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J Appl Physiol 98: 1639-1645, 2005. First published January 13, 2005; doi:10.1152/japplphysiol.00874.2004
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Effect of contractile duration on intracellular PO2 kinetics in Xenopus single skeletal myocytes

Casey A. Kindig, Richard A. Howlett, and Michael C. Hogan

Department of Medicine, Physiology Division, University of California-San Diego, La Jolla, California

Submitted 12 August 2004 ; accepted in final form 8 January 2005


   ABSTRACT
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 ABSTRACT
METHODS
 RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
It has been suggested that skeletal muscle O2 uptake (O2) kinetics follow a first-order control model. Consistent with that, O2 should show both 1) similar onset kinetics and 2) an on-off symmetry across submaximal work intensities regardless of the metabolic perturbation. To date, consensus on this issue has not been reached in whole body studies due to numerous confounding factors associated with O2 availability and fiber-type recruitment. To test whether single myocytes demonstrate similar intracellular PO2 (PIO2) on- and off-transient kinetics at varying work intensities, we studied Xenopus laevis single myocyte (n = 8) PIO2 via phosphorescence quenching during two bouts of electrically induced isometric muscle contractions of 200 (low)- and 400 (high)-ms contraction duration (1 contraction every 4 s, 15 min between trials, order randomized). The fall in PIO2, which is inversely proportional to the net increase in O2, was significantly greater (P < 0.05) during the high (24.1 ± 3.2 Torr) vs. low (17.4 ± 1.6 Torr) contraction bout. However, the mean response time (MRT; time to 63% of the overall change) for the fall in PIO2 from resting baseline to end contractions was not different (high, 77.8 ± 11.5 vs. low, 76.1 ± 13.6 s; P > 0.05) between trials. The initial rate of change at contraction onset, defined as {Delta}PIO2/MRT, was significantly greater (P < 0.05) in high compared with low. PIO2 off-transient MRT from the end of the contraction bout to initial baseline was unchanged (high, 83.3 ± 18.3 vs. low, 80.4 ± 21.6 s; P > 0.05) between high and low trials. These data revealed that PIO2 dynamics in frog isolated skeletal myocytes were invariant despite differing contraction durations and, by inference, metabolic demands. Thus these findings demonstrate that mitochondria can respond more rapidly at the initial onset of contractions when challenged with an augmented metabolic stimulus in accordance with an apparent first-order rate law.

oxygen consumption; muscle energetics; skeletal muscle fiber

AT THE ONSET OF MODERATE-INTENSITY constant-load whole body exercise, after a short cardiodynamic phase (time delay) pulmonary O2 uptake (O2) increases in monoexponential fashion. For work performed above the lactate threshold, O2 onset kinetics become more complex due to an additional, delayed "slow component," which occurs 2–3 min after exercise onset (for review, see Ref. 13). Controversy exists as to whether the speed [assessed via primary component time constant or mean response time (MRT)] of the primary O2 response is slowed as work intensity increases, particularly during heavy- and severe-intensity exercise. Indeed, although some investigations have reported slowed O2 kinetics as work rate increases (6, 12, 14, 24, 27, 29, 32, 38), others have shown unaltered O2 and/or phosphocreatine (PCr) onset kinetic values (2–4, 7, 34, 37, 39). Pioneering work proposed that muscle respiration at the transition to an altered metabolic demand is under first-order control, suggesting that respiration is rate-limited by a single step or process, possibly the creatine kinase reaction (33, 34). First-order control requires a similar time course for on- and off-transient responses to step changes in metabolism and a time course independent of ATP demand; thus deviation from invariant time constants as work rate increases would suggest the absence of first-order respiratory control. Given the equivocal O2 kinetics described above, to date, how muscle respiration is governed in the non-steady state has yet to be resolved.

There are many factors that may confound interpretation of O2 kinetics, which are independent of skeletal muscle metabolic control. Clearly, the onset of the O2 slow component, which occurs during high-intensity work, slows the overall O2 on-transient response (30, 47). Mechanisms responsible for the additional O2 cost associated with the O2 slow component have yet to be resolved (13), although recent research suggests that it may be related to muscle fiber (number and/or type) recruitment (5, 37, 40, 43). A second confounding factor, likely incurred at the onset of the heavier work domains, is a recruitment of glycolytic fibers necessary to sustain the additional work output that may slow initial kinetics given significantly less mitochondrial volume (and oxidative enzymes) and a lower phosphorus-to-O2 ratio in glycolytic muscle (4, 10). Finally, Hughson and Morrissey (24) have suggested that O2 availability may become limiting at the transition to higher intensity workloads and thus constrain the speed of the rise in O2. Thus it is apparent that whole body or whole muscle determination of O2 may not always be appropriate for making metabolic control inferences at the cellular level.

Confounding variables associated with mode of activity, fiber type, O2 availability, and a myriad of other factors cloud the metabolic control inferences from O2 kinetics data collected during whole body or whole muscle exercise. In the present study, we utilized the Xenopus laevis isolated single muscle cell preparation to investigate the effect of different contraction durations on the speed of the cellular metabolic response at the onset of contractions and also the speed of the recovery after contractions under highly controlled conditions. These isolated muscle cells lack myoglobin, and thus the fall in intracellular PO2 (PIO2) is proportional to the rise in O2 (22). Using phosphorescence quenching techniques, we studied PIO2 before, during, and after contraction bouts at two differing contractile durations. Assuming first-order control of metabolism (33, 34), we tested two general hypotheses: 1) the speed of the PIO2 kinetics would be invariant across the differing contractile durations and 2) PIO2 on- and off-kinetics would be similar.


   METHODS
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 ABSTRACT
 METHODS
RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
Female adult X. laevis were used in this investigation. All procedures were approved by the University of California-San Diego animal use and care committee and conform to National Institutes of Health standards.

Myocyte preparation.   Single muscle cells (n = 8) were isolated and prepared as described previously (19). Briefly, frogs were doubly pithed, and the lumbrical muscles (II–IV) were removed from the hind feet. Single myocytes were dissected with tendons intact in a chamber of physiological Ringer solution. Cells were microinjected with a solution consisting of 0.5 mM Pd-meso-tetra(4-carboxyphenyl)porphine bound to bovine serum albumin (for phosphorescence quenching as described below) and 10 mM fura 2 (Molecular Probes, Eugene, OR) by micropipette pressure injection (PV830 pneumatic picopump, World Precision Instruments, Sarasota, FL). The fura 2 was injected for direct visual confirmation of cell injection at an excitation light of 390 nm. After microinjection, cells were given a minimum of 1 h of recovery.

Experimental protocol.   Platinum clips were attached to the tendons of each myocyte to facilitate fiber positioning within the Ringer solution-filled chamber. One tendon was fixed, whereas the contralateral end was attached to an adjustable force transducer (model 400A, Aurora Scientific, Aurora, Ontario, Canada), allowing the muscle to be set at optimum muscle length (i.e., length at which maximal tetanic force is produced). The analog signal from the force transducer was recorded via a data acquisition system (AcqKnowledge, Biopac Systems, Santa Barbara, CA) for subsequent analysis. Fibers were perfused throughout the experiment with Ringer solution previously equilibrated with 5% CO2 and 4% O2 in N2 balance. Constant perfusion was maintained throughout the protocol to maintain the extracellular PO2 of ~30 Torr and to reduce the occurrence of an appreciable unstirred layer surrounding the cell. Tetanic isometric contractions were elicited using direct (8–10 V) stimulation of the muscle (model S48, Grass Instruments, Warwick, RI). The stimulation protocol consisted of either 200 [low (L)]- or 400 [high (H)]-ms trains of 70-Hz impulses of 1-ms duration. Myocytes were subjected to trials of ~150–180 s at a 0.25-Hz stimulation frequency with a 15-min recovery period between trials. The order of contraction duration trials was randomized between myocytes.

Assessment of PIO2.   Each myocyte was observed with a Nikon x40 fluor objective (0.70 numerical aperture). The phosphorescence quenching of the porphyrin compound within the myocyte was measured via a system consisting of a flash lamp (Oxygen Enterprises, Philadelphia, PA), a 425-nm band-pass excitation filter, a 630-nm cut-on emission filter, and a photomultiplier tube for collection of the phosphorescence signal. To calculate phosphorescence lifetimes from the intracellular O2 probe, the phosphorescent decay curves from a series of 10 flashes (15 Hz) were averaged, and a monoexponential function was fit to the subsequent best-fit decay curve (analysis software from Medical Systems, Greenvale, NY). O2 dependence of phosphorescence quenching is described by the Stern-Volmer equation where:

thus

where {tau}o and {tau} are the phosphorescence lifetimes at anoxia and a given PO2, respectively, and kq, the quenching constant (in Torr/s), is a second-order rate constant that is related to the frequency of collisions between O2 and the excited triplet state of the porphyrin and the probability of energy transfer when collisions occur. The constants kq and {tau}o were set at 690 Torr/s and 100 µs for Pd-meso-tetra(4-carboxyphenyl)porphine bound to albumin in solution for this preparation, as established previously (19). Phosphorescent decay curves were recorded every 4 s from each cell throughout the experimental period.

Data and statistical analysis.   After experimental procedures, MRT was calculated as the time to 63% of both the fall in PIO2 with contractions (on) and PIO2 recovery after contractions (off). Data are presented as means ± SE. Differences between trials in regard to the fall in PIO2 were tested via a paired t-test. Peak tension changes and on- and off-kinetics were tested via a repeated-measures two-way ANOVA. When significant F values were present, the Student-Newman-Keuls post hoc test was employed for determination of within-group differences. Data were regressed linearly using standard least-squares techniques. Additionally, y-intercept and linear slope comparisons were made based on t distributions (9). Statistical significance was accepted at P < 0.05.


   RESULTS
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DISCUSSION
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 REFERENCES
 
Peak tension did not fall significantly (P > 0.05) over the duration of the L trial (Figs. 1 and 2). In contrast, peak tension was significantly reduced (P < 0.05) at the midpoint and thereafter in the H trial (Fig. 2). The mean on- and off-transient PIO2 profiles for all fibers (n = 8) in response to the two contraction bouts are shown in Fig. 3. The fall in PIO2 was ~30% greater (P < 0.05) during the H (24.1 ± 3.2 Torr) compared with the L (17.4 ± 1.6 Torr) contraction bout (Fig. 4). Despite the greater change in PIO2 in the H compared with the L trial, the MRT of the fall in PIO2 from resting baseline to end-contractions was not different (H, 77.8 ± 11.5 vs. L, 76.1 ± 13.6 s; P > 0.05) between trials (Fig. 5). Given the unchanged MRT in the face of a greater fall in PIO2 with contractions in the H vs. L trial, the initial rate of change defined as {Delta}PIO2/MRT was significantly greater (P < 0.05) in H compared with L. In addition, the PIO2 recovery MRT values from the end of the contraction bout to initial baseline were similarly unchanged (H, 83.3 ± 18.3 vs. L, 80.4 ± 21.6 s; Fig. 5). No significant differences (all P > 0.05) between on- and off-transient PIO2 MRT values were observed either between or within the L and H bouts (Fig. 5). As shown in Fig. 6, the on- and off-transient MRT values were correlated in a positive, linear fashion (r = 0.74; slope = 1.02, P < 0.05).



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  		Fig. 1. Tension profiles for one representative myocyte. Resting and active tension for a representative cell subjected to two 3-min isometric contraction bouts with 15 min of recovery between trials.

 


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  		Fig. 2. Effect of contraction duration on the fall in peak tension. Data (means ± SE) for 8 myocytes subjected to 2 tetanic contraction bouts of either 200- or 400-ms contraction duration. Mean peak tension is normalized to initial values. There was no significant fall in tension during the 200-ms contraction bout; however, peak tension was reduced significantly (*P < 0.05) at the midpoint and thereafter compared with initial values for the 400-ms contraction bout.

 


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  		Fig. 3. Intracellular PO2 profiles at differing contraction durations. Intracellular PO2 (PIO2) profiles (means ± SE) for Xenopus muscle cells (n = 8) subjected to 2 contraction bouts. A: on-transient kinetics. B: off-kinetics.

 


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  		Fig. 4. Effect of contraction duration on the net PIO2 fall with contractions. Fall in PIO2 for 8 myocytes (means ± SE) subjected to 2 contraction bouts. PIO2 fall was significantly greater (*P < 0.05) during the high (400 ms) bout compared with the low (200 ms) bout, indicative of a proportionately greater increase in O2 consumption.

 


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  		Fig. 5. Mean response times (means ± SE; n = 8) for both the on- and off-transient PIO2 response are invariant to alterations in contraction duration. No significant differences (all P > 0.05) existed between 200- and 400-ms duration bouts for either the on- or off-kinetics. Furthermore, the on-transient mean response time was not significantly different (P > 0.05) from the off-transient time between the 2 contraction bouts.

 


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  		Fig. 6. On- and off-transient PIO2 mean response times are symmetrical regardless of contraction duration. Mean response time of the PIO2 fall (ON)/recovery (OFF) was significantly correlated (P < 0.05) in a positive linear relationship. The regression line (i.e., y-intercept and slope) did not differ significantly from the line of identity.

 

   DISCUSSION
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
This is the first study to investigate PIO2 on- and off-transient kinetics in contracting single muscle cells. The key original findings are 1) increasing contraction duration resulted in a significantly greater fall in PIO2, 2) the MRT of the fall in PIO2 at contractions onset was not affected by contraction duration, 3) the MRT of the recovery in PIO2 at the completion of the contraction bout was similar to the MRT of the fall at contractions onset, and 4) the initial rate of the PIO2 fall at contraction onset was greater in the H than in the L trial. These data suggest that Xenopus myocyte PIO2 dynamics exhibit first-order control and thereby imply that whole muscle O2 kinetics are regulated in a similar manner. Thus these findings support the notion that mitochondria can respond more rapidly at the initial onset of contractions when faced with augmented metabolic stimuli in accordance with an apparent first-order rate law.

Single-myocyte model.   The relationship between O2 and PIO2 for single myocytes lacking myoglobin, such as in Xenopus muscle, is described by Fick's law of diffusion as:

where DO2 is the muscle O2 diffusion constant and PO2 mito and PEO2 represent mitochondrial and extracellular PO2, respectively. Assuming little or no gradient between cytosolic and mitochondrial PO2, the difference between PEO2 and PIO2 is proportional to the net increase in O2 (22).

In the present investigation, the myocytes were stimulated electrically to induce isometric contractions. Thus these cells did not perform work per se. However, this method is capable of eliciting maximal O2 values in excess of 300 ml·kg–1·min–1 in the single muscle cell (26, 46). Therefore, the different contraction durations used in the present study are a reasonable surrogate of the metabolic stresses that might occur during shifts in muscle work in vivo.

The data reported in the present investigation demonstrate that PIO2 fell an additional ~30% in the H compared with L trial (Fig. 4). This fall would therefore be consistent with only a ~30% greater net O2 flux and O2 increase in the H trial despite a doubling of the contraction duration. This indicates, consistent with that reported previously in dog muscle (21), that there is a higher ATP cost for repeated contractions (increased frequency) due to the energetic cost of actomyosin ATPases and sarcoplasmic reticulum Ca2+ ATPases compared with the significantly smaller ATP requirement necessary to sustain active tension (increased duration) in the present study.

On-off symmetry and implications for a first-order metabolic control model.   Systems exhibiting first-order control dynamics are expected to manifest on-off symmetry. This should hold true as long as the work rate remains submaximal. Meyer (34) demonstrated, during electrically induced contractions of rat skeletal muscle, that the PCr concentration monoexponential time constants were similar at the onset and during recovery of contractions at three differing stimulation rates. In addition, several investigations have reported on-off symmetry of O2 kinetics at moderate exercise (12, 30, 37, 38). However, exceptions to this symmetry do exist, particularly when kinetic parameters in response to work performed above the lactate threshold are compared (4, 28, 38, 41). Most recently, two carefully controlled investigations out of the same laboratory reported on-off symmetry for cycle ergometry (37) and asymmetry for knee extensor exercise (41). In these studies, it is likely that variables incapable of being controlled influenced data acquisition and thus the interpretation of metabolic control. Although one reason for this disparity may be associated with the advent of the "slow component," Cunningham et al. (11) demonstrated that O2 off-transient kinetics are independent of the magnitude of contribution of O2 slow component during heavy exercise.

The results of the present study, which avoid many of the confounding issues discussed above, demonstrate that the MRT of the PIO2 fall in response to contractions and the MRT of PIO2 recovery at the cessation of contractions were similar at both the L and H contraction bouts (Fig. 5). This implies, therefore, that oxidative phosphorylation is rate limited by a single step abiding by an apparent first-order rate law. The question of what controls the rate of respiration remains conjectural. Putative mechanisms include kinetic limitation by cytoplasmic ADP concentration (and/or Pi) in accordance with Michaelis-Menten kinetics (8), nonequilibrium thermodynamic control via the phosphorylation potential (i.e., ATP/ADP/Pi concentration) and electron transport chain redox potential (i.e., NADH/NAD concentration) (1), alterations in Gibbs free energy of cytoplasmic ATP hydrolysis (35), and increases in cytosolic and/or intramitochondrial Ca2+ concentration (18). Moreover, Walsh et al. (47) demonstrated recently that PCr concentration and the PCr/creatine ratio (and, by inference, creatine) may play an important role in the regulation of mitochondrial ADP-stimulated respiration. Regardless of the actual mechanism of respiratory control, the present study supports the notion that the muscle metabolic response to an elevation in metabolic demand follows first-order control.

In the presenet investigation, there was an ~20–25% fall in peak isometric tension in the H compared with L trial. Unfortunately, it is difficult to avoid this confounding variable yet study single-myocyte metabolic function at two differing work intensities. One might expect this altered tension profile to affect the on-off kinetics by either altering the amplitude of the fall in PIO2 or by slowing the PIO2 recovery kinetics due to a greater breakdown in PCr and possibly higher levels of ADP. However, as reported, there were no differences in the on-off PIO2 kinetics across H and L trials. Given that the duration of the contractions bouts was relatively short (150–180 s), this loss of tension likely had little or no effect on the PIO2 results. Indeed, there was no correlation (r = 0.18) between the on-to-off PIO2 kinetics ratio and the amount of tension loss from initial values in the H group. Certainly, this may not have been the case had the amount of tension loss been exacerbated by a more rigorous contraction protocol.

On-kinetics.   To date, a single rate-limiting step has yet to be determined for the initial lag in O2 seen at exercise onset (45). It has become evident, based on work intensity, that a combination of cellular metabolic state (i.e., phosphorylation potential and redox state), enzyme activation state, and O2 availability are likely responsible for "setting" the initial rate increase in oxidative phosphorylation. The most widely manipulated variable has been O2 availability. Although several investigations have demonstrated that alterations in the fraction of inspired O2 tension will alter O2 kinetics in an O2-dependent manner (12, 31, 32), other investigations have not demonstrated differences (23, 30). In this regard, it is unclear whether the muscle cell is actually being subjected to a large variation in O2 availability (i.e., PIO2), since blood flow to working skeletal muscle is controlled in a manner that acts to maintain constant O2 delivery (20, 42). Furthermore, in a series of investigations by Grassi et al. (15–17) using an isolated muscle preparation, acute manipulations in O2 delivery/availability did not affect O2 on-kinetics during lower intensity exercise (15, 16), but increasing O2 delivery did speed O2 kinetics in response to maximal exercise (17).

The above suggests that the intensity at which aerobic work is being performed likely plays an important role in setting the O2 on-kinetic response. A large literature exists regarding the effect of work intensity on O2 kinetics. A number of studies demonstrate that the speed of the primary O2 amplitude rise is unchanged as work intensity increases (2, 3, 7, 34, 37, 39, 41). To the contrary, several investigations have reported increased primary amplitude time constants for the O2 on-transient in response to heavy compared with moderate exercise (6, 12, 14, 24, 27, 29, 32, 38). The former is consistent with first-order respiratory control. However, data interpretation and metabolic control inferences have proven tenuous given the numerous confounding variables already discussed.

It has been suggested by Hughson and Morrissey (24) that a constrained O2 delivery-to-O2 utilization ratio may be responsible for slowed O2 kinetics during heavy-intensity exercise. This has led to the view that priming of the O2 kinetic response (i.e., speeding of the primary amplitude tau) seen during supralactate threshold work is due to augmented O2 availability (14, 44). Indeed, "priming" exercise has been shown to only be effective in speeding the O2 on-transient response above the lactate threshold (e.g., Ref. 14). However, this fails to address why, in several other investigations as discussed above, O2 kinetics are unaltered across work rate domains. In the present investigation, O2 availability was controlled between trials and was set at a level that has been demonstrated to not influence the initial metabolic response to contractions, at least in isolated frog fibers (25). That may explain, in part, why PIO2 on-transient kinetics (i.e., MRTs) were not different as contraction duration was increased (Fig. 5).

Mitochondrial activation.   Because the present data were obtained in single muscle cells, and thus additional fiber recruitment at the immediate onset could not alter the volume of mitochondrial recruitment, these data demonstrate that, in accordance with a greater initial impulse (i.e., larger signal for augmented respiration) with increased contractile duration, the available mitochondria were able to respond more rapidly to the higher contractile intensity. Thus MRT was invariant between L and H bouts. This is illustrated in Fig. 7 in which mean PIO2 fell more rapidly over the initial 30 s of contractions during the H compared with L trial. This is important in that it suggests that, in whole muscle, the capacity of intact muscle to respond more rapidly to an elevated ATP demand (i.e., an invariant MRT in the face of a greater O2 or PCr amplitude) may be due, in part, to a more rapid mitochondrial response rather than muscle fiber activation level. As with the control of steady-state oxidative phosphorylation, the factors (i.e., ADP concentration, Pi, Ca2+, etc.) that may cause the initial stimulation of mitochondrial respiration remain obscure. However, our results clearly demonstrate that, with a stronger signal stimulating mitochondria resulting from increased metabolic stress, the initial mitochondrial activation in a single cell can be augmented.



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  		Fig. 7. Initial fall in PIO2 is more rapid with increased contraction duration. Normalized (from initial resting values of ~30 Torr) PIO2 (means without SE bars for clarity) are shown over the first ~30 s after contraction onset.

 
Off-kinetics.   In the present study, as shown in Fig. 5, the PIO2 off-transient MRTs were similar between the two contraction bouts. In general, it has been accepted that the O2 off-transient response for moderate exercise follows a similar time course of recovery as that seen during the on-transient (30). Interestingly, during work performed above the lactate threshold, which results in the O2 slow component, the off-kinetics are often faster than on-kinetics, i.e., the time course of recovery follows closely that of the primary on-transient component (14, 28, 30, 38). Indeed, one investigation by Ozyener and colleagues (37) reported similar pulmonary O2 off-kinetics for the primary phase across work domains spanning from moderate to severe in human subjects, as had been shown previously in horses (28). However, as discussed below, there are some reports (e.g., Ref. 12) of on-off symmetry in concert with a slowing of the on-transient O2 response as work rate was increased from moderate to heavy exercise. In addition, Rossiter et al. (41) demonstrated similar O2 off-kinetics and similar on-kinetics at both moderate and heavy work intensities, although the off-kinetics were significantly slower than the on-kinetics

One variable potentially responsible for disparate results is the manner of how off-kinetics have been characterized (i.e., how the actual data has been modeled). Indeed, several investigations have utilized different off-kinetic modeling based on the mode and/or domain of exercise. In particular, Ozyener and colleagues (37) modeled O2 off-transient kinetics using a single monoexponential fit for moderate and heavy exercise yet utilized a more complex fit for very heavy and severe domain exercise. However, Cunningham et al. (11) demonstrated that O2 off-transient kinetics are independent of the magnitude of contribution of O2 slow component during heavy exercise.

In summary, in the present investigation, we have demonstrated that, despite increasing metabolic rate via an increase in contraction duration in single skeletal myocytes, there were no differences in the PIO2 on- and off-transient kinetics between conditions and that the PIO2 on- and off-kinetics were symmetric. These invariant MRTs, despite different metabolic rates, suggest a more rapid mitochondrial activation in response to the larger initial metabolic signal, consistent with first-order kinetics, and suggest that Xenopus isolated single skeletal muscle PIO2 kinetics exhibit first-order control. Therefore, these data offer further evidence that whole muscle O2 kinetics are under a similar first-order control.


   GRANTS
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 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 GRANTS
ACKNOWLEDGMENTS
 REFERENCES
 
This work was supported, in part, by National Institute of Arthritis and Musculoskeletal and Skin Diseases Grants AR-40155 (M. C. Hogan) and 1 F32 AR-48461 (C. A. Kindig). R. A. Howlett and C. A. Kindig are Parker B. Francis pulmonary fellows.


   ACKNOWLEDGMENTS
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We thank Dr. Brandon Walsh for helpful comments regarding metabolic control.


   FOOTNOTES
 

Address for reprint requests and other correspondence: R. A. Howlett, Univ. of California-San Diego, Dept. of Medicine, Physiology Division, 9500 Gilman Dr., MC0623A, La Jolla, CA 92093-0623 (E-mail: rhowlett{at}ucsd.edu)

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.


   REFERENCES
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  1. Balaban RS. Regulation of oxidative phosphorylation in the mammalian cell. Am J Physiol Cell Physiol 258: C377–C389, 1990.
  2. Barstow TJ and Mole PA. Linear and nonlinear characteristics of oxygen uptake kinetics during heavy exercise. J Appl Physiol 71: 2099–2106, 1991.
  3. Barstow TJ, Casaburi R, and Wasserman K. O2 uptake kinetics and the O2 deficit as related to exercise intensity and blood lactate. J Appl Physiol 75: 755–762, 1993.
  4. Barstow TJ, Jones AM, Nguyen PH, and Casaburi R. Influence of muscle fiber type and pedal frequency on oxygen uptake kinetics of heavy exercise. J Appl Physiol 81: 1642–1650, 1996.
  5. Burnley M, Doust JH, Ball D, and Jones AM. Effects of prior heavy exercise on O2 kinetics during heavy exercise are related to changes in muscle activity. J Appl Physiol 93: 167–174, 2002.
  6. Carter H, Pringle JSM, Jones AM, and Doust JH. Oxygen uptake kinetics during treadmill running across exercise intensity domains. Eur J Appl Physiol 86: 347–354, 2002.
  7. Casaburi R, Barstow TJ, Robinson T, and Wasserman K. Influence of work rate on ventilatory and gas exchange kinetics. J Appl Physiol 67: 547–555, 1989.
  8. Chance B and Williams GR. Respiratory enzymes in oxidative phosphorylation. I. Kinetics of oxygen utilization. J Biol Chem 217: 383–393, 1955.
  9. Colton T. Statistics in Medicine. Boston, MA: Little Brown, 1974, p. 189–217.
  10. Crow MT and Kushmerick MJ. Chemical energetics of slow- and fast-twitch muscles of the mouse. J Gen Physiol 79: 147–166, 1982.
  11. Cunningham DA, St. Croix CM, Paterson DH, Ozyener F, and Whipp BJ. The off-transient pulmonary oxygen uptake (O2) kinetics following attainment of a particular O2 during heavy-intensity exercise in humans. Exp Physiol 85: 339–347, 2000.
  12. Engelen M, Porszasz J, Riley M, Wasserman K, Maehara K, and Barstow TJ. Effects of hypoxic hypoxia on O2 uptake and heart rate kinetics during heavy exercise. J Appl Physiol 81: 2500–2508, 1996.
  13. Gaesser GA and Poole DC. The slow component of oxygen uptake kinetics in humans. In: Exercise and Sport Sciences Reviews, edited by Holloszy JO. Baltimore, MD: Williams & Wilkins, 1996, vol. 24, p. 35–71.
  14. Gerbino A, Ward SA, and Whipp BJ. Effects of prior exercise on pulmonary gas-exchange kinetics during high-intensity exercise in humans. J Appl Physiol 80: 99–107, 1996.
  15. Grassi B, Gladden LB, Samaha M, Stary CM, and Hogan MC. Faster adjustment of O2 delivery does not affect O2 on-kinetics in isolated in situ canine muscle. J Appl Physiol 85: 1394–1403, 1998.
  16. Grassi B, Gladden LB, Stary CM, Wagner PD, and Hogan MC. Peripheral O2 diffusion does not affect O2 on-kinetics in isolated in situ canine muscle. J Appl Physiol 85: 1404–1412, 1998.
  17. Grassi B, Hogan MC, Kelley KM, Aschenbach WG, Hamann JJ, Evans RK, Patillo RE, and Gladden LB. Role of convective O2 delivery in determining O2 on-kinetics in canine muscle contracting at peak O2. J Appl Physiol 89: 1293–1301, 2000.
  18. Hansford RG. Role of calcium in respiratory control. Med Sci Sports Exerc 26: 44–51, 1994.
  19. Hogan MC. Phosphorescence quenching method for measurement of intracellular PO2 in isolated skeletal muscle fibers. J Appl Physiol 86: 720–724, 1999.
  20. Hogan MC, Roca J, Wagner PD, and West JB. Limitation of maximal O2 uptake and performance by acute hypoxia in dog muscle in situ. J Appl Physiol 65: 815–821, 1988.
  21. Hogan MC, Ingham E, and Kurdak SS. Contraction duration affects metabolic energy cost and fatigue in skeletal muscle. Am J Physiol Endocrinol Metab 274: E397–E402, 1998.
  22. Howlett RA and Hogan MC. Intracellular PO2 decreases with increasing stimulation frequency in contracting single Xenopus muscle fibers. J Appl Physiol 91: 632–636, 2001.
  23. Hughson RL and Kowalchuk JM. Kinetics of oxygen uptake for submaximal exercise in hyperoxia, normoxia and hypoxia. Can J Appl Physiol 20: 199–211, 1995.
  24. Hughson RL and Morrissey MA. Delayed kinetics of respiratory gas exchange in the transition from prior exercise. J Appl Physiol 52: 921–929, 1982.
  25. Kindig CA, Howlett RA, and Hogan MC. Effect of extracellular PO2 on the fall in intracellular PO2 in contracting single myocytes. J Appl Physiol 94: 1964–1970, 2003.
  26. Kindig CA, Kelley KM, Howlett RA, Stary SM, and Hogan MC. Assessment of O2 SM uptake dynamics in isolated single skeletal myocytes. J Appl Physiol 94: 353–357, 2003.
  27. Koga S, Shiojiri T, Shibasaki M, Kondo N, Fukuba Y, and Barstow TJ. Kinetics of oxygen uptake during supine and upright heavy exercise. J Appl Physiol 87: 253–260, 1999.
  28. Langsetmo IH and Poole DC. O2 recovery kinetics in the horse following moderate, heavy and severe exercise. J Appl Physiol 86: 1170–1177, 1999.
  29. Langsetmo I, Weigle GE, Fedde MR, Erickson HH, Barstow TJ, and Poole DC. O2 kinetics in the horse during moderate and heavy exercise. J Appl Physiol 83: 1235–1241, 1997.
  30. Linnarson D. Dynamics of pulmonary gas exchange and heart rate changes at start and end of exercise. Acta Physiol Scand Suppl 415: 1–68, 1974.
  31. Linnarsson D, Karlsson J, Fagraeus L, and Saltin B. Muscle metabolites and oxygen deficit with exercise in hypoxia and hyperoxia. J Appl Physiol 36: 399–402, 1974.
  32. MacDonald M, Pedersen PK, and Hughson RL. Acceleration of O2 kinetics in heavy submaximal exercise by hyperoxia and prior to high-intensity exercise. J Appl Physiol 83: 1318–1325, 1997.
  33. Mahler M. First-order kinetics of muscle oxygen consumption, and an equivalent proportionality between O2 and phosphorylcreatine level. Implications for the control of respiration. J Gen Physiol 86: 135–165, 1985.
  34. Meyer RA. A linear model of muscle respiration explains monoexponential phosphocreatine changes. Am J Physiol Cell Physiol 254: C548–C553, 1988.
  35. Meyer RA and Foley JM. Cellular processes integrating the metabolic response to exercsise. In: Handbook of Physiology. Exercise: Regulation and Integration of Multiple Systems. Bethesda, MD: Am. Physiol. Soc., 1996, sect. 12, chapt. 18, p. 841–869.
  36. Nagesser AS, van der Laarse WJ, and Elzinga G. Metabolic changes with fatigue in different types of single muscle fibers of Xenopus laevis. J Physiol 448: 511–523, 1992.
  37. Ozyener F, Rossiter HB, Ward SA, and Whipp BJ. Influence of exercise intensity on the on- and off-transient kinetics of pulmonary oxygen uptake in humans. J Physiol 533: 891–902, 2001.
  38. Paterson DH and Whipp BJ. Asymmetries of oxygen uptake transients at the on- and offset of heavy exercise in humans. J Physiol 443: 575–586, 1991.
  39. Piiper J, di Prampero PE, and Cerretelli P. Oxygen debt and high-energy phosphates in gastrocnemius muscle of the dog. Am J Physiol 215: 523–531, 1968.
  40. Poole DC, Barstow TJ, Gaesser GA, Willis WT, and Whipp BJ. O2 slow component: physiological and functional significance. Med Sci Sports Exerc 26: 1354–1358, 1994.
  41. Rossiter HB, Ward SA, Kowalchuk JM, Howe FA, Griffiths JR, and Whipp BJ. Dynamic asymmetry of phosphocreatine concentration and O2 uptake between the on- and off-transients of moderate and high-intensity exercise in humans. J Physiol 541: 991–1002, 2002.
  42. Rowell LB, Saltin B, Kiens B, and Christensen NJ. Is peak quadriceps flow in humans even higher during exercise with hypoxemia? Am J Physiol Heart Circ Physiol 251: H1038–H1044, 1986.
  43. Shinohara M and Moritani T. Increase in neuromuscular activity and oxygen uptake during heavy exercise. Ann Physiol Anthropol 11: 257–262, 1992.
  44. Tordi N, Perrey S, Harvey A, and Hughson RL. Oxygen uptake kinetics during two bouts of heavy cycling separated by fatiguing sprint exercise in humans. J Appl Physiol 94: 533–541, 2003.
  45. Tschakovsky ME and Hughson RL. Interaction of factors determining oxygen uptake at the onset of exercise. J Appl Physiol 86: 1101–1113, 1999.
  46. Van der Laarse WJ, Diegenbach PC, and Elzinga G. Maximum rate of oxygen consumption and quantitative histochemistry of succinate dehydrogenase in single muscle fibres of Xenopus laevis. J Muscle Res Cell Motil 10: 221–228, 1989.
  47. Walsh B, Tonkonogi M, Soderlund K, Hultman E, Saks V, and Sahlin K. Role of phosphorylcreatine and creatine in the regulation of mitochondrial respiration in human skeletal muscle. J Physiol 537: 971–978, 2001.
  48. Whipp BJ and Wasserman K. Oxygen uptake kinetics for various intensities of constant-load work. J Appl Physiol 33: 351–356, 1972.



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