Matthew T. Lewis

This study in humans was designed to evaluate: (1) the overall relationships between muscle fatigue and inorganic phosphate (Pi) and hydrogen ions (H ⁺ ) in women and men, and (2) whether the decline in contractile function for a given change in these intramuscular metabolites differs between sexes (i.e. muscle fatigue sensitivity). Sixteen healthy, young individuals (eight women) performed two consecutive (interspersed by 5 min of rest) intermittent isometric single‐leg knee‐extensor trials (60 maximal voluntary contractions; 3 s contraction, 2 s relaxation). Throughout both trials, intramuscular quadriceps [Pi] and [H ⁺ ] were quantified using phosphorus magnetic resonance spectroscopy, and quadriceps twitch force ( Q _tw ) was measured using electrical femoral nerve stimulation. The exercise‐induced reduction in Q _tw was greater in men than in women in both trials (both P < 0.048). In both sexes, the Q _tw –[Pi] relationship was unchanged across trials, while the Q _tw –[H ⁺ ] relationship shifted downwards. The decline in Q _tw for a given increase in [Pi] or [H ⁺ ] was not different between men and women. The exercise‐induced reduction in Q _tw was strongly associated only with Pi accumulation ( r = 0.761, P < 0.001). These results show that, in both sexes, Q _tw is more consistently related to Pi than to H ⁺ , and that the decrease in Q _tw for a given increase in [Pi] and [H ⁺ ] does not differ between women and men. This supports that the in vivo metabolic basis of muscle fatigue is similar across sexes, and that differences in the exercise‐induced reduction in contractile function relate to the extent of metabolic disturbance, rather than to an inherent fatigue resistance. image Key points: The decline in muscle contractile function during high‐intensity exercise (i.e. muscle fatigue) is generally less in women than in men. Sex‐related differences in the intrinsic resistance to intramuscular metabolites may explain this divergence. We evaluated (1) the overall relationships between muscle fatigue and inorganic phosphate (Pi) and hydrogen ions (H ⁺ ) in women and men, and (2) whether the decline in contractile function for a given change in intramuscular metabolites differs between sexes. In both sexes, intramuscular Pi was more consistently related to muscle fatigue compared with H ⁺ . For each metabolite (Pi or H ⁺ ), the decrease in contractile function for a given intramuscular accumulation was similar in women and men. In conclusion, in vivo the metabolic basis of muscle fatigue is similar between sexes. Sex differences in the magnitude of fatigue are therefore probably not due to an intrinsic resistance to intramuscular metabolites.

Intramuscular hydrogen ion (H⁺) and inorganic phosphate (Pi) concentrations were dissociated during exercise to challenge their relationships with peripheral and central fatigue in vivo. Ten recreationally active, healthy men (27 ± 5 years; 180 ± 4 cm; 76 ± 10 kg) performed two consecutive intermittent isometric single‐leg knee‐extensor trials (60 maximal voluntary contractions; 3 s contraction, 2 s relaxation) interspersed with 5 min of rest. Phosphorus magnetic resonance spectroscopy (³¹P‐MRS) was used to continuously quantify intramuscular [H⁺] and [Pi] during both trials. Using electrical femoral nerve stimulation, quadriceps twitch force (Q_tw) and voluntary activation (VA) were quantified at rest and throughout both trials. Decreases in Q_tw and VA from baseline were used to determine peripheral and central fatigue, respectively. Q_tw was strongly related to both [H⁺] (β coefficient: −0.9, P < 0.0001) and [Pi] (−1.1, P < 0.0001) across trials. There was an effect of trial on the relationship between Q_tw and [H⁺] (−0.5, P < 0.0001), but not Q_tw and [Pi] (0.0, P = 0.976). This suggests that, unlike the unaltered association with [Pi], a given level of peripheral fatigue was associated with a different [H⁺] in Trial 1 vs. Trial 2. VA was related to [H⁺] (−0.3, P < 0.0001), but not [Pi] (−0.2, P = 0.243), across trials and there was no effect of trial (−0.1, P = 0.483). Taken together, these results support intramuscular Pi as a primary cause of peripheral fatigue, and muscle acidosis, probably acting on group III/IV muscle afferents in the interstitial space, as a contributor to central fatigue during exercise. image Key points: We investigated the relationship between intramuscular metabolites and neuromuscular function in humans performing two maximal, intermittent, knee‐extension trials interspersed with 5 min of rest. Concomitant measurements of intramuscular hydrogen (H⁺) and inorganic phosphate (Pi) concentrations, as well as quadriceps twitch‐force (Q_tw) and voluntary activation (VA), were made throughout each trial using phosphorus magnetic resonance spectroscopy (³¹P‐MRS) and electrical femoral nerve stimulations. Although [Pi] fully recovered prior to the onset of the second trial, [H⁺] did not. Q_tw was strongly related to both [H⁺] and [Pi] across both trials. However, the relationship between Q_tw and [H⁺] shifted leftward from the first to the second trial, whereas the relationship between Q_tw and [Pi] remained unaltered. VA was related to [H⁺], but not [Pi], across both trials. These in vivo findings support the hypotheses of intramuscular Pi as a primary cause of peripheral fatigue, and muscle acidosis, probably acting on group III/IV muscle afferents, as a contributor to central fatigue.

Recently it was documented that fatiguing, high-intensity exercise resulted in a significant attenuation in maximal skeletal muscle mitochondrial respiratory capacity, potentially due to the intramuscular metabolic perturbation elicited by such intense exercise. With the utilization of intrathecal fentanyl to attenuate afferent feedback from group III/IV muscle afferents, permitting increased muscle activation and greater intramuscular metabolic disturbance, this study aimed to better elucidate the role of metabolic perturbation on mitochondrial respiratory function. Eight young, healthy males performed high-intensity cycle exercise in control (CTRL) and fentanyl-treated (FENT) conditions. Liquid chromatography-mass spectrometry and high-resolution respirometry were used to assess metabolites and mitochondrial respiratory function, respectively, pre- and postexercise in muscle biopsies from the vastus lateralis. Compared with CTRL, FENT yielded a significantly greater exercise-induced metabolic perturbation (PCr: −67% vs. −82%, Pi: 353% vs. 534%, pH: −0.22 vs. −0.31, lactate: 820% vs. 1,160%). Somewhat surprisingly, despite this greater metabolic perturbation in FENT compared with CTRL, with the only exception of respiratory control ratio (RCR) (−3% and −36%) for which the impact of FENT was significantly greater, the degree of attenuated mitochondrial respiratory capacity postexercise was not different between CTRL and FENT, respectively, as assessed by maximal respiratory flux through complex I (−15% and −33%), complex II (−36% and −23%), complex I + II (−31% and −20%), and state 3_CI+CII control ratio (−24% and −39%). Although a basement effect cannot be ruled out, this failure of an augmented metabolic perturbation to extensively further attenuate mitochondrial function questions the direct role of high-intensity exercise-induced metabolite accumulation in this postexercise response.

The impact of COVID-19 has been largely described after symptom development. Although the SARS-CoV-2 virus elevates heart rate (HR) prior to symptom onset, whether this virus evokes other presymptomatic alterations is unknown. This case study details the presymptomatic impact of COVID-19 on vascular and skeletal muscle function in a young woman [24 yr, 173.5 cm, 89 kg, body mass index (BMI): 29.6 kg·m^–2]. Vascular and skeletal muscle function were assessed as part of a separate study with the first and second visits separated by 2 wk. On the evening following the second visit, the participant developed a fever and a rapid antigen test confirmed a positive COVID-19 diagnosis. Compared with the first visit, the participant presented with a markedly elevated HR (∼30 beats/min) and a lower mean blood pressure (∼8 mmHg) at the second visit. Vascular function measured by brachial artery flow-mediated dilation, reactive hyperemia, and passive leg movement were all noticeably attenuated (25%–65%) as was leg blood flow during knee extension exercise. Muscle strength was diminished as was ADP-stimulated respiration (30%), assessed in vitro, whereas there was a 25% increase in the apparent Km. Lastly, an elevation in IL-10 was observed prior to symptom onset. Notably, 2.5 mo after diagnosis symptoms of fatigue and cough were still present. Together, these findings provide unique insight into the physiological responses immediately prior to onset of COVID-19 symptoms; they suggest that SARS-CoV-2 negatively impacts vascular and skeletal muscle function prior to the onset of common symptoms and may set the stage for the widespread sequelae observed following COVID-19 diagnosis. NEW & NOTEWORTHY This unique case study details the impact of SARS-CoV-2 infection on vascular and skeletal muscle function in a young predominantly presymptomatic woman. Prior to COVID-19 diagnosis, substantial reductions in vascular, skeletal muscle, and mitochondrial function were observed along with an elevation in IL-10. This integrative case study indicates that the presymptomatic impact of COVID-19 is widespread and may help elucidate the acute and long-term sequelae of this disease.

New Findings This Symposium Review summarizes the understanding of altered perfusion distribution at skeletal muscle arteriolar bifurcations and how this is modified by development of chronic metab...

Type 2 diabetes (T2D) is a growing health concern with nearly 400 million affected worldwide as of 2014. T2D presents with hyperglycemia and insulin resistance resulting in increased risk for blindness, renal failure, nerve damage, and premature death. Skeletal muscle is a major site for insulin resistance and is responsible for up to 80% of glucose uptake during euglycemic hyperglycemic clamps. Glucose uptake in skeletal muscle is driven by mitochondrial oxidative phosphorylation and for this reason mitochondrial dysfunction has been implicated in T2D. In this review we integrate mitochondrial function with physiologic function to present a broader understanding of mitochondrial functional status in T2D utilizing studies from both human and rodent models. Quantification of mitochondrial function is explained both in vitro and in vivo highlighting the use of proper controls and the complications imposed by obesity and sedentary lifestyle. This review suggests that skeletal muscle mitochondria are not necessarily dysfunctional but limited oxygen supply to working muscle creates this misperception. Finally, we propose changes in experimental design to address this question unequivocally. If mitochondrial function is not impaired it suggests that therapeutic interventions and drug development must move away from the organelle and toward the cardiovascular system.

Type 2 diabetes (T2D) presents with hyperglycemia and insulin resistance, affecting over 30 million people in the United States alone. Previous work has hypothesized that mitochondria are dysfunctional in T2D and results in both reduced ATP production and glucose disposal. However, a direct link between mitochondrial function and T2D has not been determined. In the current study, the Goto-Kakizaki (GK) rat model of T2D was used to quantify mitochondrial function in vitro and in vivo over a broad range of contraction-induced metabolic workloads. During high-frequency sciatic nerve stimulation, hindlimb muscle contractions at 2- and 4-Hz intensities, the GK rat failed to maintain similar bioenergetic steady states to Wistar control (WC) rats measured by phosphorus magnetic resonance spectroscopy, despite similar force production. Differences were not due to changes in mitochondrial content in red (RG) or white gastrocnemius (WG) muscles (cytochrome c oxidase, RG: 22.2 ± 1.6 vs. 23.3 ± 1.7 U/g wet wt; WG: 10.8 ± 1.1 vs. 12.1 ± 0.9 U/g wet wt; GK vs. WC, respectively). Mitochondria isolated from muscles of GK and WC rats also showed no difference in mitochondrial ATP production capacity in vitro, measured by high-resolution respirometry. At lower intensities (0.25–1 Hz) there were no detectable differences between GK and WC rats in sustained energy balance. There were similar phosphocreatine concentrations during steady-state contraction and postcontractile recovery (τ = 72 ± 6 s GK versus 71 ± 2 s WC). Taken together, these results suggest that deficiencies in skeletal muscle energetics seen at higher intensities are not due to mitochondrial dysfunction in the GK rat.

Despite extensive investigation into the impact of metabolic disease on vascular function and, by extension, tissue perfusion and organ function, interpreting results for specific risk factors can be complicated by the additional risks present in most models. To specifically determine the impact of type 2 diabetes without obesity on skeletal muscle microvascular structure/function and on active hyperemia with elevated metabolic demand, we used 17-wk-old Goto-Kakizaki (GK) rats to study microvascular function at multiple levels of resolution. Gracilis muscle arterioles demonstrated blunted dilation to acetylcholine (both ex vivo proximal and in situ distal arterioles) and elevated shear (distal arterioles only). All other alterations to reactivity appeared to reflect compromised endothelial function associated with increased thromboxane (Tx)A₂ production and oxidant stress/inflammation rather than alterations to vascular smooth muscle function. Structural changes to the microcirculation of GK rats were confined to reduced microvessel density of ~12%, with no evidence for altered vascular wall mechanics. Active hyperemia with either field stimulation of in situ cremaster muscle or electrical stimulation via the sciatic nerve for in situ gastrocnemius muscle was blunted in GK rats, primarily because of blunted functional dilation of skeletal muscle arterioles. The blunted active hyperemia was associated with impaired oxygen uptake (V̇o₂) across the muscle and accelerated muscle fatigue. Acute interventions to reduce oxidant stress (TEMPOL) and TxA₂ action (SQ-29548) or production (dazmegrel) improved muscle perfusion, V̇o₂, and muscle performance. These results suggest that type 2 diabetes mellitus in GK rats impairs skeletal muscle arteriolar function apparently early in the progression of the disease and potentially via an increased reactive oxygen species/inflammation-induced TxA₂ production/action on network function as a major contributing mechanism. NEW & NOTEWORTHY The impact of type 2 diabetes mellitus on vascular structure/function remains an area lacking clarity. Using diabetic Goto-Kakizaki rats before the development of other risk factors, we determined alterations to vascular structure/function and skeletal muscle active hyperemia. Type 2 diabetes mellitus reduced arteriolar endothelium-dependent dilation associated with increased thromboxane A₂ generation. Although modest microvascular rarefaction was evident, there were no other alterations to vascular structure/function. Skeletal muscle active hyperemia was blunted, although it improved after antioxidant or anti-thromboxane A₂ treatment.

Objective: Hypertension‐associated PA dysfunction reduces cerebral perfusion and impairs cognition. This is associated with impaired TRPV4‐mediated PA dilation; therefore, we tested the hypothesis that TRPV4 channels are important regulators of cerebral perfusion, PA structure and dilation, and cognition.Methods: Ten‐ to twelve‐month‐old male TRPV4 knockout (WKY‐Trpv4^em4Mcwi) and age‐matched control WKY rats were studied. Cerebral perfusion was measured by MRI with arterial spin labeling. PA structure and function were assessed using pressure myography and cognitive function using the novel object recognition test.Results: Cerebral perfusion was reduced in the WKY‐Trpv4^em4Mcwi rats. This was not a result of PA remodeling because TRPV4 deletion did not change PA structure. TRPV4 deletion did not change PA myogenic tone development, but PAs from the WKY‐Trpv4^em4Mcwi rats had severely blunted endothelium‐dependent dilation. The WKY‐Trpv4^em4Mcwi rats had impaired cognitive function and exhibited depressive‐like behavior. The WKY‐Trpv4^em4Mcwi rats also had increased microglia activation, and increased mRNA expression of GFAP and tumor necrosis factor alpha suggesting increased inflammation.Conclusion: Our data indicate that TRPV4 channels play a critical role in cerebral perfusion, PA dilation, cognition, and inflammation. Impaired TRPV4 function in diseases such as hypertension may increase the risk of the development of vascular dementia.