Isometric training of young rats — Effects upon hind limb muscles

The soleus, rectus femoris, and gastrocnemius muscles of young rats trained isometrically for 4 weeks were studied by light and electron microscopy.--The percentage of fast-twitch oxidative muscle fibers decreased at the cost of the fast-twitch glycolytic fibers in the rectus femoris muscle. The percentages of the slow-twitch oxidative fibers did not change significantly in any of the muscles studied. The changes in the areas of the muscle fibers were specific for the muscle and the fiber type and indicate geometrical rearrangements of the fibers in the trained muscles. The Z and M lines were broader in the soleus (containing about 85% slow-twitch oxidative fibers) than in the rectus femoris muscle (containing about 90% fast-twitch glycolytic fibers), while the sarcomere length and the pseudo-H zone were similar. The length of the myosin filaments appeared to be slightly shorter in the fast rectus femoris than in the slow soleus muscle.--The hypothesis on the temporal progress of muscle adaptation to training (Müller, 1974) was substantiated. Correlations between biochemical (Exner et al., 1973a) and histochemical parameters measuring the oxidative capacity were preserved during adaptation to training. The comparison of the histochemical results with the physiological data on similar animals (Exner et al., 1973a) suggests a complex relationship between the contraction time and the percentage of fast-twitch muscle fibers.     

MHC-based fiber type and E-C coupling characteristics in mechanically skinned muscle fibers of the rat

In this study, we investigated whether the previously established differences between fast- and slow-twitch single skeletal muscle fibers of the rat, in terms of myosin heavy chain (MHC) isoform composition and contractile function, are also detectable in excitation-contraction (E-C) coupling. We compared the contractile responsiveness of electrophoretically typed, mechanically skinned single fibers from the soleus (Sol), the extensor digitorum longus (EDL), and the white region of the sternomastoid (SM) muscle to t-system depolarization-induced activation. The quantitative parameters assessed were the amplitude of the maximum depolarization-induced force response (DIFR(max); normalized to the maximum Ca(2+)-activated force in that fiber) and the number of responses elicited until the force declined by 75% of DIFR(max) (R-D(75%)). The mean DIFR(max) values for type IIB EDL and type IIB SM fibers were not statistically different, and both were greater than the mean DIFR(max) for type I Sol fibers. The mean R-D(75%) for type IIB EDL fibers was greater than that for type I Sol fibers as well as type IIB SM fibers. These data suggest that E-C coupling characteristics of mechanically skinned rat single muscle fibers are related to MHC-based fiber type and the muscle of origin.     

Vascular flow capacity of hindlimb skeletal muscles in spontaneously hypertensive rats

Total and regional skeletal muscle flows (radiolabeled microspheres) were determined in isolated maximally vasodilated hindquarters of spontaneously hypertensive rats (SHR) and age-matched (11-12 mo) normotensive Wistar-Kyoto rats (WKY) to assess the vascular flow capacity of the skeletal muscle vascular beds. Vascular flow capacity was estimated by measuring total hindquarters and regional muscle blood flows (under conditions of maximal vasodilation with papaverine or papaverine plus isoproterenol) over a wide range of perfusion pressures in WKY and SHR. Capillary exchange capacity was estimated by determining the capillary filtration coefficient. Isogravimetric capillary pressures and segmental vascular resistances were determined in each hindquarter. Isogravimetric flows and capillary pressures were not different between WKY and SHR. However, total and precapillary vascular resistances were significantly elevated in SHR, and postcapillary resistances were not different compared with WKY. Maximal capillary filtration coefficient values for the SHR group averaged 20% lower than WKY values, suggesting that hypertension was associated with a reduction in the microvascular surface area available for fluid exchange and, therefore, the capillary exchange capacity. Over the perfusion pressures studied, total hindquarters flows averaged 60% lower in SHR than in WKY. Flows to individual skeletal muscles averaged 76% lower in SHR than in WKY regardless of the muscle fiber type. Thus, modifications exist in the hindlimb skeletal muscle vasculature of SHR that reduces the capillary exchange capacity and limit the capacity of deliver flow at a given perfusion pressure gradient.     

Effect of exercise on muscle fibre composition and enzyme activities of skeletal muscles in young rats

Young Wistar rats underwent dynamic (D) or static (S) exercise from the 5th to 35th day after birth. Histochemical and biochemical analysis were performed in the extensor digitorum longus (EDL) and the soleus muscle (SOL). Lactate dehydrogenase (LDH) (regulating anaerobic metabolism) and citrate synthase (CS) and hydroxyacyl-CoA dehydrogenase (HAD) (both regulating aerobic metabolism) activities were determined spectrophotometrically. An increase of the fast oxidative-glycolytic (FOG) muscle fibres was found in the slow SOL muscle in both trained groups, i.e. by 10% in group D and by 7% in group S in comparison with the C group. The EDL muscle fibre distribution did not differ from those of control animals in respect to the slow oxidative (SO) fibre type. A higher percentage of FOG fibres by 19% was found in group D contrary to a decreased number of the fast glycolytic (FG) muscle fibres in this trained group. The greatest increase of CS (EDL 185%, SOL 176%) and HAD (EDL 83%, SOL 178%) activities were found in group D as compared with control group (C). Only small differences were observed in LDH activity. The values of characteristic enzyme activity ratios show that dynamic training resulted in an elevation of oxidative capacity of skeletal muscle, while the static load led preferentially along the glycolytic pathway. It may be concluded that an adaptive response to the training load during early postnatal development is different due to the type of exercise (dynamic or static) and/or the type of skeletal muscle (fast or slow).     

Effect of swimming training on cardiac function and myosin ATPase activity in SHR

Male spontaneously hypertensive rats (SHR) and Wistar-Kyoto normotensive rats (WKY) were subjected to swimming training 6 times/wk, commencing at 4 wk of age, to determine whether this type of endurance exercise might alter contractile proteins and cardiac function in young adult SHR. The total duration of exercise was 190 h. Myofibrillar adenosinetriphosphatase (ATPase) activity was assayed at various free [Ca2+] ranging from 10(-7) to 10(-5) M. Ca2+-stimulated ATPase activity of actomyosin and purified myosin was determined at various Ca2+ concentrations both in the low and high ionic strength buffers. Actin-activated myosin ATPase activity of purified myosin was assayed at several concentrations of actin purified from rabbit skeletal muscle. Under all these conditions the contractile protein ATPase activity was comparable between trained and untrained WKY and SHR. Analysis of myosin isoenzymes on pyrophosphate gels showed a single band corresponding to V1 isoenzyme, and there were no differences between swimming-trained and nontrained WKY and SHR. Ventricular performance was assessed by measuring cardiac output and stroke volume after rapid intravenous volume overloading. Both cardiac index and stroke index were comparable in nontrained WKY and SHR but were significantly increased in the trained groups compared with their respective nontrained controls. These results suggest that myosin ATPase activity and distribution of myosin isoenzymes are not altered in the moderately hypertrophied left ventricle whether the hypertrophy is due to genetic hypertension (SHR) or to exercise training (trained WKY). Moreover, the data indicate that SHR, despite the persistence of a pressure overload, undergo similar increases in left ventricular mass and peak cardiac index after training, as do normotensive WKY.     

Decreased susceptibility of cultured smooth muscle cells from SHR rats to growth inhibition by heparin

Smooth muscle cell proliferation is regulated through the coordinated action of growth inhibitors and growth factors/mitogens; a specific heparin-epidermal growth factor (EGF) complementation has been proposed (Reilly et al., 1987, J. Cell. Physiol., 131:149-157). In culture, vascular smooth muscle cells (VSMC) from spontaneously hypertensive rats (SHR) proliferate more rapidly than VSMC from control Wistar Kyoto rats (WKY). We observed that, compared with WKY-derived VSMC, cells from SHR were markedly less susceptible to growth inhibition both by heparin and its homopolysaccharide analogue pentosan polysulphate (PPS). SHR-derived VSMC exhibited a reduced capacity for binding of [3H]heparin to specific extracellular surface receptors, whereas affinities for heparin were comparable between both VSMC isolates. The early (0-2 hr at 37 degrees C) kinetics of internalization did not differ between SHR- and WKY-derived VSMC, but both internalized equivalent proportions (approximately 10%) of initially surface-bound heparin. VSMC from SHR exhibited a greater capacity, without a changed affinity, for [I125]EGF binding than VSMC from WKY. Pre-exposure of VSMC to heparin or PPS decreased, in a time-dependent manner, the EGF binding capacity for both SHR and WKY (by 40-50% after 72 hr). However, in absolute terms, the EGF-binding capacity of VSMC from SHR exposed to heparinoids was similar to that of nonexposed VSMC from WKY.     

Metabolic and morphologic properties of single muscle fibers in the rat after spaceflight, Cosmos 1887

The adaptation of a slow (soleus, Sol) and a fast (medial gastrocnemius, MG) skeletal muscle to spaceflight was studied in five young male rats. The flight period was 12.5 days and the rats were killed approximately 48 h after returning to 1 g. Five other rats that were housed in cages similar to those used by the flight rats were maintained at 1 g for the same period of time to serve as ground-based controls. Fibers were classified as dark or light staining for myosin adenosine triphosphatase (ATPase). On the average, the fibers in the Sol of the flight rats atrophied twice as much as those in the MG. Further, the fibers located in the deep (close to the bone and having the highest percentage of light ATPase and high oxidative fibers in the muscle cross section) region of the MG atrophied more than the fibers located in the superficial (away from the bone and having the lowest percentage of light ATPase and high oxidative fibers in the muscle cross-section) region of the muscle. Based on quantitative histochemical assays of single muscle fibers, succinate dehydrogenase (SDH) activity per unit volume was unchanged in fibers of the Sol and MG. However, in the Sol, but not the MG, the total amount of SDH activity in a 10-microns-thick section of a fiber decreased significantly in response to spaceflight. Based on population distributions, it appears that the alpha-glycerophosphate dehydrogenase (GPD) activities were elevated in the dark ATPase fibers in the Sol, whereas the light fibers in the Sol and both fiber types in the MG did not appear to change. The ratio of GPD to SDH activities increased in the dark (but not light) fibers of the Sol and was unaffected in the MG. Immunohistochemical analyses indicate that approximately 40% of the fibers in the Sol of flight rats expressed a fast myosin heavy chain compared with 22% in control rats. Further, 31% of the fibers in the Sol of flight rats expressed both fast and slow myosin heavy chains compared with 8% in control rats. Immunohistochemical changes in the MG were minimal. These data suggest that the magnitude and direction of enzymatic activity and cell volume changes are dependent on the muscle, the region of the muscle, and the type of myosin expressed in the fibers. Further, the ability of fibers to maintain normal or even elevated activities per unit volume of some metabolic enzymes is remarkable considering the marked and rapid decrease in fiber volume.     

Exercise training improves aortic endothelium-dependent vasorelaxation and determinants of nitric oxide bioavailability in spontaneously hypertensive rats.

The present study examined in vitro vasomotor function and expression of enzymes controlling nitric oxide (NO) bioavailability in thoracic aorta of adult male normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR) that either remained sedentary (Sed) or performed 6 wk of moderate aerobic exercise training (Ex). Training efficacy was confirmed by elevated maximal activities of both citrate synthase (P = 0.0024) and beta-hydroxyacyl-CoA dehydrogenase (P = 0.0073) in the white gastrocnemius skeletal muscle of Ex vs. Sed rats. Systolic blood pressure was elevated in SHR vs. WKY (P < 0.0001) but was not affected by Ex. Despite enhanced endothelium-dependent relaxation to 10(-8) M ACh in SHR vs. WKY (P = 0.0061), maximal endothelium-dependent relaxation to 10(-4) M ACh was blunted in Sed SHR (48 +/- 12%) vs. Sed WKY (84 +/- 6%, P = 0.0067). Maximal endothelium-dependent relaxation to 10(-4) M ACh was completely restored in Ex SHR (93 +/- 9%) vs. Sed SHR (P = 0.0011). N(omega)-nitro-l-arginine abolished endothelium-dependent relaxation in all groups (P 相似文献   

Adaptation in properties of skeletal muscle to coronary artery occlusion/reperfusion in rats

The present study was designed to determine if changes in function and metabolism of heart muscle induce alterations in characteristics of skeletal muscle. We investigated the histochemical and biochemical properties of soleus (SOL) and extensor digitorum longus (EDL) muscles in Wistar rats at the chronic phase after coronary artery occlusion/reperfusion. The size of myocardial infarct region was evaluated using a high resolution pinhole single photo emission computed tomography (SPECT) system. 4 wk after left coronary artery occlusion/reperfusion, the SOL and EDL of hindlimb were dissected out and immersed in isopentane cooled with liquid nitrogen for subsequent histochemical and biochemical analysis. From SPECT imaging, the blood circulation was recovered, but the recovery of fatty acid metabolism was not observed in infarct region of heart. Citrate synthase (CS) and 3-hydroxyacyl-CoA dehydrogenase (HAD) activities in infarct region of heart were lower in the myocardial infarction (MI, n = 6) group compared with that of age-matched sham-operated (Sham, n = 6) group. In addition, heart muscle hypertrophy caused by the dysfunction in MI group was observed. In skeletal muscle, the atrophy and transition of fiber type distribution in MI group, reported in previous studies of heart failure, were not observed. However, the succinate dehydrogenase (SDH) activity in the slow twitch oxidative (SO) from SOL of MI group decreased by 9.8% and in the fast twitch oxidative glycolytic fibers (FOG), 8.0% as compared with sham group. Capillary density of the SO fibers from SOL of MI group also reduced by 18.5% and in the FOG fibers, 18.2% as compared with Sham group. Decreased capillary density in this study related significantly to decreased SDH activity of single muscle fibers in chronic phase of perfusion after surgical infarction. Our results make it clear that there is a difference in the reaction of skeletal muscle to coronary artery occlusion/reperfusion compared with chronic heart failure. However, our data would support the notion that there is a linkage between the function of heart and physiological properties of skeletal muscle.     

Skeletal muscle adaptation in rats flown on Cosmos 1667

Seven male Wistar rats were subjected to 7 days of weightlessness on the Soviet biosatellite Cosmos 1667. Muscle histomorphometry and biochemical analyses were performed on the soleus (SOL) and extensor digitorum longus (EDL) of flight rats (group F) and compared with data from three groups of terrestrial controls: one subjected to conditions similar to group F in space except for the state of weightlessness (group S) and the others living free in a vivarium (V1, V2). Relative to group V2 (its age and weight-matched control group), group F showed a greater decrease of muscle mass in SOL (23%) than in EDL (11%). In SOL a decrease in the percentage of type I fibers was counterbalanced by a simultaneous increase in type IIa fibers. The cross-sectional area of type I fiber was reduced by 24%. No statistically significant difference in capillarization and enzymatic activities was observed between the groups. In EDL a reduction in type I fiber distribution and 3-hydroxyacyl-CoA-dehydrogenase activity (27%) occurred after the flight. The small histochemical and biochemical changes reported suggest the interest in studying muscular adaptation during a flight of longer duration.     

Regulation of glucose uptake in rat slow and fast skeletal muscles

1. Regulation of glucose uptake was compared between extensor digitorum longus (EDL) and soleus (Sol) muscles in rats. 2. Insulin stimulated glucose uptake more in EDL than in Sol. 3. Under high concentrations of insulin, the glucose uptake was higher in EDL than Sol. 4. Inhibition of oxidative phosphorylation by anoxia or an uncoupler stimulated glucose uptake more in EDL than in Sol. 5. Anoxia abolished the effect of insulin on glucose uptake in both EDL and Sol. 6. The blocker to glucose transport system reduced glucose uptake more in Sol than in EDL.     

Alterations in intrinsic mitochondrial function with aging are fiber type-specific and do not explain differential atrophy between muscles

To determine whether mitochondrial dysfunction is causally related to muscle atrophy with aging, we examined respiratory capacity, H(2) O(2) emission, and function of the mitochondrial permeability transition pore (mPTP) in permeabilized myofibers prepared from four rat muscles that span a range of fiber type and degree of age-related atrophy. Muscle atrophy with aging was greatest in fast-twitch gastrocnemius (Gas) muscle (-38%), intermediate in both the fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus (Sol) muscles (-21%), and non-existent in adductor longus (AL) muscle (+47%). In contrast, indices of mitochondrial dysfunction did not correspond to this differential degree of atrophy. Specifically, despite higher protein expression for oxidative phosphorylation (oxphos) system in fast Gas and EDL, state III respiratory capacity per myofiber wet weight was unchanged with aging, whereas the slow Sol showed proportional decreases in oxphos protein, citrate synthase activity, and state III respiration. Free radical leak (H(2) O(2) emission per O(2) flux) under state III respiration was higher with aging in the fast Gas, whereas state II free radical leak was higher in the slow AL. Only the fast muscles had impaired mPTP function with aging, with lower mitochondrial calcium retention capacity in EDL and shorter time to mPTP opening in Gas and EDL. Collectively, our results underscore that the age-related changes in muscle mitochondrial function depend largely upon fiber type and are unrelated to the severity of muscle atrophy, suggesting that intrinsic changes in mitochondrial function are unlikely to be causally involved in aging muscle atrophy.     

3D Visualization and Measurement of Capillaries Supplying Metabolically Different Fiber Types in the Rat Extensor Digitorum Longus Muscle During Denervation and Reinnervation

The aim of this study was to determine whether capillarity in the denervated and reinnervated rat extensor digitorum longus muscle (EDL) is scaled by muscle fiber oxidative potential. We visualized capillaries adjacent to a metabolically defined fiber type and estimated capillarity of fibers with very high oxidative potential (O) vs fibers with very low oxidative potential (G). Capillaries and muscle fiber types were shown by a combined triple immunofluorescent technique and the histochemical method for NADH-tetrazolium reductase. Stacks of images were captured by a confocal microscope. Applying the Ellipse program, fibers were outlined, and the diameter, perimeter, cross-sectional area, length, surface area, and volume within the stack were calculated for both fiber types. Using the Tracer plug-in module, capillaries were traced within the three-dimensional (3D) volume, the length of capillaries adjacent to individual muscle fibers was measured, and the capillary length per fiber length (Lcap/Lfib), surface area (Lcap/Sfib), and volume (Lcap/Vfib) were calculated. Furthermore, capillaries and fibers of both types were visualized in 3D. In all experimental groups, O and G fibers significantly differed in girth, Lcap/Sfib, and Lcap/Vfib, but not in Lcap/Lfib. We conclude that capillarity in the EDL is scaled by muscle fiber size and not by muscle fiber oxidative potential. (J Histochem Cytochem 57:437–447, 2009)     

Effects of beta 1- vs. beta 1- beta 2-blockade on training adaptations in rat skeletal muscle

The effect of selective vs. nonselective beta-blockade on fast-twitch [extensor digitorum longus (EDL)] and slow-twitch [soleus (SOL)] muscle enzyme activities following endurance training were characterized. Citrate synthase (CS), lactate dehydrogenase (LDH), and beta-hydroxyacyl-CoA dehydrogenase (HAD) activities were compared in SOL and EDL muscles of trained (T), metoprolol-trained (MT), propranolol-trained (PT), and sedentary (C) rats. Following 8 wk of treadmill running (1 h/day, 5 days/wk at approximately 30 m/min), LDH activity was depressed approximately 20% (P less than 0.05) in both SOL and EDL in only the PT rats, indicating inhibition of beta 2-mediated anaerobic glycolysis. EDL CS activity was similarly elevated in all three trained groups compared with sedentary controls. In SOL muscle, however, a drug attenuation effect was observed so that CS activity was increased only in the T (P less than 0.01) and MT (P less than 0.05) groups. HAD enzyme activity was increased somewhat (P less than 0.10) in SOL muscle in only the T group, but more so (P less than 0.05) in EDL in all three trained groups. The above findings suggest a training-induced selectivity effect not only with respect to beta 1-vs. beta 1-beta 2-blockers, but also with respect to muscle fiber type.     

Decreased monocarboxylate transporter 1 in rat soleus and EDL muscles exposed to clenbuterol.

We hypothesized that a shift in muscle fiber type induced by clenbuterol would change monocarboxylate transporter 1 (MCT1) content and activity of lactate dehydrogenase (LDH) and isoform pattern and shift myosin heavy chain (MHC) pattern in soleus (Sol) and extensor digitorum longus (EDL) of male rats. In the clenbuterol-administered rats (2.0 mg x kg(-1) x day(-1) subcutaneously for 4 wk), the ratio of muscle weight to body weight increased in the Sol (P < 0.05) and the EDL (P < 0.01). Clenbuterol induced the appearance of fast MHC(2D) and decreased slow MHC(1) in Sol (13%) but had no effect on EDL. The MHC pattern of Sol changed from slow to fast type. Clenbuterol increased LDH-specific activity (P < 0.01) and the ratio of the muscle-type isozyme of LDH to the heart type (P < 0.05) in Sol. The LDH total activity of the EDL muscle was also increased (P < 0.05). Furthermore, MCT1 content significantly (P < 0.05) decreased in both Sol and EDL (27 and 52%, respectively). This study suggests that clenbuterol might mediate the shift of MHC from slow to fast type and the changes in the regulation of lactate metabolism. Novel to this study is the observation that clenbuterol decreases MCT1 content in the hindlimb muscles and that the decrease in MCT1 is not muscle-type specific. It may suggest that the genetic expressions of individual factors involving slow-type MHC, heart-type isozyme of LDH, and MCT1 are associated with one another but are regulated independently.     

Adaptations in rat skeletal muscle following long-term resistance exercise training

The purpose of this investigation was to determine whether long-term, heavy resistance training would cause adaptations in rat skeletal muscle structure and function. Ten male Wistar rats (3 weeks old) were trained to climb a 40-cm vertical ladder (4 days/week) while carrying progressively heavier loads secured to their tails. After 26 weeks of training the rats were capable of lifting up to 800 g or 140% of their individual body mass for four sets of 12–15 repetitions per session. No difference in body mass was observed between the trained rats and age-matched sedentary control rats. Absolute and relative heart mass were greater in trained rats than control rats. When expressed relative to body mass, the mass of the extensor digitorum longus (EDL) and soleus muscles was greater in trained rats than control rats. No difference in absolute muscle mass or maximum force-producing capacity was evident in either the EDL or soleus muscles after training, although both muscles exhibited an increased resistance to fatigue. Individual fibre hypertrophy was evident in all four skeletal muscles investigated, i.e. EDL, soleus, plantaris and rectus femoris muscles of trained rats, but muscle fibre type proportions within each of the muscles tested remained unchanged. Despite an increased ability of the rats to lift progressively heavier loads, this heavy resistance training model did not induce gross muscle hypertrophy nor did it increase the force-producing capacity of the EDL or soleus muscles. Accepted: 17 September 1997     

Dynamic exercise training in foxhounds. II. Analysis of skeletal muscle

The purpose of this study was to determine whether 8-12 wk of endurance training produces biochemical and histochemical adaptations in skeletal muscle in foxhounds. Analyses were performed on samples removed from gastrocnemius, triceps, and semitendinosus muscles of foxhounds before and after a treadmill running program. Biochemical analysis showed that training did not alter the activities of phosphofructokinase, beta-hydroxyacyl-CoA dehydrogenase, succinate dehydrogenase, or total phosphorylase. Histochemical analysis of myofibrillar actomyosin ATPase demonstrated three distinct classes of type II fibers and one type I fiber in the semitendinosus and triceps muscles and two type II and two type I fibers in the gastrocnemius muscle. Fiber type distribution and oxidative and glycolytic potentials, as indicated by nicotinamide adenine dinucleotide tetrazolium reductase or alpha-glycerophosphate dehydrogenase staining intensity, were unaltered by training. Similarly, capillary density, capillary-to-fiber ratios, and capillary area-to-fiber area ratios did not change with training. Thus, unlike humans and other mammals (i.e., rat), these foxhounds did not manifest biochemical or histochemical adaptations in skeletal muscle as the result of endurance training. This is consistent with the results of the study in which endurance training produced a 27% increase in maximal cardiac output and a 4% increase in maximal arteriovenous O2 extraction in foxhounds.     

Fiber type populations and Ca2+-activation properties of single fibers in soleus muscles from SHR and WKY rats

Electrophoretic analyses of muscle proteins in whole musclehomogenates and single muscle fiber segments were used to examine myosin heavy chain (MHC) and myosin light chain 2 (MLC2) isoform composition and fiber type populations in soleus muscles from spontaneously hypertensive rats (SHRs) and their age-matchednormotensive controls [Wistar-Kyoto (WKY) rats], at threestages in the development of high blood pressure (4 wk, 16 wk, and 24 wk of age). Demembranated (chemically skinned with 2% Triton X-100),single fiber preparations were used to determine the maximumCa²⁺-activated force percross-sectional area, calcium sensitivity, and degree of cooperativityof the contractile apparatus andCa²⁺-regulatory system withrespect to Ca²⁺. The results showthat, at all ages examined, 1) SHRsoleus contained a lower proportion of MHCI and MLC2 slow (MLC2s) and ahigher proportion of MHCIIa, MHCIId/x, and MLC2 fast (MLC2f )isoforms than the age-matched controls;2) random dissection of single fibers from SHR and WKY soleus produced four populations of fibers: type I (expressing MHCI), type IIA (expressing MHCIIa), hybrid typeI+IIA (coexpressing MHCI and MHCIIa), and hybrid type IIA+IID (coexpressing MHCIIa and MHCIId/x); and3) single fiber dissection from SHRsoleus yielded a lower proportion of type I fibers, a higher proportionof fast-twitch fibers (types IIA and IIA+IID), and a higher proportionof hybrid fibers (types I+IIA and IIA+IID) than the homologous musclesfrom the age-matched WKY rats. Because the presence of hybrid fibers isviewed as a marker of muscle transformation, these data suggest thatSHR soleus undergoes transformation well into adulthood. Our data showalso that, for a given fiber type, there are no significant differencesbetween SHR and WKY soleus muscles with respect to any of theCa²⁺-activation propertiesexamined. This finding indicates that the lower specific tensionsreported in the literature for SHR soleus muscles are not due tostrain- or hypertension-related differences in the function of thecontractile apparatus or regulatory system.     

Effect of thyroxine on basal and insulin-stimulated glucose uptake by fast and slow skeletal muscles of rats.

1. The sc injection of 1-thyroxine (2 mg/kg bw/day) for 8 days produced a significant decrease of body weight gain in young male Wistar rats. 2. In these hyperthyroid rats there was a significant decrease in the wet weight of the extensor digitorum longus (EDL) and soleus (Sol) muscles as compared with those of control rats. 3. The basal glucose uptake by the EDL and Sol muscles was unchanged in hyperthyroid rats using the wet weight of muscle as a reference. 4. In hyperthyroid rats, the insulin-stimulated uptake of glucose by both the EDL and Sol muscles was significantly decreased. This inhibition was stronger in Sol and there was no insulin stimulation of glucose uptake by Sol.     

Hypoxia-induced fibre type transformation in rat hindlimb muscles Histochemical and electro-mechanical changes

Twelve male Sprague-Dawley rats (21 days old) were randomly assigned into two experimental groups: sea level control (CONT) and hypobaric hypoxia (HYPO). The HYPO rats were kept in an hypobaric chamber maintaining a simulated altitude of 4000 m (61.1 kPa). After 10 weeks of treatment, the rat hindlimb muscles [soleus (SOL) and extensor digitorum longus (EDL)] were subjected to histochemical and electro-mechanical analyses. Results indicated that compared to CONT the HYPO SOL muscle had a significantly greater relative distribution of fast-twitch-oxidative-glycolytic (FOG) fibres (28.9% SEM 2.0 vs 18.3% SEM 1.8, P less than 0.01) with a significant decrease in slow twitch oxidative fibre distribution (69.5% SEM 2.4 vs 82.9% SEM 3.1, P less than 0.01). Compared to CONT the HYPO EDL muscle also manifested a significant increase in FOG fibre distribution (51.6% SEM 0.8 vs 46.6% SEM 1.1, P less than 0.01), but this was accompanied by a significant decrease in fast twitch glucolytic fibres (44.3% SEM 0.9 vs 49.2% SEM 1.7, P less than 0.05). These histochemical fibre type transformations accompanied significant and expected changes in the electro-mechanical parameters tested in situ, e.g. maximal twitch force, maximal rate of force development, contraction time, half relaxation time, force: frequency curve, and fatigability. It was concluded that chronic hypobaric hypoxia could have a potent influence upon the phenotype expression of muscle fibres.