Activation of the sarcoplasmic reticulum Ca2+-ATPase induced by exercise

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.

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.

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.

Effects of buthionine sulfoximine treatment on diaphragm contractility and SR Ca2+ pump function in rats

The purpose of this study was to examine the effects of glutathione (GSH) depletion and cellular oxidation on rat diaphragm contractility and sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) function in vitro under basal conditions and following fatiguing stimulation. Buthionine sulfoximine (BSO) treatment (n = 10) for 10 days (20 mM in drinking water) reduced (P < 0.05) diaphragm GSH content (nmol/mg protein) and the ratio of GSH to glutathione disulfide (GSH/GSSG) by 91% and 71%, respectively, compared with controls (CTL) (n = 10). Western blotting showed that Hsp70 expression in diaphragm was not increased (P > 0.05) with BSO treatment. As hypothesized, basal peak twitch force (g/mm(2)) was increased (P < 0.05), and fatigability in response to repetitive stimulation (350-ms trains at 100 Hz once every 1 s for 5 min) was also increased (P < 0.05) in BSO compared with CTL. Both Ca(2+) uptake and maximal SERCA activity (mumol.g protein(-1).min(-1)) measured in diaphragm homogenates that were prepared at rest were increased (P < 0.05) with BSO treatment, an effect that could be partly explained by a twofold increase (P < 0.05) in SERCA2a expression with BSO. In response to the 5-min stimulation protocol, both Ca(2+) uptake and maximal SERCA activity were increased (P < 0.05) in CTL but not (P > 0.05) in BSO diaphragm. We conclude that 1) cellular redox state is more optimal for contractile function and fatigability is increased in rat diaphragm following BSO treatment, 2) SERCA2a expression is modulated by redox signaling, and 3) regulation of SERCA function in working diaphragm is altered following BSO treatment.

Efeitos da suplementação aguda de aspartato de arginina na fadiga muscular em voluntários treinados

A atividade física influi em mecanismos específicos responsáveis pela redução da produção de força e conseqüentemente à fadiga. A preocupação em melhorar o desempenho físico tem sido propostos; observamos que estudos dão atenção para reduzir acúmulos dos metabólitos que diminuem a fadiga durante o exercício físico intenso, usando aminoácidos conhecidos por induzir mudanças metabólicas, entre eles a arginina. O presente estudo teve como objetivo estudar o efeito da suplementação aguda de aspartato de arginina em indivíduos sadios treinados submetidos a um protocolo de exaustão em um cicloergômetro. Foram utilizados 12 indivíduos treinados do sexo masculino, idade de 22,6 ± 3,5 anos. Realizaram três testes 90 minutos após a administração em dose única do aspartato de arginina ou solução placebo, em um cicloergômetro, em que incrementos de cargas foram adicionados até a exaustão. Amostras sanguíneas foram obtidas para análises bioquímicas como: creatinina, uréia, glicose e lactato. Diferenças estatísticas não foram encontradas ao comparar os valores de Freqüência Cardíaca Máxima, Tempo Máximo e Carga Máxima e também ao comparar os resultados anteriores e posteriores ao teste para uréia, creatinina e glicose. As concentrações de lactato (mmol/l) apresentaram diferença estatística ao comparar os valores pré-teste (Controle: 2,2 ± 0,14; Arginina: 2,43 ± 0,23; Placebo: 2,26 ± 0,11) com valores pós-teste (Controle 10,35 ± 0,57; Arginina: 12,07 ± 0,88; Placebo: 12,2 ± 0,96), p < 0,001. Os principais resultados deste estudo indicam que a administração aguda de aspartato de arginina não se mostrou efetiva em aumentar a tolerância à fadiga dos indivíduos avaliados e tratados no protocolo de teste incremental até a exaustão. Assim, podemos concluir que a dose utilizada não foi capaz de aumentar a tolerância à fadiga muscular.

Differential effects of repetitive activity on sarcoplasmic reticulum responses in rat muscles of different oxidative potential

We investigated the hypothesis that muscles of different oxidative potential would display differences in sarcoplasmic reticulum (SR) Ca2+ handling responses to repetitive contractile activity and recovery. Repetitive activity was induced in two muscles of high oxidative potential, namely, soleus (SOL) and red gastrocnemius (RG), and in white gastrocnemius (WG), a muscle of low oxidative potential, by stimulation in adult male rats. Measurements of SR properties, performed in crude homogenates, were made on control and stimulated muscles at the start of recovery (R0) and at 25 min of recovery (R25). Maximal Ca2+-ATPase activity (Vmax, micromol x g protein(-1) x min(-1)) at R0 was lower in stimulated SOL (105 +/- 9 vs. 135 +/- 7) and RG (269 +/- 22 vs. 317 +/- 26) and higher (P < 0.05) in WG (795 +/- 32 vs. 708 +/- 34). At R25, Vmax remained lower (P < 0.05) in SOL and RG but recovered in WG. Ca2+ uptake, measured at 2,000 nM, was depressed (P < 0.05) in SOL and RG by 34 and 13%, respectively, in stimulated muscles at R0 and remained depressed (P < 0.05) at R25. In contrast, Ca2+ uptake was elevated (P < 0.05) in stimulated WG at R0 by 9% and remained elevated (P < 0.05) at R25. Ca2+ release, unaltered in SOL and RG at both R0 and R25, was increased (P < 0.05) in stimulated WG at both R0 and R25. We conclude that SR Ca2+-handling responses to repetitive contractile activity and recovery are related to the oxidative potential of muscle.

Human muscle sarcoplasmic reticulum function during submaximal exercise in normoxia and hypoxia

In this study, the response of the sarcoplasmic reticulum (SR) to prolonged exercise, performed in normoxia (inspired O2fraction = 0.21) and hypoxia (inspired O2fraction = 0.14) was studied in homogenates prepared from the vastus lateralis muscle in 10 untrained men (peak O2consumption = 3.09 ± 0.25 l/min). In normoxia, performed at 48 ± 2.2% peak O2consumption, maximal Ca2+-dependent ATPase activity was reduced by ∼25% at 30 min of exercise compared with rest (168 ± 10 vs. 126 ± 8 μmol·g protein−1·min−1), with no further reductions observed at 90 min (129 ± 6 μmol·g protein−1·min−1). No changes were observed in the Hill coefficient or in the Ca2+concentration at half-maximal activity. The reduction in maximal Ca2+-dependent ATPase activity at 30 min of exercise was accompanied by oxalate-dependent reductions ( P < 0.05) in Ca2+uptake by ∼20% (370 ± 22 vs. 298 ± 25 μmol·g protein−1·min−1). Ca2+release, induced by 4-chloro- m-cresol and assessed into fast and slow phases, was decreased ( P < 0.05) by ∼16 and ∼32%, respectively, by 90 min of exercise. No differences were found between normoxia and hypoxia for any of the SR properties examined. It is concluded that the disturbances induced in SR Ca2+cycling with prolonged moderate-intensity exercise in human muscle during normoxia are not modified when the exercise is performed in hypoxia.

The Sarcoplasmic Reticulum in Muscle Fatigue and Disease: Role of the Sarco(endo)plasmic Reticulum Ca2+-ATPase

Skeletal muscles induced to contract repeatedly respond with a progressive loss in their ability to generate a target force or power. This condition is known simply as fatigue. Commonly, fatigue may persist for prolonged periods of time, particularly at low activation frequencies, which is called low-frequency fatigue. Failure to activate the contractile apparatus with the appropriate intracellular free calcium ([Ca2+]f) signal contributes to fatigue but the precise mechanisms involved are unknown. The sarcoplasmic reticulum (SR) is the major organelle in muscle that is responsible for the regulation of [Ca2+]f, and numerous studies have shown that SR function, both Ca2+ release and Ca2+ uptake, is impaired following fatiguing contractile activity. The major aim of this review is to provide insight into the various cellular mechanisms underlying the alterations in SR Ca2+ cycling and cytosolic [Ca2+]f that are associated both with the development of fatigue during repeated muscle contraction and with low-frequency or long-lasting fatigue. The primary focus will be on the role of the sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) in normal muscle function, fatigue, and disease. Key words: calcium release, calcium uptake, muscle relaxation, low-frequency fatigue, Brody disease

Effects of prolonged exercise and recovery on sarcoplasmic reticulum Ca2+ cycling properties in rat muscle homogenates

AIM

To examine the effects of exercise and exercise plus active and passive recovery on sarcoplasmic reticulum (SR) Ca2+-handling properties.

METHODS

Crude muscle homogenates were prepared from adult rat gastrocnemius muscle from two experiments. In one experiment, the muscle was extracted immediately after prolonged treadmill running (RUN), after a 45 min period of reduced exercise intensity (RUN+) following RUN and compared with controls (CON). In the second experiment, muscle was extracted during passive recovery following the same run protocol at 10 min (REC10), 25 min (REC25) and 45 min (REC45) and compared with CON.

RESULTS

Sarcoplasmic reticulum Ca2+-uptake was 31% higher (P < 0.05) in RUN+ compared with CON and RUN. Higher values (P < 0.05) were also found in REC25 (48%) and REC45 (50%) compared with CON. Maximal Ca2+-ATPase was increased by 23% (P < 0.05) in RUN+ compared with CON and RUN and by 65-68% (P < 0.05) in REC25 and REC45 compared with CON. A higher (P < 0.05) Hill coefficient for Ca2+-ATPase activity was observed in RUN+ (2.3 +/- 0.2) compared with CON (1.7 +/- 0.2) or RUN (1.6 +/- 0.2), but not for any REC conditions. In addition, the coupling ratio (Ca2+-uptake/Ca2+-ATPase activity) was higher (P < 0.05) in RUN+ (2.2 +/- 0.10) compared with CON (1.9 +/- 0.05) and RUN (1.9 +/- 0.08).

CONCLUSIONS

It is concluded that in crude homogenates, SR Ca2+-uptake and Ca2+-ATPase activity are elevated in recovery following prolonged running and that the elevation in these properties is more pronounced during passive compared with active recovery.

Na+-K+-ATPase in rat skeletal muscle: content, isoform, and activity characteristics

The purpose of this study was to investigate the hypothesis that muscle Na+-K+-ATPase activity is directly related to Na+-K+-ATPase content and the content of the alpha2-catalytic isoform in muscles of different fiber-type composition. To investigate this hypothesis, tissue was sampled from soleus (Sol), red gastrocnemius (RG), white gastrocnemius (WG), and extensor digitorum longus (EDL) muscles at rest from 38 male Wistar rats weighing 413 +/- 6.0 g (mean +/- SE). Na+-K+-ATPase activity was determined in homogenates (Hom) and isolated crude membranes (CM) by the regenerating ouabain-inhibitable hydrolytic activity assay (ATPase) and the 3-O-methylfluorescein K+-stimulated phosphatase (3-O-MFPase) assay in vitro. In addition, Na+-K+-ATPase content (Bmax) and the distribution of alpha1-, alpha2-, beta1-, and beta2-isoforms were determined by [3H]ouabain binding and Western blot, respectively. For the ATPase assay, differences (P < 0.05) in enzyme activity between muscles were observed in Hom (EDL > WG) and in CM (Sol > EDL = WG). For the 3-O-MFPase assay, differences (P < 0.05) were also found for Hom (Sol > RG = EDL > WG) and CM (Sol = WG > RG). For Bmax, differences in the order of RG = EDL > Sol = WG (P < 0.05) were observed. Isoform distribution was similar between Hom and CM and indicated in CM, a greater density (P < 0.05) of alpha1 in Sol than WG and EDL (P < 0.05), but more equal distribution of alpha2 between muscles. The beta1 was greater (P < 0.05) in Sol and RG, and the beta2 was greater in EDL and WG (P < 0.05). Over all muscles, the correlation (r) between Hom 3-O-MFPase and Bmax was 0.45 (P < 0.05) and between Hom alpha2 and Bmax, 0.59 (P < 0.05). The alpha1 distribution correlated to Hom 3-O-MFPase (r = 0.79, P < 0.05) CM ATPase (r = 0.69, P < 0.005) and CM 3-O-MFPase activity (r = 0.32, P < 0.05). The alpha2 distribution was not correlated with any of the Na+-K+-ATPase activity measurements. The results indicate generally poor relationships between activity and total pump content and alpha2 isoform content of the Na+-K+-ATPase. Several factors, including the type of preparation and the type of assay, appear important in this regard.

Ca2+ ATPase in Dutch Warmblood Foals Compared with Na+, K+ ATPase: Intermuscular Differences and the Effect of Exercise

We studied the effects of exercise without or with a subsequent period on pasture on Ca2+ ATPase concentration in foal skeletal muscle, and compared the results with those previously reported on Na+, K+ ATPase. Ca2+ ATPase was measured in homogenates as Ca2+-dependent steady-state phosphorylation from [gamma-32P]ATP. From day 7 after birth, 24 foals were divided into three groups: (i) staying in a box stall (Box); (ii) staying in a box stall with an exercise programme of an increasing number of sprints per day (Exercise); and (iii) staying on pasture (Pasture). Half of the foals (12 with four in each treatment group) were killed after 5 months. The remaining foals stayed on pasture until 11 months. In the 5-month Pasture group, Ca2+ ATPase concentration was 29.4 +/- 4.3 nmol/g wet weight (wt) (n = 4) in gluteus medius muscle, 25.2 +/- 3.3 nmol/g wet wt (n = 4) in semitendinosus muscle (both mixed fibre type), and 4.1 +/- 1.7 nmol/g wet wt (n = 3) in the slow masseter muscle. These values were not altered by exercise or by box rest. This was in contrast to the Na+, K+ ATPase concentration which was not different between the three muscles, but showed a 20% rise in gluteus medius and semitendinosus muscle after exercise. In the period from 5 to 11 months on pasture, there was no change in Ca2+ ATPase in any group. In conclusion, the Ca2+ ATPase concentration in foal muscle is around 6-fold higher in mixed fibres than in slow fibres. Furthermore, the enzyme is not up- or down-regulated by sprint exercise or subsequent rest.

Paradoxical effects of prior activity on human sarcoplasmic reticulum Ca2+-ATPase response to exercise

To investigate the effects of intermittent heavy exercise (HE) on sarcoplasmic reticulum (SR) maximal Ca2+-ATPase activity (Vmax) and Ca2+ uptake, a continuous two-stage standardized cycling test was performed before and after HE by untrained men [peak aerobic power (Vo -->Vo2 peak) = 42.9 +/- 2.7 ml. kg-1 x min-1]. The HE consisted of 16 bouts of cycling performed for 6 min each hour at 90% Vo2 peak. Tissue was obtained from the vastus lateralis by needle biopsy before and during each cycle test. Before HE, reductions (P < 0.05 micromol. g protein-1x min-1) of 16 and 31% were observed in Vmax and Ca2+ uptake, respectively, after 40 min of the standardized test. Resting Vmax and Ca2+ uptake were depressed (P < 0.05) by 19 and 30%, respectively, when measured 36-48 h after HE. During the standardized test, after HE, Vmax increased (P < 0.05) by 20%, whereas no change was observed in Ca2+ uptake. The HE protocol resulted in small increases (P < 0.05) and decreases (P < 0.05) in sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) 2a and SERCA1 expression, respectively, as determined by Western blotting techniques. These results indicate that SR Ca2+-sequestering function in response to a prolonged exercise test depends on prior activity status, such that rested muscles exhibit a decrease and prior exercised muscles, an increase in Ca2+-ATPase activity. Moreover, it appears that changes in SERCA content can occur in response to a sustained session of intermittent exercise.

Adaptations in human muscle sarcoplasmic reticulum to prolonged submaximal training

In this study, we employed single-leg submaximal cycle training, conducted over a 10-wk period, to investigate adaptations in sarcoplasmic reticulum (SR) Ca(2+)-regulatory proteins and processes of the vastus lateralis. During the final weeks, the untrained volunteers (age 21.4 +/- 0.3 yr; means +/- SE, n = 10) were exercising 5 times/wk and for 60 min/session. Analyses were performed on tissue extracted by needle biopsy approximately 4 days after the last training session. Compared with the control leg, the trained leg displayed a 19% reduction (P < 0.05) in homogenate maximal Ca(2+)-ATPase activity (192 +/- 11 vs. 156 +/- 18 micromol. g protein(-1). min(-1)), a 4.3% increase (P < 0.05) in pCa(50), defined as the Ca(2+) concentration at half-maximal activity (6.01 +/- 0.05 vs. 6.26 +/- 0.07), and no change in the Hill coefficient (1.75 +/- 0.15 vs. 1.76 +/- 0.21). Western blot analysis using monoclonal antibodies (7E6 and A52) revealed a 13% lower (P < 0.05) sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) 1 in trained vs. control in the absence of differences in SERCA2a. Training also resulted in an 18% lower (P < 0.05) SR Ca(2+) uptake and a 26% lower (P < 0.05) Ca(2+) release. It is concluded that a downregulation in SR Ca(2+) cycling in vastus lateralis occurs with aerobic-based training, which at least in the case of Ca(2+) uptake can be explained by reduction in Ca(2+)-ATPase activity and SERCA1 protein levels.

Mechanisms underlying increases in SR Ca2+-ATPase activity after exercise in rat skeletal muscle

Prolonged exercise followed by a brief period of reduced activity has been shown to result in an overshoot in maximal sarcoplasmic reticulum (SR) Ca(2+)-ATPase activity [maximal velocity (V(max))] in rat locomoter muscles (Ferrington DA, Reijneveld JC, Bär PR, and Bigelow DJ. Biochim Biophys Acta 1279: 203-213, 1996). To investigate the functional significance and underlying mechanisms for the increase in V(max), we analyzed Ca(2+)-ATPase activity and Ca(2+) uptake in SR vesicles from the fast rat gastrocnemius muscles after prolonged running (RUN) and after prolonged running plus 45 min of low-intensity activity (RUN+) or no activity (REC45) and compared them with controls (Con). Although no differences were observed between RUN and Con, both V(max) and Ca(2+) uptake were higher (P < 0.05) by 43 and 63%, respectively, in RUN+ and by 35 and 34%, respectively, in REC45. The increase in V(max) was accompanied by increases (P < 0.05) in the phosphorylated enzyme intermediate measured by [gamma-(32)P]ATP. No differences between groups for each condition were found for the fluorescent probes FITC and (N-cyclohexyl-N(1)-dimethylamino-alpha-naphthyl)carbodiimide, competitive inhibitors of the nucleotide-binding and Ca(2+)-binding sites on the enzyme, respectively. Similarly, no differences for the Ca(2+)-ATPase were observed between groups in nitrotyrosine and phosphoserine residues, a measure of nitrosylation and phosphorylation states, respectively. Western blots indicated no changes in relative isoform content of sarcoendoplasmic reticulum (SERCA)1 and SERCA2a. It is concluded that the increase in V(max) of the Ca(2+)-ATPase observed in recovery is not the result of changes in enzyme nitroslyation or phosphorylation, changes in ATP and Ca(2+)-binding affinity, or changes in protein content of the Ca(2+)-ATPase.

Response to exercise of patients with idiopathic hyper-CK-emia

Patients with an idiopathic increase in serum creatine kinase (CK) levels (hyper-CK-emia) have a benign prognosis, but symptoms may be disabling in daily life. Previous studies have suggested that physical exercise increases the severity of complaints in these patients. We studied whether maximal and submaximal bouts of exercise on a cycle ergometer are harmful for patients with idiopathic hyper-CK-emia. Such dynamic exercise did not lead to larger increases in serum CK activity or more complaints in 11 patients with idiopathic hyper-CK-emia, compared with 11 age-matched healthy controls. Our data suggest that exercise does not result in more extensive muscle damage in patients with idiopathic hyper-CK-emia than in healthy subjects.

Reduced activity of muscle Na(+)-K(+)-ATPase after prolonged running in rats

The purpose of this study was to investigate the hypothesis that Na(+)-K(+)-ATPase activity is reduced in muscle of different fiber composition after a single session of aerobic exercise in rats. In one experiment, untrained female Sprague-Dawley rats (weight 275 +/- 21 g; means +/- SE; n = 30) were run (Run) on a treadmill at 21 m/min and 8% grade until fatigue, or to a maximum of 2 h, which served as control (Con), or performed an additional 45 min of low-intensity exercise at 10 m/min (Run+). In a second experiment, utilizing rats of similar characteristics (weight 258 +/- 18 g; n = 32), Run was followed by passive recovery (Rec). Directly after exercise, rats were anesthetized, and tissue was extracted from Soleus (Sol), red vastus lateralis (RV), white vastus lateralis (WV), and extensor digitorum longus (EDL) and frozen for later analysis. 3-O-methylfluorescein phosphatase activity (3-O-MFPase) was determined as an indicator of Na(+)-K(+)-ATPase activity, and glycogen depletion identified recruitment of each muscle during exercise. 3-O-MFPase was decreased (P < 0.05) at Run+ by an average of 12% from Con in all muscles (P < 0.05). No difference was found between Con and Run. Glycogen was lower (P < 0.05) by 65, 57, 44, and 33% (Sol, EDL, RV, and WV, respectively) at Run, and there was no further depletion during the continued low-intensity exercise period. No differences in Na(+)-K(+)-ATPase activity was observed between Con and Rec. The results of this study indicate that inactivation of Na(+)-K(+)-ATPase can be induced by aerobic exercise in a volume-dependent manner and that the inactivation that occurs is not specific to muscles of different fiber-type composition. Inactivation of Na(+)-K(+)-ATPase suggests intrinsic structural modifications by mechanisms that are unclear.

Inactivation of sarcoplasmic reticulum Ca(2+)-atpase in low-frequency stimulated rat muscle

Continuous low-frequency stimulation (CLFS) by implanted electrodes for 12-24 h led to a significant (approximately 30%) decrease in the activity of sarcoplasmic reticulum Ca(2+)-ATPase in fast-twitch extensor digitorum longus (EDL) and tibialis anterior (TA) muscles of intact rats. The decline in catalytic activity after 24 h of CLFS was accompanied by an approximately twofold increase in dinitrophenylhydrazine-reactive carbonyl groups of the enzyme. It also correlated with an immunochemically determined 30% decrease in Ca2(+)-ATPase protein. Recovery studies after 12 h of CLFS revealed a relatively slow (48-72 h) re-establishment of normal catalytic activity. These findings suggest that the 30% decline of Ca(2+)-ATPase activity in low-frequency stimulated rat muscle led to an irreversible modification by protein oxidation. The decrease in Ca(2+)-ATPase protein most likely resulted from the degradation of inactive Ca(2+)-ATPase molecules. The relatively slow recovery of Ca(2+)-ATPase activity suggests that de novo synthesis of the enzyme may be necessary to re-attain normal activity.

Effects of 4-h ischemia and 1-h reperfusion on rat muscle sarcoplasmic reticulum function

To investigate the hypothesis that ischemia and reperfusion would impair sarcoplasmic reticulum (SR) Ca(2+) regulation in skeletal muscle, Sprague-Dawley rats (n = 20) weighing 290 +/- 3.5 g were randomly assigned to either a control control (CC) group, in which only the effects of anesthetization were studied, or to a group in which the muscles in one hindlimb were made ischemic for 4 h and allowed to recover for 1 h (I). The nonischemic, contralateral muscles served as control (C). Measurements of Ca(2+)-ATPase properties in homogenates and SR vesicles, in mixed gastrocnemius and tibialis anterior muscles, indicated no differences between groups on maximal activity, the Hill coefficient, and Ca(50), defined as the Ca(2+) concentration needed to elicit 50% of maximal activity. In homogenates, Ca(2+) uptake was lower (P < 0.05) by 20-25%, measured at 0.5 and 1.0 microM of free Ca(2+) ([Ca(2+)](f)) in C compared with CC. In SR vesicles, Ca(2+) uptake was lower (P < 0.05) by 30-38% in I compared with CC at [Ca(2+)](f) between 0.5 and 1.5 microM. Silver nitrate induced Ca(2+) release, assessed during both the initial, early rapid (phase 1), and slower, prolonged late (phase 2) phases, in homogenates and SR vesicles, indicated a higher (P < 0.05) release only in phase 1 in SR vesicles in I compared with CC. These results indicate that the alterations in SR Ca(2+) regulation, previously observed after prolonged ischemia by our group, are reversed within 1 h of reperfusion. However, the lower Ca(2+) uptake observed in long-term, nonischemic homogenates suggests that altered regulation may occur in the absence of ischemia.

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.

Alterations in sarcoplasmic reticulum function in female vastus lateralis with eccentric exercise

This study examined the alterations in sarcoplasmic reticulum (SR) Ca2+ sequestration function in homogenates during eccentric exercise and recovery and following additional eccentric exercise, and correlated these alterations with changes in force output. Eight healthy, untrained females, aged 20-25 years, cycled for a total of 60 min on an eccentric cycle ergometer (30 min at 66+/-3% VO2 peak and 30 min at 76+/-3% VO2 peak, determined during concentric exercise). Biopsies (extracted from the vastus lateralis) were taken before and after the exercise as well as on days 2, 6 and prior to and following identical exercise on day 14. Ca2+-uptake (nmol/min/mg protein) was unaffected (p > 0.05) following the first session of eccentric exercise; however, by day 2 a depression in uptake (p < 0.05) was observed which persisted throughout the remainder of the experiment. Maximal Ca2+-ATPase activity (nmol/min/mg protein) was elevated (p < 0.05) immediately following the first exercise session, remained elevated through day 2 and returned to pre-exercise levels by day 6 of recovery and increased again by day 14. No changes in either Ca2+-ATPase activity or Ca2+-uptake were observed with exercise on day 14. Both eccentric sessions, performed on days 0 and 14, resulted in similar depressions in force (p < 0.05) immediately following exercise. By day 2 force had recovered to pre-exercise levels. The results demonstrate that a prolonged alteration in SR Ca2+-uptake occurs following eccentric work that is unaccompanied by parallel changes in either SR Ca2+-ATPase activity or mechanical performance.

Post mortem changes in Ca2+ transporting proteins of sarcoplasmic reticulum in dependence on malignant hyperthermia status in pigs

Meat quality of pigs is dependent on biochemical and biophysical processes in the time course post mortem (p.m.) and is associated with the intracellular Ca2+ homeostasis. However, there is little known about changes in the Ca2+ transporting proteins controlling the Ca2+ uptake of sarcoplasmic reticulum (SR) in the time course p.m. In this study changes in the Ca2+ transporting proteins were investigated in homogenates of longissimus muscles of 4 malignant hyperthermia susceptible (MHS) and 6 malignant hyperthermia resistant (MHR) Pietrain pigs. Muscle samples were obtained at different time intervals: biopsy 2 h prior slaughtering and from the carcass immediately after exsanguination (0 h), 45 min, 4 h, and 22 h p.m. The SR Ca2+ uptake rate was measured immediately after homogenization with closed calcium release channel (CRC), with opened CRC and without manipulation of CRC. Additionally the SR Ca2+ ATPase activity was determined. The results show: (i) The ability of SR to sequester Ca2+ declined to about 60% in the first 45 min p.m. in MHS samples irrespective of CRC state, whereas in MHR samples this decline was about 5%; (ii) Ca2+ uptake and Ca2+ ATPase activity were not different between the biopsy and 0 h samples, i.e. the stress of slaughter was of no immediate influence; (iii) The Ca2+ ATPase activity of the SR declined at about the same rate as the Ca2+ uptake in both MHS and MHR pig samples in the course of time p.m.; (iv) In samples, taken immediately after exsanguination, the Ca2+ ATPase activity of MHS pigs was higher than that of MHR pigs. However, in samples taken 4 h p.m. Ca2+ ATPase activity of MHS pigs has declined to about 30% of the value at 0 h; (v) The CRC can be closed and opened in all samples up to 22 h p.m. and seems to be fully functional at all sampling times; (vi) The CRC of MHS pigs is almost fully open, whereas the CRC of MHR pigs is only partially open at all sampling times; (vii) The permeability of the SR membrane to Ca2+ (determined as the ratio of SR Ca2+ ATPase with and without ionophore A23187) is the same in both MHS and MHR and did not change with ongoing time; (viii) No uncoupling of uptake from ATP hydrolysis occurred up to 4 h p.m., but the coupling differed between MHS and MHR for all time intervals with lower values for MHS pigs. The results suggest that the decreasing Ca2+ uptake rate of homogenates, sampled at different times p.m., is essentially caused by changes in the Ca2+ pump and not by changes in the CRC or an increased phospholipid membrane permeability to Ca2+.

Altered turnover of calcium regulatory proteins of the sarcoplasmic reticulum in aged skeletal muscle

We have measured the in vivo protein turnover for the major calcium regulatory proteins of the sarcoplasmic reticulum from the skeletal muscle of young adult (7 months) and aged (28 months) Fischer 344 rats. From the time course of the incorporation and decay of protein-associated radioactivity after a pulse injection of [14C]leucine and correcting for leucine reutilization, in young rats, the apparent half-lives for calsequestrin, the 53-kDa glycoprotein, and ryanodine receptor are 5.4 +/- 0.4, 6.3 +/- 1.3, and 8.3 +/- 1.3 days, respectively. A half-life of 14.5 +/- 2.5 days was estimated for the Ca-ATPase isolated from young muscle. Differences in protein turnover associated with aging were determined using sequential injection of two different isotopic labels ([14C]leucine and [3H]leucine) to provide an estimate of protein synthesis and degradation within the same animal. The Ca-ATPase and ryanodine receptor isolated from aged muscle exhibits 27 +/- 5% and 25 +/- 3% slower protein turnover, respectively, relative to that from young muscle. In contrast, the 53-kDa glycoprotein exhibits a 25 +/- 5% more rapid turnover in aged SR, while calsequestrin exhibits no age-dependent alteration in turnover. Statistical analysis comparing the sensitivity of various methods for discriminating different rates of protein turnover validates the approach used in this study and demonstrates that the use of two isotopic labels provides at least a 6-fold more sensitive means to detect age-related differences in protein turnover relative to other methods.

Cation pumps in skeletal muscle: potential role in muscle fatigue

Two membrane bound pumps in skeletal muscle, the sarcolemma Na+-K+ adenosine triphosphatase (ATPase) and the sarcoplasmic reticulum Ca2+-ATPase, provide for the maintenance of transmembrane ionic gradients necessary for excitation and activation of the myofibrillar apparatus. The rate at which the pumps are capable of establishing ionic homeostasis depends on the maximal activity of the enzyme and the potential of the metabolic pathways for supplying adenosine triphosphate (ATP). The activity of the Ca2+-ATPase appears to be expressed in a fibre type specific manner with both the amount of the enzyme and the isoform type related to the speed of contraction. In contrast, only minimal differences exist between slow-twitch and fast-twitch fibres in Na+-K+ ATPase activity. Evidence is accumulating that both active transport of Na+ and K+ across the sarcolemma and Ca2+-uptake by the sarcoplasmic reticulum may be impaired in vivo in a task specific manner resulting in loss of contractile function. In contrast to the Ca2+-ATPase, the Na+-K+ ATPase can be rapidly upregulated soon after the onset of a sustained pattern of activity. Similar programmes of activity result in a downregulation of Ca2+-ATPase but at a much later time point. The manner in which the metabolic pathways reorganize following chronic activity to meet the changes in ATP demand by the cation pumps and the degree to which these adaptations are compartmentalized is uncertain.

Decreased conformational stability of the sarcoplasmic reticulum Ca-ATPase in aged skeletal muscle

Sarcoplasmic reticulum (SR) membranes purified from young adult (4-6 months) and aged (26-28 months) Fischer 344 male rat skeletal muscle were compared with respect to the functional and structural properties of the Ca-ATPase and its associated lipids. While we find no age-related alterations in (1) expression levels of Ca-ATPase protein, and (2) calcium transport and ATPase activities, the Ca-ATPase isolated from aged muscle exhibits more rapid inactivation during mild (37 degrees C) heat treatment relative to that from young muscle. Saturation-transfer EPR measurements of maleimide spin-labeled Ca-ATPase and parallel measurements of fatty acyl chain dynamics demonstrate that, accompanying heat inactivation, the Ca-ATPase from aged skeletal muscle more readily undergoes self-association to form inactive oligomeric species without initial age-related differences in association state of the protein. Neither age nor heat inactivation results in differences in acyl chain dynamics of the bilayer including those lipids at the lipid-protein interface. Initial rates of tryptic digestion associated with the Ca-ATPase in SR isolated from aged muscle are 16(+/- 2)% higher relative to that from young muscle. indicating more solvent exposure of a portion of the cytoplasmic domain. During heat inactivation these structural differences are amplified as a result of immediate and rapid further unfolding of the Ca-ATPase isolated from aged muscle relative to the delayed unfolding of the Ca-ATPase isolated from young muscle. Thus age-related alterations in the solvent exposure of cytoplasmic peptides of the Ca-ATPase are likely to be critical to the loss of conformational and functional stability.

Mechanical behavior of skeletal muscle during intermittent voluntary isometric contractions in humans

Changes in contractile speed and force-fusion properties were examined during repetitive isometric contractions with the knee extensors at three different target force levels. Seven healthy subjects were studied at target force levels of 30, 45, and 60% of their maximal voluntary contraction (MVC) force. Repeated 6-s contractions followed by 4-s rest were continued until exhaustion. Contractile speed was determined for contractions elicited by electrical stimulation at 1-50 Hz given during exercise and a subsequent 27-min recovery period. Contraction time remained unchanged during exercise and recovery, except for an initial rapid shift in the twitch properties. Half relaxation time (RT1/2) decreased gradually by 20-40% during exercise at 30 and 45% of MVC. In the recovery period, RT1/2 values were not fully restored to preexercise levels. During exercise at 60% MVC, the RT1/2 decreased for twitches and increased for the 50-Hz stimulation. In the recovery period after 60% MVC, RT1/2 values declined toward those seen after the 30 and 45% MVC exercise. The force oscillation amplitude in unfused tetani relative to the mean force increased during exercise at 30 and 45% MVC but remained unaltered during the 60% MVC exercise. This altered force-fusion was closely associated with the changes in RT1/2. The faster relaxation may at least partly explain the increased energy cost of contraction reported previously for the same type of exercise.

In vivo aging of rat skeletal muscle sarcoplasmic reticulum Ca-ATPase. Chemical analysis and quantitative simulation by exposure to low levels of peroxyl radicals

Sarcoplasmic reticulum (SR) Ca-ATPase of young adult (5 months) and aged (28 months) Fischer 344 male rat skeletal muscle was analyzed for posttranslational modifications as a result of biological aging and their potential functional consequences. The significant differences in the amino acid composition were a 6.8% lower content of sulfhydryl groups and a ca. 4% lower content of Arg residues of the Ca-ATPase from old as compared to young rats. Based on a total of 24 Cys residues the difference in protein thiols corresponds to a loss of 1.5 mol Cys/mol Ca-ATPase as a result of in vivo aging. The loss of Cys residues was not accompanied by a loss of enzyme activity though the 'aged' Ca-ATPase was more sensitive to heat inactivation, aggregation, and tryptic digestion. A comparison of the total sulfhydryl content of all SR proteins present revealed a 13% lower amount for SR vesicles isolated from aged rats. Compared to the alterations of Cys and Arg, there was only a slight and probably physiologically insignificant increase of protein carbonyls with aging, i.e. from 0.32 to 0.46 mol carbonyl groups per mol of Ca-ATPase. When SR vesicles from young rats were exposed to AAPH-derived peroxyl radicals, there was a loss of ca. 1.38 x 10(-4) M total SR sulfhydryl groups per 4 mg SR protein/ml (corresponding to ca. 25%) and a loss of 9.6 x 10(-5) M Ca-ATPase sulfhydryl groups (corresponding to ca. 31%) per 1.6 x 10(-5) M initiating peroxyl radicals, indicating that the stoichiometry of sulfhydryl oxidation was > or = 6 oxidized thiols per initiating AAPH-derived peroxyl radical. Besides Cys, the exposure to AAPH-derived radicals caused a slight loss of Ca-ATPase Arg, Met, and Ser residues. Most importantly, the SR Ca-ATPase exposed to this low concentration of peroxyl radicals displayed physical and functional properties quantitatively comparable to those of SR Ca-ATPase isolated from aged rats, i.e. no immediate loss of activity, increased susceptibility to heat inactivation, aggregation, and tryptic digestion. Moreover, a comparison of kinetically early tryptic fragments by HPLC-electrospray MS and N-terminal sequencing revealed that similar peptide fragments were produced from 'aged' and AAPH-oxidized Ca-ATPase which were not (or kinetically significantly later) generated from the 'young' Ca-ATPase, suggesting some conformational changes of the Ca-ATPase as a result of aging and AAPH-exposure. All except one of these peptides originated from locations remote from the nucleotide-binding and calcium-binding sites. The latter results suggest that aging and AAPH-exposure may target similar Cys residues, mainly at locations remote from the nucleotide-binding and calcium-binding sites, rationalizing the fact that Cys oxidation did not immediately cause inactivation of the Ca-ATPase. Our results provide a quantitative estimate of a net concentration of reactive oxygen species, here peroxyl radicals, which induces physical and chemical alterations of the SR Ca-ATPase quantitatively comparable to those induced by in vivo aging.

Fatty acyl-CoA-acyl-CoA-binding protein complexes activate the Ca2+ release channel of skeletal muscle sarcoplasmic reticulum

We previously reported that fatty acyl-CoA esters activate ryanodine receptor/Ca2+ release channels in a terminal cisternae fraction from rabbit skeletal muscle [Fulceri, Nori, Gamberucci, Volpe, Giunti and Benedetti (1994) Cell Calcium 15, 109-116]. Skeletal muscle cytosol contains a high-affinity fatty acyl-CoA-binding protein (ACBP) [Knudsen, Hojrup, Hansen, H.O., Hansen, H.F. and Roepstorff (1989) Biochem. J. 262, 513-519]. We show here that palmitoyl-CoA (PCoA) in a complex with a molar excess of bovine ACBP causes a discrete Ca2+ efflux or allows Ca2+ release from the Ca2+-preloaded terminal cisternae fraction by sub-optimal caffeine concentrations. Both effects were abolished by elevating the free [Mg2+] in the system, which inhibits the Ca2+ release channel activity. Sensitization towards caffeine was a function of both the concentration of the complex and the [PCoA]-to-[ACBP] ratio. In all experimental conditions the calculated free [PCoA] was no more than 50 nM, and such concentrations by themselves were inactive on Ca2+ release channels. The KD for PCoA binding was approx. 2 nM for bovine and yeast ACBP, and slightly higher (8 nM) for rat ACBP. The PCoA-rat ACBP complex behaved in the same manner as the PCoA-bovine ACBP complex, whereas the ester complexed with yeast ACBP was more active in activating/sensitizing Ca2+ efflux. A non-hydrolysable analogue of PCoA bound to (bovine) ACBP also sensitized the Ca2+ release channel towards caffeine. These findings indicate that fatty acyl-CoA-ACBP complexes either interact directly with one or more components in the terminal cisternae membranes or, through interaction with the component(s), donate the fatty acyl-CoA esters to high-affinity binding sites of the membrane, thus affecting (and possibly regulating) Ca2+ release channel activity.

Decreased contraction economy in mouse EDL muscle injured by eccentric contractions

The objective of this study was to find out whether basal and/or active energy metabolism are altered in isolated mouse extensor digitorum longus muscle injured by eccentric (Ecc) contractions. Measurements of basal O2 consumption and isometric tetanus O2 recovery cost were made at 25 degrees C on muscles that had done either 10 Ecc, 10 isometric (Iso), or no contractions (No). In parallel experiments, rates of lactate and pyruvate production were measured to estimate the anaerobic contribution. Basal O2 consumption was unaffected by the type of protocol performed (P = 0.07). However, the tetanus O2 cost per force-time integral was elevated by 30-36% for the Ecc protocol muscles over that for the Iso and No protocol muscles. When including the increased lactate production by the Ecc protocol muscles, the total energetic cost per force-time integral was 53% higher than that for the Iso protocol muscles [2.35 +/- 0.17 vs. 1.54 +/- 0.18 mumol O2/(N.m.s)]. The decreased economy was attributed to two factors. First, in skinned fibers isolated from the injured muscles, the ratio of maximal actomyosin adenosinetriphosphatase activity to force production was up by 37.5%, suggesting uncoupling of ATP hydrolysis from force production. Second, increased reliance on anaerobic metabolism along with the fluorescent microscopic study of mitochondrial membrane potential and histochemical study of ATP synthase suggested an uncoupling of oxidative phosphorylation in the injured muscles.