Functional significance of Ca2+ in long-lasting fatigue of skeletal muscle

Repeated activation of skeletal muscle causes fatigue, which involves a reduced ability to produce force and slowed contraction regarding both the speed of shortening and relaxation. One important component in skeletal muscle fatigue is a reduced sarcoplasmic reticulum (SR) Ca2+ release. In the present review we will describe different types of fatigue-induced inhibition of SR Ca2+ release. We will focus on a type of long-lasting failure of SR Ca2+ release which is called low-frequency fatigue, because this type of fatigue may be involved in the muscle dysfunction and chronic pain experienced by computer workers. Paradoxically it appears that the Ca2+ released from the SR, which is required for contraction, may actually be responsible for the failure of SR Ca2+ release during low-frequency fatigue. We will also discuss the relationship between gross morphological changes in muscle fibres and long-lasting failure of SR Ca2+ release. Finally, a model linking muscle cell dysfunction and muscle pain is proposed.

Differential activation of mitogen-activated protein kinase signalling pathways by isometric contractions in isolated slow- and fast-twitch rat skeletal muscle

Activation of mitogen-activated protein (MAP) kinases has been implicated in the signal transduction pathways linking exercise to adaptive changes of muscle protein expression. In the present study, we investigated whether contractions of isolated muscles induced phosphorylation of extracellular signal-regulated kinase 1 and 2 (ERK1/2) and p38 MAPK in a fibre-type dependent manner. Slow-twitch (soleus) and fast-twitch (epitrochlearis, extensor digitorum longus) rat skeletal muscles were exposed to intermittent tetanic stimulation. Compared with the contralateral non-stimulated muscle, contractions increased ERK1/2 phosphorylation to the same extent in fast- and slow-twitch muscles. Significant increase in phosphorylation of p38 MAPK was observed in the fast-twitch muscles only. The total amount of ERK1/2 and p38 MAPK proteins was higher in the slow-twitch soleus muscle. In conclusion, MAP kinase signalling pathways are differentially activated and expressed in slow- and fast-twitch muscles. In addition, this activation is owing to muscle contraction per se and do not demand additional external influence.

Vacuole formation in fatigued skeletal muscle fibres from frog and mouse: effects of extracellular lactate

Isolated, living muscle fibres from either Xenopus or mouse were observed in a confocal microscope and t-tubules were visualized with sulforhodamine B. Observations were made before and after fatiguing stimulation. In addition, experiments were performed on fibres observed in an ordinary light microscope with dark-field illumination. In Xenopus fibres, recovering after fatigue, t-tubules started to show dilatations 2-5 min post-fatigue. These swellings increased in size over the next 10-20 min to form vacuoles. After 2-3 h of recovery the appearance of the fibres was again normal and force production, which had been markedly depressed 10-40 min post-fatigue, was close to control. Vacuoles were not observed in mouse fibres, fatigued with the same protocol and allowed to recover. In Xenopus fibres, fatigued in normal Ringer solution and allowed to recover in Ringer solution with 30-50 mM L-lactate substituting for chloride (lactate-Ringer), the number and size of vacuoles were markedly reduced. Also, force recovery was significantly faster. Replacement of chloride by methyl sulphate or glucuronate had no effect on vacuolation. Resting Xenopus fibres exposed to 50 mM lactate-Ringer and transferred to normal Ringer solution displayed vacuoles within 5-10 min, but to a smaller extent than after fatigue. Vacuolation was not associated with marked force reduction. Mouse fibres, fatigued in 50 mM lactate-Tyrode (L-lactate substituting for chloride in Tyrode solution) and recovering in normal Tyrode solution, displayed vacuoles for a limited period post-fatigue. Vacuolation had no effect on force production. The results are consistent with the view that lactate, formed during fatigue, is transported into the t-tubules where it attracts water and causes t-tubule swelling and vacuolation. This vacuolation may be counteracted in vivo due to a gradual extracellular accumulation of lactate during fatigue.

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.

Isometric force and endurance in soleus muscle of thyroid hormone receptor-alpha(1)- or -beta-deficient mice

The specific role of each subtype of thyroid hormone receptor (TR) on skeletal muscle function is unclear. We have therefore studied kinetics of isometric twitches and tetani as well as fatigue resistance in isolated soleus muscles of R-alpha(1)- or -beta-deficient mice. The results show 20-40% longer contraction and relaxation times of twitches and tetani in soleus muscles from TR-alpha(1)-deficient mice compared with their wild-type controls. TR-beta-deficient mice, which have high thyroid hormone levels, were less fatigue resistant than their wild-type controls, but contraction and relaxation times were not different. Western blot analyses showed a reduced concentration of the fast-type sarcoplasmic reticulum Ca(2+)-ATPase (SERCa1) in TR-alpha(1)-deficient mice, but no changes were observed in TR-beta-deficient mice compared with their respective controls. We conclude that in skeletal muscle, both TR-alpha(1) and TR-beta are required to get a normal thyroid hormone response.

The use of caged adenine nucleotides and caged phosphate in intact skeletal muscle fibres of the mouse

The effects of 1-(2-nitrophenyl)ethyl ester of ATP (NPE-caged ATP), NPE-caged ADP, NPE-caged phosphate (Pi) and desoxybenzoinyl phosphate (desyl-caged Pi) on mouse skeletal muscle function were studied. All these caged compounds, when microinjected into intact single mouse muscle fibres, reduced the myoplasmic calcium during a tetanus (tetanic [Ca2+]i) and reduced force. Flash photolysis partially reversed this reduction of tetanic [Ca2+]i and force. In fibres fatigued by repeated tetani, flash photolysis of NPE-caged ATP, ADP and Pi, also caused a transient recovery of tetanic [Ca2+]i, and force. Because photolytic release of ATP, ADP and Pi produced comparable effects it seems that the mechanism of action is the reduction in concentration of the caged compound rather than the release of the biologically active molecule. Experiments on mechanically skinned rat skeletal muscle fibres with intact T-tubular/sarcoplasmic reticulum coupling showed that 1 mM NPE-caged ATP had no effect on depolarization-induced force. This result suggests that the depressant effects of the NPE-caged compounds are neither on voltage-activated Ca2+ release from the sarcoplasmic reticulum nor on myofibrillar function. Thus all the caged compounds tested inhibit excitation-contraction coupling in muscle by an unknown mechanism and this limits their value as sources of biologically important molecules. This inhibitory effect was smallest for desyl-caged Pi and under conditions of maximal activation photolytic release of Pi caused a direct inhibition of the contractile proteins. This inhibition amounted to a 1% reduction of maximum force with an increase of [Pi] of about 0.3 mM. The mean rate of force decline under these conditions was 55 s-1, which reflects the rate of cross-bridge cycling during a maximal tetanus.

Insulin increases near-membrane but not global Ca2+ in isolated skeletal muscle

It has long been debated whether changes in Ca2+ are involved in insulin-stimulated glucose uptake in skeletal muscle. We have now investigated the effect of insulin on the global free myoplasmic Ca2+ concentration and the near-membrane free Ca2+ concentration ([Ca2+]mem) in intact, single skeletal muscle fibers from mice by using fluorescent Ca2+ indicators. Insulin has no effect on the global free myoplasmic Ca2+ concentration. However, insulin increases [Ca2+]mem by approximately 70% and the half-maximal increase in [Ca2+]mem occurs at an insulin concentration of 110 microunits per ml. The increase in [Ca2+]mem by insulin persists when sarcoplasmic reticulum Ca2+ release is inhibited but is lost by perfusing the fiber with a low Ca2+ medium or by addition of L-type Ca2+ channel inhibitors. Thus, insulin appears to stimulate Ca2+ entry into muscle cells via L-type Ca2+ channels. Wortmannin, which inhibits insulin-mediated activation of glucose transport in isolated skeletal muscle, also inhibits the insulin-mediated increase in [Ca2+]mem. These data demonstrate a new facet of insulin signaling and indicate that insulin-mediated increases in [Ca2+]mem in skeletal muscle may underlie important actions of the hormone.

Vacuole formation in fatigued single muscle fibres from frog and mouse

Force recovery from fatigue in skeletal muscle may be very slow. Gross morphological changes with vacuole formation in muscle cells during the recovery period have been reported and it has been suggested that this is the cause of the delayed force recovery. To study this we have used confocal microscopy of isolated, living muscle fibres from Xenopus and mouse to visualise transverse tubules (t-tubules) and mitochondria and to relate possible fatigue-induced morphological changes in these to force depression. T-tubules were stained with either RH414 or sulforhodamine B and mitochondrial staining was with either rhodamine 123 or DiOC6(3). Fatigue was produced by repeated, short tetanic contractions. Xenopus fibres displayed a marked vacuolation which started to develop about 2 min after fatiguing stimulation, reached a maximum after about 30 min, and then receded in about 2 h. Vacuoles were never seen during fatiguing stimulation. The vacuoles developed from localised swellings of t-tubules and were mostly located in rows of mitochondria. Mitochondrial staining, however, showed no obvious alterations of mitochondrial structure. There was no clear correlation between the presence of vacuoles and force depression; for instance, some fibres showed massive vacuole formation at a time when force had recovered almost fully. Vacuole formation was not reduced by cyclosporin A, which inhibits opening of the non-specific pore in the mitochondrial inner membrane. In mouse fibres there was no vacuole formation or obvious changes in mitochondrial structure after fatigue, but still these fibres showed a marked force depression at low stimulation frequencies ('low-frequency fatigue'). Vacuoles could be produced in mouse fibres by glycerol treatment and these vacuoles were not associated with any force decline. In conclusion, vacuoles originating from the t-tubular system develop after fatigue in Xenopus but not in mouse fibres. These vacuoles are not the cause of the delayed force recovery after fatigue.

Effects of ryanodine receptor agonist 4-chloro-m-cresol on myoplasmic free Ca2+ concentration and force of contraction in mouse skeletal muscle

In single mouse skeletal muscle fibers injected with fluorescent Ca2+ indicator Indo-1, 4-chloro-m-cresol (chlorocresol, 4-CmC) and its lipophilic analogue 4-chloro-3-ethylphenol (4-CEP) increased resting myoplasmic free [Ca2+] ([Ca2+]i) in a dose-dependent manner. In this regard, 4-CEP was more potent than 4-CmC and both were more potent than caffeine. High concentrations of 4-CmC (1 mM) or 4-CEP (500 microM) caused large and irreversible increase in resting [Ca2+]i leading to contracture. 4-CmC potentiated the [Ca2+]i increase and force of contraction induced by tetanic stimulation. Unlike caffeine, 4-CmC did not affect the activity of sarcoplasmic reticulum Ca2+ pump or the myofibrillar Ca2+ sensitivity. A low concentration of 4-CEP (20 microM) had no effect on resting [Ca2+]i on its own, but it enhanced the resting [Ca2+]i increase induced by caffeine and also potentiated the [Ca2+]i increase and contraction induced by tetanic stimulation. However, a relatively high concentration of 4-CEP (200 microM) inhibited tetanic stimulation-induced [Ca2+]i increase and contraction. Dantrolene, a muscle relaxant, inhibited 4-CmC-induced [Ca2+]i increase under resting conditions. However, when 4-CEP was applied in the presence of dantrolene, there was an exaggerated increase in [Ca2+]i. We conclude that 4-CmC and 4-CEP are potent agonists that can increase [Ca2+]i rapidly and reversibly by activating ryanodine receptors in situ in intact skeletal muscle fibers. These compounds, specially 4-CmC, may be useful for mechanistic and functional studies of ryanodine receptors and excitation-contraction coupling in skeletal muscles.

Effects of CO2-induced acidification on the fatigue resistance of single mouse muscle fibers at 28 degrees C

The role of reduced muscle pH in the development of skeletal muscle fatigue is unclear. This study investigated the effects of lowering skeletal muscle intracellular pH by exposure to 30% CO2 on the number of isometric tetani needed to induce significant fatigue. Isolated single mouse muscle fibers were stimulated repetitively at intervals of 4-2.5 s by using 80-Hz, 400-ms tetani at 28 degrees C in Tyrode solution bubbled with either 5 or 30% CO2. Stimulation continued until tetanic force had fallen to 40% of the initial value. Exposure to 30% CO2 caused a significant fall in intracellular pH of approximately 0.3 pH unit but did not cause any significant changes in initial peak tetanic force. During the course of repetitive stimulation, intracellular pH fell by approximately 0.3 pH unit in both normal and acidified fibers. The number of tetani needed to reduce force to 40% of the initial value was not significantly different in 5 and 30% CO2 Tyrode. The sole effect of acidosis was to reduce the rate of relaxation of force, especially in fatigued fibers. It is concluded that, at 28 degrees C, acidosis per se does not accelerate the development of fatigue during repeated tetanic stimulation of isolated mouse skeletal muscle fibers.

Mechanisms underlying reduced maximum shortening velocity during fatigue of intact, single fibres of mouse muscle

1. The mechanism behind the reduction in shortening velocity in skeletal muscle fatigue is unclear. In the present study we have measured the maximum shortening velocity (V0) with slack tests during fatigue produced by repeated, 350 ms tetani in intact, single muscle fibres from the mouse. We have focused on two possible mechanisms behind the reduction in V0: reduced tetanic Ca2+ and accumulation of ADP. 2. During fatigue V0 initially declined slowly, reaching 90 % of the control after about forty tetani. The rate of decline then increased and V0 fell to 70 % of the control in an additional twenty tetani. The reduction in isometric force followed a similar pattern. 3. Exposing unfatigued fibres to 10 microM dantrolene, which reduces tetanic Ca2+, lowered force by about 35 % but had no effect on V0. 4. In order to see if ADP might increase rapidly during ongoing contractions, we used a protocol with a tetanus of longer duration bracketed by standard-duration tetani. V0 in these three tetani were not significantly different in control, whereas V0 was markedly lower in the longer tetanus during fatigue and in unfatigued fibres where the creatine kinase reaction was inhibited by 10 microM dinitrofluorobenzene. 5. We conclude that the reduction in V0 during fatigue is mainly due to a transient accumulation of ADP, which develops during contractions in fibres with impaired phosphocreatine energy buffering.

Effect of hydrogen peroxide and dithiothreitol on contractile function of single skeletal muscle fibres from the mouse

1. We used intact single fibres from a mouse foot muscle to study the role of oxidation-reduction in the modulation of contractile function. 2. The oxidant hydrogen peroxide (H2O2, 100-300 microM) for brief periods did not change myoplasmic Ca2+ concentrations ([Ca2+]i) during submaximal tetani. However, force increased by 27 % during the same contractions. 3. The effects of H2O2 were time dependent. Prolonged exposures resulted in increased resting and tetanic [Ca2+]i, while force was significantly diminished. The force decline was mainly due to reduced myofibrillar Ca2+ sensitivity. There was also evidence of altered sarcoplasmic reticulum (SR) function: passive Ca2+ leak was increased and Ca2+ uptake was decreased. 4. The reductant dithiothreitol (DTT, 0.5-1 mM) did not change tetanic [Ca2+]i, but decreased force by over 40 %. This was completely reversed by subsequent incubations with H2O2. The force decline induced by prolonged exposure to H2O2 was reversed by subsequent exposure to DTT. 5. These results show that the elements of the contractile machinery are differentially responsive to changes in the oxidation-reduction balance of the muscle fibres. Myofibrillar Ca2+ sensitivity appears to be especially susceptible, while the SR functions (Ca2+ leak and uptake) are less so.

Effect of nitric oxide on single skeletal muscle fibres from the mouse

1. Single skeletal muscle fibres from a mouse foot muscle were used to investigate the effects of nitric oxide on contractile function. 2. We measured force production and myoplasmic free Ca2+ concentration ([Ca2+]i) in single fibres exposed to the nitric oxide donors S-nitroso-N-acetylcysteine (SNAC) and nitroprusside. 3. The nitric oxide donors reduced myofibrillar Ca2+ sensitivity, whereas [Ca2+]i transients were increased during submaximal tetani. Force was largely unchanged. SNAC did not change maximum shortening velocity, the rate of force redevelopment, or force production at saturating [Ca2+]i. 4. The guanylyl cyclase inhibitor LY83583 increased tetanic [Ca2+]i but had no effect on Ca2+ sensitivity. LY83583 did not prevent the decrease in myofibrillar Ca2+ sensitivity in response to SNAC. The oxidizer sodium nitrite increased tetanic [Ca2+]i and decreased myofibrillar Ca2+ sensitivity. 5. We conclude that under our experimental conditions nitric oxide impairs Ca2+ activation of the actin filaments which results in decreased myofibrillar Ca2+ sensitivity.

In situ activation of the type 2 ryanodine receptor in pancreatic beta cells requires cAMP-dependent phosphorylation

Molecular mechanisms that regulate in situ activation of ryanodine receptors (RY) in different cells are poorly understood. Here we demonstrate that caffeine (10 mM) released Ca2+ from the endoplasmic reticulum (ER) in the form of small spikes in only 14% of cultured fura-2 loaded beta cells from ob/ob mice. Surprisingly, when forskolin, an activator of adenylyl cyclase was present, caffeine induced larger Ca2+ spikes in as many as 60% of the cells. Forskolin or the phosphodiesterase-resistant PKA activator Sp-cAMPS alone did not release Ca2+ from ER. 4-Chloro-3-ethylphenol (4-CEP), an agent that activates RYs in other cell systems, released Ca2+ from ER, giving rise to a slow and small increase in [Ca2+]i in beta cells. Prior exposure of cells to forskolin or caffeine (5 mM) qualitatively altered Ca2+ release by 4-CEP, giving rise to Ca2+ spikes. In glucose-stimulated beta cells forskolin induced Ca2+ spikes that were enhanced by 3,9-dimethylxanthine, an activator of RYs. Analysis of RNA from islets and insulin-secreting betaTC-3-cells by RNase protection assay, using type-specific RY probes, revealed low-level expression of mRNA for the type 2 isoform of the receptor (RY2). We conclude that in situ activation of RY2 in beta cells requires cAMP-dependent phosphorylation, a process that recruits the receptor in a functionally operative form.

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.

Mechanisms underlying the reduction of isometric force in skeletal muscle fatigue

A decline of isometric force production is one characteristic of skeletal muscle fatigue. In fatigue produced by repeated short tetani, this force decline can be divided into two components: a reduction of the cross-bridges' ability to generate force, which comes early; and a reduction of the sarcoplasmic reticulum Ca2+ release, which develops late in fatigue. Acidification due to lactic acid accumulation has been considered as an important cause of the reduced cross-bridge force production. However, in mammalian muscle it has been shown that acidification has little effect on isometric force production at physiological temperatures. By exclusion, in mammalian muscle fatigue, the reduction of force due to impaired cross-bridge function would be caused by accumulation of inorganic phosphate ions, which results from phosphocreatine breakdown. The reduction of sarcoplasmic reticulum Ca2+ release in late fatigue correlates with a decline of ATP and we speculate that the reduced Ca2+ release is caused by a local increase of the ADP/ATP ratio in the triads.

The effect of intracellular pH on contractile function of intact, single fibres of mouse muscle declines with increasing temperature

1. The effect of altered intracellular pH (pHi) on isometric contractions and shortening velocity at 12, 22 and 32 degrees C was studied in intact, single fibres of mouse skeletal muscle. Changes in pHi were obtained by exposing fibres to solutions with different CO2 concentrations. 2. Under control conditions (5% CO2), pHi (measured with carboxy SNARF-1) was about 0.3 pH units more alkaline than neutral water at each temperature. An acidification of about 0.5 pH units was produced by 30% CO2 and an alkalinization of similar size by 0% CO2. 3. In acidified fibres tetanic force was reduced by 28% at 12 degrees C but only by 10% at 32 degrees C. The force increase with alkalinization showed a similar reduction with increasing temperature. Acidification caused a marked slowing of relaxation and this slowing became less with increasing temperature. 4. Acidification reduced the maximum shortening velocity (V0) by almost 20% at 12 degrees C, but had no significant effect at 32 degrees C. Alkalinization had no significant effect on V0 at any temperature. 5. In conclusion, the effect of pHi on contraction of mammalian muscle declines markedly with increasing temperature. Thus, the direct inhibition of force production by acidification is not a major factor in muscle fatigue at physiological temperatures.

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.

Slowed relaxation in fatigued skeletal muscle fibers of Xenopus and Mouse. Contribution of [Ca2+]i and cross-bridges

Slowing of relaxation is an important characteristic of skeletal muscle fatigue. The aim of the present study was to quantify the relative contribution of altered Ca2+ handling (calcium component) and factors downstream to Ca2+ (cross-bridge component) to the slowing of relaxation in fatigued fibers of Xenopus and mouse. Two types of Xenopus fibers were used: easily fatigued, type 1 fibers and fatigue resistant, type 2 fibers. In these Xenopus fibers the free myoplasmic [Ca2+] ([Ca2+]i) was measured with indo-1, and the relaxation of Ca2(+)-derived force, constructed from tetanic [Ca2+]i records and in vivo [Ca2+]i-force curves, was analyzed. An alternative method was used in both Xenopus and mouse fibers: fibers were rapidly shortened during the initial phase of relaxation, and the time to the peak of force redevelopment was measured. These two methods gave similar results and showed proportional slowing of the calcium and cross-bridge components of relaxation in both fatigued type 1 and type 2 Xenopus fibers, whereas only the cross-bridge component was slowed in fatigued mouse fibers. Ca2+ removal from the myoplasm during relaxation was markedly less effective in Xenopus fibers as compared to mouse fibers. Fatigued Xenopus fibers displayed a reduced rate of sarcoplasmic reticulum Ca2+ uptake and increased sarcoplasmic reticulum Ca2+ leak. Some fibers were stretched at various times during relaxation. The resistance to these stretches was increased during fatigue, especially in Xenopus fibers, which indicates that longitudinal movements during relaxation had become less pronounced and this might contribute to the increased cross-bridge component of relaxation in fatigue. In conclusion, slowing of relaxation in fatigued Xenopus fibers is caused by impaired Ca2+ handling and altered cross-bridge kinetics, whereas the slowing in mouse fibers is only due to altered cross-bridge kinetics.

The role of ATP in the regulation of intracellular Ca2+ release in single fibres of mouse skeletal muscle

1. Single fibres were dissected from mouse flexor brevis muscle and injected with indo-1 and the P3-1 (2-nitrophenyl)ethyl ester of ATP (caged ATP). Myoplasmic calcium concentration ([Ca2+]i) and force were monitored during single tetani or tetani repeated until force was reduced to about 30% of control values. In vitro experiments showed that an intense, brief ultraviolet illumination (a flash) photolysed 12% of the caged ATP to ATP. 2. Fibres that had been injected with caged ATP showed concentration-dependent changes. High concentrations of caged ATP caused a reduction in [Ca2+]i during tetani (tetanic [Ca2+]i), a reduction in force in unfatigued tetani and the fibres fatigued more rapidly when stimulated repeatedly. 3. Photolytic release of ATP in unfatigued fibres caused a concentration-dependent increase in tetanic [Ca2+]i and in force. 4. When ATP was released by photolysis in a fibre fatigued by repeated tetani, it produced a concentration-dependent increase in tetanic [Ca2+]i and force. The increase in tetanic [Ca2+]i was small (63 nM per 100 microM increase in ATP) and could explain some, but not all, the increase in force. However, taking into account the fact that control flashes in the absence of caged ATP caused a small decrease in tetanic [Ca2+]i, we believe that the increase in force may be explained by the increase in tetanic [Ca2+]i. There was no evidence of changes in the sarcoplasmic reticulum Ca2+ pump rate after photolysis of caged ATP. 5. Caged ATP affects some site(s) involved in excitation-contraction coupling and the consequences are similar to muscle fatigue. When a small fraction of this caged ATP is photolysed to ATP, the consequences of fatigue are partially reversed. These observations suggest that site(s) which either bind ATP or depend on ATP hydrolysis have a key role in excitation-contraction coupling and in muscle fatigue.

C-peptide does not alter carbohydrate metabolism in isolated mouse muscle

The effects of C-peptide on carbohydrate metabolism in isolated mouse soleus muscle were studied. C-peptide, at concentrations up to 1,000 nM, had no effect on [14C]glucose incorporation into glycogen, glycogen synthase activity, or 2-deoxyglucose uptake. These data demonstrate that C-peptide has no direct effect on the measured parameters of carbohydrate metabolism in isolated mouse muscle.

Effects of repetitive tetanic stimulation at long intervals on excitation-contraction coupling in frog skeletal muscle

1. Single skeletal muscle fibres of Xenopus frogs were used to investigate the possibility that excitation-contraction (E-C) coupling can be impaired under conditions of elevated intracellular free Ca2+ ([Ca2+]i). 2. Fibres were stimulated with a train of up to 200 tetani at 10 or 20s intervals; this long-interval stimulation (LIS) scheme was chosen to minimize fatigue. After LIS, fibres were exposed to hypotonic Ringer solution for 5 min. At the end of LIS, force was about 90% of the original and the hypotonic challenge did not result in any force depression. 3. Caffeine, terbutaline and 2,5-di(tert-butyl)-1,4-benzohydroquinone increased both basal and tetanic [Ca2+]i. In ten out of thirteen fibres, the presence of any of these drugs during LIS resulted in a force reduction to about 10% of the control when fibres were returned to normal Ringer solution after the hypotonic challenge. Force production was severely depressed for at least 20 min and then recovered to control levels within 120 min. 4. Neither protease inhibitors nor a scavenger of reactive oxygen species prevented the impairment of E-C coupling. 5. It is concluded that after a period of elevated [Ca2+]i, E-C coupling in frog skeletal muscle becomes sensitive to the mechanical stress induced by exposure to hypotonic solution. The underlying molecular basis for this remains unclear.

Intracellular calibration of the calcium indicator indo-1 in isolated fibers of Xenopus muscle

Estimates of the free myoplasmic [Ca2+] ([Ca2+]i) with fluorescent dyes are complicated by the fact that some properties of these dyes are altered in the intracellular environment. In the present study indo-1 was used to measure [Ca2+]i in isolated muscle fibers from Xenopus frogs. Fluorescent ratio signals obtained from indo-1 were converted into [Ca2+]i by means of an intracellular calibration method, which involved microinjection of 0.5 M EGTA and 1 M CaCl2 to get the ratio at very low (Rmin) and high (Rmax) [Ca2+], respectively; ratios at intermediate [Ca2+] were obtained by injection of solutions with different EGTA/Ca(2+)-EGTA proportions. This calibration gave an intracellular Ca2+ dissociation constant of indo-1 of 311 nM and a [Ca2+]i at rest of 52 +/- 4 nM (mean +/- SE; n = 15). Indo-1 records during twitches were compared with records obtained with the much faster indicator mag-indo-1. This analysis suggests a Ca2+ dissociation rate of indo-1 of 52 s-1 (22 degrees C). This makes indo-1 less suitable for measurements of [Ca2+]i during twitches, whereas it is fast enough to follow most aspects of [Ca2+]i during tetani, including the relaxation phase.

Augmented force output in skeletal muscle fibres of Xenopus following a preceding bout of activity

1. The effect of a brief period of activity on subsequent isometric tetanic force production was investigated in single muscle fibres of Xenopus laevis. 2. Following a train of ten tetani separated by 4 s intervals, tetanic force was significantly augmented by about 10%. The tetanic force augmentation persisted for at least 15 min and then slowly subsided. A similar potentiation was seen following trains of five and twenty tetani. 3. During the period of tetanic force potentiation, tetanic calcium was reduced by more than 30%, and intracellular pH was reduced from 7.15 +/- 0.07 to 7.03 +/- 0.11 (n = 4). 4. Fibre swelling was greatest at 1 min and then subsided over 15-20 min and possibly accounted for a small part of the observed force potentiation. 5. A reduction in the inorganic phosphate (P1) concentration of more than 40% was found in fibres frozen in liquid nitrogen at the peak of force potentiation compared with resting fibres. 6. It is concluded that the augmentation of tetanic force found after a brief preceding bout of activity is due to a reduction in inorganic phosphate. This mechanism may underlie the improved performance observed in athletes after warm-up.

Slowing of relaxation and [Ca2+]i during prolonged tetanic stimulation of single fibres from Xenopus skeletal muscle

1. Parvalbumin (PA) has been proposed to take up Ca2+ and enhance skeletal muscle relaxation in brief contractions; as the duration of the contraction is increased, PA will become saturated with Ca2+ and no longer contribute to relaxation which therefore will be slowed. The rate of Ca2+ loading of PA is determined by the Mg2+ off rate (about 4 s-1 at 22 degrees C). In the present study we produced prolonged tetani in intact, single fibres of Xenopus frogs while measuring force and the free myoplasmic [Ca2+] ([Ca2+]i) with indo-1. 2. Mean rate constants of slowing of force relaxation with increasing tetanus duration ranged between 3.2 and 4.8 s-1, thus, similar to the Mg2+ off rate of PA. 3. The amplitude of the tail of [Ca2+]i after tetani increased with tetanus duration. This increase developed with a rate constant similar to the Mg2+ off rate of PA 4. Steady-state force-[Ca2+]i curves were produced from tetani of various frequencies and tetani produced when force was depressed after fatiguing stimulation. These curves were used to convert [Ca2+]i records into Ca(2+)-derived force. Relaxation of Ca(2+)-derived force was slowed following a time course similar to that of real force. The lag between Ca(2+)-derived and real force during relaxation was not affected by tetanus duration. 5. Tails of elevated [Ca2+]i after tetani were used to analyse the function of the SR Ca2+ pumps. This analysis showed a marked decline in the rate of Ca2+ uptake with prolonged tetani. 6. In conclusion, in Xenopus fibres the slowing of relaxation with increasing tetanus duration can be explained by altered Ca2+ handling due to PA Ca2+ loading and impaired SR Ca2+ uptake. This contrasts to our previous results in mouse fibres and the difference can be explained by a markedly lower rate of SR Ca2+ uptake resulting in higher tetanic [Ca2+]i in Xenopus fibres.