Fast and slow skeletal myosin binding protein-C and aging.

Aging is associated with skeletal muscle strength decline and cardiac diastolic dysfunction. The structural arrangements of the sarcomeric proteins, such as myosin binding protein-C (MyBP-C) are shown to be pivotal in the pathogenesis of diastolic dysfunction. Yet, the role of fast (fMyBP-C) and slow (sMyBP-C) skeletal muscle MyBP-C remains to be elucidated. Herein, we aimed to characterize MyBP-C and its paralogs in the fast tibialis anterior (TA) muscle from adult and old mice. Immunoreactivity preparations showed that the relative abundance of the fMyBP-C paralog was greater in the TA of both adult and old, but no differences were noted between groups. We further found that the expression level of cardiac myosin binding protein-C (cMyBP-C), an important modulator of cardiac output, was lowered by age. Standard SDS-PAGE along with Pro-Q Diamond phosphoprotein staining did not identify age-related changes in phosphorylated MyBP-C proteins from TA and cardiac muscles; however, it revealed that MyBP-C paralogs in fast skeletal and cardiac muscle were highly phosphorylated. Mass spectrometry further identified glycogen phosphorylase, desmin, actin, troponin T, and myosin regulatory light chain 2 as phosphorylated myofilament proteins in both ages. MyBP-C protein-bound carbonyls were determined using anti-DNP immunostaining and found the carbonyl level of fMyBP-C, sMyBP-C, and cMyBP-C to be similar between old and adult animals. In summary, our data showed some differences regarding the MyBP-C paralog expression and identified an age-related reduction of cMyBP-C expression. Future studies are needed to elucidate which are the age-driven post-translational modifications in the MyBP-C paralogs.

Effects of left ventricular assist device (LVAD) placement on myocardial oxidative stress markers.

PURPOSE: The primary purpose of this study was to examine the changes in myocardial oxidative stress during the support of a left ventricular assist device (LVAD). METHODS: Myocardial tissue was collected from the lower left ventricle of 15 adult subjects with class IV heart failure (HF) during LVAD placement (n=9) or LVAD removal (Post-LVAD; n=6). Each tissue sample was separated into cytosolic and myofibrillar subfractions and analysed for protein content and carbonylation. RESULTS: The myofibrillar proteins in the HF subjects had a significantly lower (p=0.008) level of protein carbonylation when compared to the myofibrillar proteins in Post-LVAD patients at 1.630±0.277 and 3.075±0.413 optical density, respectively. The level of protein carbonylation in myosin and actin were lower in HF (myosin: 1406.22±218.45, actin: 436±79.72 optical density) subjects compared to Post-LVAD (myosin: 2280.5±441.26, actin: 804.67±155.71 optical density) subjects (p=0.035 and p=0.018, respectively). However, once the extent of carbonylation in the myosin and actin bands were normalised to the amount of protein content, all significant difference was lost (HF moysin: 1823.89±413.42, Post-LVAD myosin: 1330.33±297.10 optical density, p=0.199 and HF actin: 3755.78±349.59, Post-LVAD actin: 4402.83±666.51 optical density, p=0.182). There was no significant difference in the cytosolic subfractions before or after normalisation of protein content. CONCLUSION: Carbonylation is elevated in the myocardium of HF and Post-LVAD subjects and it appears that LVAD support does not affect the level of myocardial oxidative stress.

Skeletal muscle sorbitol levels in diabetic rats with and without insulin therapy and endurance exercise training.

Sorbitol accumulation is postulated to play a role in skeletal muscle dysfunction associated with diabetes. The purpose of this study was to determine the effects of insulin and of endurance exercise on skeletal muscle sorbitol levels in streptozotocin-induced diabetic rats. Rats were assigned to one experimental group (control sedentary, control exercise, diabetic sedentary, diabetic exercise, diabetic sedentary no-insulin). Diabetic rats received daily subcutaneous insulin. The exercise-trained rats ran on a treadmill (1 hour, 5X/wk, for 12 weeks). Skeletal muscle sorbitol levels were the highest in the diabetic sedentary no-insulin group. Diabetic sedentary rats receiving insulin had similar sorbitol levels to control sedentary rats. Endurance exercise did not significantly affect sorbitol levels. These results indicate that insulin treatment lowers sorbitol in skeletal muscle; therefore sorbitol accumulation is probably not related to muscle dysfunction in insulin-treated diabetic individuals. Endurance exercise did not influence intramuscular sorbitol values as strongly as insulin.

Age-dependent effects of treadmill exercise during a period of inactivity.

The objective of this study was to investigate the effect of a treadmill exercise protocol to prevent muscle weakness, atrophy and alterations in calcium regulation in adult, old and very old rats. Adult (7-12 months), old (29-30 months) and very old (34-36 months) F344BNF(1) rats were randomly assigned to weight bearing (WB), weight bearing exercise (WBX), non-weight bearing (NWB) and non-weight bearing exercise (NWBX) groups. The WB group was considered the sedentary-control animals. NWB rats were hindlimb unweighted for 14 days. WBX and NWBX groups were exercised on a treadmill for approximately 15 min four times daily. The contractile properties [diameter, peak active force (P(0)), specific tension (P(0)/CSA)] of single myosin heavy chain type II fibers and Ca regulation [Ca(2+) dependent ATPase activity] were determined. Fiber diameter reduced by 24% in the very old rats with NWB. P(0) and P(0)/CSA declined in the young adult and very old rats with NWB. NWBX attenuated these changes in the young and very old rats. Ca(2+) dependent ATPase activity increased with treadmill exercise during non-weight bearing in the young animals. In conclusion, the treadmill exercise is beneficial in attenuating the non-weight bearing-induced changes in the individual MHC type II muscle fibers of the gastrocnemius muscle.

Sarcopenia of ageing: functional, structural and biochemical alterations

Aging is associated with a progressive decline of muscle mass, strength, and quality, a condition described as sarcopenia of aging. Despite the significance of skeletal muscle atrophy, the mechanisms responsible for the deterioration of muscle performance are only partially understood. The purpose of this review is to highlight cellular, molecular and biochemical changes that contribute to age-related muscle weakness.

O envelhecimento está associado ao declínio progressivo da massa, força, e qualidade muscular, uma condição descrita como sarcopenia do envelhecimento. Apesar da importante atrofia do músculo esquelético, os mecanismos responsáveis pela deterioração do desempenho muscular são somente parcialmente conhecidos. A proposta desta revisão é ressaltar as alterações celulares, moleculares e bioquímicas que contribuem para a fraqueza muscular associada ao envelhecimento.

Calcium sensitivity of force production and myofibrillar ATPase activity in muscles from Thoroughbreds with recurrent exertional rhabdomyolysis.

OBJECTIVE: To determine whether the basis for recurrent exertional rhabdomyolysis (RER) in Thoroughbreds lies in an alteration in the activation and regulation of the myofibrillar contractile apparatus by ionized calcium. ANIMALS: 4 Thoroughbred mares with RER and 4 clinically normal (control) Thoroughbreds. PROCEDURES: Single chemically-skinned type-I (slow-twitch) and type-II (fast-twitch) muscle fibers were obtained from punch biopsy specimens, mounted to a force transducer, and the tensions that developed in response to a series of calcium concentrations were measured. In addition, myofibril preparations were isolated from muscle biopsy specimens and the maximal myofibrillar ATPase activity, as well as its sensitivity to ionized calcium, were measured. RESULTS: Equine type-I muscle fibers were more readily activated by calcium than were type-II muscle fibers. However, there was no difference between the type-II fibers of RER-affected and control horses in terms of calcium sensitivity of force production. There was also no difference between muscle myofibril preparations from RER-affected and control horses in calcium sensitivity of myofibrillar ATPase activity. CONCLUSIONS AND CLINICAL RELEVANCE: An alteration in myofibrillar calcium sensitivity is not a basis for pathologic contracture development in muscles from RER-affected horses. Recurrent exertional rhabdomyolysis in Thoroughbreds may represent a novel heritable defect in the regulation of muscle excitation-contraction coupling or myoplasmic calcium concentration.

Clenbuterol in the prevention of muscle atrophy: a study of hindlimb-unweighted rats.

OBJECTIVE: To determine whether the administration of clenbuterol, a beta2-adrenergic agonist, prevents loss of muscle mass during a period of imposed inactivity. DESIGN: Randomized trial. SETTING: Basic laboratory research. ANIMALS: Thirty Fischer 344 Brown Norway F1 Hybrid rats, 12 and 30 months of age. INTERVENTIONS: The rats were randomly assigned to a control group, or to 1 of 2 experimental groups: hindlimb unweighted for 2 weeks (HU-2), or hindlimb unweighted with daily injections of clenbuterol for 2 weeks (HU-2Cl). MAIN OUTCOME MEASURES: Muscle mass weighed in milligrams and single fiber cross-sectional area histochemically evaluated. RESULTS: In both age groups, the HU-2 animals had greater muscle atrophy (decrease in muscle mass) in the soleus muscle than the extensor digitorum longus (EDL) muscle. In the HU-2Cl groups, the decline in muscle mass of both the soleus and EDL muscles was attenuated by about 4% to 20%. In the HU-2 group, single fiber cross-sectional area decreased for both fiber types (type I, 20%-40%; type II, 37%-50%) in both age groups. Clenbuterol retarded the inactivity-induced decline in single fiber cross-sectional area by 12% to 50%. In the EDL muscles of the HU-2Cl group, we found hypertrophy in both fiber types in the 30-month-old animals and in type I fibers in the 12-month-old animals. CONCLUSIONS: Clenbuterol attenuated the decrease in muscle mass and single fiber cross-sectional area in both age groups. By preventing the loss of muscle mass, clenbuterol administered early in rehabilitation may benefit severely debilitated patients imposed by inactivity. The attenuated muscle atrophy found with clenbuterol in the present study provides cellular evidence for the reported change in muscle strength after its administration after knee surgery. Thus, the administration of clenbuterol may lead to a more rapid rate of rehabilitation.

Electron paramagnetic resonance reveals age-related myosin structural changes in rat skeletal muscle fibers.

We tested the hypothesis that low specific tension (force/cross-sectional area) in skeletal muscle from aged animals results from structural changes in myosin that occur with aging. Permeabilized semimembranosus fibers from young adult and aged rats were spin labeled site specifically at myosin SH1 (Cys-707). Electron paramagnetic resonance (EPR) was then used to resolve and quantify the structural states of the myosin head to determine the fraction of myosin heads in the strong-binding (force generating) structural state during maximal isometric contraction. Fibers from aged rats generated 27 +/- 0.8% less specific tension than fibers from younger rats (P < 0.001). EPR spectral analyses showed that, during contraction, 31.6 +/- 2.1% of myosin heads were in the strong-binding structural state in fibers from young adult animals but only 22.1 +/- 1.3% of myosin heads in fibers from aged animals were in that state (P = 0.004). Biochemical assays indicated that the age-related change in myosin structure could be due to protein oxidation, as indicated by a decrease in the number of free cysteine residues. We conclude that myosin structural changes can provide a molecular explanation for age-related decline in skeletal muscle force generation.

Electron paramagnetic resonance: a high-resolution tool for muscle physiology.

Electron paramagnetic resonance: a high-resolution tool for muscle physiology. Exerc. Sport Sci. Rev., Vol. 29, No. 1, pp 3-6, 2001. Skeletal muscle function can be altered by changes in protein structure and motion. Electron paramagnetic resonance (EPR) paired with site-directed spin labeling has been used to study the relationships between (a) muscle force and myosin structure and (b) muscle relaxation and Ca-ATPase motion and structure.

Effects of inactivity on glycolytic capacity of single skeletal muscle fibers in adult and aged rats.

The purpose of this study was to determine the effects of inactivity on lactate dehydrogenase (LDH) enzyme activity (expressed in nmol/g dry weight x hour) in single skeletal musclefibers from the soleus muscle in adult and aged rats. Fourteen 12-month-old andfifteen 30-month-old Fisher 344 Brown Norway F1 Hybrid rats were randomly assigned to control, 1 week of hindlimb unweighting (HU1), or 2 weeks of hindlimb unweighting (HU2). With age, a significant decrease in LDH enzyme activity occurred in type I skeletal muscle fibers (29.5%, P < 0.05). Following HU2, individual type I skeletal muscle fibers from the 12-month-old animals showed a 33.3% increase in LDH activity. In contrast, individual type I fibers from the aged animals showed a 50.0% increase after HU1. In conclusion, the baseline levels of LDH activity were significantly less in aged versus adult rats. The timing of the skeletal muscle adaptation to inactivity was different between young and old animals, such that the older animals responded to inactivity before the younger animals. These biochemical changes may have an impact on the fatigability of the muscle following inactivity. Thefindings indicate that treatment during bed rest for the older adult may be different than that for the younger adult.

Contractile properties and protein isoforms of single skeletal muscle fibers from 12- and 30-month-old Fischer 344 brown Norway F1 hybrid rats.

This study characterizes single skeletal muscle fiber contractile properties and myosin heavy chain (MHC) isoform compositions from the soleus (SOL) and deep portion of the lateral head of the gastrocnemius (RG) muscles of 12- and 30-month-old Fischer 344 Brown Norway F1 hybrid rats (FBN F1). Thirty months of age is approximately the age of 50% survival for the FBN F1 rat. For type I MHC individual fibers from the SOL of 30-month-old animals, the diameter was 88 +/- 2 microns, peak active force was 4.4 +/- 0.2 x 10(-4) N, peak specific tension (P0) was 76 +/- 5 kN/m2, and maximal unloaded shortening velocity (V0) was 0.98 +/- 0.09 fl/s. The type I MHC fibers from the SOL of 12-month-old animals had similar properties with the exception of P0 which was 92 +/- 4 kN/m2 and V0 which was 1.65 +/- 0.12 fl/s. Contractile properties of the RG MHC type I fibers were not significantly different from MHC type I fibers from the SOL in both age groups. The V0 of the RG type IIa MHC fibers from the 12-month animals (4.05 +/- 0.36 fl/s) and 30-month animals (3.55 +/- 0.41 fl/s) and of fibers co-expressing type I MHC and type IIa MHC from 12-month animals (4.21 +/- 0.55 fl/s) and 30-month animals (2.22 +/- 0.27 fl/s) were significantly faster than that of MHC type I fibers of the respective age group. In conclusion, skeletal fibers from the 12- and 30-month-old FBN F1 rats demonstrate fiber-type specific properties with a close relationship between the MHC isoform composition and the V0.

Age-related changes in contractile properties of single skeletal fibers from the soleus muscle.

Peak absolute force, specific tension (peak absolute force per cross-sectional area), cross-sectional area, maximal unloaded shortening velocity (Vo; determined by the slack test), and myosin heavy chain (MHC) isoform compositions were determined in 124 single skeletal fibers from the soleus muscle of 12-, 24-, 30-, 36-, and 37-mo-old Fischer 344 Brown Norway F1 Hybrid rats. All fibers expressed the type I MHC isoform. The mean Vo remained unchanged from 12 to 24 mo but did decrease significantly from the 24- to 30-mo time period (from 1.71 +/- 0.13 to 0.85 +/- 0.09 fiber lengths/s). Fiber cross-sectional area remained constant until 36 mo of age, at which time there was a 20% decrease from the values at 12 mo of age (from 5,558 +/- 232 to 4,339 +/- 280 micrometer2). A significant decrease in peak absolute force of single fibers occurred between 12 and 24 mo of age (from 51 +/- 2 x 10(-5) to 35 +/- 2 x 10(-5) N) and then remained constant until 36 mo, when another 43% decrease occurred. Like peak absolute force, the specific tension decreased significantly between 12 and 24 mo by 20%, and another 32% decline was observed at 37 mo. Thus, by 24 mo, there was a dissociation between the loss of fiber cross-sectional area and force. The results suggest time-specific changes of the contractile properties with aging that are independent of each other. Underlying mechanisms responsible for the time-dependent and contractile property-specific changes are unknown. Age-related changes in the molecular dynamics of myosin may be the underlying mechanism for altered force production. The presence of more than one beta/slow MHC isoform may be the mechanism for the altered Vo with age.

The fiber-type-specific effect of inactivity and intermittent weight-bearing on the gastrocnemius muscle of 30-month-old rats.

OBJECTIVE: To characterize single skeletal muscle fiber contractile properties from the gastrocnemius muscle that occur during inactivity and intermittent weight-bearing in 30-month-old animals. DESIGN: Randomized control trial. SETTING: A controlled laboratory environment. SUBJECTS: Eighteen 30-month-old male Fisher 344 Brown Norway F1 Hybrid rats were randomly assigned to control (C), hindlimb unweighted (HU), and hindlimb unweighted with intermittent weight-bearing (HU-X) groups. INTERVENTIONS: The HU and HU-X rats were suspended for 7 days. The HU-X animals were unsuspended for four 15-minute bouts of weight-bearing. MAIN OUTCOME MEASURES: Single skeletal muscle fiber contractile properties (diameter, peak active force [P0], peak specific tension [P0/CSA], and maximal shortening velocity [V0] by fiber type) were determined from the deep portion of the lateral head of the gastrocnemius muscle (RG). RESULTS: In comparison to C animals, the ratio of gastrocnemius weight to body weight decreased by 18% and 14% following HU and HU-X, respectively. Diameter and P0 of type I fibers from the RG were reduced after HU. Attenuation of the decline in diameter and P0 was observed in type I fibers from the RG with HU-X. P0 was reduced in type IIa fibers and type I-IIa fibers with HU. Attenuation of the decline in P0 by intermittent weight-bearing in type IIa fibers and type I-IIa fibers did not occur. CONCLUSIONS: Inactivity altered the contractile properties of single skeletal muscle fibers from the gastrocnemius muscle of 30-month-old animals. The inactivity-induced alterations were present in the three fiber types. Therapeutic intervention of weight-bearing attenuated the inactivity-induced changes in the type I fibers from the gastrocnemius, but not the other fiber types.

Single soleus muscle fiber function after hindlimb unweighting in adult and aged rats.

This investigation compared how hindlimb unweighting (HU) affected the contractile function of single soleus muscle fibers from 12- and 30-mo-old Fischer 344 Brown Norway F1 Hybrid rats. After 1 wk of HU, functional properties of single permeabilized fibers were studied, and, subsequently, the fiber type was established by myosin heavy chain (MHC) analysis. After HU, the relative mass of soleus declined by 12 and 19% and the relative mass of the gastrocnemius declined by 15 and 13% in 12- and 30-mo-old animals, respectively. In 12-mo-old animals, the peak active force (5.0 +/- 0.2 x10(-4) vs. 3.8 +/- 0.2 x10(-4) N) and the peak specific tension (92 +/- 4 vs. 78 +/- 3 kN/m2) were significantly reduced in the MHC type I fibers by 24 and 15%, respectively. In 30-mo-old animals, the peak active force declined by 40% (4.7 +/- 0.2 x10(-4) vs. 2.8 +/- 0. 3 x10(-4) N) and the peak specific tension declined by 30% (79 +/- 5 vs. 55 +/- 4 kN/m2). The maximal unloaded shortening velocity of the MHC type I fibers increased in 12-mo-old animals (from 1.65 +/- 0.12 to 2.59 +/- 0.26 fiber lengths/s) and in 30-mo-old animals (from 0.90 +/- 0. 09 to 1.50 +/- 0.10 fiber lengths/s) after HU. Collectively, these data suggest that the effects of HU on single soleus skeletal muscle fiber function occur in both age groups; however, the single MHC type I fibers from the older animals show greater changes than do single MHC type I fibers from younger animals.

Contractile function of single muscle fibers after hindlimb unweighting in aged rats.

This investigation determined how muscle atrophy produced by hindlimb unweighting (HU) alters the contractile function of single muscle fibers from older animals (30 mo). After 1 wk of HU, small bundles of fibers were isolated from the soleus muscles and the deep region of the lateral head of the gastrocnemius muscles. Single glycerinated fibers were suspended between a motor lever and force transducer, functional properties were studied, and the myosin heavy chain (MHC) composition was determined electrophoretically. After HU, the diameter of type I MHC fibers of the soleus declined (88 +/- 2 vs. 80 +/- 4 microns) and reductions were observed in peak active force (47 +/- 3 vs. 28 +/- 3 mg) and peak specific tension (Po; 80 +/- 5 vs. 56 +/- 5 kN/m2). The maximal unloaded shortening velocity increased. The type I MHC fibers from the gastrocnemius showed reductions in diameter (14%), peak active force (41%), and Po (24%), whereas the type IIa MHC fibers showed reductions in peak active force and Po. Thus 1 wk of inactivity has a significant effect on the force-generating capacity of single skeletal muscle fibers from older animals in a fiber type-specific manner (type I MHC > type IIa MHC > type I-IIa MHC). The decline in the functional properties of single skeletal muscle fibers in the older animals appears to be more pronounced than what has been reported in younger animal populations.

GTP gammaS removal of D-600 block of skeletal muscle excitation-contraction coupling.

G proteins interacting with dihydropyridine receptors (DHPR) in transverse tubules (TT) of skeletal muscle may have a role in skeletal excitation-contraction (EC) coupling. The aim of this study was to determine the effects of G protein-specific nucleotides [guanosine 5'-O-(3-thiotriphosphate) (GTP gammaS) and guanosine 5'-O-(2-thiodiphosphate) (GDP betaS)] on the EC coupling mechanism in the presence of D-600, an agent that blocks EC coupling by immobilizing the voltage-sensing subunit of the DHPR in its inactivated state. By use of the mechanically peeled single-fiber preparation from rabbit adductor magnus skeletal muscle, 50 microM GTP gammaS and 500 microM GDP betaS were applied with the fiber in a D-600-induced state of blocked EC coupling. Neither nucleotide served as an independent stimulus for sarcoplasmic reticulum (SR) Ca2+ release when added to the TT polarizing bath under conditions of D-600 block. The presence of GTP gammaS or GDP betaS during a complete EC coupling cycle removed the D-600 block of EC coupling, despite continuous bath D-600. After the nucleotides were washed out, in the continued presence of D-600, the D-600 block of EC coupling was reestablished. In contrast, GTP gammaS added only during the period of TT depolarization under D-600 block did not remove the D-600 block of EC coupling, even though GTP gammaS did stimulate SR Ca2+ release. GTP gammaS had no effect on submaximum (0.5-1.0 mM) caffeine contractures and thus is unlikely to be acting through the Ca2+-induced Ca2+ release mechanism of the SR. These data suggest that the molecular binding site for GTP gammaS and GDP betaS is likely to be in the TT near the DHPR, perhaps on a G protein.

Influence of simulated bed rest and intermittent weight bearing on single skeletal muscle fiber function in aged rats.

OBJECTIVE: To characterize specific musculoskeletal contractile property changes that occur during inactivity and intermittent weight bearing in aged muscle. DESIGN: Randomized control trial. SETTING: A controlled laboratory environment. SUBJECTS: Fifteen aged rats were randomly assigned to control (CON), hindlimb unweighted (HU), and hindlimb unweighted with intermittent weight bearing (HU-IWB) groups. INTERVENTIONS: The HU and HU-IWB rats were suspended for 1 week. The HU-IWB animals were unsuspended four times daily allowing 15 minutes of weight-bearing. MAIN OUTCOME MEASURES: Muscle weights, muscle fiber diameter, peak absolute force, peak specific tension (P0), and maximal shortening velocity (V0). RESULTS: In comparison to CON animals, the soleus (SOL) wet weight was significantly (p < or = .05) reduced by 19% and 6% in HU and HU-IWB animals, respectively. SOL single fiber analysis showed no difference in fiber diameter between the three groups. However, peak absolute force and P0 of SOL type I fibers were significantly (p < or = .05) reduced in the HU group compared to CON values. V0 of SOL fibers increased with HU. In comparison to CON animals, the gastrocnemius (GAS) wet weight was significantly reduced by 9% and 8% in HU and HU-IWB animals, respectively. CONCLUSIONS: Inactivity significantly altered the contractile properties of single fibers isolated from aged mammalian SOL skeletal muscle. Furthermore, minimal weight bearing attenuated these detrimental effects induced by inactivity in the SOL. However, this weight-bearing protocol did not attenuate the inactivity-induced alterations in aged mammalian GAS skeletal muscle.

Role of sarcolemma action potentials and excitability in muscle fatigue.

The purposes of this study were to characterize the alterations in the sarcolemma action potential (AP) waveform and sarcolemma excitability as a result of fatiguing stimulation of the frog semitendinosus muscle and to relate these changes to the decrease in the force-generating ability of the muscle. Trains of APs were recorded before and after stimulation (100-ms trains, 150 Hz, 1/s for 5 min). The resting membrane potential (RMP), AP overshoot (OS), and duration at 50% of peak magnitude (DUR) were -84.3 +/- 2.0 mV, 19.5 +/- 1.9 mV, and 1.3 +/- 0.1 ms, respectively, before stimulation. The stimulation protocol caused RMP to depolarize to -75.1 +/- 2.0 mV, OS to fall to 7.3 +/- 1.9 mV, and DUR to increase to 2.5 +/- 0.4 ms. RMP and OS recovered fully in 5 min after the cessation of stimulation, whereas DUR was still prolonged. Before the stimulation protocol, AP frequency matched the stimulation frequency at all stimulation rates < or = 150 Hz. At 200-Hz stimulation, AP frequency was 192 +/- 6 Hz. After 5 min of stimulation, AP frequency matched the stimulation frequency only at < or = 60 Hz. At 100-, 150-, and 200-Hz stimulation, AP frequencies were 89 +/- 8, 84 +/- 17, and 79 +/- 15 Hz, respectively. Because of a decreased fusion frequency at fatigue, the fall in the sarcolemma AP frequency did not contribute to the decreased force. The stimulation-induced alterations in the AP waveform were moderate and unlikely to have caused fatigue. However, the alterations in AP may have been more extreme in the depths of the transverse tubules.

Effects of age and training on skeletal muscle physiology and performance.

Aging skeletal muscle exhibits decreases in muscle mass and force and changes in contractile properties. The effects of aging on the physiological characteristics of skeletal muscle are fiber type specific. This review describes the aging process in skeletal muscle; specifically, the effects of aging on the biochemical, morphological, and physiological characteristics of type I (slow-twitch) fibers and type II (fast-twitch) fibers. The effects of training on specific fiber types are also reviewed. The age-related decrease in maximum isometric force may be due, in part, to a decline in muscle mass. Decreases in muscle mass appear to occur in weight-bearing muscles and are most marked in those with a high proportion of type II fibers. The age-related fiber atrophy contributes to the decline in muscle mass. The decline in fiber size is prominent in type II fibers, whereas type I fibers are less affected. The age-related prolongation in isometric twitch properties may be due, in part, to alteration in the capacity of the sarcoplasmic reticulum for calcium release and recapture. Resistance and endurance training appear to attenuate the age-related alterations in skeletal muscle properties if the stimulus is of a sufficient intensity and duration.

Muscle fatigue in the frog semitendinosus: role of the high-energy phosphates and Pi.

The purpose of this study was to determine the concentration of ATP, phosphocreatine (PC), Pi, lactate, and glycogen in single frog skeletal muscle fibers and assess their role in the etiology of muscle fatigue. The frog semitendinosus (ST) muscle was fatigued, quick frozen at selected time points of recovery, and freeze-dried, and single fibers were dissected, weighed, and assayed for ATP, PC, lactate, Pi, and glycogen. The fatigue protocol reduced peak tetanic force (Po) to 8.5% of initial, while ATP and PC decreased from 45.18 to 33.16 and 128.90 to 28.76 mmol/kg dry wt, respectively. Lactate and Pi increased from 29.36 to 100.84 and 33.04 to 142.50 mmol/kg dry wt, respectively. It is doubtful that the small decline in ATP limited cross-bridge force production. Although a significant correlation between the recovery of PC and Po was demonstrated (r = 0.994), the time period showing the fastest rate of force recovery coincided with little change in PC. A significant correlation was demonstrated between the recovery of both total and the H2PO4- form of Pi and Po. In conclusion, the results of this study are incompatible with the hypothesis that the high-energy phosphates (ATP and PC) mediate muscle fatigue. The large increase in Pi with stimulation and the high correlation between the recovery of both total and the H2PO4- form of Pi and Po support a role for Pi in the production of skeletal muscle fatigue.