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

A chloride channel blocker prevents the suppression by inorganic phosphate of the cytosolic calcium signals that control muscle contraction

KEY POINTS

Accumulation of inorganic phosphate (P_i ) may contribute to muscle fatigue by precipitating calcium salts inside the sarcoplasmic reticulum (SR). Neither direct demonstration of this process nor definition of the entry pathway of P_i into SR are fully established.  We showed that P_i promoted Ca²⁺ release at concentrations below 10 mm and decreased it at higher concentrations. This decrease correlated well with that of [Ca²⁺ ]_SR .  Pre-treatment of permeabilized myofibres with 2 mm Cl^- channel blocker 9-anthracenecarboxylic acid (9AC) inhibited both effects of P_i .  The biphasic dependence of Ca²⁺ release on [P_i ] is explained by a direct effect of P_i acting on the SR Ca²⁺ release channel, combined with the intra-SR precipitation of Ca²⁺ salts. The effects of 9AC demonstrate that P_i enters the SR via a Cl^- pathway of an as-yet-undefined molecular nature.

ABSTRACT

Fatiguing exercise causes hydrolysis of phosphocreatine, increasing the intracellular concentration of inorganic phosphate (P_i ). P_i diffuses into the sarcoplasmic reticulum (SR) where it is believed to form insoluble Ca²⁺ salts, thus contributing to the impairment of Ca²⁺ release. Information on the P_i entrance pathway is still lacking. In amphibian muscles endowed with isoform 3 of the RyR channel, Ca²⁺ spark frequency is correlated with the Ca²⁺ load of the SR and can be used to monitor this variable. We studied the effects of P_i on Ca²⁺ sparks in permeabilized fibres of the frog. Relative event frequency (f/f_ref ) rose with increasing [P_i ], reaching 2.54 ± 1.6 at 5 mm, and then decreased monotonically, reaching 0.09 ± 0.03 at [P_i ] = 80 mm. Measurement of [Ca²⁺ ]_SR confirmed a decrease correlated with spark frequency at high [P_i ]. A large [Ca²⁺ ]_SR surge was observed upon P_i removal. Anion channels are a putative path for P_i into the SR. We tested the effect of the chloride channel blocker 9-anthracenecarboxylic acid (9AC) on P_i entrance. 9AC (400 µm) applied to the cytoplasm produced a non-significant increase in spark frequency and reduced the P_i effects on this parameter. Fibre treatment with 2 mm 9AC in the presence of high cytoplasmic Mg²⁺ suppressed the effects of P_i on [Ca²⁺ ]_SR and spark frequency up to 55 mm [P_i ]. These results suggest that chloride channels (or transporters) provide the main pathway of inorganic phosphate into the SR and confirm that P_i impairs Ca²⁺ release by accumulating and precipitating with Ca²⁺ inside the SR, thus contributing to myogenic fatigue.

Oxidative ATP synthesis in human quadriceps declines during 4 minutes of maximal contractions

KEY POINTS

During maximal exercise, skeletal muscle metabolism and oxygen consumption remain elevated despite precipitous declines in power. Presently, it is unclear whether these responses are caused by an increased ATP cost of force generation (ATP_COST ) or mitochondrial uncoupling; a process that reduces the efficiency of oxidative ATP synthesis (ATP_OX ). To address this gap, we used 31-phosphorus magnetic resonance spectroscopy to measure changes in ATP_COST and ATP_OX in human quadriceps during repeated trials of maximal intensity knee extensions lasting up to 4 min. ATP_COST remained unchanged. In contrast, ATP_OX plateaued by ∼2 min and then declined (∼15%) over the final 2 min. The maximal capacity for ATP_OX (V_max ), as well as ADP-specific rates of ATP_OX , were also significantly diminished. Collectively, these results suggest that mitochondrial uncoupling, and not increased ATP_COST , is responsible for altering the regulation of skeletal muscle metabolism and oxygen consumption during maximal exercise.

ABSTRACT

The relationship between skeletal muscle oxygen consumption and power output is augmented during exercise at workloads above the lactate threshold. Potential mechanisms for this response have been hypothesized, including increased ATP cost of force generation (ATP_COST ) and mitochondrial uncoupling, a process that reduces the efficiency of oxidative ATP synthesis (ATP_OX ). To test these hypotheses, we used phosphorus magnetic resonance spectroscopy to non-invasively measure changes in phosphate concentrations and pH in the vastus lateralis muscle of nine young adults during repeated trials of maximal, all-out dynamic knee extensions (120°s^-1 , 1 every 2 s) lasting 24, 60, 120, and 240 s. ATP_OX was measured at each time point from the initial velocity of PCr resynthesis, and ATP_COST was calculated as the sum of ATP synthesized by the creatine and adenylate kinase reactions, non-oxidative glycolysis, ATP_OX and net changes in [ATP]. Power output declined in a reproducible manner for all four trials. ATP_COST did not change over time (main effect P = 0.45). ATP_OX plateaued from 60 to 120 s and then decreased over the final 120 s (main effect P = 0.001). The maximal capacity for oxidative ATP synthesis (V_max ), as well as ADP-specific rates of ATP_OX , also decreased over time (main effect P = 0.001, both). Collectively, these results demonstrate that prolonged maximal contraction protocols impair oxidative energetics and implicate mitochondrial uncoupling as the mechanism for this response. The causes of mitochondrial uncoupling are presently unknown but may offer a potential explanation for the dissociation between skeletal muscle power output and oxygen consumption during maximal, all-out exercise protocols.

Mechanical compression during repeated sustained isometric muscle contractions and hyperemic recovery in healthy young males

BACKGROUND

An elevated intramuscular pressure during a single forearm isometric muscle contraction may restrict muscle hyperemia. However, during repeated isometric exercise, it is unclear to what extent mechanical compression and muscle vasodilatation contribute to the magnitude and time course of beat-to-beat limb hemodynamics, due to alterations in leg vascular conductance (LVC).

METHODS

In eight healthy male subjects, the time course of both beat-to-beat leg blood flow (LBF) and LVC in the femoral artery was determined between repeated 10-s isometric thigh muscle contractions and 10-s muscle relaxation (a duty cycle of 20 s) for steady-state 120 s at five target workloads (10, 30, 50, 70, and 90% of maximum voluntary contraction (MVC)). The ratio of restricted LBF due to mechanical compression across workloads was determined by the formula (relaxation LBF--contraction LBF)/relaxation LBF (%).

RESULTS

The exercise protocol was performed completely by all subjects (≤ 50% MVC), seven subjects (≤ 70% MVC), and two subjects (≤ 90% MVC). During a 10-s isometric muscle contraction, the time course in both beat-to-beat LBF and LVC displayed a fitting curve with an exponential increase (P < 0.001, r (2) ≥ 0.956) at each workload but no significant difference in mean LBF across workloads and pre-exercise. During a 10-s muscle relaxation, the time course in both beat-to-beat LBF and LVC increased as a function of workload, followed by a linear decline (P < 0.001, r (2) ≥ 0.889), that was workload-dependent, resulting in mean LBF increasing linearly across workloads (P < 0.01, r (2) = 0.984). The ratio of restricted LBF can be described as a single exponential decay with an increase in workload, which has inflection point distinctions between 30 and 50% MVC.

CONCLUSIONS

In a 20-s duty cycle of steady-state repeated isometric muscle contractions, the post-contraction hyperemia (magnitude of both LBF and LVC) during muscle relaxation was in proportion to the workload, which is in agreement with previous findings. Furthermore, time-dependent beat-to-beat muscle vasodilatation was seen, but not restricted, during isometric muscle contractions through all target workloads. Additionally, the relative contribution of mechanical obstruction and vasodilatation to the hyperemia observed in the repeated isometric exercise protocol was non-linear with regard to workload. In combination with repeated isometric exercise, the findings could potentially prove to be useful indicators of circulatory adjustment by mechanical compression for muscle-related disease.

Variation in isometric force after active shortening and lengthening and their mechanisms: a review

Introduction The isometric force history dependence of skeletal muscle has been studied along the last one hundred years. Several theories have been formulated to explain and establish the causes of the phenomenon, but not successfully, as they have not been fully accepted and demonstrated, and much controversy on such a subject still remains. Objective To present a systematic literature review on the dynamics of the mechanisms of force depression and force enhancement after active shortening and lengthening, respectively, identifying the key variables involved in the phenomenon, and to date to present the main theories and hypothesis developed trying to explaining it. Method The procedure of literature searching complied the major databases, including articles either, those which directly investigated the phenomena of force depression and force enhancement or those which presented possible causes and mechanisms associated with their respective events, from the earliest studies published until the year of 2010. Results 97 references were found according to the criteria used. Conclusion Based on this review, it is suggested that the theory of stress inhibition of actin-myosin cross-bridges is that better explain the phenomenon of force depression. Whereas regarding the force enhancement phenomenon, one theory have been well accepted, the increased number of actin-myosin cross-bridges in strong binding state influenced by the recruitment of passive elastic components, which hole is attributed to the titin filament.

Ca²⁺-pumping impairment during repetitive fatiguing contractions in single myofibers: role of cross-bridge cycling

The energy cost of contractions in skeletal muscle involves activation of both actomyosin and sarcoplasmic reticulum (SR) Ca²⁺-pump (SERCA) ATPases, which together determine the overall ATP demand. During repetitive contractions leading to fatigue, the relaxation rate and Ca²⁺ pumping become slowed, possibly because of intracellular metabolite accumulation. The role of the energy cost of cross-bridge cycling during contractile activity on Ca²⁺-pumping properties has not been investigated. Therefore, we inhibited cross-bridge cycling by incubating isolated Xenopus single fibers with N-benzyl-p-toluene sulfonamide (BTS) to study the mechanisms by which SR Ca²⁺ pumping is impaired during fatiguing contractions. Fibers were stimulated in the absence (control) and presence of BTS and cytosolic calcium ([Ca²⁺]c) transients or intracellular pH (pHi) changes were measured. BTS treatment allowed normal [Ca²⁺]c transients during stimulation without cross-bridge activation. At the time point that tension was reduced to 50% in the control condition, the fall in the peak [Ca²⁺]c and the increase in basal [Ca²⁺]c did not occur with BTS incubation. The progressively slower Ca²⁺ pumping rate and the fall in pHi during repetitive contractions were reduced during BTS conditions. However, when mitochondrial ATP supply was blocked during contractions with BTS present (BTS + cyanide), there was no further slowing in SR Ca²⁺ pumping during contractions compared with the BTS-alone condition. Furthermore, the fall in pHi was significantly less during the BTS + cyanide condition than in the control conditions. These results demonstrate that factors related to the energetic cost of cross-bridge cycling, possibly the accumulation of metabolites, inhibit the Ca²⁺ pumping rate during fatiguing contractions.

Effect of phosphate and temperature on force exerted by white muscle fibres from dogfish

Effects of Pi (inorganic phosphate) are relevant to the in vivo function of muscle because Pi is one of the products of ATP hydrolysis by actomyosin and by the sarcoplasmic reticulum Ca²⁺ pump. We have measured the Pi sensitivity of force produced by permeabilized muscle fibres from dogfish (Scyliorhinus canicula) and rabbit. The activation conditions for dogfish fibres were crucial: fibres activated from the relaxed state at 5, 12, and 20°C were sensitive to Pi, whereas fibres activated from rigor at 12°C were insensitive to Pi in the range 5–25 mmol l⁻¹. Rabbit fibres activated from rigor were sensitive to Pi. Pi sensitivity of force produced by dogfish fibres activated from the relaxed state was greater below normal body temperature (12°C for dogfish) in agreement with what is known for other species. The force-temperature relationship for dogfish fibres (intact and permeabilized fibres activated from relaxed) showed that at 12°C, normal body temperature, the force was near to its maximum value.

Modelling muscle energy-metabolism in anaerobic muscle

A mathematical model of anaerobic muscle energy-metabolism was developed to predict pH and the concentrations of nine muscle metabolites over time. Phosphorous-31 Nuclear Magnetic Resonance was used to measure time-course data for some phosphate metabolites and pH in anoxic M. semitendinosus taken from three slaughtered sheep. Muscles were held at 35 degrees C during the experiment. Measurement commenced 25 min post mortem and concluded before rigor mortis. The model was fitted to these data within experimental error, by simultaneously varying model parameter values and initial substrate concentrations. The model was used to simulate the period from death until metabolic activity ceased, in order to predict the different stages of metabolic response to anoxia. The model suggested that alkalinisation would occur in all three muscles in the first few minutes after the onset of anoxia, followed by a steady decline in pH. For two of the muscles this decline continued until rigor, with final pH values of 5.60 and 6.07. For the other muscle, pH reached a low of 5.60 near rigor but then increased to a final value of 5.73. A rise in pH after rigor has been observed but not previously explained in the literature. The modelling results suggest it was caused by the alkalising effect of adenosine monophosphate deamination being greater at low pH than the acidifying effect of inosine monophosphate dephosphorylation.

Blood flow does not limit skeletal muscle force production during incremental isometric contractions

It has been suggested that a transient limitation in blood flow during intermittent muscular contractions can contribute to muscle fatigue, and that this limitation is greater as contraction intensity increases. We investigated skeletal muscle blood flow and fatigue in 13 healthy, untrained men (21-27 years) during 16 min of intermittent (4 s contract, 6 s relax) isometric dorsiflexor contractions. Contractions began at 10% of pre-exercise maximal voluntary contraction (MVC) force and increased by 10% every 2 min. Hyperemia (i.e., post-contraction blood flow, measured by venous occlusion plethysmography) and MVC were measured at the end of each stage. Muscle volume measures were obtained using magnetic resonance imaging. After 10 min of exercise, submaximal force and post-contraction hyperemia plateaued. MVC fell from 8 min of exercise onwards (p=0.004), and this onset of fatigue preceded the plateau in submaximal force and hyperemia. Despite a large range in dorsiflexor muscle size (66.3-176.4 cm(3)) and strength (112.5-421.8 N), neither muscle size nor strength were related to fatigue. The temporal dissociation between changes in blood flow and the onset of fatigue (fall of MVC) suggest that limited blood flow was not a factor in the impaired force production observed during intermittent isometric dorsiflexor contractions in healthy young men. Additionally, post-contraction hyperemia increased linearly with increasing contraction intensity, reflecting a match between blood flow and force production throughout the protocol that was independent of fatigue.

N-acetylcysteine fails to modulate the in vitro function of sarcoplasmic reticulum of diaphragm in the final phase of fatigue

AIM

In the present study, we tested the hypothesis whether N-acetylcysteine (NAC), a non-specific antioxidant, might influence fatigue by modulating Ca2+-handling capacity by the sarcoplasmic reticulum (SR).

METHODS

In the presence (10 mm) or absence of NAC, bundles of rat diaphragm were stimulated with tetanic trains (350 ms, 30-40 Hz) at 1 train every 2 s for 300 s. SR functions, as assessed by SR Ca2+-uptake and release rates and SR Ca2+-ATPase activity, were measured in vitro on muscle homogenates.

RESULTS

Following the 300-s stimulation, the force developed by NAC-treated muscles is approximately 1.8-fold higher (P < 0.05) than that of muscles without NAC treatment. Stimulation elicited an 18-30% depression in SR function (P < 0.05). Despite the differing degrees of fatigue between NAC-treated and non-treated muscles, SR functions in these muscles were reduced to similar extents.

CONCLUSIONS

These results suggest that modulation of SR function measured in vitro may not be a major contributor to inhibition of diaphragmic fatigue with antioxidant, at least, in the final phase of fatigue where force output is remarkably reduced.

Time related changes in calcium handling in the isolated ischemic and reperfused rat heart

The main aim of this study was to assess the kinetics of intracellular free calcium (Ca(2+)i) handling by isolated rat hearts rendered ischemic for 30 min followed by 30 min of reperfusion analyzing the upstroke and downslope of the Ca(2+)i transient. Changes in mechanical performance and degradation of membrane phospholipids--estimated by tissue arachidonic acid content--were correlated with Ca(2+)i levels of the heart. The fluorescence ratio technique was applied to estimate Ca(2+)i. The disappearance of mechanical activity of the heart preceded that of the Ca(2+)i transient in the first 2 min of ischemia. The slope of upstroke of the Ca(2+)i transient, reflecting Ca2+ release, decreased by 60%, while the duration of the downslope of the transient, reflecting Ca2+ sequestration, expressed a significant prolongation (105 +/- 17 vs. 149 +/- 39 msec) during the first 3 min of ischemia. At about 20 min of ischemia end-diastolic pressure expressed a 3.5-fold increase (contracture) when the fluorescence ratio showed a 2-fold elevation. Reperfusion was accompanied with a further precipitous increase in end-diastolic pressure, while resting Ca(2+)i remained at end-ischemic levels. Increases in the arachidonic acid (AA) content of the ischemic and postischemic hearts were proportional to Ca(2+)i levels. In summary, the present findings indicate that both calcium release and removal are hampered during the early phase of ischemia. Moreover, a critical level of Ca(2+)i and a critical duration of ischemia may exist to provoke contracture of the heart. Upon reperfusion the hearts show membrane phospholipid degradation and signs of stunning exemplified by elevated AA levels, partial recovery of Ca(2+)i handling and sustained depression of mechanical performance.

The effects of HCl and CaCl(2) injections on intracellular calcium and pH in voltage-clamped snail (Helix aspersa) neurons

To investigate the mechanisms by which low intracellular pH influences calcium signaling, I have injected HCl, and in some experiments CaCl(2), into snail neurons while recording intracellular pH (pH(i)) and calcium concentration ([Ca(2+)](i)) with ion-sensitive microelectrodes. Unlike fluorescent indicators, these do not increase buffering. Slow injections of HCl (changing pH(i) by 0.1-0.2 pH units min(-1)) first decreased [Ca(2+)](i) while pH(i) was still close to normal, but then increased [Ca(2+)](i) when pH(i) fell below 6.8-7. As pH(i) recovered after such an injection, [Ca(2+)](i) started to fall but then increased transiently before returning to its preinjection level. Both the acid-induced decrease and the recovery-induced increase in [Ca(2+)](i) were abolished by cyclopiazonic acid, which empties calcium stores. Caffeine with or without ryanodine lowered [Ca(2+)](i) and converted the acid-induced fall in [Ca(2+)](i) to an increase. Injection of ortho-vanadate increased steady-state [Ca(2+)](i) and its response to acidification, which was again blocked by CPA. The normal initial response to 10 mM caffeine, a transient increase in [Ca(2+)](i), did not occur with pH(i) below 7.1. When HCl was injected during a series of short CaCl(2) injections, the [Ca(2+)](i) transients (recorded as changes in the potential (V(Ca)) of the Ca(2+)-sensitive microelectrode), were reduced by only 20% for a 1 pH unit acidification, as was the rate of recovery after each injection. Calcium transients induced by brief depolarizations, however, were reduced by 60% by a similar acidification. These results suggest that low pH(i) has little effect on the plasma membrane calcium pump (PMCA) but important effects on the calcium stores, including blocking their response to caffeine. Acidosis inhibits spontaneous calcium release via the RYR, and leads to increased store content which is unloaded when pH(i) returns to normal. Spontaneous release is enhanced by the rise in [Ca(2+)](i) caused by inhibiting the PMCA.

The mechanism of the force enhancement by MgADP under simulated ischaemic conditions in rat cardiac myocytes

In this study, the effects of MgADP and/or MgATP on the Ca2+ -dependent and Ca2+ -independent contractile force restoration were determined in order to identify the origin of the tonic force increase (i.e. ischaemic contracture) which develops during advanced stages of ischaemia. Experiments were performed at 15 degrees C during simulated ischaemic conditions in Triton-skinned right ventricular myocytes from rats. In the presence of 5 mM MgATP the maximal Ca2+ -dependent force (P(o)) of 39 +/- 2 kN m(-2) (mean +/- S.E.M.) under control conditions (pH 7.0, 15 mM phosphocreatine (CP)) decreased to 8 +/- 1 % during simulated ischaemia (pH 6.2, 30 mM inorganic phosphate (P(i)), without CP). This change was accompanied by a major reduction in Ca2+ sensitivity (pCa(50) 4.10 vs. 5.62). Substitution of MgADP for MgATP restored isometric force production and its Ca2+ sensitivity (pCa(50) 4.74 at 4 mM MgADP and 1 mM MgATP). In addition, it shifted the MgATP threshold concentration of Ca2+ -independent force development to higher levels in a concentration-dependent manner. However, Ca2+ -independent force was facilitated less by MgADP than Ca2+ -dependent force. The MgADP-induced increase in force was accompanied by marked reductions in the velocity of unloaded shortening and the rate of tension redevelopment. These data and simulations using a model of cross-bridge kinetics suggest that the ischaemic force is not a consequence of a reduction in intracellular MgATP concentration, but identify MgADP as a key modulator of the cross-bridge cycle under simulated ischaemic conditions in cardiac muscle, with a much lower inhibition constant (0.012 +/- 0.003 mM) than in skeletal muscle. Therefore, MgADP has a high potential to stabilize the force-generating cross-bridge state and to facilitate the development of ischaemic contracture, possibly involving a Ca2+ activation process in the ischaemic myocardium.

Time-dependent and indirect effect of inorganic phosphate on force production in rat gastrocnemius exercising muscle determined by 31P-MRS

The relationship of inorganic phosphate (P(i)) and its diprotonated form (H(2)PO(4)(-)) to isometric force (F) was analyzed non-invasively using 31P-magnetic resonance spectroscopy. Rat gastrocnemius muscles were electrically stimulated at six different frequencies in order to produce different levels of fatigue. A curvilinear relationship was demonstrated between force production and [P(i)] and [H(2)PO(4)(-)] accumulation. [P(i)] and [H(2)PO(4)(-)] were correlated with F at the end of the stimulation period but not when F was maximal at the early stage of the stimulation period. Interestingly, the respective [P(i)] and [H(2)PO(4)(-)] did not differ significantly between these two stages demonstrating that [P(i)] and [H(2)PO(4)(-)] cannot be considered as direct effectors of fatigue. This time-dependent and indirect effect of [P(i)] and [H(2)PO(4)(-)] on force production might be mediated by calcium ions.

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.