Limb and skeletal muscle blood flow measurements at rest and during exercise in human subjects

Cerebral blood flow in sleep: A systematic review and meta-analysis

Sleep plays an essential role in physiology, allowing the brain and body to restore itself. Despite its critical role, our understanding of the underlying processes in the sleeping human brain is still limited. Sleep comprises several distinct stages with varying depths and temporal compositions. Cerebral blood flow (CBF), which delivers essential nutrients and oxygen to the brain, varies across brain regions throughout these sleep stages, reflecting changes in neuronal function and regulation. This systematic review and meta-analysis assesses global and regional CBF across sleep stages. We included, appraised, and summarized all 38 published sleep studies on CBF in healthy humans that were not or only slightly (<24 h) sleep deprived. Our main findings are that CBF varies with sleep stage and depth, being generally lowest in NREM sleep and highest in REM sleep. These changes appear to stem from sleep stage-specific regional brain activities that serve particular functions, such as alterations in consciousness and emotional processing.

Comparative Study of Small Vessel (under 0.8 mm) Anastomosed Free Flap and Larger Vessel (over 0.8 mm) Anastomosed Free Flap: Does Supermicrosurgery Provide Sufficient Blood Flow to the Free Flap?

BACKGROUND

 This study aimed to quantify the blood flow of free flaps and compare the blood flow of small vessel (<0.8 mm) and larger vessel (>0.8 mm) anastomosed free flaps.

METHODS

 This retrospective study included patients treated successfully with a perforator free flap in the lower extremity between June 2015 and March 2017. A color duplex ultrasound system measured the flow volume through the pedicle by analyzing the mean flow peak velocity, flow volume, and flow volume per 100 g of the flap.

RESULTS

 A total of 69 patients were enrolled in this study. There was no statistical difference in peak velocity between the small vessel anastomosed free flap (25.2 ± 5.6) and larger vessel anastomosed free flap (26.5 ± 5.4). Flow volume (6.8 ± 4.2 vs. 6.3 ± 3.6) and flow volume/100 g (3.6 ± 3.9 vs. 6.2 ± 6.9) also did not show significant differences.

CONCLUSION

 Small vessel (<0.8 mm) free flaps showed similar flow velocity and flow volume to larger vessel (>0.8 mm) anastomosed free flaps. Blood flow to the small vessel anastomosed free flap was sufficient despite its small vessel size.

Physiological Responses to Supramaximal Running Exercise with End-Expiratory Breath Holding up to the Breaking Point

This study aimed to assess the physiological responses to repeated running exercise performed at supramaximal intensity and with end-expiratory breath holding (EEBH) up to the breaking point. Eight male runners participated in two running testing sessions on a motorized treadmill. In the first session, participants performed two sets of 8 repetitions at 125% of maximal aerobic velocity and with maximum EEBH. Each repetition started at the onset of EEBH and ended at its release. In the second session, participants replicated the same procedure, but with unrestricted breathing (URB). The change in cerebral and muscle oxygenation (Δ[Hbdiff]), total haemoglobin concentration (Δ[THb]) and muscle reoxygenation were continuously assessed. End-tidal oxygen (PETO2) and carbon dioxide pressure (PETCO2), arterial oxygen saturation (SpO2) and heart rate (HR) were also measured throughout exercise.On average, EEBH was maintained for 10.1 ± 1 s. At the breaking point of EEBH, PETO2 decreased to 54.1 ± 8 mmHg, whereas PETCO2 increased to 74.8 ± 3.1 mmHg. At the end of repetitions, SpO2 (nadir values 74.9 ± 5.0 vs. 95.7 ± 0.8%) and HR were lower with EEBH than with URB. Cerebral and muscle Δ[Hbdiff] were also lower with EEBH, whereas this condition induced higher cerebral and muscle Δ[THb] and greater muscle reoxygenation. This study showed that performing repeated bouts of supramaximal running exercises with EEBH up to the breaking point induced a fall in arterial, cerebral and muscle oxygenation compared with the URB condition. These phenomena were accompanied by increases in regional blood volume likely resulting from compensatory vasodilation to preserve oxygen delivery to the brain and muscles.

Cool-Water Immersion Reduces Postexercise Quadriceps Femoris Muscle Perfusion More Than Cold-Water Immersion

PURPOSE

The muscle perfusion response to postexercise cold-water immersion (CWI) is not well understood. We examined the effects of graded postexercise CWI upon global and regional quadriceps femoris muscle perfusion using positron emission tomography and [15O]H2O.

METHODS

Using a matched-group design, 30 healthy men performed cycle ergometer exercise at 70% V̇O2peak to a core body temperature of 38°C, followed by either 10 min of CWI at 8°C, 22°C, or seated rest (control). Quadriceps muscle perfusion; thigh and calf cutaneous vascular conductance; intestinal, muscle, and local skin temperatures; thermal comfort; mean arterial pressure; and heart rate were assessed at preexercise, postexercise, and after CWI.

RESULTS

Global quadriceps perfusion was reduced beyond the predefined minimal clinically relevant threshold (0.75 mL per 100 g·min-1) in 22°C water versus control (difference (95% confidence interval (CI)), -2.5 (-3.9 to -1.1) mL per 100 g·min-1). Clinically relevant decreases in muscle perfusion were observed in the rectus femoris (-2.0 (-3.0 to -1.0) mL per 100 g·min-1) and vastus lateralis (-3.5 (-4.9 to -2.0) mL per 100 g·min-1) in 8°C water, and in the vastus lateralis (-3.3 (-4.8 to -1.9) mL per 100 g·min-1) in 22°C water versus control. The mean effects for vastus intermedius and vastus medialis perfusion were not clinically relevant. Clinically relevant decreases in thigh and calf cutaneous vascular conductance were observed in both cooling conditions.

CONCLUSIONS

The present findings revealed that less noxious CWI (22°C) promoted clinically relevant postexercise decreases in global quadriceps muscle perfusion, whereas noxious cooling (8°C) elicited no effect.

Acute heat exposure improves microvascular function in skeletal muscle of aged adults

Acute heat exposure improves microvascular function in aged adults as assessed using reactive hyperemia. The cutaneous and skeletal muscle microcirculations are thought to contribute to this response, but this has never been confirmed due to the methodological challenges associated with differentiating blood flow between these vascular beds. We hypothesized that acute hot water immersion would improve endothelial-dependent, but not endothelial-independent vasodilation in the microcirculation of the vastus lateralis muscle in healthy aged adults. Participants (70 ± 5 yr) were immersed for 60 min in thermoneutral (36°C) or hot (40°C) water. Ninety minutes following immersion, skeletal muscle microdialysis was used to bypass the cutaneous circulation and directly assess endothelial-dependent and endothelial-independent vasodilation by measuring the local hyperemic response to graded infusions of acetylcholine (ACh, 27.5 and 55.0 mM) and sodium nitroprusside (SNP, 21 and 42 mM), respectively. The hyperemic response to 27.5 mM ACh did not differ between thermal conditions (P = 0.9). However, the hyperemic response to 55.0 mM ACh was increased with prior hot water immersion (thermoneutral immersion, 43.9 ± 23.2 mL/min/100 g vs. hot water immersion, 66.5 ± 25.5 mL/min/100 g; P < 0.01). Similarly, the hyperemic response to 21 mM SNP did not differ between thermal conditions (P = 0.3) but was increased following hot water immersion with the infusion of 42 mM SNP (thermoneutral immersion, 48.8 ± 25.6 mL/min/100 g vs. hot water immersion, 90.7 ± 53.5 mL/min/100 g; P < 0.01). These data suggest that acute heat exposure improves microvascular function in skeletal muscle of aged humans.NEW & NOTEWORTHY Acute heat exposure improves microvascular function in aged adults as assessed using reactive hyperemia. The cutaneous and skeletal muscle microcirculations are thought to contribute to this response, but this has never been confirmed due to the methodological challenges associated with differentiating blood flow between these vascular beds. Using the microdialysis technique to bypass the cutaneous circulation, we demonstrated that heat exposure improves endothelial-dependent and endothelial-independent vasodilation in the microcirculation of skeletal muscle in aged humans.

Spinal cord injury and vascular function: evidence from diameter-matched vessels

The effect of a spinal cord injury (SCI) on vascular function has been clouded by both the physiological and mathematical bias of assessing vasodilation in arteries with differing diameters both above and below the lesion and when comparing with healthy, nondisabled controls (CTRL). Thus, we measured vascular function, with flow-mediated vasodilation (FMD), in 10 SCI and 10 CTRL with all arteries matched for diameter (≈0.5 cm): brachial artery (BA, arm, functional limb in both groups) and popliteal artery (PA, leg, disused limb in SCI, functional limb in CTRL). PA %FMD was significantly attenuated in SCI (5.6 ± 0.6%) compared with CTRL (8.4 ± 1.3%), with no difference in the BA (SCI: 8.6 ± 0.9%; CTRL: 8.7 ± 0.7%). However, unlike the arm, where muscle mass was preserved, the legs of the SCI were significantly smaller than CTRL (∼70%). Thus, reactive hyperemia (RH), which is heavily dependent on the volume of muscle occluded, in the PA was attenuated in the SCI (144 ± 22 mL) compared with CTRL (258 ± 16 mL) but not different in the BA. Consequently, shear rate was significantly diminished in the PA of the SCI, such that %FMD/shear rate (vascular responsiveness) was actually greater in the SCI (1.5 ± 0.1% · s^-1) than CTRL (1.2 ± 0.1% · s^-1). Of note, this was significantly greater than both their own BA (0.9 ± 0.1% · s^-1) and that of the CTRL (0.9 ± 0.1% · s^-1). Therefore, examining vessels of similar size, this study reveals normal vascular function above the lesion and vascular dysfunction below the lesion. However, below the lesion there was, actually, evidence of increased vascular responsiveness in this population.NEW & NOTEWORTHY This study examined the effect of a spinal cord injury (SCI) and subsequent limb disuse on vascular function, assessed by %FMD, in diameter-matched vessels above and below the lesion in subjects with SCI and controls. The results reveal normal vascular function above the lesion and vascular dysfunction below the lesion (%FMD). However, below the lesion there was, actually, evidence of increased vascular responsiveness (%FMD/shear rate) in this population.

Assessment of resistance vessel function in human skeletal muscle: guidelines for experimental design, Doppler ultrasound, and pharmacology

The introduction of duplex Doppler ultrasound almost half a century ago signified a revolutionary advance in the ability to assess limb blood flow in humans. It is now widely used to assess blood flow under a variety of experimental conditions to study skeletal muscle resistance vessel function. Despite its pervasive adoption, there is substantial variability between studies in relation to experimental protocols, procedures for data analysis, and interpretation of findings. This guideline results from a collegial discussion among physiologists and pharmacologists, with the goal of providing general as well as specific recommendations regarding the conduct of human studies involving Doppler ultrasound-based measures of resistance vessel function in skeletal muscle. Indeed, the focus is on methods used to assess resistance vessel function and not upstream conduit artery function (i.e., macrovasculature), which has been expertly reviewed elsewhere. In particular, we address topics related to experimental design, data collection, and signal processing as well as review common procedures used to assess resistance vessel function, including postocclusive reactive hyperemia, passive limb movement, acute single limb exercise, and pharmacological interventions.

Methods for the determination of skeletal muscle blood flow: development, strengths and limitations

Since the first measurements of limb blood flow at rest and during nerve stimulation were conducted in the late 1800s, a number of methods have been developed for the determination of limb and skeletal muscle blood flow in humans. The methods, which have been applied in the study of aspects such as blood flow regulation, oxygen uptake and metabolism, differ in terms of strengths and degree of limitations but most have advantages for specific settings. The purpose of this review is to describe the origin and the basic principles of the methods, important aspects and requirements of the procedures. One of the earliest methods, venous occlusion plethysmography, is a noninvasive method which still is extensively used and which provides similar values as other more direct blood flow methods such as ultrasound Doppler. The constant infusion thermodilution method remains the most appropriate for the determination of blood flow during maximal exercise. For resting blood flow and light-to-moderate exercise, the non-invasive ultrasound Doppler methodology, if handled by a skilled operator, is recommendable. Positron emission tomography with radiolabeled water is an advanced method which requires highly sophisticated equipment and allows for the determination of muscle-specific blood flow, regional blood flows and estimate of blood flow heterogeneity within a muscle. Finally, the contrast-enhanced ultrasound method holds promise for assessment of muscle-specific blood flow, but the interpretation of the data obtained remains uncertain. Currently lacking is high-resolution methods for continuous visualization and monitoring of the skeletal muscle microcirculation in humans.

Reliability of forearm oxygen uptake during handgrip exercise: assessment by ultrasonography and venous blood gas

Assessment of forearm oxygen uptake (V˙O2 ) during handgrip exercise is a keenly investigated concept for observing small muscle mass metabolism. Although a combination of Doppler ultrasound measurements of brachial artery blood flow (Q˙) and blood gas drawn from a deep forearm vein has been utilized to calculate forearm V˙O2 for more than two decades, the applicability of this experimental design may benefit from a thorough evaluation of its reliability during graded exercise. Therefore, we evaluated the reliability of this technique during incremental handgrip exercise in ten healthy young (24 ± 3(SD) years.) males. V˙O2 and work rate (WR) exhibited a linear relationship (1.0 W: 43.8 ± 10.1 mL·min-1 ; 1.5 W: 53.8 ± 14.1 mL·min-1 ; 2.0 W: 63.4 ± 16.3 mL·min-1 ; 2.5 W: 72.2 ± 17.6 mL·min-1 ; 3.0 W: 79.2 ± 18.6 mL·min-1 ; r = 0.65, P < 0.01). In turn, V˙O2 was strongly associated with Q˙ (1.0 W: 359 ± 86 mL·min-1 ; 1.5 W: 431 ± 112 mL·min-1 ; 2.0 W: 490 ± 123 mL·min-1 ; 2.5 W: 556 ± 112 mL·min-1 ; 3.0 W: 622 ± 131 mL·min-1 ; r = 0.96; P < 0.01), whereas arteriovenous oxygen difference (a-vO2diff ) remained constant following all WRs (123 ± 11-130 ± 10 mL·L-1 ). Average V˙O2 test-retest difference was -0.4 mL·min-1 with ±2SD limits of agreement (LOA) of 8.4 and -9.2 mL·min-1 , respectively, whereas coefficients of variation (CVs) ranged from 4-7%. Accordingly, test-retest Q˙ difference was 11.9 mL·min-1 (LOA: 84.1 mL·min-1 ; -60.4 mL·min-1 ) with CVs between 4 and 7%. Test-retest difference for a-vO2diff was -0.28 mL·dL-1 (LOA: 1.26mL·dL-1 ; -1.82 mL·dL-1 ) with 3-5% CVs. In conclusion, our results revealed that forearm V˙O2 determination by Doppler ultrasound and direct venous sampling is linearly related to WR, and a reliable experimental design across a range of exercise intensities.

Temperature and blood flow distribution in the human leg during passive heat stress

The ability of direct heat stress to increase limb blood flow is well known, but the magnitude and profile of hemodynamic responses within the major vessels of the leg have not been explored. Here, we systematically characterize these responses through a wide range of heat stress levels and show that isolated leg heating confers potentially beneficial hemodynamic changes equivalent to those of moderate whole body hyperthermia, with these hemodynamic adjustments being predominantly driven by local temperature-sensitive mechanisms.

The influence of temperature on the hemodynamic adjustments to direct passive heat stress within the leg's major arterial and venous vessels and compartments remains unclear. Fifteen healthy young males were tested during exposure to either passive whole body heat stress to levels approaching thermal tolerance [core temperature (T_c) + 2°C; study 1; n = 8] or single leg heat stress (T_c + 0°C; study 2; n = 7). Whole body heat stress increased perfusion and decreased oscillatory shear index in relation to the rise in leg temperature (T_leg) in all three major arteries supplying the leg, plateauing in the common and superficial femoral arteries before reaching severe heat stress levels. Isolated leg heat stress increased arterial blood flows and shear patterns to a level similar to that obtained during moderate core hyperthermia (T_c + 1°C). Despite modest increases in great saphenous venous (GSV) blood flow (0.2 l/min), the deep venous system accounted for the majority of returning flow (common femoral vein 0.7 l/min) during intense to severe levels of heat stress. Rapid cooling of a single leg during severe whole body heat stress resulted in an equivalent blood flow reduction in the major artery supplying the thigh deep tissues only, suggesting central temperature-sensitive mechanisms contribute to skin blood flow alone. These findings further our knowledge of leg hemodynamic responses during direct heat stress and provide evidence of potentially beneficial vascular alterations during isolated limb heat stress that are equivalent to those experienced during exposure to moderate levels of whole body hyperthermia.

Evaluation of blood flow and electromyographic activity in the perioral muscles

BACKGROUND/OBJECTIVES

Although the electromyographic (EMG) activity of the perioral muscles, including the orbicularis oris and mentalis muscles, has been described in individuals with lip incompetence during lip sealing, blood flow through these muscles remains to be elucidated. The purpose of this study was to examine the blood flow associated with EMG activity in the perioral muscles using laser speckle imaging in individuals with lip incompetence.

SUBJECTS/METHODS

Blood flow and EMG activity of the superior and inferior orbicularis oris and mentalis muscles were measured with the lips in contact (C condition) and apart (O condition) in lip incompetence (experimental) and control subjects (n = 15 in each group; mean age: 29.5 years). The change ratios of blood flow and EMG activity in the C condition versus O condition (C/O ratios) were calculated and plotted in a scattergram. The Mann-Whitney U-test, Wilcoxon signed-rank test, discriminant analysis using the Mahalanobis generalized distance, and Spearman correlation were used for statistical analysis.

RESULTS

In the experimental group, blood flow and EMG activity in all muscles were significantly greater in the C condition than in the O condition. The plots of C/O ratios in the experimental group showed a distinct and wide distribution and were significantly different than those in the control group. In both groups, a significant positive correlation was observed between blood flow and EMG activity in the mentalis muscle.

CONCLUSIONS/IMPLICATIONS

The present findings suggest that observing blood flow in the mentalis muscle is an effective and easily performed method of evaluating lip incompetence.

Novel method to assess arterial insufficiency in rodent hind limb

BACKGROUND

Lack of techniques to assess maximal blood flow capacity thwarts the use of rodent models of arterial insufficiency to evaluate therapies for intermittent claudication. We evaluated femoral vein outflow (VO) in combination with stimulated muscle contraction as a potential method to assess functional hind limb arterial reserve and therapeutic efficacy in a rodent model of subcritical limb ischemia.

MATERIALS AND METHODS

VO was measured with perivascular flow probes at rest and during stimulated calf muscle contraction in young, healthy rats (Wistar Kyoto, WKY; lean Zucker rats, LZR) and rats with cardiovascular risk factors (spontaneously hypertensive [SHR]; obese Zucker rats [OZR]) with acute and/or chronic femoral arterial occlusion. Therapeutic efficacy was assessed by administration of Ramipril or Losartan to SHR after femoral artery excision.

RESULTS

VO measurement in WKY demonstrated the utility of this method to assess hind limb perfusion at rest and during calf muscle contraction. Although application to diseased models (OZR and SHR) demonstrated normal resting perfusion compared with contralateral limbs, a significant reduction in reserve capacity was uncovered with muscle stimulation. Administration of Ramipril and Losartan demonstrated significant improvement in functional arterial reserve.

CONCLUSIONS

The results demonstrate that this novel method to assess distal limb perfusion in small rodents with subcritical limb ischemia is sufficient to unmask perfusion deficits not apparent at rest, detect impaired compensation in diseased animal models with risk factors, and assess therapeutic efficacy. The approach provides a significant advance in methods to investigate potential mechanisms and novel therapies for subcritical limb ischemia in preclinical rodent models.

Local temperature-sensitive mechanisms are important mediators of limb tissue hyperemia in the heat-stressed human at rest and during small muscle mass exercise

Limb tissue and systemic blood flow increases with heat stress, but the underlying mechanisms remain poorly understood. Here, we tested the hypothesis that heat stress-induced increases in limb tissue perfusion are primarily mediated by local temperature-sensitive mechanisms. Leg and systemic temperatures and hemodynamics were measured at rest and during incremental single-legged knee extensor exercise in 15 males exposed to 1 h of either systemic passive heat-stress with simultaneous cooling of a single leg (n = 8) or isolated leg heating or cooling (n = 7). Systemic heat stress increased core, skin and heated leg blood temperatures (T_b), cardiac output, and heated leg blood flow (LBF; 0.6 ± 0.1 l/min; P < 0.05). In the cooled leg, however, LBF remained unchanged throughout (P > 0.05). Increased heated leg deep tissue blood flow was closely related to T_b (R² = 0.50; P < 0.01), which is partly attributed to increases in tissue V̇O₂ (R² = 0.55; P < 0.01) accompanying elevations in total leg glucose uptake (P < 0.05). During isolated limb heating and cooling, LBFs were equivalent to those found during systemic heat stress (P > 0.05), despite unchanged systemic temperatures and hemodynamics. During incremental exercise, heated LBF was consistently maintained ∼0.6 l/min higher than that in the cooled leg (P < 0.01), with LBF and vascular conductance in both legs showing a strong correlation with their respective local T_b (R² = 0.85 and 0.95, P < 0.05). We conclude that local temperature-sensitive mechanisms are important mediators in limb tissue perfusion regulation both at rest and during small-muscle mass exercise in hyperthermic humans.

Thermoregulatory modeling for cold stress

Modeling for cold stress has generated a rich history of innovation, has exerted a catalytic influence on cold physiology research, and continues to impact human activity in cold environments. This overview begins with a brief summation of cold thermoregulatory model development followed by key principles that will continue to guide current and future model development. Different representations of the human body are discussed relative to the level of detail and prediction accuracy required. In addition to predictions of shivering and vasomotor responses to cold exposure, algorithms are presented for thermoregulatory mechanisms. Various avenues of heat exchange between the human body and a cold environment are reviewed. Applications of cold thermoregulatory modeling range from investigative interpretation of physiological observations to forecasting skin freezing times and hypothermia survival times. While these advances have been remarkable, the future of cold stress modeling is still faced with significant challenges that are summarized at the end of this overview.

Hybrid diffuse optical techniques for continuous hemodynamic measurement in gastrocnemius during plantar flexion exercise

Occlusion calibrations and gating techniques have been recently applied by our laboratory for continuous and absolute diffuse optical measurements of forearm muscle hemodynamics during handgrip exercises. The translation of these techniques from the forearm to the lower limb is the goal of this study as various diseases preferentially affect muscles in the lower extremity. This study adapted a hybrid near-infrared spectroscopy and diffuse correlation spectroscopy system with a gating algorithm to continuously quantify hemodynamic responses of medial gastrocnemius during plantar flexion exercises in 10 healthy subjects. The outcomes from optical measurement include oxy-, deoxy-, and total hemoglobin concentrations, blood oxygen saturation, and relative changes in blood flow (rBF) and oxygen consumption rate (rV̇O2). We calibrated rBF and rV̇O2 profiles with absolute baseline values of BF and V̇O2 obtained by venous and arterial occlusions, respectively. Results from this investigation were comparable to values from similar studies. Additionally, significant correlation was observed between resting local muscle BF measured by the optical technique and whole limb BF measured concurrently by a strain gauge venous plethysmography. The extensive hemodynamic and metabolic profiles during exercise will allow for future comparison studies to investigate the diagnostic value of hybrid technologies in muscles affected by disease.

Influence of cold-water immersion on limb and cutaneous blood flow after exercise

PURPOSE

This study aimed to determine the influence of cold (8°C) and cool (22°C) water immersion on femoral artery and cutaneous blood flow after exercise.

METHODS

Twelve men completed a continuous cycle exercise protocol at 70% peak oxygen uptake until a core temperature of 38°C was attained. Subjects were then immersed semireclined into 8°C or 22°C water to the iliac crest for 10 min or rested. Rectal and thigh skin temperature, deep and superficial muscle temperature, thigh and calf skin blood flow (laser Doppler flowmetry), and superficial femoral artery blood flow (duplex ultrasound) were measured before and up to 30 min after immersion. Indices of vascular conductance were calculated (flux and blood flow/mean arterial pressure).

RESULTS

Reductions in rectal temperature were similar (0.6°C-0.7°C) in all three trials (P = 0.38). The mean ± SD thigh skin temperature during recovery was 25.4°C ± 3.8°C in the 8°C trial, which was lower than the 28.2°C ± 1.4°C and 33.78°C ± 1.0°C in the 22°C and control trials, respectively (P < 0.001). Recovery muscle temperature was also lowest in the 8°C trial (P < 0.01). Femoral artery conductance was similar after immersion in both cooling conditions and was lower (∼55%) compared with the control condition 30 min after immersion (P < 0.01). Similarly, there was greater thigh (P < 0.01) and calf (P < 0.05) cutaneous vasoconstriction during and after immersion in both cooling conditions relative to the control condition.

CONCLUSION

Colder water temperatures may be more effective in the treatment of exercise-induced muscle damage and injury rehabilitation by virtue of greater reductions in muscle temperature and not muscle blood flow.

Influence of postexercise cooling on muscle oxygenation and blood volume changes

PURPOSE

The aim of this study was to investigate the effects of postexercise cold water immersion (CWI) on tissue oxygenation and blood volume changes after intense exercise.

METHODS

Nine physically active men performed 30 min of continuous running (CR) at 70% of their maximal treadmill velocity (Vmax), followed by 10 bouts of intermittent running at Vmax. After exercise, one of the participants' legs was immersed in a cold water bath (10°C, CWI) to the level of their gluteal fold for 15 min. The contralateral leg remained outside the water bath and served as a control (CON). Vastus lateralis (VL) skin temperature (TskVL), VL oxygenation (tissue oxygenation index [TOI]), and blood volume changes (total hemoglobin [tHb] volume) were monitored continuously throughout exercise and CWI using near-infrared spectroscopy.

RESULTS

TskVL, TOI, and tHb were not significantly different between CON and CWI during continuous running and intermittent running, respectively (P > 0.05). In contrast, TskVL was significantly lower in CWI compared with CON throughout immersion, with peak differences occurring at the end of immersion (CON = 35.1 ± 0.6 vs CWI = 16.9°C ± 1.7°C, P < 0.001). tHb was significantly lower during CWI compared with CON at most time points, with peak differences of 20% ± 4% evident at the end of the 15-min immersion (P < 0.01). Likewise, TOI was significantly higher in CWI compared with CON, with peak differences of 2.5% ± 1% evident at the 12th min of immersion (P < 0.05).

CONCLUSIONS

Postexercise cooling decreased microvascular perfusion and muscle metabolic activity. These findings are consistent with the suggested mechanisms by which CWI is hypothesized to improve local muscle recovery.

Influence of hormone replacement therapy and aerobic exercise training on oxygen uptake kinetics in postmenopausal women

The purpose of this study was to determine the effects of aerobic exercise training on the adjustment of pulmonary oxygen (O2) uptake ([Formula: see text]O2p) kinetics in postmenopausal women in 2 groups: those using hormone replacement therapy (HRT) (HRT group) (n = 7, aged 56 ± 4 years) and those not using HRT (nonHRT group) (n = 8, aged 60 ± 5 years). The influence of training (cycle-ergometer 3 times per week for 6 weeks) on step transitions to both moderate-intensity (80% of the gas exchange threshold) and heavy-intensity (Δ50) cycling exercise was studied. Breath-by-breath [Formula: see text]O2p data were collected using a mass spectrometer. There were no differences in baseline characteristics between the HRT and nonHRT groups. Moderate-intensity exercise [Formula: see text]O2p kinetics were significantly speeded (p < 0.05) with the τ[Formula: see text]O2p decreasing from 46 ± 8 s before training to 32 ± 4 s after training. Similarly, during the heavy-intensity exercise, on-transient phase 2 τ[Formula: see text]O2p was reduced from before training (48 ± 7 s) to after training (38 ± 6 s). The use of HRT did not influence the effect of the endurance exercise training on τ[Formula: see text]O2p during moderate or heavy exercise in healthy postmenopausal women. To provide insight into the mechanism of adjustment, knee extension exercise was studied, and the [Formula: see text]O2p kinetics were significantly speeded (p < 0.05), with the τ[Formula: see text]O2p of the knee extension exercise decreasing from 62.2 ± 18.3 s before training to 48.0 ± 16.2 s after training. Thus, 6 weeks of exercise training resulted in appreciably faster cycling phase 2 [Formula: see text]O2p kinetics during moderate and heavy exercise in older women, independent of HRT use.

Diffuse Correlation Spectroscopy (DCS) for Assessment of Tissue Blood Flow in Skeletal Muscle: Recent Progress

Near-infrared diffuse correlation spectroscopy (DCS) is an emerging technology for monitoring blood flow in various tissues. This article reviews the recent progress of DCS for the assessment of skeletal muscle blood flow, including the developments in technology allowing use during dynamic exercise and muscular electrical stimulation, the utilization for diagnosis of muscle vascular diseases, and the applications for evaluating treatment effects. The limitations of current DCS studies and future perspective are finally discussed.

Noninvasive optical quantification of absolute blood flow, blood oxygenation, and oxygen consumption rate in exercising skeletal muscle

This study investigates a method using novel hybrid diffuse optical spectroscopies [near-infrared spectroscopy (NIRS) and diffuse correlation spectroscopy (DCS)] to obtain continuous, noninvasive measurement of absolute blood flow (BF), blood oxygenation, and oxygen consumption rate (V̇O(2)) in exercising skeletal muscle. Healthy subjects (n=9) performed a handgrip exercise to increase BF and V̇O(2) in forearm flexor muscles, while a hybrid optical probe on the skin surface directly monitored oxy-, deoxy-, and total hemoglobin concentrations ([HbO(2)], [Hb], and THC), tissue oxygen saturation (S(t)O(2)), relative BF (rBF), and relative oxygen consumption rate (rV̇O(2)). The rBF and rV̇O(2) signals were calibrated with absolute baseline BF and V̇O(2) obtained through venous and arterial occlusions, respectively. Known problems with muscle-fiber motion artifacts in optical measurements during exercise were mitigated using a novel gating algorithm that determined muscle contraction status based on control signals from a dynamometer. Results were consistent with previous findings in the literature. This study supports the application of NIRS/DCS technology to quantitatively evaluate hemodynamic and metabolic parameters in exercising skeletal muscle and holds promise for improving diagnosis and treatment evaluation for patients suffering from diseases affecting skeletal muscle and advancing fundamental understanding of muscle and exercise physiology.

Effect of hypoxia on the dynamic response of hyperaemia in the contracting human calf muscle

Although systemic hypoxia increases the muscle hyperaemic response during 'steady-state' exercise, its effect on the dynamic characteristics of this response is not clear. In the present study, we first established that hypoxia increases the steady-state hyperaemic response at low workloads during calf exercise. To study dynamic aspects of this response, eight subjects performed eight exercise trials while breathing a normoxic (fractional inspired O(2) = 0.2094) or hypoxic gas mixture (fractional inspired O(2) = 0.105). Subjects performed intermittent contractions (1 s) of the calf muscle at 20% maximal voluntary contraction, and the leg blood flow (LBF), leg vascular conductance (LVC) and EMG activities of the triceps surae muscles were measured during each contraction-relaxation period (3 s). The LBF and LVC responses were averaged for each subject and fitted using a four-phase, exponential growth and decay function. Hypoxia evoked significant increases in the change in LBF (15%) and LVC (23%) from the start to the end of exercise, as well as the amplitude of the rapid growth phase of LBF and LVC (21%). Similar, but non-significant, effects on the amplitude of the slow growth phase of LBF (P = 0.08) and LVC (P = 0.10) were observed. By contrast, hypoxia had no effect on temporal parameters of these growth phases, parameters defining the decay phases or EMG activities. These results suggest that the effect of hypoxia on exercise hyperaemia is targeted at the rapid and perhaps the slow growth phase of the response, and is not mediated by a change in the level of muscle activation.

The role of leptin in human lipid and glucose metabolism: the effects of acute recombinant human leptin infusion in young healthy males

BACKGROUND

Obese and lean humans treated with leptin have not experienced convincing weight-loss results compared with the dramatic weight losses observed in obese rodents.

OBJECTIVE

We sought to investigate the effect of acutely elevating leptin to concentrations observed in obese individuals on muscle and adipose tissue metabolism and muscle signaling in healthy lean males.

DESIGN

Healthy, lean, postabsorptive males were infused with either recombinant human leptin (rhleptin; n = 8) or saline (control; n = 8) for 4 h, which elicited leptin concentrations of ~ 20 and ~ 1 ng/mL, respectively. Systemic, skeletal muscle, and adipose tissue fat and glucose metabolism in vivo were assessed before, during, and 2 h after cessation of the infusion. Skeletal muscle biopsy specimens were obtained to quantify changes in signal transducers and activators of transcription-5'AMP-activated protein kinase (STAT-AMPK) signaling.

RESULTS

During the infusion of rhleptin, no differences in either systemic, skeletal muscle, or adipose tissue glucose or fat metabolism were observed. These observations were made despite increased activation of STAT (~ 17-fold) and AMPK (1.43-fold) after 1 h of rhleptin infusion. After the rhleptin infusion, an increase in systemic palmitate and fat oxidation was observed (P < 0.0003), which likely was caused by a concomitant increase in skeletal muscle palmitate oxidation (P < 0.02). This was observed despite lowered leptin concentrations and basal skeletal muscle STAT-AMPK signaling.

CONCLUSIONS

Elevating circulating leptin concentrations to concentrations comparable with those of obese individuals increases human in vivo skeletal muscle signaling through the AMPK pathway and causes an increase in skeletal muscle fatty acid oxidation. Abdominal adipose tissue was unaffected by the acute physiologic increase in leptin concentrations.

Physiological influence of basic perturbations assessed by non-invasive optical techniques in humans

New non-invasive techniques enabling frequent or continuous assessments of various pathophysiological conditions might be used to improve in-hospital outcome by enabling earlier and more reliable bedside detection of medical deterioration. In this preclinical study, three modern non-invasive optical techniques, laser Doppler imaging (LDI), near-infrared spectroscopy (NIRS), and tissue viability imaging (TVI), were all evaluated with respect to the influence of basic physiological perturbations (including local changes in arm positioning, skin temperature, and regional blood flow conditions) on quasi simultaneously obtained values of skin perfusion, muscle tissue oxygenation (StO2), and skin blood volume, recorded in eighteen healthy volunteers. Skin perfusion measured by LDI responded prominently to changes in positioning of the arm, whereas muscle StO2 measured by NIRS did not change significantly. Total haemoglobin count (HbT) measured by NIRS and blood volume estimated by TVI both increased significantly on lowering of the limb. On local cooling, the perfusion and blood volume were both found to increase considerably, while StO2 and HbT did not change. Local heating induced a more than 10-fold increase in skin perfusion and a small increase in blood volume. On progressive venoarterial occlusion, the perfusion, StO2, HbT, and blood volume values decreased, after transient increases in HbT and blood volume before full arterial occlusion occurred, and all values approached the baseline level on release of the occlusion with a slight overshoot of the StO2. The results obtained have potential bearing on future utilization of these non-invasive techniques in the management of severely injured and (or) critically ill patients.

Influence of cold water immersion on limb and cutaneous blood flow at rest

BACKGROUND

Cold water immersion reduces exercise-induced muscle damage. Benefits may partly arise from a decline in limb blood flow; however, no study has comprehensively investigated the influence of different degrees of cooling undertaken via cold water immersion on limb blood flow responses.

PURPOSE

To determine the influence of cold (8°C) and cool (22°C) water immersion on lower limb and cutaneous blood flow.

STUDY DESIGN

Controlled laboratory study.

METHODS

Nine men were placed in a semireclined position and lowered into 8°C or 22°C water to the iliac crest for two 5-minute periods interspersed with 2 minutes of nonimmersion. Rectal and thigh skin temperature, deep and superficial muscle temperature, heart rate, mean arterial pressure, thigh cutaneous blood velocity (laser Doppler), and superficial femoral artery blood flow (duplex ultrasound) were measured during immersion and for 30 minutes after immersion. Indices of vascular conductance were calculated (flux and blood flow/mean arterial pressure).

RESULTS

Reductions in rectal temperature (8°C, 0.2° ± 0.1°C; 22°C, 0.1° ± 0.1°C) and thigh skin temperature (8°C, 6.2° ± 0.5°C; 22°C, 3.2° ± 0.2°C) were greater in 8°C water than in 22°C (P < .01). Femoral artery conductance was reduced to a similar extent immediately after immersion (~30%) and 30 minutes after immersion (~40%) under both conditions (P < .01). In contrast, there was less thigh cutaneous vasoconstriction during and after immersion in 8°C water compared with 22°C (P = .01).

CONCLUSION

These data suggest that immersion at both temperatures resulted in similar whole limb blood flow but, paradoxically, more blood was distributed to the skin in the colder water. This suggests that colder temperatures may be associated with reduced muscle blood flow, which could provide an explanation for the benefits of cold water immersion in alleviating exercise-induced muscle damage in sports and athletic contexts.

CLINICAL RELEVANCE

Colder water temperatures may be more effective in the treatment of exercise-induced muscle damage and injury rehabilitation because of greater reductions in muscle blood flow.

Relation of femoral diameter, shear rate, and dilatory response to knee extensor exercise

INTRODUCTION

Younger women typically exhibit marked dilation of the common femoral artery (CFA) during knee extensor exercise; this is in apparent contrast to subject groups with larger (men) and/or older (older women) vessels, which on average display much smaller exercise-induced increases in CFA diameter. To gain additional insight into this variation, the present study closely examined within- and between-group relationships among CFA diameter, shear rate, and the magnitude of exercise-induced CFA dilation.

METHODS

Healthy women (15 younger and 18 older) and men (15 younger and 13 older) performed graded single-leg knee extensor exercise while CFA diameter and blood velocity were measured using Doppler ultrasound.

RESULTS

Resting CFA diameter was smaller (P < 0.05) in women (younger: 0.71 ± 0.02 cm; older: 0.72 ± 0.02 cm) compared with men (younger: 0.84 ± 0.01 cm; older: 0.97 ± 0.03 cm). Resting CFA diameter was inversely associated with peak shear rate (combined groups: r = -0.83, P < 0.001) and the magnitude of dilation in women (younger: r = -0.82; older: r = -0.73, P < 0.001) and younger men (r = -0.66, P < 0.01) but not in older men (r = -0.17, P = 0.56). The dilatory response of the CFA to graded increases in shear rate showed a continuum across groups best described by a quadratic function (r2 = 0.89). Using piecewise regression modeling, a threshold diameter of 0.79 cm was identified below which diameter is inversely related to the magnitude of CFA dilation but above which little to no dilation was present.

CONCLUSIONS

The CFA dilates to knee extensor exercise in women and men, the magnitude of which varies because of differences in shear rate across CFA diameters.

Effect of type 2 diabetes on the dynamic response characteristics of leg vascular conductance during exercise

In this study we tested the hypothesis that type 2 diabetes impairs the dynamic response of leg vascular conductance (LVC) during exercise. LVC (leg blood flow/mean arterial pressure) responses were studied during intermittent contractions of the calf muscle in subjects with type 2 diabetes (n = 9), heavy controls (n = 10) and lean controls (n = 8) using a biexponential function and an estimate of the mean response time (MRT). The time constant of the second phase of LVC was significantly greater in type 2 diabetes (66.4 ± 29.2 s) than the heavy (22.2 ± 13.4 s) and lean (21.8 ± 9.3 s) controls, resulting in a significantly greater MRT in the diabetic group (median [IQR] = 30.7 [24.6-46.5] s versus 16.3 [4.3-23.2] s and 18.4 [13.7-19.3] s). These data support the hypothesis and suggest that a slowed hyperaemic response in the exercising limb might contribute to exercise intolerance in diabetic subjects.

Local histamine H(1-) and H(2)-receptor blockade reduces postexercise skeletal muscle interstitial glucose concentrations in humans

Elevated blood flow can potentially influence skeletal muscle glucose uptake, but the impact of postexercise hyperemia on glucose availability to skeletal muscle remains unknown. Because postexercise hyperemia is mediated by histamine H(1)- and H(2)-receptors, we tested the hypothesis that postexercise interstitial glucose concentrations would be lower in the presence of combined H1- and H2-receptor blockade. To this end, 4 microdialysis probes were inserted into the vastus lateralis muscle of 14 healthy subjects (21-27 years old) immediately after 60 min of either upright cycling at 60% peak oxygen uptake (exercise, n = 7) or quiet rest (sham, n = 7). Microdialysis probes were perfused with a modified Ringer's solution containing 3 mmol L(-1) glucose, 5 mmol L(-1) ethanol, and [6-3H] glucose (200 disintegrations·min-1 microL(-1)). Two sites (blockade) received both H1- and H2-receptor antagonists (1 mmol L(-1) pyrilamine and 3 mmol L-1 cimetidine) and 2 sites (control) did not receive antagonists. Ethanol outflow/inflow ratios (an inverse surrogate of local blood flow) were higher in blockade sites than in control sites following exercise (p < 0.05), whereas blockade had no effect on ethanol outflow/inflow ratios following sham (p = 0.80). Consistent with our hypothesis, during 3 of the 5 dialysate collection periods, interstitial glucose concentrations were lower in blockade sites vs. control sites following exercise (p < 0.05), whereas blockade had no effect on interstitial glucose concentrations following sham (p = 0.79). These findings indicate that local H1- and H2-receptor activation modulates skeletal muscle interstitial glucose levels during recovery from exercise in humans and suggest that the availability of glucose to skeletal muscle is enhanced by postexercise hyperemia.

Nitric oxide contributes to the augmented vasodilatation during hypoxic exercise

We tested the hypotheses that (1) nitric oxide (NO) contributes to augmented skeletal muscle vasodilatation during hypoxic exercise and (2) the combined inhibition of NO production and adenosine receptor activation would attenuate the augmented vasodilatation during hypoxic exercise more than NO inhibition alone. In separate protocols subjects performed forearm exercise (10% and 20% of maximum) during normoxia and normocapnic hypoxia (80% arterial O(2) saturation). In protocol 1 (n = 12), subjects received intra-arterial administration of saline (control) and the NO synthase inhibitor N(G)-monomethyl-L-arginine (L-NMMA). In protocol 2 (n = 10), subjects received intra-arterial saline (control) and combined L-NMMA-aminophylline (adenosine receptor antagonist) administration. Forearm vascular conductance (FVC; ml min(-1) (100 mmHg)(-1)) was calculated from forearm blood flow (ml min(-1)) and blood pressure (mmHg). In protocol 1, the change in FVC (Delta from normoxic baseline) due to hypoxia under resting conditions and during hypoxic exercise was substantially lower with L-NMMA administration compared to saline (control; P < 0.01). In protocol 2, administration of combined L-NMMA-aminophylline reduced the DeltaFVC due to hypoxic exercise compared to saline (control; P < 0.01). However, the relative reduction in DeltaFVC compared to the respective control (saline) conditions was similar between L-NMMA only (protocol 1) and combined L-NMMA-aminophylline (protocol 2) at 10% (-17.5 +/- 3.7 vs. -21.4 +/- 5.2%; P = 0.28) and 20% (-13.4 +/- 3.5 vs. -18.8 +/- 4.5%; P = 0.18) hypoxic exercise. These findings suggest that NO contributes to the augmented vasodilatation observed during hypoxic exercise independent of adenosine.

Can venous occlusion plethysmography be used to measure high rates of arterial inflow?

To investigate whether venous occlusion plethysmography (VOP) may be used to measure high rates of arterial inflow associated with exercise, venous occlusions were performed at rest, and following dynamic handgrip exercise at 15, 30, 45, and 60% of maximum voluntary contraction (MVC) in seven healthy males. The effect of including more than one cardiac cycle in the calculation of blood flow was assessed by comparing the cumulative blood flow over one, two, three, or four cardiac cycles. The inclusion of more than one cardiac cycle at 30 and 60% MVC, and more than two cardiac cycles at 15 and 45% MVC resulted in a lower blood flow compared to using only the first cardiac cycle (P < 0.05). Despite the small time interval over which arterial inflow was measured (~1 s), this did not affect the reproducibility of the technique. Reproducibility (coefficient of variation for arterial inflow over three trials) tended to be poorer at the higher workloads, although this was not significant (12.7 +/- 6.6, 16.2 +/- 7.3, and 22.9 +/- 9.9% for the 15, 30, and 45% MVC workloads; P = 0.102). There was also a tendency for greater reproducibility with the inclusion of more cardiac cycles at the highest workload, but this did not reach significance (P = 0.070). In conclusion, when calculated over the first cardiac cycle only during venous occlusion, high rates of forearm blood flow can be measured using VOP, and this can be achieved without a significant decrease in the reproducibility of the measurement.

NOS inhibition blunts and delays the compensatory dilation in hypoperfused contracting human muscles

We previously demonstrated that skeletal muscle blood flow is restored in the exercising forearm during experimental hypoperfusion via local dilator and/or myogenic mechanisms. This study examined the role of nitric oxide (NO) in the restoration of blood flow to the active muscles during hypoperfusion. Eleven healthy subjects (10 men/1 woman; 25 +/- 1 yr of age) performed rhythmic forearm exercise (10% and 20% of maximum) while hypoperfusion was evoked by balloon inflation in the brachial artery above the elbow. Each trial included baseline, exercise, exercise with inflation, and exercise after deflation (3 min each). Forearm blood flow (FBF; ultrasound) and local (brachial artery catheter pressure, BAP) and systemic arterial pressure [mean arterial pressure (MAP); Finometer] were measured. The exercise bouts were repeated during N(G)-monomethyl-L-arginine (L-NMMA) infusion (NO synthase inhibition). Forearm vascular conductance (FVC; ml x min(-1) x 100 mmHg(-1)) was calculated from BF (ml/min) and BAP (mmHg). FBF and FVC fell acutely with balloon inflation during all trials (P < 0.01). Recovery of FBF and FVC [(inflation - nadir)/(steady-state exercise - nadir)] with L-NMMA administration was reduced during 20% exercise (FBF = 77 +/- 7% vs. 88 +/- 8%; FVC = 71 +/- 8% vs. 90 +/- 9%; P < 0.01) but not 10% exercise (FBF = 83 +/- 4% vs. 81 +/- 5%, P = 0.37; FVC = 75 +/- 10% vs. 76 +/- 7%; P = 0.44) compared with the respective control trial. The time to steady-state vasodilator response was substantially longer during the l-NMMA trials (10% = 74 +/- 4 s vs. 61 +/- 6 s; 20% = 53 +/- 4 s vs. 41 +/- 4 s; P < 0.05). Thus the magnitude and timing of the NO contribution to compensatory dilation during forearm exercise with hypoperfusion was dependent on exercise intensity. These observations suggest that NO is released by contracting muscles or that a portion of the dilation caused by ischemic metabolites is NO dependent.

Fatty acid metabolism in patients with PPARgamma mutations

CONTEXT

PPARG mutations may cause insulin resistance and dyslipidemia, but little is known about the mechanisms of the abnormalities of lipid metabolism.

OBJECTIVE

We hypothesized that in PPARG mutations, abnormal adipose tissue triglyceride storage causes insulin resistance.

DESIGN, PATIENTS, AND MAIN OUTCOME MEASURES

Whole-body and adipose tissue-specific metabolic phenotyping through arteriovenous blood sampling was made before and after a mixed meal including 13C-palmitic acid. Studies were performed in a 32-yr-old male with partial lipodystrophy and type 2 diabetes, heterozygous for the PPARG P467L mutation and in an apparently phenotypically normal 32-yr-old male heterozygous for the PPARG n.AAA553T mutation. Comparator groups were age- and sex-matched healthy participants (n=10) and type 2 diabetes sex-matched participants (n=6).

RESULTS

The P467L patient had elevated unmodulated fasting and postprandial plasma nonesterified fatty acid (NEFA) concentrations, despite a low adipose tissue NEFA output. Instead, NEFA appeared to originate directly from triglyceride-rich lipoproteins: 13C-palmitic acid accumulated rapidly in the NEFA fraction, as a sign of impaired fatty acid trapping in tissues. In contrast to the Pparg haploinsufficient mouse, the patient with n.AAA553T mutation did not exhibit paradoxically insulin sensitive and showed a mostly normal metabolic pattern.

CONCLUSIONS

The lipodystrophic PPARG P467L phenotype include excessive and uncontrolled generation of NEFA directly from triglyceride-rich lipoproteins, explaining high systemic NEFA concentrations, whereas the human PPARG haploinsufficiency is metabolically almost normal.

Measuring muscle blood flow: a key link between systemic and regional metabolism

PURPOSE OF REVIEW

To provide a brief overview of the main techniques to measure muscle blood flow in humans and highlight some of the strengths and weaknesses associated with each technique.

RECENT FINDINGS

Peak muscle blood flow values of 300 ml/min per 100 g are possible in humans during heavy exercise performed with small muscle mass. This value is far higher than that which appears in most textbooks. Accurate and reliable techniques are therefore essential in measuring muscle blood flow. Current invasive techniques commonly used include indicator dilution (thermodilution and dye dilution) and radiolabel tracer washout (e.g. 133Xe washout) methods. Although invasive techniques have provided valuable insight into tissue blood flow, noninvasive techniques such as venous occlusion plethysmography and Doppler ultrasound are frequently used and provide accurate measurements of blood flow. Newer imaging techniques (MRI, positron emission tomography, and contrast-enhanced ultrasonography) promise increased resolution of measurements of local blood flow, including in discrete tissues in which more classical techniques are not able to be used.

SUMMARY

Muscle blood flow is a key link in the interplay and regulation of systemic and local muscle metabolism. Recognizing the advantages and limitations of each technique is essential to translational researchers studying the effects of nutrition and metabolism on muscle blood flow.

The oxygen delivery response to acute hypoxia during incremental knee extension exercise differs in active and trained males

Background

It is well known that hypoxic exercise in healthy individuals increases limb blood flow, leg oxygen extraction and limb vascular conductance during knee extension exercise. However, the effect of hypoxia on cardiac output, and total vascular conductance is less clear. Furthermore, the oxygen delivery response to hypoxic exercise in well trained individuals is not well known. Therefore our aim was to determine the cardiac output (Doppler echocardiography), vascular conductance, limb blood flow (Doppler echocardiography) and muscle oxygenation response during hypoxic knee extension in normally active and endurance-trained males.

Methods

Ten normally active and nine endurance-trained males (VO_2max = 46.1 and 65.5 mL/kg/min, respectively) performed 2 leg knee extension at 25, 50, 75 and 100% of their maximum intensity in both normoxic and hypoxic conditions (FIO₂ = 15%; randomized order). Results were analyzed with a 2-way mixed model ANOVA (group × intensity).

Results

The main finding was that in normally active individuals hypoxic sub-maximal exercise (25 – 75% of maximum intensity) brought about a 3 fold increase in limb blood flow but decreased stroke volume compared to normoxia. In the trained group there were no significant changes in stroke volume, cardiac output and limb blood flow at sub-maximal intensities (compared to normoxia). During maximal intensity hypoxic exercise limb blood flow increased approximately 300 mL/min compared to maximal intensity normoxic exercise.

Conclusion

Cardiorespiratory fitness likely influences the oxygen delivery response to hypoxic exercise both at a systemic and limb level. The increase in limb blood flow during maximal exercise in hypoxia (both active and trained individuals) suggests a hypoxic stimulus that is not present in normoxic conditions.

The role of functional foods in the psychobiology of health and disease

The effect of psychological stress on health is becoming a serious concern, with figures from the World Health Organization showing that stress-related disorders affect nearly 450 million individuals worldwide. Heightened physiological stress responses and psychosocial factors have been linked to disease pathways such as hypertension and CVD. This has prompted significant interest within the scientific community, public health bodies and industry to employ interventions to control and reduce the impact of stress on health. There is now strong potential for functional foods to offer stress management benefits. Various physiological pathways have been targeted by specific dietary supplements for stress reduction, including the hypothalamic–pituitary–adrenal axis and sympathetic nervous system. Presently there are a number of ingredients, which include vitamin C, milk proteins, a number of herbal extracts (ginkgo biloba, ginseng, kava, valerian and lemon balm), andn-3 fatty acids, that have demonstrated potential stress reactivity-lowering and mood-enhancing effects, although further work is required to substantiate the efficacy in human subjects. Dietary supplements that can alleviate excessive stress responses may play an increasingly important role for the maintenance of health in a stressful environment. However, future research should employ a greater range of measures that will provide stronger evidence to substantiate functional food claims for stress relief.

Time-courses of perfusion and phosphocreatine in rat leg during low-level exercise and recovery

PURPOSE

To develop a noninvasive protocol for measuring local perfusion and metabolic demand in muscle tissue with sufficient sensitivity and time resolution to monitor kinetics at the onset of low-level exercise and during recovery.

MATERIALS AND METHODS

Capillary-level perfusion, the critical factor that determines oxygen and substrate delivery to active muscle, was measured by an arterial spin labeling (ASL) technique optimized for skeletal muscle. Phosphocreatine (PCr) kinetics, which signal the flux of oxidative phosphorylation, were measured by (31)P MR spectroscopy. Perfusion and PCr measurements were made in parallel studies before, during, and after three different intensities of low-level, stimulated exercise in rat hind limb.

RESULTS

The data reveal close coupling between the perfusion response and PCr changes. The onset and recovery time constants for PCr changes were independent of contractile force over the range of forces studied. Perfusion time constants during both onset of exercise and recovery tended to increase with contractile force.

CONCLUSION

These results demonstrate that the protocol implemented can be useful for probing the mechanisms that control skeletal muscle blood flow, the physiological limits to muscle performance, and the causes for the attenuated exercise-induced hyperemia observed in disease states.

The in vivo effects of the Pro12Ala PPARgamma2 polymorphism on adipose tissue NEFA metabolism: the first use of the Oxford Biobank

AIMS/HYPOTHESIS

To investigate the phenotypic effects of common polymorphisms on adipose tissue metabolism and cardiovascular risk factors, we set out to establish a biobank with the unique feature of allowing a prospective recruit-by-genotype approach. The first use of this biobank investigates the effects of the peroxisome proliferator-activated receptor (PPAR) Pro12Ala polymorphism on integrative tissue-specific physiology. We hypothesised that Ala12 allele carriers demonstrate greater adipose tissue metabolic flexibility and insulin sensitivity.

MATERIALS AND METHODS

From a comprehensive population register, subjects were recruited into a biobank, which was genotyped for the Pro12Ala polymorphism. Twelve healthy male Ala12 carriers and 12 matched Pro12 homozygotes underwent detailed physiological phenotyping using stable isotope techniques, and measurements of blood flow and arteriovenous differences in adipose tissue and muscle in response to a mixed meal containing [1,1,1-(13)C]tripalmitin.

RESULTS

Of 6,148 invited subjects, 1,072 were suitable for inclusion in the biobank. Among Pro12 homozygotes, insulin sensitivity correlated with HDL-cholesterol concentrations, and inversely correlated with blood pressure, apolipoprotein B, triglyceride and total cholesterol concentrations. Ala12 carriers showed no such correlations. In the meal study, Ala12 carriers had lower plasma NEFA concentrations, higher adipose tissue and muscle blood flow, and greater insulin-mediated postprandial hormone-sensitive lipase suppression along with greater insulin sensitivity than Pro12 homozygotes.

CONCLUSIONS/INTERPRETATION

This study shows that a recruit-by-genotype approach is feasible and describes the biobank's first application, providing tissue-specific physiological findings consistent with the epidemiological observation that the PPAR Ala12 allele protects against the development of type 2 diabetes.

Measuring the rate of change of haemodynamic response at the onset of exercise in normal limbs and those with intermittent claudication

Patients with claudication have an inadequate haemodynamic response to exercise. Blood flow response will not only have a magnitude, but also a rate of change. There is scope for investigating these parameters, as manipulation of the factors which control them may benefit work to improve the treatment for claudication. This work compares the responses for patients with one normal limb and one with intermittent claudication. A custom-built ergometer allows unilateral, infragenicular plantar flexion exercise, whilst common femoral artery blood flow can be measured continuously by Duplex ultrasound. This apparatus was used to measure blood flow before, at the onset of and during a 5 W square-wave exercise stimulus in 15 patients. The claudicant group had a mean steady-state gain that was approximately half that of the normal group at around 170 ml min(-1) (p < 0.001) and a response time that was much shorter (p = 0.006). A mean response time of 21.0 +/- 1.4 s was achieved in claudicant limbs compared to 31.8 +/- 2.9 s in normals. However, the average rate of change of blood flow during this response time was estimated to still be greater for the normal group, at 431.7 +/- 47.1 ml min(-2), than for the claudicant group. The differences in magnitude and rate of change of limb blood flow response to exercise in claudicants were significant and may have implications for the treatment of claudication.

Insulin-sensitizing effects of dietary resistant starch and effects on skeletal muscle and adipose tissue metabolism

BACKGROUND

Resistant starch may modulate insulin sensitivity, although the precise mechanism of this action is unknown.

OBJECTIVE

We studied the effects of resistant starch on insulin sensitivity and tissue metabolism.

DESIGN

We used a 4-wk supplementation period with 30 g resistant starch/d, compared with placebo, in 10 healthy subjects and assessed the results by using arteriovenous difference methods.

RESULTS

When assessed by euglycemic-hyperinsulinemic clamp, insulin sensitivity was higher after resistant starch supplementation than after placebo treatment (9.7 and 8.5 x 10(-2) mg glucose x kg(-1) x min(-1) x (mU insulin/L)(-1), respectively; P = 0.03); insulin sensitivity during the meal tolerance test (MTT) was 33% higher (P = 0.05). Forearm muscle glucose clearance during the MTT was also higher after resistant starch supplementation (P = 0.03) despite lower insulin concentrations (P = 0.02); glucose clearance adjusted for insulin was 44% higher. Subcutaneous abdominal adipose tissue nonesterified fatty acid (NEFA; P = 0.02) and glycerol (P = 0.05) release were lower with resistant starch supplementation, although systemic NEFA concentrations were not significantly altered. Short-chain fatty acid concentrations (acetate and propionate) were higher during the MTT (P = 0.05 and 0.01, respectively), as was acetate uptake by adipose tissue (P = 0.03). Fasting plasma ghrelin concentrations were higher with resistant starch supplementation (2769 compared with 2062 pg/mL; P = 0.03), although postprandial suppression (40-44%) did not differ significantly. Measurements of gene expression in adipose tissue and muscle were uninformative, which suggests effects at a metabolic level. The resistant starch supplement was well tolerated.

CONCLUSION

These results suggest that dietary supplementation with resistant starch has the potential to improve insulin sensitivity. Further studies in insulin-resistant persons are needed.

Methodological approaches to the study of metabolism across individual tissues in man

PURPOSE OF REVIEW

This article is intended to briefly overview available methodological approaches for the study of regional metabolism in man in vivo, and to summarize recent advances in this field of research.

RECENT FINDINGS

Several methods have been developed and currently allow for the qualitative and quantitative assessment of energy interconversions and substrate fluxes across individual tissues of man, including the measurement of arteriovenous concentration differences, microdialysis, and nuclear magnetic resonance spectroscopy of carbon, hydrogen, and phosphorus isotopes. Each method alone has been used rather extensively to examine certain aspects of organ and tissue metabolism under a variety of experimental conditions, and has contributed novel information in this regard. The most exciting development appears to be the combined use of more than one investigational technique, across one or more tissues simultaneously. A handful of recent studies have employed complex experimental designs or hybrid methodologies, ultimately demonstrating the potential for a more detailed assessment of metabolism at the local level.

SUMMARY

Clearly, advances in the use, performance, and applications of available methods are expected to provide improved and more powerful tools for the metabolic investigation of organs and tissues in humans in vivo.

Gradient-enhanced FAWSETS perfusion measurements

This work describes the use of custom-built gradients to enhance skeletal muscle perfusion measurements acquired with a previously described arterial spin labeling technique known as FAWSETS (flow-driven arterial water stimulation with elimination of tissue signal). Custom-built gradients provide active control of the static magnetic field gradient on which FAWSETS relies for labeling. This allows selective, 180 degrees modulations of the phase of the perfusion component of the signal. Phase cycling can then be implemented to eliminate all extraneous components leaving a signal that exclusively reflects capillary-level perfusion. Gradient-enhancement substantially reduces acquisition time and eliminates the need to acquire an ischemic signal to quantify perfusion. This removes critical obstacles to application of FAWSETS in organs other than skeletal muscle and makes the measurements more desirable for clinical environments. The basic physical principles of gradient-enhancement are demonstrated in flow phantom experiments and in vivo utility is demonstrated in rat hind limb during stimulated exercise.