Skeletal muscle ATP turnover by 31P magnetic resonance spectroscopy during moderate and heavy bilateral knee-extension

During constant-power high-intensity exercise, the expected increase in oxygen uptake (V̇O2) is supplemented by a V̇O2 slow component (V̇O2 sc ), reflecting reduced work efficiency, predominantly within the locomotor muscles. The intracellular source of inefficiency is postulated to be an increase in the ATP cost of power production (an increase in P/W). To test this hypothesis, we measured intramuscular ATP turnover with (31)P magnetic resonance spectroscopy (MRS) and whole-body V̇O2 during moderate (MOD) and heavy (HVY) bilateral knee-extension exercise in healthy participants (n = 14). Unlocalized (31)P spectra were collected from the quadriceps throughout using a dual-tuned ((1)H and (31)P) surface coil with a simple pulse-and-acquire sequence. Total ATP turnover rate (ATPtot) was estimated at exercise cessation from direct measurements of the dynamics of phosphocreatine (PCr) and proton handling. Between 3 and 8 min during MOD, there was no discernable V̇O2 sc (mean ± SD, 0.06 ± 0.12 l min(-1)) or change in [PCr] (30 ± 8 vs. 32 ± 7 mm) or ATPtot (24 ± 14 vs. 17 ± 14 mm min(-1); each P = n.s.). During HVY, the V̇O2 sc was 0.37 ± 0.16 l min(-1) (22 ± 8%), [PCr] decreased (19 ± 7 vs. 18 ± 7 mm, or 12 ± 15%; P < 0.05) and ATPtot increased (38 ± 16 vs. 44 ± 14 mm min(-1), or 26 ± 30%; P < 0.05) between 3 and 8 min. However, the increase in ATPtot (ΔATPtot) was not correlated with the V̇O2 sc during HVY (r(2) = 0.06; P = n.s.). This lack of relationship between ΔATPtot and V̇O2 sc , together with a steepening of the [PCr]-V̇O2 relationship in HVY, suggests that reduced work efficiency during heavy exercise arises from both contractile (P/W) and mitochondrial sources (the O2 cost of ATP resynthesis; P/O)

Data collection, handling and fitting strategies to optimize accuracy and precision of oxygen uptake kinetics estimation from breath-by-breath measurements.

Phase 2 pulmonary oxygen uptake kinetics (ϕ2 τVO2P) reflect muscle oxygen consumption dynamics and are sensitive to changes in state of training or health. This study identified an unbiased method for data collection, handling and fitting to optimize VO2P kinetics estimation. A validated computational model of VO2P kinetics and a Monte Carlo approach simulated 2 x 10(5) moderate intensity transitions using a distribution of metabolic and circulatory parameters spanning normal health. Effects of averaging (interpolation, binning, stacking or separate fitting of up to 10 transitions) and fitting procedures (bi-exponential fitting, or ϕ2 isolation by time removal, statistical or derivative methods followed by mono-exponential fitting) on accuracy and precision of ϕ2 τVO2P estimation were assessed. The optimal strategy to maximize accuracy and precision of τVO2P estimation was 1-s interpolation of 4 bouts, ensemble averaged, with the first 20 s of exercise data removed. Contradictory to previous advice, we found optimal fitting procedures removed no more than 20 s of ϕ1 data. Averaging method was less critical: interpolation, bin averaging and stacking gave similar results, each with greater accuracy compared to analyzing repeated bouts separately. The optimal procedure resulted in ϕ2 τVO2P estimates for transitions from an unloaded or loaded baseline that averaged 1.97±2.08 and 1.04±2.30 s from true, but were within 2 s of true in only 47-62% of simulations. Optimized 95% confidence intervals for τVO2P ranged from 4.08-4.51 s, suggesting a minimally important difference of ~5 s to determine significant changes in τVO2P during interventional and comparative studies

SKA2 regulated hyperactive secretory autophagy drives neuroinflammation-induced neurodegeneration

High levels of proinflammatory cytokines induce neurotoxicity and catalyze inflammation-driven neurodegeneration, but the specific release mechanisms from microglia remain elusive. Here we show that secretory autophagy (SA), a non-lytic modality of autophagy for secretion of vesicular cargo, regulates neuroinflammation-mediated neurodegeneration via SKA2 and FKBP5 signaling. SKA2 inhibits SA-dependent IL-1β release by counteracting FKBP5 function. Hippocampal Ska2 knockdown in male mice hyperactivates SA resulting in neuroinflammation, subsequent neurodegeneration and complete hippocampal atrophy within six weeks. The hyperactivation of SA increases IL-1β release, contributing to an inflammatory feed-forward vicious cycle including NLRP3-inflammasome activation and Gasdermin D-mediated neurotoxicity, which ultimately drives neurodegeneration. Results from protein expression and co-immunoprecipitation analyses of male and female postmortem human brains demonstrate that SA is hyperactivated in Alzheimer's disease. Overall, our findings suggest that SKA2-regulated, hyperactive SA facilitates neuroinflammation and is linked to Alzheimer's disease, providing mechanistic insight into the biology of neuroinflammation

Critical Power: An Important Fatigue Threshold in Exercise Physiology

The hyperbolic form of the power-duration relationship is rigorous and highly conserved across species, forms of exercise and individual muscles/muscle groups. For modalities such as cycling, the relationship resolves to two parameters, the asymptote for power (critical power, CP) and the so-called W' (work doable above CP), which together predict the tolerable duration of exercise above CP. Crucially, the CP concept integrates sentinel physiological profiles - respiratory, metabolic and contractile - within a coherent framework that has great scientific and practical utility. Rather than calibrating equivalent exercise intensities relative to metabolically distant parameters such as the lactate threshold or V[spacing dot above]O2 max, setting the exercise intensity relative to CP unifies the profile of systemic and intramuscular responses and, if greater than CP, predicts the tolerable duration of exercise until W' is expended, V[spacing dot above]O2 max is attained, and intolerance is manifested. CP may be regarded as a 'fatigue threshold' in the sense that it separates exercise intensity domains within which the physiological responses to exercise can (CP) be stabilized. The CP concept therefore enables important insights into 1) the principal loci of fatigue development (central vs. peripheral) at different intensities of exercise, and 2) mechanisms of cardiovascular and metabolic control and their modulation by factors such as O2 delivery. Practically, the CP concept has great potential application in optimizing athletic training programs and performance as well as improving the life quality for individuals enduring chronic disease

A ‘ramp-sprint’ protocol to characterise indices of aerobic function and exercise intensity domains in a single laboratory test

PurposeThe lactate threshold (LT), critical power (CP) and maximum oxygen uptake (VO₂max) together partition exercise intensity domains by their common physiological, biochemical and perceptual response characteristics. CP is the greatest power output attainable immediately following intolerance at VO₂peak, and the asymptote of 3 min all-out exercise. Thus we reasoned that a maximal 'sprint' immediately following standard ramp-incremental exercise would allow characterisation of the three aerobic indices in a single test.MethodsTen healthy men (23 ± 3 year, mean ± SD) performed 9 cycle-ergometry tests on different days: (A) two ramp-incremental tests to intolerance (20 W min(-1)), immediately followed by a 3 min maximal, variable-power effort ramp-sprint test (RST) for LT, VO₂peak and sprint-phase power (SP) determination; (B) four constant-power tests for CP and VO₂max determination; (C) constant-power tests at 10 W below LT, and 10 W below and above SP to verify intensity domain characterisation. Capillary [lactate] and breath-by-breath VO₂ were measured.ResultsReproducibility of LT, SP and VO₂max measurements between RST repeats was within 5% or less (r ≥ 0.991, p &lt; 0.001). CP (257 ± 46 W) was not different (p = 0.72) from SP (258 ± 42 W). Exercise 10 W below LT and SP resulted in steady state VO₂ and [lactate]. VO₂max (4.0 ± 0.6 L min(-1)), peak [lactate] (11 ± 2 mM) and intolerance were reached 19 ± 5 min into exercise at 10 W above SP.ConclusionsThese data suggest that the key indices of aerobic function may be accurately and reliably estimated during a single exercise test. This test may provide a basis for simplifying assessment and prescription of exercise training and experimental interventions

Selecting Constant Work Rates for Endurance Testing in COPD: The Role of the Power-Duration Relationship

Constant work rate (CWR) exercise testing is highly responsive to therapeutic interventions and reveals physiological and functional benefits. No consensus exists, however, regarding optimal methods for selecting the pre-intervention work rate. We postulate that a CWR whose tolerated duration (t(lim)) is 6 minutes (WR6) may provide a useful interventional study baseline. WR6 can be extracted from the power-duration relationship, but requires 4 CWR tests. We sought to develop prediction algorithms for easier WR6 identification using backward stepwise linear regression, one in 69 COPD patients (FEV1 45 +/- 15% pred) and another in 30 healthy subjects (HLTH), in whom cycle ergometer ramp incremental (RI) and CWR tests with t(lim) of similar to 6 minutes had been performed. Demographics, pulmonary function, and RI responses were used as predictors. We validated these algorithms against power-duration measurements in 27 COPD and 30 HLTH (critical power 43 +/- 18W and 231 +/- 43W; curvature constant 5.1 +/- 2.7 kJ and 18.5 +/- 3.1 kJ, respectively). This analysis revealed that, on average, only corrected peak work rate (= WRpeak-1 min x WRslope) in RI was required to predict WR6 (COPD SEE = 5.0W; HLTH SEE = 5.6W; R-2 > 0.96; p <0.001). In the validation set, predicted and actual WR6 were strongly correlated (COPD R-2 = 0.937; HLTH 0.978; p <0.001). However, in COPD, unlike in HLTH, there was a wide range of t(lim) values at predicted WR6: COPD 8.3 +/- 4.1 min (range 3.6 to 22.2 min), and HLTH 5.5 +/- 0.7 min (range 3.9 to 7.0 min). This analysis indicates that corrected WRpeak in an incremental test can yield an acceptable basis for calculating endurance testing work rate in HLTH, but not in COPD patients