Dynamics of microvascular oxygen pressure in the rat diaphragm.

The relative amplitudes and rates of increase of muscle blood flow (and O(2) delivery) and O(2) uptake responses determine the O(2) pressure within the muscle microvasculature (Pm(O(2))) across the rest-to-contraction transition. Skeletal muscle function is a primary determinant of pulmonary O(2) uptake kinetics; however, it has never been determined whether the dynamics of muscle Pm(O(2)) are faster in a highly oxidative muscle [e.g., diaphragm (Dia), citrate synthase activity of 39 micromol. min(-1). g(-1)] compared with less oxidative muscles [e.g., spinotrapezius (Spino), citrate synthase activity of 14 micromol. min(-1). g(-1), male Sprague-Dawley rats; Delp MD and Duan C, J Appl Physiol 80: 261-270, 1996]. Phosphorescence quenching techniques (porphyrin dendrimer, R2) were used to determine Pm(O(2)) across the transition to electrically stimulated contractions (1 Hz) within the rat Dia. After a delay of 10.4 +/- 1.3 (SE) s at the beginning of Dia contractions, Pm(O(2)) decreased close to monoexponentially from 42 +/- 2 to 27 +/- 3 Torr (P < 0.05) with an extremely fast time constant of 7.1 +/- 1.1 s. Thus Dia Pm(O(2)) decreased with significantly (P < 0.05) faster kinetics than reported previously for the Spino muscle (delay, 19.2 +/- 2.8 s; time constant Pm(O(2)), 21.7 +/- 2.1 s; Behnke BJ, Kindig CA, Musch TI, Koga S, and Poole DC, Respir Physiol 126: 53-63, 2001). With the use of two specialized muscles with similar fiber-type composition but widely disparate oxidative capacities (Delp MD and Duan C, J Appl Physiol 80: 261-270, 1996), these data demonstrate that Pm(O(2)) kinetics are significantly faster in the highly oxidative Dia compared with the low-oxidative Spino muscle and that this effect is not dependent on muscle fiber-type composition.     

Dissociation between skeletal muscle microvascular PO2 and hypoxia-induced microvascular inflammation.

Systemic hypoxia (SHx) produces microvascular inflammation in mesenteric, cremasteric, and pial microcirculations. In anesthetized rats, SHx lowers arterial blood pressure (MABP), which may alter microvascular blood flow and microvascular Po(2) (Pm(O(2))) and influence SHx-induced leukocyte-endothelial adherence (LEA). These experiments attempted to determine the individual contributions of the decreases in Pm(O(2)), venular blood flow and shear rate, and MABP to the hypoxia-induced increase in LEA. Cremaster microcirculation of anesthetized rats was visualized by intravital microscopy. Pm(O(2)) was measured by a phosphorescence-quenching method. SHx [inspired Po(2) of 70 Torr for 10 min, MABP of 65 +/- 3 mmHg, arterial Po(2) (Pa(O(2))) of 33 +/- 1 Torr] and cremaster ischemia (MABP of 111 +/- 7 mmHg, Pa(O(2)) of 86 +/- 3 Torr) produced similar Pm(O(2)): 7 +/- 2 and 6 +/- 2 Torr, respectively. However, LEA increased only in SHx (1.9 +/- 0.9 vs. 11.2 +/- 1.1 leukocytes/100 microm, control vs. SHx, P < 0.05). Phentolamine-induced hypotension (MABP of 55 +/- 4 mmHg) in normoxia lowered Pm(O(2)) to 26 +/- 6 Torr but did not increase LEA. Cremaster equilibration with 95% N(2)-5% CO(2) during air breathing (Pa(O(2)) of 80 +/- 1 Torr) lowered Pm(O(2)) to 6 +/- 1 Torr but did not increase LEA. On the other hand, when cremaster Pm(O(2)) was maintained at 60-70 Torr during SHx (Pa(O(2)) of 35 +/- 1 Torr), LEA increased from 2.1 +/- 1.1 to 11.1 +/- 1.5 leukocytes/100 microm (P < 0.05). The results show a dissociation between Pm(O(2)) and LEA and support the idea that SHx results in the release of a mediator responsible for the inflammatory response.     

Impaired muscle oxygen transfer in patients with chronic renal failure

We hypothesized that impaired O2 transport plays a role in limiting exercise in patients with chronic renal failure (CRF). Six CRF patients (25 +/- 6 yr) and six controls (24 +/- 6 yr) were examined twice during incremental single-leg isolated quadriceps exercise. Leg O2 delivery (QO2(leg)) and leg O2 uptake (VO2(leg)) were obtained when subjects breathed gas of three inspired O2 fractions (FI(O2)) (0.13, 0.21, and 1.0). On a different day, myoglobin O2 saturation and muscle bioenergetics were measured by proton and phosphorus magnetic resonance spectroscopy. CRF patients, but not controls, showed O2 supply dependency of peak VO2 (VO2(peak)) by a proportional relationship between peak VO2(leg) at each inspired O2 fraction (0.59 +/- 0.20, 0.47 +/- 0.10, 0.43 +/- 0.10 l/min, respectively) and 1) work rate (933 +/- 372, 733 +/- 163, 667 +/- 207 g), 2) QO(2leg) (0.80 +/- 0.20, 0.64 +/- 0.10, 0.59 +/- 0.10 l/min), and 3) cell PO2 (6.3 +/- 5.4, 1.7 +/- 1.3, 1.2 +/- 0.7 mmHg). CRF patients breathing 100% O2 and controls breathing 21% O2 had similar peak QO2(leg) (0.80 +/- 0.20 vs. 0.79 +/- 0.10 l/min) and similar peak VO2(leg) (0.59 +/- 0.20 vs. 0.57 +/- 0.10 l/min). However, mean capillary PO2 (47.9 +/- 4.0 vs. 38.2 +/- 4.6 mmHg) and the capillary-to-myocite gradient (40.7 +/- 6.2 vs. 34.4 +/- 4.0 mmHg) were both higher in CRF patients than in controls (P < 0.03 each). We conclude that low muscle O2 conductance, but not limited mitochondrial oxidative capacity, plays a role in limiting exercise tolerance in these patients.     

Effects of hyperthermia and hypothermia on oxygen extraction by tissues during hypovolemia

As systemic delivery of O2 (QO2 = QT X CaO2) is reduced during progressive hemorrhage, the O2 extraction ratio [(CaO2 - CVO2)/CaO2] increases until a critical delivery is reached below which O2 uptake (VO2) becomes limited by delivery (O2 supply dependence). When tissue metabolic activity and O2 demand are increased or reduced, the critical QO2 required to maintain VO2 should rise or fall accordingly, unless other changes in the distribution of a limited QO2 precipitate the onset of O2 supply dependence at a different critical extraction ratio. We compared the critical QO2 and critical extraction ratio in 23 normothermic (38 degrees C), hyperthermic (41 degrees C), or hypothermic (34 decrees C) dogs during stepwise reduction in delivery produced by bleeding, as arterial O2 content was maintained. Dogs were anesthetized, paralyzed, and mechanically ventilated. Hypothermia reduced whole-body VO2 by 31%, whereas hyperthermia increased VO2 by 20%. The critical QO2 was significantly reduced during hypothermia (5.6 +/- 0.95 ml.min-1.kg-1) (P less than 0.05) and increased during hyperthermia (8.9 +/- 1.1) (P approximately equal to 0.06) compared with normothermic controls (7.4 +/- 1.2). The extraction ratio at the onset of supply dependency was significantly increased in hyperthermia (0.76 +/- 0.05) compared with hypothermia (0.65 +/- 0.10) (P less than 0.05), and the normothermic critical extraction was 0.71 +/- 0.1. These results suggest that higher body temperatures are associated with an improved ability to maintain a VO2 independent of QO2, since a higher fraction of the delivered O2 can be extracted before the onset of O2 supply dependence, relative to lower body temperatures.     

Treatment of canine aspiration pneumonitis: fluid volume reduction vs. fluid volume expansion

The aspiration of gastric acid causes pulmonary edema and hypoxemia. One approach to the management of this syndrome is to raise cardiac output (Qt) and O2 delivery (QO2) to ensure tissue oxygenation (VO2) at the risk of increasing the edema. Another approach reduces the edema by reducing pulmonary microvascular pressure (Pmv) at the risk of reducing QO2 and VO2. We compared these approaches in 24 anesthetized, ventilated dogs with pulmonary wedge pressure (Ppw), a clinical approximation of Pmv, of 12.5 mmHg. Before and again 1 h after endobronchial instillation of 0.1 N HCl, we measured Qt, QO2, VO2, venous admixture, and in vivo extravascular lung liquid. The dogs were then randomly divided into four equal groups: 1) 12.5 mmHg Ppw, high Qt; 2) 7.5 mmHg Ppw, intermediate Qt; 3) 4.5 mmHg Ppw, low Qt; and 4) 4.5 mmHg Ppw plus dopamine, intermediate Qt. Measured values were followed for 4 more h, after which the lungs were excised to compare wet weight-to-body weight ratios (W/B). When plasmapheresis reduced Ppw at 1 h, edema did not increase further and W/B of groups 2 (21 +/- 3), 3 (18 +/- 3), and 4 (22 +/- 3) were significantly less than in group 1 (27 +/- 3) (P less than 0.001). Although Qt decreased with Ppw, increased hematocrit and reduced venous admixture maintained QO2 in group 2 but not in group 3. In group 4 an intermediate Qt maintained QO2 even at 4.5 mmHg Ppw but edema increased to the group 2 level presumably because Pmv rose with Qt on dopamine. VO2 remained constant over time in each group. These data demonstrate that canine HCl-induced pulmonary edema, measured in vivo or gravimetrically, is very sensitive to reductions in Pmv. Moreover, the lowest Pmv (and QO2) was well tolerated because an O2 supply dependency of VO2 was not observed.     

Systemic and intestinal limits of O2 extraction in the dog

When systemic delivery of O2 (QO2 = QT X CaO2, where QT is cardiac output and CaO2 is arterial O2 content) is reduced by bleeding, the systemic O2 extraction ratio [ER = (CaO2 - CVO2)/CaO2, where CVO2 is venous O2 content] increases until a critical limit is reached below which O2 uptake (VO2) becomes limited by O2 delivery. During hypovolemia, reflex increases in mesenteric arterial tone may preferentially reduce gut blood flow so that the onset of O2 supply dependence occurs in the gut before other regions. We compared the critical O2 delivery (QO2c) and critical extraction ratio (ERc) of whole body and an isolated segment (30-50 g) of small bowel in seven anesthetized paralyzed dogs ventilated with room air. Systemic QO2 was reduced in stages by controlled hemorrhage as arterial O2 content was maintained, and systemic and gut VO2 and QO2 were measured at each stage. Body QO2c was 7.9 +/- 1.9 ml X kg-1 X min-1 (ERc = 0.69 +/- 0.12), whereas gut O2 supply dependency occurred when gut QO2 was 34.3 +/- 11.3 ml X min-1 X kg gut wt-1 (ERc = 0.63 +/- 0.09). O2 supply dependency in the gut occurred at a higher systemic QO2 (9.7 +/- 2.7) than whole-body QO2c (P less than 0.05). The extraction ratio at the final stage (maximal ER) was less in the gut (0.80 +/- 0.05) than whole body (0.87 +/- 0.06). Thus during reductions in systemic QO2, gut VO2 was maintained by increases in gut extraction of O2.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pathological supply dependence of systemic and intestinal O2 uptake during endotoxemia

When systemic delivery of oxygen (QO2 = blood flow X arterial O2 content) is reduced, the systemic O2 extraction ratio [(CaO2 - CVO2)/CaO2; where CaO2 is arterial O2 content and CVO2 is venous O2 content] increases until a critical limit is reached below which O2 uptake (VO2) becomes limited by delivery. Patients with adult respiratory distress syndrome and sepsis exhibit supply dependence of VO2 even at high levels of QO2, which suggests that a peripheral O2 extraction defect may be present. We tested the hypothesis that endotoxemia might produce a similar defect in the efficacy of tissue O2 extraction by determining the whole-body critical systemic QO2 (QO2 c) and critical extraction ratio in a control group of dogs and a group receiving a 5-mg/kg dose of Escherichia coli endotoxin. QO2 c was determined in each group by measuring VO2 as QO2 was gradually reduced by bleeding. The VO2 and QO2 of an isolated segment of small intestine were also measured to determine whether O2 extraction was impaired within a local region of tissue. The dogs were anesthetized, paralyzed, and ventilated with room air. Systemic QO2 was reduced in stages by hemorrhage as hematocrit was maintained. The systemic and intestinal critical points were determined from a plot of VO2 vs. QO2. The mean systemic QO2 c and critical O2 extraction ratio of the endotoxemic group (12.8 +/- 2.0 and 0.54 +/- 0.11 ml.min-1.kg-1) were significantly different from control (6.8 +/- 1.2 and 0.78 +/- 0.04) (P less than 0.001), indicating that endotoxin administration impaired systemic extraction of O2. Endotoxin also increased base-line systemic VO2 [6.1 +/- 0.7 (before) to 7.4 +/- 0.1 (after)] (P less than 0.001). The critical and maximal intestinal O2 extraction ratios of the endotoxemic group (0.47 +/- 0.10 and 0.71 +/- 0.04) were significantly less than control (0.69 +/- 0.06 and 0.83 +/- 0.05) (P less than 0.001). In addition, intestinal reactive hyperemia disappeared in six of seven endotoxemic dogs, whereas it remained intact in all control dogs. Thus endotoxin reduced the ability of tissues to extract O2 from a limited supply at the whole body level as well as within a 40- to 50-g segment of small intestine. These results could be explained by a defect in microvascular regulation of blood flow that interfered with the optimal distribution of a limited QO2 in accordance with tissue O2 needs.     

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)).     

Effects of prior heavy exercise on VO(2) kinetics during heavy exercise are related to changes in muscle activity.

We hypothesized that the elevated primary O(2) uptake (VO(2)) amplitude during the second of two bouts of heavy cycle exercise would be accompanied by an increase in the integrated electromyogram (iEMG) measured from three leg muscles (gluteus maximus, vastus lateralis, and vastus medialis). Eight healthy men performed two 6-min bouts of heavy leg cycling (at 70% of the difference between the lactate threshold and peak VO(2)) separated by 12 min of recovery. The iEMG was measured throughout each exercise bout. The amplitude of the primary VO(2) response was increased after prior heavy leg exercise (from mean +/- SE 2.11 +/- 0.12 to 2.44 +/- 0.10 l/min, P < 0.05) with no change in the time constant of the primary response (from 21.7 +/- 2.3 to 25.2 +/- 3.3 s), and the amplitude of the VO(2) slow component was reduced (from 0.79 +/- 0.08 to 0.40 +/- 0.08 l/min, P < 0.05). The elevated primary VO(2) amplitude after leg cycling was accompanied by a 19% increase in the averaged iEMG of the three muscles in the first 2 min of exercise (491 +/- 108 vs. 604 +/- 151% increase above baseline values, P < 0.05), whereas mean power frequency was unchanged (80.1 +/- 0.9 vs. 80.6 +/- 1.0 Hz). The results of the present study indicate that the increased primary VO(2) amplitude observed during the second of two bouts of heavy exercise is related to a greater recruitment of motor units at the onset of exercise.     

Effect of endotoxin on systemic and skeletal muscle O2 extraction

Patients with the adult respiratory distress syndrome (ARDS) show a pathological dependence of O2 consumption (VO2) on O2 delivery (QO2, blood flow X arterial O2 content). In these patients, a defect in tissues' ability to extract O2 from blood can leave tissue O2 needs unmet, even at a normal QO2. Endotoxin administration produces a similar state in dogs, and we used this model to study mechanisms that may contribute to human pathology. We measured systemic and hindlimb VO2 and QO2 while reducing cardiac output by blood withdrawal. At the onset of supply dependence, the systemic QO2 was 11.4 +/- 2.7 ml.kg-1.min-1 in the endotoxin group vs. 8.0 +/- 0.7 in controls (P less than 0.05). At this point, the endotoxin-treated animals extracted only 61 +/- 11% of the arterial O2, whereas control animals extracted 70 +/- 7% (P less than 0.05). Systemic VO2 rose by 15% after endotoxin (P less than 0.05) but did not change in controls. Despite this poorer systemic ability to extract O2 by the endotoxin-treated dogs, isolated hindlimb O2 extraction at the onset of supply dependence was the same in endotoxin-treated and control dogs. At normal levels of QO2, hindlimb VO2 in endotoxin-treated dogs was 23% higher than in controls (P less than 0.05). Fractional blood flow to skeletal muscle did not differ between control and endotoxin-treated dogs. Thus skeletal muscle was not overperfused in endotoxemia and did not contribute to a systemic extraction defect by stealing blood flow from other tissues. Skeletal muscle in endotoxin-treated dogs demonstrated an increase in VO2 but no defect in O2 extraction, differing in both respects from the intestine.     

Subcutaneous PO2 as an index of the physiological limits for hemodilution in the rat.

This study was designed to test the hypothesis that changes in subcutaneous PO2 (PscO2) during progressive hemodilution will reliably predict a "critical point" at which tissue O2 consumption (VO2) becomes dependent on O2 delivery (QO2). Twelve pentobarbital-anesthetized male Sprague-Dawley rats (315-375 g) underwent stepwise exchange of plasma for blood (1.5 ml of plasma for each 1 ml of blood lost). The initial exchange was equal to 25% of the estimated circulatory blood volume, and each subsequent exchange was equal to 10% of the estimated circulatory blood volume. After nine exchanges, the hematocrit (Hct) fell from 42 +/- 1 to 6 +/- 1%. Cardiac output and O2 extraction rose significantly. PscO2 became significantly reduced (P < 0.05) after exchange of 45% of the blood volume (Hct = 16 +/- 1%). VO2 became delivery dependent when QO2 fell below 21 ml x min(-1) x kg body wt(-1) (mean Hct = 13 +/- 1%). Eight control rats undergoing 1:1 blood-blood exchange showed no change in PscO2, pH, HCO3(-), or hemodynamics. Measurement of PscO2 may be a useful guide to monitor the adequacy of QO2 during hemodilution.     

Continued divergence in VO2max of rats artificially selected for running endurance is mediated by greater convective blood O2 delivery.

We previously showed that after seven generations of artificial selection of rats for running capacity, maximal O2 uptake (VO2max) was 12% greater in high-capacity (HCR) than in low-capacity runners (LCR). This difference was due exclusively to a greater O2 uptake and utilization by skeletal muscle of HCR, without differences between lines in convective O2 delivery to muscle by the cardiopulmonary system (QO2max). The present study in generation 15 (G15) female rats tested the hypothesis that continuing improvement in skeletal muscle O2 transfer must be accompanied by augmentation in QO2max to support VO2max of HCR. Systemic O2 transport was studied during maximal normoxic and hypoxic exercise (inspired PO2 approximately 70 Torr). VO2max divergence between lines increased because of both improvement in HCR and deterioration in LCR: normoxic VO2max was 50% higher in HCR than LCR. The greater VO2max in HCR was accompanied by a 41% increase in QO2max: 96.1 +/- 4.0 in HCR vs. 68.1 +/- 2.5 ml stpd O2 x min(-1) x kg(-1) in LCR (P < 0.01) during normoxia. The greater G15 QO2max of HCR was due to a 48% greater stroke volume than LCR. Although tissue O2 diffusive conductance continued to increase in HCR, tissue O2 extraction was not significantly different from LCR at G15, because of the offsetting effect of greater HCR blood flow on tissue O2 extraction. These results indicate that continuing divergence in VO2max between lines occurs largely as a consequence of changes in the capacity to deliver O2 to the exercising muscle.     

Pathological supply dependence of O2 uptake during bacteremia in dogs

When systemic delivery of O2 [QO2 = cardiac output X arterial O2 content (CaO2)] is reduced, the systemic O2 extraction ratio [(CaO2-concentration of O2 in venous blood/CaO2] increases until a critical limit is reached below which O2 uptake (VO2) becomes limited by delivery. Many patients with adult respiratory distress syndrome exhibit supply dependence of VO2 even at high levels of QO2, which suggests that a peripheral O2 extraction defect may be present. Since many of these patients also suffer from serious bacterial infection, we tested the hypothesis that bacteremia might produce a similar defect in the ability of tissues to maintain VO2 independent of QO2, as QO2 reduced. The critical O2 delivery (QO2crit) and critical extraction ratio (ERcrit) were compared in a control group of dogs and a group receiving a continuous infusion of Pseudomonas aeruginosa (5 x 10(7) organisms/min). Dogs were anesthetized, paralyzed, and ventilated with room air. Systemic QO2 was reduced in stages by hemorrhage as hematocrit was maintained. At each stage, systemic VO2 and QO2 were measured, and the critical point was determined from a plot of VO2 vs. QO2. The mean QO2crit and ERcrit of the bacteremic group (11.4 +/- 2.2 ml.min-1.kg-1 and 0.51 +/- 0.09) were significantly different from control (7.4 +/- 1.2 and 0.71 +/- 0.10) (P less than 0.05). These results suggest that bacterial infection can reduce the ability of peripheral tissues to extract O2 from a limited supply, causing VO2 to become limited by O2 delivery at a stage when a smaller fraction of the delivered O2 has been extracted.(ABSTRACT TRUNCATED AT 250 WORDS)     

Acute effects of hydrogen peroxide on skeletal muscle microvascular oxygenation from rest to contractions

Reactive oxygen species, such as hydrogen peroxide (H(2)O(2)), exert a critical regulatory role on skeletal muscle function. Whether acute increases in H(2)O(2) modulate muscle microvascular O(2) delivery-utilization (Qo(2)/Vo(2)) matching [i.e., microvascular partial pressure of O(2) (Pmv(O(2)))] at rest and following the onset of contractions is unknown. The hypothesis was tested that H(2)O(2) treatment (exogenous H(2)O(2)) would enhance Pmv(O(2)) and slow Pmv(O(2)) kinetics during contractions compared with control. Anesthetized, healthy young Sprague-Dawley rats had their spinotrapezius muscles either exposed for measurement of blood flow (and therefore QO(2)), VO(2), and Pmv(O(2)), or exteriorized for measurement of force production. Electrically stimulated twitch contractions (1 Hz, ~7 V, 2-ms pulse duration, 3 min) were evoked following acute superfusion with Krebs-Henseleit (control) and H(2)O(2) (100 μM). Relative to control, H(2)O(2) treatment elicited disproportionate increases in QO(2) and VO(2) that elevated Pmv(O(2)) at rest and throughout contractions and slowed overall Pmv(O(2)) kinetics (i.e., ~85% slower mean response time; P < 0.05). Accordingly, H(2)O(2) resulted in ~33% greater overall Pmv(O(2)), as assessed by the area under the Pmv(O(2)) curve (P < 0.05). Muscle force production was not altered with H(2)O(2) treatment (P > 0.05), evidencing reduced economy during contractions (~40% decrease in the force/VO(2) relationship; P < 0.05). These findings indicate that, although increasing the driving force for blood-myocyte O(2) flux (i.e., Pmv(O(2))), transient elevations in H(2)O(2) impair skeletal muscle function (i.e., reduced economy during contractions), which mechanistically may underlie, in part, the reduced exercise tolerance in conditions associated with oxidative stress.     

Spinotrapezius muscle microcirculatory function: effects of surgical exteriorization

Intravital microscopy facilitates insights into muscle microcirculatory structural and functional control, provided that surgical exteriorization does not impact vascular function. We utilized a novel combination of phosphorescence quenching, microvascular oxygen pressure (microvascular PO(2)), and microsphere (blood flow) techniques to evaluate static and dynamic behavior within the exposed intact (I) and exteriorized (EX) rat spinotrapezius muscle. I and EX muscles were studied under control, metabolic blockade with 2,4-dinitrophenol (DNP), and electrically stimulated conditions with 1-Hz contractions, and across switches from 21 to 100% and 10% inspired O(2). Surgical preparation did not alter spinotrapezius muscle blood flow in either I or EX muscle. DNP elevated muscle blood flow approximately 120% (P < 0.05) in both I and EX muscles (P > 0.05 between I and EX). Contractions reduced microvascular PO(2) from 30.4 +/- 4.3 to 21.8 +/- 4.8 mmHg in I muscle and from 33.2 +/- 3.0 to 25.9 +/- 2.8 mmHg in EX muscles with no difference between I and EX. In each O(2) condition, there was no difference (each P > 0.05) in microvascular PO(2) between I and EX muscles (21% O(2): I = 37 +/- 1; EX = 36 +/- 1; 100%: I = 62 +/- 5; EX = 51 +/- 9; 10%: I = 20 +/- 1; EX = 17 +/- 2 mmHg). Similarly, the dynamic behavior of microvascular PO(2) to altered inspired O(2) was unaffected by the EX procedure [half-time (t(1/2)) to 100% O(2): I = 23 +/- 5; EX = 23 +/- 4; t(1/2) to 10%: I = 14 +/- 2; EX = 16 +/- 2 s, both P > 0.05]. These results demonstrate that the spinotrapezius muscle can be EX without significant alteration of microvascular integrity and responsiveness under the conditions assessed.     

Determinants of maximal O(2) uptake in rats selectively bred for endurance running capacity.

O(2) transport during maximal exercise was studied in rats bred for extremes of exercise endurance, to determine whether maximal O(2) uptake (VO(2 max)) was different in high- (HCR) and low-capacity runners (LCR) and, if so, which were the phenotypes responsible for the difference. VO(2 max) was determined in five HCR and six LCR female rats by use of a progressive treadmill exercise protocol at inspired PO(2) of approximately 145 (normoxia) and approximately 70 Torr (hypoxia). Normoxic VO(2 max) (in ml. min(-1). kg(-1)) was 64.4 +/- 0.4 and 57.6 +/- 1.5 (P < 0.05), whereas VO(2 max) in hypoxia was 42.7 +/- 0.8 and 35.3 +/- 1.5 (P < 0.05) in HCR and LCR, respectively. Lack of significant differences between HCR and LCR in alveolar ventilation, alveolar-to-arterial PO(2) difference, or lung O(2) diffusing capacity indicated that neither ventilation nor efficacy of gas exchange contributed to the difference in VO(2 max) between groups. Maximal rate of blood O(2) convection (cardiac output times arterial blood O(2) content) was also similar in both groups. The major difference observed was in capillary-to-tissue O(2) transfer: both the O(2) extraction ratio (0.81 +/- 0.002 in HCR, 0.74 +/- 0.009 in LCR, P < 0.001) and the tissue diffusion capacity (1.18 +/- 0.09 in HCR and 0.92 +/- 0.05 ml. min(-1). kg(-1). Torr(-1) in LCR, P < 0.01) were significantly higher in HCR. The data indicate that selective breeding for exercise endurance resulted in higher VO(2 max) mostly associated with a higher transfer of O(2) at the tissue level.     

Dissociation of changes in VO2 max, muscle QO2, and performance with training in rats

Before the start and after 4, 8, and 12 wk of a treadmill training program male rats were randomly selected and tested for running performance, maximum O2 consumption (VO2 max), running economy (VO2 submax), and skeletal muscle oxidative capacity (QO2). Data were compared with values from untrained weight-matched control rats. Maximum running time to exhaustion increased significantly (P less than 0.01) by 4 wk and again at 12 wk (P less than 0.01). Submaximal running endurance increased by 120 (4 wk), 320 (8 wk), and 372% (12 wk) (P less than 0.01). VO2 max was increased only at 12 wk (86.0 +/- 2.7 vs. 75.5 +/- 1.9 ml O2.kg-1.min-1); VO2 submax was decreased at 4 and 8 wk but not at 12 wk. Soleus QO2 was unchanged after 4 wk of training and increased by 50% at 8 wk and by 77% at 12 wk. This study is the first to show a dissociation in both the time course and the magnitude of longitudinal changes in VO2 max, VO2 submax, QO2, and maximal and submaximal running performance. We conclude that factors other than those measured explain the improvement in running performance that resulted from endurance training in these rats.     

Tissue oxygen extraction during hypovolemia: role of hemoglobin P50

When the delivery of O2 to tissues (QO2 = blood flow X O2 content) falls below a critical threshold, tissue O2 uptake (VO2) becomes limited by QO2. The mechanism responsible for this extraction limitation is not understood but may involve molecular diffusion limitation as mean capillary PO2 drops below a critical minimum level in some capillaries. We tested this hypothesis by measuring the critical QO2 necessary to maintain VO2 independent of QO2 in anesthetized, paralyzed normal dogs (n = 7) and in a second group in which PO2 at 50% saturation of hemoglobin (P50) was reduced by exchange transfusion with low-P50 erythrocytes (n = 7). QO2 was reduced in stages by removing blood volume to reduce blood flow while VO2 was measured by spirometry at each step. To the extent that O2 extraction was limited by a critical capillary PO2, we reasoned that the onset of diffusion limitation should occur at a higher QO2 with low P50, since a lower end-capillary PO2 is required to achieve the same O2 extraction. The critical QO2 (7.8 +/- 1.2 ml X min-1 X kg-1) and extraction ratio (0.63 +/- 0.06) in dogs with reduced P50 were not different from controls. At the critical delivery, mixed venous PO2 was lower in low P50 (16.1 +/- 2.9 Torr) than controls (29.9 +/- 2.3 Torr). We concluded that diffusion limitation does not initiate the early fall in VO2 below the critical QO2 and offer an alternative model to explain the onset of supply dependency.(ABSTRACT TRUNCATED AT 250 WORDS)     

VO(2) kinetics reveal a central limitation at the onset of subthreshold exercise in heart transplant recipients.

Because the cardiocirculatory response of heart transplant recipients (HTR) to exercise is delayed, we hypothesized that their O(2) uptake (VO(2)) kinetics at the onset of subthreshold exercise are slowed because of an impaired early "cardiodynamic" phase 1, rather than an abnormal subsequent "metabolic" phase 2. Thus we compared the VO(2) kinetics in 10 HTR submitted to six identical 10-min square-wave exercises set at 75% (36 +/- 5 W) of the load at their ventilatory threshold (VT) to those of 10 controls (C) similarly exercising at the same absolute (40 W; C40W group) and relative load (67 +/- 14 W; C67W group). Time-averaged heart rate, breath-by-breath VO(2), and O(2) pulse (O(2)p) data yielded monoexponential time constants of the VO(2) (s) and O(2)p increase. Separating phase 1 and 2 data permitted assessment of the phase 1 duration and phase 2 VO(2) time constant (). The VO(2) time constant was higher in HTR (38.4 +/- 7.5) than in C40W (22.9 +/- 9.6; P < or = 0. 002) or C67W (30.8 +/- 8.2; P < or = 0.05), as was the O(2)p time constant, resulting from a lower phase 1 VO(2) increase (287 +/- 59 vs. 349 +/- 66 ml/min; P < or = 0.05), O(2)p increase (2.8 +/- 0.6 vs. 3.6 +/- 1.0 ml/beat; P < or = 0.0001), and a longer phase 1 duration (36.7 +/- 12.3 vs. 26.8 +/- 6.0 s; P < or = 0.05), whereas the was similar in HTR and C (31.4 +/- 9.6 vs. 29.9 +/- 5.6 s; P = 0.85). Thus the HTR have slower subthreshold VO(2) kinetics due to an abnormal phase 1, suggesting that the heart is unable to increase its output abruptly when exercise begins. We expected a faster in HTR because of their prolonged phase 1 duration. Because this was not the case, their muscular metabolism may also be impaired at the onset of subthreshold exercise.     

Effects of eccentric exercise on microcirculation and microvascular oxygen pressures in rat spinotrapezius muscle.

A single bout of eccentric exercise results in muscle damage, but it is not known whether this is correlated with microcirculatory dysfunction. We tested the following hypotheses in the spinotrapezius muscle of rats either 1 (DH-1; n = 6) or 3 (DH-3; n = 6) days after a downhill run to exhaustion (90-120 min; -14 degrees grade): 1) in resting muscle, capillary hemodynamics would be impaired, and 2) at the onset of subsequent acute concentric contractions, the decrease of microvascular O(2) pressure (Pmv(o(2))), which reflects the dynamic balance between O(2) delivery and O(2) utilization, would be accelerated compared with control (Con, n = 6) rats. In contrast to Con muscles, intravital microscopy observations revealed the presence of sarcomere disruptions in DH-1 and DH-3 and increased capillary diameter in DH-3 (Con: 5.2 +/- 0.1; DH-1: 5.1 +/- 0.1; DH-3: 5.6 +/- 0.1 mum; both P < 0.05 vs. DH-3). At rest, there was a significant reduction in the percentage of capillaries that sustained continuous red blood cell (RBC) flux in both DH running groups (Con: 90.0 +/- 2.1; DH-1: 66.4 +/- 5.2; DH-3: 72.9 +/- 4.1%, both P < 0.05 vs. Con). Capillary tube hematocrit was elevated in DH-1 but reduced in DH-3 (Con: 22 +/- 2; DH-1: 28 +/- 1; DH-3: 16 +/- 1%; all P < 0.05). Although capillary RBC flux did not differ between groups (P > 0.05), RBC velocity was lower in DH-1 compared with Con (Con: 324 +/- 43; DH-1: 212 +/- 30; DH-3: 266 +/- 45 mum/s; P < 0.05 DH-1 vs. Con). Baseline Pmv(O(2)) before contractions was not different between groups (P > 0.05), but the time constant of the exponential fall to contracting Pmv(O(2)) values was accelerated in the DH running groups (Con: 14.7 +/- 1.4; DH-1: 8.9 +/- 1.4; DH-3: 8.7 +/- 1.4 s, both P < 0.05 vs. Con). These findings are consistent with the presence of substantial microvascular dysfunction after downhill eccentric running, which slows the exercise hyperemic response at the onset of contractions and reduces the Pmv(O(2)) available to drive blood-muscle O(2) delivery.