Changes in the expression of uncoupling proteins and lipases in porcine adipose tissue and skeletal muscle during feed deprivation*(1)

The hormone-sensitive and lipoprotein lipases are critical determinants of the metabolic adaptation to starvation. Additionally, the uncoupling proteins have emerged with potential roles in the metabolic adaptations required by energy deficiency. The objective of this study was to evaluate the expression (mRNA abundance) of uncoupling proteins 2 and 3 and that of hormone-sensitive and lipoprotein lipase in the adipose tissue and skeletal muscle of the pig in relationship to feed deprivation. Thirty-two male castrates (87 kg +/- 5%) were assigned at random to fed and feed-deprived treatment groups. After 96 hr, the pigs were euthanized and adipose and skeletal muscle tissue obtained for total RNA extraction and nuclease protection assays. Feed deprivation increased uncoupling protein 3 mRNA abundance 103-237% (P < 0.01) in longissimus and red and white semitendinosus muscle. In contrast, the increase in uncoupling protein 3 mRNA in adipose tissue was only 23% (P < 0.06), and adipose uncoupling protein 2 mRNA was not influenced (P > 0.66) by feed deprivation. The increased abundance of uncoupling protein 2 mRNA in the longissimus muscle of feed-deprived pigs was small (22%), but significant (P < 0.04). The expression of hormone-sensitive lipase was increased 46% and 64% (P < 0.04) in adipose tissue and longissimus muscle, respectively, by feed deprivation, whereas adipose lipoprotein lipase expression was reduced (P < 0.01) to 20% of that of the fed group. Longissimus lipoprotein lipase expression in the feed-deprived group was 37% of that of the fed group (P < 0.01), and similar reductions were detected in red and white semitendinosus muscle. Overall, these findings indicate that uncoupling protein 3 expression in skeletal muscle is quite sensitive to starvation in the pig, whereas uncoupling protein 2 changes are minimal. Furthermore, we conclude that hormone-sensitive lipase is upregulated at the mRNA level with prolonged feed deprivation, whereas lipoprotein lipase is downregulated.     

Effect of endurance training on the phospholipid content of skeletal muscles in the rat

Only few data are available on the effect of training on phospholipid metabolism in skeletal muscles. The aim of the present study was to examine the effect of 6 weeks of endurance training on the content of particular phospholipid fractions and on the incorporation of blood-borne [14C]-palmitic acid into the phospholipids in different skeletal muscles (white and red sections of the gastrocnemius, the soleus and the diaphragm) of the rat. Lipids were extracted from the muscles and separated using thin-layer chromatography into the following fractions: sphingomyelin, phosphatidylcholine, phosphatidylserine, phosphatidylinositol, phosphatidylethanolamine, cardiolipin and neutral lipids (this fraction being composed mostly of triacylglycerols). It was found that training did not affect the content of any phospholipid fraction in soleus muscle. It increased the content of sphingomyelin in white gastrocnemius muscle, cardiolipin and phosphatidylethanolamine in red gastrocnemius muscle and phosphatidylinositol in white gastrocnemius muscle and diaphragm. The total phospholipid content in red gastrocnemius muscle of the trained group was higher than in the control group. Training reduced the specific activity of sphingomyelin and cardiolipin in all muscles, phosphatidylcholine in soleus, red, and white gastrocnemius muscles, phosphatidylserine in all muscles, phosphatidylinositol in all except the soleus muscle, and phosphatidylethanolamine in hindleg muscles, but not in the diaphragm compared to the corresponding values in the sedentary group. It was concluded that endurance training affects skeletal muscle phospholipid content and the rate of incorporation of the blood-borne [14C]palmitic acid into the phospholipid moieties.     

Acute effect of physical exercise on glycogen content and lipoprotein lipase activity in untrained diabetic rats

The effect of acute physical exercise on skeletal muscle glycogen content and on lipoprotein lipase activity of muscle, adipose and lung tissues was studied in streptozotocin diabetic and control rats. Rats were accustomed to treadmill running for two weeks after streptozotocin treatment. For an exercise bout of moderate intensity rats were randomly divided into two groups: one was sacrificed immediately after exercise and the other 24 hours afterwards. In addition there was a nonexercised sedentary group. No depletion of glycogen was observed after exercise in the vastus lateralis muscle of control (nondiabetic) rats. No difference in glycogen utilization was found in soleus muscle between diabetic and control rats. In diabetic rats a slight decrease occurred in the lipoprotein lipase activity in adipose tissue immediately after exercise, while in control rats there was a significant decline 24 hours after exercise. In soleus muscle a slight but significant increase of lipoprotein lipase activity occurred 24 hours after exercise in diabetic rats but not in control rats. The results suggest that nonketotic streptozotocin diabetes of short duration does not influence muscle glycogen in the resting state, but glycogen utilization is disturbed in white muscle during moderate treadmill running in untrained diabetic rats. The increase in lipoprotein lipase activity after physical exercise in red muscle of diabetic rats occurs during the recovery phase.     

Rat skeletal muscle triacylglycerol utilization during exhaustive swimming

The utilization of triacylglycerol in slow oxidative, fast oxidative-glycolytic, and fast glycolytic skeletal muscle fiber types was examined in rats subjected to a prolonged exhaustive swim. Significant reductions of intramuscular triacylglycerol occurred following 2 h and 40 min of swimming in all muscles containing a predominance of slow oxidative and fast oxidative-glycolytic fibers, which possess a high capacity for free fatty acid oxidation. Triacylglycerol content in the soleus decreased by 48%, and reductions of 41, 29, and 27% were measured in the red vastus lateralis, red gastrocnemius, and plantaris muscles, respectively. In the white vastus lateralis and white gastrocnemius muscles (fast glycolytic fibers) triacylglycerol concentrations were unaffected. In all muscles the variability of intramuscular triacylglycerol measurements between animals was 20-50% and the within animal variance (right vs. left hindlimb) was similar. Analytical repeatability was approximately 10% in all muscles and significantly less than the between- and within-animal variances. It was concluded that a real biological variation exists in the triacylglycerol content of all rat skeletal muscles and that intramuscular triacylglycerol is an important energy source during prolonged exercise of moderate intensity.     

Lipoprotein lipase activity in skeletal muscles of the rat: effects of denervation and tenotomy.

The effects of denervation, tenotomy, or tenotomy with simultaneous denervation on the activity of heparin-releasable and intracellular, residual lipoprotein lipase (LPL) and triacylglycerol (TG) content were examined in rat skeletal muscles. An influence of muscle electrostimulation on denervated and tenotomized muscles was also evaluated. Activity of both LPL fractions was decreased in denervated and/or tenotomized soleus and red portion of gastrocnemius muscles. It was accompanied by a slight elevation of the intracellular TG content. Electrostimulation increased activities of both fractions of LPL in red muscles from intact hindlimbs. In stimulated denervated muscles without or with simultaneous tenotomy, activity of two LPL fractions was also enhanced, but control values were reached only in denervated soleus muscle. Electrical stimulation had no pronounced effect on LPL activity in tenotomized muscles. In conclusion, denervation and/or tenotomy decreases LPL activity in red muscles, indicating reduction of the muscle potential to utilize circulating TG. Electrostimulation only partly restores the diminished LPL activity in denervated muscles, without any effect in tenotomized ones. Thus, to maintain LPL activity in resting muscle, intact innervation and tension are needed.     

Effect of cadmium intoxication on glucose utilization in energy metabolism of muscles

The influence of cadmium intoxication on carbohydrate metabolism in skeletal muscles and liver of the male Wistar rats has been studied. Cadmium was administered as cadmium acetate in a dose of 0.3 mg Cd2+/kg body weight for three months. At the same time the control rats were injected with 0.9% NaCl. The animals were decapitated and samples of their skeletal muscles: the soleus muscle (composed mainly of red slow twitch fibers; ST) the gastrocnemius muscle containing two types of fibers (white fast twitch fibers FTb and red fast twitch fibers, FTa) and the liver were dissected out. In the samples of muscles, liver and serum contents of glycogen, glucose, pyruvate and lactate, as well as activities of hexokinase, pyruvate kinase and lactate dehydrogenase were measured. Intoxication of rats with cadmium for three months resulted in a reduction of glycolytic enzymes in the serum, ST and FTa muscle fibers and in the liver but did not change the activities of glycolytic enzymes in the FTb muscle fibers. The data obtained for the concentrations of glycogen in the liver and skeletal muscles suggest different mechanisms of cadmium influence on glycogen utilization in these organs.     

Effect of streptozotocin-diabetes on the functioning of the sphingomyelin-signalling pathway in skeletal muscles of the rat.

AIMS/HYPOTHESIS: Ceramide is the main second messenger in the sphingomyelin-transmembrane signalling pathway. The compound is likely to play a role in the induction of insulin resistance. The aim of the present study was to examine the effect of streptozotocin diabetes on the content and composition of ceramides and sphingomyelins and the activity of neutral Mg (2+)-dependent sphingomyelinase and acid sphingomyelinase in different types of skeletal muscle of the rat. METHODS: The experiments were carried out on two groups of male Wistar rats weighing 250-280 g: controls and those treated with streptozotocin at a dose of 60 mg/kg. Determinations were performed on three types of skeletal muscle: the slow-twitch oxidative (soleus), fast-twitch oxidative-glycolytic (red section of the gastrocnemius) and fast-twitch glycolytic (white section of the same muscle). The content and composition of ceramide- and sphingomyelin-fatty acids were determined using gas-liquid chromatography. The activity of the enzymes was measured using N-[(14)CH (3)]-sphingomyelin as the substrate. RESULTS: Twelve different ceramides and sphingomyelins were identified and quantified in each muscle with regard to the fatty acid residue. The ratio of total content of ceramide-saturated fatty acids to the total content of ceramide-unsaturated fatty acids was more than two. In the case of sphingomyelin, the ratio was similar to ceramide in the soleus and much higher in both sections of the gastrocnemius. Treatment with streptozotocin increased the total content of ceramide-fatty acids by 78% (p < 0.001) in the soleus, 27.5% (p < 0.01) in the red and 36.9% (p < 0.001) in the white section of the gastrocnemius. Concomitantly, the total content of sphingomyelin-fatty acids decreased by 43.8%, 31.2%, 24.8% (p < 0.001 in each case) in the respective muscles. The activity of neutral Mg (2+)-dependent sphingomyelinase was elevated by 69.5%, 105.9% and 62.3% in the soleus and red and white gastrocnemius, respectively (p < 0.001 for each muscle). The activity of acid sphingomyelinase was stable in the soleus and white gastrocnemius and decreased by 15.7% (p < 0.01) in the red gastrocnemius. CONCLUSION/INTERPRETATION: The results obtained show that insulin deficiency results in elevation in the content of ceramide in skeletal muscles. This indicates that the hormone is involved in regulation of the activity of the sphingomyelin-signalling pathway in the muscles.     

Effect of acute exercise on the content of free sphinganine and sphingosine in different skeletal muscle types of the rat.

It has previously been shown that prolonged exercise of moderate intensity reduces the content of ceramide in each type of skeletal muscle. This was accompanied by a reduction in the activity of neutral, Mg++-dependent sphingomyelinase (the major enzyme responsible for ceramide formation from sphingomyelin) in the soleus and red gastrocnemius, but not in the white gastrocnemius (A. Dobrzyń and J. Górski, Am. J. Physiol.: Endorcinol. Metab. 282: E277 - E285, 2002). No other data on regulation of ceramide metabolism in contracting muscles are available. The aim of the present study was to examine the content of sphinganine (a key precursor of ceramide on the de novo synthesis route) and the content of sphingosine (the main product of ceramide catabolism) in different skeletal muscle types after two kinds of acute exercise. The experiments were carried out on 30 male Wistar rats, 250 - 280 g of body weight. The rats were divided equally into three groups: 1 - control, 2 - run until exhaustion (1200 m/h, +10 degree incline), 3 - a group in which the sciatic nerve was stimulated 10 min with tetanic pulses (60 pulses/min). Samples were taken of the soleus and of the red and white section of the gastrocnemius. These muscles are composed mostly of the slow-twitch oxidative, fast-twitch oxidative-glycolytic and fast-twitch glycolytic fibers, respectively. Lipids were extracted with chloroform/methanol. Sphinganine and sphingosine were quantified by high-performance liquid chromatography. At rest, the content of sphinganine in the soleus was higher than in the red gastrocnemius (p < 0.05), and in the latter, it was higher than in the white gastrocnemius (p < 0.01). Prolonged exercise increased the content of sphinganine approximately 6-fold in each muscle. The resting content of sphingosine in the soleus and in the red gastrocnemius was similar--higher than in the white gastrocnemius (p < 0.001 and p < 0.01, respectively). The content of sphingosine increased over 3-fold in the soleus and nearly 2-fold in the red and white sections of the gastrocnemius. Stimulation of the sciatic nerve increased the content of both compounds approximately 2-fold in each muscle. We conclude that acute exercise increases both de novo synthesis and catabolism of ceramide in skeletal muscles. Accumulation of sphingosine in contracting muscles may contribute to the development of fatigue.     

Regulation of lipoprotein lipase in muscle and adipose tissue during exercise.

Lipoprotein lipase (LPL) is regulated in a tissue-specific manner; exercise increases LPL activity in muscle at the same time it is reduced in adipose tissue. The purpose of this study was to determine the relationship between LPL activity and LPL mRNA in muscle and adipose tissue in rats exposed to one bout of exercise. Immediately after a 2-h swim, LPL activity [pmol free fatty acids (FFA).min-1.mg tissue-1] in the exercised animals was reduced 43% in adipose tissue (110 +/- 26 to 63 +/- 17) and increased almost twofold in the soleus muscle (203 +/- 26 to 383 +/- 59) compared with sedentary control animals. At the same time, LPL mRNA was reduced 42% in adipose tissue and increased 50 and 100% in the red vastus and white vastus muscles, respectively. Twenty-four hours after the swim, LPL activity had returned to control levels in adipose tissue and the soleus muscle. At hour 24 of recovery, LPL mRNA was still reduced 23% in the adipose tissue of exercised animals but was not significantly different between exercised and control animals in any of the muscle tissues analyzed. Changes in total RNA concentration could not account for the changes in relative LPL mRNA expression. The relationship between LPL enzyme activity and LPL mRNA in muscle and adipose tissue was +0.86 and +0.93 at 0 and 24 h postexercise, respectively. Thus the tissue-specific changes in enzyme activity induced by exercise could be mediated, in part, through pretranslational control.     

No limiting role for glycogenin in determining maximal attainable glycogen levels in rat skeletal muscle

We examined whether the protein level and/or activity of glycogenin, the protein core upon which glycogen is synthesized, is limiting for maximal attainable glycogen levels in rat skeletal muscle. Glycogenin activity was 27.5 +/- 1.4, 34.7 +/- 1.7, and 39.7 +/- 1.3 mU/mg protein in white gastrocnemius, red gastrocnemius, and soleus muscles, respectively. A similar fiber type dependency of glycogenin protein levels was seen. Neither glycogenin protein level nor the activity of glycogenin correlated with previously determined maximal attainable glycogen levels, which were 69.3 +/- 5.8, 137.4 +/- 10.1, and 80.0 +/- 5.4 micromol/g wet wt in white gastrocnemius, red gastrocnemius, and soleus muscles, respectively. In additional experiments, rats were exercise trained by swimming, which resulted in a significant increase in the maximal attainable glycogen levels in soleus muscles ( approximately 25%). This increase in maximal glycogen levels was not accompanied by an increase in glycogenin protein level or activity. Furthermore, even in the presence of very high glycogen levels ( approximately 170 micromol/g wet wt), approximately 30% of the total glycogen pool continued to be present as unsaturated glycogen molecules (proglycogen). Therefore, it is concluded that glycogenin plays no limiting role for maximal attainable glycogen levels in rat skeletal muscle.     

Isolation of the cDNA encoding rat skeletal muscle myosin light chain kinase. Sequence and tissue distribution

A cDNA clone encoding skeletal muscle myosin light chain kinase (MLCK) was isolated from a rat skeletal muscle library using oligonucleotide probes. The total length of the rat skeletal muscle MLCK cDNA was 2823 base pairs with an open reading frame of 1830 base pairs. The deduced sequence of the 610-amino acid protein exhibited 96% amino acid identity to rabbit skeletal muscle MLCK in the carboxyl-terminal portion of the molecule, which contains the catalytic and the calmodulin-binding domains, and 58% identity in the amino-terminal region. Analysis of total rat mRNA revealed a single mRNA species of 3.4 kilobases that was unique to skeletal muscle. Further analysis of skeletal muscle tissue using fast-twitch glycolytic, fast-twitch oxidative glycolytic, and slow-twitch oxidative fibers isolated from rat leg revealed that the mRNA level for MLCK varied among the three fiber types. The results of kinase assays performed on the fibers showed that MLCK activity levels paralleled the MLCK mRNA levels found in each of the three types of skeletal muscle fibers studied. Fast-twitch oxidative glycolytic (gastrocnemius red) and slow-twitch oxidative (soleus) exhibited 60 and 13%, respectively, of the enzymatic activity present in fast-twitch glycolytic (gastrocnemius white) fibers.     

Muscle oxidative capacity during IL-6-dependent cancer cachexia

Many diseases are associated with catabolic conditions that induce skeletal muscle wasting. These various catabolic states may have similar and distinct mechanisms for inducing muscle protein loss. Mechanisms related to muscle wasting may also be related to muscle metabolism since glycolytic muscle fibers have greater wasting susceptibility with several diseases. The purpose of this study was to determine the relationship between muscle oxidative capacity and muscle mass loss in red and white hindlimb muscles during cancer cachexia development in the Apc(Min/+) mouse. Gastrocnemius and soleus muscles were excised from Apc(Min/+) mice at 20 wk of age. The gastrocnemius muscle was partitioned into red and white portions. Body mass (-20%), gastrocnemius muscle mass (-41%), soleus muscle mass (-34%), and epididymal fat pad (-100%) were significantly reduced in severely cachectic mice (n = 8) compared with mildly cachectic mice (n = 6). Circulating IL-6 was fivefold higher in severely cachectic mice. Cachexia significantly reduced the mitochondrial DNA-to-nuclear DNA ratio in both red and white portions of the gastrocnemius. Cytochrome c and cytochrome-c oxidase complex subunit IV (Cox IV) protein were reduced in all three muscles with severe cachexia. Changes in muscle oxidative capacity were not associated with altered myosin heavy chain expression. PGC-1α expression was suppressed by cachexia in the red and white gastrocnemius and soleus muscles. Cachexia reduced Mfn1 and Mfn2 mRNA expression and markers of oxidative stress, while Fis1 mRNA was increased by cachexia in all muscle types. Muscle oxidative capacity, mitochondria dynamics, and markers of oxidative stress are reduced in both oxidative and glycolytic muscle with severe wasting that is associated with increased circulating IL-6 levels.     

Epinephrine-activation of heparin-nonreleasable lipoprotein lipase in 3 skeletal muscle fiber types of the rat

Epinephrine was used to activate the heparin non-releasable lipoprotein lipase (LPL) in the 3 skeletal muscle fiber types of the perfused rat hindlimb. Following a 9 min washout of the capillary-bound lipoprotein lipase, the hindquarter of the rat was perfused with a buffer containing 10 nM of epinephrine. Activity of the residual LPL in soleus, red vastus lateralis, and white vastus lateralis muscles increased 75%, 96%, and 102% respectively, following epinephrine perfusion. These results suggest that skeletal muscle LPL is under hormonal control possibly through protein phosphorylation by cyclic AMP dependent protein kinase.     

Characterization of the triacylglycerol lipase activity in three types of rat skeletal muscle

The purpose of this study was to characterize the lipolytic activity of the alkaline triglyceride lipase in homogenates of three types of skeletal muscle obtained from heparin-perfused rat hindlimb. Specifically, the red portion of the vastus lateralis, the white portion of the vastus lateralis, and the soleus muscles were examined. To remove capillary-bound lipoprotein lipase from the capillary beds, muscle was perfused with an erythrocyte-free buffer containing 4% albumin, 5 units of heparin/mL, and 7.5 microM adenosine. Adenosine reduced perfusion pressure from 117 +/- 5 to 86 +/- 6 mmHg (1 mmHg = 133.32 Pa), providing evidence for an effective vasodilation. This vasodilation increased the amount of lipoprotein lipase removed from the capillary beds. By the end of the experiment, perfusates were lipoprotein lipase-free. Oxygen supply to the perfused hindlimb appeared adequate as evidenced by similar high energy phosphate values for perfused and contralateral control tissues. For example, in soleus muscle, ATP content was 4.5 +/- 0.6 vs. 4.2 +/- 0.3 mumol/g, ADP concentration was 1.0 +/- 0.2 vs. 1.4 +/- 0.2 mumol/g, and creatine phosphate level was 12.9 +/- 0.7 vs. 11.0 +/- 0.6 mumol/g for perfused and contralateral control soleus, respectively. In addition, K+ output by the hindlimb was negligible, while glycolytic flux of perfused muscle was similar to that measured in control tissue. The findings that triglyceride levels of soleus and red vastus lateralis were decreased suggest that endogenous triglyceride was providing energy for the hindlimb during perfusion. Skeletal muscle triglyceride lipase activity was stimulated by serum and heparin, inhibited by NaCl and protamine, and had a pH optimum of 8.1.(ABSTRACT TRUNCATED AT 250 WORDS)     

Skeletal muscle fiber, nerve, and blood vessel breakdown in space-flown rats

Histochemical and ultrastructural analyses were performed postflight on hind limb skeletal muscles of rats orbited for 12.5 days aboard the unmanned Cosmos 1887 biosatellite and returned to Earth 2 days before sacrifice. The antigravity adductor longus (AL), soleus, and plantaris muscles atrophied more than the non-weight-bearing extensor digitorum longus, and slow muscle fibers were more atrophic than fast fibers. Muscle fiber segmental necrosis occurred selectively in the AL and soleus muscles; primarily, macrophages and neutrophils infiltrated and phagocytosed cellular debris. Granule-rich mast cells were diminished in flight AL muscles compared with controls, indicating the mast cell secretion contributed to interstitial tissue edema. Increased ubiquitination of disrupted myofibrils implicated ubiquitin in myofilament degradation. Mitochondrial content and succinic dehydrogenase activity were normal, except for subsarcolemmal decreases. Myofibrillar ATPase activity of flight AL muscle fibers shifted toward the fast type. Absence of capillaries and extravasation of red blood cells indicated failed microcirculation. Muscle fiber regeneration from activated satellite cells was detected. About 17% of the flight AL end plates exhibited total or partial denervation. Thus, skeletal muscle weakness associated with spaceflight can result from muscle fiber atrophy and segmental necrosis, partial motor denervation, and disruption of the microcirculation.     

Time course of changes in lipoprotein lipase activity in rat skeletal muscles during denervation-reinnervation.

The effects of denervation-reinnervation after sciatic nerve crush on the activity of extracellular and intracellular lipoprotein lipase (LPL) were examined in the soleus and red portion of gastrocnemius muscles. The activity of both LPL fractions was decreased in the two muscles within 24 h after the nerve crush and remained reduced for up to 2 wk. During the reinnervation period, LPL activity was still reduced in the soleus and started to increase only on the 40th day. In the red gastrocnemius, LPL activity increased progressively with reinnervation, exceeding control values on the 30th day post-crush. The LPL activity in the soleus from the contralateral to denervated hindlimb was also affected, being increased on the postoperation day and then gradually decreased during the following days. In conclusion, the time course of changes in muscle LPL activity after nerve crush confirmed the predominant role of nerve conduction in controlling muscle potential to take up free fatty acids derived from the plasma triacylglycerols. However, other factors, such as muscle fiber composition and the fiber transformation, should also be considered in this aspect of the denervation-reinnervation process. Moreover, it was found that denervation of muscles from one hindlimb may influence LPL activity in muscles from the contralateral leg.     

Effect of sciatic nerve crush on enzyme activities of skeletal muscles of the rat.

Weight loss and reduction in specific activity of cytochrome oxidase of both red (soleus) and white muscles (gastrocnemius and plantaris) of the rat was greatest 5 days after sciatic nerve crush (p less than 0.001) and then became minimal. In neither was there a significant, concomitant loss of protein. By 19 days, the specific activity of cytochrome oxidase was the same in both muscle types. The specific activity of lactate dehydrogenase was reduced significantly (p less than 0.001) in the white muscles, to a value approaching that of the red soleus by 19 days postoperatively, but remained unaltered in the soleus. Nerve crush is proposed as a model experimental system for studying neural regulation of skeletal muscle metabolism.     

Morphological and biochemical changes in old rat muscles: effect of increased use

Male Wistar rats were strength and swim trained during a substantial period of old age to determine the influence of aging and activity on the histochemical and metabolic characteristics of a predominantly slow (soleus) and a predominantly fast (plantaris) skeletal muscle. Strength training counteracted the age-related atrophy of the fibers and the age-induced changes in fiber-type distribution of both muscles. Swim training, on the other hand, was without any effect on these parameters. The activity of both mitochondrial and cytoplasmic enzymes became lower with aging in the soleus muscle, whereas only the activity of the cytoplasmic enzymes became lower in the plantaris. Strength training reduced the aerobic capacity of both muscles, whereas swim training had the opposite effect. Aging induced a lower glycogen concentration of the lateral gastrocnemius muscle. This was avoided by swim training. The phosphocreatine and adenosine 5'-triphosphate concentrations were unchanged with aging but became higher with strength training. The activity pattern, therefore, seems to have a considerable influence on the age-related modification of the histochemical and metabolic characteristics of skeletal muscles of the rat. The effect, however, is related to the recruitment pattern of the fiber populations and the form of activity.