Effects of creatine phosphate and P(i) on Ca2+ movements and tension development in rat skinned skeletal muscle fibres

Functional adaptability of muscle fibers to long-term resistance exercise

PURPOSE

We compared the functional properties of muscle fibers from two groups of subjects that differed widely in their training history to investigate whether long-term resistance exercise alters the intrinsic contractile properties of skeletal muscle fibers.

METHODS

Vastus lateralis muscle biopsies were obtained from six sedentary males (NT group, age = 23 +/- 1 yr) and six males who had participated in regular resistance exercise training over the preceding 7.6 +/- 1.6 yr (RT group, 22 +/- 1 yr). Chemically skinned muscle fiber segments were activated with a saturating free [Ca2+] to quantify fiber peak Ca2+-activated force (P(o)), unloaded shortening velocity (V(o)), and peak power. Fiber segment myosin heavy chain (MHC) isoform content was identified by gel electrophoresis.

RESULTS

Slow and fast fibers from the RT group were larger in CSA and produced greater absolute P(o) and absolute peak power in comparison with fibers from the NT group. However, these differences were no longer evident after P(o) and peak power were normalized to fiber CSA and fiber volume, respectively. V(o)/fiber length was dependent on fiber MHC content but independent of training status.

CONCLUSION

Fiber hypertrophy was sufficient to account for intergroup differences in P(o) and peak power of slow and fast fibers. There was no evidence that the intrinsic contractility of slow or fast fibers, as evaluated by force, shortening velocity, and power normalized to the appropriate fiber dimensions, differed between RT and NT groups.

Excitation-contraction coupling and fatigue mechanisms in skeletal muscle: studies with mechanically skinned fibres

This review attempts to give an insight into the key aspects of excitation-contraction (E-C) coupling and fatigue in skeletal muscle, in particular summarizing the results and perspectives obtained from studies with mechanically skinned muscle fibres. These skinned fibre studies have provided many novel insights, such as the role of intracellular Mg2+ and ATP in the coupling mechanism, as well as how the accumulation of metabolic products, precipitation of inorganic phosphate in the sarcoplasmic reticulum (SR) and disruption of the coupling mechanism by high intracellular [Ca2+], may contribute to different types of muscle fatigue. The recent demonstration of action potential (AP)-induced Ca2+ release in skinned fibres [G.S. Posterino et al. (2000) J Physiol 527: 131-137] showed unequivocally that the normal E-C coupling mechanism [W. Melzer et al. (1995) Biochim Biophys Acta 1241: 59-116] was retained in this preparation and indicated the considerable potential of this technique. Among other things, it has been possible to show that AP activation of the voltage-sensors in the transverse-tubular (T-) system is normally sufficient to give maximal activation of the Ca2+ release channels (ryanodine receptors) in the SR and that increasing the sensitivity of the release channels to Ca2+, such as by oxidation or other means, does not increase the amount of Ca2+ released by an AP. In contrast, when the voltage-sensors are not fully activated, modulating the responsiveness of the Ca2+ release channels does affect the amount of Ca2+ release. It is suggested that some forms of muscle fatigue are caused by inadequate activation of the Ca2+ release channels due both to (a) inactivation or dysfunction of the voltage-sensors and (b) inhibitory effects on the release channels caused by local changes in the cytoplasmic environment (in particular by low [ATP] and raised concentrations of Mg2+, ATP metabolites and other factors) and by a decrease in the pool of releasable Ca2+ within the SR.

Creatine kinase injection restores contractile function in creatine-kinase-deficient mouse skeletal muscle fibres

Viable genetically engineered animals generally exhibit adaptations to the altered genotype, which may mask the role of the protein of interest. We now describe a novel method by which the direct effects of the altered genotype can be distinguished from secondary adaptive changes in isolated adult skeletal muscle cells. We studied contractile function and intracellular Ca2+ handling in single skeletal muscle fibres that are completely deficient of creatine kinase (CK; CK-/-) before and after microinjection of purified CK (injected together with the fluorescent Ca2+ indicator indo-1). The mean total CK activity after CK injection was estimated to be approximately 4 mM s-1, which is approximately 5 % of the activity in wild-type muscle fibres. After CK injection, CK-/- fibres approached the wild-type phenotype in several aspects: (a) the free myoplasmic [Ca2+] ([Ca2+]i) increased and force showed little change during a period of high-intensity stimulation (duty cycle, i.e. tetanic duration divided by tetanic interval = 0.67); (b) [Ca2+]i did not decline during a brief (350 ms) tetanus; (c) during low-intensity fatiguing stimulation (duty cycle = 0.14), tetanic [Ca2+]i increased over the first 10 tetani, and thereafter it decreased; (d) tetanic [Ca2+]i and force did not display a transient reduction in the second tetanus of low-intensity fatiguing stimulation. Conversely, tetanic force in the unfatigued state was lower in CK-/- than in wild-type fibres, and this difference persisted after CK injection. Injection of inactivated CK had no obvious effect on any of the measured parameters. In conclusion, microinjection of CK into CK-/- fibres markedly restores many, but not all, aspects of the wild-type phenotype.

Effects of glucose on contractile function, [Ca2+]i, and glycogen in isolated mouse skeletal muscle

Extensor digitorum longus muscles were stimulated to contract to fatigue and allowed to recover for 2 h in the absence or presence of 5.5 or 11 mM extracellular glucose. This was followed by a second fatigue run, which ended when the absolute force was the same as at the end of the first run. During the first fatigue run, the fluorescence ratio for indo 1 increased [reflecting an increase in myoplasmic free Ca2+ concentration ([Ca2+]i)] during the initial tetani, peaking at approximately 115% of the first tetanic value, followed by a continuous decrease to approximately 90% at fatigue. During the first fatigue run, myofibrillar Ca2+ sensitivity was significantly decreased. During the second run, the number of tetani was 57 +/- 6% of initial force in muscles that recovered in the absence of glucose and 110 +/- 6 and 119 +/- 2% of initial force in muscles that recovered in 5.5 and 11 mM glucose, respectively. Fluorescence ratios during the first, peak, and last tetani did not differ significantly between the first and second fatigue runs during any of the three conditions. Glycogen decreased by almost 50% during the first fatigue run and did not change further after recovery in the absence of glucose. After recovery in the presence of 5.5 and 11 mM glucose, glycogen increased 32 and 42% above the nonstimulated control value (P < 0.01). These data demonstrate that extracellular glucose delays the decrease of tetanic force and [Ca2+]i during fatiguing stimulation and that glycogen supercompensation following contraction can occur in the absence of insulin.

Effects of muscle activation on fatigue and metabolism in human skeletal muscle

Increasing stimulation frequency has been shown to increase fatigue but not when the changes in force associated with changes in frequency have been controlled. An effect of frequency, independent of force, may be associated with the metabolic cost resulting from the additional activations. Here, two separate experiments were performed on human medial gastrocnemius muscles. The first experiment (n = 8) was designed to test the effect of the number of pulses on fatigue. The declines in force during two repetitive, 150-train stimulation protocols that produced equal initial forces, one using 80-Hz trains and the other using 100-Hz trains, were compared. Despite a difference of 600 pulses (23.5%), the protocols produced similar rates and amounts of fatigue. In the second experiment, designed to test the effect of the number of pulses on the metabolic cost of contraction, 31P-NMR spectra were collected (n = 6) during two ischemic, eight-train stimulation protocols (80- and 100-Hz) that produced comparable forces despite a difference of 320 pulses (24.8%). No differences were found in the changes in P(i) concentration, phosphocreatine concentration, and intracellular pH or in the ATP turnover produced by the two trains. These results suggest that the effect of stimulation frequency on fatigue is related to the force produced, rather than to the number of activations. In addition, within the range of frequencies tested, increasing total activations did not increase metabolic cost.

Contractile properties of in situ perfused skeletal muscles from rats with congestive heart failure

We hypothesized that in congestive heart failure (CHF) slow-twitch but not fast-twitch muscles exhibit decreased fatigue resistance in the sense of accelerated reduction of muscle force during activity. Experiments were carried out on anaesthetized rats 6 weeks after induction of myocardial infarction or a sham operation (Sham). Animals with left ventricular end-diastolic pressure (LVEDP) > 15 mmHg under anaesthesia were selected for the CHF group. There was no muscle atrophy in CHF. Force generation by in situ perfused soleus (Sol) or extensor digitorum longus (EDL) muscles was recorded during stimulation (trains at 5 Hz for 6 s (Sol) or 10 Hz for 1.5 s (EDL) at 10 or 2.5 s intervals, respectively) for 1 h in Sol and 10 min in EDL at 37 degrees C. Initial force was almost the same in Sol from CHF and Sham rats, but relaxation was slower in CHF. Relaxation times (95-5 % of peak force) were 177 +/- 55 and 131 +/- 44 ms in CHF and Sham, respectively, following the first stimulation train. After 2 min of stimulation the muscles transiently became slower and maximum relaxation times were 264 +/- 71 and 220 +/- 45 ms in CHF and Sham, respectively (P < 0.05). After 60 min they recovered to 204 +/- 60 and 122 +/- 55 ms in CHF and Sham, respectively (P < 0.05). In CHF but not in Sham rats the force of contraction of Sol declined from the second to the sixtieth minute to 70 % of peak force. The EDL of both CHF and Sham fatigued to 24-28 % of initial force, but no differences in contractility pattern were detected. Thus, slow-twitch muscle is severely affected in CHF by slower than normal relaxation and significantly reduced fatigue resistance, which may explain the sensation of both muscle stiffness and fatigue in CHF patients.

Effect of P(i) on unloaded shortening velocity of slow and fast mammalian muscle fibers

Chemically skinned muscle fibers, prepared from the rat medial gastrocnemius and soleus, were subjected to four sequential slack tests in Ca(2+)-activating solutions containing 0, 15, 30, and 0 mM added P(i). P(i) (15 and 30 mM) had no effect on the unloaded shortening velocity (V(o)) of fibers expressing type IIb myosin heavy chain (MHC). For fibers expressing type I MHC, 15 mM P(i) did not alter V(o), whereas 30 mM P(i) reduced V(o) to 81 plus minus 1% of the original 0 mM P(i) value. This effect was readily reversible when P(i) was lowered back to 0 mM. These results are not compatible with current cross-bridge models, developed exclusively from data obtained from fast fibers, in which V(o) is independent of P(i). The response of the type I fibers at 30 mM P(i) is most likely the result of increased internal drag opposing fiber shortening resulting from fiber type-specific effects of P(i) on cross bridges, the thin filament, or the rate-limiting step of the cross-bridge cycle.

Effects of fatigue and training on sarcoplasmic reticulum Ca(2+) regulation in human skeletal muscle

Little is known about fatigue and training effects on sarcoplasmic reticulum (SR) function in human muscle, and we therefore investigated this in eight untrained controls (UT), eight endurance-trained (ET), and eight resistance-trained athletes (RT). Muscle biopsies (vastus lateralis) taken at rest and after 50 maximal quadriceps contractions (180 degrees/s, 0.5 Hz) were analyzed for fiber composition, metabolites and maximal SR Ca(2+) release, Ca(2+) uptake, and Ca(2+)-ATPase activity. Fatigue reduced (P < 0.05) Ca(2+) release (42.1 +/- 3.8%, 43.4 +/- 3.9%, 31.3 +/- 6.1%), Ca(2+) uptake (43.0 +/- 5.2%, 34.1 +/- 4.6%, 28.4 +/- 2.8%), and Ca(2+)-ATPase activity (38.6 +/- 4.2%, 48.5 +/- 5.7%, 29.6 +/- 5.0%), in UT, RT, and ET, respectively. These decreases were correlated with fatigability and with type II fiber proportion (P < 0.05). Resting SR measures were correlated with type II proportion (r > or = 0.51, P < 0.05). ET had lower resting Ca(2+) release, Ca(2+) uptake, and Ca(2+)-ATPase (P < 0.05) than UT and RT (P < 0.05), probably because of their lower type II proportion; only minor effects were found in RT. Thus SR function is markedly depressed with fatigue in controls and in athletes, is dependent on fiber type, and appears to be minimally affected by chronic training status.

Metabolic costs of force generation for constant-frequency and catchlike-inducing electrical stimulation in human tibialis anterior muscle

Metabolic costs of force generation were compared for constant-frequency and catchlike-inducing electrical stimulation. Repetitive catchlike-inducing trains consisted of 2 interpulse intervals (IPIs) at 12.5 ms, 1 IPI at 25 ms, and 5 IPIs at 50 ms. Constant-frequency trains consisted of 8 IPIs at 37.5 ms. One train was delivered to the peroneal nerve every 2.5 s for 36 times under ischemic conditions. Anaerobic adenosine triphosphate (ATP) turnover was determined using 31-phosphorus magnetic resonance spectroscopy (P-MRS) of the human tibialis anterior muscle. Compared with constant-frequency trains, catchlike-inducing trains produced a faster force generation and were more effective in maintaining the force--time integral as well as peak force. However, ATP costs of force generation were similar for the catchlike-inducing and constant-frequency stimulation (6.7 plus/minus 1.1 and 6.6 plus/minus 1.0 micromol ATP/kg wet weight/Ncenter dots, respectively, P = 0.601). This suggests that the positive effects of catchlike-inducing stimulation on force maintenance are mediated by potentiated Ca(2+) release from the sarcoplasmic reticulum rather than by lower metabolic costs of muscle force generation. Our findings also suggest that catchlike-inducing stimulation produces larger forces in fatigued muscle than constant-frequency trains and thus may be beneficial for muscle training or rehabilitation when muscle loading needs to be maintained in repetitive contractions.

Muscle fatigue: lactic acid or inorganic phosphate the major cause?

Intracellular acidosis due mainly to lactic acid accumulation has been regarded as the most important cause of skeletal muscle fatigue. Recent studies on mammalian muscle, however, show little direct effect of acidosis on muscle function at physiological temperatures. Instead, inorganic phosphate, which increases during fatigue due to breakdown of creatine phosphate, appears to be a major cause of muscle fatigue.

Muscle contractile properties during intermittent nontetanic stimulation in rat skeletal muscle

To examine changes in contractile properties and mechanisms of fatigue during submaximal nontetanic skeletal muscle activity, in situ perfused soleus (60-min protocol) and extensor digitorum longus (EDL; 10-min protocol) muscles of the rat were electrically stimulated intermittently at low frequency. The partly fused trains of contractions showed a two-phase change in appearance. During the first phase, relaxation slowed, one-half relaxation time increased, and maximal relaxation first derivative of force (dF/dt) decreased. Developed force during the trains was reduced and was closely related to the rate of relaxation in this first phase. During the second phase, relaxation became faster again, one-half relaxation time decreased, and force returned to resting levels between contractions in a train. In contrast, developed force remained reduced, so that peak force of the contractions was 51% (soleus) and 30% (EDL) of control. In the soleus muscle, the changes in contractile properties were not related to ATP, creatine phosphate, or lactate content. The changes in contractile properties fit best with a mechanism of fatigue involving changes in Ca(2+) handling by the sarcoplasmic reticulum.

Phosphocreatine degradation in type I and type II muscle fibres during submaximal exercise in man: effect of carbohydrate ingestion

1. The aim of this study was to examine the effect of carbohydrate (CHO) ingestion on changes in ATP and phosphocreatine (PCr) concentrations in different muscle fibre types during prolonged running and relate those changes to the degree of glycogen depletion. 2. Five male subjects performed two runs at 70 % maximum oxygen uptake (.V(O2,max)), 1 week apart. Each subject ingested 8 ml (kg body mass (BM))(-1) of either a placebo (Con trial) or a 5.5 % CHO solution (CHO trial) immediately before each run and 2 ml (kg BM)(-1) every 20 min thereafter. In the Con trial, the subjects ran to exhaustion (97.0 +/- 6.7 min). In the CHO trial, the run was terminated at the time coinciding with exhaustion in the Con trial. Muscle samples were obtained from the vastus lateralis before and after each trial. 3. Carbohydrate ingestion did not affect ATP concentrations. However, it attenuated the decline in PCr concentration by 46 % in type I fibres (CHO: 20 +/- 8 mmol (kg dry matter (DM))(-1); Con: 34 +/- 6 mmol (kg DM)(-1); P < 0.05) and by 36 % in type II fibres (CHO: 30 +/- 5 mmol (kg DM)(-1); Con: 48 +/- 6 mmol (kg DM)(-1); P < 0.05). 4. A 56 % reduction in glycogen utilisation in type I fibres was observed in CHO compared with Con (117 +/- 39 vs. 240 +/- 32 mmol glucosyl units (kg DM)(-1), respectively; P < 0.01), but no difference was observed in type II fibres. 5. It is proposed that CHO ingestion during exhaustive running attenuates the decline in oxidative ATP resynthesis in type I fibres, as indicated by sparing of both PCr and glycogen breakdown. The CHO-induced sparing of PCr, but not glycogen, in type II fibres may reflect differential recruitment and/or role of PCr between fibre types.

Role of phosphate and calcium stores in muscle fatigue

Intensive activity of muscles causes a decline in performance, known as fatigue, that is thought to be caused by the effects of metabolic changes on either the contractile machinery or the activation processes. The concentration of inorganic phosphate (P(i)) in the myoplasm ([P(i)](myo)) increases substantially during fatigue and affects both the myofibrillar proteins and the activation processes. It is known that a failure of sarcoplasmic reticulum (SR) Ca(2+) release contributes to fatigue and in this review we consider how raised [P(i)](myo) contributes to this process. Initial evidence came from the observation that increasing [P(i)](myo) causes reduced SR Ca(2+) release in both skinned and intact fibres. In fatigued muscles the store of releasable Ca(2+) in the SR declines mirroring the decline in SR Ca(2+) release. In muscle fibres with inoperative creatine kinase the rise of [P(i)](myo) is absent during fatigue and the failure of SR Ca(2+) release is delayed. These results can all be explained if inorganic phosphate can move from the myoplasm into the SR during fatigue and cause precipitation of CaP(i) within the SR. The relevance of this mechanism in different types of fatigue in humans is considered.

Phosphate ion channels in sarcoplasmic reticulum of rabbit skeletal muscle

1. Phosphate ions (P(i)) enter intracellular Ca2+ stores and precipitate Ca2+. Since transport pathways for P(i) across the membrane of intracellular calcium stores have not been identified and anion channels could provide such a pathway, we have examined the P(i) conductance of single anion channels from the sarcoplasmic reticulum (SR) of rabbit skeletal muscle using the lipid bilayer technique. 2. Two anion channels in skeletal muscle SR, the small conductance (SCl) and big conductance (BCl) chloride channels, were both found to have a P(i) conductance of 10 pS in 50 mM P(i). The SCl channel is a divalent anion channel which can pass HPO4(2-) as well as SO4(2-) (60 pS in 100 mM free SO4(2-)). The BCl channel is primarily a monovalent anion channel. The SCl and BCl channels are permeable to a number of small monovalent anions, showing minor selectivity between Cl-, I- and Br- (Cl- > I- > Br-) and relative impermeability to cations and large polyatomic anions (Cs+, Na+, choline+, Tris+, Hepes- and CH3O3S-). 3. The P(i) conductance of SCl and BCl channels suggests that both channel types could sustain the observed P(i) fluxes across the SR membrane. Comparison of the blocking effects of the phosphonocarboxylic acids, ATP and DIDS, on the anion channels with their effects on P(i) transport suggests that the SCl channel is the more likely candidate for the SR P(i) transport mechanism. 4. The SCl channel, with previously unknown function, provides a regulated pathway for P(i) across the SR membrane which would promote P(i) entry and thereby changes in the rapidly releasable Ca2+ store during onset and recovery from muscle fatigue. Anion channels may provide a pathway for P(i) movement into and out of Ca2+ stores in general.

Mechanisms of reduced SR Ca(2+) release induced by inorganic phosphate in rat skeletal muscle fibers

The effects of inorganic phosphate (P(i)) on Ca(2+) release from the sarcoplasmic reticulum (SR) were studied in mechanically skinned rat skeletal muscle fibers. Application of caffeine or T-tubule depolarization was used to induce Ca(2+) release from the SR, which was detected using fura 2 fluorescence. Addition of P(i) (1-40 mM) caused a reversible and concentration-dependent reduction in the caffeine-induced Ca(2+) transient. This effect was apparent at low P(i) concentration (<5 mM), which did not result in detectable precipitation of calcium phosphate within the SR. The inhibitory effect of P(i) exhibited a marked dependence on free Mg(2+) concentration. At 0.5 mM free Mg(2+), 5 mM P(i) reduced the caffeine-induced transient by 25.1 +/- 4.1% (n = 13). However, at 1.5 mM free Mg(2+), 5 mM P(i) reduced the amplitude of caffeine-induced Ca(2+) transients by 68.9 +/- 3.1% (n = 10). Depolarization-induced SR Ca(2+) release was similarly affected. These effects of P(i) may be important in skeletal muscle fatigue, if an inhibitory action of P(i) on SR Ca(2+) release is augmented by the rise in cytosolic Mg(2+) concentration, which accompanies ATP breakdown.

Effects of membrane cholesterol manipulation on excitation-contraction coupling in skeletal muscle of the toad

1. Single mechanically skinned fibres and intact bundles of fibres from the twitch region of the iliofibularis muscle of cane toads were used to investigate the effects of membrane cholesterol manipulation on excitation-contraction (E-C) coupling. The cholesterol content of membranes was manipulated with methyl-beta-cyclodextrin (MbetaCD). 2. In mechanically skinned fibres, depletion of membrane cholesterol with MbetaCD caused a dose- and time-dependent decrease in transverse tubular (t)-system depolarization-induced force responses (TSDIFRs). TSDIFRs were completely abolished within 2 min in the presence of 10 mM MbetaCD but were not affected after 2 min in the presence of a 10 mM MbetaCD-1 mM cholesterol complex. There was a very steep dependence between the change in TSDIFRs and the MbetaCD : cholesterol ratio at 10 mM MbetaCD, indicating that the inhibitory effect of MbetaCD was due to membrane cholesterol depletion and not to a pharmacological effect of the agent. Tetanic responses in bundles of intact fibres were abolished after 3-4 h in the presence of 10 mM MbetaCD. 3. The duration of TSDIFRs increased markedly soon (< 2 min) after application of 10 mM MbetaCD and 10 mM MbetaCD-cholesterol complexes, but the Ca(2+) activation properties of the contractile apparatus were minimally affected by 10 mM MbetaCD. The Ca(2+) handling abilities of the sarcoplasmic reticulum appeared to be modified after 10 min exposure to 10 mM MbetaCD. 4. Confocal laser scanning microscopy revealed that the integrity of the t-system was not compromised by either intra- or extracellular application of 10 mM MbetaCD and that a large [Ca(2+)] gradient was maintained across the t-system. 5. Membrane cholesterol depletion caused rapid depolarization of the polarized t-system as shown independently by spontaneous TSDIFRs induced by MbetaCD and by changes in the fluorescence intensity of an anionic potentiometric dye (DiBAC(4)(3)) in the presence of MbetaCD. This rapid depolarization of the t-system by cholesterol depletion was not prevented by blocking the Na(+) channels with TTX (10 microM) or the L-type Ca(2+) channels with Co(2+) (5 mM). 6. The results demonstrate that cholesterol is important for maintaining the functional integrity of the t-system and sarcoplasmic reticulum, probably by having specific effects on different membrane proteins that may be directly or indirectly involved in E-C coupling.

The use of the indicator fluo-5N to measure sarcoplasmic reticulum calcium in single muscle fibres of the cane toad

1. Single fibres from the lumbrical muscles of the cane toad (Bufo marinus) were incubated in fluo-5N AM for 2 h at 35 degrees C in order to load the indicator into the sarcoplasmic reticulum. Fluo-5N is a low-affinity calcium indicator (K(Ca) 90 microM). Successful sarcoplasmic reticulum (SR) loading was indicated by a fluorescence signal that declined during contraction. 2. Confocal microscopy showed that the dye loaded principally in lines perpendicular to the long axis of the fibre that repeated each sarcomere. This is consistent with much of the dye residing in the SR. 3. To establish the site of loading, fibres were exposed to 30 mM caffeine in the presence of 20 microM 2,5-di(tert-butyl)1,4-hydroquinone (TBQ, an SR pump inhibitor) which should release most Ca(2+) from the SR; this procedure reduced the fluorescence to 46 +/- 4 % of the control value. To determine how much indicator was in the myoplasm, fibres were exposed to 100 microg ml(-1) saponin which permeabilizes the surface membrane; saponin treatment reduced the fluorescence to 51 +/- 2 % of the control value. 4. During maximally activated tetani (100 Hz stimulation rate, 22 degrees C) the component of signal from the SR declined by 33 +/- 4 %. During relaxation the SR signal recovered in two phases with time constants of 0.38 +/- 0.14 s and 10.1 +/- 1.7 s. Partially activated tetani (30 Hz stimulation rate) showed a smaller SR signal. Application of the SR Ca(2+) pump inhibitor TBQ slowed the rate of recovery of the SR signal. 5. Muscle fatigue was produced by repeated short tetani until tension was reduced to 50 %. The SR signal during the periods between tetani declined steadily and the SR Ca(2+) signal was eventually reduced to 71 +/- 8 % of the control signal. This signal recovered in two phases when the muscle was rested. An initial phase had a time constant of 1.7 +/- 0.2 s so that by 20 s of recovery the SR Ca(2+) signal was 86 +/- 7 % of control; the second phase was slower and by 5 min the SR Ca(2+) signal was back to control values (98 +/- 5 % control). In addition the magnitude of the SR signal decline associated with each tetanus (Delta[Ca(2+)](SR)) declined monotonically throughout fatigue and returned to control after 5 min recovery. 6. This approach can monitor the SR Ca(2+) concentration in normally functioning muscle fibres with good time resolution. The method confirms other approaches that show that the free Ca(2+) available for release in the SR declines during fatigue. This reduction in [Ca(2+)](SR) will contribute to the failure of Ca(2+) delivery to the myofilaments which is an important cause of muscle fatigue.

Inhibition of creatine kinase reduces the rate of fatigue-induced decrease in tetanic [Ca(2+)](i) in mouse skeletal muscle

1. Ca(2+)-phosphate (P(i)) precipitation in the sarcoplasmic reticulum (SR) may cause reduced SR Ca(2+) release in skeletal muscle fatigue. To study this, we inhibited the creatine kinase (CK) reaction with 2,4-dinitro-1-fluorobenzene (DNFB). The hypothesis was that with inhibition of CK, phosphocreatine would not break down to creatine and P(i). Therefore P(i) transport into the SR would be limited and Ca(2+)-P(i) precipitation would not occur. 2. Intact single fibres from a mouse foot muscle were fatigued by repeated short tetani under control conditions or after exposure to DNFB (10 microM). The free myoplasmic concentrations of Ca(2+) ([Ca(2+)](i)) and Mg(2+) ([Mg(2+)](i)) were measured with indo-1 and mag-indo-1, respectively. Changes in [Mg(2+)](i) were assumed to reflect alterations in myoplasmic ATP concentration. 3. During the first 10 fatiguing tetani, tetanic [Ca(2+)](i) increased both in control and after DNFB exposure. Thereafter tetanic [Ca(2+)](i) fell and the rate of fall was about fourfold lower after DNFB exposure compared with control. 4. Under control conditions, there was a good relationship between declining tetanic [Ca(2+)](i) and increasing [Mg(2+)](i) during the final part of fatiguing stimulation. This correlation was lost after DNFB exposure. 5. In conclusion, the present data fit with a model where Ca(2+)-P(i) precipitation inhibits SR Ca(2+) release in fatigue produced by repeated short tetani. Furthermore, the results suggest that the rate of P(i) transport into the SR critically depends on the myoplasmic Mg(2+)/ATP concentration.

Role of myoplasmic phosphate in contractile function of skeletal muscle: studies on creatine kinase-deficient mice

1. Increased myoplasmic inorganic phosphate (P(i)) has been suggested to have an important role in skeletal muscle fatigue, especially in the early phase. In the present study we used intact fast-twitch muscle cells from mice completely deficient in creatine kinase (CK(-/-)) to test this suggestion. These CK(-/-) muscle cells provide a good model since they display a higher P(i) concentration in the unfatigued state and fatigue without significant increase of P(i). 2. Tetanic contractions (350 ms duration) were produced in intact single muscle fibres. The free myoplasmic [Ca(2+)] ([Ca(2+)](i)) was measured with the fluorescent indicator indo-1. The force-[Ca(2+)](i) relationship was constructed from tetani at different frequencies. 3. Compared with wild-type fibres, CK(-/-) fibres displayed lower force in 100 Hz tetani and at saturating [Ca(2+)](i) (i.e. 100 Hz stimulation during caffeine exposure), higher tetanic [Ca(2+)](i) during the first 100 ms of tetanic stimulation, reduced myofibrillar Ca(2+) sensitivity when measurements were performed 100-200 ms into tetani, and slowed force relaxation that was due to altered cross-bridge kinetics rather than delayed Ca(2+) removal from the myoplasm. 4. In wild-type fibres, a series of 10 tetani resulted in reduced tetanic force, slowed force relaxation, and increased amplitude of [Ca(2+)](i) tails after tetani. None of these changes were observed in CK(-/-) fibres. 5. Complementary experiments on isolated fast-twitch extensor digitorum longus muscles showed a reduction of tetanic force and relaxation speed in CK(-/-) muscles similar to those observed in single fibres. 6. In conclusion, increased P(i) concentration can explain changes observed in the early phase of skeletal muscle fatigue. Increased P(i) appears to be involved in both fatigue-induced changes of cross-bridge function and SR Ca(2+) handling.

Effects of ADP on sarcoplasmic reticulum function in mechanically skinned skeletal muscle fibres of the rat

1. The sarcoplasmic reticulum (SR) Ca(2+) content (expressed in terms of endogenous SR Ca(2+) content under physiologically resting conditions and measured from caffeine-induced force responses) and the effective rates of the SR Ca(2+) pump and SR Ca(2+) leak (measured from the temporal changes in SR Ca(2+) content) were determined in mechanically skinned skeletal muscle fibres of the rat at different [ADP] (< 0.10 microM to 1.04 mM). 2. The estimated SR Ca(2+) pump rate at 200 nM Ca(2+) did not change when [ADP] increased from below 0.10 microM to 10 microM but decreased by about 30 % when [ADP] increased from 10 microM to 1.04 mM. 3. The rate constant of SR Ca(2+) leak increased markedly with rising [ADP] when [Ca(2+)] in solution was 200 nM (apparent dissociation constant Kd(ADP) = 64 +/- 27 microM). Decreasing the [Ca(2+)] in solution from 200 nM to < 10 nM significantly increased the leak rate constant at all [ADP]. The SR Ca(2+) leak rate constant could be significantly reduced by blocking the SR Ca(2+) pump with 2,5-di(tert-butyl)-1,4-hydroquinone (TBQ). 4. The decrease in the SR Ca(2+) pump rate and the increase in the rate constant of SR Ca(2+) leak when the [ADP] increased from < 0.10 microM to 1.04 mM caused a 4.4-fold decrease in SR Ca(2+) loading ability at 200 nM Ca(2+). 5. The results can be fully explained by a mechanism whereby the presence of ADP causes a marked increase in the ADP-sensitive fraction of the phosphorylated pump protein, which can act as a Ca(2+)-Ca(2+) exchanger and demonstrates that ADP is an important modulator of SR function in skeletal muscle.

Interdependent effects of inorganic phosphate and creatine phosphate on sarcoplasmic reticulum Ca2+ regulation in mechanically skinned rat skeletal muscle

1. The effects of creatine phosphate (CP) and inorganic phosphate (Pi) on sarcoplasmic reticulum (SR) Ca2+ regulation were investigated in mechanically skinned muscle fibres from rat extensor digitorum longus (EDL) muscles. Changes in [Ca2+] were detected using fura-2 fluorescence, during continuous perfusion or when the solution surrounding the preparation was restricted to approximately 6 microl by stopping perfusion. 2. In solutions with 5 mM ATP and 10 mM CP, stopping the flow for 2-3 min had no effect on [Ca2+] within the bath. This suggests that SR Ca2+ uptake is balanced by an efflux under these conditions. 3. In solutions with CP, the introduction of Pi induced a small transient rise in [Ca2+], due to Ca2+ loss from the SR. Following equilibration with solutions containing Pi (> or = 5 mM), a maintained decrease in [Ca2+] occurred when the flow was stopped. This is consistent with calcium phosphate (Ca-Pi) precipitation within the SR, resulting in maintained Ca2+ uptake. 4. In the absence of CP, the [Ca2+] within the bath increased progressively when the flow was stopped. This rise in [Ca2+] was inhibited by an alternative ATP regenerating system comprising phosphoenolpyruvate (PEP) and pyruvate kinase (PK). Therefore, the loss of Ca2+ from the SR may result from local ADP accumulation and the consequent reversal of the SR Ca2+ pump. 5. In the absence of CP, the initial Ca2+ release associated with the introduction of Pi increased markedly. Following prolonged equilibration with solutions containing Pi, a rise in [Ca2+] occurred within the bath when the flow was stopped. Maintained Ca2+ uptake associated with Ca-Pi precipitation was not apparent at any level of Pi tested (1-60 mM), when CP was absent. 6. These results suggest that withdrawal of CP is associated with activation of a SR Ca2+ efflux pathway. This may involve reversal of the SR Ca2+ pump, due to local ADP accumulation. In the absence of CP, the dominant influence of Pi appears to involve further Ca2+ efflux via the SR Ca2+ pump. The possible relevance of these effects to skeletal muscle fatigue is considered.

Different Ca2+ releasing action of caffeine and depolarisation in skeletal muscle fibres of the rat

1. The relative abilities of caffeine and transverse tubular (T-) system depolarisation to induce Ca2+ release in mammalian skeletal muscle were compared in mechanically skinned fibres of the rat, in order to determine whether normal excitation-contraction (E-C) coupling is achieved by up-regulating the Ca2+-induced Ca2+ release process, as caffeine is known to do. 2. Caffeine triggered Ca2+ release in soleus (slow-twitch) fibres at much lower concentrations than in extensor digitorum longus (EDL) (fast-twitch) fibres when the sarcoplasmic reticulum (SR) of each type was loaded with Ca2+ at close to endogenous levels. The difference in caffeine sensitivity resulted at least in part from the SR being loaded endogenously at near maximal capacity in soleus fibres but at less than half of maximal capacity in EDL fibres. The caffeine sensitivity could be reversed by reversing the relative level of SR loading. 3. The ability of caffeine to induce Ca2+ release was markedly reduced by lowering the level of SR loading or by raising the free [Mg2+] from 1 to 3 mM. Caffeine, even at 30 mM, triggered little or no Ca2+ release in EDL fibres (a) at 1 mM (physiological) Mg2+ when the SR was loaded at two-thirds or less of the endogenous level, and (b) at 3 mM Mg2+ when the SR was loaded at close to the endogenous level. In contrast, depolarisation potently elicited Ca2+ release under these conditions in the same fibres. 4. The inability of 30 mM caffeine to induce Ca2+ release under certain conditions was not attributable to desensitisation or inactivation of the release channels, because there was no response even upon initial exposure to caffeine and depolarisation always remained able to trigger Ca2+ release. It instead appeared that caffeine was a relatively ineffectual stimulus in EDL fibres except under conditions where (a) the SR was heavily loaded, (b) the free [Mg2+] was low, or (c) a high [Cl-] was present. 5. These results show that the normal E-C coupling mechanism in mammalian skeletal muscle does not involve just enhancing Ca2+-induced Ca2+ release, and evidently requires the removal or bypassing of the inhibitory effect of Mg2+ on the Ca2+ release channels.

Intracellular calcium during fatigue of cane toad skeletal muscle in the absence of glucose

Mechanisms of fatigue were studied in single muscle fibres of the cane toad (Bufo marinus) in which force, intracellular calcium ([Ca2+]i), [Mg2+]i, glycogen and the rapidly releasable Ca2+ from the sarcoplasmic reticulum (SR) were measured. Fatigue was produced by repeated tetani continued until force had fallen to 50%. Two patterns of fatigue in the absence of glucose were studied. In the first fatigue run force fell to 50% in 8-10 min. Fatigue runs were then repeated until force fell to 50% in < 3 min in the final fatigue run. Addition of extracellular glucose after the final fatigue run prolonged a subsequent fatigue run. In the first fatigue run peak tetanic [Ca2+]i initially increased and then declined and at the time when force had fallen to 50% tetanic [Ca2+]i was 54+/-5% of initial value. In the final fatigue run force and peak tetanic [Ca2+]i declined more rapidly but to the same level as in first fatigue runs. At the end of the first fatigue run, the rapidly releasable SR Ca2+ store fell to 46+/-6% of the pre-fatigue value. At the end of the final fatigue run the rapidly releasable SR Ca2+ store was 109+/-16% of the pre-fatigue value. In unstimulated fibres the nonwashable glycogen content was 176+/-30 mmol glycosyl units/l fibre. After one fatigue run the glycogen content was 117+/-17 mmol glycosyl units/l fibre; at the end of the final fatigue run the glycogen content was reduced to 85+/-9 mmol glycosyl units/l fibre. [Mg2+]i did not change significantly at the end of fatigue in either the first or the final fatigue run suggesting that globally-averaged ATP does not decline substantially in either pattern of fatigue. These results suggest that different mechanisms are involved in the decline of tetanic [Ca2+]i in first compared to final fatigue runs. The SR Ca2+ store is reduced in first fatigue runs; this is not the case for the final fatigue run which is associated with a decline in glycogen and possibly related to either a non-metabolic effect of glycogen or a spatially-localised metabolic decline.

Effects of fatigue on sarcoplasmic reticulum and myofibrillar properties of rat single muscle fibers

Force decline during fatigue in skeletal muscle is attributed mainly to progressive alterations of the intracellular milieu. Metabolite changes and the decline in free myoplasmic calcium influence the activation and contractile processes. This study was aimed at evaluating whether fatigue also causes persistent modifications of key myofibrillar and sarcoplasmic reticulum (SR) proteins that contribute to tension reduction. The presence of such modifications was investigated in chemically skinned fibers, a procedure that replaces the fatigued cytoplasm from the muscle fiber with a normal medium. Myofibrillar Ca(2+) sensitivity was reduced in slow-twitch muscle (for example, the pCa value corresponding to 50% of maximum tension was 6.23 +/- 0.03 vs. 5.99 + 0.05, P < 0.01, in rested and fatigued fibers) and not modified in fast-twitch muscle. Phosphorylation of the regulatory myosin light chain isoform increased in fast-twitch muscle. The rate of SR Ca(2+) uptake was increased in slow-twitch muscle fibers (14.2 +/- 1.0 vs. 19.6 +/- 2. 5 nmol. min(-1). mg fiber protein(-1), P < 0.05) and not altered in fast-twitch fibers. No persistent modifications of SR Ca(2+) release properties were found. These results indicate that persistent modifications of myofibrillar and SR properties contribute to fatigue-induced muscle force decline only in slow fibers. These alterations may be either enhanced or counteracted, in vivo, by the metabolic changes that normally occur during fatigue development.

P(i) inhibits the SR Ca(2+) pump and stimulates pump-mediated Ca(2+) leak in rabbit cardiac myocytes

Measurements of sarcoplasmic reticulum (SR) Ca(2+) uptake were made from aliquots of dissociated permeabilized ventricular myocytes using fura 2. Equilibration with 10 mM oxalate ensured a reproducible exponential decline of [Ca(2+)] from 600 nM to a steady state of 100-200 nM after addition of Ca(2+). In the presence of 5 microM ruthenium red, which blocks the ryanodine receptor, the time course of the decline of [Ca(2+)] can be modeled by a Ca(2+)-dependent uptake process and a fixed Ca(2+) leak. Partial inhibition of the Ca(2+) pump with 1 microM cyclopiazonic acid or 50 nM thapsigargin reduced the time constant for Ca(2+) uptake but did not affect the SR Ca(2+) leak. Addition of 10 mM inorganic phosphate (P(i)) decreased the rate of Ca(2+) accumulation by the SR and increased the Ca(2+) leak rate. This effect was reversed on addition of 10 mM phosphocreatine. 10 mM P(i) had no effect on Ca(2+) leak from the SR after complete inhibition of the Ca(2+) pump. In conclusion, P(i) decreases the Ca(2+) uptake capacity of cardiac SR via a decrease in pump rate and an increase in Ca(2+) pump-dependent Ca(2+) leak.

Sodium phosphate enhances plasmid DNA expression in vivo

Intramuscular injection of plasmid DNA results in myofiber cell expression of proteins encoded by the DNA. The preferred vehicle for plasmid DNA injections has been saline (154 mM sodium chloride) or PBS (154 mM NaCl plus 10 mM sodium phosphate). Here, it is shown that injection of luciferase or beta-galactosidase encoding plasmid DNA in a 150 mM sodium phosphate vehicle into murine muscle resulted in a two- to seven-fold increase in transgene expression compared with DNA injected in saline or PBS. When the DNA encoded secreted alkaline phosphatase, preproinsulin or interferon, sodium phosphate vehicle increased their serum levels by two- to four-fold. When the DNA encoded mouse erythropoietin, sodium phosphate vehicle increased hematocrits by two-fold compared with DNA injected in saline. When the DNA encoded influenza nucleoprotein, sodium phosphate increased anti-nucleoprotein antibody titers by two-fold. The expression of luciferase from plasmid DNA instilled into lung was increased five-fold compared with that in vehicle without sodium phosphate. Incubation of plasmid DNA with muscle extract or serum showed that sodium phosphate protected the DNA from degradation. Thus, a change from sodium chloride to sodium phosphate vehicle can enhance the expression of plasmid DNA in a tissue, possibly by inhibiting DNA degradation. Gene Therapy (2000) 7, 1171-1182.

Is creatine kinase responsible for fatigue? Studies of isolated skeletal muscle deficient in creatine kinase

Creatine kinase (CK) is a key enzyme for maintaining a constant ATP/ADP ratio during rapid energy turnover. To investigate the role of CK in skeletal muscle fatigue, we used isolated whole muscles and intact single fibers from CK-deficient mice (CK(-/-)). With high-intensity electrical stimulation, single fibers from CK(-/-) mice displayed a transient decrease in both tetanic free myoplasmic [Ca(2+)] ([Ca(2+)](i), measured with the fluorescent dye indo-1) and force that was not observed in wild-type fibers. With less intense, repeated tetanic stimulation single fibers and EDL muscles, both of which are fast-twitch, fatigued more slowly in CK(-/-) than in wild-type mice; on the other hand, the slow-twitch soleus muscle fatigued more rapidly in CK(-/-) mice. In single wild-type fibers, tetanic force decreased and [Ca(2+)](i) increased during the first 10 fatiguing tetani, but this was not observed in CK(-/-) fibers. Fatigue was not accompanied by phosphocreatine breakdown and accumulation of inorganic phosphate in CK(-/-) muscles. In conclusion, CK is important for avoiding fatigue at the onset of high-intensity stimulation. However, during more prolonged stimulation, CK may contribute to the fatigue process by increasing the myoplasmic concentration of inorganic phosphate.

Impairment of Ca(2+) release in single Xenopus muscle fibers fatigued at varied extracellular PO(2)

We tested the hypothesis that the mechanisms involved in the more rapid onset of fatigue when O(2) availability is reduced in contracting skeletal muscle are similar to those when O(2) availability is more sufficient. Two series of experiments were performed in isolated, single skeletal muscle fibers from Xenopus laevis. First, relative force and free cytosolic Ca(2+) concentrations ([Ca(2+)](c)) were measured simultaneously in single fibers (n = 6) stimulated at increasing frequencies (0.25, 0.33, 0.5, and 1 Hz) at an extracellular PO(2) of either 22 or 159 Torr. Muscle fatigue (force = 50% of initial peak tension) occurred significantly sooner (P < 0.05) during the low- (237 +/- 40 s) vs. high-PO(2) treatments (280 +/- 38 s). Relative [Ca(2+)](c) was significantly decreased from maximal values at the fatigue time point during both the high- (72 +/- 4%) and low-PO(2) conditions (78 +/- 4%), but no significant difference was observed between the treatments. In the second series of experiments, using the same stimulation regime as the first, fibers (n = 6) exposed to 5 mM caffeine immediately after fatigue demonstrated an immediate but incomplete relative force recovery during both the low- (89 +/- 4%) and high-PO(2) treatments (82 +/- 3%), with no significant difference between treatments. Additionally, there was no significant difference in relative [Ca(2+)](c) between the high- (100 +/- 12% of prefatigue values) and low-PO(2) treatments (108 +/- 12%) on application of caffeine. These results suggest that in isolated, single skeletal muscle fibers, the earlier onset of fatigue that occurred during the low-extracellular PO(2) condition was modulated through similar pathways as the fatigue process during the high and involved a decrease in relative peak [Ca(2+)](c).

Mechanisms of P(i) regulation of the skeletal muscle SR Ca(2+) release channel

Inorganic phosphate (P(i)) accumulates in the fibers of actively working muscle where it acts at various sites to modulate contraction. To characterize the role of P(i) as a regulator of the sarcoplasmic reticulum (SR) calcium (Ca(2+)) release channel, we examined the action of P(i) on purified SR Ca(2+) release channels, isolated SR vesicles, and skinned skeletal muscle fibers. In single channel studies, addition of P(i) to the cis chamber increased single channel open probability (P(o); 0.079 +/- 0.020 in 0 P(i), 0. 157 +/- 0.034 in 20 mM P(i)) by decreasing mean channel closed time; mean channel open times were unaffected. In contrast, the ATP analog, beta,gamma-methyleneadenosine 5'-triphosphate (AMP-PCP), enhanced P(o) by increasing single channel open time and decreasing channel closed time. P(i) stimulation of [(3)H]ryanodine binding by SR vesicles was similar at all concentrations of AMP-PCP, suggesting P(i) and adenine nucleotides act via independent sites. In skinned muscle fibers, 40 mM P(i) enhanced Ca(2+)-induced Ca(2+) release, suggesting an in situ stimulation of the release channel by high concentrations of P(i). Our results support the hypothesis that P(i) may be an important endogenous modulator of the skeletal muscle SR Ca(2+) release channel under fatiguing conditions in vivo, acting via a mechanism distinct from adenine nucleotides.

Phosphorylating pathways and fatigue development in contracting Xenopus single skeletal muscle fibers

To investigate the differential contribution of oxidative and substrate-level phosphorylation to force production during repetitive, maximal tetanic contractions, single skeletal muscle fiber performance was examined under conditions of high-oxygen availability and anoxia. Tetanic force development (P) was measured in isolated, single type-1 muscle fibers (fast twitch; n = 6) dissected from Xenopus lumbrical muscle while being stimulated at increasing frequencies (0.25, 0.33, and 0.5 Hz), with each frequency lasting 2 min. Two separate work bouts were conducted, with the perfusate PO(2) being either 0 or 159 mmHg. No significant (P < 0. 05) difference was found in the initial peak tensions (P(0)) between the high (334 +/- 57 kPa) and the low (325 +/- 41 kPa) PO(2) treatment. No significant difference in P was observed between the treatments during the first 50 s. However, a significant difference in force production was observed between the high (P/P(0) = 0.96 +/- 0.02) and the low PO(2) condition (P/P(0) = 0.92 +/- 0.02) by 60 s of work. After 60 s, steady-state force production was maintained during the high compared with the low PO(2) condition until stimulation frequency was increased, at which point developed tension during the high PO(2) condition began to decline. Time to fatigue (P/P(0) = 0.3) was reached significantly sooner during the low (250 +/- 16 s) than the high PO(2) condition (367 +/- 28 s). These results demonstrate that during the first 50 s of 0.25-Hz contractions, substrate-level phosphorylation has the capacity to maintain force and ATP hydrolysis when oxidative phosphorylation is absent. This period was followed by an oxygen-dependent phase in which force generation was maintained during the high PO(2) condition (but not during the low PO(2) condition) until the onset of a final fatiguing phase, at which a calculated maximal rate of oxidative phosphorylation was reached.

Effect of lactate on depolarization-induced Ca(2+) release in mechanically skinned skeletal muscle fibers

It is unclear whether accumulation of lactate in skeletal muscle fibers during intense activity contributes to muscle fatigue. Using mechanically skinned fibers from rat and toad muscle, we were able to examine the effect of L(+)-lactate on excitation-contraction coupling independently of other metabolic changes. We investigated the effects of lactate on the contractile apparatus, caffeine-induced Ca(2+) release from the sarcoplasmic reticulum, and depolarization-induced Ca(2+) release. Lactate (15 or 30 mM) had only a small inhibitory effect directly on the contractile apparatus and caused appreciable (20-35%) inhibition of caffeine-induced Ca(2+) release, seemingly by a direct effect on the Ca(2+) release channels. However, 15 mM lactate had no detectable effect on Ca(2+) release when it was triggered by the normal voltage sensor mechanism, and 30 mM lactate reduced such release by only <10%. These results indicate that lactate has only a relatively small inhibitory effect on normal excitation-contraction coupling, indicating that lactate accumulation per se is not a major factor in muscle fatigue.

Characteristics of phosphate-induced Ca(2+) efflux from the SR in mechanically skinned rat skeletal muscle fibers

The effects of P(i) on sarcoplasmic reticulum (SR) Ca(2+) regulation were studied in mechanically skinned rat skeletal muscle fibers. Brief application of caffeine was used to assess the SR Ca(2+) content, and changes in concentration of Ca(2+) ([Ca(2+)]) within the cytosol were detected with fura 2 fluorescence. Introduction of P(i) (1-40 mM) induced a concentration-dependent Ca(2+) efflux from the SR. In solutions lacking creatine phosphate (CP), the amplitude of the P(i)-induced Ca(2+) transient approximately doubled. A similar potentiation of P(i)-induced Ca(2+) release occurred after inhibition of creatine kinase (CK) with 2,4-dinitrofluorobenzene. In the presence of ruthenium red or ryanodine, caffeine-induced Ca(2+) release was almost abolished, whereas P(i)-induced Ca(2+) release was unaffected. However, introduction of the SR Ca(2+) ATPase inhibitor cyclopiazonic acid effectively abolished P(i)-induced Ca(2+) release. These data suggest that P(i) induces Ca(2+) release from the SR by reversal of the SR Ca(2+) pump but not via the SR Ca(2+) channel under these conditions. If this occurs in intact skeletal muscle during fatigue, activation of a Ca(2+) efflux pathway by P(i) may contribute to the reported decrease in net Ca(2+) uptake and increase in resting [Ca(2+)].

The role of calcium stores in fatigue of isolated single muscle fibres from the cane toad

1. Intracellular calcium ([Ca2+]i) and tension were measured from single muscle fibres dissected from the cane toad (Bufo marinus). The amount of Ca2+ which could be released from the sarcoplasmic reticulum (SR) was estimated by brief (approximately 20 s) exposures to 4-chloro-m-cresol (4-CmC) or caffeine. 2. Muscle fatigue was produced by repeated tetani at 4 s or shorter intervals and continued until tension had fallen to 50% of the control. The intracellular free calcium concentration during a tetanus (tetanic [Ca2+]i) first increased and then steadily declined to 43+/-2% of control by the time tension had fallen to 50%. Over the period of fatigue the rapidly releasable Ca2+ from the SR fell to 46+/-6% of control. Tension and tetanic [Ca2+]i recovered to 93+/-3% and 100+/-4% of the control values after 20 min of rest. Over the same period rapidly releasable SR Ca2+ recovered to 98+/-12%. 3. When a similar number of tetani (200) were repeated at longer intervals (10 s), fibres showed only a small reduction in tension (to 85+/-1%) and tetanic [Ca2+]i did not change significantly. Under these conditions the rapidly releasable SR Ca2+ did not change significantly. 4. The recovery of rapidly releasable SR Ca2+ after fatigue was unaffected by removal of extracellular calcium but did not occur when oxidative phosphorylation was inhibited with cyanide. 5. These results suggest that an important cause of the decline of tetanic [Ca2+]i during fatigue is an equivalent decline in the amount of rapidly releasable SR Ca2+. The results show that the decline of rapidly releasable SR Ca2+ is related to a metabolic consequence of fatigue and are consistent with the hypothesis that Ca2+ precipitates with phosphate in the SR during fatigue.

Glycogen content and excitation-contraction coupling in mechanically skinned muscle fibres of the cane toad

1. Mechanically skinned skeletal muscle fibres from the twitch region of the iliofibularis muscle of cane toads were used to investigate the relationship between fibre glycogen content and fibre capacity to respond to transverse tubular (T-) system depolarization. 2. A large proportion of total fibre glycogen remained in mechanically skinned muscle fibres exposed to aqueous solutions. This glycogen pool (about 80% of total fibre glycogen) was very stable when the preparation was incubated in a rigor solution (pH 7.0) but decreased gradually at a rate of 0.59+/-0.20% min-1 in a relaxing solution (200 nM [Ca2+]). The rate was considerably higher (2.66+/-0.38% min(-1)) when the preparations were exposed to 30 microM [Ca2+]. An even greater rate of glycogen loss was found after T-system depolarization-induced contractions. The Ca2+-dependent loss of fibre glycogen was caused by endogenous glycogenolytic processes. 3. Silver stained SDS gels of components eluted into relaxing solution from single skinned fibres revealed a rapid (2 min) loss of parvalbumin and at least 10 other proteins varying in molecular mass between 10 and 80 kDa but there was essentially no loss of myosin heavy and light chains and actin. Subsequent elution for a further 30 min in either relaxing or maximally Ca2+-activating solution did not result in additional, appreciable detectable loss of fibre protein. 4. Depletion of fibre glycogen was associated with loss of fibre ability to respond to T-system depolarization even though the bathing solutions contained high levels of ATP (8 mM) and creatine phosphate (10 mM). 5. The capacity of mechanically skinned fibres to respond to T-system depolarization was highly positively correlated (P<0.0001) with initial fibre glycogen concentration. 6. In conclusion, the results show that (i) the capacity of skeletal muscle to respond to T-system depolarization is related directly or indirectly to the non-washable glycogen pool in fibres, (ii) this relationship holds for conditions where glycogen is not required as a source of energy and (iii) the mechanically skinned fibre preparation is well suited to study the regulation of endogenous glycogenolytic enzymes.

Influence of inorganic phosphate and pH on sarcoplasmic reticular ATPase in skinned muscle fibres of Xenopus laevis

1. The influence of 30 mM inorganic phosphate (Pi) and pH (6.2-7.4) on the rate of ATP utilization was determined in mechanically skinned bundles of myofibrils from the iliofibularis muscle of Xenopus laevis at approximately 5 C. 2. BDM (2,3-butanedione monoxime; 10 mM) depressed isometric force production and actomyosin (AM) ATPase activity equally. Therefore sarcoplasmic reticular (SR) ATPase activity could be determined by extrapolation of the total ATPase activity to zero force. 3. The SR ATPase activity without added Pi at pH 7.1 was 42 +/- 2 % of the total ATPase activity. Addition of 30 mM Pi reduced SR ATPase activity slightly, by 9 +/- 5 %, and depressed force by 62 +/- 2 % and AM ATPase activity by 21 +/- 6 %. 4. At pH 6.2, force, SR ATPase activity and AM ATPase activity were reduced by 21 +/- 5, 61 +/- 5 and 10 +/- 4 % of their respective values at pH 7.1. 5. The SR ATPase activity at 30 mM Pi and pH 6.2 was reduced markedly to 20 +/- 6 % of the value under control conditions, suggesting that the maximum rate of Ca2+ uptake during muscle fatigue was strongly depressed. This reduction was larger than expected on the basis of the effects of Pi and pH alone.

Effects of creatine phosphate on Ca2+ regulation by the sarcoplasmic reticulum in mechanically skinned rat skeletal muscle fibres

1. The effect of creatine phosphate (PCr) on sarcoplasmic reticulum (SR) Ca2+ regulation was studied in mechanically skinned skeletal muscle fibres from rat extensor digitorium longus (EDL). Preparations were perfused with solutions mimicking the intracellular milieu and the [Ca2+] within the muscle was monitored continuously using fura-2. 2. Brief application of 40 mM caffeine caused a transient increase in [Ca2+] due to SR Ca2+ release, and an associated tension response. Withdrawal of PCr resulted in (i) a slow transient release of Ca2+ from the SR (ii) a marked prolongation of the descending phase of the caffeine-induced fluorescence ratio transient and (iii) a decrease in the Ca2+ transient amplitude to 69.2 +/- 2.7 % (n = 16) of control responses. 3. Prolongation of the caffeine-induced Ca2+ transient also occurred following application of the SR Ca2+ pump inhibitor cyclopiazonic acid (CPA). This suggests that (i) the descending phase of the caffeine-induced Ca2+ transient is dependent on the rate of Ca2+ uptake by the SR and (ii) prolongation associated with PCr withdrawal may also reflect a decrease in the net Ca2+ uptake rate. 4. The effects of PCr withdrawal were mimicked by addition of the creatine kinase (CK) inhibitor 2,4-dinitro-1-fluorobenzene (DNFB). Hence, reducing the [PCr] may influence SR Ca2+ regulation by limiting local ATP regeneration by endogenous CK. After treatment with DNFB, PCr withdrawal had no effect on the Ca2+ transient, confirming that PCr does not have an additional direct effect on the SR. 5. The Ca2+ efflux associated with PCr withdrawal was insensitive to ryanodine or Ruthenium Red, but was effectively abolished by pretreatment with the SR Ca2+ pump inhibitor cyclopiazonic acid (CPA). This suggests that the Ca2+ efflux associated with PCr withdrawal is independent of the SR Ca2+ channel, but may involve reversal or inhibition of the Ca2+ ATPase. 6. These data suggest that Ca2+ regulation by the SR is strongly dependent on the supply of ATP via endogenous CK. Depletion of PCr may contribute to impaired SR Ca2+ regulation known to occur in intact skeletal muscle under conditions of fatigue.

The contribution of pH-dependent mechanisms to fatigue at different intensities in mammalian single muscle fibres

1. The contribution of intracellular pH (pHi) to the failure of Ca2+ release and inhibition of contractile proteins observed during fatigue was assessed in single intact mouse muscle fibres at 22 C. Fatigue was induced by repeated tetani at intensities designed to induce different levels of intracellular acidosis. Force and either intracellular free Ca2+ concentration ([Ca2+]i; measured using indo-1) or pHi (measured using SNARF-1) were recorded in fibres fatigued at two different intensities. 2. Intensity was varied by the repetition rate of tetani and quantified by the duty cycle (the fraction of time when the muscle was tetanized). Stimulation at the low intensity (duty cycle approximately 0.1) reduced force to 30 % of initial values in 206 +/- 21 s (60 +/- 7 tetani); at the high intensity (duty cycle approximately 0.3) force was reduced to 30% in 42 +/- 7 s (43 +/- 7 tetani) (P < 0.05; n = 14). 3. When force was reduced to 30 % of initial values, tetanic [Ca2+]i had fallen from 648 +/- 87 to 336 +/- 64 nM (48% decrease) at the low intensity but had only fallen from 722 +/- 84 to 468 +/- 60 nM (35% decrease) at the higher intensity (P < 0.05 low vs. high intensity; n = 7). 4. Fatigue resulted in reductions in Ca2+ sensitivity of the contractile proteins which were greater at the high intensity (pre-fatigue [Ca2+]i required for 50 % of maximum force (Ca50) = 354 +/- 23 nM; post-fatigue Ca50 = 421 +/- 48 nM and 524 +/- 43 nM for low and high intensities, respectively). Reductions in maximum Ca2+-activated force (Fmax) were similar at the two intensities (pre-fatigue Fmax = 328 +/- 22 microN; post-fatigue Fmax = 271 +/- 20 and 265 +/- 19 microN for low and high intensities, respectively). 5. Resting pHi was 7.15 +/- 0.05. During fatigue at the low intensity, pHi was reduced by 0.12 +/- 0.02 pH units and at the high intensity pHi was reduced by 0.34 +/- 0.07 pH units (P < 0.05; n = 5). 6. Our results indicate that the more rapid fall in force at a high intensity is due to a reduction in Ca2+ sensitivity of the contractile proteins, probably related to the greater acidosis. Our data also indicate that the failure of Ca2+ release and reduced maximum Ca2+-activated force observed during fatigue are not due to reductions in intracellular pH.

Mechanisms underlying phosphate-induced failure of Ca2+ release in single skinned skeletal muscle fibres of the rat

1. Single mechanically skinned fibres from rat extensor digitorum longus (EDL) muscles were used to investigate the mechanisms underlying inorganic phosphate (Pi) movements between the myoplasm and the sarcoplasmic reticulum (SR). Force transients elicited by caffeine/low Mg2+ application were used to assess the rate of Pi-induced inhibition of SR Ca2+ release and the subsequent recovery of Ca2+ release following removal of myoplasmic Pi. 2. Myoplasmic Pi reduced SR Ca2+ release in a concentration- and time-dependent manner. A 10 s exposure to 10, 20 and 50 mM myoplasmic Pi reduced SR Ca2+ release by 12 +/- 9, 29 +/- 5 and 82 +/- 5 %, respectively. 3. Removal of myoplasmic ATP at the time of Pi exposure significantly increased the rate and extent of SR Ca2+ release inhibition. For example, Ca2+ release was reduced by 86 +/- 6 % (n = 6) after 20 s exposure to 20 mM Pi in the absence of ATP compared with only 47 +/- 5 % (n = 5) in the presence of ATP. 4. The half and full recovery times for SR Ca2+ release following washout of myoplasmic Pi were 35 s and approximately 7 min, respectively. Recovery of Ca2+ release was unaffected by the absence of ATP during washout of Pi but was prevented when fibres were washed in the presence of high myoplasmic Pi (30 mM). Neither the Pi transporter blocker phenylphosphonic acid (PHPA) nor the anion channel blockers anthracene-9-carboxylic acid (9-AC) and 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) affected the rate of recovery of SR Ca2+ release. 5. These results show that Pi entry and exit from the SR occur primarily through a passive pathway that is insensitive to well-known anion channel blockers. Pi inhibition of SR Ca2+ release appears to be a complicated phenomenon influenced by the rate of Pi movement across the SR as well as by the rate, extent and species of Ca2+-Pi precipitate formation in the SR lumen. The more rapid inhibitory effect of Pi in the absence of myoplasmic ATP suggests that Pi may inhibit SR Ca2+ release more efficiently during the later stages of fatigue.

Mechanisms underlying the slow recovery of force after fatigue: importance of intracellular calcium

Recovery of force production after an intense bout of activity may sometimes take several days, especially at low activation frequencies ('low frequency fatigue'). This slow recovery can also be observed in isolated muscle and single muscle fibres. The origin of the force deficit is failure of excitation-contraction coupling at the level of the triads. The most likely cause of the failure is an elevated intracellular Ca2+ level, but the site of action of Ca2+ is unclear. Available evidence does not support the involvement of Ca2+-activated proteases. Ca2+-induced damage to mitochondria or swelling of t-tubules do not seem to be causative factors. Other mechanisms are discussed, including possible detrimental effects of Ca2+-activated lipases, calmodulin, and reactive oxygen species.

Events of the excitation-contraction-relaxation (E-C-R) cycle in fast- and slow-twitch mammalian muscle fibres relevant to muscle fatigue

The excitation-contraction-relaxation cycle (E-C-R) in the mammalian twitch muscle comprises the following major events: (1) initiation and propagation of an action potential along the sarcolemma and transverse (T)-tubular system; (2) detection of the T-system depolarization signal and signal transmission from the T-tubule to the sarcoplasmic reticulum (SR) membrane; (3) Ca2+ release from the SR; (4) transient rise of myoplasmic [Ca2+]; (5) transient activation of the Ca2+-regulatory system and of the contractile apparatus; (6) Ca2+ reuptake by the SR Ca2+ pump and Ca2+ binding to myoplasmic sites. There are many steps in the E-C-R cycle which can be seen as potential sites for muscle fatigue and this review explores how structural and functional differences between the fast- and slow-twitch fibres with respect to the E-C-R cycle events can explain to a great extent differences in their fatiguability profiles.

Force generation and phosphate release steps in skinned rabbit soleus slow-twitch muscle fibers

The force-generation and phosphate-release steps of the cross-bridge cycle in rabbit soleus slow-twitch muscle fibers (STF) were investigated using sinusoidal analysis, and the results were compared with those of rabbit psoas fast-twitch fibers (FTF). Single fiber preparations were activated at pCa 4.40 and ionic strength 180 mM at 20 degrees C. The effects of inorganic phosphate (Pi) concentrations on three exponential processes, B, C, and D, were studied. Results are consistent with the following cross-bridge scheme: [formula: see text] where A is actin, M is myosin, D is MgADP, and P is inorganic phosphate. The values determined are k4 = 5.7 +/- 0.5 s-1 (rate constant of isomerization step, N = 9, mean +/- SE), k-4 = 4.5 +/- 0.5 s-1 (rate constant of reverse isomerization), K4 = 1.37 +/- 0.13 (equilibrium constant of the isomerization), and K5 = 0.18 +/- 0.01 mM-1 (Pi association constant). The isomerization step (k4) in soleus STF is 20 times slower, and its reversal (k-4) is 20 times slower than psoas fibers. Consequently, the equilibrium constant of the isomerization step (K4) is the same in these two types of fibers. The Pi association constant (K5) is slightly higher in STF than in FTF, indicating that Pi binds to cross-bridges slightly more tightly in STF than FTF. By correlating the cross-bridge distribution with isometric tension, it was confirmed that force is generated during the isomerization (step 4) of the AMDP state and before Pi release in soleus STF.

Impaired calcium pump function does not slow relaxation in human skeletal muscle after prolonged exercise

This study examined the effects of prolonged exercise on human quadriceps muscle contractile function and homogenate sarcoplasmic reticulum Ca2+ uptake and Ca2+-adenosinetriphosphatase activity. Ten untrained men cycled at 75 +/- 2% (SE) peak oxygen consumption until exhaustion. Biopsies were taken from the right vastus lateralis muscle at rest, exhaustion, and 20 and 60 min postexercise. Peak tension and half relaxation time of the left quadriceps muscle were measured during electrically evoked twitch and tetanic contractions and a maximal voluntary isometric contraction at rest, exhaustion, and 10, 20, and 60 min postexercise. At exhaustion, homogenate Ca2+ uptake and Ca2+ adenosinetriphosphatase activity were reduced by 17 +/- 4 and 21 +/- 5%, respectively, and remained depressed after 60 min recovery (P </= 0.01). Muscle ATP, creatine phosphate, and glycogen were all depressed at exhaustion (P </= 0.01). Peak tension during a maximal voluntary contraction, a twitch, and a 10-Hz stimulation were reduced after exercise by 28 +/- 3, 45 +/- 6, 65 +/- 5%, respectively (P </= 0.01), but no slowing of half relaxation times were found. Thus fatigue induced by prolonged exercise reduced muscle Ca2+ uptake, but this did not cause a slower relaxation of evoked contractions.

Phosphocreatine content in single fibers of human muscle after sustained submaximal exercise

The effect of sustained submaximal exercise on muscle energetics has been studied on the single-fiber level in human skeletal muscle. Seven subjects cycled to fatigue (mean 77 min) at a work rate corresponding to approximately 75% of maximal O2 uptake. Biopsies were taken from the vastus lateralis muscle at rest, at fatigue, and after 5 min of recovery. Muscle glycogen decreased from 444 +/- 40 (SE) mmol glucosyl units/kg dry wt at rest to 94 +/- 16. Postexercise glycogen was inversely correlated (P < 0.01) to muscle content of inosine monophosphate, a catabolite of ATP. Phosphocreatine (PCr) in mixed-fiber muscle decreased at fatigue to 37% but was restored above the initial value (106.5%, P < 0.025) after 5 min of recovery. The overshoot was localized to type I fibers. The rapid reversal of PCr is in contrast to the slow recovery in contraction force. Pi increased at fatigue but less than that expected from the changes in PCr and other phosphate compounds. Mean PCr at rest was approximately 20% higher in type II than in type I fibers (86.4 +/- 3.6 and 71.6 +/- 1.8 mmol/kg dry wt, respectively, P < 0.05), but at fatigue similar PCr contents were observed in the two fiber types. Reduction in PCr in all fibers at fatigue suggests that all fibers were recruited at the end of exercise. PCr content in single fibers showed a great variability in samples at rest, exercise, and recovery. The variability was more pronounced than for ATP, and the data suggest that it is due to interfiber physiological-biochemical differences. At fatigue ATP was maintained relatively high in all single fibers, but a pronounced depletion of PCr was observed in a large number of fibers, and this may contribute to fatigue through the associated increases in Pi or/and free ADP. It is noteworthy that the increase in calculated free ADP at fatigue was similar to that after high-intensity exercise.

Phosphate transport into the sarcoplasmic reticulum of skinned fibres from rat skeletal muscle

The rate, magnitude and pharmacology of inorganic phosphate (Pi) transport into the sarcoplasmic reticulum were estimated in single, mechanically skinned skeletal muscle fibres of the rat. This was done, indirectly, by using a technique that measured the total Ca2+ content of the sarcoplasmic reticulum and by taking advantage of the 1:1 stoichiometry of Ca2+ and Pi transport into the sarcoplasmic reticulum lumen during Ca-Pi precipitation-induced Ca2+ loading. The apparent rate of Pi entry into the sarcoplasmic reticulum increased with increasing myoplasmic [Pi] in the 10 mM-50 mM range at a fixed, resting myoplasmic pCa of 7.15, as judged by the increase in the rate of Ca-Pi precipitation-induced sarcoplasmic reticulum Ca2+ uptake. At 20 mM myoplasmic [Pi] the rate of Pi entry was calculated to be at least 51 microM s-1 while the amount of Pi loaded appeared to saturate at around 3.5 mM (per fibre volume). These values are approximations due to the complex kinetics of formation of different species of Ca-Pi precipitate formed under physiological conditions. Phenylphosphonic acid (PhPA, 2.5 mM) inhibited Pi transport by 37% at myoplasmic pCa 6.5 and also had a small, direct inhibitory effect on the sarcoplasmic reticulum Ca2+ pump (16%). In contrast, phosphonoformic acid (PFA, 1 mM) appeared to enhance both the degree of Pi entry and the activity of the sarcoplasmic reticulum Ca2+ pump, results that were attributed to transport of PFA into the sarcoplasmic reticulum lumen and its subsequent complexation with Ca2+. Thus, results from these studies indicate the presence of a Pi transporter in the sarcoplasmic reticulum membrane of mammalian skeletal muscle fibres that is (1) active at physiological concentrations of myoplasmic Pi and Ca2+ and (2) partially inhibited by PhPA. This Pi transporter represents a link between changes in myoplasmic [Pi] and subsequent changes in sarcoplasmic reticulum luminal [Pi]. It might therefore play a role in the delayed metabolic impairment of sarcoplasmic reticulum Ca2+ release seen during muscle fatigue, which should occur abruptly once the Ca-Pi solubility product is exceeded in the sarcoplasmic reticulum lumen.

Increased tetanic force and reduced myoplasmic [P(i)] following a brief series of tetani in mouse soleus muscle

Muscle performance is improved after a brief period of exercise (warm-up). One factor that is known to strongly affect force production is the myoplasmic concentration of inorganic phosphate ([P(i)]). Improved performance after warm-up may therefore be due to a reduction of [P(i)]. Herein, we show that after a warm-up protocol (15 tetani at 2-s intervals), tetanic force is increased by approximately 6% (P < 0.05) and [P(i)] is almost halved (P < 0.05) in isolated mouse soleus muscle. A warm-up protocol with longer intervals (15 tetani at 5-s intervals) reduced tetanic force and did not alter [P(i)]. We conclude that a reduction of [P(i)] contributes to the force-potentiating effect of warm-up.

Role of intracellular calcium and metabolites in low-frequency fatigue of mouse skeletal muscle

We have examined the extent to which prolonged reductions in low-frequency force (i.e., low-frequency fatigue) result from increases in intracellular free Ca2+ concentration ([Ca2+]i) and alterations in muscle metabolites. Force and [Ca2+]i were measured in mammalian single muscle fibers in response to short, intermediate, and long series of tetani that elevated the [Ca2+]i-time integral to 5, 17, and 29 microM x s, respectively. Only the intermediate and long series resulted in prolonged (>60 x min) reductions in Ca2+ release and low-frequency fatigue. When fibers recovered from the long series of tetani without glucose, Ca2+ release was reduced to a greater extent and force was reduced at high and low frequencies. These findings indicate that the decrease in sarcoplasmic reticulum Ca2+ release associated with fatigue has at least two components: 1) a metabolic component, which, in the presence of glucose, recovers within 1 h, and 2) a component dependent on the elevation of the [Ca2+]i-time integral, which recovers more slowly. It is this Ca2+-dependent component that is primarily responsible for low-frequency fatigue.