Contractile function and low-intensity exercise effects of old dystrophic (mdx) mice

Alan Hayes and David A. Williams

Muscle and Cell Physiology Laboratory, Department of Physiology, The University of Melbourne, Parkville, Victoria 3052, Australia

    ABSTRACT
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Abstract
Introduction
Materials & Methods
Results
Discussion
References

Old mdx mice display a severe myopathy almost identical to Duchenne's muscular dystrophy. This study examined the contractile properties of old mdx muscles and investigated any effects of low-intensity exercise. Isometric contractile properties of the extensor digitorum longus (EDL) and soleus muscles were tested in adult (8-10 mo) and old (24 mo, split into sedentary and exercised groups) mdx mice. The EDL and soleus from old mdx mice exhibited decreased absolute twitch and tetanic forces, and the soleus exhibited a >50% decrease in relative forces (13.4 ± 0.4 vs. 6.0 ± 0.9 N/cm2) compared with adult mice. Old mdx muscles also showed longer contraction times and a higher percentage of type I fibers. Normal and mdx mice completed 10 wk of swimming, but mdx mice spent significantly less time swimming than normal animals (7.8 ± 0.4 vs. 15.8 ± 1.1 min, respectively). However, despite their severe dystrophy, mdx muscles responded positively to the low-intensity exercise. Relative tetanic tensions were increased (~25% and ~45% for the EDL and soleus, respectively) after the swimming, although absolute forces were unaffected. Thus these results indicate that, even with a dystrophin-deficient myopathy, mdx muscles can still respond to low-intensity exercise. This study shows that the contractile function of muscles of old mdx mice displays many similarities to that of human dystrophic patients and provides further evidence that the use of non-weight-bearing, low-intensity exercises, such as swimming, has no detrimental effect on dystrophic muscle and could be a useful therapeutic aid for sufferers of muscular dystrophy.

Duchenne's muscular dystrophy; swimming; contraction; muscle wasting; dystrophin-deficient muscle

    INTRODUCTION
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Abstract
Introduction
Materials & Methods
Results
Discussion
References

DESPITE THE DISCOVERY OF the affected gene and its product, dystrophin, a cure for Duchenne's muscular dystrophy (DMD) remains elusive. Some investigators have utilized drug or exercise treatments to alleviate the symptoms and delay the progression of the disease until suitable genetic therapies are developed. Several recent studies from this and other laboratories have shown that exercise can have beneficial effects on the function of dystrophic muscles (13, 16-18, 28). However, the majority of these studies involved the use of hindlimb muscles from the mdx mouse for their investigations. This mouse exhibits a gene defect homologous to patients suffering from DMD (7, 29) and as such lacks the protein dystrophin from the muscle sarcolemma (19). Although these animals suffer vigorous muscle degeneration from an early age, rapid regeneration almost fully compensates for this degeneration (1, 11). Because their muscles appear to be resistant to further degeneration, mdx mice show little muscle weakness throughout the first year of their lives. Because this corresponds to a time at which most of the exercise studies mentioned above were undertaken, extrapolating the results to humans has been a contentious point.

Recent studies (21, 25) have shown that, as they reach 2 yr of age, mdx hindlimb muscles reproduce the pathophysiology of DMD. Typical clinical structural changes observed in DMD muscles are muscle necrosis, motor weakness and early death, and connective tissue infiltration, and some accumulation of adipose tissue was observed, to varying degrees, in the muscles of mdx mice. It was concluded that, late in life, mdx mice develop a muscular dystrophy with a phenotype similar to DMD dystrophinopathy, and investigations with this model could provide an exciting new avenue to an understanding of the pathophysiology of DMD and the development of suitable treatments.

Therefore, the aims of this study were twofold. The first goal was to investigate the contractile properties of old mdx mice. Given the pathophysiological changes observed by Lefaucheur et al. (21) and Pastoret and Sebille (25), detriments in muscular contractile function would also be expected. The second goal was to subject a group of old mdx mice to a very low intensity swimming protocol, to address the question of whether exercise has detrimental effects or is of benefit to dystrophic muscles suffering from a severe dystrophinopathy. Here we report that, whereas old mdx mice do suffer from severe contractile impairment, their muscles are still capable of responding to low-intensity exercise.

    MATERIALS AND METHODS
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Abstract
Introduction
Materials & Methods
Results
Discussion
References

Animal groups. Male mdx mice were allocated to groups based on age: 8- to 10-mo-old sedentary adult mice (dystrophic adult, n = 8) and 24-mo-old mice. The older mice were randomly separated into sedentary (dystrophic aged, n = 8) and exercised (dystrophic exercised, n = 7) groups. Age-matched normal (C57BL/10) mice were also divided into adult (normal adult, n = 7) and old (normal aged, n = 6) groups. All mice were kept in their cages and had access to food and water ad libitum. All experiments conformed to the use and handling of animal guidelines of The National Health and Medical Research Council of Australia.

Exercise protocol. The dystrophic exercised group first underwent exercise when they reached 24 mo of age. Mice were introduced into a large Perspex tank (1.4 × 0.6 × 0.64 m) filled with water (maintained at 35 ± 1°C) to a depth that was longer than the length of a mouse from nose to tip of the tail. Mice completed a 10-wk training program, in which they exercised once daily for as long as possible, until it was considered hazardous to continue the exercise session. Unlike rats, which freely dive and investigate the bottom of the tank, mice rarely fall below the surface of the water, unless "dunked" by their companions, and return to the surface within 1-2 s. Therefore, with the safety of the mice paramount, individual mice were considered fatigued and were removed from the tank if they took >2 s to return to the surface of the water or failed to maintain their heads above water for >10-15 s, as previously described for exhausted rats (22). This is a more stringent test than for rats, which are considered exhausted if they fail to return to the surface of the water 10 s after being submerged (24). However, due to their age and swimming performance (see RESULTS), and to ensure the survival of all animals undertaking the swimming, mice were given a "rest" day following 2 consecutive days on which swimming was performed. A group of normal animals (n = 7) aged 24 mo was also exercised for 10 wk to allow comparison of the exercise performance of mdx mice.

Contractile and histochemical methods. After the 10-wk exercise program of the exercised groups, animals in all groups (adult = 10-12 mo of age, aged and exercised = 26-27 mo of age) were killed by cervical dislocation, and the isometric contractile properties of the fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus muscles were measured as reported previously (16-18). Briefly, muscles were attached to a sensitive force transducer (FT003; Grass Instruments, Quincy, MA) and were bathed in Krebs-Henseleit mammalian Ringer solution (maintained between 20 and 22°C), which was aerated with Carbogen (5% CO2-95% O2; Commonwealth Industrial Gases, Australia). At the muscle's optimum length (the length at which the muscle produced peak force), supramaximal stimuli delivered via two parallel platinum electrodes were used to elicit a series of twitch and tetanic stimulations. Peak twitch tension (Pt), the time taken to achieve this peak tension (TTP), and the time taken for the muscle to relax from this peak tension to one-half the amplitude of that peak tension (RT1/2) were recorded from a twitch contraction. The absolute maximum isometric tetanic tension (Po) was measured from a tetanic stimulation (elicited at a frequency of 90 Hz at a pulse duration of 2.75 ms and 70 Hz at a duration of 3.2 ms for the EDL and soleus, respectively). Both Pt and Po tensions were normalized with respect to muscle mass, which has been used previously in descriptions of dystrophic muscle (1, 17, 18, 26), to avoid the assumption that the muscle density is unaltered by fat and connective tissue infiltration. This is of particular importance in the old dystrophic animals, which have been shown to have extensive accumulation of connective tissue and some fat infiltration (21, 25). However, forces were also normalized with respect to an estimate of cross-sectional area, according to the equation MM/Lo × D, where MM is the muscle mass, Lo is the optimal length, and D is the density of skeletal muscle (1.06 g/cm3), and these results were reported where they differed from those normalized with respect to muscle mass.

After the twitch and tetanus measurements, the fatigue characteristics of the muscles were also measured. The regime consisted of subjecting the muscle to a 1-s tetanus at a rate of 12/min for 5 min. At 1-min intervals, beginning with the prefatigue measurement, the tetanic response was recorded and the maximum tetanic force level was measured. After a 20-min rest period, a tetanic stimulation was performed to ensure that the protocol did not cause irreversible damage to the muscle. Thus fatigue was defined as the reversible decay in tension plotted against time, with the maximum value of the tetanus recorded at each minute expressed relative to the Po of the initial tetanus.

After the measurement of contractile properties, the muscles were blotted dry, weighed, and snap frozen in isopentane cooled in liquid nitrogen. Sections from the EDL and soleus muscles were stained for myosin ATPase activity as outlined by Hamalainen and Pette (15). Analysis of muscle fiber types was performed as previously described (see Ref. 18). Fiber type proportions (percentage) and fiber percentage area, defined as the percentage of the muscle cross-sectional area occupied by a particular fiber type, were calculated.

Statistics. All values are reported as means ± SE. Differences between mdx and C57BL/10 mice, and between old and adult groups, were measured by a two-way analysis of variance (ANOVA). In the event of interactions between the variables, differences between groups were measured by a one-way ANOVA, followed by a Student-Newman-Keuls post hoc comparison test. Differences between dystrophic aged and dystrophic exercised groups were measured by a Student's t-test. In all cases, a probability value of P < 0.05 was required for differences to be considered significant.

    RESULTS
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Abstract
Introduction
Materials & Methods
Results
Discussion
References

Swimming performance. Compared with adult mice, which can easily swim for >1 h/day (16, 17), the old mice of either strain were unable to swim continuously for periods >25 min. Hence the exercise would be considered low intensity (although it was not necessarily "low intensity" for the old mice). Although mice were removed if they struggled to return to the surface after temporarily "sinking" (see MATERIALS AND METHODS), they were also removed if the normal hindlimb action of swimming was not performed. Variations of the normal swimming action were noted, and included mice rolling onto their sides and only using the forelimb and hindlimb from one side of their body to propel themselves, an activity commonly seen in both control and mdx mice. In addition, mdx animals lay almost horizontally on the surface of the water, arching their backs and flicking their tail for movement. This latter deviation from normal swimming action was observed in all the mdx animals during the 10 wk of exercise but never in the normal animals. The old mdx mice (7.8 ± 0.4 min; 12.5 min maximum) were unable to complete the same amount of daily swimming in a single bout as normal mice of the same age (15.8 ± 1.1 min; 25 min maximum).

Morphometry. There were no differences in the body masses of normal adult and normal aged mice. Similarly, there were no differences between the groups for either absolute or relative muscle masses for the two muscles tested (see Table 1).

                              
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Table 1.   Morphometric measurements of all groups

The body masses of dystrophic aged mice were significantly lower than those of dystrophic adult mice. The absolute muscle masses of the EDL from dystrophic aged mice were lower than for dystrophic adult mice (see Table 1). Another fast-twitch muscle, the plantaris, also displayed a decreased mass (P < 0.001) due to aging (17.9 ± 1.8 and 25.9 ± 1.5 mg for dystrophic aged and dystrophic adult, respectively). These differences reflected the overall loss of body mass, such that the relative EDL and plantaris muscle masses were unaffected by aging. In contrast, there was no difference between the masses of dystrophic adult and dystrophic aged soleus muscles, such that the relative soleus muscle mass was significantly higher (P < 0.01) in dystrophic aged compared with dystrophic adult mice. Swimming had no effect on the body or muscle masses of any of the muscles tested in the old dystrophic animals (see Table 1).

There was no difference in body mass between normal adult and dystrophic adult mice, whereas dystrophic aged mice had a lower body mass (P < 0.01) than normal aged mice. Dystrophic adult mice had larger EDL and soleus muscle masses (P < 0.001) than normal adult mice. The absolute mass of the dystrophic aged soleus was still larger (P < 0.001) than in the normal aged group, whereas no difference was observed in the EDL absolute muscle mass. Again, this loss of EDL mass reflected the general loss of body mass, such that the relative muscle masses of both the EDL and soleus were greater (P < 0.001) in the dystrophic aged mice than in normal aged mice.

Isometric contractile characteristics: EDL. Absolute Pt and Po were larger in the normal aged mice compared with the normal adult mice (see Table 2). The Pt/MM was also significantly larger (P < 0.05) in the normal aged mice, but this difference was not apparent when expressed relative to cross-sectional area (5.6 ± 0.5 and 6.3 ± 0.3 N/cm2 for normal adult and normal aged, respectively). As there were few apparent effects attributed to aging in the normal mice, there is nothing to suggest that the density of the muscles would be altered. Hence the forces expressed relative to cross-sectional area should be considered the true relative force level in normal animals. There were no differences in relative Po between the groups (see Fig. 1). The TTP was longer in the normal aged group compared with normal adult mice.

                              
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Table 2.   Isometric contractile properties of muscles from all groups

Both TTP and RT1/2 were significantly longer in dystrophic aged than in dystrophic adult mice (see Table 2). Absolute Pt and Po of the dystrophic aged EDL were significantly lower than in the dystrophic adult group. However, this was attributable to the loss of muscle mass, such that the relative forces were not different between the groups. Although swimming had no significant effects on the absolute forces produced (see Table 2), it did increase both Pt/MM (P < 0.05) and Po/MM (see Fig. 1) in dystrophic exercised mice compared with the dystrophic aged group. Similar trends were observed in the forces expressed relative to cross-sectional area (2.3 ± 0.2 and 3.0 ± 0.3 N/cm2 for Pt of dystrophic aged and dystrophic exercised, respectively, P < 0.06; see Fig. 1). The Pt/Po ratio was significantly lower (P < 0.001) in dystrophic aged compared with dystrophic adult mice.


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Fig. 1.   Tetanic forces (Po) of extensor digitorum longus (EDL; A) and soleus (B) muscles expressed relative to cross-sectional area (CSA; open bars) and muscle mass (MM; closed bars). Significant difference between old mdx mice that were exercised (dystrophic exercised) and that were sedentary (dystrophic aged): * P < 0.05. See text for differences due to aging and between normal and dystrophic animals. Exer, exercised.

Comparisons between the two strains showed that dystrophic adult mice exhibited lower absolute (P < 0.05) and relative (P < 0.001) tensions than the normal adult mice. These deficiencies in force production (P < 0.001) were also observed in the dystrophic aged compared with the normal aged animals. The TTP (P < 0.01) and RT1/2 (P < 0.05) were faster in the dystrophic adult mice, as was the TTP (P < 0.01) in the dystrophic aged mice compared with their age-matched normal counterparts. The dystrophic aged mice also exhibited a reduced Pt/Po ratio (P < 0.001) compared with normal aged mice.

Isometric contractile characteristics: soleus. The TTP of the normal aged mice was significantly shorter than for normal adult mice (see Table 2). No other differences in the contractile properties were observed between the two normal groups.

The TTP and RT1/2 were slower in dystrophic aged than dystrophic adult mice. Swimming prevented the change in RT1/2, such that dystrophic exercised mice relaxed faster than dystrophic aged mice (see Table 2). Dystrophic aged animals had lower absolute and relative Pt and Po than dystrophic adult mice. However, as with the EDL, the swimming program improved the force-generating capacity of old mice, such that the forces expressed relative to both muscle mass and cross-sectional area [Pt (P < 0.05) and Po (see Fig. 1)] from dystrophic exercised mice were larger than in the dystrophic aged animals.

Dystrophic adult mice had larger absolute and relative twitch and tetanic forces (P < 0.001) than normal adult mice. There was no difference in the absolute forces generated between the dystrophic aged and normal aged mice. In contrast, dystrophic aged Pt/MM (P < 0.01) and Po/MM (P < 0.05) were both lower than values for normal aged mice. Relative to cross-sectional area, only Pt of dystrophic aged mice (0.9 ± 0.1 N/cm2) was significantly lower (P < 0.05) than for normal aged mice (1.5 ± 0.3 N/cm2). Although the TTP was faster (P < 0.01) in the dystrophic adult mice, both the TTP (P < 0.01) and RT1/2 (P < 0.001) were slower in the dystrophic aged mice than in their age-matched normal counterparts.

Fatigue characteristics. The EDL of normal aged animals was more fatigue resistant than the normal adult EDL at the second minute (P < 0.05) of the fatigue protocol. No other differences were observed with aging in normal animals.

Similarly, the EDL of dystrophic aged animals was more fatigue resistant than the dystrophic adult EDL at the second minute (P < 0.05) of the fatigue protocol. The soleus muscles from the dystrophic aged group exhibited significantly greater resistance to fatigue than the dystrophic adult mice at all time points of the fatigue protocol (see Fig. 2). Swimming had no effect on the fatigue properties of either muscle in the old dystrophic animals.


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Fig. 2.   Fatigue profiles of EDL (A) and soleus (B) muscles. Values are peak tetanic tensions at each minute of the stimulation protocol expressed as a percentage of the initial peak tension. Significant difference between dystrophic (Dyst) adult and dystrophic aged mice: * P < 0.05, *** P < 0.001. Significant difference between age-matched dystrophic and normal animals: # P < 0.05, ## P < 0.01, ### P < 0.001.

The EDL from both dystrophic adult and dystrophic aged mice exhibited less fatigue resistance than their normal age-matched counterparts at the first (P < 0.001) and second (P < 0.01) time points of the fatigue protocol. The dystrophic adult soleus had lower fatigue resistance at the first minute (P < 0.05) and fifth minute (P < 0.01) of the fatigue protocol. In contrast, the soleus from dystrophic aged mice exhibited enhanced resistance to fatigue compared with normal aged animals at the first and fifth minutes (P < 0.05) of the fatigue protocol.

Histochemistry. In the C57BL/10 mice, there were no differences observed in the histochemical fiber proportions or percentage fiber areas between any of the groups for either of the muscle types analyzed.

There were no differences in either the fiber type proportions or percentage fiber areas of the EDL from dystrophic aged compared with dystrophic adult animals, except for an increase in the muscle section occupied by type I fibers in the old animals (see Fig. 3). In the soleus muscle, dystrophic aged mice exhibited both larger proportions (P < 0.05) and larger percentage areas (see Fig. 3) of type I fibers than dystrophic adult mice. There were no differences between dystrophic aged and dystrophic exercised mice for either muscle.


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Fig. 3.   Percentage of muscle cross-sectional area occupied by type I (open bars), type IIa (solid bars), type IIb (crosshatched bars), and intermediate (hatched bars) fibers in EDL (A) and soleus (B) muscles. Significant difference in percentage of type I fibers between dystrophic adult and dystrophic aged mice: * P < 0.05, ** P < 0.01. There were no differences between normal adult and normal aged mice. Dystrophic mice had significantly higher percentages of type IIa (EDL), type I, and intermediate (soleus) fibers and concomitant decreases in type IIb (EDL) and type IIa (soleus) fibers (see text for P values).

The EDL muscle from both dystrophic adult and dystrophic aged mice had higher type IIa (P < 0.05) and smaller type IIb (P < 0.01) proportions than the age-matched normal mice. Similarly, both the adult and old dystrophic animals displayed larger type IIa and smaller type IIb percentage areas than the normal mice (P < 0.001). In the soleus muscle, both dystrophic adult and dystrophic aged mice exhibited increased proportions of type I (P < 0.001) and intermediate (regenerating) fibers (P < 0.05) and a concomitant decrease in the proportion of type IIa fibers (P < 0.001) compared with age-matched normal mice. Identical changes and levels of significance were observed in the percentage of the muscle occupied by the various fiber types.

    DISCUSSION
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Abstract
Introduction
Materials & Methods
Results
Discussion
References

The results of this study show that old mdx animals suffer from severe impairment in contractile function, consistent with a typical dystrophin-deficient myopathy such as DMD. As these changes occur at an age at which the muscle function of normal mice has been unaffected, old mdx mice provide an excellent model with which to investigate DMD. Furthermore, the contractile properties of the old mdx mice can still be altered by very-low-intensity swimming, indicating that low levels of controlled, non-weight-bearing activity can be beneficial for dystrophic muscle.

Comparison between normal and dystrophic mice. Differences in the adult strains are similar to those previously reported under these experimental conditions (11, 16, 18, 20). The lower fatigue resistance of adult mdx mice at several of the time points is perhaps surprising, given that increased proportions of oxidative fiber types have commonly been reported in mdx mice and observed again in this study. This may be due to impairments in energy metabolism of dystrophin-deficient muscle, because a recent study has shown that there is a limitation of chemical reaction rates through both glycolytic and tricarboxylic acid cycle pathways in mdx muscles (9). Differences between the age-matched old mice from the two strains, which included decreased body mass, decreased force production, longer contraction times, and greater fatigue resistance in the mdx animals, simply illustrates the enhanced deterioration of function of the mdx mice with aging.

Contractile properties of old dystrophic muscle. Although the mass and contractile properties of the normal mice undergo few changes up to 2 yr of age, both fast- and slow-twitch dystrophic muscles exhibit large decreases in both mass and force. The reasons that mdx muscles display a severe myopathy with aging are unclear, but may include an impaired regenerative capacity. For instance, muscles from aged animals have been shown to exhibit less recovery after injury (5). Repeated bouts of degeneration and regeneration throughout the life span of the mdx mice could exhaust, or severely compromise, the satellite cell population and hence the ability of the dystrophic muscles to regenerate.

There is a general loss of muscle mass with aging in humans (see Refs. 2 and 6), and the mdx mice from this study also exhibited this phenomenon. The results from this study show that the fast-twitch muscles were more affected (25-30% decrease) than the slow-twitch soleus (10% decrease). The preferential atrophy of the fast-twitch fibers is also evident in the fact that the percentage of the muscle cross-section occupied by type I fibers increased with aging in both the EDL and soleus muscles of mdx mice (see Fig. 3).

As a result of decreased muscle mass, the forces produced by the mdx muscles examined in this study were all lower in the old animals. However, in the soleus muscle, the decreased muscle mass did not account for all of the decrease in force, such that the relative muscle forces were also significantly lower in the old animals. The reduction in specific force in the soleus, and not the EDL, with age is most likely due to the preferential loss of fast-twitch fibers in dystrophic muscle. In the adult mdx animal, the EDL exhibits a decrease in relative forces compared with normal animals, due possibly to the presence of embryonic and neonatal muscle fibers (12) due to the periods of degeneration and regeneration that occur during its life. Because embryonic and neonatal fibers have been shown to have a low capacity for force generation (27), it is reasonable to expect that the EDL would display lower forces, despite the maintenance of higher muscle mass, as it contains mostly fast-twitch fibers that are susceptible to damage and hence undergo regeneration. Furthermore, as the drastic changes that occur as the animals age are likely due to a loss of this regenerating capacity, the EDL loses muscle mass and hence force, yet suffers no further decrease in relative force. In contrast, the soleus muscle hypertrophies and produces larger relative forces than normal mice in adulthood, presumably as compensation for the loss of force being produced by fast-twitch muscles. When the regenerative capabilities are lost in old age, the soleus also begins to lose much of its muscle substance. However, since it loses more of its type II fibers, there is a greater degree of force decrement than muscle mass lost and a subsequent loss of relative force production.

A further reason for the greater loss of force than mass would be the infiltration of fat and connective tissue. Although the magnitude of connective tissue infiltration, which increases with aging, is thought not to be large enough to affect the amount of noncontractile material in normal muscle (6), this infiltration occurs earlier in the mdx mouse (23) and has been shown to be extensive in old mdx mice (21, 25). Functional evidence of increased connective tissue may be reflected in the reduced twitch-to-tetanus ratio of the old mdx compared with normal mice. Extensive connective and elastic tissue in a muscle tends to reduce the absolute force observed in a single twitch. In addition to the extensive fibrosis, increased fat infiltration (21, 25) may also result in the loss of force production unparalleled by the loss of muscle mass.

Other alterations in the contractile properties of old mdx animals are most likely the result of the contribution of increased proportions and percentage areas of type I fibers, and the elevated connective tissue levels, within the muscles. In both the EDL and soleus, the TTP was significantly slower in the old mdx mice, most likely due to the increased time taken to stretch the connective tissue elements. Similarly, the RT1/2 was significantly slower in the old mdx mice, perhaps indicating impairments in the calcium-handling properties of the sarcoplasmic reticulum.

Effects of low-intensity exercise on old dystrophic muscle. An important result of the present study is that low-intensity swimming was still able to improve relative force production in both the EDL and soleus muscles, without causing any detrimental effects in mice displaying dystrophin deficiency and a severe myopathy almost identical to DMD.

Because some of the loss of skeletal muscle mass and force with aging is presumed to result from decreases in normal daily activity (3), it is no surprise that studies investigating the effects of increased activity levels in aged subjects have reported beneficial effects. For example, studies into resistance training of the elderly have reported improvements in muscle strength (8, 14). Endurance training has also been shown to be effective in improving muscle function in the elderly (10). Because resistance or weight-training programs would be impractical for dystrophic patients with little functional muscle mass, the present study employed a very-low-intensity endurance swimming protocol. In this study, even 10 min or less of swimming each day led to improvements in the relative force production of dystrophic muscle. Despite the absence of changes in muscle mass, both the EDL and soleus muscles exhibited improvements in force-generating ability, such that relative tetanic forces were larger than in the old, sedentary mdx animals. Some of this increase may result from improved calcium handling of the muscle, as the slowing of the RT1/2 in the soleus was prevented by swimming, such that it was significantly faster than in dystrophic aged animals and not significantly different from the adult mdx animals. However, with no other significant differences attributable to the swimming observed, it must be assumed that most of the improvements were due to improved histological appearance of the muscle, with less connective tissue and fat infiltration, and greater apparent survival of fully functional muscle fibers.

It is clear that the low levels of activity performed by the old mdx animals were responsible for the improvements in relative force production observed. This study has shown that, although mdx animals suffer from the functional deterioration consistent with a severe, dystrophin-deficient myopathy associated with connective tissue and fat infiltration (making them an excellent model of DMD), their muscles remain responsive to low-intensity exercise. However, although relative force production was increased, it must be noted that there was no change in the absolute forces generated by the exercised old mdx animals. Because human dystrophic muscle shows several similarities to old mdx muscles, these observations further support the usefulness of a non-weight-bearing, low-intensity exercise such as swimming as a potential therapeutic adjunct for human patients. However, the lack of any increase in absolute force production illustrates the fact that any functional improvements will be small and emphasizes the need to begin any exercise therapy as early as possible, when there is sufficient muscle still present to be positively influenced.

    ACKNOWLEDGEMENTS

Present address and address for reprint requests: A. Hayes, Dept. of Biomedical Sciences, Victoria Univ. of Technology (F064), PO Box 14428, Melbourne City MC, Melbourne, Victoria 8001, Australia.

    FOOTNOTES

Received 15 May 1997; accepted in final form 7 January 1998.

    REFERENCES
Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References

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AJP Cell Physiol 274(4):C1138-C1144
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