Heart failure with preserved ejection fraction diminishes peripheral hemodynamics and accelerates exercise-induced neuromuscular fatigue.

This study investigated the impact of HFpEF on neuromuscular fatigue and peripheral hemodynamics during small muscle mass exercise not limited by cardiac output. Eight HFpEF patients (NYHA II-III, ejection-fraction: 61 ± 2%) and eight healthy controls performed dynamic knee extension exercise (80% peak workload) to task failure and maximal intermittent quadriceps contractions (8 × 15 s). Controls repeated knee extension at the same absolute intensity as HFpEF. Leg blood flow (QL) was quantified using Doppler ultrasound. Pre/postexercise changes in quadriceps twitch torque (ΔQtw; peripheral fatigue), voluntary activation (ΔVA; central fatigue), and corticospinal excitability were quantified. At the same relative intensity, HFpEF (24 ± 5 W) and controls (42 ± 6 W) had a similar time-to-task failure (∼10 min), ΔQtw (∼50%), and ΔVA (∼6%). This resulted in a greater exercise-induced change in neuromuscular function per unit work in HFpEF, which was significantly correlated with a slower QL response time. Knee extension exercise at the same absolute intensity resulted in an ∼40% lower QL and greater ΔQtw and ΔVA in HFpEF than in controls. Corticospinal excitability remained unaltered during exercise in both groups. Finally, despite a similar ΔVA, ΔQtw was larger in HFpEF versus controls during isometric exercise. In conclusion, HFpEF patients are characterized by a similar development of central and peripheral fatigue as healthy controls when tested at the same relative intensity during exercise not limited by cardiac output. However, HFpEF patients have a greater susceptibility to neuromuscular fatigue during exercise at a given absolute intensity, and this impairs functional capacity. The patients' compromised QL response to exercise likely accounts, at least partly, for the patients' attenuated fatigue resistance.NEW & NOTEWORTHY The susceptibility to neuromuscular fatigue during exercise is substantially exaggerated in individuals with heart failure with a preserved ejection fraction. The faster rate of fatigue development is associated with the compromised peripheral hemodynamic response characterizing these patients during exercise. Given the role of neuromuscular fatigue as a factor limiting exercise, this impairment likely accounts for a significant portion of the exercise intolerance typical for this population.

Intensity-dependent alterations in the excitability of cortical and spinal projections to the knee extensors during isometric and locomotor exercise.

We investigated the role of exercise intensity and associated central motor drive in determining corticomotoneuronal excitability. Ten participants performed a series of nonfatiguing (3 s) isometric single-leg knee extensions (ISO; 10-100% of maximal voluntary contractions, MVC) and cycling bouts (30-160% peak aerobic capacity, W peak). At various exercise intensities, electrical potentials were evoked in the vastus lateralis (VL) and rectus femoris (RF) via transcranial magnetic stimulation (motor-evoked potentials, MEP), and electrical stimulation of both the cervicomedullary junction (cervicomedullary evoked potentials, CMEP) and the femoral nerve (maximal M-waves, M max). Whereas M max remained unchanged in both muscles (P > 0.40), voluntary electromyographic activity (EMG) increased in an exercise intensity-dependent manner for ISO and cycling exercise in VL and RF (both P < 0.001). During ISO exercise, MEPs and CMEPs progressively increased in VL and RF until a plateau was reached at ∼ 75% MVC; further increases in contraction intensity did not cause additional changes (P > 0.35). During cycling exercise, VL-MEPs and CMEPs progressively increased by ∼ 65% until a plateau was reached at W peak. In contrast, RF MEPs and CMEPs progressively increased by ∼ 110% throughout the tested cycling intensities without the occurrence of a plateau. Furthermore, alterations in EMG below the plateau influenced corticomotoneuronal excitability similarly between exercise modalities. In both exercise modalities, the MEP-to-CMEP ratio did not change with exercise intensity (P > 0.22). In conclusion, increases in exercise intensity and EMG facilitates the corticomotoneuronal pathway similarly in isometric knee extension and locomotor exercise until a plateau occurs at a submaximal exercise intensity. This facilitation appears to be primarily mediated by increases in excitability of the motoneuron pool.

Central and peripheral hemodynamic responses to passive limb movement: the role of arousal.

The exact role of arousal in central and peripheral hemodynamic responses to passive limb movement in humans is unclear but has been proposed as a potential contributor. Thus, we used a human model with no lower limb afferent feedback to determine the role of arousal on the hemodynamic response to passive leg movement. In nine people with a spinal cord injury, we compared central and peripheral hemodynamic and ventilatory responses to one-leg passive knee extension with and without visual feedback (M+VF and M-VF, respectively) as well as in a third trial with no movement or visual feedback but the perception of movement (F). Ventilation (Ve), heart rate, stroke volume, cardiac output, mean arterial pressure, and leg blood flow (LBF) were evaluated during the three protocols. Ve increased rapidly from baseline in M+VF (55 ± 11%), M-VF (63 ± 13%), and F (48 ± 12%) trials. Central hemodynamics (heart rate, stroke volume, cardiac output, and mean arterial pressure) were unchanged in all trials. LBF increased from baseline by 126 ± 18 ml/min in the M+VF protocol and 109 ± 23 ml/min in the M-VF protocol but was unchanged in the F protocol. Therefore, with the use of model that is devoid of afferent feedback from the legs, the results of this study reveal that, although arousal is invoked by passive movement or the thought of passive movement, as evidenced by the increase in Ve, there is no central or peripheral hemodynamic impact of this increased neural activity. Additionally, this study revealed that a central hemodynamic response is not an obligatory component of movement-induced LBF.

Hyperoxic interval training in chronic obstructive pulmonary disease patients with oxygen desaturation at peak exercise.

High-intensity work might not be preserved in chronic obstructive pulmonary disease (COPD) during whole-body exercise due to ventilatory limitations that exceed metabolic limitations, resulting in reduced training adaptations. The purpose of the present study was to address the hyperoxic effect during training and testing in COPD patients with hypoxemia at peak exercise. Six COPD and eight coronary artery disease (CAD) patients completed 24 aerobic high-intensity interval training sessions, 4x4 min in hyperoxia at 85-95% of the peak heart rate and peak exercise tested in normoxia and hyperoxia pre- and post-training. VO2peak increased in the COPD group by 19% (13-31%) and in the CAD group by 15% (7-29%), [0.98(0.68-1.52)-1.17(0.89-1.78) and 2.11(1.57-2.64)-2.44(1.92-3.39) L/min], respectively. VO2peak was higher in hyperoxia at pre- and post-test (1.22(0.80-1.87) and 1.37(1.01-1.94) L/min) in the COPD group. Work economy improved by 10% in both groups. Quality of life improved in the COPD group in terms of physical and mental health status by 24% and 35%. Hyperoxic aerobic high-intensity interval training in COPD patients with hypoxemia at peak exercise increases VO2peak, peak workload, work economy and quality of life. Acute hyperoxia increases VO2peak, peak workload at pre- and post-test compared with normoxia in the COPD patients, indicating an oxygen supply limitation to VO2peak.

Age-related endothelial dysfunction in human skeletal muscle feed arteries: the role of free radicals derived from mitochondria in the vasculature.

AIM: This study sought to determine the role of free radicals derived from mitochondria in the vasculature in the recognized age-related endothelial dysfunction of human skeletal muscle feed arteries (SMFAs). METHODS: A total of 44 SMFAs were studied with and without acute exposure to the mitochondria-targeted antioxidant MitoQ and nitric oxide synthase (NOS) blockade. The relative abundance of proteins from the electron transport chain, phosphorylated (p-) to endothelial (e) NOS ratio, manganese superoxide dismutase (MnSOD) and the mitochondria-derived superoxide (O2-) levels were assessed in SMFA. Endothelium-dependent and endothelium-independent SMFA vasodilation was assessed in response to flow-induced shear stress, acetylcholine (ACh) and sodium nitroprusside (SNP). RESULTS: MitoQ restored endothelium-dependent vasodilation in the old to that of the young when stimulated by both flow (young: 68 ± 5; old: 25 ± 7; old + MitoQ 65 ± 9%) and ACh (young: 97 ± 4; old: 59 ± 10; old + MitoQ: 98 ± 5%), but did not alter the initially uncompromised, endothelium-independent vasodilation (SNP). Compared to the young, MitoQ in the old diminished the initially elevated mitochondria-derived O2- levels and appeared to attenuate the breakdown of MnSOD. Furthermore, MitoQ increased the ratio of p-eNOS to NOS and the restoration of endothelium-dependent vasodilation in the old by MitoQ was ablated by NOS blockade. CONCLUSION: This study demonstrated that MitoQ reverses age-related vascular dysfunction by what appears to be an NO-dependent mechanism in human SMFAs. These findings suggest that mitochondria-targeted antioxidants may have utility in terms of counteracting the attenuated blood flow and vascular dysfunction associated with advancing age.

Myoglobin O2 desaturation during exercise. Evidence of limited O2 transport.

The assumption that cellular oxygen pressure (PO2) is close to zero in maximally exercising muscle is essential for the hypothesis that O2 transport between blood and mitochondria has a finite conductance that determines maximum O2 consumption. The unique combination of isolated human quadriceps exercise, direct measures of arterial, femoral venous PO2, and 1H nuclear magnetic resonance spectroscopy to detect myoglobin desaturation enabled this assumption to be tested in six trained men while breathing room air (normoxic, N) and 12% O2 (hypoxic, H). Within 20 s of exercise onset partial myoglobin desaturation was evident even at 50% of maximum O2 consumption, was significantly greater in H than N, and was then constant at an average of 51 +/- 3% (N) and 60 +/- 3% (H) throughout the incremental exercise protocol to maximum work rate. Assuming a myoglobin PO2 where 50% of myoglobin binding sites are bound with O2 of 3.2 mmHg, myoglobin-associated PO2 averaged 3.1 +/- .3 (N) and 2.1 +/- .2 mmHg (H). At maximal exercise, measurements of arterial PO2 (115 +/- 4 [N] and 46 +/- 1 mmHg [H]) and femoral venous PO2 (22 +/- 1.6 [N] and 17 +/- 1.3 mmHg [H]) resulted in calculated mean capillary PO2 values of 38 +/- 2 (N) and 30 +/- 2 mmHg(H). Thus, for the first time, large differences in PO2 between blood and intracellular tissue have been demonstrated in intact normal human muscle and are found over a wide range of exercise intensities. These data are consistent with an O2 diffusion limitation across the 1-5-microns path-length from red cell to the sarcolemma that plays a role in determining maximal muscle O2 uptake in normal humans.

Incremental large and small muscle mass exercise in patients with heart failure: evidence of preserved peripheral haemodynamics and metabolism.

AIM: Doubt still remains as to whether peripheral vascular and skeletal muscle dysfunction accompanies the compromised cardiac function associated with heart failure with reduced ejection fraction (HFrEF). The aim of this study was to examine the effect of HFrEF on the haemodynamic and metabolic responses to exercise with both a large (cycle) and a small [knee extensor (KE)] muscle mass in comparison with well-matched healthy controls (Ctrls). METHODS: Utilizing blood sampling and thermodilution blood flow measurements, we studied incremental cycle and KE exercise in 12 patients with HFrEF (ejection fraction: 25 ± 3%) and eight Ctrls. RESULTS: Incremental cycle exercise in both groups [heart failure with reduced ejection fraction (HFrEF): 23 ± 1 to 116 ± 10; Ctrls: 22 ± 1 to 137 ± 5 W] resulted in a similar rise in blood flow (HFrEF: 1525 ± 132 to 4216 ± 408; Ctrls: 1774 ± 161 to 4713 ± 448 mL min(-1)), oxygen uptake (HFrEF: 206 ± 24 to 586 ± 34; Ctrls: 252 ± 21 to 747 ± 89 mL min(-1)) and lactate efflux across the leg (HFrEF: 479 ± 122 to 4929 ± 1255; Ctrls: 537 ± 155 to 5776 ± 1010 mm min(-1)). Vascular resistance fell similarly in both groups with increasing exercise intensity (HFrEF: 66 ± 10 to 24 ± 3; Ctrls: 69 ± 12 to 24 ± 4 mmHg L(-1) min(-1) ). Incremental KE exercise also revealed similar haemodynamic and metabolic responses in both Ctrls and patients. CONCLUSION: Although assessed in a relatively small cohort, these data reveal that, when compared with well-matched healthy Ctrls, alterations in peripheral haemodynamics and skeletal muscle metabolism during exercise may not be an obligatory accompaniment to HFrEF.

In vivo and in vitro evidence that intrinsic upper- and lower-limb skeletal muscle function is unaffected by ageing and disuse in oldest-old humans.

AIM: To parse out the impact of advanced ageing and disuse on skeletal muscle function, we utilized both in vivo and in vitro techniques to comprehensively assess upper- and lower-limb muscle contractile properties in 8 young (YG; 25 ± 6 years) and 8 oldest-old mobile (OM; 87 ± 5 years) and 8 immobile (OI; 88 ± 4 years) women. METHODS: In vivo, maximal voluntary contraction (MVC), electrically evoked resting twitch force (RT), and physiological cross-sectional area (PCSA) of the quadriceps and elbow flexors were assessed. Muscle biopsies of the vastus lateralis and biceps brachii facilitated the in vitro assessment of single fibre-specific tension (Po). RESULTS: In vivo, compared to the young, both the OM and OI exhibited a more pronounced loss of MVC in the lower limb [OM (-60%) and OI (-75%)] than the upper limb (OM = -51%; OI = -47%). Taking into account the reduction in muscle PCSA (OM = -10%; OI = -18%), only evident in the lower limb, by calculating voluntary muscle-specific force, the lower limb of the OI (-40%) was more compromised than the OM (-13%). However, in vivo, RT in both upper and lower limbs (approx. 9.8 N m cm(-2) ) and Po (approx. 123 mN mm(-2) ), assessed in vitro, implies preserved intrinsic contractile function in all muscles of the oldest-old and were well correlated (r = 0.81). CONCLUSION: These findings suggest that in the oldest-old, neither advanced ageing nor disuse, per se, impacts intrinsic skeletal muscle function, as assessed in vitro. However, in vivo, muscle function is attenuated by age and exacerbated by disuse, implicating factors other than skeletal muscle, such as neuromuscular control, in this diminution of function.

Rationally designed "dipeptoid" analogues of CCK. alpha-Methyltryptophan derivatives as highly selective and orally active gastrin and CCK-B antagonists with potent anxiolytic properties.

This paper describes the synthesis and structure-activity relationships (SAR) leading to the first rational design of "dipeptoid" analogues of the neuropeptide cholecystokinin (CCK). Compounds [R-(R*,S*)]-4-[2-[3-(1H-indol-3-yl)-2-methyl-1-oxo-2-[(tricyclo [3.3.1.1(3,7)]dec-2-yloxy)carbonyl]amino]propyl]amino]-3- phenylpropyl]-amino]-4-oxo-2-butenoic acid, [R-(R*,R*)]-4-[2-[3-(1H-indol-3-yl)-2-methyl-1-oxo-2-[(tricyclo [3.3.1.1(3,7)]dec-2-oxy)carbonyl]amino]propyl]amino]-1- phenylethyl]amino]-4-oxo-2-butenoic acid, and [R-(R*,R*)]-4-[2-[3-(1H-indol-3-yl)-2-methyl-1-oxo-2-[(tricyclo [3.3.1.1(3,7)]dec-2-yloxy)carbonyl]amino]propyl]amino]-1- phenylethyl]amino]-4-oxobutanoic acid (29d) have CCK-B binding affinities of IC50 = 0.8, 0.7, and 1.7 nM with a CCK-A/CCK-B ratio of 550, 1100, and 2500, respectively. Compound 27 is well-absorbed and is equiactive by the subcutaneous (sc) and intravenous (iv) routes of administration in the Ghosh and Schild test in rats in inhibiting pentagastrin stimulated gastric acid secretion with ED50 = 0.07 (0.01-0.34) mumol/kg. Compound 29d is anxiolytic in mice in the black-white test box over the range 0.0001-30 mg/kg sc, comparable in activity to diazepam over the range 0.125-1 mg/kg ip), and also active in this test when dosed orally over a wide range from 0.0001 to 10 mg/kg.

Skeletal muscle phosphocreatine recovery in exercise-trained humans is dependent on O2 availability.

In skeletal muscle, phosphocreatine (PCr) recovery from submaximal exercise has become a reliable and accepted measure of muscle oxidative capacity. During exercise, O2 availability plays a role in determining maximal oxidative metabolism, but the relationship between O2 availability and oxidative metabolism measured by 31P-magnetic resonance spectroscopy (MRS) during recovery from exercise has never been studied. We used 31P-MRS to study exercising human gastrocnemius muscle under conditions of varied fractions of inspired O2 (FIO2) to test the hypothesis that varied O2 availability modulates PCr recovery from submaximal exercise. Six male subjects performed three bouts of 5-min steady-state submaximal plantar flexion exercise followed by 5 min of recovery in a 1.5-T magnet while breathing three different FIO2 concentrations (0.10, 0. 21, and 1.00). Under each FIO2 treatment, the PCr recovery time constants were significantly different, being longer in hypoxia [33. 5 +/- 4.1 s (SE)] and shorter in hyperoxia (20.0 +/- 1.8 s) than in normoxia (25.0 +/- 2.7 s) (P

Skeletal muscle intracellular PO(2) assessed by myoglobin desaturation: response to graded exercise.

The relationship between skeletal muscle intracellular PO(2) (iPO(2)) and progressive muscular work has important implications for the understanding of O(2) transport and utilization. Presently there is debate as to whether iPO(2) falls progressively with increasing O(2) demand or reaches a plateau from moderate to maximal metabolic demand. Thus, using (1)H magnetic resonance spectroscopy of myoglobin (Mb), we studied cellular oxygenation during progressive single-leg knee extensor exercise from unweighted to 100% of maximal work rate in six active human subjects. In all subjects, the Mb peak at 73 ppm was not visible at rest, whereas the peak was small or indistinguishable from the noise in the majority of subjects during progressive exercise from unweighted to 50-60% of maximum work rate. In contrast, beyond this exercise intensity, a Mb peak of consistent magnitude was discernible in all subjects. When a Mb half saturation of 3.2 Torr was used, the calculated skeletal muscle PO(2) was variable before 60% of maximum work rate but in general was relatively high (>18 Torr, the measurable PO(2) with the poorest signal-to-noise ratio, in the majority of cases), whereas beyond this exercise intensity iPO(2) fell to a relatively uniform and invariant level of 3.8 +/- 0.5 Torr across all subjects. These results do not support the concept of a progressive linear fall in iPO(2) across increasing work rates. Instead, this study documents variable but relatively high iPO(2) from rest to moderate exercise and again confirms that from 50-60% of maximum work rate iPO(2) reaches a plateau that is then invariant with increasing work rate.

Initial fall in skeletal muscle force development during ischemia is related to oxygen availability.

We examined the hypothesis that the initial decline (first 1-2 min) in force development that occurs in working muscle when blood flow is halted is caused by O2 availability and not another factor related to blood flow. This was tested by reducing O2 delivery (muscle blood flow X arterial O2 content) to working muscle by either stopping blood flow [ischemia (I)] or maintaining blood flow with low arterial O2 content [hypoxemia (H)]. If initial decline in force development were similar between these two methods of reducing O2 delivery, it would suggest O2 availability as the common pathway. Isolated dog gastrocnemius muscle was stimulated at approximately 60-70% of maximal O2 uptake (1 isometric tetanic contraction every 2 s) until steady-state conditions of muscle blood flow and developed force were attained (approximately 3 min). Two conditions were then sequentially imposed on the working muscle: I, induced by shutting off pump controlling arterial perfusion of the muscle and clamping venous outflow, and H, induced by perfusing the muscle with deoxygenated blood (collected before testing while animal breathed N2) at steady-state blood flow level. Rates of the fall in force production in 17 matched conditions of H and I (approximately 40 s for each condition) were compared in 6 muscles tested. The blood perfusing the muscle during H had arterial PO2 = 8 +/- 1 (SE) Torr, arterial PCO2 = 37 +/- 1 Torr, and arterial pH = 7.39 +/- 0.03. The rate of decline in developed force was not significantly different (P = 0.46) between the 17 matched conditions of H (0.66 +/- 0.10 g force.g mass-1.s-1) and I (0.79 +/- 0.15 g force.g mass-1.s-1). These findings suggest that the initial fall in developed force in working skeletal muscle that occurs with ischemia is related to O2 availability.

Human muscle performance and PCr hydrolysis with varied inspired oxygen fractions: a 31P-MRS study.

The purpose of this study was to use 31P-magnetic resonance spectroscopy to examine the relationships among muscle PCr hydrolysis, intracellular H+ concentration accumulation, and muscle performance during incremental exercise during the inspiration of gas mixtures containing different fractions of inspired O2 (FIO2). We hypothesized that lower FIO2 would result in a greater disruption of intracellular homeostasis at submaximal workloads and thereby initiate an earlier onset of fatigue. Six subjects performed plantar flexion exercise on three separate occasions with the only variable altered for each exercise bout being the FIO2 (either 0.1, 0.21, or 1.00 O2 in balance N2). Work rate was increased (1-W increments starting at 0 W) every 2 min until exhaustion. Time to exhaustion (and thereby workload achieved) was significantly (P < 0.05) greater as FIO2 was increased. Muscle phosphocreatine (PCr) concentration, Pi concentration, and pH at exhaustion were not significantly different among the three FIO2 conditions. However, muscle PCr concentration and pH were significantly reduced at identical submaximal workloads (and thereby equivalent rates of respiration) above 4-5 W during the lowest FIO2 condition compared with the other two FIO2 conditions. These results demonstrate that exhaustion during all FIO2 occurred when a particular intracellular environment was achieved and suggest that during the lowest FIO2 condition, the greater PCr hydrolysis and intracellular acidosis at submaximal workloads may have contributed to the significantly earlier time to exhaustion.

Phosphocreatine hydrolysis during submaximal exercise: the effect of FIO2.

There is evidence that the concentration of the high-energy phosphate metabolites may be altered during steady-state submaximal exercise by the breathing of different fractions of inspired O2 (FIO2). Whereas it has been suggested that these changes may be the result of differences in time taken to achieve steady-state O2 uptake (V(O2)) at different FIO2 values, we postulated that they are due to a direct effect of O2 tension. We used 31P-magnetic resonance spectroscopy during constant-load, steady-state submaximal exercise to determine 1) whether changes in high-energy phosphates do occur at the same V(O2) with varied FIO2 and 2) that these changes are not due to differences in V(O2) onset kinetics. Six male subjects performed steady-state submaximal plantar flexion exercise [7.2 +/- 0.6 (SE) W] for 10 min while lying supine in a 1.5-T clinical scanner. Magnetic resonance spectroscopy data were collected continuously for 2 min before exercise, 10 min during exercise, and 6 min during recovery. Subjects performed three different exercise bouts at constant load with the FIO2 switched after 5 min of the 10-min exercise bout. The three exercise treatments were 1) FIO2 of 0.1 switched to 0.21, 2) FIO2 of 0.1 switched to 1.00, and 3) FIO2 of 1.00 switched to 0.1. For all three treatments, the FIO2 switch significantly (P

Lactate efflux from exercising human skeletal muscle: role of intracellular PO2.

It remains controversial whether lactate formation during progressive dynamic exercise from submaximal to maximal effort is due to muscle hypoxia. To study this question, we used direct measures of arterial and femoral venous lactate concentration, a thermodilution blood flow technique, phosphorus magnetic resonance spectroscopy (MRS), and myoglobin (Mb) saturation measured by 1H nuclear MRS in six trained subjects performing single-leg quadriceps exercise. We calculated net lactate efflux from the muscle and intracellular PO2 with subjects breathing room air and 12% O2. Data were obtained at 50, 75, 90, and 100% of quadriceps maximal O2 consumption at each fraction of inspired O2. Mb saturation was significantly lower in hypoxia than in normoxia [40 +/- 3 vs. 49 +/- 3% (SE)] throughout incremental exercise to maximal work rate. With the assumption of a PO2 at which 50% of Mb-binding sites are bound with O2 of 3.2 Torr, Mb-associated PO2 averaged 3.1 +/- 0.3 and 2.3 +/- 0.2 Torr in normoxia and hypoxia, respectively. Net blood lactate efflux was unrelated to intracellular PO2 across the range of incremental exercise to maximum (r = 0.03 and 0.07 in normoxia and hypoxia, respectively) but linearly related to O2 consumption (r = 0.97 and 0.99 in normoxia and hypoxia, respectively) with a greater slope in 12% O2. Net lactate efflux was also linearly related to intracellular pH (r = 0.94 and 0.98 in normoxia and hypoxia, respectively). These data suggest that with increasing work rate, at a given fraction of inspired O2, lactate efflux is unrelated to muscle cytoplasmic PO2, yet the efflux is higher in hypoxia. Catecholamine values from comparable studies are included and indicate that lactate efflux in hypoxia may be due to systemic rather than intracellular hypoxia.

Cellular PO2 as a determinant of maximal mitochondrial O(2) consumption in trained human skeletal muscle.

Previously, by measuring myoglobin-associated PO(2) (P(Mb)O(2)) during maximal exercise, we have demonstrated that 1) intracellular PO(2) is 10-fold less than calculated mean capillary PO(2) and 2) intracellular PO(2) and maximum O(2) uptake (VO(2 max)) fall proportionately in hypoxia. To further elucidate this relationship, five trained subjects performed maximum knee-extensor exercise under conditions of normoxia (21% O(2)), hypoxia (12% O(2)), and hyperoxia (100% O(2)) in balanced order. Quadriceps O(2) uptake (VO(2)) was calculated from arterial and venous blood O(2) concentrations and thermodilution blood flow measurements. Magnetic resonance spectroscopy was used to determine myoglobin desaturation, and an O(2) half-saturation pressure of 3.2 Torr was used to calculate P(Mb)O(2) from saturation. Skeletal muscle VO(2 max) at 12, 21, and 100% O(2) was 0.86 +/- 0.1, 1.08 +/- 0.2, and 1.28 +/- 0.2 ml. min(-1). ml(-1), respectively. The 100% O(2) values approached twice that previously reported in human skeletal muscle. P(Mb)O(2) values were 2.3 +/- 0.5, 3.0 +/- 0.7, and 4.1 +/- 0.7 Torr while the subjects breathed 12, 21, and 100% O(2), respectively. From 12 to 21% O(2), VO(2) and P(Mb)O(2) were again proportionately related. However, 100% O(2) increased VO(2 max) relatively less than P(Mb)O(2), suggesting an approach to maximal mitochondrial capacity with 100% O(2). These data 1) again demonstrate very low cytoplasmic PO(2) at VO(2 max), 2) are consistent with supply limitation of VO(2 max) of trained skeletal muscle, even in hyperoxia, and 3) reveal a disproportionate increase in intracellular PO(2) in hyperoxia, which may be interpreted as evidence that, in trained skeletal muscle, very high mitochondrial metabolic limits to muscle VO(2) are being approached.

Local perfusion and metabolic demand during exercise: a noninvasive MRI method of assessment.

A noninvasive magnetic resonance imaging (MRI) method to assess the distribution of perfusion and metabolic demand (Q/VO(2)) in exercising human skeletal muscle is described. This method combines two MRI techniques that can provide accurate multiple localized measurements of Q/VO(2) during steady-state plantar flexion exercise. The first technique, (31)P chemical shift imaging, permits the acquisition of comparable phosphorus spectra from multiple voxels simultaneously. Because phosphocreatine (PCr) depletion is directly proportional to ATP hydrolysis, its relative depletion can be used as an index of muscle O(2) uptake (VO(2)). The second MRI technique allows the measurement of both spatially and temporally resolved muscle perfusion in vivo by using arterial spin labeling. Promising validity and reliability data are presented for both MRI techniques. Initial results from the combined method provide evidence of a large variation in Q/VO(2), revealing areas of apparent under- and overperfusion for a given metabolic turnover. Analysis of these data in a similar fashion to that employed in the assessment of ventilation-to-perfusion matching in the lungs revealed a similar second moment of the perfusion distribution and PCr distribution on a log scale (log SD(Q) and log SD(PCr)) (0.47). Modeling the effect of variations in log SD(Q) and log SD(PCr) in terms of attainable VO(2), assuming no diffusion limits, indicates that the log SD(Q) and log SD(PCr) would allow only 92% of the target VO(2) to be achieved. This communication documents this novel, noninvasive method for assessing Q/VO(2), and initial data suggest that the mismatch in Q/VO(2) may play a significant role in determining O(2) transport and utilization during exercise.

Increased VO2 max with right-shifted Hb-O2 dissociation curve at a constant O2 delivery in dog muscle in situ.

If the diffusive component of O2 transport in muscle is important in determining exercise capacity, an increased capillary-to-tissue PO2 difference should enhance gas exchange from blood to skeletal muscle during exercise. Thus a rightward shift in the O2 dissociation curve should theoretically increase O2 extraction and improve maximal O2 uptake (VO2 max). To test this hypothesis, we used the canine gastrocnemius muscle to study maximal exercise in eight dogs at a normal P50 (33.1 +/- 0.4 Torr) and with the O2 dissociation curve shifted to the right by an allosteric modifier of hemoglobin (Hb) (methylpropionic acid, RSR-13; P50 = 53.2 +/- 5.0 Torr). Four control dogs were also studied before and after infusion of vehicle. O2 (100%) was inspired during exercise to maintain arterial saturation in both conditions. The muscle was surgically isolated and electrically stimulated (tetanic train: 0.2-ms stimuli for 200-ms duration at 50 Hz, once per s). To maintain O2 delivery (pre-RSR-13 = 19.1 +/- 2.9; RSR-13 = 19.6 +/- 2.5 ml . 100 g-1 . min-1), the muscle was pump perfused. At a constant O2 delivery, RSR-13 significantly increased percent O2 extraction (pre-RSR-13 = 61 +/- 4.0; RSR-13 = 75.5 +/- 4.7) and muscle VO2 max (pre-RSR-13 = 11.8 +/- 2.1; RSR-13 = 14.2 +/- 1.5 ml . 100 g-1 . min-1). This improvement in VO2 max with increased P50 demonstrates its O2 supply dependence when P50 is normal and the importance of O2 diffusive transport to muscle at maximal exercise.

Muscle O2 uptake kinetics in humans: implications for metabolic control.

Muscle O2 uptake (VO2) kinetics in response to an augmented energetic requirement (on-transition) has never been directly determined in humans. We have developed a constant-infusion thermodilution technique that allowed rapid measurements of leg blood flow (Qleg) and, in conjunction with frequent serial measurement of arteriovenous O2 content difference across the leg [(Ca - Cv)O2leg], permitted the determination of the VO2 of the leg (VO2leg) at 3- to 4-s time intervals. VO2leg kinetics during the on-transition was taken as a close approximation of muscle VO2 (VO2mus) kinetics. Alveolar VO2 (VO2A), Qleg, leg O2 delivery [(Q.CaO2leg)], (Ca - Cv)O2leg, and VO2leg kinetics were determined in six trained subjects [age 22.8 +/- 4.4 (SD) yr; maximal O2 uptake 59.1 +/- 5.3 ml.kg-1.min-1] during the transition from unloaded pedaling to a workload (loaded pedaling; LP) (183 +/- 20 W) well below the previously determined ventilatory threshold. For all variables, two distinct phases were recognized. During the first 10-15 s of loaded pedaling (phase I), VO2A, Qleg, and (Q.CaO2)leg increased rapidly, whereas VO2leg increased only slightly and (Ca - Cv)O2leg actually decreased. After phase I, all variables showed a monoexponential increase (phase II), with similar time courses [slightly faster for (Ca - CV)O2leg]. In a consideration of both phases, the half times of the responses among variables were not significantly different: 25.5 +/- 2.6 s for VO2A, 26.6 +/- 7.6 s for Qleg, 26.9 +/- 8.3 s for (Q.CaO2leg, 23.5 +/- 1.3 s for (Ca - Cv)O2leg, and 27.9 +/- 5.7 s for VO2leg. We conclude that during the on-transition the kinetics of VO2A and VO2leg, as measured by these methods, are similar. The analysis of the early phase (first 10-15 s) of the on-transition indicates that bulk delivery of O2 to the working muscles is not limiting VO2leg kinetics. However, the present results cannot discriminate between maldistribution of blood flow/VO2 vs. inertia the intracellular oxidative machinery as the limiting factor.

Evidence of O2 supply-dependent VO2 max in the exercise-trained human quadriceps.

Maximal O2 delivery and O2 uptake (VO2) per 100 g of active muscle mass are far greater during knee extensor (KE) than during cycle exercise: 73 and 60 ml. min-1. 100 g-1 (2.4 kg of muscle) (R. S. Richardson, D. R. Knight, D. C. Poole, S. S. Kurdak, M. C. Hogan, B. Grassi, and P. D. Wagner. Am. J. Physiol. 268 (Heart Circ. Physiol. 37): H1453-H1461, 1995) and 28 and 25 ml. min-1. 100 g-1 (7.5 kg of muscle) (D. R. Knight, W. Schaffartzik, H. J. Guy, R. Predilleto, M. C. Hogan, and P. D. Wagner. J. Appl. Physiol. 75: 2586-2593, 1993), respectively. Although this is evidence of muscle O2 supply dependence in itself, it raises the following question: With such high O2 delivery in KE, are the quadriceps still O2 supply dependent at maximal exercise? To answer this question, seven trained subjects performed maximum KE exercise in hypoxia [0.12 inspired O2 fraction (FIO2)], normoxia (0.21 FIO2), and hyperoxia (1.0 FIO2) in a balanced order. The protocol (after warm-up) was a square wave to a previously determined maximum work rate followed by incremental stages to ensure that a true maximum was achieved under each condition. Direct measures of arterial and venous blood O2 concentration in combination with a thermodilution blood flow technique allowed the determination of O2 delivery and muscle VO2. Maximal O2 delivery increased with inspired O2: 1.3 +/- 0.1, 1.6 +/- 0.2, and 1.9 +/- 0.2 l/min at 0.12, 0.21, and 1.0 FIO2, respectively (P < 0.05). Maximal work rate was affected by variations in inspired O2 (-25 and +14% at 0.12 and 1.0 FIO2, respectively, compared with normoxia, P < 0.05) as was maximal VO2 (VO2 max): 1.04 +/- 0.13, 1. 24 +/- 0.16, and 1.45 +/- 0.19 l/min at 0.12, 0.21, and 1.0 FIO2, respectively (P < 0.05). Calculated mean capillary PO2 also varied with FIO2 (28.3 +/- 1.0, 34.8 +/- 2.0, and 40.7 +/- 1.9 Torr at 0.12, 0.21, and 1.0 FIO2, respectively, P < 0.05) and was proportionally related to changes in VO2 max, supporting our previous finding that a decrease in O2 supply will proportionately decrease muscle VO2 max. As even in the isolated quadriceps (where normoxic O2 delivery is the highest recorded in humans) an increase in O2 supply by hyperoxia allows the achievement of a greater VO2 max, we conclude that, in normoxic conditions of isolated KE exercise, KE VO2 max in trained subjects is not limited by mitochondrial metabolic rate but, rather, by O2 supply.