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1 Department of Pulmonary Diseases, University Hospital Nijmegen, Nijmegen 6500 HB, The Netherlands; and Departments of 2 Anesthesiology, and 3 Physiology and Biophysics, Mayo Clinic and Foundation, Rochester, Minnesota 55905
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
ACKNOWLEDGEMENTS
FOOTNOTES
REFERENCES
ABSTRACT 
Van Balkom, Roland H. H., Wen-Zhi Zhan, Y. S. Prakash, P. N. Richard Dekhuijzen, and Gary C. Sieck. Corticosteroid effects on isotonic contractile properties of rat diaphragm muscle. J. Appl. Physiol. 83(4):
1062-1067, 1997.
The effects of corticosteroids (CS) on diaphragm
muscle (Diam) fiber morphology
and contractile properties were evaluated in three groups of rats:
controls (Ctl), surgical sham and weight-matched controls (Sham), and
CS-treated (6 mg · kg
1 · day1
prednisolone at 2.5 ml/h for 3 wk). In the CS-treated
Diam, there was a selective
atrophy of type IIx and IIb fibers, compared with a generalized atrophy
of all fibers in the Sham group. Maximum isometric force was reduced by
20% in the CS group compared with both Ctl and Sham. Maximum
shortening velocity in the CS Diam was slowed by ~20% compared with Ctl and Sham. Peak power output of
the CS Diam was only 60% of Ctl
and 70% of Sham. Endurance to repeated isotonic contractions improved
in the CS-treated Diam compared
with Ctl. We conclude that the atrophy of type IIx and IIb fibers in
the Diam can only partially
account for the CS-induced changes in isotonic contractile properties.
Other factors such as reduced myofibrillar density or altered
cross-bridge cycling kinetics are also likely to contribute to the
effects of CS treatment.
prednisolone; skeletal muscle; fiber type; shortening velocity; fatigue; endurance
INTRODUCTION CORTICOSTEROID (CS) treatment is common in the clinical
setting, despite a variety of contraindications, including skeletal muscle myopathy. Recently, considerable attention has focused on the
possibility that CS treatment impairs diaphragm muscle (Diam) function in patients with
chronic obstructive pulmonary disease (1). In these patients, CS
treatment appears to contribute to
Diam weakness, further reducing
their functional reserve capacity. To date, animal studies have
examined only the effects of CS treatment on isometric properties of
the Diam. However, an examination
of only the isometric properties of the
Diam may not reveal the true impact of CS treatment. The force-velocity relationship is an essential
characteristic of Diam contractile
properties, and, to date, there is very little information concerning
the effects of CS treatment on the ability of the
Diam to shorten. This may explain
the equivocal results of animal studies reporting either no effect of
CS treatment on maximum isometric specific force (Po; force normalized for muscle
cross-sectional area) of the Diam (2, 3, 10, 13, 22) or only a
small reduction in specific force (20).
As in other skeletal muscles, the maximum shortening velocity
(Vmax) of
Diam fibers displays a strong
association with myosin heavy chain (MHC) isoform composition (8, 18).
In the Diam, type IIx and IIb
fibers, expressing the MHC2X and
MHC2B isoforms, respectively (16,
19), have a faster
Vmax than type I
and IIa fibers, expressing the
MHCslow and
MHC2A isoforms, respectively. An
effect of CS treatment on the force-velocity relationship of the
Diam is suggested by the selective
atrophy of type IIx and/or IIb fibers (2, 3, 12, 14, 20, 22).
Accordingly, we hypothesize that, in the
Diam, CS treatment is associated
with a slowing of
Vmax.
Fiber type differences in
Vmax also
correspond to differences in power output, with type IIx and IIb fibers
generating greater power than type I and IIa fibers (18, 19). If CS
treatment selectively affects the size of type IIx and IIb fibers, then the power output of the Diam
should be reduced. The increased power output of type IIx and IIb
Diam fibers is also associated with greater energetic demands, compared with type I and IIa fibers (18). Thus a reduction in the relative contribution of type IIx and IIb
fibers to total Diam mass should
result in an overall reduction in energy requirements. If muscle
fatigue is related to an imbalance between energy supply and energy
demand, the effects of CS treatment may be reflected by an improvement
in fatigue resistance (rate of force decline) or endurance (duration of
sustained power output). Indeed, previous studies have reported an
improvement in isometric fatigue resistance of the rat
Diam after CS treatment (13, 22).
However, since energy requirements increase with power output (4, 18),
the effects of CS treatment on improving endurance should be even more
pronounced during repetitive isotonic shortening.
In the present study, we evaluated the effects of CS treatment on the
isotonic contractile and endurance properties of the rat
Diam. We hypothesized that CS
treatment induces a selective atrophy of type IIx and IIb
Diam fibers and that, as a result, there is a slowing of
Vmax, a decrease
in power output, and an improvement in isotonic endurance.
METHODS Male Sprague-Dawley rats (initial body weights 315 ± 5 g) were
divided into three groups: 1)
untreated controls (Ctl; n = 8);
2) surgical sham and weight-matched
controls (Sham; n = 8); and
3) CS-treated (CS;
n = 8). All animals were housed in
separate cages under a 12:12-h light-dark cycle, fed with Purina rat
chow, and provided with water ad libitum. Animals in the Ctl and CS groups were provided food ad libitum, whereas rats in the Sham group
were food restricted to match their weight growth curve with that of
the CS group. Body weights were monitored daily in all groups.
All procedures used in this study were approved by the Institutional
Animal Care and Use Committee of the Mayo Clinic and were in strict
accordance with the American Physiological Society animal care
guidelines. Surgical procedures were performed under aseptic
conditions. The recovery of animals from surgery was carefully monitored.
CS treatment. Animals were
anesthetized by the administration of ketamine (60 mg/kg im) and
xylazine (2.5 mg/kg im), and a miniosmotic pump (Alzet 2M4) was
implanted sucutaneously in the neck. In the CS group, the miniosmotic
pump contained a 37.5 mg/ml aqueous suspension of prednisolone sodium
succinate (Upjohn), whereas in the Sham group the pump contained a
sterile physiological saline solution. Based on a flow rate of 2.5 µl/h for the osmotic pump, a dose of 6 mg/kg prednisolone was
provided continuously for a 3-wk period. Measurements of the remaining
amount of solution in the pump at the end of the 3-wk treatment period
were used to estimate total drug delivery. At the terminal experiment,
blood samples were obtained to measure prednisolone,
3,3 Fiber type composition and morphology.
After the 3-wk treatment period, the rats were anesthetized with
pentobarbital sodium (70 mg/kg), and the right
Diam was rapidly excised. Muscle
segments were dissected from the midcostal region, and the resting
excised length of the strip was measured by using digital calipers. The muscle strips were then stretched to 1.5 times this excised length [an approximation for optimal fiber length
(Lo)
(15)], pinned on cork, and rapidly frozen in melting isopentane
cooled to its melting point by liquid nitrogen.
Transverse sections of muscle fibers were cut at 6 µm by using a
cryostat (Reichert Jung 2000E) kept at The fluorescently stained sections were visualized by using an Olympus
BH-2 microscope. Images of the stained muscle sections were digitized
into a 1,024 × 1,024 array of picture elements (pixels) by using
a charge-coupled diode camera attached to a calibrated
image-processing system (19). With the use of a ×20 microscope
objective, each pixel had a projected area of 0.15 µm2. The cross-sectional area of
individual muscle fibers was determined from the number of pixels
within the delineated boundary of the fiber. To determine fiber type
proportions, ~500 muscle fibers were sampled from each
Diam. Cross-sectional areas were
measured for at least 25 fibers of each type within a given muscle. The relative contribution of each fiber type to the total area of the
muscle segment (an estimate of total mass when
Lo was similar) was calculated based on the proportion and average cross-sectional area
of each fiber type.
MHC isoform composition. The
techniques for determination of MHC isoform composition of the rat
Diam have been previously described (8, 19). Briefly, myosin was extracted from scissor-minced Diam tissue, the extracts were
centrifuged, and supernatants were recovered. After overnight storage
to allow precipitation of myosin filaments, the solution was
centrifuged, and the pellet was dissolved in a sample buffer, boiled,
and then stored frozen. Different MHC isoforms were separated by sodium
dodecyl sulfate-polyacrylamide gel electrophoresis. The identity of
specific MHC bands in silver-stained gels had been previously
determined by using immunoblotting techniques (9, 19). The relative
composition of the different MHC isoforms was determined by
densitometry, normalizing the average density of each band for the
total peak densities for all the isoforms combined.
Contractile and endurance properties.
Muscle strips (~3 mm wide) were dissected from the midcostal region,
with fiber insertions at the costal margin and central tendon left
intact. The muscle strip was mounted vertically in a glass tissue
chamber containing oxygenated mammalian Ringer solution of the
following composition (mM): 135 Na+, 5 K+, 2 Ca2+, 1 Mg2+, 121 Cl The muscle was stimulated directly by using platinum plate electrodes
placed in close apposition on either side of the muscle. Rectangular
current pulses (0.5-ms duration) were generated by using a Grass S88
stimulator and amplified by a current amplifier (Mayo Foundation,
Section of Engineering). The stimulus intensity producing the maximum
twitch force response was determined, and the stimulus intensity was
set at ~125% of this value for the remainder of the experiment
(~220 mA). Muscle preload was adjusted by using the micromanipulator
until Lo for
maximal twitch force was achieved.
The Cambridge system was controlled by using custom-built software
(LabView), implemented on an IBM 486 personal computer. Length and
force were independently controlled, allowing the Cambridge system to
operate either in isometric or isotonic modes, respectively. Length and
force outputs were digitized by using a data-acquisition board
(National Instruments) at a sampling frequency of 1 kHz.
Peak isometric twitch force (Pt)
and Po (600-ms duration train)
were measured. The force-velocity relationship of the
Diam was then determined. While
the muscle was maximally stimulated at 75 Hz for 330 ms, afterloads
were clamped at values ranging from 3 to 100% of
Po. A shorter stimulus duration
was used to accommodate the limited range of lever movement of the
Cambridge system during muscle shortening. At least 1 min intervened
between each load level. The velocity of shortening at each load clamp was calculated as the change in muscle length (normalized for Lo) during a
50-ms period. To eliminate the dynamics of connective and other
noncontractile tissue in the muscle, the time window for this
measurement was set to begin at 25 ms after the first detectable change
in length. Vmax
was calculated by fitting the force-velocity curve by using the
modified Hill equation and extrapolating the fitted curve to zero-load
(21).
Power output during isotonic contraction was calculated as the product
of force and velocity, and the load clamp level yielding maximum power
was determined. The load clamp was set to this value, and endurance was
assessed during repetitive isotonic shortening induced by stimulating
the muscle at 75 Hz in 330-ms duration trains repeated every second.
The time at which power output declined to zero (no detectable muscle
shortening) was defined as endurance time.
After the experiment, the muscle was weighed, and cross-sectional area
was estimated based on the following formula: muscle weight
(g)/[Lo
(cm) · 1.056 (g/cm3)]. Forces were then
normalized for cross-sectional area of the muscle segments.
Statistical analysis. Data were
compared by using a one-way analysis of variance followed by Duncan's
multiple-range test. Repeated-measures analysis of variance was used
for analysis of force-frequency, force-velocity, and force-power
relationships, as well as for the analysis of the decline in maximum
power output during the isotonic fatigue test. Statistical significance
was tested at the 0.05 level. All data were expressed as means ± SE.
RESULTS Efficacy of CS treatment. After 3 wk
of treatment, there was very little residual solution (<5% of total
volume) remaining in the miniosmotic pumps. Prednisolone levels
measured in blood serum of Ctl and Sham animals were below detectable
levels (<0.5 µg/dl). In contrast, the serum prednisolone level
measured in the CS-treated animals at the time of the terminal
experiment was 4.9 ± 1 µg/dl. Serum
T3 and
T4 levels were not
significantly different across the three experimental groups (Ctl:
T3 46 ± 3 ng/dl,
T4 4.0 ± 0.2 µg/dl; Sham:
T3 48 ± 6 ng/dl,
T4 4.2 ± 0.4 µg/dl; and CS:
T3 47 ± 4 ng/dl,
T4 3.9 ± 0.4 µg/dl).
Body weights. Over the 3-wk
experimental period, body weights of Ctl animals increased by 26% (315 ± 7 g initial and 397 ± 9 g final body weights). In the CS and
Sham animals, body weight gain was significantly reduced compared with
Ctl (P < 0.05), increasing by only 6 and 4%, respectively (CS: 319 ± 5 g initial and 338 ± 9 g
final body weights; Sham: 313 ± 7 g initial and 327 ± 9 g final
body weights).
Fiber type composition and morphology.
In all three experimental groups, fiber types could be readily
classified by immunoreactivity for the different MHC antibodies. The
incidence of coexpression of MHC isoforms appeared to be very low
(<1%) in all three groups. However, it was not possible to detect
coexpression of MHC2X and MHC2B isoforms by
immunohistochemistry, and it is likely that such coexpression was more
frequent (19). There were no differences across groups in the
proportions of different fiber types (Table 1).
,5-triiodo-L-thyronine (T3), and thyroxine
(T4).
20°C. The muscle sections were then reacted with antibodies to different MHC isoforms: 1) mouse
anti-MHCslow immunoglobulin (Ig) G
(Novocastra) for identification of type I fibers by positive
immunoreactivity; 2) mouse
anti-MHC2A IgG (7) for
identification of type IIa fibers by positive immunoreactivity; 3) mouse
anti-MHCall-2X IgG (16) for
identification of type IIx fibers by negative immunoreactivity; and
4) mouse
anti-MHC2B IgM (16) for
identification of type IIb fibers by positive immunoreactivity. After a
2- to 3-h incubation with the primary antibody, the sections were
washed in 0.1 M phosphate buffer and incubated further in Cy3-conjugated donkey anti-mouse IgG or IgM.
, 25 HCO3, 11 glucose, 0.3 glutamic acid,
0.4 glutamate, and
N, N-bis(2-hydroxyethyl)-2-aminoethane-sulfonic acid buffer (pH = 7.4). A 0.0008% solution of
d-tubocurarine chloride was added to
prevent neuromuscular transmission. The solution was oxygenated with
95% O2-5%
CO2 and maintained at 26°C.
The origin of the muscle bundle along the costal margin was attached to
a metal clamp mounted in series with a micromanipulator at the base of
the tissue chamber. The central tendon was glued to a thin, stiff
plastic rod that was firmly fixed to the lever arm of a dual-mode
length-force servo-control system (Cambridge Technologies, model 300B).
Table 1.
Effect of CS treatment on fiber type proportions, cross-sectional
areas, and relative contributions to total Diam
cross-sectional area
Treatment
Type
I
Type IIa
Type IIx
Type IIb
Fiber
type proportions,
%total
Ctl
36.1 ± 0.5
32.5 ± 0.7
23.5 ± 1.5
6.8 ± 1.6
Sham
37.4 ± 1.5
31.0 ± 1.2
24.1 ± 1.5
7.5 ± 1.4
CS
40.4 ± 2.2
29.9 ± 1.5
24.1 ± 1.9
5.5 ± 1.5
Fiber
cross-sectional area,
µm2
Ctl
875 ± 28
821 ± 35
2,666 ± 163
3,388 ± 263
Sham
600 ± 19*
693 ± 18*
1,710 ± 75*
2,685 ± 186*
CS
772 ± 62
770 ± 67
1,668 ± 202*
2,284 ± 307*
Relative
contribution to total Diam area,
%total
Ctl
21.7 ± 0.9
18.6 ± 0.6
44.2 ± 4.1
15.5 ± 3.8
Sham
21.6 ± 1.6
20.6 ± 2.0
39.5 ± 3.3
19.2 ± 4.1
CS
29.8 ± 2.3*,
21.7 ± 1.4
37.2 ± 3.2
11.3 ± 3.1
Values are means ± SE. Ctl, control group; Sham, Sham group; CS,
corticosteroid-treated group; Diam, diaphragm muscle.
*
P < 0.05 compared with Ctl group.
P < 0.05 compared with Sham group.
In the CS-treated animals, cross-sectional areas of type IIx and IIb Diam fibers were significantly smaller than those of type IIx and IIb fibers in Ctl (P < 0.05; Table 1). In contrast, cross-sectional areas of type I and IIa fibers in the CS Diam were comparable to similar fiber types in Ctl animals. In the Sham Diam, there was a generalized atrophy of all fiber types compared with Ctl (P < 0.05; Table 1). Type I fibers in the Sham Diam were also smaller than type I fibers in the CS Diam (P < 0.05; Table 1). Cross-sectional areas of type IIa, IIx, and IIb fibers in the CS Diam were comparable to similar fiber types in Sham animals.
In the CS Diam, the relative contribution of type I fibers to total Diam cross-sectional area increased (P < 0.05; Table 1). Otherwise, there were no differences across groups in the relative contribution of different fiber types to total Diam cross-sectional area. However, the combined contribution of type IIx and IIb fiber areas was ~60% of total Diam cross-sectional area in Ctl and Sham animals but only ~48% in the CS group (Table 1).
MHC isoform composition. On the basis of electrophoretic separation, the relative expression of the MHC2B isoform decreased in the CS-treated Diam (P < 0.05; Table 2). The MHC isoform composition of Ctl and Sham Diam was comparable (Table 2).
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Contractile and endurance properties. After 3 wk of CS treatment, Pt and Po of the Diam were reduced compared with both Ctl and Sham groups (P < 0.05, Table 3). Pt and Po were not different between Ctl and Sham animals. Compared with Ctl and Sham groups, the force-velocity relationships of the CS Diam were shifted to the left (P < 0.05; Fig. 1A). The Vmax of the CS Diam was significantly slower than that of both Ctl and Sham Diam (P < 0.05, Fig. 1B). In all Diam, peak power output occurred at ~33% of Po and at 33% of Vmax (Fig. 2). Peak power output of the CS Diam was significantly lower than that of both Ctl and Sham groups (Fig. 2; P < 0.05). The peak power output of the Sham Diam was also slightly lower than that of Ctl animals (Fig. 2; P < 0.05).
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With repetitive shortening contractions, maximum power output of the
Diam rapidly declined in all three
groups (Fig. 3;
P < 0.05). After 60 s of repetitive
contractions, Diam power output was comparable in all three groups (Fig. 3). However, given the differences in the initial peak power output of each group, the rate of
decline in power was slower in the CS
Diam compared with both Sham and
Ctl animals (Fig. 3; P < 0.05).
Endurance time of the CS Diam was
120 ± 6 s compared with 96 ± 4 s for Ctl
(P < 0.05) and 108 ± 7 s for
Sham (Fig. 3).
DISCUSSION
The results of the present study support our hypotheses that CS treatment induces a selective atrophy of type IIx and IIb fibers in the rat Diam, which is associated with a slowing of Vmax, a reduction in power output, and an improvement in isotonic endurance. However, the CS-induced changes in Diam isotonic properties were disproportionately greater than the changes in type IIx and IIb fiber morphology and MHC isoform expression. Therefore, we conclude that, in addition to the selective atrophy of type IIx and IIb fibers, CS treatment exerts an influence on cross-bridge cycling kinetics.
Across the 3-wk period, the normal increase in body weight observed in Ctl rats was blunted by CS treatment. The final body weight of the CS-treated animals was ~15% lower than that of Ctl rats. Because alterations in nutritional status alone can affect morphology and function of the rat Diam (2, 11, 17), interpretation of the direct effects of CS treatment is confounded. However, the morphological and contractile adaptations of the Diam in the Sham group, where body weight was matched to that of the CS group by food restriction, were generally dissimilar to those observed in the CS-treated animals. These results suggest that the effects of CS treatment on Diam structural and functional properties cannot be solely attributed to a nonselective catabolic effect.
The CS-induced selective atrophy of type IIx and IIb Diam fibers observed in the present study is in general agreement with several previous studies (2, 3, 12, 14, 20, 22). However, these previous studies did not classify fiber types based on expression of different MHC isoform, nor did they distinguish between type IIx and IIb fibers. Standard histochemical classification of fiber types based on the pH lability of myofibrillar adenosinetriphosphatase (ATPase) staining, as used in these studies, cannot distinguish type IIx fibers, which are abundant in the rat Diam (9, 19). In addition, the MHC2B isoform is often coexpressed with the MHC2X isoform. Therefore, it is not surprising that type IIx and IIb fibers displayed a similar pattern of atrophy in response to CS treatment. In the Sham Diam, there was a generalized atrophy of all fiber types, which has also been previously observed (2, 10, 11, 17, 20, 22). When combined, type IIx and IIb fibers comprised ~60% of both Ctl and Sham Diam but only ~48% of the CS Diam. In the CS-treated Diam, there was a reduction in the relative expression of the MHC2B isoform, whereas no changes in MHC isoform expression were observed in the Sham group. When combined, MHC2X and MHC2B isoforms comprised ~40% of the CS Diam compared with ~49% of Ctl and ~46% of Sham Diam. The relatively modest change in MHC isoform composition of the CS Diam was consistent with the normal T3 and T4 levels of these animals. Clearly, the CS-induced morphological adaptations of type IIx and IIb Diam fibers and the alterations in MHC isoform expression were not as pronounced as the changes in isotonic contractile properties.
Three weeks of CS treatment resulted in a 20% reduction in Po compared with both Ctl and Sham groups. These results are in agreement with the previous report of van Balkom et al. (20) but contrast with several other studies in which no effect of CS treatment on Diam Po was observed (2, 3, 10, 13, 22). The reasons for these discrepant results are unclear but may relate to the type, dose, and duration of CS treatment used. It is unlikely that the reduction in specific force of the CS-treated Diam observed in the present study was attributable only to the selective atrophy of type IIx and IIb fibers or the reduction in MHC2B isoform expression. A reduction in specific force could also arise from a number of alternative mechanisms, including a decrease in myofibrillar density and/or changes in cross-bridge cycling kinetics. Lieu and colleagues (12) reported that CS treatment is associated with a reduction of myofibrillar and sarcoplasmic protein concentration in the rat Diam, albeit not as pronounced as in the plantaris muscle. Such alterations in myofibrillar and sarcoplasmic protein concentration could reflect a decrease in the number of available cross bridges and/or changes in calcium handling.
The force-velocity relationship of the Diam was altered by CS treatment such that Vmax was slowed by ~20% and peak power output was reduced by 40% compared with Ctl animals. The slowing of Vmax in the CS Diam is generally consistent with the selective atrophy of type IIx and IIb fibers and the reduction in MHC2B expression. However, the slowing of Vmax induced by CS treatment was substantially greater than that which would be predicted by the relatively modest reduction in MHC2B expression. Therefore, it is unlikely that the slowing of Vmax in the CS-treated Diam was solely attributable to a selective atrophy of type IIx and IIb fibers and/or the reduction in MHC2B expression. In muscle fibers, Vmax is correlated with actomyosin ATPase activity (18) and cross-bridge cycling rate. Type I and IIa fibers have lower actomyosin ATPase activities than type IIx and IIb fibers (18, 19) and, as a result, a slower Vmax. It is possible that the slowing of Vmax in the CS-treated Diam reflects a decrease in actomyosin ATPase activity of muscle fibers independent of MHC isoform expression.
In all groups, the Diam displayed very rapid fatigue during repetitive isotonic contractions at a load corresponding to peak power output. During shortening contractions, muscle fiber energy utilization increases (4, 18); thus the rapid fatigue may be related to an imbalance between energy utilization and energy production. CS-treated animals displayed a slower rate of power decrement during repetitive isotonic contractions compared with Ctl and Sham groups and prolonged endurance time compared with Ctl rats. These results are in general agreement with the improved fatigue resistance during repetitive isometric contractions noted in previous studies (13, 22). However, the results of the present study are in contrast to the report of Ferguson and colleagues (5), who found that CS-treated rabbits displayed less endurance to an incremental inspiratory threshold load. However, Diam fatigue was not directly verified in this study, and respiratory failure, used to define endurance, could have resulted from a number of mechanisms other than Diam fatigue.
The results of the present study suggest that CS treatment reduces energy utilization during repetitive isotonic contractions and thus improves the balance between energy supply and energy demand. A reduction in energy utilization would result from the selective atrophy of type IIx and IIb fibers, which have higher actomyosin ATPase activities (18, 19). In addition, as suggested above, CS treatment may directly reduce actomyosin ATPase activity independent of MHC isoform expression. Other studies have also suggested that CS treatment impairs muscle energy utilization. For example, after CS treatment, there is an accumulation of glycogen (5) and a reduction in creatine kinase activity (6). There may also be an effect of CS treatment on energy production. For example, it has been reported that CS treatment reduces citrate synthase activity in the rat Diam (12, 20). However, no effect of CS treatment on succinate dehydrogenase activity was observed (10).
In conclusion, CS treatment causes a reduction in specific force, a slowing of Vmax, a decrease in power output, and an improvement in endurance during repetitive isotonic contractions. These contractile adaptations are generally consistent with the selective atrophy of type IIx and IIb fibers and the reduction in MHC2B expression that was observed in the CS Diam. However, the contractile adaptations are disproportionately greater than the morphological changes induced by CS treatment. Therefore, we conclude that the impairment of Diam function associated with CS treatment involves additional mechanisms including a reduction in myofibrillar density and/or a slowing of cross-bridge cycling kinetics.
ACKNOWLEDGEMENTS
The authors are grateful to Yun-Hua Fang for her assistance in this study.
FOOTNOTES
Address for reprint requests: G. C. Sieck, Anesthesia Research, Mayo Clinic, Rochester, MN 55905 (E-mail: sieck.gary{at}mayo.edu).
Received 16 December 1996; accepted in final form 8 May 1997.
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