Expiratory flow limitation results in dynamic hyperinflation, dyspnoea and premature exercise intolerance. We aimed to measure whether expiratory resistance reduces locomotor power via limiting maximal voluntary motor activity, exacerbating muscle fatigue, or both. Healthy volunteers ( n = 14; 23 (3) years) performed a series of very heavy‐domain constant power cycling exercise tests with and without an imposed expiratory flow resistance (7 cmH 2 O/L/s). The decline in maximal evocable isokinetic power at intolerance during each experimental condition was apportioned to: (1) the power equivalent from a reduction in maximum voluntary muscle activation (termed ‘activation fatigue’); and (2) the deficit in expected power at a given isokinetic muscle activity (muscle fatigue). Imposed expiratory resistance reduced exercise tolerance (487 (145) vs. 575 (137) s; 95% confidence interval of the difference (CI diff ) 52, 125 s; P = 0.0002). At isotime‐control, imposed expiratory resistance resulted in a greater decline in inspiratory reserve volume (CI diff 0.20, 0.94 L; P = 0.007), and increased dyspnoea (Borg CR‐10; CI diff 0.7, 3.0; P = 0.006) than without. Muscle fatigue was unaffected (CI diff −20, 17 W; P = 0.873), but activation fatigue was greater with expiratory resistance (CI diff 1, 49 W; P = 0.044) and related to the reduction in inspiratory reserve volume ( r 2 = 0.53; P = 0.028). As a result, locomotor power reserve was reduced with expiratory resistance (253 (83) vs. 201 (92) W; CI diff −10, 113; P = 0.09). Imposed expiratory resistive loading initiated a cascade of abnormal lung mechanics and symptoms. These abnormalities conflate to reduce exercise tolerance through limiting maximal voluntary motor activity.

During mechanical ventilation (MV), force developed by the diaphragm decreases over time much faster than locomotor muscles, known as ventilator-induced diaphragm dysfunction (VIDD). VIDD is accompanied by an increase in intramyofiber protein S-nitrosylation, a modification of cysteines by nitric oxide (NO). During the treatment of acute respiratory distress syndrome (ARDS), inhaled NO is commonly used to improve gas exchange, but little is known about the effects of increased NO availability to the diaphragm during MV and the intracellular mechanisms to protect against excessive S-nitrosylation. We hypothesize that the enzyme S-nitrosoglutathione reductase (GSNOR) protects against excessive protein S-nitrosylation in diaphragm myofibers in conditions where NO availability is enhanced during MV, thereby preserving contractile function. We tested this hypothesis by enhancing NO availability and blocking GSNOR activity in mice during MV. Mice were treated with GSNOR inhibitor (SPL-334; GSNORi) or both GSNORi and NO donor isosorbide dinitrate (GSNORi-ISDN) together, and mechanically ventilated for 2h. E x vivo diaphragm force and mitochondrial respiration were measured. Male (C57BL/6J) 3-4 month old mice (n=27) were anesthetized and subjected to MV for 0 (non-MV), 2, 4, or 6h (150 breaths/min, 10 cmH2O PIP, 3.5 cmH2O PEEP. 8 mL/kg VT). Alternatively, mice were treated with PBS/10% DMSO (DMSO, n=6) or 25 μg SPL-334 in PBS/DMSO (GSNORi, n=6) or 25 μg SPL-334 + 1.7 mg ISDN in PBS/DMSO (GSNORi-ISDN; n=6), and then subjected to MV for 2 h. After MV, mice were euthanized, and diaphragm strips were used for force, and permeabilized fiber bundles for mitochondrial oxidative phosphorylation and H2O2 generation measurements. Peak tetanic force was decreased by MV ~23% at 4 h and ~40% at 6 h (P=0.0183 and P=0.0062, respectively, one-way ANOVA, Tukey post-test) vs 0 h. However, peak force was not different between DMSO, GSNORi and GSNORi-ISDN at 2 h MV. Coupled-phosphorylating mitochondrial respiration (Kruskal-Wallis H=2.2, p=0.35) was not different but H2O2 flux was highest in GSNORi-ISDN vs DMSO and GSNORi (Kruskal-Wallis H=6.6, p=0.03). GSNORi and ISDN treatment during MV did not produce any changes to VIDD and to mitochondrial respiration, but increased ROS production. If exogenous NO is not provided, inhibiting GSNOR in vivo alone does not affect diaphragm function ex vivo during short MV. Support: TRDRP (T32IR5221 and T29KT0397CA, to L.N.) and SDSU 2023 SEED Grant (to L.N.). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

AbstractTolerance to high‐intensity constant power exercise can be characterized by the hyperbolic power‐duration (or velocity–duration) relationship. The hyperbola is defined by the asymptote (critical power or velocity) and the curvature constant (W′ or D′). The effects of thermoregulatory stress on middle‐distance running performance are equivocal—possibly due to the complexities of the hyperbolic velocity–duration relationship for these relatively short duration events. We aimed to measure the effects of heat stress on the velocity–duration relationship in amateur runners. Fifteen participants (23 ± 6 years) completed a series of constant‐velocity running bouts to intolerance in three heat indices (MILD: 20°C, VERY HOT: 38°C, EXTREME: 55°C). Critical velocity (CV) in MILD (3.52 ± 0.86 m/s) was higher than VERY HOT (3.39 ± 0.82 m/s) and EXTREME (3.29 ± 1.05 m/s; F[2.28] = 3.80, p < 0.035) with no effect of thermal stress on D′ (F[2.28] = 2.48, p = 0.11). In amateur competitive/recreational runners, heat stress of ≥38°C heat index negatively affected CV. Thus, even during relatively short events, such as middle‐distance running where fluid loss is not a primary concern, heat stress may negatively impact performance.

Pulmonary arterial hypertension (PAH) is associated with significant morbidity and mortality. PAH is characterized by pulmonary artery remodeling, elevated right ventricular pressure (RVP) and, ultimately, cardiac failure. Pulmonary endothelial cells can sense danger or damage caused by mechanical injury or pathogens through alarmin cytokines. These cytokines can signal proliferation to restore barrier integrity or aberrant hyperproliferation and remodeling. We hypothesized that IL-33 signals pulmonary artery endothelial cells to proliferate under hypertensive conditions during the remodeling response and rise in RVP. To test this hypothesis, pulmonary hypertension (PH) was induced in C57Bl/6J, IL-33 receptor gene deleted (ST2-/- ) and MYD88 gene deleted (MYD88-/- ) mice by exposure to 10% O2 and SU5416 injections (SUHX). RVP, arterial wall thickness, endothelial cell proliferation and IL-33 levels and signaling were evaluated. In response to SUHX. RVP increased in C57Bl/6J mice in response to SUHX (49% male and 70% female; p < 0.0001) and this SUHX response was attenuated in ST2-/- mice (29% male p = 0.003; 30% female p = 0.001) and absent in MYD88-/- mice. Wall thickness was increased in SUHX C57Bl/6J mice (p = 0.005), but not in ST2-/- or MYD88-/- mice. Proliferating cells were detected in C57Bl/6J mice by flow cytometry (CD31+ /BrDU+ ; p = 0.02) and immunofluorescence methods (Ki-67+). IL-33 was increased by SUHX (p = 0.03) but a genotype effect was not observed (p = 0.76). We observed that in hPAECs, IL-33 expression is regulated by both IL-33 and DLL4. These data suggest IL-33/ST2 signaling is essential for the endothelial cell proliferative response in PH.

Pulmonary arterial hypertension (PAH) is a progressive disease of the pulmonary vasculature that leads to right ventricular failure. Skeletal muscle maladaptations limit physical activity and may contribute to disease progression. The role of alarmin/inflammatory signaling in PAH respiratory muscle dysfunction is unknown. We hypothesized that diaphragm mitochondrial and contractile functions are impaired in SU5416/hypoxia-induced pulmonary hypertension due to increased systemic IL-33 signaling. We induced pulmonary hypertension in adult C57Bl/6 J (WT) and ST2 (IL1RL1) gene ablated mice by SU5416/hypoxia (SuHx). We measured diaphragm fiber mitochondrial respiration, inflammatory markers, and contractile function ex vivo. SuHx reduced coupled and uncoupled permeabilized myofiber respiration by -40 %. During coupled respiration with complex I substrates, ST2-/- attenuated SuHx inhibition of mitochondrial respiration (genotype x treatment interaction F[1,67] = 3.3, p = 0.07, eta 2 = 0.04). Flux control ratio and coupling efficiency were not affected by SuHx or genotype. A higher substrate control ratio for succinate was observed in SuHx fibers and attenuated in ST2-/- fibers (F [1,67] = 5.3, p < 0.05, eta 2 = 0.07). Diaphragm TNF alpha, but not IL-33 or NFkB, was increased in SuHx vs. DMSO in both genotypes (F[1,43] = 4.7, p < 0.05, eta 2 = 0.1). Diaphragm force-frequency relationships were right-shifted in SuHx vs. WT (F[3,440] = 8.4, p < 0.05, eta 2 = 0.0025). There was no effect of ST2-/- on the force-frequency relationship. Force decay during a fatigue protocol at 100 Hz, but not at 40 Hz, was attenuated by SuHx vs. DMSO in both genotypes (F[1,41] = 5.6, p < 0.05, eta 2 = 0.11). SuHx mice exhibit a modest compensation in diaphragm contractility and mitochondrial dysfunction during coupled respiration; the latter partially regulated through ST2 signaling.

Tolerance for high-intensity constant power exercise is dictated by the hyperbolic relationship between power and duration. This relationship is defined by two parameters: the asymptote, termed critical power, and the curvature constant (W′). Critical power distinguishes predictable tolerance to high-intensity work from power outputs resulting in unpredictable exercise tolerance. The amount of work that can be completed above critical power is W′. Interestingly, the pacing strategy for which the participant expends W' affects exercise tolerance in short bouts. However, these improvements seem not to persist during exercise bouts 6 min or longer. Purported mechanisms for this modulation in tolerance are speeding/slowing of oxidative metabolism. One result of speeded kinetics of metabolism could be the preservation of voluntary locomotor power. Objective To measure locomotor muscle fatigue dynamics and exercise tolerance during a fast-start pacing strategy compared to a constant power task. Methods Volunteers (N=15, 27±4yr, 176±7cm, 77±12kg) completed constant and fast-start exercise tasks to the limit of tolerance above critical power. Maximal isokinetic power was measured throughout as an index of locomotor fatigue. Results Compared to constant power pacing, a fast-start pacing strategy resulted in augmented tolerance to supra-critical power exercise of (CI95 23, 79 s). Tolerance to constant power exercise was reproducible and was not different before and after the fast-start strategy bout. The decline in maximal isokinetic power was not different during constant power and fast-start pacing strategies, nor was there an interaction of pacing strategy and time (F[5, 168]=0.5, p=0.75). Maximal isokinetic power was lower during the fast-start strategy vs constant power when the data were normalized to baseline isokinetic power (F[1,168]=6.2, p=0.01). However, no interaction of pacing strategy and time was present (F[5,168]=0.39, p=0.85). Conclusions While fast-start pacing improved exercise tolerance and augmented the supra-critical power work tolerance, the dynamics of locomotor fatigue were not different between conditions. Interestingly, our data argue against a fixed volume of work (W') above critical power and lend more evidence that W' is a result of a complex conflation of fatigue-related processes rather than a simple work capacity.

ABSTRACT Introduction The mechanism(s) of exercise intolerance at V˙O2max remain poorly understood. In health, standard ramp-incremental (RI) exercise is limited by fatigue-induced reductions in maximum voluntary cycling power. Whether neuromuscular fatigue also limits exercise when the RI rate is slow and RI peak power at intolerance is lower than standard RI exercise, is unknown. Methods In twelve healthy participants, maximal voluntary cycling power was measured during a short (~6 s) isokinetic effort at 80 rpm (Piso) at baseline and, using an instantaneous switch from cadence-independent to isokinetic cycling, immediately at the limit of RI exercise with RI rates of 50, 25, and 10 W·min−1 (RI-50, RI-25, and RI-10). Breath-by-breath pulmonary gas exchange was measured throughout. Results Baseline Piso was not different among RI rates (analysis of variance; P > 0.05). Tolerable duration increased with decreasing RI rate (RI-50, 411 ± 58 s vs RI-25, 732 ± 93 s vs RI-10, 1531 ± 288 s; P < 0.05). At intolerance, V˙O2peak was not different among RI rates (analysis of variance; P > 0.05), but RI peak power decreased with RI rate (RI-50, 361 ± 48 W vs RI-25, 323 ± 39 W vs RI-10, 275 ± 38 W; P < 0.05). Piso at intolerance was 346 ± 43 W, 353 ± 45 W, and 392 ± 69 W for RI-50, RI-25, and RI-10, respectively (P < 0.05 for RI-10 vs RI-50 and RI-25). At intolerance, in RI-50 and RI-25, Piso was not different from RI peak power (P > 0.05), thus there was no “power reserve.” In RI-10, Piso was greater than RI peak power at intolerance (P < 0.001), that is, there was a “power reserve.” Conclusions In RI-50 and RI-25, the absence of a power reserve suggests the neuromuscular fatigue-induced reduction in Piso coincided with V˙O2max and limited the exercise. In RI-10, the power reserve suggests neuromuscular fatigue was insufficient to limit the exercise, and additional mechanisms contributed to intolerance at V˙O2max.

Expiratory flow limitation is a key characteristic in obstructive pulmonary diseases. To study abnormal lung mechanics isolated from heterogeneities of obstructive disease, we measured pulmonary function in healthy adults with expiratory loading. Thirty-seven volunteers (25±5 yr) completed spirometry and body plethysmography under control and threshold expiratory loading of 7, 11 cmH2O, and a subset at 20 cmH2O (n = 11). We analyzed the shape of the flow-volume relationship with rectangular area ratio (RAR; Ma et al., Respir Med 2010). Airway resistance was increased (p<0.0001) with 7 and 11 cmH2O loading vs control (9.20±1.02 and 11.76±1.68 vs. 2.53± 0.80 cmH2O/L/s). RAR was reduced (p = 0.0319) in loading vs control (0.45±0.07 and 0.47±0.09L vs. 0.48±0.08). FEV1 was reduced (p<0.0001) in loading vs control (3.24±0.81 and 3.23±0.80 vs. 4.04±1.05 L). FVC was reduced (p<0.0001) in loading vs control (4.11±1.01 and 4.14±1.03 vs. 5.03±1.34 L). Peak expiratory flow (PEF) was reduced (p<0.0001) in loading vs control (6.03±1.67 and 6.02±1.84 vs. 8.50±2.81 L/s). FEV1/FVC (p<0.0068) was not clinically significant and FRC (p = 0.4) was not different in loading vs control. Supra-physiologic loading at 20 cmH2O did not result in further limitation. Expiratory loading reduced FEV1, FVC, PEF, but there were no clinically meaningful differences in FEV1/FVC, FRC, or RAR. Imposed expiratory loading likely leads to high airway pressures that resist dynamic airway compression. Thus, a concave expiratory flow-volume relationship was consistently absent-a key limitation for model comparison with pulmonary function in COPD. Threshold loading may be a useful strategy to increase work of breathing or induce dynamic hyperinflation.

INTRODUCTION There is no required training for breath-hold diving, making dissemination of safety protocols difficult. A recommended breath-hold dive time limit of 60 s was proposed for amateur divers. However, this does not consider the metabolic-rate dependence of oxygen stores depletion. We aimed to measure the effect of apnoea time and metabolic rate on arterial and tissue oxygenation. METHODS Fifty healthy participants (23 (SD 3) y, 22 women) completed four periods of apnoea for 60 s (or to tolerable limit) during rest and cycle ergometry at 20, 40, and 60 W. Apnoea was initiated after hyperventilation to achieve PETCO2 of approximately 25 mmHg. Pulse oximetry, frontal lobe oxygenation, and pulmonary gas exchange were measured throughout. We defined hypoxia as SpO2 < 88%. RESULTS Static and exercise (20, 40, 60 W) breath-hold break times were 57 (SD 7), 50 (11), 48 (11), and 46 (11) s (F [2.432, 119.2] = 32.0, P < 0.01). The rise in PETCO2 from initiation to breaking of apnoea was dependent on metabolic rate (time × metabolic rate interaction; F [3,147] = 38.6, P < 0.0001). The same was true for the fall in SpO2 (F [3,147] = 2.9, P = 0.03). SpO2 fell to < 88% on 14 occasions in eight participants, all of whom were asymptomatic. CONCLUSIONS Independent of the added complexities of a fall in ambient pressure on ascent, the effect of apnoea time on hypoxia depends on the metabolic rate and is highly variable among individuals. Therefore, we contend that a universally recommended time limit for breath-hold diving or swimming is not useful to guarantee safety.

We report a systematic combinatorial exploration of affinity enhancement of antibodies by insertions and deletions (InDels). Transposon-based introduction of InDels via the method TRIAD (transposition-based random insertion and deletion mutagenesis) was used to generate large libraries with random in-frame InDels across the entire single-chain variable fragment gene that were further recombined and screened by ribosome display. Knowledge of potential insertion points from TRIAD libraries formed the basis of exploration of length and sequence diversity of novel insertions by insertional-scanning mutagenesis (InScaM). An overall 256-fold affinity improvement of an anti-IL-13 antibody BAK1 as a result of InDel mutagenesis and combination with known point mutations validates this approach, and suggests that the results of this InDel mutagenesis and conventional exploration of point mutations can synergize to generate antibodies with higher affinity.

Expiratory flow limitation is a key characteristic in chronic obstructive pulmonary disease (COPD). Increased airway resistance occurs due to bronchoconstriction, destruction of elastic tissue in the airways, and mucus hypersecretion from goblet cells caused by irritation of the epithelium. Obstruction can lead to dynamic hyperinflation, dyspnea, and exercise intolerance. However, increased airway resistance is just one of many abnormalities in COPD and asthma ‐ COPD in particular is an exceedingly heterogeneous disease. Therefore, isolating the effects of expiratory airflow limitation is challenging.PURPOSEIn order to study abnormal lung mechanics in isolation, we measured pulmonary function with and without expiratory loading in healthy adults.METHODSForty‐eight volunteers (26±5 yr, 171.8±10.2 cm, 72.2±13.3 kg) completed spirometry and body plethysmography according to the ATS/ERS standards under three randomized conditions: control, expiratory loading of 7 and 11 cmH2O. The expiratory load was added by installing a threshold inspiratory muscle trainer in reverse. We analyzed the shape of the flow‐volume (F‐V) loops with rectangular area ratio (RAR) using custom MATLab software.RESULTSAirway resistance was increased (F[1.5, 27.5]= 446.0, p<0.05) with 7 and 11 cmH2O vs control (9.20±1.02 and 11.76±1.68 cmH2O vs. 2.53± 0.80 cmH2O). RAR (F[1.9, 77.9]= 3.71, p>0.05) was reduced with 7 and 11 cmH2O vs control (0.45±0.07 and 0.47±0.09L vs. 0.48±0.08L). FEV1 was reduced (F[1.6, 60.38.6]= 67.54, p<0.05) with 7 and 11 cmH2O vs control (3.26±0.82 and 3.23±0.79L vs. 4.03±1.04L). FVC was also reduced (F[1.5, 55.31] = 77.71, p<0.05) with 7 and 11 cmH2O vs control (4.15±1.01 and 4.17±1.02L L vs. 5.05±1.33L L). PEF was reduced (F[1.5, 55.99] = 72.69, p<0.05) with 7 and 11 cmH2O vs control (6.02±1.65 and 5.95±1.85L vs. 8.39±2.85L L). FEV1/FVC (F[1.9, 69.17] = 5.5, p> 0.05) and FRC (F[1.863, 33.53]= 0.93, p<0.05) were not different between resistance conditions or compared to the control.CONCLUSIONSExpiratory loading reduced FEV1, FVC, PEF and provided a doseresponse in airway resistance and RAR. There were no differences in FEV1/FVC. While RAR was reduced with expiratory loading, the magnitude was of little clinical importance. Therefore, a concave expiratory F‐V relationship was consistently absent – a key limitation for model comparison with pulmonary function in COPD. This is most likely due to the imposed resistance being applied outside of the airways, leading to higher airway pressures. The higher airway pressures are likely to resist dynamic airway compression and either maintain the equal pressure point position or possibly move it proximally.

PURPOSE We aimed to measure 1) the dynamics of locomotor fatigue during constant supra-critical power cycling, and 2) the magnitude of any reserve in locomotor power at intolerance to constant and ramp-incremental cycling in recreationally-active volunteers. METHODS Fifteen participants (7 women and 8 men, 22±3 yr, 3.34±0.67 L.min V[Combining Dot Above]O2peak) completed ramp-incremental and very-heavy constant power (205±46 W) exercise to the limit of tolerance. Immediately following intolerance, the ergometer was switched into the isokinetic mode and participants completed a short (~5 s) maximal isokinetic effort at 70 rpm. The time course of locomotor fatigue during constant supra-critical power exercise was characterized with these short maximal isokinetic sprints at 30, 60, 120, 180 s and at the limit of tolerance. Each bout was terminated following the isokinetic sprint. RESULTS Constant power exercise duration was 312±37 s. Isokinetic power production at 30, 60, 120, 180 s and the limit of tolerance (at 312±37 s) was 609±165, 503±195, 443±157, 449±133, and 337±94 W, respectively. Of the total decline in isokinetic power, ~36% occurred within the first minute of exercise and significant (p<0.05) reductions in isokinetic power occurred at all time-points vs the baseline maximal isokinetic power (666±158 W). Additionally, a significant power reserve of 132±74 W (64% of the task requirement) and 119±80 W (47%) was present at the limit of constant power and ramp-incremental exercise, respectively. CONCLUSIONS Locomotor fatigue occurred rapidly during supra-critical power exercise with pseudo-exponential kinetics. Instantaneous isokinetic power production at the limit of tolerance exceeded that of the task requirement, regardless of the constant, or ramp work rate profile. Thus, the perceptual and physiologic limits were dissociated at the limit of tolerance in recreationally-active volunteers.

Diaphragm dysfunction accompanies cardiopulmonary disease and impaired oxygen delivery. Vascular endothelial growth factor (VEGF) regulates oxygen delivery through angiogenesis, capillary maintenance, and contraction-induced perfusion. We hypothesized that myofiber-specific VEGF deficiency contributes to diaphragm weakness and fatigability. Diaphragm protein expression, capillarity and fiber morphology, mitochondrial respiration and hydrogen peroxide (H2O2) generation, and contractile function were compared between adult mice with conditional gene ablation of skeletal myofiber VEGF (SkmVEGF-/-; n = 12) and littermate controls (n = 13). Diaphragm VEGF protein was ~50% lower in SkmVEGF-/- than littermate controls (1.45 ± 0.65 vs. 3.04 ± 1.41 pg/total protein; P = 0.001). This was accompanied by an ~15% impairment in maximal isometric specific force (F[1,23] = 15.01, P = 0.001) and a trend for improved fatigue resistance (P = 0.053). Mean fiber cross-sectional area and type I fiber cross-sectional area were lower in SkmVEGF-/- by ~40% and ~25% (P < 0.05). Capillary-to-fiber ratio was also lower in SkmVEGF-/- by ~40% (P < 0.05), and thus capillary density was not different. Sarcomeric actin expression was ~30% lower in SkmVEGF-/- (P < 0.05), whereas myosin heavy chain and MAFbx were similar (measured via immunoblot). Mitochondrial respiration, citrate synthase activity, PGC-1α, and hypoxia-inducible factor 1α were not different in SkmVEGF-/- (P > 0.05). However, mitochondrial-derived reactive oxygen species (ROS) flux was lower in SkmVEGF-/- (P = 0.0003). In conclusion, myofiber-specific VEGF gene deletion resulted in a lower capillary-to-fiber ratio, type I fiber atrophy, actin loss, and contractile dysfunction in the diaphragm. In contrast, mitochondrial respiratory function was preserved alongside lower ROS generation, which may play a compensatory role to preserve fatigue resistance in the diaphragm.NEW & NOTEWORTHY Diaphragm weakness is a hallmark of diseases in which oxygen delivery is compromised. Vascular endothelial growth factor (VEGF) modulates muscle perfusion; however, it remains unclear whether VEGF deficiency contributes to the onset of diaphragm dysfunction. Conditional skeletal myofiber VEGF gene ablation impaired diaphragm contractile function and resulted in type I fiber atrophy, a lower number of capillaries per fiber, and contractile protein content. Mitochondrial function was similar and reactive oxygen species flux was lower. Diaphragm VEGF deficiency may contribute to the onset of respiratory muscle weakness.

Respiratory muscle weakness is a hallmark of COPD, heart failure, and critical illness. Vascular endothelial growth factor (VEGF) may be an important therapeutic target in chronic cardiopulmonary disease as it is essential for neovascularization and vessel repair following trauma. We hypothesized that respiratory muscle, which is required to persistently function, would exhibit impaired mitochondrial, structural, and contractile function in skeletal myofiber VEGF gene deleted mice.We measured diaphragm biochemistry, structure, and contractile function in adult mice with conditional deletion of the VEGF‐A gene in skeletal myofibers (SkmVEGF−/−). Male SkmVEGF−/− mice (n=12) were compared to wild type (WT) controls (n=13) – both on a C57BL/6J background. Fiber bundles were isolated and electrically stimulated in vitro to evaluate force‐frequency and fatigue profiles. Mitochondrial O2 consumption and H2O2 flux (index of mitochondrial ROS) were simultaneously measured in saponin‐permeabilized fiber bundles (Oroboros, Innsbruck, AT). Fiber structure and biochemistry were measured using standard techniques (IHC, Western, ELISA).Diaphragm VEGF protein was lower in SkmVEGF−/− than WT mice (1.5±0.2 vs. 3.0±0.4 pg/total protein, p<0.05). Compared to WT mice, the force‐frequency relationship was depressed in SkmVEGF−/− (F[7,161]=15.0, p<0.05), with maximal specific force reduced by ~10 % (24±1 vs. 21±1 N/cm2; p<0.05). However, fatigue resistance tended to be greater following SkmVEGF−/− (F[2,115]=3.3, p=0.06). Fiber type proportions were unchanged, but type I fiber cross‐sectional area was reduced in SkmVEGF−/− (653±45 vs 493±53 μm2; p<0.05). Compared to WT, sarcomeric actin protein expression was reduced by ~30% in SkmVEGF−/− (p<0.05) while myosin heavy chain, MAFbx, MuRF1, PGC1α, and HIF1α protein remained unchanged (p>0.05). Mitochondrial respiration was not different between SkmVEGF−/− and WT (F[1,80]=0.9, p>0.05) in each of the respiratory states. However, H2O2 flux was reduced in SkmVEGF−/− (F[1,75]=14.6, p<0.05).Skeletal myofiber‐specific VEGF deletion contributed to diaphragm weakness, but tended to improve fatigue resistance in this highly oxidative muscle. Reductions in oxidative fiber size, sarcomeric actin protein content, and ROS generation following VEGF gene deletion may contribute to protecting mitochondrial respiratory function in the diaphragm. The adaptations may be part of compensatory mechanisms mitigating larger deficits in force generation and fatigue resistance.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Tolerance to high‐intensity exercise is described by a hyperbolic power‐tolerable duration (P‐tLIM) relationship. The asymptote of the P‐tLIM relationship, termed critical power, demarcates sustainable from non‐sustainable exercise. However, characterizing this relationship is time consuming – typically 4 constant power exercise tests to intolerance are required. Alternatively, at the limit of tolerance to supra‐critical power exercise, a 3 min all‐out sprint approximates critical power (Murgatroyd et al., Eur J Appl Physiol 114:1863–74, 2014). We aimed to determine whether maximal isokinetic power (30 s in duration) measured immediately at the limit of tolerance approximates critical power.Ten participants (3 females, 7 males, 22±2 yr, 73±11 kg, VO2peak 3.62±0.87 L.min−1) completed constant supra‐critical power exercise tests to intolerance, immediately followed by a maximal isokinetic effort at 80 rpm for 30 s. Mean isokinetic power over the final 20 s was compared to the critical power asymptote determined from four variable‐duration constant power tests to intolerance. Mean bias and limits of agreement were calculated using the method of Bland & Altman.Critical power estimated from the P‐tLIM relationship was 165±54 W with 95% confidence interval of 157, 172 W. Of the four constant power tests used to define P‐tLIM, isokinetic power was not different to critical power only following the longest duration trial (p>0.05, 186±65 vs 165±54 W). However, mean bias and limits of agreement were 21 W and ±88 W, respectively. In the remaining trials, isokinetic power immediately following the limit of tolerance was greater than critical power (p<0.05).Isokinetic power measured immediately following the limit of tolerance consistently overestimated critical power. While not different to critical power, the closest estimation showed more than 20 W bias with limits of agreement nearly 12 times the span of the 95% confidence interval. Thus, brief maximal isokinetic power (30 s) immediately following the limit of tolerance does not approximate critical power.

At the limit of tolerance to ramp‐incremental exercise, maximal voluntary isokinetic power is not different to the power required by the task in endurance‐trained athletes (Ferguson et al., J Appl Physiol 120 (1):70–7, 2016). This is consistent with the physiologic limits for power production having been attained at the limit of ramp‐incremental exercise. The dynamics of the decline in locomotor power, and whether the physiologic limits are similarly attained during constant‐power cycling are unknown. We aimed to measure 1) the dynamics of locomotor fatigue during constant supra‐critical power cycling, and 2) the magnitude of any reserve in locomotor power immediately at the tolerable limit of both constant and ramp‐incremental cycling in recreationally active volunteers. Fifteen participants (7 females, 8 males, 22 ± 3 yr, 65.9 ± 12.4 kg, VO 2peak 3.34 ± 0.67 L.min −1 ) completed ramp‐incremental (25 W.min −1 ) and constant power (205 ± 46 W) exercise to the limit of tolerance. Immediately following intolerance, participants completed a short (<5 s) maximal isokinetic effort at 70 rpm. The time course of locomotor fatigue during constant supra‐critical power exercise was characterized using short maximal isokinetic sprints interleaved at 30 s, 60 s, 120 s, 180 s, and at the limit of tolerance. Constant power exercise duration was 312 ± 37 s. Isokinetic power production at 30, 60, 120, 180 s and the limit of tolerance was 609 ± 165, 503 ± 195, 443 ± 157, 449 ± 133, and 337 ± 94 W, respectively. Of the total decline in isokinetic power, ~50% occurred within the first minute of exercise and significant (p<0.05) reductions in isokinetic power occurred at all time‐points vs the baseline maximal isokinetic power (666 ± 158 W). Additionally, a significant power reserve of 132 ± 74 (64 % of the task requirement) and 119 ± 80 W (47 %) was present at the limit of constant power and ramp‐incremental exercise. Locomotor fatigue occurred rapidly during supra‐critical power exercise with pseudo‐exponential kinetics. The dynamics of locomotor fatigue were similar to the dynamics reported for primary fatigue‐related intramuscular metabolites, suggesting a close mechanistic link between the intramuscular milieu and voluntary muscle power. Instantaneous isokinetic power production at the limit of tolerance exceeded that of the task requirement, regardless of the work rate profile. Thus, the perceptual and physiologic limits were dissociated at the limit of tolerance in recreationally active volunteers.

Expiratory flow resistance (e.g. asthma, chronic obstructive pulmonary disease) results in dyspnea, dynamic hyperinflation, and premature exercise intolerance during heavy intensity exercise. This cascade seems to be primarily precipitated by abnormal lung mechanics that result in exacerbated symptoms and reduced maximal evocable limb motor activity. We do not know whether this cascade behaves in the same way during moderate exercise where only modest locomotor fatigue is expected to develop. We hypothesized that expiratory flow resistance would result in dynamic hyperinflation, dyspnea, and exacerbated locomotor fatigue during moderate intensity exercise in a similar manner to that during heavy intensity exercise. Volunteers (N=10, 27±6 yr) completed constant power moderate exercise (50% peak ramp power) with 3 levels of imposed expiratory flow resistance (7, 9, 11 cm H 2 O·L·s −1 ) or control (no imposed flow resistance). We measured operating lung volumes during exercise with inspiratory capacity maneuvers. Participants reported dyspnea and leg effort using a Borg 10‐point scale. At the termination of exercise we used a rapid switch from hyperbolic to isokinetic cycling to measure the decline in peak isokinetic power (P iso ). Decline in P iso was not different when expiratory flow resistance was imposed ( p >0.05). Inspiratory capacity was smaller at the end of exercise in the most severe resistance (11 cm H 2 O·L·s −1 ) as compared to control (2.31±1.52 vs. 3.15±0.71 L; p <0.05). Similarly, inspiratory reserve volume was smaller at the end of exercise in the most severe resistance as compared to control (−0.07±1.28 vs. 0.82±0.43 L; p <0.05). Dyspnea was greater at the end of exercise in both mild and severe resistance (7 and 11 cm H 2 O·L·s −1 ) as compared to control (5.7±2.5 and 5.2±1.9 vs. 3.7±1.6; p <0.05). P iso was not related to inspiratory reserve volume, inspiratory capacity, or dyspnea ( p >0.05). Imposed severe expiratory flow resistance results in hyperinflation and exacerbated dyspnea in healthy volunteers. Unlike what we have found during heavy exercise, however, abnormal lung mechanics and dyspnea were not related to maximal locomotor power during moderate exercise.