The RyR1 P3528S Substitution Alters Mouse Skeletal Muscle Contractile Properties and RyR1 Ion Channel Gating

The recessive Ryanodine Receptor Type 1 (RyR1) P3527S mutation causes mild muscle weakness in patients and increased resting cytoplasmic [Ca2+] in transformed lymphoblastoid cells. In the present study, we explored the cellular/molecular effects of this mutation in a mouse model of the mutation (RyR1 P3528S). The results were obtained from 73 wild type (WT/WT), 82 heterozygous (WT/MUT) and 66 homozygous (MUT/MUT) mice with different numbers of observations in individual data sets depending on the experimental protocol. The results showed that WT/MUT and MUT/MUT mouse strength was less than that of WT/WT mice, but there was no difference between genotypes in appearance, weight, mobility or longevity. The force frequency response of extensor digitorum longus (EDL) and soleus (SOL) muscles from WT/MUT and MUT/MUT mice was shifter to higher frequencies. The specific force of EDL muscles was reduced and Ca2+ activation of skinned fibres shifted to a lower [Ca2+], with an increase in type I fibres in EDL muscles and in mixed type I/II fibres in SOL muscles. The relative activity of RyR1 channels exposed to 1 µM cytoplasmic Ca2+ was greater in WT/MUT and MUT/MUT mice than in WT/WT mice. We suggest the altered RyR1 activity due to the P2328S substitution could increase resting [Ca2+] in muscle fibres, leading to changes in fibre type and contractile properties.

Wild white-capped noddies keep a cool head in a heated situation

Sunning, or sunbathing, is a behavior observed in diverse birds from at least 50 taxonomic families. While sunning, birds exhibit signs of heat stress, notably panting, indicating a risk of overheating. Given that even modest increases in brain temperature can impair brain function, sunning birds may have mechanisms that selectively cool the brain. Sunning birds could cool the brain using active physiological mechanisms (e.g., an ophthalmic rete or sleeping) or passive adaptations, such as light-colored plumage over the cranium. White-capped noddies are tropical seabirds that sunbathe in direct sunlight on cloudless days. Using infrared thermography on wild birds, we found that the white cap is 20 °C cooler than that of the black back while sunning. A deceased bird showed the same thermal profile, indicating that this difference arises from dichromatic coloration and not underlying physiology. Thus, the white cap may extend the duration of time noddies can sunbathe and keep the brain cool, near core body temperature, while allowing the rest of the body to heat up, perhaps to displace or kill parasites.

Effects of high-intensity intermittent exercise on the contractile properties of human type I and type II skeletal muscle fibers

In vitro studies have shown that alterations in redox state can cause a range of opposing effects on the properties of the contractile apparatus in skeletal muscle fibers. To test whether and how redox changes occurring in vivo affect the contractile properties, vastus lateralis muscle fibers from seven healthy young adults were examined at rest (PRE) and following (POST) high-intensity intermittent cycling exercise. Individual mechanically skinned muscle fibers were exposed to heavily buffered solutions at progressively higher free [Ca²⁺] to determine their force-Ca²⁺ relationship. Following acute exercise, Ca²⁺ sensitivity was significantly decreased in type I fibers (by 0.06 pCa unit) but not in type II fibers (0.01 pCa unit). Specific force decreased after the exercise in type II fibers (-18%) but was unchanged in type I fibers. Treatment with the reducing agent dithiothreitol (DTT) caused a small decrease in Ca²⁺-sensitivity in type II fibers at PRE (by ∼0.014 pCa units) and a significantly larger decrease at POST (∼0.035 pCa units), indicating that the exercise had increased S-glutathionylation of fast troponin I. DTT treatment also increased specific force (by ∼4%), but only at POST. In contrast, DTT treatment had no effect on either parameter in type I fibers at either PRE or POST. In type I fibers, the decreased Ca²⁺ sensitivity was not due to reversible oxidative changes and may have contributed to a decrease in power production during vigorous exercises. In type II fibers, exercise-induced redox changes help counter the decline in Ca²⁺-sensitivity while causing a small decline in maximum force.NEW & NOTEWORTHY This study identified important cellular changes occurring in human skeletal muscle fibers following high-intensity intermittent exercise: 1) a decrease in contractile apparatus Ca²⁺ sensitivity in type I but not type II fibers, 2) a decrease in specific force only in type II muscle fibers, and 3) a redox-dependent increase in Ca²⁺ sensitivity occurring only in type II fibers, which would help maintain muscle performance by countering the normal metabolite-induced decline in Ca²⁺ sensitivity.

Contractile properties and sarcoplasmic reticulum calcium content in type I and type II skeletal muscle fibres in active aged humans

KEY POINTS

Muscle weakness in old age is due in large part to an overall loss of skeletal muscle tissue, but it remains uncertain how much also stems from alterations in the properties of the individual muscle fibres. This study examined the contractile properties and amount of stored intracellular calcium in single muscle fibres of Old (70 ± 4 years) and Young (22 ± 3 years) adults. The maximum level of force production (per unit cross-sectional area) in fast twitch fibres in Old subjects was lower than in Young subjects, and the fibres were also less sensitive to activation by calcium. The amount of calcium stored inside muscle fibres and available to trigger contraction was also lower in both fast- and slow-twitch muscle fibres in the Old subjects. These findings indicate that muscle weakness in old age stems in part from an impaired capacity for force production in the individual muscle fibres.

ABSTRACT

This study examined the contractile properties and sarcoplasmic reticulum (SR) Ca(2+) content in mechanically skinned vastus lateralis muscle fibres of Old (70 ± 4 years) and Young (22 ± 3 years) humans to investigate whether changes in muscle fibre properties contribute to muscle weakness in old age. In type II fibres of Old subjects, specific force was reduced by ∼17% and Ca(2+) sensitivity was also reduced (pCa50 decreased ∼0.05 pCa units) relative to that in Young. S-Glutathionylation of fast troponin I (TnIf ) markedly increased Ca(2+) sensitivity in type II fibres, but the increase was significantly smaller in Old versus Young (+0.136 and +0.164 pCa unit increases, respectively). Endogenous and maximal SR Ca(2+) content were significantly smaller in both type I and type II fibres in Old subjects. In fibres of Young, the SR could be nearly fully depleted of Ca(2+) by a combined caffeine and low Mg(2+) stimulus, whereas in fibres of Old the amount of non-releasable Ca(2+) was significantly increased (by > 12% of endogenous Ca(2+) content). Western blotting showed an increased proportion of type I fibres in Old subjects, and increased amounts of calsequestrin-2 and calsequestrin-like protein. The findings suggest that muscle weakness in old age is probably attributable in part to (i) an increased proportion of type I fibres, (ii) a reduction in both maximum specific force and Ca(2+) sensitivity in type II fibres, and also a decreased ability of S-glutathionylation of TnIf to counter the fatiguing effects of metabolites on Ca(2+) sensitivity, and (iii) a reduction in the amount of releasable SR Ca(2+) in both fibre types.

Ca2+-dependent proteolysis of junctophilin-1 and junctophilin-2 in skeletal and cardiac muscle

Excessive increases in intracellular [Ca(2+)] in skeletal muscle fibres cause failure of excitation-contraction coupling by disrupting communication between the dihydropyridine receptors in the transverse tubular system and the Ca(2+) release channels (RyRs) in the sarcoplasmic reticulum (SR), but the exact mechanism is unknown. Previous work suggested a possible role of Ca(2+)-dependent proteolysis in this uncoupling process but found no proteolysis of the dihydropyridine receptors, RyRs or triadin. Junctophilin-1 (JP1; ∼90 kDa) stabilizes close apposition of the transverse tubular system and SR membranes in adult skeletal muscle; its C-terminal end is embedded in the SR and its N-terminal associates with the transverse tubular system membrane. Exposure of skeletal muscle homogenates to precisely set [Ca(2+)] revealed that JP1 undergoes Ca(2+)-dependent proteolysis over the physiological [Ca(2+)] range in tandem with autolytic activation of endogenous μ-calpain. Cleavage of JP1 occurs close to the C-terminal, yielding a ∼75 kDa diffusible fragment and a fixed ∼15 kDa fragment. Depolarization-induced force responses in rat skinned fibres were abolished following 1 min exposure to 40 μm Ca(2+), with accompanying loss of full-length JP1. Supraphysiological stimulation of rat skeletal muscle in vitro by repeated tetanic stimulation in 30 mm caffeine also produced marked proteolysis of JP1 (and not RyR1). In dystrophic mdx mice, JP1 proteolysis is seen in limb muscles at 4 and not at 10 weeks of age. Junctophilin-2 in cardiac and skeletal muscle also undergoes Ca(2+)-dependent proteolysis, and junctophilin-2 levels are reduced following cardiac ischaemia-reperfusion. Junctophilin proteolysis may contribute to skeletal muscle weakness and cardiac dysfunction in a range of circumstances.

ROS-mediated decline in maximum Ca2+-activated force in rat skeletal muscle fibers following in vitro and in vivo stimulation

We hypothesised that normal skeletal muscle stimulated intensely either in vitro or in situ would exhibit reactive oxygen species (ROS)-mediated contractile apparatus changes common to many pathophysiological conditions. Isolated soleus (SOL) and extensor digitorum longus (EDL) muscles of the rat were bubbled with 95% O₂ and stimulated in vitro at 31°C to give isometric tetani (50 Hz for 0.5 s every 2 s) until maximum force declined to ≤30%. Skinned superficial slow-twitch fibers from the SOL muscles displayed a large reduction (∼41%) in maximum Ca²⁺-activated specific force (F_max), with Ca²⁺-sensitivity unchanged. Fibers from EDL muscles were less affected. The decrease in F_max in SOL fibers was evidently due to oxidation effects on cysteine residues because it was reversed if the reducing agent DTT was applied prior to activating the fiber. The GSH∶GSSG ratio was ∼3-fold lower in the cytoplasm of superficial fibers from stimulated muscle compared to control, confirming increased oxidant levels. The presence of Tempol and L-NAME during in vitro stimulation prevented reduction in F_max. Skinned fibers from SOL muscles stimulated in vivo at 37°C with intact blood supply also displayed reduction in F_max, though to a much smaller extent (∼12%). Thus, fibers from muscles stimulated even with putatively adequate O₂ supply display a reversible oxidation-induced decrease in F_max without change in Ca²⁺-sensitivity, consistent with action of peroxynitrite (or possibly superoxide) on cysteine residues of the contractile apparatus. Significantly, the changes closely resemble the contractile deficits observed in a range of pathophysiological conditions. These findings highlight how readily muscle experiences ROS-related deficits, and also point to potential difficulties when defining muscle performance and fatigue.

S-glutathionylation of troponin I (fast) increases contractile apparatus Ca2+ sensitivity in fast-twitch muscle fibres of rats and humans

Oxidation can decrease or increase the Ca2+ sensitivity of the contractile apparatus in rodent fast-twitch (type II) skeletal muscle fibres, but the reactions and molecular targets involved are unknown. This study examined whether increased Ca2+ sensitivity is due to S-glutathionylation of particular cysteine residues. Skinned muscle fibres were directly activated in heavily buffered Ca2+ solutions to assess contractile apparatus Ca2+ sensitivity. Rat type II fibres were subjected to S-glutathionylation by successive treatments with 2,2′-dithiodipyridine (DTDP) and glutathione (GSH), and displayed a maximal increase in pCa50 (−log10 [Ca2+] at half-maximal force) of ∼0.24 pCa units, with little or no effect on maximum force or Hill coefficient. Partial similar effect was produced by exposure to oxidized gluthathione (GSSG, 10 mM) for 10 min at pH 7.1, and near-maximal effect by GSSG treatment at pH 8.5. None of these treatments significantly altered Ca2+ sensitivity in rat type I fibres. Western blotting showed that both the DTDP–GSH and GSSG–pH 8.5 treatments caused marked S-glutathionylation of the fast troponin I isoform (TnI(f)) present in type II fibres, but not of troponin C (TnC) or myosin light chain 2. Both the increased Ca2+ sensitivity and glutathionylation of TnI(f) were blocked by N-ethylmaleimide (NEM). S-nitrosoglutathione (GSNO) also increased Ca2+ sensitivity, but only in conditions where it caused S-glutathionylation of TnI(f). In human type II fibres from vastus lateralis muscle, DTDP–GSH treatment also caused similar increased Ca2+ sensitivity and S-glutathionylation of TnI(f). When the slow isoform of TnI in type I fibres of rat was partially substituted (∼30%) with TnI(f), DTDP–GSH treatment caused a significant increase in Ca2+ sensitivity (∼0.08 pCa units). TnIf in type II fibres from toad and chicken muscle lack Cys133 present in mammalian TnIf, and such fibres showed no change in Ca2+ sensitivity with DTDP–GSH nor any S-glutathionylation of TnI(f) (latter examined only in toad). Following 40 min of cycling exercise in human subjects (at ∼60% peak oxygen consumption), TnI(f) in vastus lateralis muscle displayed a marked increase in S-glutathionylation (∼4-fold). These findings show that S-glutathionylation of TnI(f), most probably at Cys133, increases the Ca2+ sensitivity of the contractile apparatus, and that this occurs in exercising humans, with likely beneficial effects on performance.

Modulation of contractile apparatus Ca2+ sensitivity and disruption of excitation-contraction coupling by S-nitrosoglutathione in rat muscle fibres

S-Nitrosoglutathione (GSNO) is generated in muscle and may S-glutathionylate and/or S-nitrosylate various proteins involved in excitation–contraction (EC) coupling, such as Na+-K+-ATPases, voltage-sensors (VSs) and Ca2+ release channels (ryanodine receptors,RyRs), possibly changing their properties. Using mechanically skinned fibres from rat extensor digitorum longus muscle, we sought to identify which EC coupling processes are most susceptible to GSNO-modulated changes and whether these changes could be important in muscle function and fatigue. For comparison, we examined the effect of other oxidation, nitrosylation, or glutathionylation treatments (S-nitroso-N-acetyl-penicillamine (SNAP), hydrogen peroxide,2,2-dithiodipyridine and reduced glutathione) on twitch and tetanic force, action potential (AP) repriming, sarcoplasmic reticulum (SR) Ca2+ loading and leakage, and contractile apparatus properties. None of the treatments detectably altered AP repriming, indicating that t-system excitability was relatively insensitive to such oxidative modification. Importantly, the overall effect on twitch and tetanic force of a given treatment was determined primarily by its action on Ca2+ sensitivity of the contractile apparatus. For example, S-nitrosylation with the NO• donor,SNAP, caused matching decreases in the contractile Ca2+ sensitivity and twitch response, and GSNO applied ∼10 min after preparation had very similar effects. The only exception was when GSNO was applied immediately after preparation, which resulted in irreversible decreases in twitch and tetanic responses even though it concomitantly increased Ca2+ sensitivity by∼0.1 pCaunits, the latter evidently due to S-glutathionylation of the contractile apparatus. This decrease in AP-mediated force responses was due to impaired VS–RyR coupling and was accompanied by increased Ca2+ leakage through RyRs. Such oxidation-related impairment of coupling could be responsible for prolonged low frequency fatigue in certain circumstances.

Hydroxyl radical and glutathione interactions alter calcium sensitivity and maximum force of the contractile apparatus in rat skeletal muscle fibres

Studies on intact muscle fibres indicate that reactive oxygen species (ROS) produced during muscle activity, or applied exogenously, can cause decreased force responses primarily by reducing the Ca(2+) sensitivity of the contractile apparatus. Identification of the molecular basis of this effect is complicated by the fact that studies on skinned muscle fibres in general have not observed reduced contractile Ca(2+) sensitivity when applying ROS, predominantly H(2)O(2). Here, using skinned fibres from rat extensor digitorum longus (EDL) and soleus muscle, it is shown that although H(2)O(2) (> or = 100 microm) has little effect by itself, when added in the presence of myoglobin it causes marked reduction in the Ca(2+) sensitivity of the contractile apparatus, probably due to production of hydroxyl radicals (OH(*)). Maximum force production is also reduced, but only with larger or more prolonged treatments. The effects are not prevented by tempol, a potent superoxide scavenger. Dithiotreitol (DTT) produces little reversal of the sensitivity change if applied afterwards, but it does substantially reverse all the changes if applied before the fibre undergoes an activation sequence. When glutathione (GSH, 5 mM) is present, exposure of EDL fibres to H(2)O(2) and myoglobin causes an increase in Ca(2+) sensitivity, with longer treatments causing a subsequent decrease, whereas in soleus fibres it causes only decreases in sensitivity and maximum force. The increased Ca(2+) sensitivity in EDL fibres is evidently due to the summed actions of (i) a potentiating effect of glutathionylation, which can be reversed by DTT and only occurs in fast-twitch fibres, and (ii) a less reversible reduction in sensitivity. Western blotting showed that reductions in Ca(2+) sensitivity were not due to loss of troponin-C. The present findings help provide a mechanistic basis for diverse findings on the effects of ROS in muscle fibres and implicate OH(*) radicals and glutathione as likely mediators of the effects.

Chloride conductance in the transverse tubular system of rat skeletal muscle fibres: importance in excitation-contraction coupling and fatigue

Contraction in skeletal muscle fibres is governed by excitation of the transverse-tubular (t-) system, but the properties of the t-system and their importance in normal excitability are not well defined. Here we investigate the properties of the t-system chloride conductance using rat skinned muscle fibres in which the sarcolemma has been mechanically removed but the normal excitation-contraction coupling mechanism kept functional. When the t-system chloride conductance was eliminated, either by removal of all Cl(-) or by block of the chloride channels with 9-anthracene carboxylic acid (9-AC) or by treating muscles with phorbol 12,13-dibutyrate, there was a marked reduction in the threshold electric field intensity required to elicit a t-system action potential (AP) and twitch response. Calculations of the t-system chloride conductance indicated that it constitutes a large proportion of the total chloride conductance observed in intact fibres. Blocking the chloride conductance increased the size of the twitch response and was indicative that Cl(-) normally carries part of the repolarizing current across the t-system membrane on each AP. Block of the t-system chloride conductance also reduced tetanic force responses at higher frequency stimulation (100 Hz) and greatly reduced twitch responses in the period shortly after a brief tetanus, owing to rapid loss of t-system excitability during the AP train. Blocking activity of the Na(+)-K(+) pump in the t-system membrane caused loss of excitability owing to K(+) build-up in the sealed t-system, and this occurred approximately 3-4 times faster when the chloride conductance was blocked. These findings show that the t-system chloride conductance plays a vital role during normal activity by countering the effects of K(+) accumulation in the t-system and maintaining muscle excitability.

Na+-K+ pumps in the transverse tubular system of skeletal muscle fibers preferentially use ATP from glycolysis

The Na(+)-K(+) pumps in the transverse tubular (T) system of a muscle fiber play a vital role keeping K(+) concentration in the T-system sufficiently low during activity to prevent chronic depolarization and consequent loss of excitability. These Na(+)-K(+) pumps are located in the triad junction, the key transduction zone controlling excitation-contraction (EC) coupling, a region rich in glycolytic enzymes and likely having high localized ATP usage and limited substrate diffusion. This study examined whether Na(+)-K(+) pump function is dependent on ATP derived via the glycolytic pathway locally within the triad region. Single fibers from rat fast-twitch muscle were mechanically skinned, sealing off the T-system but retaining normal EC coupling. Intracellular composition was set by the bathing solution and action potentials (APs) triggered in the T-system, eliciting intracellular Ca(2+) release and twitch and tetanic force responses. Conditions were selected such that increased Na(+)-K(+) pump function could be detected from the consequent increase in T-system polarization and resultant faster rate of AP repriming. Na(+)-K(+) pump function was not adequately supported by maintaining cytoplasmic ATP concentration at its normal resting level ( approximately 8 mM), even with 10 or 40 mM creatine phosphate present. Addition of as little as 1 mM phospho(enol)pyruvate resulted in a marked increase in Na(+)-K(+) pump function, supported by endogenous pyruvate kinase bound within the triad. These results demonstrate that the triad junction is a highly restricted microenvironment, where glycolytic resynthesis of ATP is critical to meet the high demand of the Na(+)-K(+) pump and maintain muscle excitability.

Transverse tubular system depolarization reduces tetanic force in rat skeletal muscle fibers by impairing action potential repriming

When muscle fibers are repeatedly stimulated, they may become depolarized and force output decline. Excitation of the transverse tubular system (T-system) is critical for activation, but its role in muscle fatigue is poorly understood. Here, mechanically skinned fibers from rat fast-twitch muscle were used, because the sarcolemma is absent but the T-system retains normal excitability and its properties can be studied in isolation. The T-system membrane was fully polarized by bathing the skinned fiber in an internal solution with 126 mM K+ (control solution) or set at partially depolarized levels (approximately −63 and −58 mV) in solutions with 66 or 55 mM K+, respectively, and action potentials (APs) were triggered in the sealed T-system by field stimulation. Prolonged depolarization of the T-system reduced tetanic force proportionately more than twitch force, with greater effect at higher stimulation frequency (responses at 20 and 100 Hz reduced to 71 and 62% in 66 mM K+ and to 54 and 35% in 55 mM K+, respectively). Double-pulse stimulation showed that depolarization increased the repriming period (estimated minimum time before a second AP can be produced) from ∼4 ms to ∼7.5 and 15 ms in the 66 and 55 mM K+ solutions, respectively. These results demonstrate that T-system depolarization reduces tetanic force by impairing AP repriming, rather than by preventing AP generation per se or by inactivating the T-system voltage sensors. The findings also explain why it is advantageous to reduce the rate of motoneuron stimulation to muscles during repeated or prolonged periods of activity.

Long-lasting muscle fatigue: partial disruption of excitation-contraction coupling by elevated cytosolic Ca2+ concentration during contractions

The repeated elevation of cytosolic Ca2+ concentration ([Ca2+]i) above resting levels during contractile activity has been associated with long-lasting muscle fatigue. The mechanism underlying this fatigue appears to involve elevated [Ca2+]i levels that induce disruption of the excitation-contraction (E-C) coupling process at the triad junction. Unclear, however, are which aspects of the activity-related [Ca2+]i changes are responsible for the deleterious effects, in particular whether they depend primarily on the peak [Ca2+]i reached locally at particular sites or on the temporal summation of the increased [Ca2+] in the cytoplasm as a whole. In this study, we used mechanically skinned fibers from rat extensor digitorum longus muscle, in which the normal E-C coupling process remains intact. The [Ca2+]i was raised either by applying a set elevated [Ca2+] throughout the fiber or by using action potential stimulation to induce the release of sarcoplasmic reticulum Ca2+ by the normal E-C coupling system with or without augmentation by caffeine or buffering with BAPTA. Herein we show that elevating [Ca2+]i in the physiological range of 2–20 μM irreversibly disrupts E-C coupling in a concentration-dependent manner but requires exposure for a relatively long time (1–3 min) to cause substantial uncoupling. The effectiveness of Ca2+ released via the endogenous system in disrupting E-C coupling indicates that the relatively high [Ca2+]i attained close to the release site at the triad junction is a more important factor than the increase in bulk [Ca2+]i. Our results suggest that during prolonged vigorous activity, the many repeated episodes of relatively high triadic [Ca2+] can disrupt E-C coupling and lead to long-lasting fatigue.

Calcium phosphate precipitation in the sarcoplasmic reticulum reduces action potential-mediated Ca2+release in mammalian skeletal muscle

During vigorous exercise, Piconcentration levels within the cytoplasm of fast-twitch muscle fibers may reach ≥30 mM. Cytoplasmic Pimay enter the sarcoplasmic reticulum (SR) and bind to Ca2+to form a precipitate (CaPi), thus reducing the amount of releasable Ca2+. Using mechanically skinned rat fast-twitch muscle fibers, which retain the normal action potential-mediated Ca2+release mechanism, we investigated the consequences of Piexposure on normal excitation-contraction coupling. The total amount of Ca2+released from the SR by a combined caffeine/low-Mg2+concentration stimulus was reduced by ∼20%, and the initial rate of force development slowed after 2-min exposure to 30 mM Pi(with or without the presence creatine phosphate). Peak (50 Hz) tetanic force was also reduced (by ∼25% and ∼45% after 10 and 30 mM Piexposure, respectively). Tetanic force responses produced after 30 mM Piexposure were nearly identical to those observed in the same fiber after depletion of total SR Ca2+by ∼35%. Ca2+content assays revealed that the total amount of Ca2+in the SR was not detectably changed by exposure to 30 mM Pi, indicating that Ca2+had not leaked from the SR but instead formed a precipitate with the Pi, reducing the amount of available Ca2+for rapid release. These results suggest that CaPiprecipitation that occurs within the SR could contribute to the failure of Ca2+release observed in the later stages of metabolic muscle fatigue. They also demonstrate that the total amount of Ca2+stored in the SR cannot drop substantially below the normal endogenous level without reducing tetanic force responses.

Effect of low cytoplasmic [ATP] on excitation-contraction coupling in fast-twitch muscle fibres of the rat

In this study we investigated the roles of cytoplasmic ATP as both an energy source and a regulatory molecule in various steps of the excitation-contraction (E-C) coupling process in fast-twitch skeletal muscle fibres of the rat. Using mechanically skinned fibres with functional E-C coupling, it was possible to independently alter cytoplasmic [ATP] and free [Mg2+]. Electrical field stimulation was used to elicit action potentials (APs) within the sealed transverse tubular (T-) system, producing either twitch or tetanic (50 Hz) force responses. Measurements were also made of the amount of Ca2+ released by an AP in different cytoplasmic conditions. The rate of force development and relaxation of the contractile apparatus was measured using rapid step changes in [Ca2+]. Twitch force decreased substantially (approximately 30%) at 2 mm ATP compared to the level at 8 mm ATP, whereas peak tetanic force only declined by approximately 10% at 0.5 mm ATP. The rate of force development of the twitch and tetanus was slowed only slightly at [ATP] > or = 0.5 mm, but was slowed greatly (> 6-fold) at 0.1 mm ATP, the latter being due primarily to slowing of force development by the contractile apparatus. AP-induced Ca2+ release was decreased by approximately 10 and 20% at 1 and 0.5 mm ATP, respectively, and by approximately 40% by raising the [Mg2+] to 3 mm. Adenosine inhibited Ca2+ release and twitch responses in a manner consistent with its action as a competitive weak agonist for the ATP regulatory site on the ryanodine receptor (RyR). These findings show that (a) ATP is a limiting factor for normal voltage-sensor activation of the RyRs, and (b) large reductions in cytoplasmic [ATP], and concomitant elevation of [Mg2+], substantially inhibit E-C coupling and possibly contribute to muscle fatigue in fast-twitch fibres in some circumstances.

Effect of Carnosine on Excitation–Contraction Coupling in Mechanically-Skinned Rat Skeletal Muscle

This study investigated whether carnosine alters individual processes involved in normal excitation-contraction (E-C) coupling in mammalian skeletal muscle fibres. Mechanically-skinned fibre preparations were utilized because they allow carnosine to be precisely and readily applied to the cytoplasmic environment as desired, whilst still retaining the normal E-C coupling mechanism. Carnosine caused an increase (approximately +0.02 to approximately +0.09 pCa units) in Ca2+ -sensitivity of the contractile apparatus in a concentration-dependent manner (i.e. with 4, 8 and 16 mM respectively). Force responses elicited by 8 mM caffeine-induced Ca2+ release from the sarcoplasmic reticulum (SR) were potentiated in the presence of carnosine (compared to the bracketing responses in the absence of carnosine). Force responses elicited by transverse tubular (T-) system depolarization via the dihydropyridine receptors (DHPRs), either by ionic (Na+) substitution or by action potentials (APs), were also potentiated in a concentration-dependent manner in the presence of carnosine. The potentiation of the force responses in all of the various experiments was seemingly totally explicable by the increase in Ca2+-sensitivity of the contractile apparatus caused by carnosine. Thus, these results show that carnosine potentiates force responses solely by 'sensitizing' the contractile apparatus to Ca2+ ions and under physiological conditions does not cause additional Ca2+ release from the SR.

L(+)-lactate does not affect twitch and tetanic responses in mechanically skinned mammalian muscle fibres

This study investigated whether a high intracellular concentration of L(+)-lactate (30 mM) affects normal excitation-contraction coupling in skeletal muscle. Electrical stimulation was used to elicit action potentials in the (sealed) transverse-tubular system of mechanically skinned muscle fibres, giving rise to twitch and tetanic force responses. As the sarcolemma was absent, lactate could be applied to the cytoplasmic environment via the bathing solution (at a constant pH of 7.1) and its effect examined independently of other metabolic changes that occur during muscle fatigue. The presence of 30 mM lactate had virtually no effect on direct activation of the contractile apparatus by Ca2+. Lactate also had no significant effect on either the rate of rise or the peak of the twitch response, with the only detectable effect being a slight (13%) slowing in its relaxation rate. As the amplitude of the twitch response (approximately 60% of maximum force) may be regarded as a sensitive indicator of the amount of Ca2+ released by an action potential, there was evidently to change in Ca2+ release in the presence of lactate. Lactate also had no significant effect on the rate of rise and peak force of the tetanic response or on its subsequent relaxation. Additional experiments, in which the sarcoplasmic reticulum was emptied of Ca2+ (in a caffeine solution) and reloaded repeatedly, showed no significant effect of 30 mM lactate on Ca2+ uptake. This study shows that the presence of L(+)-lactate does not inhibit excitation-contraction coupling in mechanically skinned fibres.

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.