Endurance training in humans: aerobic capacity and structure of skeletal muscle

The adaptation of muscle structure, power output, and mass-specific rate of maximal O2 consumption (VO2max/Mb) with endurance training on bicycle ergometers was studied for five male and five female subjects. Biopsies of vastus lateralis muscle and VO2max determinations were made at the start and end of 6 wk of training. The power output maintained on the ergometer daily for 30 min was adjusted to achieve a heart rate exceeding 85% of the maximum for two-thirds of the training session. It is proposed that the observed preferential proliferation of subsarcolemmal vs. interfibrillar mitochondria and the increase in intracellular lipid deposits are two possible mechanisms by which muscle cells adapt to an increased use of fat as a fuel. The relative increase of VO2max/Mb (14%) with training was found to be smaller by more than twofold than the relative increase in maximal maintained power (33%) and the relative change in the volume density of total mitochondria (+40%). However, the calculated VO2 required at an efficiency of 0.25 to produce the observed mass-specific increase in maximal maintained power matched the actual increase in VO2max/Mb (8.0 and 6.5 ml O2 X min-1 X kg-1, respectively). These results indicate that despite disparate relative changes the absolute change in aerobic capacity at the local level (maintained power) can account for the increase in aerobic capacity observed at the general level (VO2max).     

Phenotypic effects of the "mini-muscle" allele in a large HR x C57BL/6J mouse backcross

From outbred Hsd:ICR mice, we selectively bred 4 replicate lines for high running (High-Runner [HR] lines) on wheels while maintaining 4 nonselected lines as controls (C lines). An apparent Mendelian recessive, the "mini-muscle" (MM) allele, whose main phenotypic effect is to reduce hindlimb muscle mass by 50%, was discovered in 2 HR lines and 1 C line. This gene of major effect has gone to fixation in one selected line, remains polymorphic in another, and is now undetectable in the one C line. Homozygotes exhibit various pleiotropic effects, including a doubling of mass-specific muscle aerobic capacity, and larger hearts, livers, and spleens. To create a population suitable for mapping the genomic location of the MM allele and to better characterize its pleiotropic effects, we crossed females fixed for the MM allele with male C57BL/6J. F(1) males were then backcrossed to the MM parent females. Backcross (BC) mice (N = 404) were dissected, and a 50:50 ratio of normal to MM phenotype was observed with no overlap in relative muscle mass. In the BC, analysis of covariance revealed that MM individuals ran significantly more on days 5 and 6 of a 6-day exposure to running wheels (as in the routine selective-breeding protocol), were smaller in body mass, and had larger ventricles and spleens.     

Oxygen transport during exercise in large mammals. I. Adaptive variation in oxygen demand

This study investigated mechanisms used by horses and steers to increase O2 uptake and delivery (VO2) from resting to maximal rates and identified the mechanisms that enable horses to achieve higher maximal rates of O2 consumption (VO2max) than steers. VO2 and circulatory variables were measured while Standardbred trotting horses and steers (450-kg body mass) stood quietly and ran on a treadmill at speeds up to those eliciting VO2max. As VO2 increased in both species, heart rate and circulating hemoglobin (Hb) concentration increased, thereby increasing O2 delivery by the circulation, while cardiac stroke volume remained unchanged. At VO2max arterial PCO2 increased from its resting value in horses but was unchanged in steers, and arterial PO2 decreased in both species. Although the horses hypoventilated and were hypoxemic at VO2max, no significant decrease in arterial Hb saturation occurred. VO2max of the horses was 2.6 times higher than that of the steers and was associated with a 100% larger cardiac output, 100% larger stroke volume, and 40% higher Hb concentration, whereas heart rates at VO2max were identical in the two species. The higher cardiac output of the horses at VO2max resulted from a 1.2-fold higher mean arterial pressure and 1.6-fold lower peripheral tissue resistance (associated with a larger skeletal muscle capillary bed). Both the magnitude of the difference in VO2max between horses and steers and the mechanisms used to achieve it are the same as observed in smaller pairs of mammalian species with large variation in aerobic capacity.     

Effect of hemoglobin concentration on maximal O2 uptake in canine gastrocnemius muscle in situ

O2 delivery to maximally working muscle was decreased by altering hemoglobin (Hb) concentration and arterial PO2 (PaO2) to investigate whether the reductions in maximal O2 uptake (VO2max) that occur with lowered [Hb] are in part related to changes in the effective muscle O2 diffusing capacity (DmO2). Two sets of experiments were conducted. In the initial set (n = 8), three levels of Hb [5.8 +/- 0.3, 9.4 +/- 0.1, and 14.4 +/- 0.6 (SE) g/100 ml] in the blood were used in random order to pump perfuse, at equal muscle blood flows and PaO2, maximally working isolated dog gastrocnemius muscle. VO2max declined with decreasing [Hb], but the relationship between VO2max and both the effluent venous PO2 (PvO2) and the calculated mean capillary PO2 (PcO2) was not linear through the origin and, therefore, not compatible with a single value of DmO2 (as calculated by Bohr integration using a model based on Fick's law of diffusion). To clarify these results, a second set of experiments (n = 6) was conducted in which two levels of Hb (14.0 +/- 0.6 and 6.9 +/- 0.6 g/100 ml) were each combined with two levels of oxygenation (PaO2 79 +/- 8 and 29 +/- 2 Torr) and applied in random sequence to again pump perfuse maximally working dog gastrocnemius muscle at constant blood flow. In these experiments, the relationship between VO2max and both PvO2 and calculated PcO2 for each [Hb] was consistent with a constant estimate of DmO2 as PaO2 was reduced, but the calculated DmO2 for the lower [Hb] was 33% less than that at the higher [Hb] (P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Mean myoglobin oxygen tension during exercise at maximal oxygen uptake.

Changes in intracellular Po2 in myoglobin containing skeletal muscle during exercise were estimated in normal nonathlete subjects from measurements of shifts of CO between blood and muscle under conditions where the total body CO stores remained constant. Exercise was performed on a bicycle ergometer. In 1.5-2 and 6-7 min runs at Vo2 max with the subject breathing 21% O2, mean MbCO/HbCO increased 146 +/- 7 and 163 +/- 11% of resting values, respectively (P less than 0.05). With the subjects breathing 13-14% O2, in 1.5-2 and 6-7 min runs, Vo2 max fell an average of 4.3 +/- 5.1% and 12.0 +/- 5.2%, respectively, and mean MbCO/HbCO increased to 233 +/- 18% and 210 +/- 52% of resting value, respectively (P less than 0.05). These findings suggest that mean myoglobin Po2 fell during exercise at Vo2 max, with the subjects breathing 21% O2 and the decrease in mean myoglobin Po2 was greater with the subject breathing 13-14% O2. There was considerable variability in different subjects and in some, the data were not consistent with intracellular O2 availability limiting aerobic metabolism. The data support a postulate that there are several limiting factors for the aerobic capacity, including intracellular O2 availability.     

Artificial selection for high activity favors mighty mini-muscles in house mice

After 14 generations of selection for voluntary wheel running, mice from the four replicate selected lines ran, on average, twice as many revolutions per day as those from the four unselected control lines. To examine whether the selected lines followed distinct strategies in the correlated responses of the size and metabolic capacities of the hindlimb muscles, we examined mice from selected lines, housed for 8 wk in cages with access to running wheels that were either free to rotate ("wheel access" group) or locked ("sedentary"). Thirteen of twenty individuals in one selected line (line 6) and two of twenty in another (line 3) showed a marked reduction ( approximately 50%) in total hindlimb muscle mass, consistent with the previously described expression of a small-muscle phenotype. Individuals with these "mini-muscles" were not significantly smaller in total body mass compared with line-mates with normal-sized muscles. Access to free wheels did not affect the relative mass of the mini-muscles, but did result in typical mammalian training effects for mitochondrial enzyme activities. Individuals with mini-muscles showed a higher mass-specific muscle aerobic capacity as revealed by the maximal in vitro rates of citrate synthase and cytochrome c oxidase. Moreover, these mice showed the highest activities of hexokinase and carnitine palmitoyl transferase. Females with mini-muscles showed the highest levels of phosphofructokinase, and males with mini-muscles the highest levels of pyruvate dehydrogenase. As shown by total muscle enzyme contents, the increase in mass-specific aerobic capacity almost completely compensated for the reduction caused by the "loss" of muscle mass. Moreover, the mini-muscle mice exhibited the lowest contents of lactate dehydrogenase and glycogen phosphorylase. Interestingly, metabolic capacities of mini-muscled mice resemble those of muscles after endurance training. Overall, our results demonstrate that during selection for voluntary wheel running, distinct adaptive paths that differentially exploit the genetic variation in morphological and physiological traits have been followed.     

Acute vs. chronic effects of elevated hemoglobin O(2) affinity on O(2) transport in maximal exercise.

These studies were conducted to compare the effects on systemic O(2) transport of chronically vs. acutely increased Hb O(2) affinity. O(2) transport during maximal normoxic and hypoxic [inspired PO(2) (PI(O(2))) = 70 and 55 Torr, respectively] exercise was studied in rats with Hb O(2) affinity that was increased chronically by sodium cyanate (group 1) or acutely by transfusion with blood obtained from cyanate-treated rats (group 2). Group 3 consisted of normal rats. Hb O(2) half-saturation pressure (P(50); Torr) during maximal exercise was approximately 26 in groups 1 and 2 and approximately 46 in group 3. In normoxia, maximal blood O(2) convection (TO(2 max) = cardiac output x arterial blood O(2) content) was similar in all groups, whereas in hypoxia TO(2 max) was significantly higher in groups 1 and 2 than in group 3. Tissue O(2) extraction (arteriovenous O(2) content/arterial O(2) content) was lowest in group 1, intermediate in group 2, and highest in group 3 (P < 0.05) at all exercise PI(O(2)) values. In normoxia, maximal O(2) utilization (VO(2 max)) paralleled O(2) extraction ratio and was lowest in group 1, intermediate in group 2, and highest in group 3 (P < 0.05). In hypoxia, the lower O(2) extraction ratio values of groups 1 and 2 were offset by their higher TO(2 max); accordingly, their differences in VO(2 max) from group 3 were attenuated or reversed. Tissue O(2) transfer capacity (VO(2 max)/mixed venous PO(2)) was lowest in group 1 and comparable in groups 2 and 3. We conclude that lowering Hb P(50) has opposing effects on TO(2 max) and O(2) extraction ratio, with the relative magnitude of these changes, which varies with PI(O(2)), determining VO(2 max). Although the lower O(2) extraction ratio of groups 2 vs. 3 suggests a decrease in tissue PO(2) diffusion gradient secondary to the low P(50), the lower O(2) extraction ratio of groups 1 vs. 2 suggests additional negative effects of sodium cyanate and/or chronically low Hb P(50) on tissue O(2) transfer.     

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.     

Anemia causes a relative decrease in blood lactate concentration during exercise

The purpose of the present study was to examine to what degree a reduction in systemic oxygen transport capacity influences the absolute and relative levels (% of maximal oxygen uptake) of submaximal blood lactate accumulation. Anemia was induced by repeated venesections in eight healthy males. After 9-10 weeks of anemia, hemoglobin concentration [Hb] was restored by retransfusion of packed erythrocytes. The [Hb] values obtained were, before venesections, in control (C) = 145 +/- 10, in the anemic state (A) = 110 +/- 8, and after retransfusion (R) = 143 +/- 8 g X l-1 respectively. In all states, muscle biopsies were taken and measurements made of VO2max and VO2 at a running velocity corresponding to a blood lactate concentration of 4 mM (upsilon Hla 4.0). In the A condition Vo2max decreased by 19% as compared to C (P less than 0.01). upsilon Hla 4.0 was 14% lower in A as compared to C and R (p less than 0.01). VO2 at upsilon Hla 4.0 was 13% lower in A as compared to C (P less than 0.01). However, VO2 at upsilon Hla 4.0 expressed as a percentage of VO2max was increased (P less than 0.01) in the anemic state, the values obtained being C = 83.3%, A = 89.8% and R = 84.8%. Ventilation at upsilon Hla 4.0 was higher in A as compared to C and R (P less than 0.05). R and C values were not significantly different for any of the values presented above. The maximal activity of citrate synthase in muscle did not differ between the three different conditions.(ABSTRACT TRUNCATED AT 250 WORDS)     

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

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

Oxygen transport to skeletal muscle working at VO(2)max in acute hypoxia: theoretical predictions

The influence of acute hypoxia (30 < or = PaO(2) < or = 100 mmHg) on the values of VO(2)max and parameters of oxygen transport in muscle working at VO(2)max was studied. We investigated muscle working under different values of blood flow F (60 < or = F < or = 120 ml/min per 100 g), blood pH (7.0-7.6), and different diffusion conditions. Investigations were performed on a computer model of O(2) delivery to and O(2) consumption in the working muscle. VO(2)max, PvO(2), pO(2)- and VO(2)-distribution in muscle fiber were calculated. It was shown that the greater the degree of arterial hypoxemia, the lower the muscle VO(2)max and blood pO(2) values. When working at VO(2)max, the average and minimal values of tissue pO(2) depend on PaO(2). The greater the blood flow through muscle, the greater the VO(2)max. However, with an increasing degree of arterial hypoxemia, the effect of F and blood pH on the value of VO(2)max is weakened. The diffusion conditions produced a powerful influence on the VO(2)max value. At reduced PaO(2) they are the most important limiting factors of O(2) supply to muscle working at maximal effort.     

Aerobic metabolism of human quadriceps muscle: in vivo data parallel measurements on isolated mitochondria

The aim of the present study was to examine whether parameters of isolated mitochondria could account for the in vivo maximum oxygen uptake (VO2max) of human skeletal muscle. VO2max and work performance of the quadriceps muscle of six volunteers were measured in the knee extensor model (range 10-18 mmol O2 x min(-1) x kg(-1) at work rates of 22-32 W/kg). Mitochondria were isolated from the same muscle at rest. Strong correlations were obtained between VO2max and a number of mitochondrial parameters (mitochondrial protein, cytochrome aa3, citrate synthase, and respiratory activities). The activities of citrate synthase, succinate dehydrogenase, and pyruvate dehydrogenase, measured in isolated mitochondria, corresponded to, respectively, 15, 3, and 1.1 times the rates calculated from VO2max. The respiratory chain activity also appeared sufficient. Fully coupled in vitro respiration, which is limited by the rate of ATP synthesis, could account for, at most, 60% of the VO2max. This might be due to systematic errors or to loose coupling of the mitochondrial respiration under intense exercise.     

A Novel Intronic Single Nucleotide Polymorphism in the Myosin heavy polypeptide 4 Gene Is Responsible for the Mini-Muscle Phenotype Characterized by Major Reduction in Hind-Limb Muscle Mass in Mice

Replicated artificial selection for high levels of voluntary wheel running in an outbred strain of mice favored an autosomal recessive allele whose primary phenotypic effect is a 50% reduction in hind-limb muscle mass. Within the High Runner (HR) lines of mice, the numerous pleiotropic effects (e.g., larger hearts, reduced total body mass and fat mass, longer hind-limb bones) of this hypothesized adaptive allele include functional characteristics that facilitate high levels of voluntary wheel running (e.g., doubling of mass-specific muscle aerobic capacity, increased fatigue resistance of isolated muscles, longer hind-limb bones). Previously, we created a backcross population suitable for mapping the responsible locus. We phenotypically characterized the population and mapped the Minimsc locus to a 2.6-Mb interval on MMU11, a region containing ∼100 known or predicted genes. Here, we present a novel strategy to identify the genetic variant causing the mini-muscle phenotype. Using high-density genotyping and whole-genome sequencing of key backcross individuals and HR mice with and without the mini-muscle mutation, from both recent and historical generations of the HR lines, we show that a SNP representing a C-to-T transition located in a 709-bp intron between exons 11 and 12 of the Myosin heavy polypeptide 4 (Myh4) skeletal muscle gene (position 67,244,850 on MMU11; assembly, December 2011, GRCm38/mm10; ENSMUSG00000057003) is responsible for the mini-muscle phenotype, Myh4^Minimsc. Using next-generation sequencing, our approach can be extended to identify causative mutations arising in mouse inbred lines and thus offers a great avenue to overcome one of the most challenging steps in quantitative genetics.     

Work performance in iron deficiency of increasing severity

The effect of iron deficiency on work capacity was studied in groups of rats that had received diets with iron contents ranging between 9 and 50 mg/kg diet from 3 to 6 wk of age. Maximal O2 consumption (VO2max) declined only 16% with a decrease in hemoglobin (Hb) from 14 to 8 g/dl and fell sharply only below a Hb of 7 g/dl. Duration until exhaustion in a treadmill exercise of submaximal intensity (endurance) showed no significant depression between a Hb of 14 and 10 g/dl. However, endurance declined abruptly by 73% between a Hb of 10 and 8 g/dl. The VO2max results are in accord with known compensatory mechanisms that help to maintain delivery of O2 to tissues until anemia becomes severe. The sharp fall in endurance with relatively mild iron deficiency suggests a lack of similarly effective compensations for decreased oxidative capacity of muscle.     

Muscle respiratory capacity and VO2 max in identically trained young and old rats

Old rats have a decreased hindlimb muscle respiratory capacity and whole-body maximal O2 consumption (VO2 max). The decline in spontaneous physical activity in old rats might contribute to these age-related changes. The magnitude of the age-related decline is not uniform in all skeletal muscle respiratory enzymes, and the decrease in palmitate oxidation is particularly great. This study was designed to determine if young and old rats subjected to the same exercise-training protocol would attain similar values for VO2 max and several markers of muscle respiratory capacity. Four- and 18-mo-old Fischer 344 rats underwent an identical 6-mo program of treadmill running. After training, both age groups had increased VO2 max above sedentary age-matched controls. However, the old trained rats had a lower VO2 max than identically trained young rats. In contrast to VO2 max, the two trained groups attained similar values for gastrocnemius citrate synthase, cytochrome oxidase, 3-hydroxyacyl-CoA dehydrogenase, palmitate oxidation, and total carnitine concentration. Thus, when the young and old rats performed an identical exercise protocol within the capacity of the old animals, differences in skeletal muscle respiratory capacity were eliminated. The dissimilarity in VO2 max between the identically trained groups was apparently caused by age-related differences in factors other than muscle respiratory capacity.     

Ambulatory estimates of maximal aerobic power from foot -ground contact times and heart rates in running humans.

Seeking to develop a simple ambulatory test of maximal aerobic power (VO(2 max)), we hypothesized that the ratio of inverse foot-ground contact time (1/t(c)) to heart rate (HR) during steady-speed running would accurately predict VO(2 max). Given the direct relationship between 1/t(c) and mass-specific O(2) uptake during running, the ratio 1/t(c). HR should reflect mass-specific O(2) pulse and, in turn, aerobic power. We divided 36 volunteers into matched experimental and validation groups. VO(2 max) was determined by a treadmill test to volitional fatigue. Ambulatory monitors on the shoe and chest recorded foot-ground contact time (t(c)) and steady-state HR, respectively, at a series of submaximal running speeds. In the experimental group, aerobic fitness index (1/t(c). HR) was nearly constant across running speed and correlated with VO(2 max) (r = 0.90). The regression equation derived from data from the experimental group predicted VO(2 max) from the 1/t(c). HR values in the validation group within 8.3% and 4.7 ml O(2) x kg(-1) x min(-1) (r = 0.84) of measured values. We conclude that simultaneous measurements of foot-ground constant times and heart rates during level running at a freely chosen constant speed can provide accurate estimates of maximal aerobic power.     

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

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

The ergogenic effect of recombinant human erythropoietin on VO2max depends on the severity of arterial hypoxemia

Treatment with recombinant human erythropoietin (rhEpo) induces a rise in blood oxygen-carrying capacity (CaO(2)) that unequivocally enhances maximal oxygen uptake (VO(2)max) during exercise in normoxia, but not when exercise is carried out in severe acute hypoxia. This implies that there should be a threshold altitude at which VO(2)max is less dependent on CaO(2). To ascertain which are the mechanisms explaining the interactions between hypoxia, CaO(2) and VO(2)max we measured systemic and leg O(2) transport and utilization during incremental exercise to exhaustion in normoxia and with different degrees of acute hypoxia in eight rhEpo-treated subjects. Following prolonged rhEpo treatment, the gain in systemic VO(2)max observed in normoxia (6-7%) persisted during mild hypoxia (8% at inspired O(2) fraction (F(I)O(2)) of 0.173) and was even larger during moderate hypoxia (14-17% at F(I)O(2) = 0.153-0.134). When hypoxia was further augmented to F(I)O(2) = 0.115, there was no rhEpo-induced enhancement of systemic VO(2)max or peak leg VO(2). The mechanism highlighted by our data is that besides its strong influence on CaO(2), rhEpo was found to enhance leg VO(2)max in normoxia through a preferential redistribution of cardiac output toward the exercising legs, whereas this advantageous effect disappeared during severe hypoxia, leaving augmented CaO(2) alone insufficient for improving peak leg O(2) delivery and VO(2). Finally, that VO(2)max was largely dependent on CaO(2) during moderate hypoxia but became abruptly CaO(2)-independent by slightly increasing the severity of hypoxia could be an indirect evidence of the appearance of central fatigue.     

Effect of increased Hb-O2 affinity on VO2max at constant O2 delivery in dog muscle in situ

We investigated the effect of increasing hemoglobin- (Hb) O2 affinity on muscle maximal O2 uptake (VO2max) while muscle blood flow, [Hb], HbO2 saturation, and thus O2 delivery (muscle blood flow X arterial O2 content) to the working muscle were kept unchanged from control. VO2max was measured in isolated in situ canine gastrocnemius working maximally (isometric tetanic contractions). The muscles were pump perfused, in alternating order, with either normal blood [O2 half-saturation pressure of hemoglobin (P50) = 32.1 +/- 0.5 (SE) Torr] or blood from dogs that had been fed sodium cyanate (150 mg.kg-1.day-1) for 3-4 wk (P50 = 23.2 +/- 0.9). In both conditions (n = 8) arterial PO2 was set at approximately 200 Torr to fully saturate arterial blood, which thereby produced the same arterial O2 contents, and muscle blood flow was set at 106 ml.100 g-1.min-1, so that O2 delivery in both conditions was the same. VO2max was 11.8 +/- 1.0 ml.min-1.100 g-1 when perfused with the normal blood (control) and was reduced by 17% to 9.8 +/- 0.7 ml.min-1.100 g-1 when perfused with the low-P50 blood (P less than 0.01). Mean muscle effluent venous PO2 was also significantly less (26 +/- 3 vs. 30 +/- 2 Torr; P less than 0.01) in the low-P50 condition, as was an estimate of the capillary driving pressure for O2 diffusion, the mean capillary PO2 (45 +/- 3 vs. 51 +/- 2 Torr). However, the estimated muscle O2 diffusing capacity was not different between conditions.(ABSTRACT TRUNCATED AT 250 WORDS)     

Morphometry, ultrastructure, myosin isoforms, and metabolic capacities of the "mini muscles" favoured by selection for high activity in house mice

Prolonged selective breeding of mice (Mus musculus) for high levels of voluntary wheel running has favoured an unusual phenotype ("mini muscles"), apparently caused by a single Mendelian recessive allele, in which most hind-limb muscles are markedly reduced in mass, but have increased mass-specific activities of mitochondrial enzymes. We examined whether these changes reflect changes in fibre size, number or ultrastructure in normal and "mini-muscle" mice within the two (of four) selectively bred lines (lab designations L3 and L6) that exhibit the phenotype at generations 26 and 27. In both lines, the gastrocnemius and plantaris muscles are smaller in mass (by >50% and 20%, respectively) in affected individuals. The mass-specific activities of mitochondrial enzymes in the gastrocnemius and plantaris muscles were increased in the mini phenotype in both lines, with stronger effects in the gastrocnemius muscle. In the gastrocnemius, the % myosin heavy chain (MHC) IIb was reduced by 50% in L3 and by 30% in L6, whereas the % MHC IIa and I were higher, particularly in L3. Fibre number in the plantaris muscle did not significantly differ between mini and normal muscles, although muscle mass was a significant positive correlate of fibre number. Small fibres were more abundant in mini than normal muscles in L3. Mitochondrial volume density was significantly higher in mini than normal muscle fibres in L3, but not in L6. Microscopy revealed a surprising attribute of the mini muscles: an abundance of small, minimally differentiated, myofibril-containing cells positioned in a disorderly fashion, particularly in the surface layer. We hypothesise that these unusual cells may be satellite cells or type IIb fibres that did not complete their differentiation. Together, these observations suggest that mice with the mini phenotype have reduced numbers of type IIb fibres in many of their hind-limb muscles, leading to a decrease in mass and an increase in mass-specific aerobic capacity in muscles that typically have a high proportion of type IIb fibres. Moreover, the several statistically significant interactions between muscle phenotype and line indicate that the effect of the underlying allele is altered by genetic background.