Malignant Hyperthermia Susceptible Swine Research Articles

In vitro effects of levosimendan on muscle of malignant hyperthermia susceptible and non-susceptible swine

BackgroundThe calcium sensitizer levosimendan is increasingly used to improve hemodynamics in patients with acutely decompensated heart failure. By binding to cardiac troponin C the conformation of the calcium-troponin C complex is stabilized, which leads to acceleration of actin-myosin crossbrigde formation and increased force generating capacity of muscle fibers. Besides indications in cardiac failure, beneficial effects of levosimendan in skeletal muscle disorders are currently evaluated. The aim of this study was to investigate differential effects of levosimendan on skeletal muscle of pigs with and without susceptibility to malignant hyperthermia (MH) in order to identify possible risks of this emerging drug for patients with predisposition to MH.MethodsMuscle bundles of 17 pigs (9 MH susceptible (MHS); 8 MH non-susceptible (MHN)) were excised under general anesthesia and examined in the tissue bath with increasing concentrations of levosimendan (0.065; 0.125; 0.5; 1.0; 10 and 50 μg/ml). Baseline tension and twitch force were monitored continuously. Data are presented as median and interquartile range. Statistical evaluation was performed using D’Agostino & Pearson test for normal distribution and student’s t test and 2-way ANOVA for differences between the groups. P < 0.05 was considered significant.ResultsThere were no differences between the groups concerning length, weight, initial twitch force and pre-drug resting tension of the investigated muscle strips. After an initial decrease in both groups, twitch amplitude was significantly higher in MHN (− 3.0 [− 5.2–0.2] mN) compared to MHS (− 7.5 [− 10.8- -4.5] mN) (p = 0.0034) muscle at an applied levosimendan concentration of 50 μg/ml. A marked increase in resting tension was detected following levosimendan incubation with 50 μg/ml in MHS muscle bundles (3.3 [0.9–6.1] mN) compared to MHN (− 0.7 [− 1.3–0.0] mN) (p < 0.0001).ConclusionsThis in vitro investigation revealed the development of significant contractures in muscle bundles of MHS pigs after incubation with levosimendan. However, the effect appeared only at supra-therapeutic concentrations and further research is needed to determine the impact of levosimendan on MHS individuals in vivo.

Ca2+ Influx via the Na+/Ca2+ Exchanger Is Enhanced in Malignant Hyperthermia Skeletal Muscle

Malignant hyperthermia (MH) is potentially fatal pharmacogenetic disorder of skeletal muscle caused by intracellular Ca(2+) dysregulation. NCX is a bidirectional transporter that effluxes (forward mode) or influxes (reverse mode) Ca(2+) depending on cellular activity. Resting intracellular calcium ([Ca(2+)]r) and sodium ([Na(+)]r) concentrations are elevated in MH susceptible (MHS) swine and murine muscles compared with their normal (MHN) counterparts, although the contribution of NCX is unclear. Lowering [Na(+)]e elevates [Ca(2+)]r in both MHN and MHS swine muscle fibers and it is prevented by removal of extracellular Ca(2+) or reduced by t-tubule disruption, in both genotypes. KB-R7943, a nonselective NCX3 blocker, reduced [Ca(2+)]r in both swine and murine MHN and MHS muscle fibers at rest and decreased the magnitude of the elevation of [Ca(2+)]r observed in MHS fibers after exposure to halothane. YM-244769, a high affinity reverse mode NCX3 blocker, reduces [Ca(2+)]r in MHS muscle fibers and decreases the amplitude of [Ca(2+)]r rise triggered by halothane, but had no effect on [Ca(2+)]r in MHN muscle. In addition, YM-244769 reduced the peak and area under the curve of the Ca(2+) transient elicited by high [K(+)]e and increased its rate of decay in MHS muscle fibers. siRNA knockdown of NCX3 in MHS myotubes reduced [Ca(2+)]r and the Ca(2+) transient area induced by high [K(+)]e. These results demonstrate a functional NCX3 in skeletal muscle whose activity is enhanced in MHS. Moreover reverse mode NCX3 contributes to the Ca(2+) transients associated with K(+)-induced depolarization and the halothane-triggered MH episode in MHS muscle fibers.

Ca2+ Influx Mediated by Reverse Mode of Na+/Ca2+ Exchanger is Enhanced in Malignant Hyperthermia Skeletal Muscle

Malignant hyperthermia (MH), is potentially fatal pharmacogenetic disorder of skeletal muscle, which has been associated to intracellular Ca2+ dysregulation. The Na+/Ca2+ exchange (NCX) is a bidirectional transporter that normally extrudes Ca2+ from the cell (forward mode), but also brings Ca2+ into the cell (reverse mode) under special conditions such as intracellular Na+ accumulation or membrane depolarization. Resting intracellular Ca2+ concentration ([Ca2+]r) and intracellular Na+ concentration ([Na+]r) were elevated in MH susceptible (MHS) swine and mouse muscles compared to Wt (MHN) muscle fibers. Lowering extracellular Na+ induced an elevation of [Ca2+]r in both MHN and MHS swine and rodent muscle fibers that could be prevented by removal of extracellular Ca2+. Local application of KB-R7943, a nonselective NCX blocker, reduced [Ca2+]r in both MHN and MHS muscle fibers and the magnitude of the elevation of [Ca2+]r observed during a MH episode. YM-2444769 an NCX blocker that preferentially inhibits NCX3 reverse mode did not reduce resting [Ca2+]r in MHN, but it does in MHS muscle fibers, and decreased the amplitude of the elevation [Ca2+]r induced by halothane in these muscles. These results demonstrate the existence of a functional NCX in skeletal muscle, which appears to be enhanced in MHS muscle fibers. In addition, they represent the first evidence that NCX -reverse mode- contributes to the dysfunction of [Ca2+]r and [Na+]r and intracellular Ca2+ elevation induced by halothane in MHS muscle.Funding: NIH AR43140, AR052534 (PDA, JRL)

When best evidence is rarer than the disease

To the Editor, Dr. Bryson raises important questions in his editorial, ‘‘Rarity and review: evidence-based practice for uncommon diseases’’. For many rare disorders, such as carcinoid heart disease, ‘‘Best Evidence’’ is a case series that took 18 years to assemble! Animal models of rare diseases provide another source of evidence. The porcine model of malignant hyperthermia (MH) has been used to develop human diagnostic tests and showed the efficacy of dantrolene therapy at a time when human MH mortality was 80%. Most of what we know about MH comes from such studies and from epidemiologic work. There will never be a human randomized controlled trial (RCT) to determine the optimal dose of dantrolene to treat MH. Instead, MH in susceptible swine was considered similar enough to human MH to extrapolate those results with dramatic effect on mortality. Now new ex vivo models, such as tissue myotubes or cell cultures, provide functional assays to study MH pathophysiology and therapeutics. The Oxford Centre for Evidence-Based Medicine (OCEBM) Levels of Evidence (2011) and the PRISMA statement, cited by Dr. Bryson, are silent on such nonclinical studies. The OCEBM lists systematic review of human RCTs as an example of Level 1 evidence but describes ‘‘mechanism-based reasoning’’ as the lowest level (Level 5). It does not dismiss such weaker evidence, but it does not specify how it should be evaluated. Recently, the United States Agency for Healthcare Research and Quality (AHRQ) released a white paper outlining a framework to evaluate ‘‘the evidential weight of mechanistic knowledge’’. It provides a definition of evidence based on inferential effect (how things work), not study design (how information was produced), as is the case in evidence-based medicine (EBM). It suggests that the strength of inferential evidence is ‘‘represented by the degree to which the probability of a clinical effect is modified by evidence from the component steps’’. Those steps are a formalized evaluation of mechanistic evidence that includes seeking a ‘‘translation’’ step that significantly raises the probability that the evidence could be applied to humans. Pharmaceutical companies already practice in a similar way to bring new compounds to market. The AHRQ considers that such a framework could link basic science, EBM, and comparative effectiveness research. The translation of evidence from such mechanistic knowledge could help strengthen systematic reviews of rare disorders.

Preparation of the Drager Fabius CE and Drager Zeus anaesthetic machines for patients susceptible to malignant hyperthermia

Malignant hyperthermia may follow exposure to trace quantities of inhalational anaesthetics. In susceptible patients, the complete avoidance of these triggers is advised when possible; however, failing this, it is essential to washout or purge the anaesthesia machine of residual inhalational anaesthetics. This study examined the washout profile of sevoflurane from the Drager Fabius CE and the Drager Zeus machines. The washout profile of sevoflurane was measured from the Fabius CE and Zeus anaesthesia machines following a standard period of exposure. The disposable tubing, CO2 absorber and other components of each machine were then replaced to examine their impact on the retention of sevoflurane. The effect of autoclaving the ventilator diaphragm and non-disposable ventilator tube or substituting for a new diaphragm and ventilation tube were examined in later parts of this study. University teaching hospital. Time taken to reach 5 parts per million of sevoflurane when machines underwent standard washout with fresh gas flush. The concentration of sevoflurane reached 5 parts per million in the Fabius CE machines after an mean (SD) of 140 min (46) at a fresh gas flow (FGF) of 10 l min(-1). The time taken for sevoflurane to reach 5 parts per million was significantly reduced when the ventilator diaphragm and non-disposable tube were replaced with either new or autoclaved components [14 or 22 min, respectively (P = 0.017, P = 0.031)]. The concentration of sevoflurane reached 5 parts per million in the Zeus machines after an mean (SD) of 85 min (6) at a fresh gas flow of 10 l min(-1). When the fresh gas flow was increased to 18 l min(-1) (the maximum allowable), the time to reach 5 parts per million was reduced to 16 min. When preparing the Fabius CE for the malignant hyperthermia susceptible patient, remove the vaporiser, replace the disposable tubing, the reservoir bag and the CO2 absorber. Replace the ventilator diaphragm and non-disposable ventilator tube with new or autoclaved components and flush the machine at 10 l min(-1) for at least 36 min. When preparing the Zeus, remove the vaporiser, replace the disposable tubing, the reservoir bag and CO2 absorber and flush at a fresh gas flow of 10 l min(-1) for at least 90 min. In both the Fabius and Zeus, continue at a fresh gas flow of 10 l min(-1) for the duration of the operation.

In-Vitro and In-Vivo Effects of the Local Anaesthetic Dibucaine on Malignant Hyperthermia–Susceptible and Normal Swine

Background: Ten case reports have described clinical signs and symptoms of malignant hyperthermia (MH) that developed following local anaesthetics. In this study, the In-Vitro effects of dibucaine, an amide local anaesthetic, were compared in skeletal-muscle samples from MH-susceptible (MHS) and normal (MHN) swine. In addition, we investigated the In-Vivo MH trigger potency of dibucaine in MHS swine. Methods: Trigger-free general anaesthesia was induced in six MHS and six MHN swine. Muscle biopsies were obtained. In the In-Vivo experiment, cumulative doses of dibucaine (1, 2, 4, 8, 16, and 32 mg/kg) were administered i.v. In the In-Vitro contracture test (IVCT), dibucaine was added cumulatively to muscle specimens from eight MHS and eight MHN muscles (bath concentrations: 0.1, 0.2, 0.4, 0.6, and 1.0 mmol/l). Results: In the In-Vivo test, no signs or symptoms of MH were observed. In-Vitro contractures developed in all muscle specimens, but they developed significantly faster and were more intense in MHS than in MHN muscles. Conclusion: Dibucaine is not a MH trigger in swine. The IVCT with dibucaine demonstrated overlaps between the diagnostic groups, so that the possibility of 100 % accurate discrimination between MHS and MHN individuals by this means does not seem likely.

Phosphodiesterase-III-inhibition with amrinone leads to contracture development in skeletal muscle preparations of malignant hyperthermia susceptible swine

The phosphodiesterase-III (PDE-III) inhibitor enoximone-induced marked contractures in skeletal muscle specimens of malignant hyperthermia (MH) susceptible (MHS) human beings and swine. Whether this is a substance specific effect of enoximone or caused by inhibition of PDE-III remained unclear. Therefore, the effects of the PDE-III inhibitor amrinone in porcine MH normal (MHN) and MHS skeletal muscles were investigated. MH-trigger-free general anaesthesia was performed in eight MHS and eight MHN swine. The MH status of the swine was determined by detection of the Arg615-Cys point mutation on chromosome 6 indicating MH susceptibility. Skeletal muscle specimens were excised for the in vitro contracture tests with amrinone. Amrinone was added cumulatively every 5 min to muscle specimens in order to obtain organ bath concentrations between 20 and 400 micromol L(-1). The in vitro effects of amrinone on muscle contractures and twitches were measured. Amrinone-induced contractures in all skeletal muscle preparations. MHS muscles developed contractures at significantly lower bath concentrations of amrinone than MHN muscles. Contractures of MHS compared to MHN muscles were significantly larger at bath concentrations of 80, 100, 150, 200 and 400 micromol L(-1) amrinone. Muscle twitches remained unchanged up to and including 200 micromol L(-1) amrinone. Inhibition of PDE-III in general elicited higher contractures in MHS than in MHN muscles. Therefore, a contribution of PDE-III and the cyclic adenosine monophosphate (cAMP) system in the pathophysiology of MH must be suspected.

Inhibition of Sarcoplasmic Ca2+Adenosine Triphosphatase in Porcine Skeletal Muscle Samples with Cyclopiazonic Acid Enables In Vitro Malignant Hyperthermia Discrimination

Inhibition of Sarcoplasmic Ca2+Adenosine Triphosphatase in Porcine Skeletal Muscle Samples with Cyclopiazonic Acid Enables In Vitro Malignant Hyperthermia Discrimination

Dantrolene Use in 3,4-Methylenedioxymethamphetamine (“Ecstasy”)–Mediated Hyperthermia

We thank Dr. Rusyniak et al. for their critical comments on our study about the induction of malignant hyperthermia (MH) in susceptible swine by 3,4-methylenedioxymethamphetamine (MDMA) (“ecstasy”).1However, some of the criticisms of our study must be relativized.First, to our knowledge, this is the first controlled study investigating the association between MDMA-induced hypermetabolic syndrome and MH. MH crisis is an acute clinical complication; therefore, the experimental setting for this study was following the clinical situation, and diagnosis of MH in our experiment could only be based on clinical parameters. The definition of the clinical cutoff parameters for MH crisis in our study was following the recommendations for clinical diagnosis of human MH crisis and previous animal studies. Increasing doses of MDMA induced a hypermetabolic state in MH-susceptible (MHS) as well as MH-normal (MHN) swine. However, the changes in the MHN swine after receiving a higher dose of MDMA (12 mg/kg) were moderate compared with the changes in MHS swine after 8 mg/kg MDMA, and all MHS swine fulfilled the defined criteria for MH.The only known differentiation between MHS and MHN swine is the presence of the Arg615-Cys point mutation on chromosome 6 leading to a functional impairment of the skeletal muscle ryanodine receptor (RyR1). We share the opinion of Dr. Rusyniak et al. that MDMA-induced hypermetabolism is not solely mediated through RyR1 complexes. However, the different reactions of MHS and MHN swine in our study are an indirect hint for activation of RyR1 after in vivo MDMA administration. The current study was aimed to prove whether MDMA is capable of inducing an MH syndrome, not to clarify the exact pathomechanism, i.e. , a possible mediation via the RyR1. Whether the RyR1 activation could be attributed to a direct effect of MDMA at the skeletal muscle or to a secondary effect of central stimulation, hyperthermia, or an MDMA-metabolite must therefore be clarified in future studies.The definition of an MH “trigger” is not as clear as mentioned in the letter of Dr. Rusyniak et al. From a clinical point of view, an MH trigger is a substance that is able to induce an MH crisis in a genetically determined individual in a clinically relevant dosage without any relevant cofactors. Following this definition, MDMA triggered MH in MHS swine in our study. We agree that cumulative intravenous administration of MDMA is not the common method of MDMA abuse. However, this course of action and measurement of corresponding MDMA plasma concentrations allowed us to determine a dose response and to underline the clinical relevance.The therapeutic regimen of MDMA-induced MH in our study was based on the standard clinical therapy of MH. Standardized therapy of MH in the MHS swine performed with dantrolene, sodium bicarbonate, and hyperventilation partly removed the clinical signs of MH immediately. The body temperature of the swine remained unchanged 15 min after therapy induction. We agree that the short observation time without the possibility to detect changes in body temperature was a weakness in our study design.Whether administration of dantrolene is useful in all patients with MDMA-induced hyperthermia could not be answered by our study. However, in a life-threatening clinical situation, “simple hyperthermia” could not be distinguished from “true malignant hyperthermia.” Therefore, in our opinion, dantrolene might be a lifesaving therapy option, and consequently, administration of dantrolene should be considered with respect to patient safety in cases of MDMA-mediated hyperthermic syndrome.* University Hospital Hamburg-Eppendorf, Hamburg, Germany. fiege@uke.uni-hamburg.de

Induction of Malignant Hyperthermia in Susceptible Swine by 3,4-Methylenedioxymethamphetamine (“Ecstasy”)

3,4-Methylenedioxymethamphetamine (MDMA, "ecstasy") can mediate acute toxic effects such as muscle rigidity, metabolic acidosis, and hyperthermia. Because of close clinical similarities, an association between MDMA intoxication and malignant hyperthermia (MH) was suggested. The aim of this study was to investigate whether MDMA is a trigger of MH in susceptible swine. MH-nontriggering general anesthesia was performed in six MH-susceptible (MHS) and six MH-normal swine. The animals were exposed to MDMA in cumulative doses of 0.5, 1, 2, 4, 8, and 12 mg/kg. The clinical occurrence of MH was defined by achievement of two of three conditions: central venous Pco2 >/=75 mmHg, central venous pH </= 7.20, and increase of body core temperature >/= 2.0 degrees C. Once MH occurred, a standardized therapy with dantrolene, sodium bicarbonate, and hyperventilation with 100% oxygen was initialized. Administration of 8 mg/kg MDMA triggered MH in all MHS swine. The MH-normal swine also developed clinical signs of hypermetabolism, but even after administration of 12 mg/kg MDMA, changes were moderate compared with the MHS swine. Dantrolene therapy of MDMA-induced MH crisis in the MHS swine partially counteracted the clinical signs of MH immediately. MDMA induces MH in genetically susceptible swine in relevant doses. Therefore, MHS patients should avoid use of MDMA or related drugs. Patients with a personal or family history of MDMA-induced hyperthermia should be tested for a diagnosis of MH susceptibility. Dantrolene is effective in therapy of MDMA-induced porcine MH.

In vitro and in vivo effects of the phosphodiesterase-III inhibitor enoximone on malignant hyperthermia-susceptible swine.

In human skeletal muscles, the phosphodiesterase-III inhibitor enoximone induces in vitro contracture development, and it has been suggested that enoximone could trigger malignant hyperthermia (MH). In this study, the in vitro and in vivo effects of enoximone in MH-normal (MHN) and MH-susceptible (MHS) swine were investigated. Malignant hyperthermia trigger-free general anesthesia was performed in MHS and MHN swine. Skeletal muscle specimens were excised for an in vitro contracture test with 0.6 mm enoximone. Thereafter, MHS and MHN swine were exposed to cumulative administration of 0.5, 1, 2, 4, 8, 16, and 32 mg/kg enoximone intravenously. Clinical occurrence of MH was defined by a Pco(2) greater than 70 mmHg, a pH less than 7.20, and an increase in body temperature of more than 2.0 degrees C. Enoximone induced marked contractures in all MHS muscle specimens in vitro. In contrast, only small or no contracture development was observed in MHN muscle specimens, without an overlap in contractures between MHS and MHN muscles. However, in vivo, no clinical differences were found between MHS and MHN swine following cumulative enoximone doses. According to the defined criteria, none of the swine developed MH during the experiment. Furthermore, high enoximone doses induced progressive circulatory insufficiency, and after receiving 32 mg/kg enoximone, all animals died of cardiovascular failure. The cumulative enoximone doses used in this study were 30- to 50-fold higher than therapeutic doses in humans. Enoximone does not trigger MH in genetically determined swine. However, enoximone might be useful for in vitro diagnosis of MH.

Cyclopiazonic Acid Induce Contractures in Isolated Skeletal Muscles of Malignant Hyperthermia Susceptible Swine

Cyclopiazonic Acid Induce Contractures in Isolated Skeletal Muscles of Malignant Hyperthermia Susceptible Swine

Can 4-chloro-3-ethylphenol be used in-vitro to differentiate between malignant hyperthermia susceptible and non-susceptible swine?

Can 4-chloro-3-ethylphenol be used in-vitro to differentiate between malignant hyperthermia susceptible and non-susceptible swine?

In vitro effects of 4-chloro-3-ethylphenol in skeletal muscle preparations from malignant hyperthermia susceptible and normal swine

In vitro effects of 4-chloro-3-ethylphenol in skeletal muscle preparations from malignant hyperthermia susceptible and normal swine

In vitro effects of 4-chloro-3-ethylphenol in skeletal muscle preparations from malignant hyperthermia susceptible and normal swine

The in vitro contracture test with halothane and caffeine is the current gold standard for diagnosis of malignant hyperthermia. This test has a sensitivity of 99.0% but a specificity of only 93.6%. Therefore, an alternative drug is desirable which distinguishes between malignant hyperthermia-susceptible and malignant hyperthermia-normal subjects with a higher specificity and sensitivity. 4-chloro-3-ethylphenol has recently been shown to trigger Ca2+-induced Ca2+-release in skeletal muscle terminal cisternae and to increase the myoplasmic free Ca2+ concentration in skeletal muscle fibres. The purpose of this study was to investigate the ability of 4-chloro-3-ethylphenol to distinguish between malignant hyperthermia-susceptible and malignant hyperthermia-normal porcine muscle specimen in the in vitro contracture test. Ten malignant hyperthermia-susceptible and 14 malignant hyperthermia-normal swine were anaesthetized and muscle biopsies were taken. For the in vitro contracture test muscle specimens were exposed to cumulative concentrations of 4-chloro-3-ethylphenol (12.5 to 200 micromol L(-1)). 4-chloro-3-ethylphenol produced contractures in a concentration-dependent manner in the malignant hyperthermia-susceptible muscle bundles. In contrast, cumulative 4-chloro-3-ethylphenol did not generate contractures in malignant hyperthermia-normal specimens. Contractures were significantly greater (P < 0.05) in the malignant hyperthermia-susceptible compared to the malignant hyperthermia-normal preparations in all 4-chloro-3-ethylphenol concentration steps from 50 micromol L(-1) to 200 micromol L(-1). There was no overlap between the two groups above a concentration of 75 micromol L(-1) in cumulative 4-chloro-3-ethylphenol in vitro contracture tests. It remains to be verified whether an in vitro contracture test with 4-chloro-3-ethylphenol can also discriminate between malignant hyperthermia-susceptible and malignant hyperthermia-normal humans. Since no prior tested agent revealed a clear differentiation in contracture development without overlap, the 4-chloro-3-ethylphenol test might be a promising new approach to the diagnosis of malignant hyperthermia.

Prolonging viability of swine muscle biopsy specimens in malignant hyperthermia testing.

In North America, the caffeine halothane contracture test (CHCT) is the standard test for the diagnosis of malignant hyperthermia (MH). Current CHCT protocol recommends that the test be completed within 5 h of muscle excision. The purpose of this study was to investigate whether the period of skeletal muscle viability could be extended to 24 h. We tested the gracilis muscle from normal (n = 8) and MH-susceptible swine (n = 8). After baseline (1-2 h after excision) CHCT, the remaining muscles were placed into one of the following four treatment groups. In Groups 1 and 2, the muscles remained under tension and were stored in Krebs buffer (pH 7.4) at 23 degrees C-25 degrees C (clamped-warm) and 4 degrees C (clamped-cold), respectively. In Groups 3 and 4, the muscle strips were dissected, and the ends were tied with silk sutures, cut from the clamp, and placed in Krebs buffer at 23 degrees C-25 degrees C (free-warm) and 4 degrees C (free-cold), respectively. The responses of the treatment groups to halothane (3%) and caffeine (0.5-32 mM) were tested 22-26 h after excision. The clamped-warm storage was the only storage method to correctly diagnose MH susceptibility in all muscle strips tested. This finding was also confirmed in muscle stored under clamped-warm conditions and shipped overnight to another testing center for a parallel CHCT.

Hypersensitivity of malignant hyperthermia-susceptible swine skeletal muscle to caffeine is mediated by high resting myoplasmic [Ca2+

Malignant hyperthermia (MH) is an inherited pharmacogenetic syndrome that is triggered by halogenated anesthetics and/or depolarizing muscle relaxants. MH-susceptible (MHS) skeletal muscle has been shown to be more sensitive to caffeine-induced contracture than muscle from nonsusceptible (MHN) subjects and is the basis for the most commonly used clinical diagnostic test to determine MH susceptibility. We studied the effects of caffeine on myoplasmic free calcium concentration ([Ca2+]i) in MHN and MHS swine muscle fibers by means of Ca2+-selective microelectrodes before and after K+-induced partial depolarization. [Ca2+]i in untreated MHN fibers was 123 +/- 8 nm versus 342 +/- 33 nm in MHS fibers. Caffeine (2 mm) caused an increase in [Ca2+]i in both groups (296 +/- 41 nm MHN vs. 1,159 +/- 235 nm MHS) with no change in resting membrane potential. When either MHN or MHS, muscle fibers were incubated in 10 mm K+ [Ca2+]i transiently increased to 272 +/- 22 nm in MHN and 967 +/- 38 nm in MHS for 6-8 min. Exposure of MHN fibers to 2 mm caffeine while resting [Ca2+]i was elevated induced an increment in [Ca2+]i to 940 +/- 37 nm. After 6-8 min of exposure to 10 mm K+, [Ca2+]i returned to control levels in all fibers, and the effect of 2 mm caffeine on resting [Ca2+]i returned to control, despite continued partial membrane depolarization. These results suggest that the increased "sensitivity" to caffeine of MHS swine muscle fibers is a nonspecific response related, at least in part, to the high resting [Ca2+]i and not an increased caffeine sensitivity of the sarcoplasmic reticulum Ca2+ release channel per se.

Xenon does not trigger malignant hyperthermia in susceptible swine.

Xenon is a noble gas with anesthetic properties currently under investigation for use in humans. This study was performed to evaluate whether xenon may trigger malignant hyperthermia in susceptible swine. Nine malignant hyperthermia-sensitive swine (Pietrain) were initially anesthetized with pentobarbital and then ventilated with 70% xenon in oxygen for 2 h. Heart rate, mean arterial pressure, cardiac output, body temperature, arterial and mixed-venous blood gases, and plasma catecholamine and lactate levels were measured every 10 min both during xenon-oxygen ventilation and after a 30-min xenon washout phase followed by subsequent administration of halothane (1% inspired) and succinylcholine (3 mg/kg intravenous). During the investigation, no malignant hyperthermia-specific therapy was instituted. Xenon exposure did not induce any changes in metabolic and hemodynamic parameters nor elevations of the plasma catecholamine levels indicative for an episode of malignant hyperthermia. By contrast, in all animals, within 20 min after the administration of halothane and succinylcholine, fulminant and fatal malignant hyperthermia episodes were initiated. The authors conclude that xenon does not trigger malignant hyperthermia in susceptible swine.

4-chloro-m-cresol triggers malignant hyperthermia in susceptible swine at doses greatly exceeding those found in drug preparations.

Chlorocresols are used as preservatives in numerous commercial drugs that have been shown to induce myoplasmic Ca2+ release; the most potent isoform is 4-chloro-m-cresol. The aims of this study were to (1) examine the in vivo effects of 4-chloro-m-cresol on swine susceptible to malignant hyperthermia and (2) contrast in vivo versus in vitro dose-response curves. Susceptible swine (weight: 38.5 kg+/-3.55 kg) were anesthetized and monitored for variations in physiological responses, including end-tidal CO2, heart rate, blood pressure, blood chemistry, and temperatures. In the first animals studied, 4-chloro-m-cresol, at equivalent cumulative doses of 0.14, 0.28, 0.57, 1.14, 2.27, 4.54, and 9.08 mg/kg (n = 3; 12.5, 25, 50, 100, 200, 400, and 800 micromol) were administered, and in a second group, larger doses were used: 1.14, 3.41, 7.95, 17.04 (n = 4), and/or 35.22 (n = 1) mg/kg (100, 300, 700, 1,500, and/or 3,100 micromol). For comparison, in vitro rectus abdominis muscle preparations obtained from normal and susceptible swine were exposed to 4-chloro-m-cresol, at cumulative concentrations of 6.25, 12.5, 25, 50, 100, 200, 400, 800, and 1,600 micromol; standard caffeine and halothane contracture testing was also performed. Episodes of malignant hyperthermia were not triggered in response to administration of low doses of 4-chloro-m-cresol, but transient cardiovascular reactions (e.g., tachycardia, arrhythmias, and hypotension) were observed. Subsequently, episodes in these animals were triggered when halothane (0.87; 1 MAC) and succinylcholine (2 mg/kg) were given. Animals administered the higher doses of 4-chloro-m-cresol all had fulminant episodes of malignant hyperthermia that were fatal, when equivalent cumulative concentrations were greater than 1,500 micromol. The levels of 4-chloro-m-cresol in the plasma rapidly decreased: e.g., 5 min postadministration of the 1,500-micromol dose, the mean plasma level was only 52+/-18 micromol (n = 4). Hemolysis was detected following 4-chloro-m-cresol administration at concentrations > 200 micromol. In vitro, muscle from susceptible animals elicited contractures > 200 mg at 50-micromol bath concentrations of 4-chloro-m-cresol (n = 29), whereas normal muscle did not elicit such contractures until bath concentrations were > 800 micromol (n = 10). 4-chloro-m-cresol is a trigger of malignant hyperthermia in susceptible swine, but only when serum concentrations are far above those likely to be encountered in humans. A relatively low concentration of 4-chloro-m-cresol, 50 micromol, is sufficient to activate sarcoplasmic [Ca+2] release in vitro (e.g., contractures); this same bolus dose administered in vivo (0.57 mg/kg) has minimal effects due to the rapid decrease in its plasma levels.

This Month in ANESTHESIOLOGY

This Month in ANESTHESIOLOGY