How myofilaments operate at short mammalian skeletal muscle lengths is unknown. A common assumption is that thick (myosin-containing) filaments get compressed at the Z-disc. We provide ...ultrastructural evidence of sarcomeres contracting down to 0.44 µm-approximately a quarter of thick filament resting length-in long-lasting contractions while apparently keeping a regular, parallel thick filament arrangement. Sarcomeres produced force at such extremely short lengths. Furthermore, sarcomeres adopted a bimodal length distribution with both modes below lengths where sarcomeres are expected to generate force in classic force-length measurements. Mammalian fibres did not restore resting length but remained short after deactivation, as previously reported for amphibian fibres, and showed increased forces during passive re-elongation. These findings are incompatible with viscoelastic thick filament compression but agree with predictions of a model incorporating thick filament sliding through the Z-disc. This more coherent picture of mechanical mammalian skeletal fibre functioning opens new perspectives on muscle physiology.
Key points
Dietary nitrate supplementation increases plasma nitrite concentration, which provides an oxygen‐independent source of nitric oxide and can delay skeletal muscle fatigue.
Nitrate ...supplementation has been shown to increase myofibre calcium release and force production in mouse skeletal muscle during contractions at a supra‐physiological oxygen tension, but it is unclear whether nitrite exposure can delay fatigue development and improve myofibre calcium handling at a near‐physiological oxygen tension.
Single mouse muscle fibres acutely treated with nitrite had a lower force and cytosolic calcium concentration during single non‐fatiguing contractions at a near‐physiological oxygen tension.
Nitrite treatment delayed fatigue development during repeated fatiguing isometric contractions at near‐physiological, but not at supra‐physiological, oxygen tension in combination with better maintenance of myofilament calcium sensitivity and sarcoplasmic reticulum calcium pumping.
These findings improve understanding of the mechanisms by which increased skeletal muscle nitrite exposure might be ergogenic and imply that this is related to improved calcium handling.
Dietary nitrate (NO3−) supplementation, which increases plasma nitrite (NO2−) concentration, has been reported to attenuate skeletal muscle fatigue development. Sarcoplasmic reticulum (SR) calcium (Ca2+) release is enhanced in isolated single skeletal muscle fibres following NO3− supplementation or NO2− incubation at a supra‐physiological PO2 but it is unclear whether NO2− incubation can alter Ca2+ handling and fatigue development at a near‐physiological PO2. We hypothesised that NO2− treatment would improve Ca2+ handling and delay fatigue at a physiological PO2 in intact single mouse skeletal muscle fibres. Each muscle fibre was perfused with Tyrode solution pre‐equilibrated with either 20% (PO2∼150 Torr) or 2% O2 (PO2 = 15.6 Torr) in the absence and presence of 100 µM NaNO2. At supra‐physiological PO2 (i.e. 20% O2), time to fatigue was lowered by 34% with NaNO2 (control: 257 ± 94 vs. NaNO2: 159 ± 46 s, Cohen's d = 1.63, P < 0.05), but extended by 21% with NaNO2 at 2% O2 (control: 308 ± 217 vs. NaNO2: 368 ± 242 s, d = 1.14, P < 0.01). During the fatiguing contraction protocol completed with NaNO2 at 2% O2, peak cytosolic Ca2+ concentration (Ca2+c) was not different (P > 0.05) but Ca2+c accumulation between contractions was lower, concomitant with a greater SR Ca2+ pumping rate (P < 0.05) compared to the control condition. These results demonstrate that increased exposure to NO2− blunts fatigue development at near‐physiological, but not at supra‐physiological, PO2 through enhancing SR Ca2+ pumping rate in single skeletal muscle fibres. These findings extend our understanding of the mechanisms by which increased NO2− exposure can mitigate skeletal muscle fatigue development.
Key points
Dietary nitrate supplementation increases plasma nitrite concentration, which provides an oxygen‐independent source of nitric oxide and can delay skeletal muscle fatigue.
Nitrate supplementation has been shown to increase myofibre calcium release and force production in mouse skeletal muscle during contractions at a supra‐physiological oxygen tension, but it is unclear whether nitrite exposure can delay fatigue development and improve myofibre calcium handling at a near‐physiological oxygen tension.
Single mouse muscle fibres acutely treated with nitrite had a lower force and cytosolic calcium concentration during single non‐fatiguing contractions at a near‐physiological oxygen tension.
Nitrite treatment delayed fatigue development during repeated fatiguing isometric contractions at near‐physiological, but not at supra‐physiological, oxygen tension in combination with better maintenance of myofilament calcium sensitivity and sarcoplasmic reticulum calcium pumping.
These findings improve understanding of the mechanisms by which increased skeletal muscle nitrite exposure might be ergogenic and imply that this is related to improved calcium handling.
Sprint interval training (SIT) causes fragmentation of the skeletal muscle sarcoplasmic reticulum Ca2+ release channel, ryanodine receptor 1 (RyR1), 24 h post‐exercise, potentially signalling ...mitochondrial biogenesis by increasing cytosolic Ca2+. Yet, the time course and skeletal muscle fibre type‐specific patterns of RyR1 fragmentation following a session of SIT remain unknown. Ten participants (n = 4 females; n = 6 males) performed a session of SIT (6 × 30 s ‘all‐out’ with 4.5 min rest after each sprint) with vastus lateralis muscle biopsy samples collected before and 3, 6 and 24 h after exercise. In whole muscle, full‐length RyR1 protein content was significantly reduced 6 h (mean (SD); −38 (38)%; P < 0.05) and 24 h post‐SIT (−30 (48)%; P < 0.05) compared to pre‐exercise. Examining each participant's largest response in pooled samples, full‐length RyR1 protein content was reduced in type II (−26 (30)%; P < 0.05) but not type I fibres (−11 (40)%; P > 0.05). Three hours post‐SIT, there was also a decrease in sarco(endo)plasmic reticulum Ca2+ ATPase 1 in type II fibres (−23 (17)%; P < 0.05) and sarco(endo)plasmic reticulum Ca2+ ATPase 2a in type I fibres (−19 (21)%; P < 0.05), despite no time effect for either protein in whole muscle samples (P > 0.05). PGC1A mRNA content was elevated 3 and 6 h post‐SIT (5.3‐ and 3.7‐fold change from pre, respectively; P < 0.05 for both), but peak PGC1A mRNA expression was not significantly correlated with peak RyR1 fragmentation (r2 = 0.10; P > 0.05). In summary, altered Ca2+‐handling protein expression, which occurs primarily in type II muscle fibres, may influence signals for mitochondrial biogenesis as early as 3–6 h post‐SIT in humans.
Key points
Sprint interval training (SIT) has been shown to cause fragmentation of the sarcoplasmic reticulum calcium‐release channel, ryanodine receptor 1 (RyR1), 24 h post‐exercise, which may act as a signal for mitochondrial biogenesis.
In this study, the time course was examined of RyR1 fragmentation in human whole muscle and pooled type I and type II skeletal muscle fibres following a single session of SIT.
Full‐length RyR1 protein content was significantly lower than pre‐exercise by 6 h post‐SIT in whole muscle, and fragmentation was detectable in type II but not type I fibres, though to a lesser extent than in whole muscle.
The peak in PGC1A mRNA expression occurred earlier than RyR1 fragmentation.
The increased temporal resolution and fibre type‐specific responses for RyR1 fragmentation provide insights into its importance to mitochondrial biogenesis in humans.
figure legend Western blotting was performed on whole muscle and pooled type I and II muscle fibre preparations derived from human vastus lateralis muscle biopsy samples collected before and after a single session of sprint interval training (SIT). Full‐length ryanodine receptor 1 (RyR1) protein content was reduced 6 and 24 h post‐exercise in whole muscle samples compared to baseline, despite a heterogeneous time course among individuals. This RyR1 fragmentation proceeded and outlasted the increase in peroxisome proliferator‐activated γ receptor coactivator 1α (PGC1A) mRNA expression. When examining the time point of each individual's peak response, RyR1 fragmentation was evident in type II, but not type I, muscle fibres. These findings suggest that, in humans, mitochondrial biogenesis could be influenced by RyR1 fragmentation 3–6 h post‐SIT in a fibre type‐dependent manner. Created with BioRender.com.
We investigated the susceptibility of type I and type II skeletal myofibres to atrophy in hens with hepatic fibrosis induced by bile duct ligation (BDL). Seven hens, approximately 2 years old, were ...randomly assigned to BDL (n = 4) and sham surgery (SHAM) (n = 3) groups. Mean body weight and mean liver weight as a percentage of mean body weight were significantly lower in the BDL group than in the SHAM group at 4 weeks post surgery (P = 0.002, P = 0.005, respectively). Mean plasma aspartate aminotransferase activity was slightly higher, while total cholesterol (P <0.001), total bilirubin (P = 0.022) and NH3 (P = 0.048) concentrations were significantly higher in the BDL group than in the SHAM group. Liver lesions were induced in all hens in the BDL group. The weights of the pectoralis (PCT) (P = 0.049) and flexor perforans et perforatus digiti III (FPPD III) muscles (P = 0.006) as a percentage of body weight were significantly decreased in the BDL group. A significantly reduced mean myofibre cross-sectional area in the PCT of BDL hens (P = 0.005) was indicative of atrophy. No significant differences were observed in the fibre type composition of the PCT, supracoracoideus or FPPD III muscles between the SHAM and BDL groups. However, there was an approximate 43% increase in the number of type I fibres in the femorotibialis lateralis of the BDL group and small angular type II fibres and large round type I fibres in this muscle were characteristic of peripheral neuropathy. The results suggest that type II fibres are more susceptible to atrophy than type I fibres in this model of hepatic fibrosis.
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