| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
1 Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas, TX 75309-9034, USA
2 Department of Kinesiology, Texas A & M University, College Station, TX, USA
3 Departments of Anatomy & Physiology and Kinesiology, Kansas State University, Manhattan, KS 66506-5802, USA
 |
Abstract |
|---|
 Top Abstract IntroductionMethodsResultsDiscussionReferences |
|---|
, increased microvascular blood–muscle O2 flux is the product of both augmented O2 delivery 
and fractional O2 extraction. Whole body and exercising limb measurements demonstrate that 
and fractional O2 extraction increase as linear and hyperbolic functions, respectively, of 
. Given the presence of disparate vascular control mechanisms among different muscle fibre types, we tested the hypothesis that, in response to muscle contractions, 
would be lower and fractional O2 extraction (as assessed via microvascular O2 pressure, PmvO2) higher in fast- versus slow-twitch muscles. Radiolabelled microsphere and phosphorescence quenching techniques were used to measure 
and PmvO2, respectively at rest and across the transition to 1 Hz twitch contractions at low (Lo, 2.5 V) and high intensities (Hi, 4.5 V) in rat (n
= 20) soleus (Sol, slow-twitch, type I), mixed gastrocnemius (MG, fast-twitch, type IIa) and white gastrocnemius (WG, fast-twitch, type IIb) muscle. At rest and for Lo and Hi (steady-state values) transitions, PmvO2 was lower (all P < 0.05) in MG (mmHg: rest, 22.5 ± 1.0; Lo, 15.3 ± 1.3; Hi, 10.2 ± 1.6) and WG (mmHg: rest, 19.0 ± 1.3; Lo, 12.2 ± 1.1; Hi, 9.9 ± 1.1) than in Sol (rest, 33.1 ± 3.2 mmHg; Lo, 19.0 ± 2.3 mmHg; Hi, 18.7 ± 1.8 mmHg), despite lower 
and 
in MG and WG under each set of conditions. These data suggest that during submaximal metabolic rates, the relationship between 
and O2 extraction is dependent on fibre type (at least in the muscles studied herein), such that muscles comprised of fast-twitch fibres display a greater fractional O2 extraction (i.e. lower PmvO2) than their slow-twitch counterparts. These results also indicate that the greater sustained PmvO2 in Sol may be important for ensuring high blood–myocyte O2 flux and therefore a greater oxidative contribution to energetic requirements.
(Received 19 November 2004;
accepted after revision 5 January 2005;
first published online 6 January 2005)
Corresponding author D. C. Poole: Department of Anatomy and Physiology, College of Veterinary Medicine, Kansas State University, Manhattan, KS 66506-5802, USA. Email: poole{at}vet.k-state.edu
|
Introduction |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
and metabolic rate 
is strong and has been well characterized for both the whole body and the exercising limbs. In the exercising human, leg 
and 
are linearly related by the equation 
, where S represents the slope (5.3) and I the intercept (2.8 l min–1; data from Knight et al. 1992). This equation stipulates that fractional O2 extraction (or arteriovenous O2 difference, 
) must increase as a hyperbolic function of 
according to the approximation equation, 
(Knight et al. 1992; Poole, 1997; see Whipp & Ward, 1982 for original derivation of this equation). However, human leg muscles are comprised of a complex mosaic of type I and type II fibres and, to date, methodological limitations have precluded determination of whether the 
(or O2 delivery, 
)-to-
relationship differs between the different muscle fibre types. This information is important because the 
-to-
ratio determines the microvascular O2 pressure (PmvO2), which in turn facilitates blood–tissue O2 flux and thus strongly impacts cellular metabolic regulation.
Free of the methodological limitations noted above, animal studies indicate that during both rest and exercise there exists a pronounced stratification of 
across different muscles and muscle fibre types (e.g. Armstrong & Laughlin, 1983; Poole et al. 2000; reviewed by Laughlin et al. 1997). This heterogeneity of 
probably arises, in part, from differential specific control of arteriolar vasodilation for different fibre types. For example, arterioles from slow-twitch (type I) fibres have a greater endothelial nitric oxide synthase (eNOS) mRNA expression and a higher sensitivity and maximal responsiveness to endothelium-dependent vasodilation than their fast-twitch counterparts (Delp et al. 2000; Wunsch et al. 2000; Woodman et al. 2001). In addition, first-order arterioles isolated from slow-twitch muscles demonstrate a reduced sensitivity (i.e. less vasoconstriction) to noradrenaline (Delp, 1999) compared to their fast-twitch counterparts. Moreover, in contrast to fast-twitch muscle, virtually no contraction-induced ‘sympatholysis’ (Thomas et al. 1994) is present in the slow-twitch soleus. Based on the augmented contraction-induced hyperaemia noted in the slow-twitch soleus versus the fast-twitch white and mixed gastrocnemius (reviewed by Laughlin et al. 1997), it is generally to be expected that 
(and thus 
/
) will be higher in muscles comprised of slow-twitch fibres. Indeed, we have demonstrated, during moderate intensity contractions, that 
is elevated relative to 
in slow (soleus) compared with fast-twitch (peroneal) muscle (Behnke et al. 2003; McDonough et al. 2004). It has also been shown that exercise intensity exerts a profound influence upon muscle 
(Armstrong & Laughlin, 1983). Of particular interest are the findings that both muscle fibre type and oxidative capacity strongly impact both the intensity-dependent pattern of 
distribution (Armstrong & Laughlin, 1983) and 
at maximal exercise (Poole et al. 2000).
In light of the above findings, the present investigation was designed to determine the relationships among 
and PmvO2 across a broad range of metabolic demands (rest, and low and high stimulation intensities) in muscles selected specifically to span the range of fibre types present in mammalian muscle (soleus, mixed gastrocnemius and white gastrocnemius). Given the heterogeneity in 
between these muscles, we wished to test the specific hypothesis that PmvO2 would fall to lower levels during contractions in fast-twitch (type II, mixed and white gastrocnemius) compared to slow-twitch muscles, with the greatest effect manifest in the white gastrocnemius (type IIb) muscle. In addition, we hypothesized that the kinetics of PmvO2, across the rest–contraction transient, would be significantly faster (indicative of poorer 
-to-
matching) in both fast-twitch gastrocnemius muscles compared with the slow-twitch soleus.
|
Methods |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
Surgical preparation
All rats (female Sprague-Dawley, n = 20, wt = 317 ± 10 g) were anaesthetized prior to experimentation with pentobarbitone sodium (40 mg kg–1
I.P. to effect). In order to ensure that an adequate anaesthetic plane was maintained throughout the experiment, ocular and pedal reflexes were tested. If indicated, pentobarbitone was administered in a supplemental dosage (5–10 mg kg–1) as needed. The carotid and tail (caudal) arteries were catheterized, with the use of an introducer, with polyethylene tubing (PE-10 connected to PE-50). This allowed for the infusion of the phosphorescent probe (palladium meso-tetra (4-carboxyphenyl) porphine dendrimer (R2); 15 mg kg–1), measurement of arterial blood pressure (Digi-Med BPA model 200, Louisville, KY, USA) and withdrawal of arterial blood for blood gas measurement (Nova Stat Profile M, Waltham, MA, USA). In addition, the catheters allowed us to measure muscle blood flow 
via the radio-labelled microsphere technique.
The muscles chosen for the present experiment (soleus, Sol; mixed gastrocnemius, MG; and white gastrocnemius, WG) were chosen on the basis of their fibre type composition and oxidative enzyme activities (Delp & Duan, 1996). The Sol is a postural muscle whose primary functions are plantar flexion and ankle stabilization. Soleus is composed primarily of slow-twitch fibres (84% type I, 7% type IIa and 9% type IId/x) and has a citrate synthase activity (CSa; the marker of oxidative capacity used herein) of 21.3 µmol min–1 g–1. The MG is a powerful muscle of plantar flexion and, as such, has a primarily fast-twitch fibre composition (3% type I, 6% type IIa, 34% type IId/x and 57% type IIb), with a CSa of 25.7 µmol min–1 g–1. The WG is a plantar flexor of low oxidative capacity (CSa, 8.1 µmol min–1 g–1) and has a fibre type composition that is purely fast-twitch in nature (8% type IId/x, 92% type IIb; Delp & Duan, 1996). Each muscle was exposed for PmvO2 measurements in the following manner. For experiments on the MG and WG, the leg was shaved and the skin and fascia overlying the ‘calf’ was opened in the sagittal plane. After the skin was opened, the tibial nerve was isolated and a stimulating electrode was attached. The ground electrode was attached distally, near the Achilles tendon. For experiments on the Sol, the skin and fascia overlying the peroneal muscle group were opened (coronal plane). After these were opened, the peroneal muscle was reflected so as to facilitate access to the Sol (Behnke et al. 2003). Care was taken to minimize the extent of the surgery in all cases. The exposed tissue was superfused with a Krebs–Henseleit bicarbonate-buffered solution (38°C, equilibrated with 5% CO2–N2 balance) and body temperature was maintained at 
38°C by use of a heating pad.
Contractions protocol
Prior to beginning the experimental protocol, the R2 probe was introduced into the blood via the arterial catheter. The rat was then moved to a purpose-built ergometer and secured so as to limit movement, but not impede ventilation or cardiodynamics. To limit leg movement to the plantar flexors, the ankle was pinned to a wooden support, such that the ankle was fixed (same position each time). The phosphorimeter light guide was then positioned (within 1–3 mm) above the belly of the muscle of interest. Approximately 15 min later, the Sol, MG or WG was stimulated at 1 Hz for 3 min (2 ms pulse duration) using a Grass S88 stimulator (Quincy, MA, USA) at two different contraction intensities, 2.5 V (Lo) and 4.5 V (Hi). These contraction intensities were chosen on the basis of an incremental contraction test (+1 V min–1) and correspond to approximately 30 and 65% of the stimulation voltage that produced a minimal PmvO2 (P. McDonough, B. J. Behnke, T. I. Musch & D. C. Poole; unpublished observations). These stimulation voltages were chosen simply to span a relatively large intensity range and could be taken to resemble, in certain respects, low and moderately heavy intensity exercise. At the conclusion of the experimental protocol all animals were killed with an overdose of pentobarbitone sodium (>80 mg kg–1).
Muscle blood flow and oxygen consumption
Muscle blood flow 
and oxygen consumption 
were measured in a separate set of experiments as previously described (Behnke et al. 2002a, 2003). Briefly, 
was determined using the radiolabelled microsphere technique (Musch & Terrell, 1992) and was measured at rest and just before the cessation of the 3 min contractions protocol in all three muscles and expressed as milliliters of blood per minute per 100 g tissue (ml min–1 (100 g)–1). Three different radiolabelled, 15 µm diameter microspheres (46Sc, 85Sr and 141Ce; New England Nuclear, Boston, MA, USA) were agitated via sonication and 2.5 x 105 microspheres were injected into the ascending aorta at the specified time point. Tissue radiation counts were performed using a gamma scintillation counter (Packard Auto Gamma Spectrometer, Cobra model 5003, Perkin-Elmer Life and Analytical Sciences, Shelton, CT, USA). The correct placement of the carotid catheter in the aorta was verified after the experiment, while adequate mixing of microspheres was verified via inspection of kidney blood flows, with a difference <15% between R and L kidney being considered acceptable.
Muscle 
was determined in the following fashion. Arterial O2 content (CaO2) was measured directly (carotid arterial blood) and mixed venous O2 content 
was calculated from PmvO2 (since PmvO2 is a valid approximation of mixed venous partial pressure of O2 (PO2; McDonough et al. 2001) using the rat O2 dissociation curve (constructed using an n value (Hill coefficient) of 2.6, the measured [Hb], P50 (the PO2 at which Hb is 50% Saturated with O2) of 38 and an O2 carrying capacity of 1.39 ml O2 (g Hb)–1). 
was then calculated via the principle of mass balance using the Fick equation (i.e. 
). Muscle O2 diffusion index 
was defined as 
/PmvO2 and, as such, provides an index of diffusive O2 transport per unit of driving pressure.
Principle and measurement of phosphorescence quenching
The principles of phosphorescence quenching have been detailed previously (Poole et al. 1995; McDonough et al. 2001; Behnke et al. 2003). Specifically, the Stern-Volmer relationship (Rumsey et al. 1988) describes the nature of the quantitative relationship between probe phosphorescence and PmvO2:
|
|
|
|
For the R2 probe, t0 is 601 µs and kQ 409 mmHg s–1 and, under the physiological conditions studied herein (Lo et al. 1997), PmvO2 is wholly dependent upon the lifetime of the phosphorescence decay, which is inversely proportional to the prevailing PO2.
Importantly, R2 is restricted to the intravascular space within the muscle, which allows measurement of microvascular O2 tension (PmvO2; (Poole et al. 2004)). A Frequency Domain Phosphorimeter (PMOD 1000; Oxygen Enterprises, Ltd, Philadelphia, PA, USA) was employed, with the light guide focused on a circular area of exposed muscle of 2 mm diameter. Within this area (principally composed of capillary blood, since this compartment constitutes the majority of intramuscular blood volume; Poole et al. 1995), a sample is obtained up to 500 µm deep. The PMOD 1000 modulates the excitation frequencies between 100 Hz and 20 kHz, which can measure PmvO2 values from 0 to 240 mmHg. PmvO2 was measured continuously, with data reported at 2 s intervals throughout.
Citrate synthase measurement
Citrate synthase was measured in duplicate from homogenates prepared from the Sol, MG and WG muscles according to the methodology of Srere (1969). Citrate synthase activity (CSa) was measured using a spectrophotometer (Spectramax 190 Molecular devices plate reader) in 300 µl aliquots at 30°C. Activity levels were expressed as µmol per minute per gram wet weight.
Curve fitting and statistical analysis
For the PmvO2 data, curve fitting was accomplished using KaleidaGraph software (version 3.5; Synergy Software, Reading, PA, USA) and was performed on each data set using a one-component model:
|
|
|
|
1 and 2 are the amplitudes of the fast and slow PmvO2 components, TD1 and TD2 are the independent time delays and 
1 and 2 are the time constants for each component. Goodness of fit was determined by three criteria: (1) the coefficient of determination (i.e. r2); (2) the sum of the squared residuals; and (3) visual inspection and analysis of the residual fit to a linear model. Mean response time (MRT) was calculated using the formula from MacDonald et al. (1997):
|
|
1 and 2, TD1 and TD2 and 1 and 2 are as defined above and tot
=
1
+
2. If a one-component model provided the best fit, the MRT equation reduced to the following:
|
|
PmvO2/ for the on-transient to contractions (Behnke et al. 2003).
PmvO2 values during resting and steady-state contractions (e.g. baseline and ), as well as modelling dependent results (e.g. TD, and MRT), were analysed using standard analysis of variance techniques between muscles (Sol, MG and WG). When a significant F value was demonstrated by the ANOVA, a Student–Newman–Keuls (SNK) post hoc test was performed to determine differences among mean values. Pearson product–moment correlations were performed upon selected variables. Statistical significance was accepted at P
0.05.
|
Results |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
Lo-contraction protocol (Lo).
At rest, microvascular O2 pressure (PmvO2) values were highest in the Sol, intermediate in MG and lowest in WG (Table 1). The nadir in PmvO2 was significantly lower in both MG and WG than in Sol. In addition, both MG and WG frequently undershot the final steady-state PmvO2 during the transition (Table 1 and Fig. 1A). This undershoot of final steady-state PmvO2 occurred in 50% of the MG and WG preparations, but never occurred in Sol (Table 1). In terms of dynamics, for both MG and WG, the PmvO2 mean response time (MRT) was significantly faster compared to Sol (Table 1). This faster MRT was due to a significantly shorter time constant () and/or time delay (TD) in MG and WG compared to Sol (Table 1). However, the PmvO2 was significantly greater in Sol compared to both MG and WG (Table 1), such that the relative rate of PmvO2 fall (dPO2/dt, equal to dPmvO2/) was not different between muscles (Table 1).
|
|
(Table 1).
Lo- versus Hi-contraction protocol.
While baseline PmvO2 was not different between experimental conditions, both the nadir PmvO2 and the steady-state PmvO2 were significantly lower in Hi versus Lo for MG only (Table 1). However, MRT was significantly shorter for all three muscles in Hi compared to Lo (Table 1). Moreover, while PmvO2 was significantly greater in Hi versus Lo for MG and WG, PmvO2 was significantly less in Hi versus Lo for Sol (Table 1). Since was significantly shorter for MG and WG in Hi versus Lo, the greater PmvO2 resulted in a significantly greater dPO2/dt in these two muscles in Hi versus Lo conditions, which was not the case for Sol (Table 1).
Muscle blood flow and oxygen delivery
Lo-contraction protocol.
Muscle blood flow 
was significantly greater in Sol than in either MG or WG at rest and during contractions, while 
was greater only at rest in WG compared to MG (Fig. 2; all P < 0.05). In the absence of altered CaO2, these differences were reflected in oxygen delivery (
; Fig. 3: theoretical schema; Fig. 4: actual data), as 
was greater in Sol versus MG and WG at rest (5.8 ± 1.4 versus 0.7 ± 0.1 and 1.3 ± 0.2 ml min–1 (100 g)–1; Sol versus MG and WG) and during contractions (12.3 ± 3.0 versus 7.2 ± 1.2 and 8.1 ± 1.9 ml min–1 (100 g)–1; Sol versus MG and WG; all P < 0.05). Again, as for 
, at rest, 
was higher in WG than in MG (P < 0.05). The increase of 
(Fig. 2) and 
from rest to Lo was significant in all three muscles (P < 0.05).
|
|
|
was significantly elevated in Sol compared to both MG and WG during contractions. As regards 
, Sol was elevated versus both MG and WG during contractions (16.1 ± 2.6 versus 9.6 ± 1.4 and 9.6 ± 1.6 ml min–1 (100 g)–1; Sol versus MG and WG). As in Lo, 
(Fig. 2) and 
were increased from rest in all muscles (Fig. 2; P < 0.05).
Lo- versus Hi-contraction protocol.
Both 
and 
increased from Lo to Hi (Fig. 2) for all muscles, while 
and 
were significantly greater in Sol compared to both MG and WG for both Lo and Hi (Fig. 2).
Muscle oxygen consumption
Lo-contraction protocol.
Muscle oxygen consumption 
was significantly higher for Sol versus MG and WG at rest (3.1 ± 0.4 versus 0.5 ± 0.02 and 1.1 ± 0.03 ml O2 min–1 (100 g)–1; Sol versus MG and WG) and during contractions (10.2 ± 0.6 versus 6.4 ± 0.2 and 7.6 ± 0.1 ml O2 min–1 (100 g)–1; Sol versus MG and WG; Fig. 5; all P < 0.05). However, in contrast to the situation for 
and 
was significantly higher in WG compared to MG during contractions (P < 0.05).
|
was significantly higher for Sol versus both MG and WG during contractions (13.5 ± 0.5 versus 9.0 ± 0.2 and 9.2 ± 0.1 ml O2 min–1 (100 g)–1; Sol versus MG and WG; Fig. 5; all P < 0.05). However, in contrast to the situation at rest and during the Lo-contraction protocol, 
was not different between WG and MG during Hi (Fig. 5).
Lo- versus Hi-contraction protocol.
was significantly elevated in Hi versus Lo in all muscles (Fig. 5). This increase in 
was due to simultaneous (albeit of differing magnitude) increases in both 
and 
(Fig. 4).
Muscle diffusion index
Lo-contraction protocol.
At rest, muscle diffusion index 
was greater in Sol than in WG and both were greater than in MG (Table 2). This difference was eradicated during the contraction period, when 
was not different between muscles during Lo (Table 2 and Fig. 4). In all three muscles, 
was increased from rest to Lo.
|
was increased from rest (Table 2). In addition, 
was greater in both MG and WG (which were not different) compared to Sol (Table 2).
Lo- versus Hi-contraction protocol.
was increased from Lo to Hi in both fast-twitch muscles (MG and WG). However, 
was not different in Sol between Lo and Hi (Fig. 4).
Citrate synthase activity
Citrate synthase (CSa) was significantly greater in Sol (24.3 ± 0.5 µmol min–1 g–1) than either MG (17.9 ± 3.2 µmol min–1 g–1) or WG (11.0 ± 0.7 µmol min–1 g–1). In addition, CSa was significantly greater in MG than in WG (P < 0.05).
|
Discussion |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
was lower. Thus, these fast-twitch muscles demonstrated a greater fractional O2 extraction, a stratagem that necessitates a more pronounced increase in 
, in addition to an increased ATP contribution from non-aerobic sources (i.e. phosphocreatine (PCr) hydrolysis and glycolysis; Wilson, 1994). (2) PmvO2 kinetics at the onset of contractions were more rapid (i.e. reduced MRT, Table 2) for MG and WG than for Sol. Moreover, there was a significant transient undershoot of PmvO2 below the steady-state response in both MG and WG that was absent in Sol, an observation previously noted only in the pathological conditions of diabetes (Behnke et al. 2002b) and heart failure (Diederich et al. 2002; Behnke et al. 2004) for the mixed fibre type spinotrapezius muscle. In total, these findings suggest that the slow-twitch Sol, as compared to the fast-twitch MG and WG, is better able to defend PmvO2 (and thus the O2 driving pressure into the myocyte) and adapt (i.e. make adjustments in oxidative metabolism) to metabolic transients in a manner which minimizes the reliance upon non-aerobic energy sources and, thus, the reduction in cellular energy state. Effects of contraction intensity on PmvO2
The present investigation is the first study to investigate the effect of muscular contractions of different intensity upon PmvO2 dynamics in skeletal muscle. The results indicate that a higher contraction intensity (i.e. Hi versus Lo) results in: (1) a further lowering of PmvO2 in fast- (MG and WG) but not in slow-twitch muscle (Sol); and (2) a speeding of MRT in all muscles. In this regard, Sol appears to have the ability to adjust to an increase in energetic demands with little (rest-to-Lo transition) or no increase (Lo-to-Hi transition) in fractional O2 extraction. Thus, Sol exhibits a proportionally larger increase in 
, while the two fast-twitch muscle portions demonstrate a relatively modest 
response and therefore mandate a greater increase in fractional O2 extraction (i.e. 
; Fig. 4). Furthermore, since PmvO2 was significantly different between muscle fibre types during both contraction protocols (Table 1), the relative rate of fall in PmvO2 (dPO2/dt) was compared in an effort to normalize the speed with which O2 extraction increases among muscles. When compared in this fashion, the different strategies by which slow- and fast-twitch muscles adapt to an increase in energetic demands are clearly evident. Specifically, while dPO2/dt was the same for both Lo and Hi for Sol, dPO2/dt was significantly larger in the Hi protocol for both MG and WG compared to Lo (Table 1). Thus, these findings indicate that the slow-twitch Sol adjusted to an increase in energetic demand with little (or no) additional reduction in PmvO2 and a less pronounced speeding of PmvO2 kinetics. Oppositely, the two fast-twitch muscles (WG and MG) relied upon rapid and large changes in fractional O2 extraction (i.e. fast PmvO2 kinetics and an increased 
).
While the present study is the first to describe the changes in PmvO2 and its kinetics with an increase in contraction intensity in vivo, Kindig et al. (2003b), using an isolated single fibre preparation, noted that extracellular PO2 (PeO2; essentially a static external O2 source) fell faster and to progressively lower values in fast- versus slow-twitch Xenopus laevis muscle as stimulation intensity was increased. In addition, ‘peak’
was 20% higher in the slow-twitch fibres. Furthermore, they noted that a reduction in the initial PeO2 (again loosely analogous to 
) progressively reduced the MRT for the fall in PeO2 (Kindig et al. 2003a). Thus, the findings of Kindig et al. (2003a,b) are consistent with those of the present study where we demonstrate a decreased extracellular O2 pressure-head in fast-twitch muscle that is accentuated with increased contraction intensity. A reduction in this critical O2 driving pressure (PmvO2 herein) would be expected to reduce 
according to Fick's law of diffusion (Wagner et al. 1991) and consequently stimulate substrate level metabolism (i.e. greater PCr degradation and increased glycogenolysis/glycolysis) to a much greater degree (Wilson et al. 1977) in fast- (MG and WG) versus slow-twitch muscles (Sol). Additionally, the results of both the present study and those of Kindig et al. (2003a,b) show that this effect will be exacerbated with an increase in contraction intensity in fast-, but not slow-twitch muscle.
Basis for, and consequences of, differing PmvO2 responses among muscles
It is becoming increasingly apparent that the interrelationship between muscle 
and 
in the steady state and across the rest–contractions transient is complex and, at best, only partly understood. At present, our understanding of the control of 
is perhaps less contentious than that of 
. Specifically, steady-state 
is a function of basal O2 requirements plus any additional work/tension-induced O2 demands, which are principally determined by the myofilament cross-bridge activity and muscle ‘efficiency’ (reviewed by Kushmerick, 1983; Poole, 1997), while 
kinetics in healthy muscle are considered to be determined primarily by what has been termed muscle oxidative enzyme inertia (reviewed by Grassi, 2000). With respect to vascular control, there exist myriad mechanisms that include mechanical (muscle pump), neural (sympathetic), propagated (conducted vasodilation) and vasomotor control (e.g. endothelium dependent and independent), which are thought to contribute to the hyperaemia of exercise and increased 
(reviewed by Thomas & Segal, 2004). As mentioned in the Introduction, Sol is endowed with a superior ability to undergo a more rapid and greater contraction-induced hyperaemia than MG or WG (Delp et al. 2000; Wunsch et al. 2000; Woodman et al. 2001). The present investigation illustrates the functional consequences of this elevated hyperaemic response (i.e. increased PmvO2 and reduced reliance upon O2 extraction) and is in agreement with research wherein an increased 
during contractions led to a better maintenance of cellular energy state and reduced fatigue (Hogan et al. 1992; Haseler et al. 1998).
Irrespective of PmvO2 kinetics, PmvO2 and therefore the pressure driving O2 diffusion into the myocyte, is uniformly lower across the rest-to-exercise transition in fast- versus slow-twitch locomotory muscle (Behnke et al. 2003; present study). This would impair O2 flux in MG and WG, insofar as that flux is determined by the capillary-to-myocyte PO2 gradient (Wagner, 1995). However, the capillary-to-myocyte PO2 gradient acts in concert with the diffusion characteristics 
of the muscle to facilitate capillary-to-myocyte O2 flux. 
is believed to be determined by: (1) the capillary haematocrit and capillary length per fibre volume, the product of which gives the number of red blood cells adjacent to a muscle fibre at a given time (Federspiel & Popel, 1986; Groebe & Thews, 1990); and (2) the size of the ‘functionally O2-carrier depleted region’, which will be reduced at lower intramyocyte PO2 values such that 
increases (Honig et al. 1997). Figure 4 demonstrates the different strategies by which PmvO2 and 
interact to generate the measured 
in fast- versus slow-twitch muscle and how these relationships change from Lo to Hi stimulation intensities. In the Sol, PmvO2 is relatively high and capillary-to-myocyte O2 flux increases from Lo to Hi mostly as a result of increased convective O2 delivery (see Sol, Fig. 4A). Alternatively, in MG and WG, if decreased PmvO2 reflects a reduced intracellular PO2 (and it likely does) the saturation state of myoglobin will be lower and O2 flux will be improved via an increased O2 conductance of myoglobin as the thickness of the functionally O2-carrier depleted region is decreased (i.e. enhanced 
, see MG and WG, Fig. 4B and C). Thus, large reductions in PmvO2 across the transient (particularly during Hi; see Figs 4A, B and C) lead to a greater reliance upon 
to meet O2 demand (MG and WG), while high convective O2 delivery obviated this requirement in Sol. This greater reliance upon increased 
and decreased PmvO2 (and therefore intracellular PO2), in concert with the finding that PmvO2 falls faster and to lower levels (sometimes decreasing transiently below steady-state levels) in fast- versus slow-twitch locomotory muscles (see Figs 1 and 6) may have profound functional consequences and may help explain several key physiological and pathophysiological observations. For example, as mentioned above, a reduced microvascular O2 pressure-head would act to impair both blood-to-myocyte and intracellular O2 flux. In turn, a low intracellular PO2 may serve to slow 
kinetics (Behnke et al. 2002a), thus elevating the O2 deficit and forcing a greater reliance upon substrate level phosphorylation. Ultimately, this will cause a greater perturbation (PCr, ADPfree, Pi and H+) of the intracellular milieu and accelerate glycogen depletion and the onset of fatigue (Wilson et al. 1977). It is possible, therefore, that the slowed 
kinetics and more pronounced intracellular perturbations found under conditions which predispose the individual towards recruitment of fast-twitch fibres (i.e. heavy/severe exercise (Whipp & Mahler, 1980; Barstow & Mole, 1991), diabetes (Behnke et al. 2002b) or heart failure (Diederich et al. 2002; Behnke et al. 2004)] may result, in part, from the altered PmvO2 profiles characteristic of this fibre type.
|
As mentioned above, with respect to muscle function, the matching of 
to 
as reflected by PmvO2 is extremely important because it sets the O2 pressure-head that drives blood-to-tissue O2 transfer. However, one limitation of the present investigation was the inability to follow the dynamics of 
in separatum at the onset of contractions and so only the resting and steady-state 
were measured. Consequently, we have refrained from making mechanistic interpretations regarding the 
profile beyond the fact that, to produce the observed profile of PmvO2 in fast- versus slow-twitch muscle (i.e. faster fall to a lower contracting value in fast-twitch muscle), 
dynamics must be slower and the dynamic range reduced compared to 
. When the required technology is developed, it would obviously be important to explore these issues further. In addition, since the phosphorimeter only covers a small area of the muscle being studied (see Methods), it could be argued that our results are only applicable to the specific section of muscle sampled. However, we have published several papers that demonstrate a remarkable between-animal consistency of PmvO2 (Behnke et al. 2001, 2003; Diederich et al. 2002; Geer et al. 2002; present study), which suggests that the PmvO2 values measured herein are indicative of an ‘average’
PmvO2 for that muscle. Lastly, the determinants of muscle diffusing capacity are complex. Structural variables, such as capillary length per fibre volume and capillary-to-fibre surface contact, in combination with functional indices of capillary haemodynamics (e.g. capillary red blood cell haematocrit, flux and velocity), which are considered important determinants of blood-to-myocyte O2 transfer (Federspiel & Popel, 1986; Groebe & Thews, 1990; Mathieu-Costello et al. 1991), could lend critical insight into the recruitment of O2 diffusing capacity during exercise transients. Unfortunately, these variables could not be resolved, due to technical limitations, in the present investigation.
Conclusions
The regulation of the 
-to-
relationship appears to be fundamentally different in the fast- versus slow-twitch muscles examined herein, with PmvO2 falling faster and to lower levels in fast-twitch muscles (Fig. 6). This finding is consistent with the lower exercise-induced hyperaemia during submaximal contractions reported in certain fast-twitch muscles and may help to explain the presence of slowed 
kinetics and greater metabolic perturbation found during physiological and pathophysiological conditions that recruit a greater fast-twitch fibre population.
|
References |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
Barstow
TJ
&
Mole
PA (1991). Linear and nonlinear characteristics of oxygen uptake kinetics during heavy exercise. J Appl Physiol
71, 2099–2106.
Behnke BJ, Barstow TJ, Kindig CA, McDonough P, Musch TI & Poole DC (2002a). Dynamics of oxygen uptake following exercise onset in rat skeletal muscle. Respir Physiol Neurobiol 133, 229–239.[CrossRef][Medline]
Behnke
BJ, Delp
MD, McDonough
P, Spier
SA, Poole
DC
&
Musch
TI (2004). Effects of chronic heart failure on microvascular oxygen exchange dynamics in muscles of contrasting fiber type. Cardiovasc Res
61, 325–332.
Behnke BJ, Kindig CA, McDonough P, Poole DC & Sexton WL (2002b). Dynamics of microvascular oxygen pressure during rest-contraction transition in skeletal muscle of diabetic rats. Am J Physiol 283, H926–H932.
Behnke BJ, Kindig CA, Musch TI, Koga S & Poole DC (2001). Dynamics of microvascular oxygen pressure across the rest-exercise transition in rat skeletal muscle. Respir Physiol 126, 53–63.[CrossRef][Medline]
Behnke BJ, McDonough P, Padilla DJ, Musch TI & Poole DC (2003). Oxygen exchange profile in muscles of contrasting fibre types. J Physiol 547, 597–605.
Delp
MD (1999). Myogenic and vasoconstrictor responsiveness of skeletal muscle arterioles is diminished by hindlimb unloading. J Appl Physiol
86, 1178–1184.
Delp MD, Colleran PN, Wilkerson MK, McCurdy MR & Muller-Delp J (2000). Structural and functional remodeling of skeletal muscle microvasculature is induced by simulated microgravity. Am J Physiol 278, H1866–H1873.
Delp
MD
&
Duan
C (1996). Composition and size of type I, IIA, IID/X and IIB fibers and citrate synthase activity of rat muscle. J Appl Physiol
80, 261–270.
Diederich
ER, Behnke
BJ, McDonough
P, Kindig
CA, Barstow
TJ
et al. (2002). Dynamics of microvascular oxygen partial pressure in contracting skeletal muscle of rats with chronic heart failure. Cardiovasc Res
56, 479–486.
Federspiel WJ & Popel AS (1986). A theoretical analysis of the effect of the particulate nature of blood on oxygen release in capillaries. Microvasc Res 32, 164–189.[CrossRef][Medline]
Geer
CM, Behnke
BJ, McDonough
P
&
Poole
DC (2002). Dynamics of microvascular oxygen pressure in the rat diaphragm. J Appl Physiol
93, 227–232.
Grassi B (2000). Skeletal muscle VO2 on-kinetics: set by O2 delivery or by O2 utilization? New insight into an old issue. Med Sci Sports Exerc 32, 108–116.
Groebe K & Thews G (1990). Calculated intra- and extracellular PO2 gradients in heavily working red muscle. Am J Physiol 259, H84–H92.[Medline]
Haseler
LJ, Richardson
RS, Videen
JS
&
Hogan
MC (1998). Phosphocreatine hydrolysis during submaximal exercise: the effect of Fio2. J Appl Physiol
85, 1457–1463.
Hogan
MC, Arthur
PG, Bebout
DE, Hochachka
PW
&
Wagner
PD (1992). Role of O2 in regulating tissue respiration in dog muscle working in situ. J Appl Physiol
73, 728–736.
Honig CR, Gayeski TEJ & Groebe K, ed. (1997). Myoglobin and Oxygen Gradients, vol. II. Lippincott-Raven, Philadelphia.
Kindig
CA, Howlett
RA
&
Hogan
MC (2003a). Effect of extracellular Po2 on the fall in intracellular Po2 in contracting single myocytes. J Appl Physiol
94, 1964–1970.
Kindig
CA, Kelley
KM, Howlett
RA, Stary
CM
&
Hogan
MC (2003b). Assessment of O2 uptake dynamics in isolated single skeletal myocytes. J Appl Physiol
94, 353–357.
Knight
DR, Poole
DC, Schaffartzik
W, Guy
HJ, Prediletto
R
et al. (1992). Relationship between body and leg VO2 during maximal cycle ergometry. J Appl Physiol
73, 1114–1121.
Kushmerick MJ (1983). Energetics of Muscle Contraction. American Physiological Society, Bethesda.
Laughlin MH, McAllister RM & Delp MD, ed. (1997). Heterogeneity of Blood Flow in Striated Muscle. Lippincott-Raven, Philadelphia.
Lo L-W, Vinogradov SA, Koch CJ & Wilson DF (1997). A new, water soluble, phosphor for oxygen measurements in vivo. Adv Exp Med Biol 411, 577–583.[Medline]
MacDonald
M, Pedersen
PK
&
Hughson
RL (1997). Acceleration of VO2 kinetics in heavy submaximal exercise by hyperoxia and prior high-intensity exercise. J Appl Physiol
83, 1318–1325.
Mathieu-Costello O, Ellis CG, Potter RF, MacDonald IC & Groom AC (1991). Muscle capillary-to-fiber perimeter ratio: morphometry. Am J Physiol 261, H1617–H1625.[Medline]
McDonough P, Behnke BJ, Kindig CA & Poole DC (2001). Rat muscle microvascular Po2 kinetics during the exercise off-transient. Exp Physiol 86, 349–356.[Abstract]
McDonough
P, Behnke
BJ, Musch
TI
&
Poole
DC (2004). Recovery of microvascular Po2 during the exercise off-transient in muscles of different fiber type. J Appl Physiol
96, 1039–1044.
Musch TI & Terrell JA (1992). Skeletal muscle blood flow abnormalities in rats with a chronic myocardial infarction: rest and exercise. Am J Physiol 262, H411–H419.[Medline]
Poole DC (1997). Influence of exercise training on skeletal muscle oxygen delivery and utilization. In The Lung: Scientific Foundations, ed. Crystal R G, West J B et al. E, pp. 1957–1967. Lippincott-Raven, Philadelphia.
Poole DC, Behnke BJ, McDonough P, McAllister RM & Wilson DF (2004). Measurement of muscle microvascular oxygen pressures: Compartmentalization of phosphorescent probe. Microcirc 11, 317–326.[CrossRef][Medline]
Poole
DC, Sexton
WL, Behnke
BJ, Ferguson
CS, Hageman
KS
&
Musch
TI (2000). Respiratory muscle blood flows during physiological and chemical hyperpnea in the rat. J Appl Physiol
88, 186–194.
Poole
DC, Wagner
PD
&
Wilson
DF (1995). Diaphragm microvascular plasma PO2 measured in vivo. J Appl Physiol
79, 2050–2057.
Rumsey
WL, Vanderkooi
JM
&
Wilson
DF (1988). Imaging of phosphorescence: a novel method for measuring oxygen distribution in perfused tissue. Science
241, 1649–1651.
Srere PA (1969). Citrate synthase. Meth Enzymol 13, 3–5.
Thomas
GD, Hansen
J
&
Victor
RG (1994). Inhibition of 
2-adrenergic vasoconstriction during contraction of glycolytic, not oxidative, rat hindlimb muscle. Am J Physiol
266, H920–H929.[Medline]
Thomas
GD
&
Segal
SS (2004). Neural control of muscle blood flow during exercise. J Appl Physiol
97, 731–738.
Wagner PD (1995). Muscle O2 transport and O2 dependent control of metabolism. Med Sci Sports Exerc 27, 47–53.
Wagner PD, Hoppeler H & Saltin B, ed. (1991). Determinants of Maximal Oxygen Uptake. Raven Press, Ltd, New York.
Whipp BJ & Mahler M, ed. (1980). Dynamics of Pulmonary Gas Exchange During Exercise, vol. II. Academic Press, New York.
Whipp
BJ
&
Ward
SA (1982). Cardiopulmonary coupling during exercise. J Exp Biol
100, 175–193.
Wilson DF (1994). Factors affecting the rate and energetics of mitochondrial oxidative phosphorylation. Med Sci Sports Exerc 26, 37–43.
Wilson DF, Erecinska M, Brown C & Silver IA (1977). Effect of oxygen tension on cellular energetics. Am J Physiol 233, C135–C140.[Medline]
Woodman
CR, Schrage
WG, Rush
JWE, Ray
CA, Price
EM
et al. (2001). Hindlimb unweighting decreases endothelium-dependent dilation and eNOS expression in soleus not gastrocnemius. J Appl Physiol
91, 1091–1098.
Wunsch SA, Muller-Delp J & Delp MD (2000). Time course of vasodilatory responses in skeletal muscle arterioles: role in hyperemia at onset of exercise. Am J Physiol 279, H1715–H1723.
|
Acknowledgements |
|---|
Author's present address
P. McDonough: Pulmonary & Critical Care Medicine, H8.130, University of Texas Southwestern Medical Center, Dallas, TX 75390-9034, USA. Email: paul.mcdonough{at}utsouthwestern.edu
This article has been cited by other articles:
|
|
 |
) for Sol, MG and WG at rest in response to the Lo- and Hi-contraction protocols 
denotes significant difference from Lo; 
denotes significantly greater 
compared to both fast-twitch muscles (P < 0.05).
difference) and diffusive (
) determinants of oxygen uptake 
and the y-intercept 
. These relationships permit analysis of O2 transport differences among fibre types (see 
can increase via an increase in 
alone, while the vertical distance a–c shows how an increase in 
and 
will impact 
. Finally, the line a–d demonstrates how an increase in 
alone will serve to increase 
.
for the Sol (see 
predominantly through an increase in 
with a relatively modest increase in 
. B, graphical representation of the same relationship in MG. Note how the MG relies to a much greater extent upon an increase in 
to increase 
. C, graphical representation of the above relationships for WG. Note that WG does not increase either 
or 
very much and so 
does not increase to nearly the extent demonstrated by MG and Sol.
) for Sol, MG and WG in response to both the Lo- and Hi-contraction protocols 
denotes significant difference from MG only (all P < 0.05).