Attenuation of force deficit after lengthening contractions in soleus muscle from trained rats.

The purposes of this study were 1) to determine the extent to which endurance training reduces the functional deficit induced by lengthening contractions in the soleus (Sol) muscle and 2) to determine whether young and old rats training at a comparable relative exercise intensity would demonstrate a similar protective effect from lengthening-contraction-induced injury. Young (3-mo-old) and old (23-mo-old) male Fischer 344 rats were randomly assigned to either a control or exercise training group [young control (YC), old control (OC), young trained (YT), old trained (OT)]. Exercise training consisted of 10 wk of treadmill running (15% grade, 45 min/day, and 5 days/wk) such that by the end of training the young and old rats were exercising at 27 and 15 m/min, respectively. After training, contractile properties of the Sol muscle were measured in vitro at 26 degrees C. The percent decrease in maximal isometric specific force (P(o)) was determined after a series of 20 lengthening contractions (20% strain from optimal muscle length, 1 contraction every 5 s). After the lengthening-contraction protocol, Sol muscle P(o) was decreased by approximately 26% (19.6 vs. 14.6 N/cm(2)) and 28% (14.8 vs. 9.6 N/cm(2)) in the YC and OC rats, respectively. After exercise training, the reduction in P(o) was significantly (P < 0.05) attenuated to a similar degree ( approximately 13%) in both YT rats (18.7 vs. 16.2 N/cm(2)) and OT rats (15.8 vs. 13.7 N/cm(2)). It is concluded that exercise training attenuates the force deficit after repeated lengthening contractions to a comparable extent in young and old rats training at a similar exercise intensity.     

The effect of swimming activity on bone architecture in growing rats

The effect of non-habitual physical activity on bone architecture in the rat humeral shaft was examined. Two groups of rats were trained to swim for 1 h a day, for 20 weeks, at two training levels. The control group consisted of sedentary rats. Parameters of cross-sectional bone morphology (cross-section areas, principal area moments of inertia and their ratio) were used to evaluate the response of bone architecture to mechanical loading. The strength of bone was assessed by measuring the ultimate compressive force and stress. The cortical cross-section area and principal moments of inertia were found to be significantly higher in the swimming groups than in the controls. Examination of the ratio between the major and minor moments of inertia revealed a pronounced change in the shape of the bone cross-section which became more rounded following swimming training. The ultimate compressive force was significantly higher in the swimming rats while the changes in ultimate stress were not significant. Our results indicate a gain of bone strength due to increased periosteal apposition and modified bone tissue distribution. The marked changes in bone morphology are attributed to the different nature of the forces and moments exerted on the humerus during swimming compared to those prevailing during normal locomotion.     

Effect of exercise training on passive stiffness in locomotor skeletal muscle: role of extracellular matrix

The purpose ofthis study was to evaluate the effect of endurance exercise training onboth locomotor skeletal muscle collagen characteristics and passivestiffness properties in the young adult and old rat. Young(3-mo-old) and senescent (23-mo-old) male Fischer 344 rats wererandomly assigned to either a control or exercise training group[young control (YC), old control (OC), young trained (YT), oldtrained (OT)]. Exercise training consisted of treadmill runningat ~70% of maximal oxygen consumption (45 min/day, 5 days/wk, for 10 wk). Passive stiffness (stress/strain) of the soleus (Sol) muscle fromall four groups was subsequently measured in vitro at 26°C.Stiffness was significantly greater for Sol muscles in OC rats comparedwith YC rats, but in OT rats exercise training resulted in muscles withstiffness characteristics not different from those in YC rats. Solmuscle collagen concentration and the level of the nonreduciblecollagen cross-link hydroxylysylpyridinoline (HP) significantlyincreased from young adulthood to senescence. Although training had noeffect on Sol muscle collagen concentration in either age group, itresulted in a significant reduction in the level of Sol muscle HP in OTrats. In contrast, exercise had no effect on HP in the YT animals.These findings indicate that 10 wk of endurance exercise significantlyalter the passive viscoelastic properties of Sol muscle in old but notin young adult rats. The coincidental reduction in the principalcollagen cross-link HP also observed in response to training in OTmuscle highlights the potential role of collagen in influencing passivemuscle viscoelastic properties.

     

Physical training and connective tissues in young mice: biochemistry of long bones

The effect of physical training on collagen, ground substance, and nucleic acid concentrations in long bones was studied in male mice of NMRI strain. The mice to be trained and their controls were about 2 wk old at the beginning of training, which took place on a 5 degrees inclined treadmill 5 days/wk for 3-22 wk. The duration of daily exercise was increased progressively over 3 wk. The final daily exercise bouts were 50 and 80 min for moderate programs and 180 min for the intensive program at a speed of 30 cm/s. Increased concentrations of nitrogen and hexosamines were found at both training intensities, especially after prolonged training. No conclusive changes in nucleic acid concentrations were observed after training. The hexosamine-hydroxyproline ratio was higher and the hydroxyproline-nitrogen ratio lower in the long bones of trained animals compared to the controls. In conclusion these data suggest that prolonged physical activity affects the organic matrix of long bones by maintaining above average concentrations of glycosaminoglycans in matured bones.     

Aging and respiratory muscle metabolic plasticity: effects of endurance training.

We examined the oxidative and antioxidant enzyme activities in respiratory and locomotor muscles in response to endurance training in young and aging rats. Young adult (4-mo-old) and old (24-mo-old) female Fischer 344 rats were divided into four groups: 1) young trained (n = 12), 2) young untrained (n = 12), 3) old trained (n = 10), and 4) old untrained (n = 6). Both young and old endurance-trained animals performed the same training protocol during 10 wk of continuous treadmill exercise (60 min/day, 5 days/wk). Compared with young untrained animals, the young trained group had significantly elevated (P less than 0.05) activities of 3-hydroxyacyl-CoA dehydrogenase (HADH), glutathione peroxidase (GPX), and citrate synthase (CS) in both the costal diaphragm and the plantaris muscle. In contrast, training had no influence (P greater than 0.05) on the activity of lactate dehydrogenase within the costal diaphragm in young animals. In the aging animals, training did not alter (P greater than 0.05) activities of CS, HADH, GPX, or lactate dehydrogenase in the costal diaphragm but significantly (P less than 0.05) increased CS, HADH, and GPX activities in the plantaris muscle. Furthermore, training resulted in higher activities of CS and HADH in the intercostal muscles in the old trained than in the old untrained animals. Finally, activities of CS, HADH, and GPX were significantly (P less than 0.05) lower in the plantaris in the old untrained than in the young untrained animals; however, CS, HADH, and GPX activities were greater (P less than 0.05) in the costal diaphragm in the old sedentary than in the young untrained animals.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of heavy-resistance exercise training on muscle fiber composition in young rats

The purpose of this investigation was to determine whether heavy-resistance exercise training alters the skeletal muscle fiber composition of young rats. Ten male Long Evans rats (3 wk old) were trained to lift progressively heavier weights, which were secured to the rats' tails, while they ascended a 40-cm 90 degree mesh incline 20 times/day 5 days/wk for a food reward. After 8 wk of training, they lifted 406 +/- 19 (SD) g in addition to their body weight (261 +/- 9 g). Compared with 10 sedentary pair-fed rats, no hypertrophy of forelimb muscles (biceps brachii and brachialis) was observed, but rectus femoris wet and dry weights were greater (P less than 0.01) in the trained group. In the deep region of the rectus femoris, type I fiber area was similar between groups, but the trained rats had both a lower (P less than 0.05) percentage of type I fibers and a smaller (P less than 0.05) portion of the total area occupied by type I fibers. The percentage of type IIb fibers in the deep region of the rectus femoris was also similar between groups, but the portion of the deep area composed of type IIb fibers was greater (P less than 0.05) in the trained rats. In the superficial region of the rectus femoris, the trained rats' type IIb fibers were larger (P less than 0.01) and occupied a greater (P less than 0.05) portion of the superficial muscle area.(ABSTRACT TRUNCATED AT 250 WORDS)     

Myosin heavy chain composition in the rat diaphragm: effect of age and exercise training.

Increases in aerobic capacity in both young and senescent rats consequent to endurance exercise training are now known to occur not only in locomotor skeletal muscle but also in diaphragm. In the current study the effects of aging and exercise training on the myosin heavy chain (MHC) composition were determined in both the costal and crural diaphragm regions of female Fischer 344 rats. Exercise training [treadmill running at 75% maximal oxygen consumption (1 h/day, 5 day/wk, x 10 wk)] resulted in similar increases in plantaris muscle citrate synthase activity in both young (5 mo) and old (23 mo) trained animals (P < 0.05). Computerized densitometric image analysis of fast and slow MHC bands revealed the ratio of fast to slow MHC to be significantly higher (P < 0.005) in the crural compared with costal diaphragm region in both age groups. In addition, a significant age-related increase (P < 0.05) in percentage of slow MHC was observed in both diaphragm regions. However, exercise training failed to change the relative proportion of slow MHC in either the costal or crural region.     

Blood circulation of long bones in trained growing rats and mice.

The effect of physical training on the blood circulation of long bones was studied in growing rats and mice of NMRI-strain. The animals to be trained and their controls were about 2 weeks old at the beginning of the training. The training took place on a 5 degree inclined treadmill 5 days a week for 3 weeks in experiment I and 7 weeks in experiments II and III. The duration of the daily exercise was progressively increased over 3 weeks. The final exercise bouts were 80 min for moderate and 180 min for intensive training programs. The circulating red cell volume (ml/100 g bone) of the humeral, femoral and tibial bones of the trained animals was lower compared to the controls in all three experiments mainly due to reduced hematocrit values. The circulating blood volume (ml/100 g bone) decreased in the tibial bones of the trained animals in experiment I and showed a decreasing tendency in experiment III, but no significant differences between the groups were observed in the humeral and femoral bones. Yet, when related to the volume of the bones the circulating blood volume (ml/100 ccm bone) was significantly higher in the femoral bones of the trained animals, while the changes in the humeral bones were negligible (experiment III). The results suggest that the vascularity of long bones is affected by physical training. The varying responses in different bones are perhaps due to the amount of mechanical stress during physical activity.     

Discrimination between strontium and calcium in suckling rats

Discrimination between strontium (Sr) and calcium (Ca) was examined in suckling rats and compared with that in older rats after weaning. Concentrations of Sr and Ca and the Sr/Ca ratios in serum and femur of 10-d old and 21-d old rats were determined. The Sr concentrations and Sr/Ca ratios in the serum and femur of 10-d old rats were lower than those of 21-d old rats, that could be explained by the fact that 10-d old rats ingested only maternal milk in which the Sr/Ca ratio was much lower than the laboratory diet. The relative ratios of Sr/Ca in serum and femur to that in the diet were found to be higher in 10-d old rats compared with those in 21-d old rats, and also higher than those in the older rats after weaning, as described in our previous publication. This result may reflect that discrimination between Sr and Ca during intestinal absorption is lacking in very young animals before weaning and develops after this age. Renal discrimination between Sr and Ca in the suckling rats at 10 d of age was evaluated by determining the relationship between the relative clearances of Ca and Sr. The mathematical model proposed by Walser and Robinson was applied on these results and the parameter for the equation, that is, the discrimination constant, was shown to be higher in 10-d old rats compared to those in young (7 wk of age) and adult (25 wk of age) rats. This result suggests that the discrimination of Sr in favor of Ca during the tubular reabsorptive process may not be fully developed in the very young rats before weaning.     

Influence of age and exercise training on lipid metabolism in Fischer-344 rats

The influence of training on fatty acid and glyceride synthesis by liver and adipose tissue homogenates of young and old Fischer-344 rats was examined. Four groups of rats (10 animals/group) were studied: young untrained, young trained, old untrained, and old trained. Training of each group was for 10 wk at 75% maximal O2 uptake. Young rats were killed at 6 mo of age and old rats were killed at 27 mo of age. Fatty acid synthesis was assessed by measuring the activities of acetyl-CoA carboxylase, fatty acid synthase, ATP citrate-lyase, "malic" enzyme, and glucose-6-phosphate dehydrogenase. Glyceride synthesis was evaluated by determining the rate of incorporation of [14C]glycerol 3-phosphate into lipids. In addition, lipoprotein lipase activity was measured in acetone-ether powders of adipose tissue from the four groups of rats. In liver, training had no effect on fatty acid or glyceride synthesis in either group. However, aging caused a significant decrease in the activities of four of the lipogenic enzymes but had no effect on glyceride synthesis. Training caused an increase in fatty acid synthase and glyceride synthesis in adipose tissue, and aging decreased lipoprotein lipase activity. It was concluded that training enhances the synthetic capacity of lipids by adipose tissue but that aging had a more profound effect in that the activities of the enzymes involved in these processes were lower in the old rats. Furthermore, the decreased activity of lipoprotein lipase in the older rats may explain the higher plasma triglyceride levels that were observed in these animals.     

Differential expression of stress proteins in rat myocardium after free wheel or treadmill run training.

High-intensity treadmill exercise increases the expression of a cardioprotective, inducible 72-kDa stress protein (SP72) in cardiac muscle. This investigation examined whether voluntary free wheel exercise training would be sufficient to confer a similar response. Male Sprague-Dawley rats were randomly assigned to either treadmill (TM-Tr) or free wheel (FW-Tr) training groups. By the end of the 8-wk training period, TM-Tr animals ran 1 h/day, 5 days/wk up a 10% grade, covering a distance of 8,282 m/wk. FW-Tr rats ran, on average, 5,300 m/wk, with one-third of the animals covering distances similar to those for the TM-Tr group. At the time of death, hearts of trained and caged sedentary control (Sed) animals were divided into left (LV) and right (RV) ventricles. Citrate synthase activity and the relative immunoblot contents of SP72, SP73 (the constitutive isoform of the SP70 family), and a 75-kDa mitochondrial chaperone (SP75) were subsequently determined. LV and RV did not differ on any measure, and SP73, SP75, and citrate synthase were not affected by training. Cardiac SP72 levels were elevated over fourfold in both ventricles of TM-Tr compared with RV of FW-Sed rats. Despite the animals having run a similar total distance, cardiac SP72 content in FW-Tr rats was not different from that in Sed animals. These data indicate that voluntary exercise training is insufficient to elicit an elevation of SP72 in rat heart and suggest that exercise intensity may be a critical factor in evoking the cardioprotective SP72 response.     

Metabolic effects of treadmill exercise training on the diabetic heart.

This study determined whether exercise training in rats would prevent the accumulation of lipids and depressed glucose utilization found in hearts from diabetic rats. Diabetes was induced by intravenous streptozotocin (60 mg/kg). Trained diabetic rats were run on a treadmill for 60 min, 27 m/min, 10% grade, 6 days/wk for 10 wk. Training of diabetic rats had no effect on glycemic control but decreased plasma lipids. In vivo myocardial long-chain acylcarnitine, acyl-CoA, and high-energy phosphate levels were similar in sedentary control, sedentary diabetic, and trained diabetic groups. The levels of myocardial triacylglycerol were similar in sedentary control and diabetic rats but decreased in trained diabetic rats. Hearts were perfused with buffer containing diabetic concentrations of glucose (22 mM) and palmitate (1.2 mM). D-[U-14C] glucose oxidation rates (14CO2 production) were depressed in hearts from sedentary diabetic rats relative to sedentary control rats. Hearts from trained diabetic rats exhibited increased glucose oxidation relative to those of sedentary diabetic rats, but this improvement was below that of the sedentary control rats. [9,10(-3)H]palmitate oxidation rates (3H2O production) were identical in all three groups. These findings suggest that exercise training resulted in a partial normalization of myocardial glucose utilization in diabetic rats.     

Papillary mechanics and cardiac morphology of infarcted rat hearts after training

Infarction of the left ventricle was induced by ligation of the coronary artery in male Sprague-Dawley rats under ketamine-xylazine anesthesia. Three weeks after surgery, animals were assigned to a trained (n = 21; running at 20 m/min, 10% grade, 1 h/day, 5 days/wk) or nontrained group (n = 23) for an additional 8 wk. A third, sham-operated control group (n = 16) remained cage sedentary for 11 wk. Ventricular mass was greater in the trained and nontrained infarct groups [1,335 +/- 57.3 and 1,414 +/- 56.1 mg, respectively (mean +/- SE)] compared with the control group (1,155 +/- 50.9 mg) (P less than or equal to 0.05). The diameter of septal fibers was 13% greater in the trained and 17% greater in the nontrained infarct groups compared with control. The specific peak developed force and maximum rate of force development of left ventricular papillary muscle in vitro were 75 and 62% greater in both infarcted groups compared with the control group; these variables were unaffected by training. Myofibrillar adenosine triphosphatase activity of septum was 20% lower in both infarct groups compared with sham-operated animals. We conclude that exercise training did not alter the magnitude of morphological and physiological adaptations to infarction.     

Ultrastructural muscle damage in young vs. older men after high-volume, heavy-resistance strength training.

This study assessed ultrastructural muscle damage in young (20-30 yr old) vs. older (65-75 yr old) men after heavy-resistance strength training (HRST). Seven young and eight older subjects completed 9 wk of unilateral leg extension HRST. Five sets of 5-20 repetitions were performed 3 days/wk with variable resistance designed to subject the muscle to near-maximal loads during every repetition. Biopsies were taken from the vastus lateralis of both legs, and muscle damage was quantified via electron microscopy. Training resulted in a 27% strength increase in both groups (P < 0.05). In biopsies before training in the trained leg and in all biopsies from untrained leg, 0-3% of muscle fibers exhibited muscle damage in both groups (P = not significant). After HRST, 7 and 6% of fibers in the trained leg exhibited damage in the young and older men, respectively (P < 0.05, no significant group differences). Myofibrillar damage was primarily focal, confined to one to two sarcomeres. Young and older men appear to exhibit similar levels of muscle damage at baseline and after chronic HRST.     

Residual stress distribution in rabbit limb bones

The presence of the residual stresses in bone tissue has been noted and the authors have reported that there are residual stresses in bone tissue. The aim of our study is to measure the residual stress distribution in the cortical bone of the extremities of vertebrates and to describe the relationships with the osteon population density. The study used the rabbit limb bones (femur, tibia/fibula, humerus, and radius/ulna) and measured the residual stresses in the bone axial direction at anterior and posterior positions on the cortical surface. The osteons at the sections at the measurement positions were observed by microscopy. As a result, the average stresses at the hindlimb bones and the forelimb bones were 210 and 149 MPa, respectively. In the femur, humerus, and radius/ulna, the residual stresses at the anterior position were larger than those at the posterior position, while in the tibia, the stress at the posterior position was larger than that at the anterior position. Further, in the femur and humerus, the osteon population densities in the anterior positions were larger than those in the posterior positions. In the tibia, the osteon population density in the posterior position was larger than that in the anterior position. Therefore, tensile residual stresses were observed at every measurement position in the rabbit limb bones and the value of residual stress correlated with the osteon population density (r=0.55, P<0.01).     

Exercise training and beta-blocker treatment ameliorate age-dependent impairment of beta-adrenergic receptor signaling and enhance cardiac responsiveness to adrenergic stimulation

Cardiac beta-adrenergic receptor (beta-AR) signaling and left ventricular (LV) responses to beta-AR stimulation are impaired with aging. It is shown that exercise and beta-AR blockade have a favorable effect on cardiac and vascular beta-AR signaling in several cardiovascular diseases. In the present study, we examined the effects of these two different strategies on beta-AR dysregulation and LV inotropic reserve in the aging heart. Forty male Wistar-Kyoto aged rats were randomized to sedentary, exercise (12 wk treadmill training), metoprolol (250 mg.kg(-1).day(-1) for 4 wk), and exercise plus metoprolol treatment protocols. Ten male Wistar-Kyoto sedentary young rats were also used as a control group. Old trained, old metoprolol-treated, and old trained plus metoprolol-treated rats showed significantly improved LV maximal and minimal first derivative of the pressure rise responses to beta-AR stimulation (isoproterenol) compared with old untrained animals. We found a significant reduction in cardiac sarcolemmal membrane beta-AR density and adenylyl cyclase activity in old untrained animals compared with young controls. Exercise training and metoprolol, alone or combined, restored cardiac beta-AR density and G-protein-dependent adenylyl cyclase activation in old rats. Although cardiac membrane G-protein-receptor kinase 2 levels were not upregulated in untrained old compared with young control rats, both exercise and metoprolol treatment resulted in a dramatic reduction of G-protein-receptor kinase 2 protein levels, which is a further indication of beta-AR signaling amelioration in the aged heart induced by these treatment modalities. In conclusion, we demonstrate for the first time that exercise and beta-AR blockade can similarly ameliorate beta-AR signaling in the aged heart, leading to improved beta-AR responsiveness and corresponding LV inotropic reserve.     

Bones benefits gained by jump training are preserved after detraining in young and adult rats.

We investigated the osteogenic responses to jump training and subsequent detraining in young and adult male rats to test the following hypotheses: 1) jump training has skeletal benefits; 2) these skeletal benefits are preserved with subsequent detraining throughout bone morphometric changes; and 3) there are no differences between young and adult rats during detraining in terms of the maintenance of exercise-induced changes. Twelve-week-old (young) and 44-wk-old (adult) rats were divided into the following four groups: young-sedentary, young-exercised, adult-sedentary, and adult-exercised. The exercised groups performed jump training (height = 40 cm, 10 jumps/day, 5 days/wk) for 8 wk followed by 24 wk of being sedentary. Tibial bone mineral content and bone mineral density in vivo significantly increased with jump training, and the effects were maintained after detraining in both the young and adult exercised groups, although the benefits of training became somewhat diminished. After 24 wk of detraining, the beneficial effects of training on bone mass and strength were preserved and associated with morphometric changes, such as periosteal perimeter, cortical area, and moment of inertia. There were no significant age-exercise interactions in such parameters, except for the periosteal perimeter. These results suggest that there are few differences in bone accommodation and maintenance by training and detraining between young and adult rats.     

Determinants of VO2max in rats after high-intensity sprint training

The hemodynamic response to maximal exercise was determined in rats that were subjected to high-intensity sprint training (HIST) and rats that served as sedentary controls. Training consisted of five 1-min bouts of treadmill running at work loads (15% grade, 97 m/min) in excess of the animals' maximal O2 uptake (VO2max) interspersed with 90 s of rest. Training was performed 6 days/wk for 6 wk. After the training regimen, all rats were acutely instrumented with catheters in the right carotid artery and right ventricle. O2 uptakes, hemodynamic parameters, arterial and mixed venous O2 concentrations, blood gases, and acid-base status were determined at rest and during submaximal and maximal exercise. Results demonstrated that VO2max of HIST rats was significantly greater than that found for sedentary control rats. This increase in VO2max was due to an increase in maximal cardiac output (Qmax), since maximal arteriovenous O2 difference was similar between trained and sedentary rats. The increase in Qmax was due to an increase in maximal stroke volume (SVmax), because maximal heart rate in trained rats was similar to that in sedentary control rats. Citrate synthase and phosphofructokinase activities measured in the white gastrocnemius, plantaris, and soleus muscles of trained and sedentary rats were similar. These results suggest that the increase in VO2max produced with HIST in rats is strongly linked to an increase in central cardiac function as indicated by an increase in Qmax and SVmax.     

Training-induced alterations in young and senescent rat diaphragm muscle.

The current study sought to examine the effects of chronic endurance treadmill running on oxidative capacity and capillary density in specific diaphragm muscle fiber types in young (5 mo) and senescent (greater than or equal to 23 mo) female Fischer 344 rats. Both young and senescent animals trained at approximately 75% of maximal O2 consumption for 1 h/day 5 days/wk for 10 wk. Plantaris citrate synthase activity was significantly increased (P less than 0.01) in both young and old trained groups. Densitometric analysis of succinate dehydrogenase (SDH) activity in diaphragm type I, IIa, and IIb muscle fibers was done using a computerized image-processing system. There were no age-related differences in SDH activity between the young and old groups for any of the fiber types. In addition, SDH activity was found to be significantly increased (P less than 0.05) in all three fiber types in both the young and senescent trained animals compared with their sedentary counterparts. Fiber size and capillary density did not differ between young and senescent rats, nor did exercise affect this measure. Each fiber, irrespective of type, had an average of approximately four capillaries in contact with it. However, type IIb fibers had a significantly lower capillary density per unit area than type I or IIa muscle fibers. The results indicate that the senescent costal diaphragm maintains its ability to adapt to an increased metabolic demand brought about by locomotor exercise. Of further interest is the finding that training adaptations occurred in all three fiber types, suggesting that increased work of breathing from moderate exercise leads to recruitment of all three fiber types.(ABSTRACT TRUNCATED AT 250 WORDS)