Intracellular PO2 kinetics at different contraction frequencies in Xenopus single skeletal muscle fibers.

Increasing contraction frequency in single skeletal muscle fibers has been shown to increase the magnitude of the fall in intracellular Po(2) (Pi(O(2))), reflecting a greater metabolic rate. To test whether Pi(O(2)) kinetics are altered by contraction frequency through this increase in metabolic stress, Pi(O(2)) was measured in Xenopus single fibers (n = 11) during and after contraction bouts at three different frequencies. Pi(O(2)) was measured via phosphorescence quenching at 0.16-, 0.25-, and 0.5-Hz tetanic stimulation. The kinetics of the change in Pi(O(2)) from resting baseline to end-contraction values and end contraction to rest were described as a mean response time (MRT) representing the time to 63% of the change in Pi(O(2)). As predicted, the fall in Pi(O(2)) from baseline following contractions was progressively greater at 0.5 and 0.25 Hz than at 0.16 Hz (32.8 +/- 2.1 and 29.3 +/- 2.0 Torr vs. 23.6 +/- 2.2 Torr, respectively) since metabolic demand was greater. The MRT for the decrease in Pi(O(2)) was progressively faster at the higher frequencies (0.5 Hz: 45.3 +/- 4.5 s; 0.25 Hz: 63.3 +/- 4.1 s; 0.16 Hz: 78.0 +/- 4.1 s), suggesting faster accumulation of stimulators of oxidative phosphorylation. The MRT for Pi(O(2)) off-kinetics (0.5 Hz: 84.0 +/- 11.7 s; 0.25 Hz: 79.1 +/- 8.4 s; 0.16 Hz: 81.1 +/- 8.3 s) was not different between trials. These data demonstrate in single fibers that the rate of the fall in Pi(O(2)) is dependent on contraction frequency, whereas the rate of recovery following contractions is independent of either the magnitude of the fall in Pi(O(2)) from baseline or the contraction frequency. This suggests that stimulation frequency plays an integral role in setting the initial metabolic response to work in isolated muscle fibers, possibly due to temporal recovery between contractions, but it does not determine recovery kinetics.     

Effect of extracellular PO2 on the fall in intracellular PO2 in contracting single myocytes.

The purpose of this investigation was to study the effects of altered extracellular Po(2) (Pe(O(2))) on the intracellular Po(2) (Pi(O(2))) response to contractions in single skeletal muscle cells. Single myocytes (n = 12) were dissected from lumbrical muscles of adult female Xenopus laevis and injected with 0.5 mM Pd-meso-tetra(4-carboxyphenyl)porphine for assessment of Pi(O(2)) via phosphorescence quenching. At a Pe(O(2)) of approximately 20 (low), approximately 40 (moderate), and approximately 60 (high) Torr, tetanic contractions were induced at a frequency of 0.67 Hz for approximately 2 min with a 5-min recovery between bouts (blocked order design). The Pi(O(2)) response to contractions was characterized by a time delay followed by a monoexponential decline to steady-state (SS) values. The fall in Pi(O(2)) to SS values was significantly greater at each progressively greater Pe(O(2)) (all P < 0.05). The mean response time (time delay + time constant) was significantly faster in the low (35.2 +/- 5.1 s; P < 0.05 vs. high) and moderate (43.3 +/- 6.4 s; P < 0.05 vs. high) compared with high Pe(O(2)) (61.8 +/- 9.4 s) and was correlated positively (r = 0.965) with the net fall in Pi(O(2)). However, the initial rate of change of Pi(O(2)) (calculated as net fall in Pi(O(2))/time constant) was not different (P > 0.05) among Pe(O(2)) trials. These latter data suggest that, over the range of 20-60 Torr, Pe(O(2)) does not play a deterministic role in setting the initial metabolic response to contractions in isolated frog myocytes. Additionally, these results suggest that oxidative phosphorylation in these myoglobin-free myocytes may be compromised by Pe(O(2)) at values nearing 60 Torr.     

Relationship between intracellular PO2 recovery kinetics and fatigability in isolated single frog myocytes.

In single frog skeletal myocytes, a linear relationship exists between "fatigability" and oxidative capacity. The purpose of this investigation was to study the relationship between the intracellular Po(2) (Pi(O(2))) offset kinetics and fatigability in single Xenopus laevis myocytes to test the hypothesis that Pi(O(2)) offset kinetics would be related linearly with myocyte fatigability and, by inference, oxidative capacity. Individual myocytes (n = 30) isolated from lumbrical muscle were subjected to a 2-min bout of isometric peak tetanic contractions at either 0.25- or 0.33-Hz frequency while Pi(O(2)) was measured continuously via phosphorescence quenching techniques. The mean response time (MRT; time to 63% of the overall response) for Pi(O(2)) recovery from contracting values to resting baseline was calculated. After the initial square-wave constant-frequency contraction trial, each cell performed an incremental contraction protocol [i.e., frequency increase every 2 min from 0.167, 0.25, 0.33, 0.5, 1.0, and 2.0 Hz until peak tension fell below 50% of initial values (TTF)]. TTF values ranged from 3.39 to 10.04 min for the myocytes. The Pi(O(2)) recovery MRT ranged from 26 to 146 s. A significant (P < 0.05), negative relationship (MRT = -12.68TTF + 168.3, r(2) = 0.605) between TTF and Pi(O(2)) recovery MRT existed. These data demonstrate a significant correlation between fatigability and oxidative phosphorylation recovery kinetics consistent with the notion that oxidative capacity determines, in part, the speed with which skeletal muscle can recover energetically to alterations in metabolic demand.     

Effects of acute creatine kinase inhibition on metabolism and tension development in isolated single myocytes.

This study investigated the effects of acute creatine kinase (CK) inhibition (CKi) on contractile performance, cytosolic Ca2+ concentration ([Ca2+]c), and intracellular PO2 (PIO2) in Xenopus laevis isolated myocytes during a 2-min bout of isometric tetanic contractions (0.33-Hz frequency). Peak tension was similar between trials during the first contraction but was significantly (P < 0.05) attenuated for all subsequent contractions in CKi vs. control (Con). The fall in PIO2 (DeltaPIO2) from resting values was significantly greater in Con (26.0 +/- 2.2 Torr) compared with CKi (17.8 +/- 1.8 Torr). However, the ratios of Con to CKi end-peak tension (1.53 +/- 0.11) and DeltaPO2 (1.49 +/- 0.11) were similar, suggesting an unaltered aerobic cost of contractions. Additionally, the mean response time (MRT) of DeltaPIO2was significantly faster in CKi vs. Con during both the onset (31.8 +/- 5.5 vs. 49.3 +/- 5.7 s; P < 0.05) and cessation (21.2 +/- 4.1 vs. 68.0 +/- 3.2 s; P < 0.001) of contractions. These data demonstrate that initial phosphocreatine hydrolysis in single skeletal muscle fibers is crucial for maintenance of sarcoplasmic reticulum Ca2+ release and peak tension during a bout of repetitive tetanic contractions. Furthermore, as PIO2 fell more rapidly at contraction onset in CKi compared with Con, these data suggest that CK activity temporally buffers the initial ATP-to-ADP concentration ratio at the transition to an augmented energetic demand, thereby slowing the initial mitochondrial activation by mitigating the energetic control signal (i.e., ADP concentration, phosphorylation potential, etc.) between sites of ATP supply and demand.     

Recovery of microvascular PO2 during the exercise off-transient in muscles of different fiber type.

The speed with which muscle energetic status recovers after exercise is dependent on oxidative capacity and vascular O(2) pressures. Because vascular control differs between muscles composed of fast- vs. slow-twitch fibers, we explored the possibility that microvascular O(2) pressure (Pmv(O(2)); proportional to the O(2) delivery-to-O(2) uptake ratio) would differ during recovery in fast-twitch peroneal (Per: 86% type II) compared with slow-twitch soleus (Sol: 84% type I). Specifically, we hypothesized that, in Per, Pmv(O(2)) would be reduced immediately after contractions and would recover more slowly during the off-transient from contractions compared with Sol. The Per and Sol muscles of six female Sprague-Dawley rats (weight = approximately 220 g) were studied after the cessation of electrical stimulation (120 s; 1 Hz) to compare the recovery profiles of Pmv(O(2)). As hypothesized, Pmv(O(2)) was lower throughout recovery in Per compared with Sol (end contraction: 13.4 +/- 2.2 vs. 20.2 +/- 0.9 Torr; end recovery: 24.0 +/- 2.4 vs. 27.4 +/- 1.2 Torr, Per vs. Sol; P 相似文献   

Oxygen uptake kinetics during two bouts of heavy cycling separated by fatiguing sprint exercise in humans.

We tested the hypothesis that O(2) uptake (Vo(2)) kinetics at the onset of heavy exercise would be altered in a state of muscle fatigue and prior metabolic acidosis. Eight well-trained cyclists completed two identical bouts of 6-min cycling exercise at >85% of peak Vo(2) separated by three successive bouts of 30 s of sprint cycling. Not only was baseline Vo(2) elevated after prior sprint exercises but also the time constant of phase II Vo(2) kinetics was faster (28.9 +/- 2.4 vs. 22.2 +/- 1.7 s; P < 0.05). CO(2) output (Vco(2)) was significantly reduced throughout the second exercise bout. Subsequently Vo(2) was greater at 3 min and increased less after this after prior sprint exercise. Cardiac output, estimated by impedance cardiography, was significantly higher in the first 2 min of the second heavy exercise bout. Normalized integrated surface electromyography of four leg muscles and normalized mean power frequency were not different between exercise bouts. Vo(2) and Vco(2) kinetic responses to heavy exercise were markedly altered by prior multiple sprint exercises.     

Assessment of O2 uptake dynamics in isolated single skeletal myocytes.

The purpose of this research was to develop a technique for rapid measurement of O(2) uptake (Vo(2)) kinetics in single isolated skeletal muscle cells. Previous attempts to measure single cell Vo(2) have utilized polarographic-style electrodes, thereby mandating large fluid volumes and relatively poor sensitivity. Thus our laboratory has developed an approximately 100-microl, well-stirred chamber for the measurement of Vo(2) in isolated Xenopus laevis myocytes using a phosphorescence quenching technique [Ringer solution with 0.05 mM Pd-meso-tetra(4-carboxyphenyl)porphine] to monitor the fall in extracellular Po(2) (which is proportional to cellular Vo(2) within the sealed chamber). Vo(2) in single living myocytes dissected from Xenopus lumbrical muscles was measured from rest across a bout of repetitive tetanic contractions (0.33 Hz) and in response to a ramp protocol utilizing an increasing contraction frequency. In response to the square-wave contraction bout, the increase in Vo(2) to steady state (SS) was 16.7 +/- 1.3 ml x 100 g(-1) x min(-1) (range 13.0-21.9 ml x 100 g(-1) x min(-1); n = 6). The rise in Vo(2) at contractions onset (n = 6) was fit with a time delay (2.1 +/- 1.2 s, range 0.0-7.7 s) plus monoexponential rise to SS (time constant = 9.4 +/- 1.5 s, range 5.2-14.9 s). Furthermore, in two additional myocytes, Vo(2) increased progressively as contraction frequency increased (ramp protocol). This technique for measuring Vo(2) in isolated, single skeletal myocytes represents a novel and powerful investigative tool for gaining mechanistic insight into mitochondrial function and Vo(2) dynamics without potential complications of the circulation and other myocytes.     

Muscle capillary blood flow kinetics estimated from pulmonary O2 uptake and near-infrared spectroscopy.

The near-infrared spectroscopy (NIRS) signal (deoxyhemoglobin concentration; [HHb]) reflects the dynamic balance between muscle capillary blood flow (Q(cap)) and muscle O(2) uptake (Vo(2)(m)) in the microcirculation. The purposes of the present study were to estimate the time course of Q(cap) from the kinetics of the primary component of pulmonary O(2) uptake (Vo(2)(p)) and [HHb] throughout exercise, and compare the Q(cap) kinetics with the Vo(2)(p) kinetics. Nine subjects performed moderate- (M; below lactate threshold) and heavy-intensity (H, above lactate threshold) constant-work-rate tests. Vo(2)(p) (l/min) was measured breath by breath, and [HHb] (muM) was measured by NIRS during the tests. The time course of Q(cap) was estimated from the rearrangement of the Fick equation [Q(cap) = Vo(2)(m)/(a-v)O(2), where (a-v)O(2) is arteriovenous O(2) difference] using Vo(2)(p) (primary component) and [HHb] as proxies of Vo(2)(m) and (a-v)O(2), respectively. The kinetics of [HHb] [time constant (tau) + time delay [HHb]; M = 17.8 +/- 2.3 s and H = 13.7 +/- 1.4 s] were significantly (P < 0.001) faster than the kinetics of Vo(2) [tau of primary component (tau(P)); M = 25.5 +/- 8.8 s and H = 25.6 +/- 7.2 s] and Q(cap) [mean response time (MRT); M = 25.4 +/- 9.1 s and H = 25.7 +/- 7.7 s]. However, there was no significant difference between MRT of Q(cap) and tau(P)-Vo(2) for both intensities (P = 0.99), and these parameters were significantly correlated (M and H; r = 0.99; P < 0.001). In conclusion, we have proposed a new method to noninvasively approximate Q(cap) kinetics in humans during exercise. The resulting overall Q(cap) kinetics appeared to be tightly coupled to the temporal profile of Vo(2)(m).     

Aging potentiates the effect of congestive heart failure on muscle microvascular oxygenation.

Congestive heart failure (CHF) is most prevalent in aged individuals and elicits a spectrum of cardiovascular and muscular perturbations that impairs the ability to deliver (Qo(2)) and utilize (Vo(2)) oxygen in skeletal muscle. Whether aging potentiates the CHF-induced alterations in the Qo(2)-to-Vo(2) relationship [which determines microvascular Po(2) (Pmv(O(2)))] in resting and contracting skeletal muscle is unclear. We tested the hypothesis that old rats with CHF would demonstrate a greater impairment of skeletal muscle Pmv(O(2)) than observed in young rats with CHF. Phosphorescence quenching was utilized to measure spinotrapezius Pmv(O(2)) at rest and across the rest-to-contractions (1-Hz, 4-6 V) transition in young (Y) and old (O) male Fischer 344 Brown-Norway rats with CHF induced by myocardial infarction (mean left ventricular end-diastolic pressure >20 mmHg for Y(CHF) and O(CHF)). In CHF muscle, aging significantly reduced resting Pmv(O(2)) (32.3 +/- 3.4 Torr for Y(CHF) and 21.3 +/- 3.3 Torr for O(CHF); P < 0.05) and in both Y(CHF) and O(CHF) compared with their aged-matched counterparts, CHF reduced the rate of the Pmv(O(2)) fall at the onset of contractions. Moreover, across the on-transient and in the subsequent steady state, Pmv(O(2)) values in O(CHF) vs. Y(CHF) were substantially lower (for steady-state, 20.4 +/- 1.7 Torr for Y(CHF) and 16.4 +/- 2.0 Torr for O(CHF); P < 0.05). At rest and during contractions in CHF, the pressure driving blood-muscle O(2) diffusion (Pmv(O(2))) is substantially decreased in old animals. This finding suggests that muscle dysfunction and exercise intolerance in aged CHF patients might be due, in part, to the failure to maintain a sufficiently high Pmv(O(2)) to facilitate blood-muscle O(2) exchange and support mitochondrial ATP production.     

Effects of antioxidants on contracting spinotrapezius muscle microvascular oxygenation and blood flow in aged rats

Aged rats exhibit a decreased muscle microvascular O(2) partial pressure (Pmv(O(2))) at rest and during contractions compared with young rats. Age-related reductions in nitric oxide bioavailability due, in part, to elevated reactive O(2) species, constrain muscle blood flow (Qm). Antioxidants may restore nitric oxide bioavailability, Qm, and ameliorate the reduced Pmv(O(2)). We tested the hypothesis that antioxidants would elevate Qm and, therefore, Pmv(O(2)) in aged rats. Spinotrapezius muscle Pmv(O(2)) and Qm were measured, and oxygen consumption (Vm(O(2))) was estimated in anesthetized male Fisher 344 x Brown Norway hybrid rats at rest and during 1-Hz contractions, before and after antioxidant intravenous infusion (76 mg/kg vitamin C and 52 mg/kg tempol). Before infusion, contractions evoked a biphasic Pmv(O(2)) that fell from 30.6 +/- 0.9 Torr to a nadir of 16.8 +/- 1.2 Torr with an "undershoot" of 2.8 +/- 0.7 Torr below the subsequent steady-state (19.7 +/- 1.2 Torr). The principal effect of antioxidants was to elevate baseline Pmv(O(2)) from 30.6 +/- 0.9 to 35.7 +/- 0.8 Torr (P < 0.05) and reduce or abolish the undershoot (P < 0.05). Antioxidants reduced Qm and Vm(O(2)) during contractions (P < 0.05), while decreasing force production 16.5% (P < 0.05) and elevating the force production-to-Vm(O(2)) ratio (0.92 +/- 0.03 to 1.06 +/- 0.6, P < 0.05). Thus antioxidants increased Pmv(O(2)) by altering the balance between muscle O(2) delivery and Vm(O(2)) at rest and during contractions. It is likely that this effect arose from antioxidants reducing myocyte redox below the level optimal for contractile performance and directly (decreased tension) or indirectly (altered balance of vasoactive mediators) influencing O(2) delivery and Vm(O(2)).     

VO(2) on-kinetics in isolated canine muscle in situ during slowed convective O(2) delivery

The purpose of this study was to examine O(2) uptake (Vo(2)) on-kinetics when the spontaneous blood flow (and therefore O(2) delivery) on-response was slowed by 25 and 50 s. The isolated gastrocnemius muscle complex (GS) in situ was studied in six anesthetized dogs during transitions from rest to a submaximal metabolic rate (≈50-70% of peak Vo(2)). Four trials were performed: 1) a pretrial in which resting and steady-state blood flows were established, 2) a control trial in which the blood flow on-kinetics mean response time (MRT) was set at 20 s (CT20), 3) an experimental trial in which the blood flow on-kinetics MRT was set at 45 s (EX45), and 4) an experimental trial in which the blood flow on-kinetics MRT was set at 70 s (EX70). Slowing O(2) delivery via slowing blood flow on-kinetics resulted in a linear slowing of the Vo(2) on-kinetics response (R = 0.96). Average MRT values for CT20, EX45, and EX70 Vo(2) on-kinetics were (means ± SD) 17 ± 2, 23 ± 4, and 26 ± 3 s, respectively (P < 0.05 among all). During these transitions, slowing blood flow resulted in greater muscle deoxygenation (as indicated by near-infrared spectroscopy), suggesting that lower intracellular Po(2) values were reached. In this oxidative muscle, Vo(2) and O(2) delivery were closely matched during the transition period from rest to steady-state contractions. In conjunction with our previous work showing that speeding O(2) delivery did not alter Vo(2) on-kinetics under similar conditions, it appears that spontaneously perfused skeletal muscle operates at the nexus of sufficient and insufficient O(2) delivery in the transition from rest to contractions.     

Effects of arterial hypotension on microvascular oxygen exchange in contracting skeletal muscle.

In healthy animals under normotensive conditions (N), contracting skeletal muscle perfusion is regulated to maintain microvascular O2 pressures (PmvO2) at levels commensurate with O2 demands. Hypovolemic hypotension (H) impairs muscle contractile function; we tested whether this condition would alter the matching of O2 delivery (Qo2) to O2 utilization (Vo2), as determined by PmvO2 at the onset of muscle contractions. PmvO2 in the spinotrapezius muscles of seven female Sprague-Dawley rats (280+/-6 g) was measured every 2 s across the transition from rest to 1-Hz twitch contractions. Measurements were made under N (mean arterial pressure, 97+/-4 mmHg) and H (induced by arterial section; mean arterial pressure, 58+/-3 mmHg, P<0.05) conditions; PmvO2 profiles were modeled using a multicomponent exponential fitted with independent time delays. Hypotension reduced muscle blood flow at rest (24+/-8 vs. 6+/-1 ml-1.min-1.100 g-1 for N and H, respectively; P<0.05) and during contractions (74+/-20 vs. 22+/-4 ml-1.min-1.100 g-1 for N and H, respectively; P<0.05). H significantly decreased resting PmvO2 and steady-state contracting PmvO2(19.4+/-2.4 vs. 8.7+/-1.6 Torr for N and H, respectively, P<0.05). At the onset of contractions, H reduced the time delay (11.8+/-1.7 vs. 5.9+/-0.9 s for N and H, respectively, P<0.05) before the fall in PmvO2 and accelerated the rate of PmvO2 decrease (time constant, 12.6+/-1.4 vs. 7.3+/-0.9 s for N and H, respectively, P<0.05). Muscle Vo2 was reduced by 71% at rest and 64% with contractions in H vs. N, and O2 extraction during H averaged 78% at rest and 94% during contractions vs. 51 and 78% in N. These results demonstrate that H constrains the increase of skeletal muscle Qo2 relative to that of Vo2 at the onset of contractions, leading to a decreased PmvO2. According to Fick's law, this scenario will decrease blood-myocyte O2 flux, thereby slowing Vo2 kinetics and exacerbating the O2 deficit generated at exercise onset.     

Speeding of VO2 kinetics during moderate-intensity exercise subsequent to heavy-intensity exercise is associated with improved local O2 distribution

The relationship between the adjustment of muscle deoxygenation (Δ[HHb]) and phase II V(O(2p)) during moderate-intensity exercise was examined before (Mod 1) and after (Mod 2) a bout of heavy-intensity "priming" exercise. Moderate intensity V(O(2p)) and Δ[HHb] kinetics were determined in 18 young males (26 ± 3 yr). V(O(2p)) was measured breath-by-breath. Changes in Δ[HHb] of the vastus lateralis muscle were measured by near-infrared spectroscopy. V(O(2p)) and Δ[HHb] response profiles were fit using a monoexponential model, and scaled to a relative % of the response (0-100%). The Δ[HHb]/Vo(2) ratio for each individual (reflecting the local matching of O(2) delivery to O(2) utilization) was calculated as the average Δ[HHb]/Vo(2) response from 20 s to 120 s during the exercise on-transient. Phase II τV(O(2p)) was reduced in Mod 2 compared with Mod 1 (P < 0.05). The effective τ'Δ[HHb] remained the same in Mod 1 and Mod 2 (P > 0.05). During Mod 1, there was an "overshoot" in the Δ[HHb]/Vo(2) ratio (1.08; P < 0.05) that was not present during Mod 2 (1.01; P > 0.05). There was a positive correlation between the reduction in the Δ[HHb]/Vo(2) ratio and the smaller τV(O(2p)) from Mod 1 to Mod 2 (r = 0.78; P < 0.05). This study showed that a smaller τV(O(2p)) during a moderate bout of exercise subsequent to a heavy-intensity priming exercise was associated with improved microvascular O(2) delivery during the on-transient of exercise, as suggested by a smaller Δ[HHb]/Vo(2) ratio.     

Control of microvascular PO₂ kinetics following onset of muscle contractions: role for AMPK

The microvascular partial pressure of oxygen (Pmv(o(2))) kinetics following the onset of exercise reflects the relationship between muscle O(2) delivery and uptake (Vo(2)). Although AMP-activated protein kinase (AMPK) is known as a regulator of mitochondria and nitric oxide metabolism, it is unclear whether the dynamic balance of O(2) delivery and Vo(2) at exercise onset is dependent on AMPK activation level. We used transgenic mice with muscle-specific AMPK dominant-negative (AMPK-DN) to investigate a role for skeletal muscle AMPK on Pmv(o(2)) kinetics following onset of muscle contractions. Phosphorescence quenching techniques were used to measure Pmv(o(2)) at rest and across the transition to twitch (1 Hz) and tetanic (100 Hz, 3-5 V, 4-ms pulse duration, stimulus duration of 100 ms every 1 s for 1 min) contractions in gastrocnemius muscles (each group n = 6) of AMPK-DN mice and wild-type littermates (WT) under isoflurane anesthesia with 100% inspired O(2) to avoid hypoxemia. Baseline Pmv(o(2)) before contractions was not different between groups (P > 0.05). Both muscle contraction conditions exhibited a delay followed by an exponential decrease in Pmv(o(2)). However, compared with WT, AMPK-DN demonstrated 1) prolongation of the time delay before Pmv(o(2)) began to decline (1 Hz: WT, 3.2 ± 0.5 s; AMPK-DN, 6.5 ± 0.4 s; 100 Hz: WT, 4.4 ± 1.0 s; AMPK-DN, 6.5 ± 1.4 s; P < 0.05), 2) a faster response time (i.e., time constant; 1 Hz: WT, 19.4 ± 3.9 s; AMPK-DN, 12.4 ± 2.6 s; 100 Hz: WT, 15.1 ± 2.2 s; AMPK-DN, 9.0 ± 1.7 s; P < 0.05). These findings are consistent with the presence of substantial mitochondrial and microvascular dysfunction in AMPK-DN mice, which likely slows O(2) consumption kinetics (i.e., oxidative phosphorylation response) and impairs the hyperemic response at the onset of contractions thereby sowing the seeds for exercise intolerance.     

Adaptation of pulmonary O2 uptake kinetics and muscle deoxygenation at the onset of heavy-intensity exercise in young and older adults.

The purpose was to examine the adaptation of pulmonary O(2) uptake (Vo(2p)) and deoxygenation of the vastus lateralis muscle at the onset of heavy-intensity, constant-load cycling exercise in young (Y; 24 +/- 4 yr; mean +/- SD; n = 5) and older (O; 68 +/- 3 yr; n = 6) adults. Subjects performed repeated transitions on 4 separate days from 20 W to a work rate corresponding to heavy-intensity exercise. Vo(2p) was measured breath by breath. The concentration changes in oxyhemoglobin, deoxyhemoglobin (HHb), and total hemoglobin/myoglobin were determined by near-infrared spectroscopy (Hamamatsu NIRO-300). Vo(2p) data were filtered, interpolated to 1 s, and averaged to 5-s bins. HHb-near-infrared spectroscopy data were filtered and averaged to 5-s bins. A monoexponential model was used to fit Vo(2p) [phase 2, time constant (tau) of Vo(2p)] and HHb [following the time delay (TD) from exercise onset to the start of an increase in HHb] data. The tauVo(2p) was slower (P < 0.001) in O (49 +/- 8 s) than Y (29 +/- 4 s). The HHb TD was similar in O (8 +/- 3 s) and Y (7 +/- 1 s); however, the tau HHb following TD was faster (P < 0.05) in O (8 +/- 2 s) than Y (14 +/- 2 s). The slower Vo(2p) kinetics and faster muscle deoxygenation in O compared with Y during heavy-intensity exercise imply that the kinetics of muscle perfusion are slowed relatively more than those of Vo(2p) in O. This suggests that the slowed Vo(2p) kinetics in O may be a consequence of a slower adaptation of local muscle blood flow relative to that in Y.     

Kinetics of muscle deoxygenation are accelerated at the onset of heavy-intensity exercise in patients with COPD: relationship to central cardiovascular dynamics.

Patients with chronic obstructive pulmonary disease (COPD) have slowed pulmonary O(2) uptake (Vo(2)(p)) kinetics during exercise, which may stem from inadequate muscle O(2) delivery. However, it is currently unknown how COPD impacts the dynamic relationship between systemic and microvascular O(2) delivery to uptake during exercise. We tested the hypothesis that, along with slowed Vo(2)(p) kinetics, COPD patients have faster dynamics of muscle deoxygenation, but slower kinetics of cardiac output (Qt) following the onset of heavy-intensity exercise. We measured Vo(2)(p), Qt (impedance cardiography), and muscle deoxygenation (near-infrared spectroscopy) during heavy-intensity exercise performed to the limit of tolerance by 10 patients with moderate-to-severe COPD and 11 age-matched sedentary controls. Variables were analyzed by standard nonlinear regression equations. Time to exercise intolerance was significantly (P < 0.05) lower in patients and related to the kinetics of Vo(2)(p) (r = -0.70; P < 0.05). Compared with controls, COPD patients displayed slower kinetics of Vo(2)(p) (42 +/- 13 vs. 73 +/- 24 s) and Qt (67 +/- 11 vs. 96 +/- 32 s), and faster overall kinetics of muscle deoxy-Hb (19.9 +/- 2.4 vs. 16.5 +/- 3.4 s). Consequently, the time constant ratio of O(2) uptake to mean response time of deoxy-Hb concentration was significantly greater in patients, suggesting a slower kinetics of microvascular O(2) delivery. In conclusion, our data show that patients with moderate-to-severe COPD have impaired central and peripheral cardiovascular adjustments following the onset of heavy-intensity exercise. These cardiocirculatory disturbances negatively impact the dynamic matching of O(2) delivery and utilization and may contribute to the slower Vo(2)(p) kinetics compared with age-matched controls.     

Effects of intermittent hypoxic training on amino and fatty acid oxidative combustion in human permeabilized muscle fibers.

The effects of concurrent hypoxic/endurance training on mitochondrial respiration in permeabilized fibers in trained athletes were investigated. Eighteen endurance athletes were divided into two training groups: normoxic (Nor, n = 8) and hypoxic (H, n = 10). Three weeks (W1-W3) of endurance training (5 sessions of 1 h to 1 h and 30 min per week) were completed. All training sessions were performed under normoxic [160 Torr inspired Po(2) (Pi(O(2)))] or hypoxic conditions ( approximately 100 Torr Pi(O(2)), approximately 3,000 m) for Nor and H group, respectively, at the same relative intensity. Before and after the training period, an incremental test to exhaustion in normoxia was performed, muscle biopsy samples were taken from the vastus lateralis, and mitochondrial respiration in permeabilized fibers was measured. Peak power output (PPO) increased by 7.2% and 6.6% (P < 0.05) for Nor and H, respectively, whereas maximal O(2) uptake (Vo(2 max)) remained unchanged: 58.1 +/- 0.8 vs. 61.0 +/- 1.2 ml.kg(-1).min(-1) and 58.5 +/- 0.7 vs. 58.3 +/- 0.6 ml.kg(-1).min(-1) for Nor and H, respectively, between pretraining (W0) and posttraining (W4). Maximal ADP-stimulated mitochondrial respiration significantly increased for glutamate + malate (6.27 +/- 0.37 vs. 8.51 +/- 0.33 mumol O(2).min(-1).g dry weight(-1)) and significantly decreased for palmitate + malate (3.88 +/- 0.23 vs. 2.77 +/- 0.08 mumol O(2).min(-1).g dry weight(-1)) in the H group. In contrast, no significant differences were found for the Nor group. The findings demonstrate that 1) a 3-wk training period increased the PPO at sea level without any changes in Vo(2 max), and 2) a 3-wk hypoxic exercise training seems to alter the intrinsic properties of mitochondrial function, i.e., substrate preference.     

Effect of age on O(2) uptake kinetics and the adaptation of muscle deoxygenation at the onset of moderate-intensity cycling exercise.

Phase 2 pulmonary O(2) uptake (Vo(2(p))) kinetics are slowed with aging. To examine the effect of aging on the adaptation of Vo(2(p)) and deoxygenation of the vastus lateralis muscle at the onset of moderate-intensity constant-load cycling exercise, young (Y) (n = 6; 25 +/- 3 yr) and older (O) (n = 6; 68 +/- 3 yr) adults performed repeated transitions from 20 W to work rates corresponding to moderate-intensity (80% estimated lactate threshold) exercise. Breath-by-breath Vo(2(p)) was measured by mass spectrometer and volume turbine. Deoxy (HHb)-, oxy-, and total Hb and/or myoglobin were determined by near-infrared spectroscopy (Hamamatsu NIRO-300). Vo(2(p)) data were filtered, interpolated to 1 s, and averaged to 5-s bins. HHb data were filtered and averaged to 5-s bins. Vo(2(p)) data were fit with a monoexponential model for phase 2, and HHb data were analyzed to determine the time delay from exercise onset to the start of an increase in HHb and thereafter were fit with a single-component exponential model. The phase 2 time constant for Vo(2(p)) was slower (P < 0.01) in O (Y: 26 +/- 7 s; O: 42 +/- 9 s), whereas the delay before an increase in HHb (Y: 12 +/- 2 s; O: 11 +/- 1 s) and the time constant for HHb after the time delay (Y: 13 +/- 10 s; O: 9 +/- 3 s) were similar in Y and O. However, the increase in HHb for a given increase in Vo(2(p)) (Y: 7 +/- 2 microM x l(-1) x min(-1); O: 13 +/- 4 microM x l(-1) x min(-1)) was greater (P < 0.01) in O compared with Y. The slower Vo(2(p)) kinetics in O compared with Y adults was accompanied by a slower increase of local muscle blood flow and O(2) delivery discerned from a faster and greater muscle deoxygenation relative to Vo(2(p)) in O.     

Measurement of activation energy and oxidative phosphorylation onset kinetics in isolated muscle fibers in the absence of cross-bridge cycling

This study utilized N-benzyl-p-toluene sulfonamide (BTS), a potent inhibitor of cross-bridge cycling, to measure 1) the relative metabolic costs of cross-bridge cycling and activation energy during contraction, and 2) oxygen uptake kinetics in the presence and absence of myosin ATPase activity, in isolated Xenopus laevis muscle fibers. Isometric tension development and either cytosolic Ca2+ concentration ([Ca2+]c) or intracellular Po2 (PiO2) were measured during contractions at 20 degrees C in control conditions (Con) and after exposure to 12.5 microM BTS. BTS attenuated tension development to 5+/-0.4% of Con but did not affect either resting or peak [Ca2+]c during repeated isometric contractions. To determine the relative metabolic cost of cross-bridge cycling, we measured the fall in PiO2) (DeltaPiO2; a proxy for Vo2) during contractions in Con and BTS groups. BTS attenuated DeltaP(iO2) by 55+/-6%, reflecting the relative ATP cost of cross-bridge cycling. Thus, extrapolating DeltaPiO2 to a value that would occur at 0% tension suggests that actomyosin ATP requirement is approximately 58% of overall ATP consumption during isometric contractions in mixed fiber types. BTS also slowed the fall in PiO2) (time to 63% of overall DeltaPiO2) from 75+/-9 s (Con) to 101+/-9 s (BTS) (P<0.05), suggesting an important role of the products of ATP hydrolysis in determining the Vo2 onset kinetics. These results demonstrate in isolated skeletal muscle fibers that 1) activation energy accounts for a substantial proportion (approximately 42%) of total ATP cost during isometric contractions, and 2) despite unchanged [Ca2+]c transients, a reduced rate of ATP consumption results in slower Vo2 onset kinetics.     

Pulmonary VO2 dynamics during treadmill and arm exercise in peripheral arterial disease.

Slowed pulmonary O(2) uptake (Vo(2)) kinetics in peripheral arterial disease (PAD) have been attributed to impaired limb blood flow and/or peripheral muscle metabolic abnormalities. Although PAD results from atherosclerotic occlusive disease in the arteries to the lower extremities, systemic abnormalities affecting whole body O(2) delivery or vascular function in PAD could also partially explain the exercise impairment. To date, the effects of these systemic abnormalities have not been evaluated. To test the hypothesis that the slowed pulmonary Vo(2) kinetics in PAD reflects local and not systemic abnormalities, Vo(2) kinetics were evaluated after the onset of constant-load exercise of the upper and lower limbs in PAD patients and healthy controls (Con). Ten PAD patients and 10 Con without significant cardiopulmonary dysfunction performed multiple transitions from rest to moderate-intensity arm ergometry and treadmill exercise to assess their Vo(2) kinetic responses. Reactive hyperemic (RH) blood flow was assessed in the arms and legs as a measure of endothelial function. Compared with Con, PAD Vo(2) kinetic phase 2 time constants were prolonged during treadmill exercise (PAD 34.3 +/- 9.2 s vs. Con 19.6 +/- 3.5 s; P < 0.01) but not arm exercise (PAD 38.5 +/- 7.5 s vs. Con 32.5 +/- 9.0 s; P > 0.05). RH blood flow was significantly reduced in the legs (PAD 20.7 +/- 8.3 vs. Con 46.1 +/- 17.1 ml.100 ml(-1).min(-1); P < 0.01) and arms of PAD subjects (PAD 34.0 +/- 8.6 vs. Con 50.8 +/- 12.2 ml.100 ml(-1).min(-1); P < 0.01) compared with Con, but RH limb flow was not correlated with arm or treadmill Vo(2) kinetic responses in either group. In summary, slowed pulmonary Vo(2) kinetics in PAD patients occur only with exercise of the lower limbs affected by the arterial occlusive disease process and are not slowed with exercise of the unaffected upper extremities compared with controls. Furthermore, the slowed pulmonary Vo(2) kinetics of the lower extremity could not be explained by any abnormalities in resting cardiac or pulmonary function and were not related to the magnitude of reduction in limb vascular reactivity.