Division of Physiology, Department of Medicine, University of California, San Diego, La Jolla, California 92093-0623
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ABSTRACT |
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It has been suggested that during a skeletal muscle contraction the metabolic energy cost at the onset may be greater than the energy cost related to holding steady-state force. The purpose of the present study was to investigate the effect of contraction duration on the metabolic energy cost and fatigue process in fully perfused contracting muscle in situ. Canine gastrocnemius muscle (n = 6) was isolated, and two contractile periods (3 min of isometric, tetanic contractions with 45-min rest between) were conducted by each muscle in a balanced order design. The two contractile periods had stimulation patterns that resulted in a 1:3 contraction-to-rest ratio, with the difference in the two contractile periods being in the duration of each contraction: short duration 0.25-s stimulation/0.75-s rest vs. long duration 1-s stimulation/3-s rest. These stimulation patterns resulted in the same total time of stimulation, number of stimulation pulses, and total time in contraction for each 3-min period. Muscle O2 uptake, the fall in developed force (fatigue), the O2 cost of developed force, and the estimated total energy cost (ATP utilization) of developed force were significantly greater (P < 0.05) with contractions of short duration. Lactate efflux from the working muscle and muscle lactate concentration were significantly greater with contractions of short duration, such that the calculated energy derived from glycolysis was three times greater in this condition. These results demonstrate that contraction duration can significantly affect both the aerobic and anaerobic metabolic energy cost and fatigue in contracting muscle. In addition, it is likely that the greater rate of fatigue with more rapid contractions was a result of elevated glycolytic production of lactic acid.
oxygen uptake; exercise; acid-base balance; adenosine 5'-triphosphate; lactate; lactic acid; mitochondrial respiration; phosphocreatine; glycolysis
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INTRODUCTION |
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IN MOST CONDITIONS of muscle contractile activity,
O2 uptake
(O2) and the total metabolic
energy cost of the working muscle are tightly coupled with the
developed force of the muscle (for example, see Ref. 17). However, it
has been suggested that anywhere from 20 to 50% (2, 3, 8, 19, 20, 27)
of the ATP used during a contraction may be used for ion transport
related to activation and relaxation, independent of the ATP hydrolysis by the actomyosin adenosinetriphosphatase (ATPase) necessary for force
generation. In addition, there is evidence (7, 13) that total ATPase
activity in a single contraction is greater at the onset of contraction
than the ATPase activity needed to subsequently maintain the
steady-state tetanic force that is developed. Although these processes
have been studied by thermodynamic methods (see Ref. 19), isolated
fibers and muscles in solution (3, 8, 10, 13, 23), there have been few
studies investigating these energetics using intact muscle in vivo or
in situ. It would follow then that repetitive contractions of short
duration may be more energy consuming than contractions of long
duration, if the total time of contraction remains constant. Fales et
al. (11) and Stainsby and Fales (26) demonstrated in working skeletal muscle in situ that
O2 is
lower per stimulus the longer an isometric contraction is maintained.
However, only single contractions of different durations were compared,
and anaerobic metabolism was not measured. Chastiotis et al. (4) and
Bergstrom and Hultman (2) demonstrated in electrically stimulated,
totally ischemic contracting human quadriceps a significantly greater
energy cost for a series of contractions of short duration compared
with long when total contraction times were kept equal. In examining
the energetics of terrestrial locomotion, Heglund et al. (14)
demonstrated that the metabolic cost of constant speed locomotion
increases as animals turn on and off their muscles at a faster rate.
These researchers (2, 4, 14) have suggested that short-duration contractions may require higher amounts of total energy due to the high
energy cost of ion pumping in each activation-contraction cycle. This
extra energy cost may become important to working muscle because a
greater energy cost may affect the ability of working muscle to
maintain a steady state of force development, as previously shown in
ischemic contracting muscle (2, 4).
Although these various studies (2, 4, 11, 14, 26) suggest that contraction duration may influence muscle energetics, there has been no information gathered from normally perfused contracting whole muscle concerning the total metabolic energy requirements of the activation-contraction cycle and the subsequent effects on steady-state muscle function. It was the purpose of the present study to use fully perfused contracting muscle in situ (in which both anaerobic and aerobic energy supply pathways could be assessed) to investigate the effect of two different contraction durations, while keeping the total contraction time equal, on metabolic energy cost and fatigue.
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METHODS |
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Six adult mongrel dogs of either sex with a weight range of 10-18 kg were anesthetized with 30 mg/kg pentobarbital sodium. Maintenance doses were given as required. The dogs were intubated with cuffed endotracheal tubes, and ventilation was maintained with a Harvard 613 ventilator at a rate that achieved normal values of PO2 and PCO2. Esophageal temperature was maintained near 37°C by the use of heating pads. The animals were given heparin at a dosage of 1,500 U/kg after the surgery.
Surgical preparation. The left gastrocnemius-flexor digitorum superficialis muscle complex (for convenience referred to as gastrocnemius) was isolated as described previously (18). Briefly, the muscle was isolated from nearby muscle groups, and all vessels draining into the popliteal vein except for those from the gastrocnemius were ligated to isolate the venous outflow from the gastrocnemius. The arterial circulation to the gastrocnemius was isolated by ligating all vessels from the femoral and popliteal artery that did not enter the gastrocnemius. The left popliteal vein was cannulated, and the venous outflow from the isolated muscle was returned to the animal via a jugular catheter.
The right femoral artery was catheterized for arterial blood sampling. This catheter was connected to the left femoral artery so that the isolated muscle was perfused by blood from this contralateral artery. Perfusion was accomplished either directly from the contralateral (systemic pressure, self perfused) or via a Sigmamotor pump to control flow. A pressure transducer in this line at the head of the muscle constantly monitored perfusion pressure. A carotid artery was also catheterized to monitor systemic blood pressure. The left sciatic nerve, which innervates the gastrocnemius, was doubly ligated and cut between ties. To prevent cooling and drying, all exposed tissues were covered with saline-soaked gauze and with a sheet of saran. After the muscle was surgically isolated, the Achilles tendon was attached to an isometric myograph to measure force development. The hindlimb was fixed at the knee and ankle and attached to the myograph with struts to minimize movement. Weights were used at the end of each experiment to calibrate the force myograph. The isometric force developed by each muscle was normalized to the weight of that muscle. Before each contraction period, the resting muscle was passively stretched until a tension setting of ~10 g force per gram muscle mass (estimated before the experiment) was recorded. This ensured that the initial tension development was not affected by slippage in the system that might have occurred during the prior contraction period. This resting muscle length was slightly less than the length at which the contractile response was greatest.Experimental protocol. Isometric muscle contractions (tetanic) were elicited by stimulation of the sciatic nerve with square-wave impulses (4-6 V) of 0.2-ms duration at a rate of 40 impulses/s. Each muscle (n = 6) was stimulated for two contraction periods (45-min rest between each period) in a balanced order design. Each of the two contractile periods had stimulation patterns that resulted in a 1:3 contraction-to-relaxation ratio, with the difference in the two contractile periods being in the contraction duration (short duration 0.25-s stimulation/0.75-s rest vs. long duration 1-s stimulation/3-s rest). The total time of stimulation, number of stimulation pulses, and total contraction time were the same for each 3-min contraction period.
Before the first contraction period the blood supply to the isolated muscle was switched from self perfusion to pump perfusion, and enough time was allowed for conditions to stabilize at a blood flow similar to the self-perfused level. All blood flows were measured from the venous outflow using a graduated cylinder to collect the blood for a 20-s time period. Muscle blood flow was set at a level that achieved a perfusion pressure of 120 mmHg during the first contraction period. The same blood flow was used in the subsequent contraction period so that muscle blood flow was matched for both conditions.Measurements.
Arterial blood samples from the arterial line entering the muscle and
venous samples from the left popliteal vein as close to the
gastrocnemius as possible were drawn anaerobically at the end of each
rest period and during the last few seconds of each of the two
stimulation periods. These samples were kept on ice for the brief time
before measurement. Venous blood flow measurements were made at the
same time the blood samples were drawn by timed blood collections into
a graduated cylinder. Barbee et al. (1), using muscle contractions
similar to those used in this investigation, determined that a near
steady-state flow and
O2 had been
achieved by the end of 2 min.
Statistics. Repeated measures analysis of variance was used for the statistical analysis. Values are expressed as means ± SE. In all statistical analyses, the 0.05 level of significance was used.
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RESULTS |
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Mean weight of the gastrocnemius muscles
(n = 6) removed after the end of the
experiment was 82 ± 3 (SE) g. The principal variables of
O2 transport, gas exchange, and
acid-base balance in the blood perfusing the muscles during the two
stimulation conditions are presented in Table
1. The arterial
PO2 (and arterial
O2 content) and muscle blood flow
were not significantly different between the two conditions, so that
O2 delivery (16.1 ± 0.8 ml · 100 g1 · min
1
during short-duration contractions vs. 16.8 ± 0.7 during long) to
the working muscle was also no different. Muscle perfusion pressure was
also no different between conditions (133 ± 6 mmHg during
short-duration contractions vs. 138 ± 7 during long). However, there was a significantly higher
O2 and
O2 extraction ratio (
O2/O2
delivery) during the short-contraction duration period (see Table 1).
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Table 2 lists the biochemical, force, and bioenergetic parameters for the two stimulation paradigms. There was no difference in the initial developed force between the two contraction periods (indicating complete recovery between treatments); however, force development during contractions of short duration fell to a lower level over the 3-min time period, indicating significantly greater fatigue, as shown in Fig. 1. Developed force fell to 70% of initial developed force during the short-duration contraction period vs. 91% during the long-duration contraction period. In addition to the measured changes in developed force, estimates of the tension-time integral were made from the chart recordings. The tension-time integral for a single contraction (averaged from 5 contractions) fell from the initial maximal value of 15.5 ± 0.5 g force/s to 10.9 ± 0.8 during the short-duration contraction period compared with the respective values of 55.0 ± 2.5 and 51.4 ± 1.9 during the long- duration contraction period. These values followed the same patterns as developed force, and the tension-time estimates indicated that the time for activation and relaxation for a single contraction was not different between the two stimulation patterns and that there were no significant changes in the time to achieve maximal force or relaxation rates as the fatigue process occurred in the two stimulation conditions. The measured values of developed force were therefore representative of a comparative index of isometric energy demand.
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As indicated in Table 2, intracellular levels of ATP did not change significantly from rest to work and were never different between the two conditions. Muscle [PCr] fell significantly during the short-contraction duration condition but was not significantly different from the values during the other contraction period. Therefore, muscle Pi concentration ([Pi]) (which increases proportionally with decreasing PCr) was also no different at the end of the two contractile periods. Muscle [lactate] increased significantly from rest to work during both contractile conditions and was significantly higher at the end of the short-duration contraction period compared with the long, as was lactate efflux. Calculated intracellular [H+] was also significantly higher during short- duration contractions compared with long.
The O2 cost for the developed force was significantly greater (39%), as shown in Table 2, during the contractions of short duration. The calculated ATP utilization rate was 32% greater (P < 0.05), and the ratio of ATP utilization to developed force was 70% (P < 0.05) greater during the short-duration contractions compared with long.
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DISCUSSION |
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This study demonstrated for the first time in fully aerobic,
contracting muscle that contractions of short duration resulted in
significantly higher O2,
O2/developed force,
calculated rate of ATP utilization, calculated energy cost (ATP
utilization/developed force), and fatigue compared with contractions of
long duration but with the same total contraction time. The estimated
energy cost, from calculations of both aerobic and anaerobic metabolic energy-generating pathways, to produce an equal amount of force per
time was 70% greater (P < 0.01) in
the contractions of short duration compared with long duration. In
addition, contractions of short duration resulted in significantly
higher intracellular [lactate], muscle lactate efflux, and
a threefold increase in glycolysis that likely influenced the faster
rate of fatigue.
Energy cost of contractions. The total energy cost of a muscle contraction reflects the summed ATPase activity of the muscle, which consists of actomyosin, Ca2+, and membrane ion transport ATPases. Although it has been well demonstrated that the energy cost of continuous muscular work is generally proportional to the amount of work being done, there have been relatively few studies examining the energy requirements of the nonactomyosin ATPases. Thermodynamic studies (measurements of activation and maintenance heat) have suggested that this energy cost may be 20-50% of the total energy cost (see Ref. 19). Because of the relatively small movement of membrane ions, it is thought that the vast majority of nonactomyosin ATPase activity is from the resequestering of Ca2+ by the sarcoplasmic reticulum Ca2+ pumps (19). In addition to the energy cost of ion pumping, there is evidence that the actomyosin ATPase rate may be higher at the onset of a single contraction (10, 13), with this ATPase rate falling as the tetanic contractile force is maintained. However, this higher rate at the onset of a contraction (labile heat production) becomes significantly reduced as single contractions follow each other in a continuous pattern (9).
With this in mind, it has been postulated that the frequency of the activation-contraction cycle can strongly influence the total energy requirement of the work being conducted. In comparisons of single contractions of different durations, Fales et al. (11) and Stainsby and Fales (26) demonstrated that O2 consumption is lower per stimulus in long isometric tetanic contractions vs. short. Chastiotis et al. (4) and Bergstrom and Hultman (2) demonstrated in electrically stimulated human quadriceps, contracting with complete blood flow restriction (ischemia), a greater energy cost for contractions of short duration vs. long when the total contraction time was the same. They (2, 4) suggested that this may have been due to the energy cost of activation and relaxation (the main component being Ca2+ pumping) being a significant percentage of the total energy cost. Because normal exercising muscle does not work in an anaerobic environment, the intent of the present study was to examine these questions concerning muscle energetics by using an in situ muscle preparation in which both the anaerobic and aerobic contribution to energy expenditure could be calculated. The fourfold difference between the time the muscle was in the contracted state in the present study (0.25 vs. 1 s) was the same as the fourfold difference used by Bergstrom and Hultman (2). We found that the energy cost (ATP utilization) for the contractions of short duration was significantly greater than that calculated for contractions of long duration, consistent with the findings of Bergstrom and Hultman (2). In addition, our present study shows a significantly greater contribution to ATP production from both aerobic (Fatigue.
One important finding from the present study was that the higher energy
cost of the short-duration contractile period resulted in a
significantly greater fall in developed force (to 70% of initial force
development over the 3-min contraction period, Fig. 1) compared with
the long-duration contractions (to 91% of initial). This occurred even
though O2 was significantly
greater in the short-duration contractile period. The factors that
reduce force development over time (fatigue) when stimulation patterns
remain constant in contracting muscle are complex (30, 32). The
relative contribution of these factors to the fatigue process is highly dependent on the conditions of the work (intensity, duration, etc.).
Typically, fatigue during relatively high-intensity work results from
an imbalance between the processes of ATP demand and ATP production. In
the present study, the higher energy cost of the short-duration
contractions resulted in both higher rates of oxidative phosphorylation
and glycolysis (a threefold increase as estimated by the higher
intracellular [lactate] and lactate efflux) in an attempt
to accommodate the higher ATP demand. The modulator of the reduced
force generation under the conditions of short-duration contractions
was likely related to the altered intracellular environment resulting
from the higher energy demand. Increased levels of
H+ and
Pi have been implicated (31, 33)
in the etiology of high-intensity contractile fatigue (by interfering
with Ca2+ release,
Ca2+ binding, or actomyosin
binding; see Refs. 11 and 32). In addition, increases in muscle
[lactate] may decrease contractility independent of changes
in pH (16). Because muscle
[Pi] was not different
during the two contractile conditions of this study, it is likely that
the elevated rate of glycolysis and the subsequent increases in muscle
[lactate] and
[H+] during the short-
duration contraction period contributed to the significantly greater
reduction in force development.
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ACKNOWLEDGEMENTS |
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This research was supported by the National Institutes of Health Grants AR-40155 and HL-17731. S. S. Kurdak was supported by Çukurova University, Faculty of Medicine, Adana, Turkey.
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FOOTNOTES |
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Address for reprint requests: M. C. Hogan, Dept. of Medicine, 0623-A, Univ. of California, San Diego, 9500 Gilman Dr., La Jolla, CA 92093-0623.
Received 8 September 1997; accepted in final form 11 November 1997.
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