Mg2+ dependence of Ca2+ release from the sarcoplasmic reticulum induced by sevoflurane or halothane in skeletal muscle from humans susceptible to malignant hyperthermia

Molecular Aspects Implicated in Dantrolene Selectivity with Respect to Ryanodine Receptor Isoforms

Dantrolene is an intra-cellularly acting skeletal muscle relaxant used for the treatment of the rare genetic disorder, malignant hyperthermia (MH). In most cases, MH susceptibility is caused by dysfunction of the skeletal ryanodine receptor (RyR1) harboring one of nearly 230 single-point MH mutations. The therapeutic effect of dantrolene is the result of a direct inhibitory action on the RyR1 channel, thus suppressing aberrant Ca2+ release from the sarcoplasmic reticulum. Despite the almost identical dantrolene-binding sequence exits in all three mammalian RyR isoforms, dantrolene appears to be an isoform-selective inhibitor. Whereas RyR1 and RyR3 channels are competent to bind dantrolene, the RyR2 channel, predominantly expressed in the heart, is unresponsive. However, a large body of evidence suggests that the RyR2 channel becomes sensitive to dantrolene-mediated inhibition under certain pathological conditions. Although a consistent picture of the dantrolene effect emerges from in vivo studies, in vitro results are often contradictory. Hence, our goal in this perspective is to provide the best possible clues to the molecular mechanism of dantrolene’s action on RyR isoforms by identifying and discussing potential sources of conflicting results, mainly coming from cell-free experiments. Moreover, we propose that, specifically in the case of the RyR2 channel, its phosphorylation could be implicated in acquiring the channel responsiveness to dantrolene inhibition, interpreting functional findings in the structural context.

Sevoflurane induced neurotoxicity in neonatal mice links to a GSK3β/Drp1-dependent mitochondrial fission and apoptosis

Mitochondria damage and apoptosis were found associated with sevoflurane induced neurotoxicity in developing brains of rodent and neuro cell lines. The detailed upstream mechanism remains unclear. This study explored whether sevoflurane induces neurotoxicity by activating a GSK3β (glycogen synthase kinase 3β)/Drp1 (dynamin-related protein-1)-dependent mitochondrial fission and apoptosis. Our results showed that sevoflurane exposure promoted mitochondria fission in hippocampus of neonatal mice, resulted in a prolonged escape latency from P32 (32-day-postnatal) to P35, and decreased platform crossing times on P36 as compared to the control treatment. Additionally, sevoflurane upregulated GSK3β stability and activation, promoted phosphorylation of Drp1 at Ser⁶¹⁶ along with its translocation to mitochondria and resulted in increasing cytochrome c and cleaved casepase-3 in hippocampus of neonatal mice and in human SK-N-SH cells. Simultaneously, sevoflurane promoted the interaction between Drp1 and GSK3β. Furthermore, GSK3β activated phosphorylation of Drp1 at Ser⁶¹⁶, induced mitochondrial fission, loss of mitochondrial membrane potential (MMP) and apoptosis in SK-N-SH cells, which was attenuated by TDZD-8, an inhibitor of GSK3β. In conclusion, sevoflurane induced neurotoxicity links to a GSK3β/Drp1 dependent mitochondrial fission and apoptosis.

The Use of Continuous Blood Purification for the Treatment of Malignant Hyperthermia in an Infant

BACKGROUND

Malignant hyperthermia (MH) is a rare and potentially life-threatening pharmacogenetic disorder encountered during general anesthesia, with the incidence higher in children than in adults. Dantrolene is the specific antagonist of MH, but it is not readily available in China, thus developing alternative treatment protocols is of great practical importance.

CASE PRESENTATION

Herein, the authors report a two-month-old infant who underwent holmium laser epiglottis retrofitting through a bronchoscope, but developed limb muscular stiffness, tachypnea, tachycardia, and hyperthermia after sevoflurane exposure. After the diagnosis of MH, corresponding supportive treatment was implemented. Because there was no dantrolene available, continuous blood purification and mechanical ventilation were performed. A few days later, the boy recovered without any complications.

CONCLUSION

Based on the authors' successful clinical practice, the authors consider continuous blood purification as a reliable treatment for MH. But its feasibility still needs to be clarified after multicenter clinical observations.

Magnesium sulfate - An effective agent could delay the progression of fulminant malignant hyperthermia

Malignant hyperthermia (MH) is a life-threatening disease that occurs during general anaesthesia following exposure to succinylcholine (SCh), a depolarizing muscle relaxant, and volatile anaesthetics. Susceptibility to MH most commonly arises from mutations in the RyR1 gene, the Ca²⁺ release channel of skeletal muscle. Fulminant MH (f-MH) is the most dangerous form of MH, which presents a hypermetabolic cascade state, including very high temperature and carbon dioxide production, increased heart rate and oxygen consumption, mixed acidosis, rigid muscles, and rhabdomyolysis. Dantrolene is the only specific drug therapy for MH on the market. Without dantrolene, the reported mortality of f-MH is as high as 42.3%. Based on the participation of catecholamine in the hyperhaemodynamic response of f-MH and the demonstrated effective control of catecholamine release of magnesium sulfate, combined with the fact that magnesium and calcium have opposite effects on muscle contraction, I hypothesized that magnesium sulfate could be a choice for delaying the progression of f-MH while waiting for dantrolene treatment.

Dantrolene requires Mg2+ to arrest malignant hyperthermia

Malignant hyperthermia (MH) is a clinical syndrome of skeletal muscle that presents as a hypermetabolic response to volatile anesthetic gases, where susceptible persons may develop lethally high body temperatures. Genetic predisposition mainly arises from mutations on the skeletal muscle ryanodine receptor (RyR). Dantrolene is administered to alleviate MH symptoms, but its mechanism of action and its influence on the Ca²⁺ transients elicited by MH triggers are unknown. Here, we show that Ca²⁺ release in the absence of Mg²⁺ is unaffected by the presence of dantrolene but that dantrolene becomes increasingly effective as cytoplasmic-free [Mg²⁺] (free [Mg²⁺]_cyto) passes mM levels. Furthermore, we found in human muscle susceptible to MH that dantrolene was ineffective at reducing halothane-induced repetitive Ca²⁺ waves in the presence of resting levels of free [Mg²⁺]_cyto (1 mM). However, an increase of free [Mg²⁺]_cyto to 1.5 mM could increase the period between Ca²⁺ waves. These results reconcile previous contradictory reports in muscle fibers and isolated RyRs, where Mg²⁺ is present or absent, respectively, and define the mechanism of action of dantrolene is to increase the Mg²⁺ affinity of the RyR (or "stabilize" the resting state of the channel) and suggest that the accumulation of the metabolite Mg²⁺ from MgATP hydrolysis is required to make dantrolene administration effective in arresting an MH episode.

Pharmacogenetics and pathophysiology of CACNA1S mutations in malignant hyperthermia

A review of the pharmacogenetics (PGt) and pathophysiology of calcium voltage-gated channel subunit alpha1 S (CACNA1S) mutations in malignant hyperthermia susceptibility type 5 (MHS5; MIM #60188) is presented. Malignant hyperthermia (MH) is a life-threatening hypermetabolic state of skeletal muscle usually induced by volatile, halogenated anesthetics and/or the depolarizing neuromuscular blocker succinylcholine. In addition to ryanodine receptor 1 (RYR1) mutations, several CACNA1S mutations are known to be risk factors for increased susceptibility to MH (MHS). However, the presence of these pathogenic CACNA1S gene variations cannot be used to positively predict MH since the condition is genetically heterogeneous with variable expression and incomplete penetrance. At present, one or at most six CACNA1S mutations display significant linkage or association either to clinically diagnosed MH or to MHS as determined by contracture testing. Additional pathogenic variants in CACNA1S, either alone or in combination with genes affecting Ca²⁺ homeostasis, are likely to be discovered in association to MH as whole exome sequencing becomes more commonplace.

Malignant hyperthermia, a Scandinavian update

BACKGROUND

Malignant Hyperthermia (MH) is a rare pharmacogenetic disorder, triggered by halogenated anesthetics and/or succinylcholine. In susceptible individuals, these drugs can activate an explosive life threatening clinical reaction. Leading symptoms are hypercarbia, muscle rigidity, and metabolic acidosis. MH is inherited in an autosomal-dominant manner and linked to mutations in the large ryanodine 1 gene (RYR1) gene in the majority of cases. Very few MH patients have been found to carry mutations in the CACNA1S gene.

METHODS

For this review a large litterature search was carried out and the Swedish MH database consisting of 436 probands who have undergone in vitro muscle contraction test (IVCT) during 1984-2014 was analyzed.

RESULTS

Twelve different MH causative mutations have been found in Swedish patients so far. These mutations lead to a disturbed calcium balance in striated muscle tissue. A muscle biopsy for the IVCT or finding of an approved causative mutation are required for the diagnosis.

CONCLUSION

A Malignant Hyperthermia susceptible (MHS) patient should be anesthetized with trigger-free anesthesia. There are a few reports of MH-like reactions in patients unrelated to anesthesia. The outcome is dependent on early recognizing of the reaction and fast disconnection of the trigger agents and administration of dantrolene.

Activation and propagation of Ca2+ release from inside the sarcoplasmic reticulum network of mammalian skeletal muscle

Skeletal muscle fibres are large and highly elongated cells specialized for producing the force required for posture and movement. The process of controlling the production of force within the muscle, known as excitation-contraction coupling, requires virtually simultaneous release of large amounts of Ca(2+) from the sarcoplasmic reticulum (SR) at the level of every sarcomere within the muscle fibre. Here we imaged Ca(2+) movements within the SR, tubular (t-) system and in the cytoplasm to observe that the SR of skeletal muscle is a connected network capable of allowing diffusion of Ca(2+) within its lumen to promote the propagation of Ca(2+) release throughout the fibre under conditions where inhibition of SR ryanodine receptors (RyRs) was reduced. Reduction of cytoplasmic [Mg(2+)] ([Mg(2+)]cyto) induced a leak of Ca(2+) through RyRs, causing a reduction in SR Ca(2+) buffering power argued to be due to a breakdown of SR calsequestrin polymers, leading to a local elevation of [Ca(2+)]SR. The local rise in [Ca(2+)]SR, an intra-SR Ca(2+) transient, induced a local diffusely rising [Ca(2+)]cyto. A prolonged Ca(2+) wave lasting tens of seconds or more was generated from these events. Ca(2+) waves were dependent on the diffusion of Ca(2+) within the lumen of the SR and ended as [Ca(2+)]SR dropped to low levels to inactivate RyRs. Inactivation of RyRs allowed re-accumulation of [Ca(2+)]SR and the activation of secondary Ca(2+) waves in the persistent presence of low [Mg(2+)]cyto if the threshold [Ca(2+)]SR for RyR opening could be reached. Secondary Ca(2+) waves occurred without an abrupt reduction in SR Ca(2+) buffering power. Ca(2+) release and wave propagation occurred in the absence of Ca(2+)-induced Ca(2+) release. These observations are consistent with the activation of Ca(2+) release through RyRs of lowered cytoplasmic inhibition by [Ca(2+)]SR or store overload-induced Ca(2+) release. Restitution of SR Ca(2+) buffering power to its initially high value required imposing normal resting ionic conditions in the cytoplasm, which re-imposed the normal resting inhibition on the RyRs, allowing [Ca(2+)]SR to return to endogenous levels without activation of store overload-induced Ca(2+) release. These results are discussed in the context of how pathophysiological Ca(2+) release such as that occurring in malignant hyperthermia can be generated.

Sevoflurane is less sensitive than halothane for in vitro detection of malignant hyperthermia susceptibility

BACKGROUND

Sevoflurane is a known triggering agent of malignant hyperthermia (MH). The present study analyzed different effects of sevoflurane on skeletal muscle of MH susceptible and nonsusceptible individuals in vitro and compared the results to the standardized test protocol with halothane and caffeine. A potential influence of a present ryanodine receptor type 1 (RyR1) mutation was investigated.

METHODS

Muscle bundles of 24 MH-susceptible patients with or without an RyR1 mutation, 35 MH-nonsusceptible and 10 MH-equivocal patients were exposed either to sevoflurane 8 vol% bolus or increasing doses of 2, 4, 6, and 8 vol%. In MH-positive patients, a screening for mutations in the RyR1 gene was performed.

RESULTS

The in vitro parameters initial length, weight, predrug resting tension, and predrug twitch height did not differ between the groups. Sevoflurane caused significant contractures in MH-susceptible but not in MH-nonsusceptible muscle after increasing doses [1.4 (0.3-6.0) vs. 0 (0-0) mN] and after bolus application [6.9 (2.4-21.4) vs. 0 (0-0) mN]. However, only 50% of the susceptible patients developed contractures ≥ 2 mN after increasing concentrations while 83% did so after rapid bolus administration. Presence of an RyR1 mutation was detected in 36% of the examined MH-positive patients but had no influence on developing contractures.

CONCLUSION

Sevoflurane-induced contractures do not reliably detect MH susceptibility on an individual level. Therefore, sevoflurane is no suitable alternative for diagnostic use. Mutation-specific effects regarding contracture sizes after incubation with sevoflurane, halothane, or caffeine were not found.

Succinylcholine in malignant hyperthermia: evaluation of a novel in vivo model

INTRODUCTION

Malignant hyperthermia (MH) is a potentially lethal anesthetic complication characterized by muscle hypermetabolism and generalized rigor. The exact mechanism of succinylcholine as an MH trigger cannot be examined in existing in vitro models. Therefore, a novel in vivo model was used to examine the metabolic response to succinylcholine.

METHODS

With institutional review board approval, 6 MH susceptible (MHS) and 6 MH non-susceptible (MHN) pigs were anesthetized with hemodynamic and systemic metabolic monitoring. Microdialysis catheters were placed intramuscularly. After equilibration, succinylcholine, halothane, and Ringer solution were injected. Lactate was measured in the dialysate and statistically analyzed by Mann-Whitney U-test (significance level P < 0.05).

RESULTS

Hemodynamic and systemic metabolic parameters were not different between the groups throughout the experiment. In the MHS pigs, halothane induced a significant increase of lactate. In MHN pigs, no substance induced a reaction.

CONCLUSIONS

Halothane, but not succinylcholine, induced a hypermetabolic reaction in this model. Therefore, the role of succinylcholine as an MH trigger remains questionable.

Magnesium does not influence the clinical course of succinylcholine-induced malignant hyperthermia

BACKGROUND

Malignant hyperthermia (MH) is a potentially lethal hypermetabolic syndrome. Volatile anesthetics and/or succinylcholine lead to an increase of the intracellular calcium concentration resulting in activation of various intracellular processes. A production of carbon dioxide, and later lactate, are early signs of increased cellular energy consumption. On a cellular level, magnesium acts as a physiological calcium inhibitor resulting in less-intense calcium liberation from the sarcoplasmic reticulum. In this study, we examined the effects of IV magnesium administration on the clinical course of an MH crisis.

METHODS

Sixteen Pietrain pigs (10 MH-susceptible [MHS] and 6 MH-nonsusceptible [MHN]) were anesthetized without an MH trigger substance. Invasive hemodynamic monitoring was established before 4 mg/kg succinylcholine was administered. Four of the MHS pigs received 10 mg/kg magnesium sulfate 10 minutes later. Hemodynamic changes (heart rate, mean arterial blood pressure, and oxygen saturation as measured by pulse oximetry) were continuously monitored. Venous and arterial blood gases (pH, Pco(2), Po(2), base excess, and lactate) were taken at 15-minute intervals. The H test and U test were used with P < 0.05 for significant differences among the groups.

RESULTS

No differences among the groups were seen for weight, hemodynamic, and metabolic variables before administration of succinylcholine. In all MHS animals, succinylcholine led to a marked decrease of mean arterial blood pressure and increase of heart rate. Animals in both MHS groups developed combined metabolic and respiratory acidosis. Succinylcholine had no effect on animals in the MHN group. Hemodynamic and metabolic values were not different between the 2 MHS groups but were between groups MHS and MHN.

CONCLUSION

Succinylcholine led to a hemodynamic and metabolic reaction in only MHS pigs. Treatment with magnesium did not influence the clinical course. The intervention had no beneficial effect in the acute phase of an MH crisis.

Store-operated Ca2+ entry in malignant hyperthermia-susceptible human skeletal muscle

In malignant hyperthermia (MH), mutations in RyR1 underlie direct activation of the channel by volatile anesthetics, leading to muscle contracture and a life-threatening increase in core body temperature. The aim of the present study was to establish whether the associated depletion of sarcoplasmic reticulum (SR) Ca(2+) triggers sarcolemmal Ca(2+) influx via store-operated Ca(2+) entry (SOCE). Samples of vastus medialis muscle were obtained from patients undergoing assessment for MH susceptibility using the in vitro contracture test. Single fibers were mechanically skinned, and confocal microscopy was used to detect changes in [Ca(2+)] either within the resealed t-system ([Ca(2+)](t-sys)) or within the cytosol. In normal fibers, halothane (0.5 mM) failed to initiate SR Ca(2+) release or Ca(2+)(t-sys) depletion. However, in MH-susceptible (MHS) fibers, halothane induced both SR Ca(2+) release and Ca(2+)(t-sys) depletion, consistent with SOCE. In some MHS fibers, halothane-induced SR Ca(2+) release took the form of a propagated wave, which was temporally coupled to a wave of Ca(2+)(t-sys) depletion. SOCE was potently inhibited by "extracellular" application of a STIM1 antibody trapped within the t-system but not when the antibody was denatured by heating. In conclusion, (i) in human MHS muscle, SR Ca(2+) depletion induced by a level of volatile anesthetic within the clinical range is sufficient to induce SOCE, which is tightly coupled to SR Ca(2+) release; (ii) sarcolemmal STIM1 has an important role in regulating SOCE; and (iii) sustained SOCE from an effectively infinite extracellular Ca(2+) pool may contribute to the maintained rise in cytosolic [Ca(2+)] that underlies MH.

Calcium-Induced Calcium Release in Skeletal Muscle

Calcium-induced calcium release (CICR) was first discovered in skeletal muscle. CICR is defined as Ca2+ release by the action of Ca2+ alone without the simultaneous action of other activating processes. CICR is biphasically dependent on Ca2+ concentration; is inhibited by Mg2+, procaine, and tetracaine; and is potentiated by ATP, other adenine compounds, and caffeine. With depolarization of the sarcoplasmic reticulum (SR), a potential change of the SR membrane in which the luminal side becomes more negative, CICR is activated for several seconds and is then inactivated. All three types of ryanodine receptors (RyRs) show CICR activity. At least one RyR, RyR1, also shows non-CICR Ca2+ release, such as that triggered by the t-tubule voltage sensor, by clofibric acid, and by SR depolarization. Maximum rates of CICR, at the optimal Ca2+ concentration in the presence of physiological levels of ATP and Mg2+ determined in skinned fibers and fragmented SR, are much lower than the rate of physiological Ca2+ release. The primary event of physiological Ca2+ release, the Ca2+ spark, is the simultaneous opening of multiple channels, the coordinating mechanism of which does not appear to be CICR because of the low probability of CICR opening under physiological conditions. The coordination may require Ca2+, but in that case, some other stimulus or stimuli must be provided simultaneously, which is not CICR by definition. Thus CICR does not appear to contribute significantly to physiological Ca2+ release. On the other hand, CICR appears to play a key role in caffeine contracture and malignant hyperthermia. The potentiation of voltage-activated Ca2+ release by caffeine, however, does not seem to occur through secondary CICR, although the site where caffeine potentiates voltage-activated Ca2+ release might be the same site where caffeine potentiates CICR.