Intracellular PO(2) decreases with increasing stimulation frequency in contracting single Xenopus muscle fibers

Strain-Dependent Variation in Acute Ischemic Muscle Injury

Limited efficacy of clinical interventions for peripheral arterial disease necessitates a better understanding of the environmental and genetic determinants of tissue pathology. Existing research has largely ignored the early skeletal muscle injury response during hind limb ischemia (HLI). We compared the hind limb muscle response, after 6 hours of ischemia, in two mouse strains that differ dramatically in their postischemic extended recovery: C57BL/6J and BALB/cJ. Perfusion, measured by laser Doppler and normalized to the control limb, differed only slightly between strains after HLI (<12% across all measures). Similar (<10%) effect sizes in lectin-perfused vessel area and no differences in tissue oxygen saturation measured by reflectance spectroscopy were also found. Muscles from both strains were functionally impaired after HLI, but greater muscle necrosis and loss of dystrophin-positive immunostaining were observed in BALB/cJ muscle compared with C57BL/6J. Muscle cell-specific dystrophin loss and reduced viability were also detected in additional models of ischemia that were independent of residual perfusion differences. Our results indicate that factors other than the completeness of ischemia alone (ie, background genetics) influence the magnitude of acute ischemic muscle injury. These findings may have implications for future development of therapeutic interventions for limb ischemia and for understanding the phasic etiology of chronic and acute ischemic muscle pathophysiology.

Skeletal muscle microvascular and interstitial PO2 from rest to contractions

KEY POINTS

Oxygen pressure gradients across the microvascular walls are essential for oxygen diffusion from blood to tissue cells. At any given flux, the magnitude of these transmural gradients is proportional to the local resistance. The greatest resistance to oxygen transport into skeletal muscle is considered to reside in the short distance between red blood cells and myocytes. Although crucial to oxygen transport, little is known about transmural pressure gradients within skeletal muscle during contractions. We evaluated oxygen pressures within both the skeletal muscle microvascular and interstitial spaces to determine transmural gradients during the rest-contraction transient in anaesthetized rats. The significant transmural gradient observed at rest was sustained during submaximal muscle contractions. Our findings support that the blood-myocyte interface provides substantial resistance to oxygen diffusion at rest and during contractions and suggest that modulations in microvascular haemodynamics and red blood cell distribution constitute primary mechanisms driving increased transmural oxygen flux with contractions.

ABSTRACT

Oxygen pressure (PO2) gradients across the blood-myocyte interface are required for diffusive O₂ transport, thereby supporting oxidative metabolism. The greatest resistance to O₂ flux into skeletal muscle is considered to reside between the erythrocyte surface and adjacent sarcolemma, although this has not been measured during contractions. We tested the hypothesis that O₂ gradients between skeletal muscle microvascular (PO2 mv ) and interstitial (PO2 is ) spaces would be present at rest and maintained or increased during contractions. PO2 mv and PO2 is   were determined via phosphorescence quenching (Oxyphor probes G2 and G4, respectively) in the exposed rat spinotrapezius during the rest-contraction transient (1 Hz, 6 V; n = 8). PO2 mv was higher than PO2 is in all instances from rest (34.9 ± 6.0 versus 15.7 ± 6.4) to contractions (28.4 ± 5.3 versus 10.6 ± 5.2 mmHg, respectively) such that the mean PO2 gradient throughout the transient was 16.9 ± 6.6 mmHg (P < 0.05 for all). No differences in the amplitude of PO2 fall with contractions were observed between the microvasculature and interstitium (10.9 ± 2.3 versus 9.0 ± 3.5 mmHg, respectively; P > 0.05). However, the speed of the PO2 is fall during contractions was slower than that of PO2 mv (time constant: 12.8 ± 4.7 versus 9.0 ± 5.1 s, respectively; P < 0.05). Consistent with our hypothesis, a significant transmural gradient was sustained (but not increased) from rest to contractions. This supports that the blood-myocyte interface is the site of a substantial PO2 gradient driving O₂ diffusion during metabolic transients. Based on Fick's law, elevated O₂ flux with contractions must thus rely primarily on modulations in effective diffusing capacity (mainly erythrocyte haemodynamics and distribution) as the PO2 gradient is not increased.

Regulation of metabolism: the rest-to-work transition in skeletal muscle

Mitochondrial oxidative phosphorylation is programmed to set and maintain metabolic homeostasis, and understanding that program is essential for an integrated view of cellular and tissue metabolism. The behavior predicted by a mechanism-based model for oxidative phosphorylation is compared with that experimentally measured for skeletal muscle when work is initiated. For the model, initiation of work is simulated by imposing a rate of ATP utilization of either 0.6 (equivalent of 13.4 ml O2·100 g tissue(-1)·min(-1) or 6 μmol O2·g tissue(-1)·min(-1)) or 0.3 mM ATP/s. Creatine phosphate ([CrP]) decrease, both experimentally measured and predicted by the model, can be fit to a single exponential. Increase in ATP synthesis begins immediately but can show a "lag period," during which the rate accelerates. The length of the lag period is similar for both experiment and model; in the model, the lag depends on intramitochondrial [NAD(+)]/[NADH], mitochondrial content, and size of the creatine pool ([CrP] + [Cr]) as well as the resting [CrP]/[Cr]. For in vivo conditions, increase in oxygen consumption may be linearly correlated with a decrease in [CrP] and an increase in inorganic phosphate ([Pi]) and [Cr]. The decrease in [CrP], resting and working steady state [CrP], and the increase in oxygen consumption are dependent on the Po2 in the inspired gas (experimental) or tissue Po2 (model). The metabolic behavior predicted by the model is consistent with available experimental measurements in muscle upon initiation of work, with the model providing valuable insight into how metabolic homeostasis is set and maintained.

On-off asymmetries in oxygen consumption kinetics of single Xenopus laevis skeletal muscle fibres suggest higher-order control

The mechanisms controlling skeletal muscle oxygen consumption (V(o)₂) during exercise are not well understood. We determined whether first-order control could explain V(o)₂kinetics at contractions onset (V(o)₂(on)) and cessation (V(o)₂off)) in single skeletal muscle fibres differing in oxdidative capacity, and across stimulation intensities up to V(o)₂(max). Xenopus laevis fibres (n = 21) were suspended in a sealed chamber with a fast response P(o)₂ electrode to measure V(o)₂ every second before, during and after stimulated isometric contractions. A first-order model did not well characterize on-transient V(o)₂ kinetics. Including a time delay (TD) in the model provided a significantly improved characterization than a first-order fit without TD (F-ratio; P < 0.05), and revealed separate 'activation' and 'exponential' phases in 15/21 fibres contracting at V(o)₂(max) (mean ± SD TD: 14 ± 3s). On-transient kinetics (τV(o)₂(on)) was weakly and linearly related to V(o)₂(max) (R² = 0.271, P = 0.015). Off-transient kinetics, however, were first-order, and τV(o)₂(off) was greater in low-oxidative (V(o)₂max < 0.05 nmol mm⁻³s⁻¹ than high-oxidative fibres (V(o)₂(max > 0.10 nmol mm ⁻³ s⁻¹; 170 ± 70 vs. 29 ± 6 s, P < 0.001). 1/ τV(o)₂(off) was proportional to V(o)₂(max) (R² = 0.727, P < 0.001), unlike in the on-transient. The calculated oxygen deficit was larger (P < 0.05) than the post-contraction volume of consumed oxygen at all intensities except V(o)₂(max). These data show a clear dissociation between the kinetic control of V(o)₂at the onset and cessation of contractions and across stimulation intensities. More complex models are therefore required to understand the activation of mitochondrial respiration in skeletal muscle at the start of exercise.

Increasing temperature speeds intracellular PO2 kinetics during contractions in single Xenopus skeletal muscle fibers

Precise determination of the effect of muscle temperature (T(m)) on mitochondrial oxygen consumption kinetics has proven difficult in humans, in part due to the complexities in controlling for T(m)-related variations in blood flow, fiber recruitment, muscle metabolism, and contractile properties. To address this issue, intracellular Po(2) (P(i)(O(2))) was measured continuously by phosphorescence quenching following the onset of contractions in single Xenopus myofibers (n = 24) while controlling extracellular temperature. Fibers were subjected to two identical contraction bouts, in random order, at 15°C (cold, C) and 20°C (normal, N; n = 12), or at N and 25°C (hot, H; n = 12). Contractile properties were determined for every contraction. The time delay of the P(i)(O(2)) response was significantly greater in C (59 ± 35 s) compared with N (35 ± 26 s, P = 0.01) and H (27 ± 14 s, P = 0.01). The time constant for the decline in P(i)(O(2)) was significantly greater in C (89 ± 34 s) compared with N (52 ± 15 s; P < 0.01) and H (37 ± 10 s; P < 0.01). There was a linear relationship between the rate constant for P(i)(O(2)) kinetics and T(m) (r = 0.322, P = 0.03). Estimated ATP turnover was significantly greater in H than in C (P < 0.01), but this increased energy requirement alone with increased T(m) could not account for the differences observed in P(i)(O(2)) kinetics among conditions. These results demonstrate that P(i)(O(2)) kinetics in single contracting myofibers are dependent on T(m), likely caused by temperature-induced differences in metabolic demand and by temperature-dependent processes underlying mitochondrial activation at the start of muscle contractions.

Mitochondrial activation at the onset of contractions in isolated myofibres during successive contractile periods

At the onset of skeletal muscle repetitive contractions, there is a significant delay in the time to achieve oxidative phosphorylation steady state. The purpose of the present study was to examine the factors that limit oxidative phosphorylation at the onset of contractions. NAD(P)H was measured in real time during two contractile periods (2 min each) separated by 5 min of rest in intact single muscle fibres (n = 7) isolated from Xenopus laevis. The fibres were then loaded with the dye tetramethylrhodamine methyl ester perchlorate (TMRM) to evaluate the kinetics of the mitochondrial membrane potential (Δψ (m)) during two further successive contractile periods. At the onset of contractions in the first period, NAD(P)H exhibited a time delay (14.1 ± 1.3 s) before decreasing toward a steady state. In contrast, Δψ(m) decreased immediately after the first contraction and started to be reestablished after 10.7 ± 0.9 s, with restoration to the pre-stimulation values after approximately 32 s. In the second contractile period (5 min after the first), NAD(P)H decreased immediately (i.e. no time delay) after the first contraction and had a significantly shorter time constant compared to the first contractile bout (3.3 ± 0.3 vs. 5.0 ± 0.2 s, P < 0.05). During the second bout, Δψ(m) remained unchanged from pre-stimulation values. These results suggest: (1) that at the onset of contractions, oxidative phosphorylation is primarily limited by the activity of the electron transport chain complexes rather than by a limited level of substrates; and (2) when the muscle is 'primed' by previous contractile activity, the faster enhancement of the cellular respiratory rate is due to intrinsic factors within the myofibre.

Reactive oxygen species formation during tetanic contractions in single isolated Xenopus myofibers

Contracting skeletal muscle produces reactive oxygen species (ROS) that have been shown to affect muscle function and adaptation. However, real-time measurement of ROS in contracting myofibers has proven to be difficult. We used amphibian (Xenopus laevis) muscle to test the hypothesis that ROS are formed during contractile activity in isolated single skeletal muscle fibers and that this contraction-induced ROS formation affects fatigue development. Single myofibers were loaded with 5 μM dihydrofluorescein-DA (Hfluor-DA), a fluorescent probe that reacts with ROS and results in the formation of fluorescein (Fluor) to precisely monitor ROS generation within single myofibers in real time using confocal miscroscopy. Three identical periods of maximal tetanic contractions (1 contraction/3 s for 2 min, separated by 60 min of rest) were conducted by each myofiber (n = 6) at 20°C. Ebselen (an antioxidant) was present in the perfusate (10 μM) during the second contractile period. Force was reduced by ∼30% during each of the three contraction periods, with no significant difference in fatigue development among the three periods. The Fluor signal, indicative of ROS generation, increased significantly above baseline in both the first (42 ± 14%) and third periods (39 ± 10%), with no significant difference in the increase in fluorescence between the first and third periods. There was no increase of Fluor in the presence of ebselen during the second contractile period. These results demonstrated that, in isolated intact Xenopus myofibers, 1) ROS can be measured in real time during tetanic contractions, 2) contractile activity induced a significant increase above resting levels of ROS production, and 3) ebselen treatment reduced ROS generation to baseline levels but had no effect on myofiber contractility and fatigue development.

The O2 cost of the tension-time integral in isolated single myocytes during fatigue

One proposed explanation for the Vo(2) slow component is that lower-threshold motor units may fatigue and develop little or no tension but continue to use O(2), thereby resulting in a dissociation of cellular respiration from force generation. The present study used intact isolated single myocytes with differing fatigue resistance profiles to investigate the relationship between fatigue, tension development, and aerobic metabolism. Single Xenopus skeletal muscle myofibers were allocated to a fast-fatiguing (FF) or a slow-fatiguing (SF) group, based on the contraction frequency required to elicit a fall in tension to 60% of peak. Phosphorescence quenching of a porphyrin compound was used to determine Delta intracellular Po(2) (Pi(O(2)); a proxy for Vo(2)), and developed isometric tension was monitored to allow calculation of the time-integrated tension (TxT). Although peak DeltaPi(O(2)) was not different between groups (P = 0.36), peak tension was lower (P < 0.05) in SF vs. FF (1.97 +/- 0. 17 V vs. 2. 73 +/- 0.30 V, respectively) and time to 60% of peak tension was significantly longer in SF vs. FF (242 +/- 10 s vs. 203 +/- 10 s, respectively). Before fatigue, both DeltaPi(O(2)) and TxT rose proportionally with contraction frequency in SF and FF, resulting in DeltaPi(O(2))/TxT being identical between groups. At fatigue, TxT fell dramatically in both groups, but DeltaPi(O(2)) decreased proportionately only in the FF group, resulting in an increase in DeltaPi(O(2))/TxT in the SF group relative to the prefatigue condition. These data show that more fatigue-resistant fibers maintain aerobic metabolism as they fatigue, resulting in an increased O(2) cost of contractions that could contribute to the Vo(2) slow component seen in whole body exercise.

Models of muscle contraction and energetics

How does skeletal muscle manage to regulate the pathways of ATP synthesis during large-scale changes in work rate while maintaining metabolic homeostasis remains unknown. The classic model of metabolic regulation during muscle contraction states that accelerating ATP utilization leads to increasing concentrations of ADP and Pi, which serve as substrates for oxidative phosphorylation and thus accelerate ATP synthesis. An alternative model states that both the ATP demand and ATP supply pathways are simultaneously activated. Here, we review experimental and computational models of muscle contraction and energetics at various organizational levels and compare them with respect to their pros and cons in facilitating understanding of the regulation of energy metabolism during exercise in the intact organism.

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.

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.

Determinants of Oxidative Phosphorylation Onset Kinetics in Isolated Myocytes

At the onset of constant-load exercise, pulmonary oxygen uptake (VO(2)) exhibits a monoexponential increase, following a brief time delay, to a new steady state. To date, the specific factors controlling VO(2) onset kinetics during the transition to higher rates of work remain largely unknown. To study the control of respiration in the absence of confounding factors such as blood flow heterogeneity and fiber type recruitment patterns, the onset kinetics of mitochondrial respiration were studied at the start of contractions in isolated single myocytes. Individual myocytes were microinjected with a porphyrin compound to allow phosphorescent measurement of intracellular PO(2) (P(i)O(2), an analog of VO(2)). Peak tension and P(i)O(2) were continuously monitored under a variety of conditions designed to test the role of work intensity, extracellular PO(2), cellular metabolites, and enzyme activation on the regulation of VO(2) onset kinetics.

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(Pi[Formula: see text]) 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 CKivs. control (Con). The fall in Pi[Formula: see text](ΔPi[Formula: see text]) 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 CKiend-peak tension (1.53 ± 0.11) and ΔPi[Formula: see text](1.49 ± 0.11) were similar, suggesting an unaltered aerobic cost of contractions. Additionally, the mean response time (MRT) of ΔPi[Formula: see text]was significantly faster in CKivs. 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 Pi[Formula: see text]fell more rapidly at contraction onset in CKicompared 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.

The effects of sustained exercise and hypoxia upon oxygen tensions in the red muscle of rainbow trout

Teleost fish possess discrete blocks of oxidative red muscle (RM) and glycolytic white muscle, whereas tetrapod skeletal muscles are mixed oxidative/glycolytic. It has been suggested that the anatomy of RM in teleost fish could lead to higher intramuscular O2 partial pressures (PO2) than in mammalian skeletal muscles. This study provides the first direct experimental support for this suggestion by using novel optical fibre sensors to discover a mean (+/- S.E.M., N=6) normoxic steady-state red muscle PO2 (PrmO2) of 61+/-10 mmHg (1 mmHg=133.3 Pa) in free-swimming rainbow trout Oncorhynchus mykiss. This is significantly higher than literature reports for mammalian muscles, where the PO2 never exceeds 40 mmHg. Aerobic RM powers sustained swimming in rainbow trout. During graded incremental exercise, PrmO2 declined from 62+/-5 mmHg at the lowest swim speed down to 45+/-3 mmHg at maximum rates of aerobic work, but then rose again to 51+/-5 mmHg at exhaustion. These measurements of PrmO2 during exercise indicated, therefore, that O2 supply to the RM was not a major limiting factor at exhaustion in trout. The current study found no evidence that teleost haemoglobins with a Root effect cause extremely elevated O2 tensions in aerobic tissues. Under normoxic conditions, PrmO2 was significantly lower than arterial PO2 (119+/-5 mmHg), and remained lower when the arterial to tissue PO2 gradient was reduced by exposure to mild hypoxia. When two sequential levels of mild hypoxia (30 min at a water PO2 of 100 mmHg then 30 min at 75 mmHg) caused PaO2 to fall to 84+/-2 mmHg then 61+/-3 mmHg, respectively, this elicited simultaneous reductions in PrmO2,to 51+/-6 mmHg then 41+/-5 mmHg, respectively. Although these hypoxic reductions in PrmO2 were significantly smaller than those in PaO2, the effect could be attributed to the sigmoid shape of the trout haemoglobin-O2 dissociation curve.

Effect of dissociating cytosolic calcium and metabolic rate on intracellular PO2 kinetics in single frog myocytes

The purpose of this investigation was to utilize 2,3-butanedione monoxime (BDM; an inhibitor of contractile activation) to dissociate cytosolic [Ca(2+)] ([Ca(2+)](c)) from the putative respiratory regulators that arise from muscle contraction-induced ATP utilization in order to determine the relative contribution of [Ca(2+)](c) on intracellular P(O(2)) (P(iO(2))) kinetics during the transition from rest to contractions in single skeletal myocytes isolated from Xenopus laevis lumbrical muscle. Myocytes were subjected to electrically induced isometric tetanic contractions (0.25 Hz; 2-min bouts) while peak tension and either [Ca(2+)](c) (n= 7; ratiometric fluorescence microscopy) or P(iO(2)) (n= 7; phosphorescence microscopy) was measured continuously. Cells were studied under both control and 3 mm BDM conditions in randomized order. Initial (control, 100 +/- 0%; BDM, 72.6 +/- 4.6%), midpoint (control, 86.7 +/- 1.8%; BDM, 61.6 +/- 4.1%) and end (control, 85.0 +/- 2.8%; BDM, 57.5 +/- 5.0%) peak tensions (normalized to initial control values) were significantly reduced (P < 0.05) with BDM compared with control (n= 14). Despite the reduced peak tension, peak [Ca(2+)](c) was not altered (P > 0.05) between control and BDM trials. Thus, the peak tension-to-peak [Ca(2+)](c) ratio was reduced with BDM compared with control. The absolute fall in P(iO(2)) with contractions, which is proportional to the rise in , was significantly reduced with BDM (13.2 +/- 1.3 mmHg) compared with control (22.0 +/- 2.0 mmHg). However, P(iO(2)) onset kinetics (i.e. mean response time (MRT)) was not altered between BDM (66.8 +/- 8.0 s) and control (64.9 +/- 6.3 s) trials. Therefore, the initial rate of change (defined as the fall in P(iO(2))/MRT) was significantly slower in BDM fibres compared with control. These data demonstrate in these isolated single skeletal muscle fibres that unchanged peak [Ca(2+)](c) in the face of reduced metabolic feedback from the contractile sites evoked with BDM did not alter P(iO(2)) onset kinetics in isolated single frog myocytes, suggesting that metabolic signals arising from the contractile sites play a more substantial role than [Ca(2+)](c) in the signalling pathway to oxidative phosphorylation during the transition from rest to repeated tetanic contractions.

Effect of contractile duration on intracellular PO2 kinetics in Xenopus single skeletal myocytes

It has been suggested that skeletal muscle O(2) uptake (Vo(2)) kinetics follow a first-order control model. Consistent with that, Vo(2) should show both 1) similar onset kinetics and 2) an on-off symmetry across submaximal work intensities regardless of the metabolic perturbation. To date, consensus on this issue has not been reached in whole body studies due to numerous confounding factors associated with O(2) availability and fiber-type recruitment. To test whether single myocytes demonstrate similar intracellular Po(2) (Pi(O(2))) on- and off-transient kinetics at varying work intensities, we studied Xenopus laevis single myocyte (n = 8) Pi(O(2)) via phosphorescence quenching during two bouts of electrically induced isometric muscle contractions of 200 (low)- and 400 (high)-ms contraction duration (1 contraction every 4 s, 15 min between trials, order randomized). The fall in Pi(O(2)), which is inversely proportional to the net increase in Vo(2), was significantly greater (P < 0.05) during the high (24.1 +/- 3.2 Torr) vs. low (17.4 +/- 1.6 Torr) contraction bout. However, the mean response time (MRT; time to 63% of the overall change) for the fall in Pi(O(2)) from resting baseline to end contractions was not different (high, 77.8 +/- 11.5 vs. low, 76.1 +/- 13.6 s; P > 0.05) between trials. The initial rate of change at contraction onset, defined as DeltaPi(O(2))/MRT, was significantly greater (P < 0.05) in high compared with low. Pi(O(2)) off-transient MRT from the end of the contraction bout to initial baseline was unchanged (high, 83.3 +/- 18.3 vs. low, 80.4 +/- 21.6 s; P > 0.05) between high and low trials. These data revealed that Pi(O(2)) dynamics in frog isolated skeletal myocytes were invariant despite differing contraction durations and, by inference, metabolic demands. Thus these findings demonstrate that mitochondria can respond more rapidly at the initial onset of contractions when challenged with an augmented metabolic stimulus in accordance with an apparent first-order rate law.

NAD(P)H fluorescence imaging of mitochondrial metabolism in contracting Xenopus skeletal muscle fibers: effect of oxygen availability

The blue autofluorescence (351 nm excitation, 450 nm emission) of single skeletal muscle fibers from Xenopus was characterized to be originating from mitochondrial NAD(P)H on the basis of morphological and functional correlations. This fluorescence signal was used to estimate the oxygen availability to isolated single Xenopus muscle fibers during work level transitions by confocal microscopy. Fibers were stimulated to generate two contractile periods that were only different in the PO2 of the solution perfusing the single fibers (PO2 of 30 or 0-2 Torr; pH = 7.2). During contractions, mean cellular NAD(P)H increased significantly from rest in the low PO2 condition with the core (inner 10%) increasing to a greater extent than the periphery (outer 10%). After the cessation of work, NAD(P)H decreased in a manner consistent with oxygen tensions sufficient to oxidize the surplus NAD(P)H. In contrast, NAD(P)H decreased significantly with work in 30 Torr PO2. However, the rate of NAD(P)H oxidation was slower and significantly increased with the cessation of work in the core of the fiber compared with the peripheral region, consistent with a remaining limitation in oxygen availability. These results suggest that the blue autofluorescence signal in Xenopus skeletal muscle fibers is from mitochondrial NAD(P)H and that the rate of NAD(P)H oxidation within the cell is influenced by extracellular PO2 even at high extracellular PO2 during the contraction cycle. These results also demonstrate that although oxygen availability influences the rate of NAD(P)H oxidation, it does not prevent NAD(P)H from being oxidized through the process of oxidative phosphorylation at the onset of contractions.

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.

Dichloroacetate accelerates the fall in intracellular PO2 at onset of contractions in Xenopus single muscle fibers

mM DCA, whereas the second group [control (Con); n = 10] was incubated for 30 min in Ringer solution only. After incubation, fibers were electrically stimulated to elicit tetanic contractions (0.5 Hz) for 2 min during which PiO2 was monitored. PiO2 before contractions began was 32.0 +/- 1.8 and 29.0 +/- 1.8 Torr for DCA and Con, respectively, and fell to 6.0 +/- 1.3 and 8.8 +/- 2.4 Torr (no significant difference), respectively, after steady state was reached. The kinetics of the fall, determined by both the time delay (from the start of contractions to the initial decrease in PiO2) and the tau (63% of the change to a steady state in PiO2), were calculated. In DCA cells, the tau was significantly (P < 0.05) faster than Con (22.1 +/- 3.6 vs. 39.7 +/- 5.8 s). In contrast, the time delay was not significantly (P > 0.45) different between the two groups (11.4 +/- 1.7 vs. 12.6 +/- 2.3 s, respectively). The amount of fatigue, reflected by a decrease in force production from initial, was not significantly different between groups. These data suggest that by stimulating pyruvate dehydrogenase with DCA in isolated single skeletal muscle cells, the faster fall in PiO2 is indicative of oxidative metabolism being more rapidly activated. This is the first evidence that oxygen uptake at the onset of contractions may be altered by DCA during moderate- to high-intensity contractile activity.

Calibrated histochemistry applied to oxygen supply and demand in hypertrophied rat myocardium

Oxygen supply and demand of individual cardiomyocytes during the development of myocardial hypertrophy is studied using calibrated histochemical methods. An oxygen diffusion model is used to calculate the critical extracellular oxygen tension (PO(2,crit)) required by cardiomyocytes to prevent hypoxia during hypertrophic growth, and determinants of PO(2,crit) are estimated using calibrated histochemical methods for succinate dehydrogenase activity, cardiomyocyte cross-sectional area, and myoglobin concentration. The model calculation demonstrates that it is essential to calibrate the histochemical methods, so that absolute values for the relevant parameters are obtained. The succinate dehydrogenase activity, which is proportional to the maximum rate of oxygen consumption, and the myoglobin concentration hardly change while the cardiomyocytes grow. The cross-sectional area of the cardiomyocytes, which increases up to threefold in the right ventricular wall due to pulmonary hypertension in monocrotaline-treated rats, is the most important determinant of PO(2,crit) in this model of myocardial hypertrophy. The relationship between oxygen supply and demand at the level of the cardiomyocyte can be investigated using paired determinations of spatially integrated succinate dehydrogenase activity and capillary density. Hypoxia-inducible factor 1alpha can be demonstrated by immunohistochemistry in cardiomyocytes with high PO(2,crit) and increased spatially integrated succinate dehydrogenase activity, indicating that limited oxygen supply affects gene expression in these cells. We conclude that a mismatch of oxygen supply and demand may develop during hypertrophic growth, which can play a role in the transition from myocardial hypertrophy to heart failure.