Jayson R. Gifford

Cardiac, skeletal, and smooth muscle mitochondrial respiration: are all mitochondria created equal?

Unlike cardiac and skeletal muscle, little is known about vascular smooth muscle mitochondrial respiration. Therefore, the present study examined mitochondrial respiratory rates in smooth muscle of healthy human feed arteries and compared with that of healthy cardiac and skeletal muscles. Cardiac, skeletal, and smooth muscles were harvested from a total of 22 subjects (53 ± 6 yr), and mitochondrial respiration was assessed in permeabilized fibers. Complex I + II, state 3 respiration, an index of oxidative phosphorylation capacity, fell progressively from cardiac to skeletal to smooth muscles (54 ± 1, 39 ± 4, and 15 ± 1 pmol·s(-1)·mg(-1), P < 0.05, respectively). Citrate synthase (CS) activity, an index of mitochondrial density, also fell progressively from cardiac to skeletal to smooth muscles (222 ± 13, 115 ± 2, and 48 ± 2 μmol·g(-1)·min(-1), P < 0.05, respectively). Thus, when respiration rates were normalized by CS (respiration per mitochondrial content), oxidative phosphorylation capacity was no longer different between the three muscle types. Interestingly, complex I state 2 normalized for CS activity, an index of nonphosphorylating respiration per mitochondrial content, increased progressively from cardiac to skeletal to smooth muscles, such that the respiratory control ratio, state 3/state 2 respiration, fell progressively from cardiac to skeletal to smooth muscles (5.3 ± 0.7, 3.2 ± 0.4, and 1.6 ± 0.3 pmol·s(-1)·mg(-1), P < 0.05, respectively). Thus, although oxidative phosphorylation capacity per mitochondrial content in cardiac, skeletal, and smooth muscles suggest all mitochondria are created equal, the contrasting respiratory control ratio and nonphosphorylating respiration highlight the existence of intrinsic functional differences between these muscle mitochondria. This likely influences the efficiency of oxidative phosphorylation and could potentially alter ROS production.

Symmorphosis and skeletal muscle V̇O2 max : in vivo and in vitro measures reveal differing constraints in the exercise-trained and untrained human.

The concept of symmorphosis predicts that the capacity of each step of the oxygen cascade is attuned to the task demanded of it during aerobic exercise at maximal rates of oxygen consumption ( V̇O2 max ) such that no single process is limiting or in excess at V̇O2 max . The present study challenges the applicability of this concept to humans by revealing clear, albeit very different, limitations and excesses in oxygen supply and consumption among untrained and endurance‐trained humans. Among untrained individuals, V̇O2 max is limited by the capacity of the mitochondria to consume oxygen, despite an excess of oxygen supply, whereas, among trained individuals, V̇O2 max is limited by the supply of oxygen to the mitochondria, despite an excess of mitochondrial respiratory capacity.

α1-Adrenergic Responsiveness in Human Skeletal Muscle Feed Arteries: The Impact of Reducing Extracellular PH

•  What is the central question of this study? In human arteries involved in the regulation of muscle blood flow, there is a lack of data about whether acidosis alters vascular sensitivity to vasoactive agents, as well as altering endothelium dependent vasorelaxation. Little is known about the interaction of metabolites and vascular function in human skeletal muscle feed arteries. •  What is the main finding and its importance? Increasing acidosis attenuated the response and sensitivity of the arteries to phenylephrine; this effect was selective to the receptor over smooth muscle. Acidosis did not alter endothelium dependent vasorelaxation. Impaired vasoconstriction coupled with intact vasorelaxation, promotes decreased vascular tone with exposure to acidosis, and may contribute to sympatholysis during exercise.

Further Peripheral Vascular Dysfunction in Heart Failure Patients With a Continuous-Flow Left Ventricular Assist Device: The Role of Pulsatility.

OBJECTIVES Using flow-mediated vasodilation (FMD) and reactive hyperemia (RH), this study aimed to provide greater insight into left ventricular assist device (LVAD)-induced changes in peripheral vascular function. BACKGROUND Peripheral endothelial function is recognized to be impaired in patients with heart failure with reduced ejection fraction (HFrEF), but the peripheral vascular effects of continuous-flow LVAD implantation, now used as either a bridge to transplantation or as a destination therapy, remain unclear. METHODS Sixty-eight subjects (13 New York Heart Association [NYHA] functional class II HFrEF patients, 19 NYHA functional class III/IV HFrEF patients, 20 NYHA functional class III/IV HFrEF patients post-LVAD implantation, and 16 healthy age-matched control subjects) underwent FMD and RH testing in the brachial artery with blood flow velocity, artery diameters, and pulsatility index (PI) assessed by ultrasound Doppler. RESULTS PI was significantly lower in the LVAD group (2.0 ± 0.4) compared with both the HFrEF II (8.6 ± 0.8) and HFrEF III/IV (8.1 ± 0.9) patients, who, in turn, had significantly lower PI than the control subjects (12.8 ± 0.9). Likewise, LVAD %FMD/shear rate (0.09 ± 0.01 %Δ/s(-1)) was significantly reduced compared with all other groups (control subjects, 0.24 ± 0.03; HFrEF II, 0.17 ± 0.02; and HFrEF III/IV, 0.13 ± 0.02 %Δ/s(-1)), and %FMD/shear rate significantly correlated with PI (r = 0.45). RH was unremarkable across groups. CONCLUSIONS Although central hemodynamics are improved in patients with HFrEF by a continuous-flow LVAD, peripheral vascular function is further compromised, which is likely due, at least in part, to the reduction in pulsatility that is a characteristic of such a mechanical assist device.

Impact of age on the vasodilatory function of human skeletal muscle feed arteries

Although advancing age is often associated with attenuated skeletal muscle blood flow and skeletal muscle feed arteries (SMFAs) have been recognized to play a regulatory role in the vasculature, little is known about the impact of age on the vasodilatory capacity of human SMFAs. Therefore, endothelium-dependent and -independent vasodilation were assessed in SMFAs (diameter: 544 ± 63 μm) obtained from 24 (equally represented) young (33 ± 2 yr) and old (71 ± 2 yr) subjects in response to three stimuli: 1) flow-induced shear stress, 2) ACh, and 3) sodium nitropusside (SNP). Both assessments of endothelium-dependent vasodilation, flow (young subjects: 68 ± 1% and old subjects: 32 ± 7%) and ACh (young subjects: 92 ± 3% and old subjects: 73 ± 4%), were significantly blunted (P < 0.05) in SMFAs of old compared with young subjects, with no such age-related differences in endothelium-independent vasodilation (SNP). In response to an increase in flow-induced shear stress, vasodilation kinetics (time constant to reach 63% of the amplitude of the response: 55 ± 1 s in young subjects and 92 ± 7 s in old subjects) and endothelial nitric oxide synthase (eNOS) activation (phospho-eNOS(s1177)/total eNOS: 1.0 ± 0.1 in young subjects and 0.2 ± 0.1 in old subjects) were also significantly attenuated in old compared with young subjects (P < 0.05). Furthermore, the vessel superoxide concentration was greater in old subjects (old subjects: 3.9 ± 1.0 area under curve/mg and young subjects: 1.7 ± 0.1 area under the curve/mg, P < 0.05). These findings reveal that the endothelium-dependent vasodilatory capacity, including vasodilation kinetics but not smooth muscle function, of human SMFAs is blunted with age and may be due to free radicals. Given the potential regulatory role of SMFAs in skeletal muscle blood flow, these findings may explain, at least in part, the often observed attenuated perfusion of skeletal muscle with advancing age that may contribute to exercise intolerance in the elderly.

Quadriceps exercise intolerance in patients with chronic obstructive pulmonary disease: the potential role of altered skeletal muscle mitochondrial respiration

This study sought to determine if qualitative alterations in skeletal muscle mitochondrial respiration, associated with decreased mitochondrial efficiency, contribute to exercise intolerance in patients with chronic obstructive pulmonary disease (COPD). Using permeabilized muscle fibers from the vastus lateralis of 13 patients with COPD and 12 healthy controls, complex I (CI) and complex II (CII)-driven State 3 mitochondrial respiration were measured separately (State 3:CI and State 3:CII) and in combination (State 3:CI+CII). State 2 respiration was also measured. Exercise tolerance was assessed by knee extensor exercise (KE) time to fatigue. Per milligram of muscle, State 3:CI+CII and State 3:CI were reduced in COPD (P < 0.05), while State 3:CII and State 2 were not different between groups. To determine if this altered pattern of respiration represented qualitative changes in mitochondrial function, respiration states were examined as percentages of peak respiration (State 3:CI+CII), which revealed altered contributions from State 3:CI (Con 83.7 ± 3.4, COPD 72.1 ± 2.4%Peak, P < 0.05) and State 3:CII (Con 64.9 ± 3.2, COPD 79.5 ± 3.0%Peak, P < 0.05) respiration, but not State 2 respiration in COPD. Importantly, a diminished contribution of CI-driven respiration relative to the metabolically less-efficient CII-driven respiration (CI/CII) was also observed in COPD (Con 1.28 ± 0.09, COPD 0.81 ± 0.05, P < 0.05), which was related to exercise tolerance of the patients (r = 0.64, P < 0.05). Overall, this study indicates that COPD is associated with qualitative alterations in skeletal muscle mitochondria that affect the contribution of CI and CII-driven respiration, which potentially contributes to the exercise intolerance associated with this disease.

Changes in dermal interstitial ATP levels during local heating of human skin

•  Heating human skin results in a localized increase in blood flow (hyperaemia) to the skin. •  Experiments in mouse skin and cultured human cells suggest that skin cells known as keratinocytes release the chemical ATP, a known cutaneous vasodilator, and that the release of this ATP increases when temperature‐sensing vanilloid type III and IV transient receptor potential ion channels (TRPV3, TRPV4 respectively) are activated by heating. Studies also suggest that this ATP release is necessary for temperature sensation. •  We hypothesized that, like mouse skin locally heating human skin to temperatures that activate heat‐sensing TRPV3 and TRPV4 channels would be associated with an accumulation of ATP in the interstitial space that would be related to temperature sensation. •  We also hypothesized that the accumulation of ATP would be associated with the magnitude of heat‐induced hyperaemia to the area. •  We report that, unlike mouse skin, such local heating does not result in an accumulation of ATP in the interstitial space of human skin, and therefore such an accumulation is not necessary for cutaneous temperature sensation and local reactive hyperaemia. •  We also report that warming skin from 31°C results in dilatation at temperatures as low as 35°C, which is several degrees lower than previously reported. This suggests that TRPV3 and/or TRPV4 channels have a role in heat‐induced hyperaemia or that such heating sensitizes TRPV1 channels to respond to temperatures below their typical threshold.

The role of muscle mass in exercise-induced hyperemia

Exercise-induced hyperemia is often normalized for muscle mass, and this value is sometimes evaluated at relative exercise intensities to take muscle recruitment into account. Therefore, this study sought to better understand the impact of muscle mass on leg blood flow (LBF) during exercise. LBF was assessed by Doppler ultrasound in 27 young healthy male subjects performing knee-extensor (KE) exercise at three absolute (5, 15, and 25 W) and three relative [20, 40, and 60% of maximum KE (KEmax)] workloads. Thigh muscle mass (5.2-8.1 kg) and LBF were significantly correlated at rest (r = 0.54; P = 0.004). Exercise-induced hyperemia was linearly related to absolute workload, but revealed substantial between-subject variability, documented by the coefficient of variation (5 W: 17%; 15 W: 16%; 25 W: 16%). Quadriceps muscle mass (1.5-2.7 kg) and LBF were not correlated at 5, 15, or 25 W (r = 0.09-0.01; P = 0.7-0.9). Normalizing blood flow for quadriceps muscle mass did not improve the coefficient of variation at each absolute workload (5 W: 21%; 15 W: 21%; 25 W: 22%), while the additional evaluation at relative exercise intensities resulted in even greater variance (20% KEmax: 29%; 40% KEmax: 29%; 60% KEmax: 27%). Similar findings were documented when subjects were parsed into high and low aerobic capacity. Thus, in contrast to rest, blood flow during exercise is unrelated to muscle mass, and simply normalizing for muscle mass or comparing normalized blood flow at a given relative exercise intensity has no effect on the inherent blood flow variability. Therefore, during exercise, muscle mass does not appear to be a determinant of the hyperemic response.

Exercise training improves vascular mitochondrial function.

Exercise training is recognized to improve cardiac and skeletal muscle mitochondrial respiratory capacity; however, the impact of chronic exercise on vascular mitochondrial respiratory function is unknown. We hypothesized that exercise training concomitantly increases both vascular mitochondrial respiratory capacity and vascular function. Arteries from both sedentary (SED) and swim-trained (EX, 5 wk) mice were compared in terms of mitochondrial respiratory function, mitochondrial content, markers of mitochondrial biogenesis, redox balance, nitric oxide (NO) signaling, and vessel function. Mitochondrial complex I and complex I + II state 3 respiration and the respiratory control ratio (complex I + II state 3 respiration/complex I state 2 respiration) were greater in vessels from EX relative to SED mice, despite similar levels of arterial citrate synthase activity and mitochondrial DNA content. Furthermore, compared with the SED mice, arteries from EX mice displayed elevated transcript levels of peroxisome proliferative activated receptor-γ coactivator-1α and the downstream targets cytochrome c oxidase subunit IV isoform 1,isocitrate dehydrogenase(Idh)2, and Idh3a, increased manganese superoxide dismutase protein expression, increased endothelial NO synthase phosphorylation (Ser(1177)), and suppressed reactive oxygen species generation (all P< 0.05). Although there were no differences in EX and SED mice concerning endothelium-dependent and endothelium-independent vasorelaxation, phenylephrine-induced vasocontraction was blunted in vessels from EX compared with SED mice, and this effect was normalized by NOS inhibition. These training-induced increases in vascular mitochondrial respiratory capacity and evidence of improved redox balance, which may, at least in part, be attributable to elevated NO bioavailability, have the potential to protect against age- and disease-related challenges to arterial function.

Human skeletal muscle feed arteries: evidence of regulatory potential

Recently, it has been recognized that human skeletal muscle feed arteries can be harvested during exploratory surgery for melanoma. This approach provides vessels for in vitro study from a wide spectrum of relatively healthy humans. Although, the regulatory role of skeletal muscle feed arteries in rodent models has been documented, whether such vessels in humans possess this functionality is unknown.