An assay for sarcoplasmic reticulum Ca2(+)-ATPase activity in muscle homogenates

Summer fades, deer change: Photoperiodic control of cellular seasonal acclimatization of skeletal muscle

We found major seasonal changes of polyunsaturated fatty acids (PUFAs) in muscular phospholipids (PL) in a large non-hibernating mammal, the red deer (Cervus elaphus). Dietary supply of essential linoleic acid (LA) and α-linolenic acid (ALA) had no, or only weak influence, respectively. We further found correlations of PL PUFA concentrations with the activity of key metabolic enzymes, independent of higher winter expression. Activity of the sarcoplasmic reticulum (SR) Ca++-ATPase increased with SR PL concentrations of n-6 PUFA, and of cytochrome c oxidase and citrate synthase, indicators of ATP-production, with concentrations of eicosapentaenoic acid in mitochondrial PL. All detected cyclic molecular changes were controlled by photoperiod and are likely of general relevance for mammals living in seasonal environments, including humans. During winter, these changes at the molecular level presumably compensate for Arrhenius effects in the colder peripheral body parts and thus enable a thrifty life at lower body temperature.

Weight loss increases skeletal muscle mitochondrial energy efficiency in obese mice

Weight loss from an overweight state is associated with a disproportionate decrease in whole-body energy expenditure that may contribute to the heightened risk for weight regain. Evidence suggests that this energetic mismatch originates from lean tissue. Although this phenomenon is well documented, the mechanisms have remained elusive. We hypothesized that increased mitochondrial energy efficiency in skeletal muscle is associated with reduced expenditure under weight loss. Wildtype (WT) male C57BL6/N mice were fed with high fat diet for 10 weeks, followed by a subset of mice that were maintained on the obesogenic diet (OB) or switched to standard chow to promote weight loss (WL) for additional 6 weeks. Mitochondrial energy efficiency was evaluated using high-resolution respirometry and fluorometry. Mass spectrometric analyses were employed to describe the mitochondrial proteome and lipidome. Weight loss promoted ~50% increase in the efficiency of oxidative phosphorylation (ATP produced per O2 consumed, or P/O) in skeletal muscle. However, weight loss did not appear to induce significant changes in mitochondrial proteome, nor any changes in respiratory supercomplex formation. Instead, it accelerated the remodeling of mitochondrial cardiolipin (CL) acyl-chains to increase tetralinoleoyl CL (TLCL) content, a species of lipids thought to be functionally critical for the respiratory enzymes. We further show that lowering TLCL by deleting the CL transacylase tafazzin was sufficient to reduce skeletal muscle P/O and protect mice from diet-induced weight gain. These findings implicate skeletal muscle mitochondrial efficiency as a novel mechanism by which weight loss reduces energy expenditure in obesity.

Influence of vitamin D on sarcopenia pathophysiology: A longitudinal study in humans and basic research in knockout mice

BACKGROUND

Vitamin D is an essential nutrient in musculoskeletal function; however, its relationship to sarcopenia remains ambiguous, and the mechanisms and targets of vitamin D activity have not been elucidated. This study aimed to clarify the role of vitamin D in mature skeletal muscle and its relationship with sarcopenia.

METHODS

This epidemiological study included 1653 community residents who participated in both the fifth and seventh waves of the National Institute for Longevity Sciences, Longitudinal Study of Aging and had complete background data. Participants were classified into two groups: vitamin D-deficient (serum 25-hydroxyvitamin D < 20 ng/mL) and non-deficient (serum 25-hydroxyvitamin D ≥ 20 ng/mL); they underwent propensity-score matching for background factors (age, sex, height, weight, comorbidities, smoker, alcohol intake, energy intake, vitamin D intake, steps, activity, season and sarcopenia). Changes in muscle strength and mass over the 4-year period were compared. For basic analysis, we generated Myf6^CreERT2 Vitamin D Receptor (VDR)-floxed (Vdr^mcKO ) mice with mature muscle fibre-specific vitamin D receptor knockout, injected tamoxifen into 8-week-old mice and analysed various phenotypes at 16 weeks of age.

RESULTS

Grip strength reduction was significantly greater in the deficient group (-1.55 ± 2.47 kg) than in the non-deficient group (-1.13 ± 2.47 kg; P = 0.019). Appendicular skeletal muscle mass reduction did not differ significantly between deficient (-0.05 ± 0.79 kg) and non-deficient (-0.01 ± 0.74 kg) groups (P = 0.423). The incidence of new cases of sarcopenia was significantly higher in the deficient group (15 vs. 5 cases; P = 0.039). Skeletal muscle phenotyping of Vdr^mcKO mice showed no significant differences in muscle weight, myofibre percentage or myofibre cross-sectional area; however, both forelimb and four-limb muscle strength were significantly lower in Vdr^mcKO mice (males: forelimb, P = 0.048; four-limb, P = 0.029; females: forelimb, P < 0.001; four-limb, P < 0.001). Expression profiling revealed a significant decrease in expression of sarcoendoplasmic reticulum Ca²⁺ -ATPase (SERCA) 1 (P = 0.019) and SERCA2a (P = 0.049) genes in the Vdr^mcKO mice. In contrast, expression of non-muscle SERCA2b and myoregulin genes showed no changes.

CONCLUSIONS

Vitamin D deficiency affects muscle strength and may contribute to the onset of sarcopenia. Vitamin D-VDR signalling has minimal influence on the regulation of muscle mass in mature myofibres but has a significant influence on muscle strength.

Sirtuin 3 overexpression preserves maximal sarco(endo)plasmic reticulum calcium ATPase activity in the skeletal muscle of mice subjected to high fat-high sucrose-feeding

Sarco(endo)plasmic reticulum calcium (Ca²⁺) ATPase (SERCA) transports Ca²⁺ in muscle. Impaired SERCA activity contributes to diabetic myopathy. Sirtuin (SIRT) 3 regulates muscle metabolism and function. However, it is unknown if SIRT3 regulates muscle SERCA activity. We determined if SIRT3 overexpression enhances SERCA activity in mouse gastrocnemius muscle and if SIRT3 overexpression preserves gastrocnemius SERCA activity in a model of type 2 diabetes, induced by high fat-high sucrose (HFHS)-feeding. We also determined if the acetylation status of SERCA proteins in mouse gastrocnemius is altered by SIRT3 overexpression or HFHS-feeding. Wild-type (WT) mice and SIRT3 transgenic (SIRT3_TG) mice, overexpressing SIRT3 in skeletal muscle, were fed a standard- or HFHS-diet for 4-months. SIRT3_TG and WT mice developed obesity and glucose intolerance after 4-months of HFHS-feeding. SERCA V_max was higher in gastrocnemius of SIRT3TG mice, compared to WT mice. HFHS-fed mice had lower SERCA1a protein levels and lower SERCA V_max in their gastrocnemius than control-fed mice. The decrease in SERCA V_max in gastrocnemius muscle due to HFHS-feeding was attenuated by SIRT3 overexpression in HFHS-fed SIRT3_TG mice. SERCA1a and SERCA2a acetylation in mouse gastrocnemius was not altered by genotype or diet. These findings suggest SIRT3 overexpression improves SERCA function in diabetic mouse skeletal muscle.

Muscle-specific sirtuin 3 overexpression does not attenuate the pathological effects of high-fat/high-sucrose feeding but does enhance cardiac SERCA2a activity

Obesity, type 2 diabetes, and heart disease are linked to an unhealthy diet. Sarco(endo)plasmic reticulum calcium (Ca2+ ) ATPase 2a (SERCA2a) controls cardiac function by transporting Ca2+ in cardiomyocytes. SERCA2a is altered by diet and acetylation, independently; however, it is unknown if diet alters cardiac SERCA2a acetylation. Sirtuin (SIRT) 3 is an enzyme that might preserve health under conditions of macronutrient excess by modulating metabolism via regulating deacetylation of target proteins. Our objectives were to determine if muscle-specific SIRT3 overexpression attenuates the pathological effects of high fat-high sucrose (HFHS) feeding and if HFHS feeding alters cardiac SERCA2a acetylation. We also determined if SIRT3 alters cardiac SERCA2a acetylation and regulates cardiac SERCA2a activity. C57BL/6J wild-type (WT) mice and MCK-mSIRT3-M1-Flag transgenic (SIRT3TG ) mice, overexpressing SIRT3 in cardiac and skeletal muscle, were fed a standard-diet or a HFHS-diet for 4 months. SIRT3TG and WT mice developed obesity, glucose intolerance, cardiac dysfunction, and pathological cardiac remodeling after 4 months of HFHS feeding, indicating muscle-specific SIRT3 overexpression does not attenuate the pathological effects of HFHS-feeding. Overall cardiac lysine acetylation was increased by 63% in HFHS-fed mice (p = 0.022), though HFHS feeding did not alter cardiac SERCA2a acetylation. Cardiac SERCA2a acetylation was not altered by SIRT3 overexpression, whereas SERCA2a Vmax was 21% higher in SIRT3TG (p = 0.039) than WT mice. This suggests that SIRT3 overexpression enhanced cardiac SERCA2a activity without direct SERCA2a deacetylation. Muscle-specific SIRT3 overexpression may not prevent the complications associated with an unhealthy diet in mice, but it appears to enhance SERCA2a activity in the mouse heart.

Orthograde signal of dihydropyridine receptor increases Ca2+ leakage after repeated contractions in rat fast-twitch muscles in vivo

The purpose of this study was to investigate the mechanism underlying sarcoplasmic reticulum (SR) Ca2+ leakage after in vivo contractions. Rat gastrocnemius muscles were electrically stimulated in vivo, and then mechanically skinned fibers and SR microsomes were prepared from the muscles excised 30 min after repeated high-intensity contractions. The mechanically skinned fibers maintained the interaction between dihydropyridine receptors (DHPRs) and ryanodine receptors (RyRs), whereas the SR microsomes did not. Interestingly, skinned fibers from the stimulated muscles showed increased SR Ca2+ leakage, whereas Ca2+ leakage decreased in SR microsomes from the stimulated muscles. To enhance the orthograde signal of DHPRs, SR Ca2+ leakage in the skinned fiber was measured 1) under a continuously depolarized condition and 2) in the presence of nifedipine. As a result, in either of the two conditions, SR Ca2+ leakage in the rested fibers reached a level similar to that in the stimulated fibers. Furthermore, the increased SR Ca2+ leakage from the stimulated fibers was alleviated by treatment with 1 mM tetracaine (Tet) but not by treatment with 3 mM free Mg2+ (3 Mg). Tet exerted a greater inhibitory effect on the DHPR signal to RyR than 3 Mg, although their inhibitory effects on RyR were almost similar. These results suggest that the increased Ca2+ leakage after muscle contractions is mainly caused by the orthograde signal of DHPRs to RyRs.

Assessment of Na+/K+ ATPase Activity in Small Rodent and Human Skeletal Muscle Samples

INTRODUCTION

In skeletal muscle, the Na/K ATPase (NKA) plays essential roles in processes linked to muscle contraction, fatigue, and energy metabolism; however, very little information exists regarding the regulation of NKA activity. The scarcity of information regarding NKA function in skeletal muscle likely stems from methodological constraints, as NKA contributes minimally to total cellular ATP utilization, and therefore contamination from other ATPases prevents the assessment of NKA activity in muscle homogenates. Here we introduce a method that improves accuracy and feasibility for the determination of NKA activity in small rodent muscle samples (5-10 mg) and in human skeletal muscle.

METHODS

Skeletal muscle homogenates from mice (n = 6) and humans (n = 3) were used to measure NKA and sarcoplasmic reticulum Ca ATPase (SERCA) activities with the addition of specific ATPase inhibitors to minimize "background noise."

RESULTS

We observed that myosin ATPase activity was the major interfering factor for estimation of NKA activity in skeletal muscle homogenates, as the addition of 25 μM of blebbistatin, a specific myosin ATPase inhibitor, considerably minimized "background noise" (threefold) and enabled the determination of NKA maximal activity with values three times higher than previously reported. The specificity of the assay was demonstrated after the addition of 2 mM ouabain, which completely inhibited NKA. On the other hand, the addition of blebbistatin did not affect the ability to measure SERCA function. The coefficient of variation for NKA and SERCA assays were 6.2% and 4.4%, respectively.

CONCLUSION

The present study has improved the methodology to determine NKA activity. We further show the feasibility of measuring NKA and SERCA activities from a common muscle homogenate. This methodology is expected to aid in our long-term understanding of how NKA affects skeletal muscle metabolic homeostasis and contractile function in diverse situations.

Phospholipid methylation regulates muscle metabolic rate through Ca2+ transport efficiency

The biophysical environment of membrane phospholipids affects structure, function, and stability of membrane-bound proteins.^1,2 Obesity can disrupt membrane lipids, and in particular, alter the activity of sarco/endoplasmic reticulum (ER/SR) Ca²⁺-ATPase (SERCA) to affect cellular metabolism.^3-5 Recent evidence suggests that transport efficiency (Ca²⁺ uptake / ATP hydrolysis) of skeletal muscle SERCA can be uncoupled to increase energy expenditure and protect mice from diet-induced obesity.^6,7 In isolated SR vesicles, membrane phospholipid composition is known to modulate SERCA efficiency.^8-11 Here we show that skeletal muscle SR phospholipids can be altered to decrease SERCA efficiency and increase whole-body metabolic rate. The absence of skeletal muscle phosphatidylethanolamine (PE) methyltransferase (PEMT) promotes an increase in skeletal muscle and whole-body metabolic rate to protect mice from diet-induced obesity. The elevation in metabolic rate is caused by a decrease in SERCA Ca²⁺-transport efficiency, whereas mitochondrial uncoupling is unaffected. Our findings support the hypothesis that skeletal muscle energy efficiency can be reduced to promote protection from obesity.

Resveratrol prevents palmitic-acid-induced cardiomyocyte contractile impairment

Long-chain saturated fatty acids, especially palmitic acid (PA), contribute to cardiomyocyte lipotoxicity. This study tests the effects of PA on adult rat cardiomyocyte contractile function and proteins associated with calcium regulating cardiomyocyte contraction and relaxation. Adult rat cardiomyocytes were pretreated with resveratrol (Resv) and then treated with PA. For the reversal study, cardiomyocytes were incubated with PA prior to treatment with Resv. Cardiomyocyte contractility, ratio of rod- to round-shaped cardiomyocytes, and Hoechst staining were used to measure functional and morphological changes in cardiomyocytes. Protein expression of sarco-endoplasmic reticulum ATPase 2a (SERCA2a), native phospholamban (PLB) and phosphorylated PLB (pPLB ser16 and pPLB thr17), and troponin I (TnI) and phosphorylated TnI (pTnI) were measured. SERCA2a activity was also measured. Our results show that PA (200 μM) decreased the rate of cardiomyocyte relaxation, reduced the number of rod-shaped cardiomyocytes, and increased the number of cells with condensed nuclei; pre-treating cardiomyocytes with Resv significantly prevented these changes. Post-treatment with Resv did not reverse morphological changes induced by PA. Protein expression levels of SERCA2a, PLB, pPLBs, TnI, and pTnI were unchanged by PA or Resv. SERCA2a activity assay showed that V_max and Iono ratio were increased with PA and pre-treatment with Resv prevented this increase. In conclusion, our results show that Resv protect cardiomyocytes from contractile dysfunction induced by PA.

Exercise-induced increases in the expression and activity of cardiac sarcoplasmic reticulum calcium ATPase 2 is attenuated in AMPKα2kinase-dead mice

Exercise enhances cardiac sarcoplasmic reticulum Ca2+-ATPase 2a (SERCA2a) function through unknown mechanisms. The present study tested the hypothesis that the positive effects of exercise on SERCA2a expression and function in the left ventricle is dependent on adenosine-monophosphate-activated protein kinase (AMPK) α2 function. AMPKα2kinase-dead (KD) transgenic mice, which overexpress inactivated AMPKα2subunit, and wild-type C57Bl/6 (WT) mice were randomized into sedentary groups or groups with access to running wheels. After 5 months, exercised KD mice exhibited shortened deceleration time compared with sedentary KD mice. In left ventricular tissue, the ratio of phosphorylated AMPKαThr172:total AMPKα was 65% lower (P < 0.05) in KD mice compared with WT mice. The left ventricle of KD mice had 37% lower levels of SERCA2a compared with WT mice. Although exercise increased SERCA2a protein levels in WT mice by 53%, this response of exercise was abolished in exercised KD mice. Exercise training reduced total phospholamban protein content by 23% in both the WT and KD mice but remained 20% higher overall in KD mice. Collectively, these data suggest that AMPKα influences SERCA2a and phospholamban protein content in the sedentary and exercised heart, and that exercise-induced changes in SERCA2a protein are dependent on AMPKα function.

Energetic drain driving hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is the most common form of hereditary cardiomyopathy and is mainly caused by mutations of genes encoding cardiac sarcomeric proteins. HCM is characterized by hypertrophy of the left ventricle, frequently involving the septum, that is not explained solely by loading conditions. HCM has a heterogeneous clinical profile, but diastolic dysfunction and ventricular arrhythmias represent two dominant features of the disease. Preclinical evidence indicates that the enhanced Calcium (Ca²⁺ ) sensitivity of the myofilaments plays a key role in the pathophysiology of HCM. Notably, this is not always a direct consequence of sarcomeric mutations, but can also result from secondary mutation-driven alterations. Here, we review experimental and clinical evidence indicating that increased myofilament Ca²⁺ sensitivity lies upstream of numerous cellular derangements which potentially contribute to the progression of HCM toward heart failure and sudden cardiac death.

Muscle nonshivering thermogenesis in a feral mammal

Muscle nonshivering thermogenesis (NST) was recently suggested to play an important role in thermoregulation of species lacking brown adipose tissue (BAT). The mechanism, which is independent of muscle contractions, produces heat based on the activity of an ATPase pump in the sarcoplasmic reticulum (SERCA1a) and is controlled by the protein sarcolipin. To evaluate whether muscle NST could indeed play an important role in thermoregulation in species lacking BAT, we investigated the thermogenic capacities of newborn wild boar piglets. During cold exposure over the first 5 days of life, total heat production was improved while shivering intensity decreased, indicating an increasing contribution of NST. Sampling skeletal muscle tissue for analyses of SERCA activity as well as gene expression of SERCA1a and sarcolipin, we found an age-related increase in all three variables as well as in body temperature. Hence, the improved thermogenesis during the development of wild boars is not due to shivering but explained by the observed increase in SERCA activity. Our results suggest that muscle NST may be the primary mechanism of heat production during cold stress in large mammals lacking BAT, strengthening the hypothesis that muscle NST has likely played an important role in the evolution of endothermy.

Dietary Lipids Affect the Onset of Hibernation in the Garden Dormouse (Eliomys quercinus): Implications for Cardiac Function

Dietary lipids strongly influence patterns of hibernation in heterotherms. Increased dietary uptake of n-6 polyunsaturated fatty acids (PUFAs), particularly of linoleic acid (LA, C18:2 n-6), enables animals to reach lower body temperatures (T_b), lengthens torpor bout duration, and results in lower energy expenditure during hibernation. Conversely, dietary n-3 PUFA impacts negatively on hibernation performance. PUFA in surrounding phospholipids (PLs) presumably modulate the temperature-dependent activity of the sarcoplasmic reticulum (SR) Ca²⁺ ATPase 2 (SERCA2) and thus determine the threshold T_b still allowing proper heart function during torpor. We tested the effect of diets enriched with 10% of either corn oil (“CO,” high n-6 PUFA, e.g., LA) or menhaden oil [“MO,” long-chain n-3 PUFA, e.g., docosahexaenoic acid (DHA)] on hibernation performance and SERCA2 activity levels during torpor in garden dormice, an insectivorous, fat-storing hibernator. Prior to hibernation, individuals fed the MO diet showed an almost nine-times higher DHA levels and 30% lower LA proportions in white adipose tissue (WAT), reflecting the fatty acid composition of SR membranes, compared to CO-diet fed animals. When fed the MO diet, dormice significantly delayed their mean onset of hibernation by almost 4 days (range: 0–12 days), compared with CO-diet fed animals. Hibernation onset correlated positively with WAT-DHA levels and negatively with WAT-LA proportions prior to hibernation. Subsequently, hibernating patterns were similar between the two dietary groups, despite a significant difference in WAT-LA but not in WAT-DHA levels in mid-hibernation. SR-PL fatty acid composition and SERCA2 activity were identical in torpid individuals from the two dietary groups in mid-hibernation. In line with our previous findings on Syrian hamsters, a granivorous, food-storing hibernator, SERCA2 activity correlated positively with LA and negatively with DHA levels of SR-PL in torpid dormice, although SERCA2 activity was about three-times higher in garden dormice than in Syrian hamsters at similar PL-DHA proportions. Similarly, minimal T_b during torpor decreased as SERCA2 activity increased. We conclude that: (1) fatty acid composition of SR membranes modulates cardiac SERCA2 activity, hence determining the minimum T_b tolerated by hibernators, and (2) high DHA levels prevent hibernators from entering into torpor, but the critical levels differ substantially between species.

Comparative proteomic profiling reveals a role for Cisd2 in skeletal muscle aging

Skeletal muscle has emerged as one of the most important tissues involved in regulating systemic metabolism. The gastrocnemius is a powerful skeletal muscle composed of predominantly glycolytic fast‐twitch fibers that are preferentially lost among old age. This decrease in gastrocnemius muscle mass is remarkable during aging; however, the underlying molecular mechanism is not fully understood. Strikingly, there is a ~70% decrease in Cisd2 protein, a key regulator of lifespan in mice and the disease gene for Wolfram syndrome 2 in humans, within the gastrocnemius after middle age among mice. A proteomics approach was used to investigate the gastrocnemius of naturally aged mice, and this was compared to the autonomous effect of Cisd2 on gastrocnemius aging using muscle‐specific Cisd2 knockout (mKO) mice as a premature aging model. Intriguingly, dysregulation of calcium signaling and activation of UPR/ER stress stand out as the top two pathways. Additionally, the activity of Serca1 was significantly impaired and this impairment is mainly attributable to irreversibly oxidative modifications of Serca. Our results reveal that the overall characteristics of the gastrocnemius are very similar when naturally aged mice and the Cisd2 mKO mice are compared in terms of pathological alterations, ultrastructural abnormalities, and proteomics profiling. This suggests that Cisd2 mKO mouse is a unique model for understanding the aging mechanism of skeletal muscle. Furthermore, this work substantiates the hypothesis that Cisd2 is crucial to the gastrocnemius muscle and suggests that Cisd2 is a potential therapeutic target for muscle aging.

HDAC Inhibition Improves the Sarcoendoplasmic Reticulum Ca2+-ATPase Activity in Cardiac Myocytes

SERCA2a is the Ca²⁺ ATPase playing the major contribution in cardiomyocyte (CM) calcium removal. Its activity can be regulated by both modulatory proteins and several post-translational modifications. The aim of the present work was to investigate whether the function of SERCA2 can be modulated by treating CMs with the histone deacetylase (HDAC) inhibitor suberanilohydroxamic acid (SAHA). The incubation with SAHA (2.5 µM, 90 min) of CMs isolated from rat adult hearts resulted in an increase of SERCA2 acetylation level and improved ATPase activity. This was associated with a significant improvement of calcium transient recovery time and cell contractility. Previous reports have identified K464 as an acetylation site in human SERCA2. Mutants were generated where K464 was substituted with glutamine (Q) or arginine (R), mimicking constitutive acetylation or deacetylation, respectively. The K464Q mutation ameliorated ATPase activity and calcium transient recovery time, thus indicating that constitutive K464 acetylation has a positive impact on human SERCA2a (hSERCA2a) function. In conclusion, SAHA induced deacetylation inhibition had a positive impact on CM calcium handling, that, at least in part, was due to improved SERCA2 activity. This observation can provide the basis for the development of novel pharmacological approaches to ameliorate SERCA2 efficiency.

Role of calpain in eccentric contraction-induced proteolysis of Ca2+-regulatory proteins and force depression in rat fast-twitch skeletal muscle

The aim of this study was to examine the in vivo effects of eccentric contraction (ECC) on calpain-dependent proteolysis of Ca2+-regulatory proteins and force production in fast-twitch skeletal muscles. Rat extensor digitorum longus muscles were exposed to 200 repeated ECC in situ and excised immediately [recovery 0 (REC0)] or 3 days [recovery 3 (REC3)] after cessation of ECC. Calpain inhibitor (CI)-treated rats were intraperitoneally injected with MDL-28170 before ECC and during REC3. Tetanic force was markedly reduced at REC0 and remained reduced at REC3. CI treatment ameliorated the ECC-induced force decline but only at REC3. No evidence was found for proteolysis of dihydropyridine receptor (DHPR), junctophilin (JP)1, JP2, ryanodine receptor (RyR), sarcoplasmic reticulum Ca2+-ATPase (SERCA)1a, or junctional face protein-45 at REC0. At REC3, ECC resulted in decreases in DHPR, JP1, JP2, RyR, and SERCA1a. CI treatment prevented the decreases in DHPR, JP1, and JP2, whereas it had little effect on RyR and SERCA1a. These findings suggest that DHPR, JP1, and JP2, but not RyR and SERCA1a, undergo calpain-dependent proteolysis in in vivo muscles subjected to ECC and that impaired function of DHPR and/or JP might cause prolonged force deficits with ECC.

NEW & NOTEWORTHY Calpain-dependent proteolysis is one of the contributing factors to muscle damage that occurs with eccentric contraction (ECC). It is unclear, however, whether calpains account for proteolysis of Ca2+-regulatory proteins in in vivo muscles subjected to ECC. Here, we provide evidence that dihydropyridine receptor and junctophilin, but not ryanodine receptor and sarcoplasmic reticulum Ca2+-ATPase, undergo calpain-dependent proteolysis.

Effects of endurance training on reduction of plasma glucose during high intensity constant and incremental speed tests in Wistar rats

The aim of this research was to investigate the effects of endurance training on reduction of plasma glucose during high intensity constant and incremental speed tests in Wistar rats. We hypothesized that plasma glucose might be decreased in the exercised group during heavy (more intense) exercise. Twenty-four 10-week-old male Wistar rats were randomly assigned to sedentary and exercised groups. The prescription of endurance exercise training intensity was determined as 60% of the maximum intensity reached at the incremental speed test. The animals were trained by running on a motorized treadmill, five days/week for a total period of 67 weeks. Plasma glucose during the constant speed test in the exercised group at 20 m/min was reduced at the 14th, 21st and 28th min compared to the sedentary group, as well at 25 m/min at the 21st and 28th min. Plasma glucose during the incremental speed test was decreased in the exercised group at the moment of exhaustion (48th min) compared to the sedentary group (27th min). Endurance training positively modulates the mitochondrial activity and capacity of substrate oxidation in muscle and liver. Thus, in contrast to other studies on high load of exercise, the effects of endurance training on the decrease of plasma glucose during constant and incremental speed tests was significantly higher in exercised than in sedentary rats and associated with improved muscle and hepatic oxidative capacity, constituting an important non-pharmacological intervention tool for the prevention of insulin resistance, including type 2 diabetes mellitus.

Cellular properties of extensor carpi radialis brevis and trapezius muscles in healthy males and females

In this study, we sought to determine whether differences in cellular properties associated with energy homeostasis could explain the higher incidence of work-related myalgia in trapezius (TRAP) compared with extensor carpi radialis brevis (ECRB). Tissue samples were obtained from the ECRB (n = 19) and TRAP (n = 17) of healthy males and females (age 27.9 ± 2.2 and 28.1 ± 1.5 years, respectively; mean ± SE) and analyzed for properties involved in both ATP supply and utilization. The concentration of ATP and the maximal activities of creatine phosphokinase, phosphorylase, and phosphofructokinase were higher (P < 0.05) in ECRB than TRAP. Succinic dehydrogenase, citrate synthase, and cytochrome c oxidase were not different between muscles. The ECRB also displayed a higher concentration of Na(+)-K(+)-ATPase and greater sarcoplasmic reticulum Ca(2+) release and uptake. No differences existed between muscles for either monocarboxylate transporters or glucose transporters. It is concluded that the potentials for high-energy phosphate transfer, glycogenolysis, glycolysis, and excitation-contraction coupling are higher in ECRB than TRAP. Histochemical measurements indicated that the muscle differences are, in part, related to differing amounts of type II tissue. Depending on the task demands, the TRAP may experience a greater metabolic and excitation-contraction coupling strain than the ECRB given the differences observed.

Muscle dysfunction associated with adjuvant-induced arthritis is prevented by antioxidant treatment

Background

In addition to the primary symptoms arising from inflamed joints, muscle weakness is prominent and frequent in patients with rheumatoid arthritis (RA). Here, we investigated the mechanisms of arthritis-induced muscle dysfunction in rats with adjuvant-induced arthritis (AIA).

Methods

AIA was induced in the knees of rats by injection of complete Freund’s adjuvant and was allowed to develop for 21 days. Muscle contractile function was assessed in isolated extensor digitorum longus (EDL) muscles. To assess mechanisms underlying contractile dysfunction, we measured redox modifications, redox enzymes and inflammatory mediators, and activity of actomyosin ATPase and sarcoplasmic reticulum (SR) Ca²⁺-ATPase.

Results

EDL muscles from AIA rats showed decreased tetanic force per cross-sectional area and slowed twitch contraction and relaxation. These contractile dysfunctions in AIA muscles were accompanied by marked decreases in actomyosin ATPase and SR Ca²⁺-ATPase activities. Actin aggregates were observed in AIA muscles, and these contained high levels of 3-nitrotyrosine and malondialdehyde-protein adducts. AIA muscles showed increased protein expression of NADPH oxidase 2/gp91^phox, neuronal nitric oxide synthase, tumor necrosis factor α (TNF-α), and high-mobility group box 1 (HMGB1). Treatment of AIA rats with EUK-134 (3 mg/kg/day), a superoxide dismutase/catalase mimetic, prevented both the decrease in tetanic force and the formation of actin aggregates in EDL muscles without having any beneficial effect on the arthritis development.

Conclusions

Antioxidant treatment prevented the development of oxidant-induced actin aggregates and contractile dysfunction in the skeletal muscle of AIA rats. This implies that antioxidant treatment can be used to effectively counteract muscle weakness in inflammatory conditions.

SERCA1 overexpression minimizes skeletal muscle damage in dystrophic mouse models

Duchenne muscular dystrophy (DMD) is characterized by progressive muscle wasting secondary to repeated muscle damage and inadequate repair. Elevations in intracellular free Ca²⁺ have been implicated in disease progression, and sarcoplasmic/endoplasmic reticulum Ca²⁺-ATPase 1 (SERCA1) overexpression has been shown to ameliorate the dystrophic phenotype in mdx mice. The purpose of this study was to assess the effects of SERCA1 overexpression in the more severe mdx/Utr(-/-) mouse model of DMD. Mice overexpressing SERCA1 were crossed with mdx/Utr ± mice to generate mdx/Utr(-/-)/+SERCA1 mice and compared with wild-type (WT), WT/+SERCA1, mdx/+SERCA1, and genotype controls. Mice were assessed at ∼12 wk of age for changes in Ca²⁺ handling, muscle mass, quadriceps torque, markers of muscle damage, and response to repeated eccentric contractions. SERCA1-overexpressing mice had a two- to threefold increase in maximal sarcoplasmic reticulum Ca²⁺-ATPase activity compared with WT which was associated with normalization in body mass for both mdx/+SERCA1 and mdx/Utr(-/-)/+SERCA1. Torque deficit in the quadriceps after eccentric injury was 2.7-fold greater in mdx/Utr(-/-) vs. WT mice, but only 1.5-fold greater in mdx/Utr(-/-)/+SERCA1 vs. WT mice, an attenuation of 44%. Markers of muscle damage (% centrally nucleated fibers, necrotic area, and serum creatine kinase levels) were higher in both mdx and mdx/Utr(-/-) vs. WT, and all were attenuated by overexpression of SERCA1. These data indicate that SERCA1 overexpression ameliorates functional impairments and cellular markers of damage in a more severe mouse model of DMD. These findings support targeting intracellular Ca²⁺ control as a therapeutic approach for DMD.

Products of lipid peroxidation, but not membrane susceptibility to oxidative damage, are conserved in skeletal muscle following temperature acclimation

Changes in oxidative capacities and phospholipid remodeling accompany temperature acclimation in ectothermic animals. Both responses may alter redox status and membrane susceptibility to lipid peroxidation (LPO). We tested the hypothesis that phospholipid remodeling is sufficient to offset temperature-driven rates of LPO and, thus, membrane susceptibility to LPO is conserved. We also predicted that the content of LPO products is maintained over a range of physiological temperatures. To assess LPO susceptibility, rates of LPO were quantified with the fluorescent probe C11-BODIPY in mitochondria and sarcoplasmic reticulum from oxidative and glycolytic muscle of striped bass (Morone saxatilis) acclimated to 7°C and 25°C. We also measured phospholipid compositions, contents of LPO products [i.e., individual classes of phospholipid hydroperoxides (PLOOH)], and two membrane antioxidants. Despite phospholipid headgroup and acyl chain remodeling, these alterations do not counter the effect of temperature on LPO rates (i.e., LPO rates are generally not different among acclimation groups when normalized to phospholipid content and compared at a common temperature). Although absolute levels of PLOOH are higher in muscles from cold- than warm-acclimated fish, this difference is lost when PLOOH levels are normalized to total phospholipid. Contents of vitamin E and two homologs of ubiquinone are more than four times higher in mitochondria prepared from oxidative muscle of warm- than cold-acclimated fish. Collectively, our data demonstrate that although phospholipid remodeling does not provide a means for offsetting thermal effects on rates of LPO, differences in phospholipid quantity ensure a constant proportion of LPO products with temperature variation.

Excitation–contraction coupling properties in women with work-related myalgia: a preliminary study

We investigated the potential role of selected excitation–contraction coupling processes in females with work-related myalgia (WRM) by comparing WRM with healthy controls (CON) using tissue from extensor carpi radialis brevis (ECRB) and trapezius (TRAP) muscles. For the ECRB, age (mean ± SE) was 29.6 ± 3.5 years for CON (n = 9) and 39.2 ± 2.8 years for WRM (n = 13), while for the TRAP, the values were 26.0 ± 2.1 years for CON (n = 7) and 44.6 ± 2.9 years for WRM (n = 11). For the sarcoplasmic reticulum (SR) of the ECRB, WRM displayed concentrations (nmol·(mg protein)−1·min−1) that were lower (P < 0.05) for Total (202 ± 4.4 vs 178 ± 7.1), Basal (34 ± 1.6 vs 30.1 ± 1.3), and maximal Ca2+-ATPase activity (Vmax, 168 ± 4.9 vs 149 ± 6.3), and Ca2+-uptake (5.06 ± 0.31 vs 4.13 ± 0.29), but not SERCA1a and SERCA2a isoforms, by comparison with CON. When age was incorporated as a co-variant, Total, Basal, and Ca2+-uptake remained different from CON (P < 0.05), but not Vmax (P = 0.13). For TRAP, none of the ATPase properties differed between groups (P > 0.05) either before or following adjustment for age. No differences (P > 0.05) were observed between the groups for Ca2+-release in the SR for either TRAP or ECRB. Similarly, no deficiencies, regardless of muscle, were noted for either the Na+–K+-ATPase content or the α and β subunit isoform distribution in WRM. This preliminary study provides a basis for further research, with expanded numbers, investigating the hypothesis that abnormalities in SR Ca2+-regulation are involved in the cellular etiology of WRM.

Differential regulation of the fiber type-specific gene expression of the sarcoplasmic reticulum calcium-ATPase isoforms induced by exercise training

The regulatory role of adenosine monophosphate-activated protein kinase (AMPK)-α2 on sarcoplasmic reticulum calcium-ATPase (SERCA) 1a and SERCA2a in different skeletal muscle fiber types has yet to be elucidated. Sedentary (Sed) or exercise-trained (Ex) wild-type (WT) and AMPKα2-kinase dead (KD) transgenic mice, which overexpress a mutated and inactivated AMPKα2 subunit, were utilized to characterize how genotype or exercise training influenced the regulation of SERCA isoforms in gastrocnemius. As expected, both Sed and Ex KD mice had >40% lower AMPK phosphorylation and 30% lower SERCA1a protein than WT mice (P < 0.05). In contrast, SERCA2a protein was not different among KD and WT mice. Exercise increased SERCA1a and SERCA2a protein content among WT and KD mice, compared with their Sed counterparts. Maximal SERCA activity was lower in KD mice, compared with WT. Total phospholamban protein was higher in KD mice than in WT and lower in Ex compared with Sed mice. Exercise training increased phospholamban Ser(16) phosphorylation in WT mice. Laser capture microdissection and quantitative PCR indicated that SERCA1a mRNA expression among type I fibers was not altered by genotype or exercise, but SERCA2a mRNA was increased 30-fold in WT+Ex, compared with WT+Sed. In contrast, the exercise-stimulated increase for SERCA2a mRNA was blunted in KD mice. Exercise upregulated SERCA1a and SERCA2a mRNA among type II fibers, but was not altered by genotype. Collectively, these data suggest that exercise differentially influences SERCA isoform expression in type I and type II fibers. Additionally, AMPKα2 influences the regulation of SERCA2a mRNA in type I skeletal muscle fibers following exercise training.

American Thyroid Association Guide to investigating thyroid hormone economy and action in rodent and cell models

BACKGROUND

An in-depth understanding of the fundamental principles that regulate thyroid hormone homeostasis is critical for the development of new diagnostic and treatment approaches for patients with thyroid disease.

SUMMARY

Important clinical practices in use today for the treatment of patients with hypothyroidism, hyperthyroidism, or thyroid cancer are the result of laboratory discoveries made by scientists investigating the most basic aspects of thyroid structure and molecular biology. In this document, a panel of experts commissioned by the American Thyroid Association makes a series of recommendations related to the study of thyroid hormone economy and action. These recommendations are intended to promote standardization of study design, which should in turn increase the comparability and reproducibility of experimental findings.

CONCLUSIONS

It is expected that adherence to these recommendations by investigators in the field will facilitate progress towards a better understanding of the thyroid gland and thyroid hormone dependent processes.

Multiple causes of fatigue during shortening contractions in rat slow twitch skeletal muscle

Fatigue in muscles that shorten might have other causes than fatigue during isometric contractions, since both cross-bridge cycling and energy demand are different in the two exercise modes. While isometric contractions are extensively studied, the causes of fatigue in shortening contractions are poorly mapped. Here, we investigate fatigue mechanisms during shortening contractions in slow twitch skeletal muscle in near physiological conditions. Fatigue was induced in rat soleus muscles with maintained blood supply by in situ shortening contractions at 37°C. Muscles were stimulated repeatedly (1 s on/off at 30 Hz) for 15 min against a constant load, allowing the muscle to shorten and perform work. Fatigue and subsequent recovery was examined at 20 s, 100 s and 15 min exercise. The effects of prior exercise were investigated in a second exercise bout. Fatigue developed in three distinct phases. During the first 20 s the regulatory protein Myosin Light Chain-2 (slow isoform, MLC-2s) was rapidly dephosphorylated in parallel with reduced rate of force development and reduced shortening. In the second phase there was degradation of high-energy phosphates and accumulation of lactate, and these changes were related to slowing of muscle relengthening and relaxation, culminating at 100 s exercise. Slowing of relaxation was also associated with increased leak of calcium from the SR. During the third phase of exercise there was restoration of high-energy phosphates and elimination of lactate, and the slowing of relaxation disappeared, whereas dephosphorylation of MLC-2s and reduced shortening prevailed. Prior exercise improved relaxation parameters in a subsequent exercise bout, and we propose that this effect is a result of less accumulation of lactate due to more rapid onset of oxidative metabolism. The correlation between dephosphorylation of MLC-2s and reduced shortening was confirmed in various experimental settings, and we suggest MLC-2s as an important regulator of muscle shortening.

Membrane phospholipid fatty acid composition regulates cardiac SERCA activity in a hibernator, the Syrian hamster (Mesocricetus auratus)

Polyunsaturated fatty acids (PUFA) have strong effects on hibernation and daily torpor. Increased dietary uptake of PUFA of the n-6 class, particularly of Linoleic acid (LA, C18:2 n-6) lengthens torpor bout duration and enables animals to reach lower body temperatures (T_b) and metabolic rates. As previously hypothesized, this well-known influence of PUFA may be mediated via effects of the membrane fatty acid composition on sarcoplasmic reticulum (SR) Ca²⁺⁻ATPase 2a (SERCA) in the heart of hibernators. We tested the hypotheses that high proportions of n-6 PUFA in general, or specifically high proportions of LA (C18:2 n-6) in SR phospholipids (PL) should be associated with increased cardiac SERCA activity, and should allow animals to reach lower minimum T_b in torpor. We measured activity of SERCA from hearts of hibernating and non-hibernating Syrian hamsters (Mesocricetus auratus) in vitro at 35°C. Further, we determined the PL fatty acid composition of the SR membrane of these hearts. We found that SERCA activity strongly increased as the proportion of LA in SR PL increased but was negatively affected by the content of Docosahexaenoic acid (DHA; C22:6 n-3). SR PL from hibernating hamsters were characterized by high proportions of LA and low proportions of DHA. As a result, SERCA activity was significantly higher during entrance into torpor and in torpor compared to inter-bout arousal. Also, animals with increased SERCA activity reached lower T_b during torpor. Interestingly, a subgroup of hamsters which never entered torpor but remained euthermic throughout winter displayed a phenotype similar to animals in summer. This was characterized by lower proportions of LA and increased proportions of DHA in SR membranes, which is apparently incompatible with torpor. We conclude that the PUFA composition of SR membranes affects cardiac function via modulating SERCA activity, and hence determines the minimum T_b tolerated by hibernators.

Exercise training prevents the development of cardiac dysfunction in the low-dose streptozotocin diabetic rats fed a high-fat diet

This study tested the hypothesis that exercise training would prevent the development of diabetes-induced cardiac dysfunction and altered expression of sarcoplasmic reticulum Ca(2 +)-transport proteins in the low-dose streptozotocin-induced diabetic rats fed a high-fat diet (HFD+STZ). Male Sprague-Dawley rats (4 weeks old; 125-150 g) were made diabetic using a high-fat diet (40% fat, w/w) and a low-dose of streptozotocin (35 mg·(kg body mass)(-1)) by intravenous injection. Diabetic animals were divided among a sedentary group (Sed+HFD+STZ) or an exercise-trained group (Ex+HFD+STZ) that accumulated 3554 ± 338 m·day(-1) of voluntary wheel running (mean ± SE). Sedentary animals fed a low-fat diet served as the control (Sed+LFD). Oral glucose tolerance was impaired in the sedentary diabetic group (1179 ± 29; area under the curve (a.u.c.)) compared with that in the sedentary control animals (1447 ± 42 a.u.c.). Although left ventricular systolic function was unchanged by diabetes, impaired E/A ratios (i.e., diastolic function) and rates of pressure decay (-dP/dt) indicated the presence of diastolic dysfunction. Diabetes also reduced SERCA2a protein content and maximal SERCA2a activity (V(max)) by 21% and 32%, respectively. In contrast, the change in each parameter was attenuated by exercise training. Based on these data, it appears that exercise training prevented the development of diabetic cardiomyopathy and the dysregulation of sarcoplasmic reticulum protein content in an inducible animal model of type 2 diabetes.

Muscle cellular properties in the ice hockey player: a model for investigating overtraining?

In this study, we hypothesized that athletes involved in 5-6 months of sprint-type training would display higher levels of proteins and processes involved in muscle energy supply and utilization. Tissue was sampled from the vastus lateralis of 13 elite ice hockey players (peak oxygen consumption = 51.8 ± 1.3 mL·kg(-1)·min(-1); mean ± standard error) at the end of a season (POST) and compared with samples from 8 controls (peak oxygen consumption = 45.5 ± 1.4 mL·kg(-1)·min(-1)) (CON). Compared with CON, higher activities were observed in POST (p < 0.05) only for succinic dehydrogenase (3.32 ± 0.16 mol·(mg protein)(-1)·min(-1) vs. 4.10 ± 0.11 mol·(mg protein)(-1)·min(-1)) and hexokinase (0.73 ± 0.05 mol·(mg protein)(-1)·min(-1) vs. 0.90 ± 0.05mol·(mg protein)(-1)·min(-1)) but not for phosphorylase, phosphofructokinase, and creatine phosphokinase. No differences were found in Na(+),K(+)-ATPase concentration (β(max): 262 ± 36 pmol·(g wet weight)(-1) vs. 275 ± 27 pmol·(g wet weight)(-1)) and the maximal activity of the sarcoplasmic reticulum Ca(2+)-ATPase (98.1 ± 6.1 µmol·(g protein)(-1)·min(-1) vs. 102 ± 3.3 µmol·(g protein)(-1)·min(-1)). Cross-sectional area was lower (p < 0.05) in POST but only for the type IIA fibres (6312 ± 684 μm(2) vs. 5512 ± 335 μm(2)), while the number of capillary counts per fibre and the capillary to fibre area ratio were generally higher (p < 0.05). These findings suggest that elite trained ice hockey players display elevations only in support of glucose-based aerobic metabolism that occur in the absence of alterations in excitation-contraction processes.

An evaluation of muscle maintenance costs during fiber hypertrophy in the lobster Homarus americanus: are larger muscle fibers cheaper to maintain?

Large muscle fiber size imposes constraints on muscle function while imparting no obvious advantages, making it difficult to explain why muscle fibers are among the largest cell type. Johnston and colleagues proposed the 'optimal fiber size' hypothesis, which states that some fish have large fibers that balance the need for short diffusion distances against metabolic cost savings associated with large fibers. We tested this hypothesis in hypertrophically growing fibers in the lobster Homarus americanus. Mean fiber diameter was 316±11 μm in juveniles and 670±26 μm in adults, leading to a surface area to volume ratio (SA:V) that was 2-fold higher in juveniles. Na(+)/K(+)-ATPase activity was also 2-fold higher in smaller fibers. (31)P-NMR was used with metabolic inhibitors to determine the cost of metabolic processes in muscle preparations. The cost of Na(+)/K(+)-ATPase function was also 2-fold higher in smaller than in larger diameter fibers. Extrapolation of the SA:V dependence of the Na(+)/K(+)-ATPase over a broad fiber size range showed that if fibers were much smaller than those observed, maintenance of the membrane potential would constitute a large fraction of whole-animal metabolic rate, suggesting that the fibers grow large to reduce maintenance costs. However, a reaction-diffusion model of aerobic metabolism indicated that fibers in adults could attain still larger sizes without diffusion limitation, although further growth would have a negligible effect on cost. Therefore, it appears that decreased fiber SA:V makes larger fibers in H. americanus less expensive to maintain, which is consistent with the optimal fiber size hypothesis.

Muscle fatigue and excitation-contraction coupling responses following a session of prolonged cycling

AIM

The mechanisms underlying the fatigue that occurs in human muscle following sustained activity are thought to reside in one or more of the excitation-contraction coupling (E-C coupling) processes. This study investigated the association between the changes in select E-C coupling properties and the impairment in force generation that occurs with prolonged cycling.

METHODS

Ten volunteers with a peak aerobic power (VO(2peak)) of 2.95 ± 0.27 L min(-1) (mean ± SE), exercised for 2 h at 62 ± 1.3%. Quadriceps function was assessed and tissue properties (vastus lateralis) were measured prior to (E1-pre) and following (E1-post) exercise and on three consecutive days of recovery (R1, R2 and R3).

RESULTS

While exercise failed to depress the maximal activity (V(max) ) of the Na(+) ,K(+) -ATPase (P = 0.10), reductions (P < 0.05) were found at E1-post in V(max) of sarcoplasmic reticulum Ca(2+) -ATPase (-22%), Ca(2+) -uptake (-26%) and phase 1(-33%) and 2 (-38%) Ca(2+) -release. Both V(max) and Ca(2+) -release (phase 2) recovered by R1, whereas Ca(2+) -uptake and Ca(2+) -release (phase 1) remained depressed (P < 0.05) at R1 and at R1 and R2 and possibly R3 (P < 0.06) respectively. Compared with E1-pre, fatigue was observed (P < 0.05) at 10 Hz electrical stimulation at E1-post (-56%), which persisted throughout recovery. The exercise increased (P < 0.05) overall content of the Na(+), K(+)-ATPase (R1, R2 and R3) and the isoforms β2 (R1, R2 and R3) and β3 (R3), but not β1 or the α-isoforms (α1, α2 and α3).

CONCLUSION

These results suggest a possible direct role for Ca(2+)-release in fatigue and demonstrate a single exercise session can induce overlapping perturbations and adaptations (particularly to the Na(+), K(+)-ATPase).

Malleability of human skeletal muscle sarcoplasmic reticulum to short-term training

This study investigated the hypothesis that adaptations would occur in the sarcoplasmic reticulum in vastus lateralis soon after the onset of aerobic-based training consistent with reduced Ca2+-cycling potential. Tissue samples were extracted prior to (0 days) and following 3 and 6 days of cycling performed for 2 h at 60%–65% of peak aerobic power (VO2peak) in untrained males (VO2peak = 47 ± 2.3 mL·kg–1·min–1; mean ± SE, n = 6) and assessed for changes (nmol·mg protein–1·min–1) in maximal Ca2+-ATPase activity (Vmax), Ca2+-uptake, and Ca2+-release (phase 1 and phase 2) as well as the sarcoplasmic (endoplasmic) reticulum Ca2+-ATPase (SERCA) isoforms. Training resulted in reductions (p < 0.05) in SERCA1a at 6 days (–14%) but not at 3 days. For SERCA2a, reductions (p < 0.05) were also noted only at 6 days (–7%). For Vmax, depressions (p < 0.05) were found at 6 days (172 ± 11) but not at 3 days (176 ± 13; p < 0.10) compared with 0 days (192 ± 11). These changes were accompanied by a lower (p < 0.05) Ca2+-uptake at both 3 days (–39%) and 6 days (–48%). A similar pattern was found for phase 1 Ca2+-release with reductions (p < 0.05) of 37% observed at 6 days and 23% (p = 0.21) at 3 days of training, respectively. In a related study using the same training protocol and participant characteristics, microphotometric determinations of Vmax indicated reductions (p < 0.05) in type I at 3 days (–27%) and at 6 days (–34%) and in type IIA fibres at 6 days (–17%). It is concluded that in response to aerobic-based training, sarcoplasmic reticulum Ca2+-cycling potential is reduced by adaptations that occur soon after training onset.

Slowed relaxation and preserved maximal force in soleus muscles of mice with targeted disruption of the Serca2 gene in skeletal muscle

Sarcoplasmic reticulum Ca(2+) ATPases (SERCAs) play a major role in muscle contractility by pumping Ca(2+) from the cytosol into the sarcoplasmic reticulum (SR) Ca(2+) store, allowing muscle relaxation and refilling of the SR with releasable Ca(2+). Decreased SERCA function has been shown to result in impaired muscle function and disease in human and animal models. In this study, we present a new mouse model with targeted disruption of the Serca2 gene in skeletal muscle (skKO) to investigate the functional consequences of reduced SERCA2 expression in skeletal muscle. SkKO mice were viable and basic muscle structure was intact. SERCA2 abundance was reduced in multiple muscles, and by as much as 95% in soleus muscle, having the highest content of slow-twitch fibres (40%). The Ca(2+) uptake rate was significantly reduced in SR vesicles in total homogenates. We did not find any compensatory increase in SERCA1 or SERCA3 abundance, or altered expression of several other Ca(2+)-handling proteins. Ultrastructural analysis revealed generally well-preserved muscle morphology, but a reduced volume of the longitudinal SR. In contracting soleus muscle in vitro preparations, skKO muscles were able to fully relax, but with a significantly slowed relaxation time compared to controls. Surprisingly, the maximal force and contraction rate were preserved, suggesting that skKO slow-twitch fibres may be able to contribute to the total muscle force despite loss of SERCA2 protein. Thus it is possible that SERCA-independent mechanisms can contribute to muscle contractile function.

Remodeling of calcium handling in skeletal muscle through PGC-1α: impact on force, fatigability, and fiber type

Regular endurance exercise remodels skeletal muscle, largely through the peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α). PGC-1α promotes fiber type switching and resistance to fatigue. Intracellular calcium levels might play a role in both adaptive phenomena, yet a role for PGC-1α in the adaptation of calcium handling in skeletal muscle remains unknown. Using mice with transgenic overexpression of PGC-1α, we now investigated the effect of PGC-1α on calcium handling in skeletal muscle. We demonstrate that PGC-1α induces a quantitative reduction in calcium release from the sarcoplasmic reticulum by diminishing the expression of calcium-releasing molecules. Concomitantly, maximal muscle force is reduced in vivo and ex vivo. In addition, PGC-1α overexpression delays calcium clearance from the myoplasm by interfering with multiple mechanisms involved in calcium removal, leading to higher myoplasmic calcium levels following contraction. During prolonged muscle activity, the delayed calcium clearance might facilitate force production in mice overexpressing PGC-1α. Our results reveal a novel role of PGC-1α in altering the contractile properties of skeletal muscle by modulating calcium handling. Importantly, our findings indicate PGC-1α to be both down- as well as upstream of calcium signaling in this tissue. Overall, our findings suggest that in the adaptation to chronic exercise, PGC-1α reduces maximal force, increases resistance to fatigue, and drives fiber type switching partly through remodeling of calcium transients, in addition to promoting slow-type myofibrillar protein expression and adequate energy supply.

Catechins attenuate eccentric exercise-induced inflammation and loss of force production in muscle in senescence-accelerated mice

Catechins have a great variety of biological actions. We evaluated the potential benefits of catechin ingestion on muscle contractile properties, oxidative stress, and inflammation following downhill running, which is a typical eccentric exercise, in senescence-accelerated prone mice (SAMP). Downhill running (13 m/min for 60 min; 16° decline) induced a greater decrease in the contractile force of soleus muscle and in Ca(2+)-ATPase activity in SAMP1 compared with the senescence-resistant mice (SAMR1). Moreover, compared with SAMR1, SAMP1 showed greater downhill running-induced increases in plasma CPK and LDH activity, malondialdehyde, and carbonylated protein as markers of oxidative stress; and in protein and mRNA expression levels of the inflammatory mediators such as tumor necrosis factor-α and monocyte chemoattractant protein-1 in muscle. SAMP1 exhibited aging-associated vulnerability to oxidative stress and inflammation in muscle induced by downhill running. Long-term (8 wk) catechin ingestion significantly attenuated the downhill running-induced decrease in muscle force and the increased inflammatory mediators in both plasma and gastrocnemius muscle. Furthermore, catechins significantly inhibited the increase in oxidative stress markers immediately after downhill running, accompanied by an increase in glutathione reductase activity. These findings suggest that long-term catechin ingestion attenuates the aging-associated loss of force production, oxidative stress, and inflammation in muscle after exercise.

Cellular responses in skeletal muscle to a season of ice hockey

We hypothesized that a season of ice hockey would result in extensive remodeling of muscle. Tissue sampled from the vastus lateralis of 15 players (age = 20.6 ± 0.4 years; mean ± SE) prior to (PRE) and following (POST) a season was used to characterize specific adaptations. Measurement of representative metabolic pathway enzymes indicated higher maximal activities in POST than in PRE (p < 0.05) for succinic dehydrogenase (3.26 ± 0.31 vs. 3.91 ± 0.11 mol mg protein(-1) min(-1)), citrate synthase (7.26 ± 0.70 vs. 8.70 ± 0.55 mol mg protein(-1) min(-1)), and phosphofructokinase (12.8 ± 1.3 vs. 14.4 ± 0.96 mol mg protein(-1) min(-1)) only. The season resulted in an increase in Na+-K+-ATPase concentration (253 ± 6.3 vs. 265 ± 6.0 pmol g(-1) wet weight), a decrease (p < 0.05) in maximal activity of the sarcoplasmic reticulum Ca2+-ATPase (107 ± 4.2 micromol g protein(-1) min(-1) vs. 92.0 ± 4.6 micromol g protein(-1) min(-1)), and no change in the distribution (%) of fibre types. A smaller (p < 0.05) cross-sectional area (CSA) for both type I (-11.7%) and type IIA (-18.2%) fibres and a higher (p < 0.05) capillary count/CSA for type I (+17.9%) and type IIA (+17.2%) were also found over the season. No changes were found in peak oxygen consumption (51.4 ± 1.2 mL kg(-1) min(-1) vs. 52.3 ± 1.3 mL kg(-1) min(-1)). The results suggest, based on the alterations in oxidative and perfusion potentials and muscle mass, that the dominant adaptations are in support of oxidative metabolism, which occurs at the expense of fibre CSA and possibly force-generating potential.

Age-related changes in contraction and relaxation of rat diaphragm

Age-related changes of physiological and biochemical properties were examined in the diaphragm muscle, which has particularly high activation compared to that of other skeletal muscles. The diaphragm from 10-week-, 50-week- and 100-week-old male Wistar rats were used to measure in vitro isometric contractile properties, sarcoplasmic reticulum (SR) Ca2+-ATPase activity, and myosin heavy chain (MHC) isoform composition. Although there were no significant differences in specific twitch tension of the diaphragm among the groups, there was significant reduction in specific tetanic tension in the 50-week to 100-week groups. The contraction time and 1/2 relaxation time of twitch contraction extended with aging, and significant differences were found between 10-week-old and 100-week-old diaphragms. Regarding the activity of SR Ca2+-ATPase, the pattern of age-related change was similar to that in the 1/2 relaxation time and there was a significant difference between 10-week-old and 100-week-old diaphragms. There was a significant increase in the relative composition of the MHC I isoform in 100-week-diaphragms compared to that in 10-week-old diaphragms and a concomitant decrease in the relative composition of fast myosin was noted. These findings demonstrated that older diaphragms have slower contraction and relaxation speeds, and these alterations were attributed to changes in SR Ca2+-ATPase activity and MHC isoform composition.

Cellular assessment of muscle in COPD: case studies of two males

The objective of this paper is to provide an overview of the recent developments in muscle physiology and biochemistry in general, and with respect to chronic obstructive pulmonary disease (COPD) specifically. As a way of illustration, we have presented data on the remodeling that occurs in vastus lateralis in two patients with COPD (COPD #1, forced expiratory volume in one second/forced vital capacity [FEV₁/FVC] = 63%; COPD #2, FEV₁/FVC = 41%) exhibiting differences in muscle wasting as compared to healthy controls (CON; FEV₁/FVC = 111 ± 2.2%, n = 4). Type I fibers percentages were lower in both COPD #1 (16.7) and COPD #2 (24.9) compared to CON (57.3 ± 5.2). Cross sectional area of the type I fibers of the patients ranged between 65%–68% of CON and for the type II subtypes (IIA, IIAX, IIX) between 74% and 89% (COPD #1) and 17%–32% (COPD #2). A lower number of capillary contacts were observed for all fiber types in COPD #1 but not COPD #2. Lower concentrations of adenosine triphosphate (ATP) (24%–26%) and phosphocreatine (18%–20%), but not lactate occurred in COPD. In contrast to COPD #1, who displayed normal glucose transporter content, GLUT1 and GLUT4 were only 71% and 54%, respectively of CON in COPD #2. Lower monocarboxylate contents were found for MCT1 in both COPD #1 (63%) and COPD #2 (41%) and for MCT4 (78%) in COPD #1. Maximal oxidative enzyme activities (V_max) for COPD #2 ranged between 37% (succinic dehydrogenase) and 70% (cytochrome C oxidase) of CON. For the cytosolic enzymes, V_max ranged between 89% (hexokinase) to 31% (pyruvate kinase) of CON. Depressions were also observed in V_max of the Na⁺-K⁺-ATPase for COPD #1 (66% of CON) but not COPD #2 (92% of CON) while V_max of the Ca²⁺-ATPase was near normal in COPD #1 (84% CON). It is concluded that disturbances can occur in muscle to a wide range of excitation, contraction and metabolic processes in COPD.

No relationship between enzyme activity and structure of nucleotide binding site in sarcoplasmic reticulum Ca(2+)-ATPase from short-term stimulated rat muscle

AIM

We examined whether structural alterations to the adenine nucleotide binding site (ANBS) within sarcoplasmic (endo) reticulum Ca(2+)-ATPase (SERCA) would account for contraction-induced changes in the catalytic activity of the enzyme as assessed in vitro.

METHODS

Repetitive contractions were induced in rat gastrocnemius by electrical nerve stimulation. Measurements of sarcoplasmic reticulum properties were performed on control and stimulated muscles immediately after or at 30 min after the cessation of 5-min stimulation. In order to examine the properties at the ANBS, the binding capacity of SERCA to fluorescence isothiocyanate (FITC), a competitive inhibitor at the ANBS, was analysed in microsomes.

RESULTS

Short-term electrical stimulation evoked a 23.9% and 32.6% decrease (P < 0.05) in SERCA activity and in the FITC binding capacity, respectively, in the superficial region of the muscle. Whereas SERCA activity reverted to normal levels during 30-min recovery, a restoration of the FITC binding capacity did not occur.

CONCLUSION

The discordant changes between the enzyme activity and the FITC binding suggest that, at least during recovery after exercise, changes in SERCA activity may not correlate closely with structural alterations to the ANBS within the enzyme.

Sarcoplasmic Reticulum Ca2+-ATPase Activity and Glycogen Content in Various Fiber Types after Intensive Exercise in Thoroughbred Horses

To find a new parameter indicating muscle fitness in Thoroughbred horses, we examined time-dependent recovery of glycogen content and sarcoplasmic reticulum (SR) Ca²⁺-ATPase activity of skeletal muscle after intensive treadmill running. Two repeated 50-sec running sessions (13 m/sec) were performed on a flat treadmill (approximately 90%VO₂max). Muscle samples of the middle gluteal muscle were taken before exercise (pre) and 1 min, 20 min, 60 min, and 24 hr after exercise. Muscle fiber type composition was determined in the pre muscle samples by immunohistochemical staining with monoclonal antibody to myosin heavy chain. SR Ca²⁺-ATPase activity of the muscle and glycogen content of each muscle fiber type were determined with biochemical analysis and quantitative histochemical staining, respectively. As compared to the pre value, the glycogen content of each muscle fiber type was reduced by 15–27% at 1 min, 20 min, and 60 min after the exercise and recovered to the pre value at 24 hr after exercise test. These results indicate that 24 hr is enough time to recover glycogen content after short-term intensive exercise. The mean value of the SR Ca²⁺-ATPase activity showed a slight decrease (not significant) immediately after exercise, and complete recovery at 60 min after exercise. There were no significant relationship between the changes in glycogen content of each muscle fiber type and SR Ca²⁺-ATPase. Although further studies are needed, SR Ca²⁺-ATPase is not a useful parameter to detect muscle fitness, at least in Thoroughbred horses.

Abnormal sarcoplasmic reticulum Ca2+-sequestering properties in skeletal muscle in chronic obstructive pulmonary disease

The objective of this study was to investigate the hypothesis that alterations in sarcoplasmic reticulum (SR) Ca2+-cycling properties would occur in skeletal muscle in patients with moderate to severe chronic obstructive pulmonary disease (COPD). To investigate this hypothesis, tissue samples were obtained from the vastus lateralis of 8 patients with COPD [age 65.6 ± 3.2 yr; forced expiratory volume in 1 s (FEV1)/forced vital capacity (FVC) = 44 ± 2%; mean ± SE] and 10 healthy age-matched controls (CON, age 67.5 ± 2.5 yr; FEV1/FVC = 77 ± 2%), and homogenates were analyzed for a wide range of SR properties. Compared with CON, COPD displayed (in μmol·g protein−1·min−1) a 16% lower maximal Ca2+-ATPase activity [maximal velocity ( Vmax), 158 ± 10 vs. 133 ± 7, P < 0.05] and a 17% lower Ca2+uptake (4.65 ± 0.039 vs. 3.85 ± 0.26, P < 0.05) that occurred in the absence of differences in Ca2+release. The lower Vmaxin COPD was also accompanied by an 11% lower ( P < 0.05) Ca2+sensitivity, as measured by the Hill coefficient (defined as the relationship between Ca2+-ATPase activity and free cytosolic Ca2+concentration for 10–90% Vmax). For the sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) isoforms, SERCA1a was 16% higher ( P < 0.05) and SERCA2a was 14% lower ( P < 0.05) in COPD. It is concluded that moderate to severe COPD results in abnormalities in SR Ca2+-ATPase properties that cannot be explained by changes in the SERCA isoform phenotypes. The reduced catalytic properties of SERCA in COPD suggest a disturbance in Ca2+cycling, possibly resulting in impairment in Ca2+-mediated mechanical function and/or second messenger regulated processes.

Alterations in in vitro function and protein oxidation of rat sarcoplasmic reticulum Ca2+-ATPase during recovery from high-intensity exercise

The hypothesis tested in this study was that the extent to which sarcoplasmic reticulum (SR) Ca(2+)-ATPase is oxidized would correlate with a decline in its activity. For this purpose, changes in the SR Ca(2+)-sequestering ability and the contents of carbonyl and sulfhydryl groups during recovery after exercise were examined in the superficial portions of vastus lateralis muscles from rats subjected to 5 min running at an intensity corresponding to maximal oxygen uptake (50 m min(-1), 10% gradient). A single bout of exercise elicited a 22.4% reduction (P < 0.05) in SR Ca(2+)-ATPase activity. The decreased activity progressively reverted to normal levels during recovery after exercise, reaching normal levels after 60 min of recovery. This change was paralleled by a depressed SR Ca(2+)-uptake rate, and the proportional alteration in these two variables resulted in no change in the ratio of Ca(2+)-uptake rate to Ca(2+)-ATPase activity. The contents of SR Ca(2+)-ATPase protein and sulfhydryl groups in microsomes were unchanged after exercise and during recovery periods. In contrast, the content of carbonyl groups in SR Ca(2+)-ATPase behaved in an opposite manner to that of SR Ca(2+)-ATPase activity. An approximately 80% augmentation (P < 0.05) in the carbonyl group content occurred immediately after exercise. The elevated carbonyl content decreased towards normal levels during 60 min of recovery. These results are strongly suggestive that oxidation of SR Ca(2+)-ATPase is responsible, at least in part, for a decay in the SR Ca(2+)-pumping function produced by high-intensity exercise and imply that oxidized proteins may be repaired during recovery from exercise.

Muscle metabolic, SR Ca2+-cycling responses to prolonged cycling, with and without glucose supplementation

This study investigated the effects of prolonged exercise, with and without glucose supplementation, on metabolism and sarcoplasmic reticulum (SR) Ca2+-handling properties in working vastus lateralis muscle. Fifteen untrained volunteers [peak O2consumption (V̇o2peak) = 3.45 ± 0.17 l/min; mean ± SE] cycled at ∼60% V̇o2peakon two occasions, during which they were provided with either an artificially sweetened placebo beverage (NG) or a 6% glucose (G) beverage (∼1.00 g carbohydrate/kg body mass). Beverage supplementation started at 30 min of exercise and continued every 15 min thereafter. SR Ca2+handling, metabolic, and substrate responses were assessed in tissue extracted from the vastus lateralis at rest, after 30 min and 90 min of exercise, and at fatigue in both conditions. Plasma glucose during G was 15–23% higher ( P < 0.05) than those observed during NG following 60 min of exercise until fatigue. Cycle time to fatigue was increased ( P < 0.05) by ∼19% during G (137 ± 7 min) compared with NG (115 ± 6 min). Prolonged exercise reduced ( P < 0.05) maximal Ca2+-ATPase activity (−18.4%), SR Ca2+uptake (−27%), and both Phase 1 (−22.2%) and Phase 2 (−34.2%) Ca2+-release rates during NG. The exercise-induced reductions in SR Ca2+-cycling properties were not altered during G. The metabolic responses to exercise were all unaltered by glucose supplementation, since no differences in respiratory exchange ratios, carbohydrate and lipid oxidation rates, and muscle metabolite and glycogen contents were observed between NG and G. These results indicate that the maintenance of blood glucose homeostasis by glucose supplementation is without effect in modifying the muscle metabolic, endogenous glycogen, or SR Ca2+-handling responses.

Preconditioning results in S-nitrosylation of proteins involved in regulation of mitochondrial energetics and calcium transport

Nitric oxide has been shown to be an important signaling messenger in ischemic preconditioning (IPC). Accordingly, we investigated whether protein S-nitrosylation occurs in IPC hearts and whether S-nitrosoglutathione (GSNO) elicits similar effects on S-nitrosylation and cardioprotection. Preceding 20 minutes of no-flow ischemia and reperfusion, hearts from C57BL/6J mice were perfused in the Langendorff mode and subjected to the following conditions: (1) control perfusion; (2) IPC; or (3) 0.1 mmol/L GSNO treatment. Compared with control, IPC and GSNO significantly improved postischemic recovery of left ventricular developed pressure and reduced infarct size. IPC and GSNO both significantly increased S-nitrosothiol contents and S-nitrosylation levels of the L-type Ca2+ channel alpha1 subunit in heart membrane fractions. We identified several candidate S-nitrosylated proteins by proteomic analysis following the biotin switch method, including the cardiac sarcoplasmic reticulum Ca2+-ATPase, alpha-ketoglutarate dehydrogenase, and the mitochondrial F1-ATPase alpha1 subunit. The activities of these enzymes were altered in a concentration-dependent manner by GSNO treatment. We further developed a 2D DyLight fluorescence difference gel electrophoresis proteomic method that used DyLight fluors and a modified biotin switch method to identify S-nitrosylated proteins. IPC and GSNO produced a similar pattern of S-nitrosylation modification and cardiac protection against ischemia/reperfusion injury, suggesting that protein S-nitrosylation may play an important cardioprotective role in heart.

Muscle sarcoplasmic reticulum calcium regulation in humans during consecutive days of exercise and recovery

The study investigated the hypothesis that three consecutive days of prolonged cycle exercise would result in a sustained reduction in the Ca(2+)-cycling properties of the vastus lateralis in the absence of changes in the sarcoplasmic (endoplasmic) reticulum Ca(2+)-ATPase (SERCA) protein. Tissue samples were obtained at preexercise (Pre) and postexercise (Post) on day 1 (E1) and day 3 (E3) and during recovery day 1 (R1), day 2 (R2), and day 3 (R3) in 12 active but untrained volunteers (age 19.2 +/- 0.27 yr; mean +/- SE) and analyzed for changes (nmol.mg protein(-1).min(-1)) in maximal Ca(2+)-ATPase activity (V(max)), Ca(2+) uptake and Ca(2+) release (phase 1 and phase 2), and SERCA isoform expression (SERCA1a and SERCA2a). At E1, reductions (P < 0.05) from Pre to Post in V(max) (150 +/- 7 vs. 121 +/- 7), Ca(2+) uptake (7.79 +/- 0.28 vs. 5.71 +/- 0.33), and both phases of Ca(2+) release (phase 1, 20.3 +/- 1.3 vs. 15.2 +/- 1.1; phase 2, 7.70 +/- 0.60 vs. 4.99 +/- 0.48) were found. In contrast to V(max), which recovered at Pre E3 and then remained stable at Post E3 and throughout recovery, Ca(2+) uptake remained depressed (P < 0.05) at E3 Pre and Post and at R1 as did phase 2 of Ca(2+) release. Exercise resulted in an increase (P < 0.05) in SERCA1a (14% at R2) but not SERCA2a. It is concluded that rapidly adapting mechanisms protect V(max) following the onset of regular exercise but not Ca(2+) uptake and Ca(2+) release.

Effects of high-intensity training and acute exercise on in vitro function of rat sarcoplasmic reticulum

To evaluate the effects of high-intensity training and/or a single bout of exercise on in vitro function of the sarcoplasmic reticulum (SR), the rats were subjected to 8 weeks of interval running program (final training: 2.5-min running x 4 sets per day, 50 m/min at 10% incline). Following training, SR function, i.e., Ca2+-ATPase activity and Ca2+-uptake and release rates, was examined in homogenates of the superficial region of the vastus lateralis muscle from rats subjected to a single bout of treadmill running (50 m/min at 10% incline) for 2.5 min or to exhaustion. Training brought about a 12.4% increase (P < 0.05) in SR Ca2+-uptake rate in rested muscles. This change was not accompanied by alterations in Ca2+-ATPase activity, Ca2+-release rate, Ca2+ dependence of enzyme and protein contents of Ca2+-ATPase and ryanodine receptor. A single bout of high-intensity exercise to exhaustion evoked significant reductions (P < 0.05) in SR function, irrespective of whether or not the animals were trained. For 2.5-min run and exhausted rats, no differences existed between SR functions of untrained and trained muscles. These data suggest that high-intensity training may be capable of enhancing SR Ca2+-sequestering ability, and may not protect against decreasing SR function with high-intensity exercise.

Ultrastructural changes and sarcoplasmic reticulum Ca2+ regulation in red vastus muscle following eccentric exercise in the rat

This study examined the effects of a bout of low-intensity, prolonged downhill exercise on sarcoplasmic reticulum (SR) Ca(2+)-ATPase activity, Ca(2+) uptake and release in rat red vastus muscle. Ionophore stimulation was determined to assess vesicle integrity by measuring the ratio of Ca(2+)-ATPase activities in the presence and absence of A23187. Observations of the muscle ultrastructure were made to evaluate muscle damage at the level of the myofibrils and SR. Adult male Sprague-Dawley rats (weight, 395 +/- 5.9 g) were either assigned as non-exercise controls or subjected to 90 min of downhill treadmill exercise (-16 deg; 15 m min(-1)), and then killed immediately, 4, 24, 48, 72 or 144 h after exercise (n = 7). Calcium uptake was significantly lower (P < 0.05) compared with control values (19.25 +/- 1.38 nmol min(-1) (mg protein)(-1)), by 29 and 36% immediately and 4 h postexercise, respectively, and remained depressed (P < 0.05) 24 h postexercise. Calcium release was also significantly lower (P < 0.05) compared with control values (31.06 +/- 2.36 nmol min(-1) (mg protein)(-1)), by 37 and 39% immediately and 4 h postexercise, respectively, and remained depressed (P < 0.05) 24 h postexercise. Ca(2+)-ATPase activity measured with ionophore was 31% lower (P < 0.05) 4 h postexercise, and remained lower (P < 0.05) 24 h postexercise. The ratio of Ca(2+)-ATPase activities in the presence and absence of A23187 was not significantly changed after exercise, indicating that membrane integrity was not altered by the exercise. Focal dilatations of the SR were observed immediately and 4 h following exercise, implying that SR may be susceptible to damage in the localized regions of overstretched sarcomeres. The results demonstrate that a bout of low-intensity, prolonged downhill exercise results in a long-lasting depression of SR function that is not fully restored after 2 days of recovery, which may underlie some functional impairments induced by eccentric exercise.

Comparative effects of a low-carbohydrate diet and exercise plus a low-carbohydrate diet on muscle sarcoplasmic reticulum responses in males

We employed a glycogen-depleting session of exercise followed by a low-carbohydrate (CHO) diet to investigate modifications that occur in muscle sarcoplasmic reticulum (SR) Ca2+-cycling properties compared with low-CHO diet alone. SR properties were assessed in nine untrained males [peak aerobic power (V̇o2 peak) = 43.6 ± 2.6 (SE) ml·kg−1·min−1] during prolonged cycle exercise to fatigue performed at ∼58% V̇o2 peakafter 4 days of low-CHO diet (Lo CHO) and after glycogen-depleting exercise plus 4 days of low-CHO (Ex+Lo CHO). Compared with Lo CHO, Ex+Lo CHO resulted in 12% lower ( P < 0.05) resting maximal Ca2+-ATPase activity ( Vmax= 174 ± 12 vs. 153 ± 10 μmol·g protein−1·min−1) and smaller reduction in Vmaxinduced during exercise. A similar effect was observed for Ca2+uptake. The Hill coefficient, defined as slope of the relationship between cytosolic free Ca2+concentration and Ca2+-ATPase activity, was higher ( P < 0.05) at rest (2.07 ± 0.15 vs. 1.90 ± 0.10) with Ex+Lo CHO, an effect that persisted throughout the exercise. The coupling ratio, defined as the ratio of Ca2+uptake to Vmax, was 23–30% elevated ( P < 0.05) at rest and during the first 60 min of exercise with Ex+Lo CHO. The ∼27 and 34% reductions ( P < 0.05) in phase 1 and phase 2 Ca2+release, respectively, observed during exercise with Lo CHO were not altered by Ex+Lo CHO. These results indicate that when prolonged exercise precedes a short-term Lo CHO diet, Ca2+sequestration properties and efficiency are improved compared with those during Lo CHO alone.