The anaerobic threshold (AT) remains a widely recognized, and contentious, concept in exercise physiology and medicine. As conceived by Karlman Wasserman, the AT coalesced the increase of blood ...lactate concentration (La−), during a progressive exercise test, with an excess pulmonary carbon dioxide output (V̇CO2). Its principal tenets were: limiting oxygen (O2) delivery to exercising muscle→increased glycolysis, La− and H+ production→decreased muscle and blood pH→with increased H+ buffered by blood HCO3−→increased CO2 release from blood→increased V̇CO2 and pulmonary ventilation. This schema stimulated scientific scrutiny which challenged the fundamental premise that muscle anoxia was requisite for increased muscle and blood La−. It is now recognized that insufficient O2 is not the primary basis for lactataemia. Increased production and utilization of La− represent the response to increased glycolytic flux elicited by increasing work rate, and determine the oxygen uptake (V̇O2) at which La− accumulates in the arterial blood (the lactate threshold; LT). However, the threshold for a sustained non‐oxidative contribution to exercise energetics is the critical power, which occurs at a metabolic rate often far above the LT and separates heavy from very heavy/severe‐intensity exercise. Lactate is now appreciated as a crucial energy source, major gluconeogenic precursor and signalling molecule but there is no ipso facto evidence for muscle dysoxia or anoxia. Non‐invasive estimation of LT using the gas exchange threshold (non‐linear increase of V̇CO2 versus V̇O2) remains important in exercise training and in the clinic, but its conceptual basis should now be understood in light of lactate shuttle biology.
figure legend As originally posited and defended most ardently by Karlman Wasserman and Brian J. Whipp, the anaerobic threshold (AT) concept represents the epitome of integrative physiological control. The graph at the upper right portrays blood La−, arterial CO2 partial pressure (PCO2) and ventilation (V̇E) as a function of V̇O2 increasing from rest to maximum (V̇O2max). The AT is identified (black arrow) by the departure of V̇E from linearity without PCO2 decreasing. As shown rightwards from the ‘Original theory’ box, the AT held that lack of (or very low) muscle O2, via the Pasteur effect, increased muscle and blood lactic acid, then H+ dissociated from the lactic acid and was buffered by bicarbonate thus increasing CO2 evolution from the blood; this stimulated the ‘extra’ V̇E at AT simultaneous with the increase in La−. Currently we understand that neither anoxia nor dysoxia underpins increased muscle La− production and efflux, but rather its increase in the blood is attributed to the rate of La− appearance being greater than disappearance. Also, as long appreciated, the variability in, and complexities of, the control of breathing dictate that the V̇CO2 versus V̇O2 relation (gas exchange threshold) more reliably approximates the lactate threshold than does the ventilatory profile. At the bottom, the modern understanding of metabolism is that La− is a central element linking glycolysis (Gly) and oxidative phosphorylation (OxPhos) energetics. Via cell signalling, La− also plays roles in short‐ and long‐term adaptations in both health and disease.
Exercise Intolerance in Untreated OSA Elbehairy, Amany F.; Geneidy, Nesma M.; Elhoshy, Mona S. ...
Chest,
January 2023, 2023-01-00, Letnik:
163, Številka:
1
Journal Article
Recenzirano
Reduced exercise capacity has been reported previously in patients with OSA hypopnea syndrome (OSAHS), although the underlying mechanisms are unclear.
What are the underlying mechanisms of reduced ...exercise capacity in untreated patients with OSAHS? Is there a role for systemic or pulmonary vascular abnormalities?
This was a cross-sectional observational study in which 14 patients with moderate to severe OSAHS and 10 control participants (matched for age, BMI, smoking history, and FEV1) underwent spirometry, incremental cycle cardiopulmonary exercise test (CPET) with arterial line, resting echocardiography, and assessment of arterial stiffness (pulse wave velocity PWV and augmentation index AIx).
Patients (age, 50 ± 11 years; BMI, 30.5 ± 2.7 kg/m2; smoking history, 2.4 ± 4.0 pack-years; FEV1 to FVC ratio, 0.78 ± 0.04; FEV1, 85 ± 14% predicted, mean ± SD for all) had mean ± SD apnea hypopnea index of 43 ± 19/h. At rest, PWV, AIx, and mean pulmonary artery pressure (PAP) were higher in patients vs control participants (P < .05). During CPET, patients showed lower peak work rate (WR) and oxygen uptake and greater dyspnea ratings compared with control participants (P < .05 for all). Minute ventilation (V·E), ventilatory equivalent for CO2 output (V·E/V·CO2), and dead space volume (VD) to tidal volume (VT) ratio were greater in patients vs control participants during exercise (P < .05 for all). Reduction in VD to VT ratio from rest to peak exercise was greater in control participants compared with patients (0.24 ± 0.08 vs 0.04 ± 0.14, respectively; P = .001). Dyspnea intensity at the highest equivalent WR correlated with corresponding values of V·E/V·CO2 (r = 0.65; P = .002), and dead space ventilation (r = 0.70; P = .001). Age, PWV, and mean PAP explained approximately 70% of the variance in peak WR, whereas predictors of dyspnea during CPET were rest-to-peak change in VD to VT ratio and PWV (R2 = 0.50; P < .001).
Patients with OSAHS showed evidence of pulmonary gas exchange abnormalities during exercise (in the form of increased dead space) and resting systemic vascular dysfunction that may explain reduced exercise capacity and increased exertional dyspnea intensity.
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The combined study of carbon (C) and oxygen (O) isotopes in plant organic matter has emerged as a powerful tool for understanding plant functional responses to environmental change. The approach ...relies on established relationships between leaf gas exchange and isotopic fractionation to derive a series of model scenarios that can be used to infer changes in photosynthetic assimilation and stomatal conductance driven by changes in environmental parameters (CO2, water availability, air humidity, temperature, nutrients). We review the mechanistic basis for a conceptual model, in light of recently published research, and discuss where isotopic observations do not match our current understanding of plant physiological response to the environment. We demonstrate that (1) the model was applied successfully in many, but not all studies; (2) although originally conceived for leaf isotopes, the model has been applied extensively to tree‐ring isotopes in the context of tree physiology and dendrochronology. Where isotopic observations deviate from physiologically plausible conclusions, this mismatch between gas exchange and isotope response provides valuable insights into underlying physiological processes. Overall, we found that isotope responses can be grouped into situations of increasing resource limitation versus higher resource availability. The dual‐isotope model helps to interpret plant responses to a multitude of environmental factors.
Summary statement
We analysed >300 studies making use of the dual carbon and oxygen isotope model (DI‐model). Based on the isotopic patterns, we can draw conclusions on how environmental variables impact photosynthesis and stomatal conductance. The concept was well received and applied to interpret plant environment interactions. Yet, some studies reported nonplausible model results. Evaluating all publications applying the DI‐model revealed most causes for nonplausibility.
Drought has become an important factor limiting crop yields in China. As an important greenhouse horticultural crop in China, the research of tomato ( Solanum lycopersicum L. cv. Jinpeng No.10) is of ...great theoretical and practical significance. In the study, four different relative soil moisture contents (74% to 80%, 55% to 61%, 47% to 52%, and 25% to 30%) were used to induce drought stress. We investigated changes in photosynthetic gas exchange, chlorophyll fluorescence, and other related physiological parameters in response to different relative soil moisture contents. Drought inhibited the photosynthesis of tomato significantly, as shown by a clear decline in the net photosynthetic rate. Our results indicated stomatal limitation and nonstomatal limitation were responsible for the photosynthesis reduction.
In the mammalian lung, an apparently homogenous mesh of capillary vessels surrounds each alveolus, forming the vast respiratory surface across which oxygen transfers to the blood
. Here we use ...single-cell analysis to elucidate the cell types, development, renewal and evolution of the alveolar capillary endothelium. We show that alveolar capillaries are mosaics; similar to the epithelium that lines the alveolus, the alveolar endothelium is made up of two intermingled cell types, with complex 'Swiss-cheese'-like morphologies and distinct functions. The first cell type, which we term the 'aerocyte', is specialized for gas exchange and the trafficking of leukocytes, and is unique to the lung. The other cell type, termed gCap ('general' capillary), is specialized to regulate vasomotor tone, and functions as a stem/progenitor cell in capillary homeostasis and repair. The two cell types develop from bipotent progenitors, mature gradually and are affected differently in disease and during ageing. This cell-type specialization is conserved between mouse and human lungs but is not found in alligator or turtle lungs, suggesting it arose during the evolution of the mammalian lung. The discovery of cell type specialization in alveolar capillaries transforms our understanding of the structure, function, regulation and maintenance of the air-blood barrier and gas exchange in health, disease and evolution.
Lactate or gas exchange threshold (GET) and critical power (CP) are closely associated with human exercise performance. We tested the hypothesis that the limit of tolerance (T
) during cycle exercise ...performed within the exercise intensity domains demarcated by GET and CP is linked to discrete muscle metabolic and neuromuscular responses. Eleven men performed a ramp incremental exercise test, 4-5 severe-intensity (SEV; >CP) constant-work-rate (CWR) tests until T
, a heavy-intensity (HVY; <CP but >GET) CWR test until T
, and a moderate-intensity (MOD; <GET) CWR test until T
Muscle biopsies revealed that a similar (
> 0.05) muscle metabolic milieu (i.e., low pH and PCr and high lactate) was attained at T
(approximately 2-14 min) for all SEV exercise bouts. The muscle metabolic perturbation was greater at T
following SEV compared with HVY, and also following SEV and HVY compared with MOD (all
< 0.05). The normalized M-wave amplitude for the vastus lateralis (VL) muscle decreased to a similar extent following SEV (-38 ± 15%), HVY (-68 ± 24%), and MOD (-53 ± 29%), (
> 0.05). Neural drive to the VL increased during SEV (4 ± 4%;
< 0.05) but did not change during HVY or MOD (
> 0.05). During SEV and HVY, but not MOD, the rates of change in M-wave amplitude and neural drive were correlated with changes in muscle metabolic (PCr, lactate) and blood ionic/acid-base status (lactate, K
) (
< 0.05). The results of this study indicate that the metabolic and neuromuscular determinants of fatigue development differ according to the intensity domain in which the exercise is performed.
The gas exchange threshold and the critical power demarcate discrete exercise intensity domains. For the first time, we show that the limit of tolerance during whole-body exercise within these domains is characterized by distinct metabolic and neuromuscular responses. Fatigue development during exercise greater than critical power is associated with the attainment of consistent "limiting" values of muscle metabolites, whereas substrate availability and limitations to muscle activation may constrain performance at lower intensities.
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