to developing diabetes. Type 2 diabetes is a broad class of diseases characterized by disturbance of glucose homeostasis, i.e., having either hyperglycemia and/or decreased sensitivity to insulin. Given the role of oxidative phosphorylation in glucose sensing, tissue oxygen deprivation may predispose individuals to developing diabetes as well as contributing to the disease itself. This is particularly true in age-related diabetes because the incidence of vascular insufficiency increases markedly with increasing age. NEW & NOTEWORTHY Glucose sensing requires glucose metabolism through glycolysis and oxidative phosphorylation. Dependence of the latter on oxygen concentration imposes an oxygen dependence on glucose sensing. We have used a validated computational model to quantify that dependence. Evidence is presented that tissue oxygenation plays an important role in predisposition of individuals to developing type 2 diabetes and in progression of the disease.
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Affiliation(s)
- David F Wilson
- Perelman School of Medicine, Department of Biochemistry and Biophysics, University of Pennsylvania , Philadelphia, Pennsylvania
| | - Franz M Matschinsky
- Perelman School of Medicine, Department of Biochemistry and Biophysics, University of Pennsylvania , Philadelphia, Pennsylvania
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Matschinsky FM, Wilson DF. The Central Role of Glucokinase in Glucose Homeostasis: A Perspective 50 Years After Demonstrating the Presence of the Enzyme in Islets of Langerhans. Front Physiol 2019; 10:148. [PMID: 30949058 PMCID: PMC6435959 DOI: 10.3389/fphys.2019.00148] [Citation(s) in RCA: 162] [Impact Index Per Article: 32.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/24/2018] [Accepted: 02/07/2019] [Indexed: 01/05/2023] Open
Abstract
It is hypothesized that glucokinase (GCK) is the glucose sensor not only for regulation of insulin release by pancreatic β-cells, but also for the rest of the cells that contribute to glucose homeostasis in mammals. This includes other cells in endocrine pancreas (α- and δ-cells), adrenal gland, glucose sensitive neurons, entero-endocrine cells, and cells in the anterior pituitary. Glucose transport is by facilitated diffusion and is not rate limiting. Once inside, glucose is phosphorylated to glucose-6-phosphate by GCK in a reaction that is dependent on glucose throughout the physiological range of concentrations, is irreversible, and not product inhibited. High glycerol phosphate shuttle, pyruvate dehydrogenase, and pyruvate carboxylase activities, combined with low pentose-P shunt, lactate dehydrogenase, plasma membrane monocarboxylate transport, and glycogen synthase activities constrain glucose-6-phosphate to being metabolized through glycolysis. Under these conditions, glycolysis produces mostly pyruvate and little lactate. Pyruvate either enters the citric acid cycle through pyruvate dehydrogenase or is carboxylated by pyruvate carboxylase. Reducing equivalents from glycolysis enter oxidative phosphorylation through both the glycerol phosphate shuttle and citric acid cycle. Raising glucose concentration increases intramitochondrial [NADH]/[NAD+] and thereby the energy state ([ATP]/[ADP][Pi]), decreasing [Mg2+ADP] and [AMP]. [Mg2+ADP] acts through control of KATP channel conductance, whereas [AMP] acts through regulation of AMP-dependent protein kinase. Specific roles of different cell types are determined by the diverse molecular mechanisms used to couple energy state to cell specific responses. Having a common glucose sensor couples complementary regulatory mechanisms into a tightly regulated and stable glucose homeostatic network.
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Affiliation(s)
- Franz M Matschinsky
- Department of Biochemistry and Biophysics, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA, United States
| | - David F Wilson
- Department of Biochemistry and Biophysics, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA, United States
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Wilson DF, Matschinsky FM. Metabolic homeostasis: oxidative phosphorylation and the metabolic requirements of higher plants and animals. J Appl Physiol (1985) 2018; 125:1183-1192. [DOI: 10.1152/japplphysiol.00352.2018] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/17/2022] Open
Abstract
A model of oxidative phosphorylation and its regulation is presented, which is consistent with the experimental data on metabolism in higher plants and animals. The variables that provide real-time control of metabolic status are: intramitochondrial [NAD+]/[NADH], energy state ([ATP]/[ADP][Pi]), and local oxygen concentration ([O2]). ATP consumption and respiratory chain enzyme content are tissue specific (liver vs. heart muscle), and the latter is modulated by chronic alterations in ATP consumption (i.e., endurance training etc.). ATP consumption affects the energy state, which increases or decreases as necessary to match synthesis with consumption. [NAD+]/[NADH], local [O2], and respiratory chain content determine the energy state at which match of synthesis and utilization is achieved. Tissues vary widely in their ranges of ATP consumption. Expressed as the turnover of cytochrome c, the rates may change little (7 to 12/s) (liver) or a lot (1 to >300/s) (flight muscle of birds, bats, and insects). Ancillary metabolic pathways, including creatine or arginine kinase, glycerol phosphate shuttle, fatty acid, and citric acid cycle dehydrogenases, are responsible for meeting tissue-specific differences in maximal rate and range in ATP utilization without displacing metabolic homeostasis. Intramitochondrial [NAD+]/[NADH], [ATP], and [Pi] are adjusted to keep [ADP] and [AMP] similar for all tissues despite large differences in ranges in ATP utilization. This is essential because [ADP] and [AMP], particularly the latter, have major roles in regulating the activity of many enzymes and signaling pathways (AMP deaminase, AMP dependent protein kinases, etc.) common to all higher plants and animals. NEW & NOTEWORTHY Oxidative phosphorylation has an intrinsic program that sets and stabilizes cellular energy state ([ATP]/[ADP][Pi]), and thereby metabolic homeostasis. A computational model consistent with regulation of oxidative phosphorylation in higher plants and animals is presented. Focus is on metabolism ancillary to oxidative phosphorylation by which it was integrated into preexisting metabolic regulation and adapted by evolution to develop cells and tissues with differing rates of ATP utilization: i.e., liver versus brain versus muscle.
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Affiliation(s)
- David F. Wilson
- Department of Biochemistry and Biophysics, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania
| | - Franz M. Matschinsky
- Department of Biochemistry and Biophysics, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania
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Golub AS, Dodhy SC, Pittman RN. Oxygen dependence of respiration in rat spinotrapezius muscle contracting at 0.5-8 twitches per second. J Appl Physiol (1985) 2018; 125:124-133. [PMID: 29494286 DOI: 10.1152/japplphysiol.01136.2016] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022] Open
Abstract
The oxygen dependence of respiration was obtained in situ in microscopic regions of rat spinotrapezius muscle for different levels of metabolic activity produced by electrical stimulation at rates from 0.5 to 8 Hz. The rate of O2 consumption (V̇o2) was measured with phosphorescence quenching microscopy (PQM) as the rate of O2 disappearance in a muscle with rapid flow arrest. The phosphorescent oxygen probe was loaded into the interstitial space of the muscle to give O2 tension (Po2) in the interstitium. A set of sigmoid curves relating the Po2 dependence of V̇o2 was obtained with a Po2-dependent region below a characteristic Po2 (~30 mmHg) and a Po2-independent region above this Po2. The V̇o2(Po2) plots were fit by the Hill equation containing O2 demand (rest to 8 Hz: 216 ± 26 to 636 ± 77 nl O2/cm3 s) and the Po2 value corresponding to O2 demand/2 (rest to 8 Hz: 22 ± 4 to 11 ± 1 mmHg). The initial Po2 and V̇o2 pairs of values measured at the moment of flow arrest formed a straight line, determining the rate of oxygen supply. This line had a negative slope, equal to the oxygen conductance for the O2 supply chain. For each level of tissue blood flow the set of possible values of Po2 and V̇o2 consists of the intersection points between this O2 supply line and the set of V̇o2 curves. An electrical analogy for the intraorgan O2 supply and consumption is an inverting transistor amplifier, which allows the use of graphic analysis methods for prediction of the behavior of the oxygen processing system in organs. NEW & NOTEWORTHY The sigmoidal shape of curves describing oxygen dependence of muscle respiration varies from basal to maximal workload and characterizes the oxidative metabolism of muscle. The rate of O2 supply depends on extracellular O2 tension and is determined by the overall oxygen conductance in the muscle. The dynamics of oxygen consumption is determined by the supply line that intersects the oxygen demand curves. An electrical analogy for the oxygen supply/consumption system is an inverting transistor amplifier.
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Affiliation(s)
- Aleksander S Golub
- Department of Physiology and Biophysics, Medical College of Virginia Campus, Virginia Commonwealth University , Richmond, Virginia
| | - Sami C Dodhy
- Department of Physiology and Biophysics, Medical College of Virginia Campus, Virginia Commonwealth University , Richmond, Virginia
| | - Roland N Pittman
- Department of Physiology and Biophysics, Medical College of Virginia Campus, Virginia Commonwealth University , Richmond, Virginia
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Jones AM, Vanhatalo A. The 'Critical Power' Concept: Applications to Sports Performance with a Focus on Intermittent High-Intensity Exercise. Sports Med 2018; 47:65-78. [PMID: 28332113 PMCID: PMC5371646 DOI: 10.1007/s40279-017-0688-0] [Citation(s) in RCA: 132] [Impact Index Per Article: 22.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
Abstract
The curvilinear relationship between power output and the time for which it can be sustained is a fundamental and well-known feature of high-intensity exercise performance. This relationship 'levels off' at a 'critical power' (CP) that separates power outputs that can be sustained with stable values of, for example, muscle phosphocreatine, blood lactate, and pulmonary oxygen uptake ([Formula: see text]), from power outputs where these variables change continuously with time until their respective minimum and maximum values are reached and exercise intolerance occurs. The amount of work that can be done during exercise above CP (the so-called W') is constant but may be utilized at different rates depending on the proximity of the exercise power output to CP. Traditionally, this two-parameter CP model has been employed to provide insights into physiological responses, fatigue mechanisms, and performance capacity during continuous constant power output exercise in discrete exercise intensity domains. However, many team sports (e.g., basketball, football, hockey, rugby) involve frequent changes in exercise intensity and, even in endurance sports (e.g., cycling, running), intensity may vary considerably with environmental/course conditions and pacing strategy. In recent years, the appeal of the CP concept has been broadened through its application to intermittent high-intensity exercise. With the assumptions that W' is utilized during work intervals above CP and reconstituted during recovery intervals below CP, it can be shown that performance during intermittent exercise is related to four factors: the intensity and duration of the work intervals and the intensity and duration of the recovery intervals. However, while the utilization of W' may be assumed to be linear, studies indicate that the reconstitution of W' may be curvilinear with kinetics that are highly variable between individuals. This has led to the development of a new CP model for intermittent exercise in which the balance of W' remaining ([Formula: see text]) may be calculated with greater accuracy. Field trials of athletes performing stochastic exercise indicate that this [Formula: see text] model can accurately predict the time at which W' tends to zero and exhaustion is imminent. The [Formula: see text] model potentially has important applications in the real-time monitoring of athlete fatigue progression in endurance and team sports, which may inform tactics and influence pacing strategy.
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Affiliation(s)
- Andrew M Jones
- Sport and Health Sciences, College of Life and Environmental Sciences, University of Exeter, Heavitree Road, Exeter, EX12LU, UK.
| | - Anni Vanhatalo
- Sport and Health Sciences, College of Life and Environmental Sciences, University of Exeter, Heavitree Road, Exeter, EX12LU, UK
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Wilson DF, Cember ATJ, Matschinsky FM. Glutamate dehydrogenase: role in regulating metabolism and insulin release in pancreatic β-cells. J Appl Physiol (1985) 2018; 125:419-428. [PMID: 29648519 DOI: 10.1152/japplphysiol.01077.2017] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/23/2023] Open
Abstract
Regulation of insulin release and glucose homeostasis by pancreatic β-cells is dependent on the metabolism of glucose by glucokinase (GK) and the influence of that activity on oxidative phosphorylation. Genetic alterations that result in hyperactivity of mitochondrial glutamate dehydrogenase (GDH-1) can cause hypoglycemia-hyperammonemia following high protein meals, but the role of GDH-1 remains poorly understood. GDH-1 activity is strongly inhibited by GTP, to near zero in the absence of ADP, and cooperatively activated ( n = 2.3) by ADP. The dissociation constant for ADP is near 200 µM in vivo, but leucine and its nonmetabolized analog 2-amino-2-norbornane-carboxylic acid (BCH) can activate GDH-1 by increasing the affinity for ADP. Under physiological conditions, as [ADP] increases GDH-1 activity remains very low until ~35 µM (threshold) and then increases rapidly. A model for GDH-1 and its regulation has been combined with a previously published model for glucose sensing that coupled GK activity and oxidative phosphorylation. The combined model (GK-GDH-core) shows that GK activity determines the energy state ([ATP]/[ADP][Pi]) in β-cells for glucose concentrations > 5 mM ([ADP] < 35 µM). As glucose falls < 5 mM the [ADP]-dependent increase in GDH-1 activity prevents [ADP] from rising above ~70 µM. Thus, GDH-1 dynamically buffers β-cell energy metabolism during hypoglycemia, maintaining the energy state and the basal rate of insulin release. GDH-1 hyperactivity suppresses the normal increase in [ADP] in hypoglycemia. This leads to hypoglycemia following a high protein meal by increasing the basal rate of insulin release (β-cells) and decreasing glucagon release (α-cells). NEW & NOTEWORTHY A model of β-cell metabolism and regulation of insulin release is presented. The model integrates regulation of oxidative phosphorylation, glucokinase (GK), and glutamate dehydrogenase (GDH-1). GDH-1 is near equilibrium under physiological conditions, but the activity is inhibited by GTP. In hypoglycemia, however, GK activity is low and [ADP], a potent activator of GDH-1, increases. Reducing equivalents from GDH dynamically buffers the intramitochondrial [NADH]/[NAD+], and thereby the energy state, preventing hypoglycemia-induced substrate deprivation.
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Affiliation(s)
- David F Wilson
- Department of Biochemistry and Biophysics, Perelman School of Medicine, University of Pennsylvania , Philadelphia, Pennsylvania
| | - Abigail T J Cember
- Department of Biochemistry and Biophysics, Perelman School of Medicine, University of Pennsylvania , Philadelphia, Pennsylvania
| | - Franz M Matschinsky
- Department of Biochemistry and Biophysics, Perelman School of Medicine, University of Pennsylvania , Philadelphia, Pennsylvania
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Wilson DF, Cember ATJ, Matschinsky FM. The thermodynamic basis of glucose-stimulated insulin release: a model of the core mechanism. Physiol Rep 2018; 5:5/12/e13327. [PMID: 28655753 PMCID: PMC5492210 DOI: 10.14814/phy2.13327] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/30/2017] [Revised: 05/23/2017] [Accepted: 05/24/2017] [Indexed: 11/24/2022] Open
Abstract
A model for glucose sensing by pancreatic β-cells is developed and compared with the available experimental data. The model brings together mathematical representations for the activities of the glucose sensor, glucokinase, and oxidative phosphorylation. Glucokinase produces glucose 6-phosphate (G-6-P) in an irreversible reaction that determines glycolytic flux. The primary products of glycolysis are NADH and pyruvate. The NADH is reoxidized and the reducing equivalents transferred to oxidative phosphorylation by the glycerol phosphate shuttle, and some of the pyruvate is oxidized by pyruvate dehydrogenase and enters the citric acid cycle. These reactions are irreversible and result in a glucose concentration-dependent reduction of the intramitochondrial NAD pool. This increases the electrochemical energy coupled to ATP synthesis and thereby the cellular energy state ([ATP]/[ADP][Pi]). ATP and Pi are 10-100 times greater than ADP, so the increase in energy state is primarily through decrease in ADP The decrease in ADP is considered responsible for altering ion channel conductance and releasing insulin. Applied to the reported glucose concentration-dependent release of insulin by perifused islet preparations (Doliba et al. 2012), the model predicts that the dependence of insulin release on ADP is strongly cooperative with a threshold of about 30 μmol/L and a negative Hill coefficient near -5.5. The predicted cellular energy state, ADP, creatine phosphate/creatine ratio, and cytochrome c reduction, including their dependence on glucose concentration, are consistent with experimental data. The ability of the model to predict behavior consistent with experiment is an invaluable resource for understanding glucose sensing and planning experiments.
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Affiliation(s)
- David F Wilson
- Department of Biochemistry and Biophysics, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania
| | - Abigail T J Cember
- Department of Biochemistry and Biophysics, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania
| | - Franz M Matschinsky
- Department of Biochemistry and Biophysics, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania
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Wilson DF. Oxidative phosphorylation: regulation and role in cellular and tissue metabolism. J Physiol 2017; 595:7023-7038. [PMID: 29023737 PMCID: PMC5709332 DOI: 10.1113/jp273839] [Citation(s) in RCA: 156] [Impact Index Per Article: 22.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/09/2017] [Accepted: 08/30/2017] [Indexed: 12/12/2022] Open
Abstract
Oxidative phosphorylation provides most of the ATP that higher animals and plants use to support life and is responsible for setting and maintaining metabolic homeostasis. The pathway incorporates three consecutive near equilibrium steps for moving reducing equivalents between the intramitochondrial [NAD+ ]/[NADH] pool to molecular oxygen, with irreversible reduction of oxygen to bound peroxide at cytochrome c oxidase determining the net flux. Net flux (oxygen consumption rate) is determined by demand for ATP, with feedback by the energy state ([ATP]/[ADP][Pi ]) regulating the pathway. This feedback affects the reversible steps equally and independently, resulting in the rate being coupled to ([ATP]/[ADP][Pi ])3 . With increasing energy state, oxygen consumption decreases rapidly until a threshold is reached, above which there is little further decrease. In most cells, [ATP] and [Pi ] are much higher than [ADP] and change in [ADP] is primarily responsible for the change in energy state. As a result, the rate of ATP synthesis, plotted against [ADP], remains low until [ADP] reaches about 30 μm and then increases rapidly with further increase in [ADP]. The dependencies on energy state and [ADP] near the threshold can be fitted by the Hill equation with a Hill coefficients of about -2.6 and 4.2, respectively. The homeostatic set point for metabolism is determined by the threshold, which can be modulated by the PO2 and intramitochondrial [NAD+ ]/[NADH]. The ability of oxidative phosphorylation to precisely set and maintain metabolic homeostasis is consistent with it being permissive of, and essential to, development of higher plants and animals.
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Affiliation(s)
- David F. Wilson
- Department of Biochemistry and Biophysics, Perelman School of MedicineUniversity of PennsylvaniaPhiladelphiaPA19104USA
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Regulation of oxidative phosphorylation through each-step activation (ESA): Evidences from computer modeling. PROGRESS IN BIOPHYSICS AND MOLECULAR BIOLOGY 2017; 125:1-23. [DOI: 10.1016/j.pbiomolbio.2016.12.001] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/28/2016] [Accepted: 12/06/2016] [Indexed: 01/20/2023]
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Wilson DF. Oxidative phosphorylation: unique regulatory mechanism and role in metabolic homeostasis. J Appl Physiol (1985) 2016; 122:611-619. [PMID: 27789771 DOI: 10.1152/japplphysiol.00715.2016] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/12/2016] [Revised: 10/12/2016] [Accepted: 10/24/2016] [Indexed: 11/22/2022] Open
Abstract
Oxidative phosphorylation is the primary source of metabolic energy, in the form of ATP, in higher plants and animals, but its regulation in vivo is not well understood. A model has been developed for oxidative phosphorylation in vivo that predicts behavior patterns that are both distinctive and consistent with experimental measurements of metabolism in intact cells and tissues. A major regulatory parameter is the energy state ([ATP]/[ADP][Pi], where brackets denote concentration). Under physiological conditions, the [ATP] and [Pi] are ~100 times that of [ADP], and most of the change in energy state is through change in [ADP]. The rate of oxidative phosphorylation (y-axis) increases slowly with increasing [ADP] until a threshold is reached and then increases very rapidly and linearly with further increase in [ADP]. The dependence on [ADP] can be characterized by a threshold [ADP] (T) and control strength (CS), the normalized slope above threshold (Δy/(Δx/T). For normoxic cells without creatine kinase, T is ~30 µM and CS is ~10 s-1 Myocytes and cells with larger ranges of rates of ATP utilization, however, have the same [ADP]- and [AMP]-dependent mechanisms regulating metabolism and gene expression. To compensate, these cells have creatine kinase, and hydrolysis/synthesis of creatine phosphate increases the change in [Pi] and thereby CS. Cells with creatine kinase have [ADP] and [AMP], which are similar to cells without creatine kinase, despite the large differences in metabolic rate. 31P measurements in human muscles during work-to-rest and rest-to-work transitions are consistent with predictions of the model.NEW & NOTEWORTHY A model developed for oxidative phosphorylation in vivo is shown to predict behavior patterns that are both novel and consistent with experimental measurements of metabolism in working muscle and other cells. The dependence of the rate on ADP concentration shows a pronounced threshold with a steep, nearly linear increase above the threshold. The threshold determines the homeostatic set point, and the slope above threshold determines how much metabolism changes in response to varied energy demand.
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Affiliation(s)
- David F Wilson
- Department of Biochemistry and Biophysics, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania
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Wilson DF. Regulation of metabolism: the work-to-rest transition in skeletal muscle. Am J Physiol Endocrinol Metab 2016; 310:E633-E642. [PMID: 26837809 DOI: 10.1152/ajpendo.00512.2015] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 12/14/2015] [Accepted: 01/28/2016] [Indexed: 11/22/2022]
Abstract
The behavior of oxidative phosphorylation predicted by a model for the mechanism and kinetics of cytochrome c oxidase is compared with the experimentally observed behavior during the work-to-rest transition in skeletal muscle. For both experiment and model, when work stops, the increase in creatine phosphate and decrease in creatine and inorganic phosphate concentrations ([CrP], [Cr], and [Pi]) begin immediately. The rate of change for each is maximal and then progressively slows as the increasing energy state ([ATP]/[ADP][Pi]) suppresses the rate of oxidative phosphorylation. The time courses can be reasonably fitted to single exponential curves with similar time constants. The energy state in the working and resting steady states at constant Po2 are dependent on the intramitochondrial [NAD+]/[NADH], mitochondrial content, and size of the creatine pool ([CrP] + [Cr]). The rate of change in [CrP] is linearly correlated with [CrP] and with [Pi] and [Cr]. The time constant for [CrP] increase in the resting and working steady states, and the rate of decrease in oxygen consumption are similarly dependent on the Po2 in the inspired gas (experimental) or tissue Po2 (model). Myoglobin strongly buffers intracellular Po2 below ∼15 torr, truncating the low end of the oxygen distribution in the tissue and suppressing intra- and intermyocyte oxygen gradients. The predictions of the model are consistent with the experimental data throughout the work/rest transition, providing valuable insights into the regulation of cellular and tissue metabolism.
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Affiliation(s)
- David F Wilson
- Department of Biochemistry and Biophysics, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania
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Korzeniewski B, Rossiter HB. Each-step activation of oxidative phosphorylation is necessary to explain muscle metabolic kinetic responses to exercise and recovery in humans. J Physiol 2015; 593:5255-68. [PMID: 26503399 PMCID: PMC4704516 DOI: 10.1113/jp271299] [Citation(s) in RCA: 35] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/15/2015] [Accepted: 10/22/2015] [Indexed: 11/08/2022] Open
Abstract
KEY POINTS The basic control mechanisms of oxidative phosphorylation (OXPHOS) and glycolysis during work transitions in human skeletal muscle are still a matter of debate. We used simulations of skeletal muscle bioenergetics to identify key system features that contribute to this debate, by comparing kinetic model outputs with experimental human data, including phosphocreatine, pH, pulmonary oxygen uptake and fluxes of ATP production by OXPHOS (vOX), anaerobic glycolysis and creatine kinase in moderate and severe intensity exercise transitions. We found that each-step activation of particular OXPHOS complexes, NADH supply and glycolysis, and strong (third-order) glycolytic inhibition by protons was required to reproduce observed phosphocreatine, pH and vOX kinetics during exercise. A slow decay of each-step activation during recovery, which was slowed further following severe exercise, was necessary to reproduce the experimental findings. Well-tested computer models offer new insight in the control of the human skeletal muscle bioenergetic system during physical exercise. ABSTRACT To better understand muscle bioenergetic regulation, a previously-developed model of the skeletal muscle cell bioenergetic system was used to simulate the influence of: (1) each-step activation (ESA) of NADH supply (including glycolysis) and oxidative phosphorylation (OXPHOS) complexes and (2) glycolytic inhibition by protons on the kinetics of ATP synthesis from OXPHOS, anaerobic glycolysis and creatine kinase. Simulations were fitted to previously published experimental data of ATP production fluxes and metabolite concentrations during moderate and severe intensity exercise transitions in bilateral knee extension in humans. Overall, the computer simulations agreed well with experimental results. Specifically, a large (>5-fold) direct activation of all OXPHOS complexes was required to simulate measured phosphocreatine and OXPHOS responses to both moderate and severe intensity exercise. In addition, slow decay of ESA was required to fit phosphocreatine recovery kinetics, and the time constant of ESA decay was slower following severe (180 s) than moderate (90 s) exercise. Additionally, a strong inhibition of (anaerobic) glycolysis by protons (glycolytic rate inversely proportional to the cube of proton concentration) provided the best fit to the experimental pH kinetics, and may contribute to the progressive increase in oxidative ATP supply during acidifying contractions. During severe-intensity exercise, an 'additional' ATP usage (a 27% increase at 8 min, above the initial ATP supply) was necessary to explain the observed V̇O2 slow component. Thus, parallel activation of ATP usage and ATP supply (ESA), and a strong inhibition of ATP supply by anaerobic glycolysis, were necessary to simulate the kinetics of muscle bioenergetics observed in humans.
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Affiliation(s)
- Bernard Korzeniewski
- Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, Kraków, Poland
| | - Harry B Rossiter
- Rehabilitation Clinical Trials Centre, Division of Respiratory & Critical Care Physiology & Medicine, Los Angeles Biomedical Research Institute at Harbor-UCLA Medical Centre, Torrance, CA, USA
- School of Biomedical Sciences, Faculty of Biological Sciences, University of Leeds, Leeds, UK
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