1. Alterations of Intra and Extra Mitochondrial Enzyme in the Muscle Fibersof Rat Hind Limbs: Role of Exercise




Название1. Alterations of Intra and Extra Mitochondrial Enzyme in the Muscle Fibersof Rat Hind Limbs: Role of Exercise
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Indian Journal of Gerontology

2007, Vol. 22, No. 1. pp 1 - 13


Alterations of Intra and Extra Mitochondrial Enzyme in the Muscle Fibers of Rat Hind Limbs: Role of Exercise Training Under Age Induced Oxidative Stress Conditions


K. Mallikarjuna, K. Nishanth*, T. Bhaskar Reddy and

K. Sathyavelu Reddy

Division of Exercise Physiology, Department of Zoology

Sri Venkateswara University, Tirupati – 517502, A.P, India


* Department of Biotechnology, Sri Vidyanikethan Engineering College

A. Rangampet, Tirupati

ABSTRACT


Advancing age is associated with decline in physiological functions and cause oxidative stress. It is well documented that physical exercise training enhances the capacity to resist oxidative stress and is useful at every stage of life. Keeping in view of this, the present study was under taken to investigate the effect of endurance exercise training on the status of selected metabolic enzymes in aging rat skeletal muscle fibers. Wistar male albino rats of two different age groups i.e., young (3 months) and old/moderately aged (18 months) were divided in to two groups; sedentary control (SC) and exercise trained (ExT). Metabolic enzymes glucose-6-phosphate dehydrogenase (G-6-PDH), succinate dehydrogenase (SDH) and malate dehydrogenase (MDH) estimated in the hind limb muscle fibers of soleus (SOL), red gastrocnemius (RG) and white gastrocnemius WG) were decreased with advancement of age in all the muscle fibers; whereas, these enzymes were found to be significantly (p<0.01) elevated in both the exercise trained groups. The decreased metabolic enzymes in old rats are indicative of low mitochondrial performance caused due to high levels of ROS during aging. Induction of the enzymes with exercise training was more in oxidative soleus muscles of young and old rats than the glycolytic muscle fibers. It is envisaged that regular exercise training may enhance the metabolic enzyme activities and play a vital role in maintaining the mitochondrial functions at active state even in old animals.

Key words : Aging, Exercise, G-6-PDH, Mitochondrial enzymes, Skeletal muscle fibers.

Aging is an integral part of the process of growth and development that is terminated by death. In humans and animals, aging is associated with pronounced morphological and functional changes in skeletal musculature including loss of muscle mass, muscle strength and decreased speed of contraction (Fielding and Meydani, 1997; Song et al., 2006). The loss of muscle mass is due to reduction in both the number and the size of muscle fibers. The muscle fibers of rat hind limb, soleus (SOL), red gastrocnemius (RG) and white gastrocnemius (WG) are also considered as Type-I (slow oxidative–SO), Type-IIa (fast oxidative glycolytic–FOG) and Type-IIb (fast glycolytic–FG) muscle fibers respectively (Armstrong and Phelps, 1984; Delp and Duan, 1996). There are greater losses of Type II fibers compared with Type I fibers with age and Type II fibers show more mitochondrial deletions and mitochondrial damage (Payne et al., 2003). The decline in mitochondrial content and function impairs muscle oxidative and endurance capacity and is therefore, likely to contribute to the increase in muscle fatigue ability that occurs with aging (Rooyackers et al., 1996).

Endurance exercise training has been shown to be very powerful stimulant of muscle growth and muscle protein synthesis. Strenuous physical exercise and sports are associated with a dramatic increase in oxygen uptake both at the whole-body level and in skeletal muscle. Evidences indicate the increase of oxygen flux in locomotor muscle during strenuous exercise results in the production of reactive oxygen species (ROS). Mitochondria are a major intracellular source for ROS production during oxidative phosphorylation not only in exercise but also in aging; and the increased production of ROS is implicated in the aging process (Bakala et al., 2003). The resulting large increase in ROS may stimulate antioxidant enzyme synthesis; influence their degradation or both and can alter the structure and function of lipids, proteins and nucleic acids, leading to cellular injury and even cell death (Powers et al., 2005; Lee and Wei, 2007). The changes that occur in mitochondria during stress conditions reflect on mitochondrial enzymes and may lead to alter in the mitochondrial succinate and malate dehydrogenase activities. Hence, these enzymes (TCA cycle intermediates) considered as markers for mitochondria.

Although many investigators focused on metabolic differences and antioxidant enzymes alterations in young and old rats under stress conditions (Hammeren et al., 1992; Jhansi Lakshmi, 1998; Mallikarjuna et al., 2004), the specific effects of endurance exercise training on energy and mitochondrial enzymes in different muscle fibers during aging, have not been completely understood. Because of the importance of the specific muscle fibers involved in neuro muscular disease and in exercise, the present investigation was taken up to investigate the effect of treadmill exercise on extra mitochondrial G-6-PDH and intra mitochondrial SDH and MDH enzyme activities in different muscle fibers of aging rats. The soleus muscle was selected because it is predominantly made up of slow twitch and highly oxidative fibers, the red gastrocnemius muscles as representative of fast twitch and oxidative fibers whereas, the white gastrocnemius muscle consists mainly of fast twitch and glycolytic fibers.

MATERIAL AND METHODS

Animal care and treatment:

Pathogen free, Wistar strain male albino rats (n=24) of two different age groups i.e., young (3 months) weighing 17010gm and moderately aged (18 months) weighing 24010gm were used in the current investigation (Approved by the Institutional Animal Ethics Committee). We assumed such a division of the age groups according to the studies of Cao and Cutler (1995) and Koprowska et al. (2004). The skeletal muscle growth/maturity occurs in between 3-6 months and other physiological changes occur in the aging rats from 6 months onwards. The rats were housed in clean polypropylene cages, 6 rats per cage and maintained under temperature controlled room (27  20C) with a photoperiod of 12 hrs light and 12hrs dark cycle. The rats were fed with a standard rat pellet diet and water ad libitum.

Rats from each age (3-months and 18-months) were divided into two groups of six in each group. The first group of rats from each age (young n=6; old n=6) served as sedentary control (SC). The second group of rats from each age (young n=6; old n=6) were considered as exercise trained (ExT) group and subjected to treadmill exercise training with medium uphill constant gradient of 7.5% at a running speed of 23m/min, 30min/day and 5 days/week for a period of 12 weeks. All exercised animals in both age groups completed 12 weeks period of exercise training protocol. The running program was scheduled between 6.00 and 8.00 A.M. The treadmill was custom built at the University Scientific Instrumentation Center (USIC), Sri Venkateswara University Campus, Tirupati.

Muscle Sampling Procedure

The animals were sacrificed after 24 hrs of the last training session along with sedentary control rats by cervical dislocation and selected muscle fibers of soleus (SOL), red gastrocnemius (RG) and white gastrocnemius (WG) were quickly removed from the hind limb by using dissecting microscope. The excised muscle fibers (SOL, RG and WG) were rapidly cleaned free of fat, tendon and surface fascia in ice-cold rat ringer solution, blotted and immediately frozen in liquid nitrogen. Tissues were stored at –800C until biochemical analysis. Total time for muscle excision, dissection and freezing was < 5 min.

Biochemical Assays

Selected metabolic enzyme activities, such as glucose 6 phosphate dehydrogenase (G-6-PDH, EC:1.1.1.49), succinate dehydrogenase (SDH, EC:1.3.99.1) and malate dehydrogenase (MDH, EC: 1.1.1.37) were measured using the methods of Lohr and Waller (1965), Nachlas et al. (1960) and Prameelamma and Swami (1975) respectively.

Statistical Analysis

The ANOVA was carried out by using SPSS package and the data has been analyzed for the significance of the main effects i.e., age and exercise along with their interaction. Results were expressed as the mean  SD of six observations and the values of significance were evaluated. The difference was considered significant at p < 0.01.
RESULTS

The specific activity of G-6-PDH remarkably decreased in the old rats compared to young ones. With endurance exercise training G-6-PDH activity was significantly elevated in both the age groups of rats than that of their respective controls. Among muscle fibers, oxidative soleus muscles exhibited maximum percent elevation with exercise training in the young and also in moderately aged rats. Nevertheless, the elevation was more in young rats.

Table 1 : Changes in Glucose-6-Phosphate Dehydrogenase (G6-PDH) activity in selected skeletal muscle fibers of sedentary control and exercise trained male albino rats


Skeletal Young rats Moderately aged rats

muscle (3 months) (18 months)

fibers Sedentary Exercise Sedentary Exercise control trained control trained

Soleus 0.619 0.004 0.9680.006* 0.5760.005 0.7810.003*(SOL) (+56.34) (+35.72)

Red 0.5860.004 0.8890.003* 0.4840.003 0.3260.004* Gastrocnemius (+51.86) (+29.52)

(RG)

White 0.4980.003 0.7110.005* 0.4050.004 0.5130.004* Gastrocnemius (+42.93) (+26.89)

(WG)

Values are expressed in units of superoxide anion reduced /mg protein/min.

Values are mean  SD; * significant at p < 0.001.

Values in parentheses denote percent change.

Table 2 : Changes in Succinate Dehydrogenase (SDH) activity in selected skeletal muscle fibers of sedentary control and exercise trained

Young rats Moderately aged rats

Skeletal (3 months) (18 months)

muscle fibers Sedentary Exercise Sedentary Exercise control trained control trained

Soleus 0.613±0.045 1.290±0.052* 0.549±0.023 0.917±0.039*

(SOL) (+110.44) (+67.03)


Red 0.415±0.025 0.661±0.064* 0.361±0.039 0.548±0.062*

Gastrocnemius (+60.24) (+51.38)

(RG)


White 0.397±0.059 0.726±0.061* 0.320±0.067 0.432±0.056* Gastrocnemius (+82.66) (+34.57)

(WG)

Values are expressed in units of superoxide anion reduced /mg protein/min.

Values are mean  SD; * significant at p < 0.001.

Values in parentheses denote percent change over sedentary control


As a function of age, the activities of SDH was remarkably decreased in aged rats. But endurance exercise training elevated the SDH activities in all muscle fibers of both the age groups. Drastic increase of SDH activity with exercise training was noticed in slow twitch oxidative muscle fibers. Moreover this elevation was more in young exercised rats than aged rats.

The specific activity of MDH decreased with advancement of age in all muscle fibers; but exercise training elevated the MDH activity in young and aged muscle fibers. MDH showed maximum percent elevation in highly oxidative soleus muscle than other muscle fibers in the young age group.

Table 3 : Changes in Malate Dehydrogenase (MDH) activity in selected skeletal muscle fibers of sedentary control and exercise trained

Young rats Moderately aged rats

Skeletal (3 months) (18 months)

muscle fibers Sedentary Exercise Sedentary Exercise control trained control trained

Soleus (SOL) 0.540  0.036 1.006  0.095* 0.414  0.034 0.595  0.034* (+86.296) (+43.719)

Red 0.377  0.038 0.568  0.041* 0.315  0.014 0.5020.022* Gastrocnemius (+50.529) (+59.365)

(RG)

White 0.5345  0.031 0503  0.028* 0.275  0.036 0.404 0.017* Gastrocnemius (+45.797) (+46.909)

(WG)

Values are expressed in units of superoxide anion reduced /mg protein/min.

Values are mean  SD; * significant at p < 0.001

Values in parentheses denote percent change over sedentary control

DISCUSSION

Glucose-6-phosphate dehydrogenase (G-6-PDH) enzyme is extra mitochondrial in location and highly specific for NADP as an electron acceptor. It has been traditionally thought that this is a typical “house keeping” enzyme that is regulated solely by the ratio of NADPH and NADP (Kletzien et al. 1994; Tian et al., 1999). In the present investigation, the decline in the activity of G-6-PDH was noticed in all muscle fibers of aged animals. This decrease could be attributed to the reduced availability of NADP. Similar age related decrease in hepatic G-6-PDH activity was reported by Gurumurthy (2001) in aged female rats. Aging is associated with the accumulation of inactive or less active forms of numerous enzymes and cause oxidative stress. Oxidative stress caused by aging, may result in the decrease of protein thiols, which may consequently impair many enzymes. In such a situation, G-6-PDH inhibition is closely associated with decreased protein thiols. A decrease in protein thiol content is a consequence of an increase in intracellular oxidants. The inhibition of G-6-PDH may decrease cellular reducing equivalents, thus limiting anti-oxidative defense mechanism (Tian et al., 1999). Endurance exercise training significantly augmented the G-6-PDH activities in both age groups. Salvemini et al.(1999) reported that G-6-PDH induction served to maintain and regenerate the intracellular GSH pool. Several pieces of evidences indicate that the formation of GSH from its oxidized form GSSG, is dependent on NADPH produced by the pentose phosphate pathway and this pathway can be activated in response to GSH depletion. There is a correlation between GSH and G-6-PDH activity, when G-6-PDH activity reaches maximum induction, GSH pool is restored, suggesting that GSH equilibrium could be depend on G-6-PDH expression. The increased GSH content with exercise training is also an indicative of increased G-6-PDH activity in tissues (Mallikarjuna et al., 2004). It is that G-6-PDH activity has a predominant role in the control of GSH output and is essential in maintaining an intracellular redox potential.

Succinate dehydrogenase (SDH) is a marker enzyme of mitochondria in the tissues. The activity of SDH in mitochondria is usually far greater than the other enzymes in both the developing and adult animals. Since the specific activity of SDH indicates the state of oxidative metabolism in mitochondria, any alterations in its activity under age induced oxidative stress conditions reflects the extent of derangement in mitochondrial metabolism. The present study illustrates that the specific activity of SDH was dramatically decreased with advancement of age. The decrease in the SDH activity of old rats reflects the general decline in oxidative potential of senescent tissues due to marked structural changes in the mitochondria such as enlargement, matrix vacuolation and shortened cristae with the advancement of age (Wardlaw et al., 1986). The decrease in SDH activity was also reported by Jhansi Lakshmi (1998) and Mallikarjuna (2005) in senescent rats. This is may be due to diversion of TCA cycle intermediates such as -ketoglutarate, for its conversion into glutamate and glutamine to counter the toxic effects of ammonia produced during aging there by depleting TCA cycle intermediates. Aging may be considered as a type of stress, which leads to hypoxic condition in the tissues and consequent reduction in mitochondrial oxidoreductase activities (Mallikarjuna, 2005). The increased SDH activity during exercise training indicates increased aerobic efficiency of the skeletal muscle fibers. Previously, Hammeren et al. (1992) and Sullivan et al., (1995) reported significant increase of SDH activity with exercise in different tissues of both young and old rats. Stuewe et al. (2000) reported that chronic exercise training induces many mitochondrial enzymes and enhanced mitochondrial proteins turnover which may also contribute to elevated SDH activity. Thus, the increased activity of SDH in young rats with endurance exercise training suggests increased mitochondrial oxidative potential and energy synthesis, utilization of carbohydrates and fats as substrates. During dynamic exercise at intensities more than 50% of peak oxygen uptake, there is a net increase in the total concentrations of TCA cycle intermediates in the tissues (Howarth et al. 2004). As an intermediate substance of TCA cycle, the increased SDH activity may be due to the increased availability of the substrate for SDH activity in the citric acid cycle.

Malate dehydrogenase (MDH) is a mitochondrial enzyme and plays an important role in TCA cycle. It has been demonstrated that several mitochondrial soluble NAD+ dependant dehydrogenases including MDH are specifically associated with NADH: ubiquinone oxidoreductase (Kotlayar et al., 2004). Mitochondrial MDH activity was remarkably decreased in older rats compared to the younger ones. Sailaja (1997) reported a decreased MDH activity in the skeletal muscles old albino rats. Ji et al. (1991) observed significant drop in MDH activity in the tissues of senile rats whereas training increased the enzyme activity levels. The decrease in MDH activity in senile rats suggests the lower utilization of malate in the Kreb’s cycle. The drop in the MDH activity denotes fluctuations of oxidative metabolism and also reflects the turnover of carbohydrates and energy out put (Murray et al., 2000).

It has been demonstrated that impairment in mitochondrial respiration and oxidative phosphorylation elicits an increase in oxidative stress (Yan and Sohal, 1998). Moreover, oxidative damage and large-scale depletion and duplication of mitochondrial DNA have been found to increase with age in various tissues of mammals. All these functional derangements occuring in mitochondria may lead to cause decrease mitochondrial enzymes including MDH during aging. Endurance exercise training significantly elevated the MDH activity in all muscle fibers in both age groups. Mallikarjuna (2005) and Gurumurthy (2001) reported increased hepatic and cardiac MDH activities with exercise training in albino rats. The increased MDH activity with response to exercise training suggests that higher utilization of malate in the tissues, which reflects the up-regulation of oxidative metabolism and the turnover of carbohydrates and energy output which is required during development and as well as exercise (Murray et al., 2000). Previously, Stuewe et al. (2000) reported that NAD concentration is a key factor in the activation of mitochondrial MDH. It was reported that the activity of mitochondrial enzymes increased with endurance exercise training in animals (Powers et al., 2005). Increased mitochondrial capacity also indicates the increased activity of MDH in the cell with endurance exercise training. The elevated mitochondrial activity also suggest a positive shift from anaerobic to aerobic utilization of energy substrates after continuous repetition of exercise regimen which could change or compensate physiological inefficiency that occur in aging phenomenon.

conclusions

In the present study, the selected dehydrogenases such as extra mitochondrial G-6-PDH and intra mitochondrial SDH and MDH enzymes were drastically decreased in older rats and elevated with exercise training. These results suggest that mitochondrial enzymes in old animals are less able to meet energy demands. The increased mitochondrial enzymes with exercise training even in old animals suggest that mitochondrial work efficiency and oxidative phosphorylation was improved. From this results, one could infer that regular exercise training provide a favorable atmosphere to mitochondria to cope with oxidative stress which is caused by high amount of free radicals during aging. It also suggests that muscle energy metabolism and TCA cycle intermediates are increased with exercise training even in aging rats.

Acknowledgements

One of the authors (Dr. K. Sathyavelu Reddy) is thankful to the University Grants Commission (New Delhi) for the financial support in the form of a Major Research Grant [UGC–MRP F-3 -125/2001 (SR-II) dt 29-03-2001] to carry out this work.

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Indian Journal of Gerontology

2008, Vol. 22, No. 1. pp 14 -26
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