Halothane-induced Contractures Research Articles

Halothane modulation of skeletal muscle ryanodine receptors: dependence on Ca2+, Mg2+, and ATP

Malignant hyperthermia (MH) susceptibility is a genetic disorder of skeletal muscle associated with mutations in the ryanodine receptor isoform 1 (RyR1) of sarcoplasmic reticulum (SR). In MH-susceptible skeletal fibers, RyR1-mediated Ca(2+) release is highly sensitive to activation by the volatile anesthetic halothane. Indeed, studies with isolated RyR1 channels (using simple Cs(+) solutions) found that halothane selectively affects mutated but not wild-type RyR1 function. However, studies in skeletal fibers indicate that halothane can also activate wild-type RyR1-mediated Ca(2+) release. We hypothesized that endogenous RyR1 agonists (ATP, lumenal Ca(2+)) may increase RyR1 sensitivity to halothane. Consequently, we studied how these agonists affect halothane action on rabbit skeletal RyR1 reconstituted into planar lipid bilayers. We found that cytosolic ATP is required for halothane-induced activation of the skeletal RyR1. Unlike RyR1, cardiac RyR2 (much less sensitive to ATP) responded to halothane even in the absence of this agonist. ATP-dependent halothane activation of RyR1 was enhanced by cytosolic Ca(2+) (channel agonist) and counteracted by Mg(2+) (channel inhibitor). Dantrolene, a muscle relaxant used to treat MH episodes, did not affect RyR1 or RyR2 basal activity and did not interfere with halothane-induced activation. Studies with skeletal SR microsomes confirmed that halothane-induced RyR1-mediated SR Ca(2+) release is enhanced by high ATP-low Mg(2+) in the cytosol and by increased SR Ca(2+) load. Thus, physiological or pathological processes that induce changes in cellular levels of these modulators could affect RyR1 sensitivity to halothane in skeletal fibers, including the outcome of halothane-induced contracture tests used to diagnose MH susceptibility.

Inhibition of sarcoplasmic Ca2+-ATPase increases caffeine- and halothane-induced contractures in muscle bundles of malignant hyperthermia susceptible and healthy individuals

BackgroundMalignant hyperthermia (MH) is triggered by halogenated anaesthetics and depolarising muscle relaxants, leading to an uncontrolled hypermetabolic state of skeletal muscle. An uncontrolled sarcoplasmic Ca2+ release is mediated via the ryanodine receptor. A compensatory mechanism of increased sarcoplasmic Ca2+-ATPase activity was described in pigs and in transfected cell lines. We hypothesized that inhibition of Ca2+ reuptake via the sarcoplasmic Ca2+-ATPase (SERCA) enhances halothane- and caffeine-induced muscle contractures in MH susceptible more than in non-susceptible skeletal muscle.MethodsWith informed consent, surplus muscle bundles of 7 MHS (susceptible), 7 MHE (equivocal) and 16 MHN (non-susceptible) classified patients were mounted to an isometric force transducer, electrically stimulated, preloaded and equilibrated. Following 15 min incubation with cyclopiazonic acid (CPA) 25 μM, the European MH standard in-vitro-contracture test protocol with caffeine (0.5; 1; 1.5; 2; 3; 4 mM) and halothane (0.11; 0.22; 0.44; 0.66 mM) was performed. Data as median and quartiles; Friedman- and Wilcoxon-test for differences with and without CPA; p < 0.05.ResultsInitial length, weight, maximum twitch height, predrug resting tension and predrug twitch height of muscle bundles did not differ between groups. CPA increased halothane- and caffeine-induced contractures significantly. This increase was more pronounced in MHS and MHE than in MHN muscle bundles.ConclusionInhibition of the SERCA activity by CPA enhances halothane- and caffeine-induced contractures especially in MHS and MHE skeletal muscle and may help for the diagnostic assignment of MH susceptibility. The status of SERCA activity may play a significant but so far unknown role in the genesis of malignant hyperthermia.

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

FK506 (tacrolimus) increases halothane-induced Ca2+ release from skeletal muscle sarcoplasmic reticulum.

FK506 binding protein is closely associated with the sarcoplasmic reticulum ryanodine receptor-channel and can modulate its function. The ryanodine receptor is stabilized by its association with FK506 binding protein. The immunosuppressant drugs FK506 (tacrolimus) and rapamycin can promote dissociation of FK506 binding protein from the ryanodine receptor 1 and by this mechanism increase sensitivity of ryanodine receptor 1 to agonists such as caffeine. Furthermore, it was shown recently that treatment of normal human skeletal muscle with FK506 and rapamycin increased halothane-induced contracture. The authors investigated the effect of the immunosuppressants FK506 and rapamycin on halothane-induced Ca2+ release in skeletal muscle sarcoplasmic reticulum vesicles. Skeletal muscle terminal cisterns were isolated from New Zealand White rabbits. Ca2+ uptake and release was monitored in skeletal muscle sarcoplasmic reticulum vesicles using the fluo-3 fluorescent technique. Western Blot analysis of FK506 binding protein was performed using standard protocol. The authors observed that treatment of skeletal muscle sarcoplasmic reticulum vesicles with FK506 and rapamycin enhances halothane-induced Ca2+ release by about five times. Furthermore, the Ca2+ release induced by halothane in the presence of FK506 was inhibited by several antagonists of the ryanodine receptor, such as ruthenium red, spermine, and Mg2+. Dissociation of FK506 binding protein from its binding site in skeletal muscle sarcoplasmic reticulum vesicles can modulate halothane-induced Ca2+ release through the ryanodine receptor. Data are discussed in relation to the role of the FK506 binding protein in modulating the effect of halothane on the ryanodine receptor and the development of malignant hyperthermia phenotype.

5-HT2 receptor antagonist-mediated inhibition of halothane-induced contractures in skeletal muscle specimens from malignant hyperthermia susceptible patients.

Administration of 5-HT2 receptor agonists induced malignant hyperthermia (MH) in susceptible pigs. Furthermore, the 5-HT2 receptor antagonist ritanserin prevented 5-HT-induced porcine MH. It has been shown that 5-HT2 receptor agonists induce marked contractures in skeletal muscle specimens from MH susceptible (MHS) but not in specimens from normal patients. The purpose of this study was to investigate the effects of ritanserin on halothane-induced contractures in muscle specimens from MHS patients. Twenty-five patients aged 8-56 years (29.5+/-13.6) classified as MHS by the in vitro contracture test (IVCT) with halothane and caffeine according to the protocol of the European MH Group participated in this study. Muscle specimens were pretreated with ritanserin 10 micromol/l (n= 14), 20 micromol/l (n=14) and 100 micromol/l (n=12) for 10 min and subsequently halothane was added incrementally (0.11-0.22-0.44 mmol/l) to the tissue bath as described in the European MH protocol. The results of the halothane contracture test were used as control. Following administration of halothane, muscle contractures reached a maximum of 16.9+/-4.2 mN. Ritanserin led to a significant inhibition of halothane-induced contractures in MHS muscles. Following pretreatment with ritanserin, halothane-induced contracture maximum was significantly smaller with 7.5+/-3.1 mN after 10 micromol/l ritanserin, 4.9+/-1.5 mN after 20 micromol/l ritanserin and 0.5+/- 0.2 mN after 100 micromol/l ritanserin than without pretreatment. Administration of ritanserin induced at all concentrations a decrease in muscle twitch height. Increase in muscle twitch following halothane was reduced in a concentration-dependent manner by ritanserin. The presented findings indicate that 5-HT might be involved in the mechanisms of halothane-induced MH in humans. Further studies have to determine the pathophysiological role of the 5-HT system in MH, and whether ritanserin could be an alternative for treatment or prevention of halothane-induced MH.

Abnormal regulation of muscle contraction in horses with recurrent exertional rhabdomyolysis

Abstract Objective To determine whether abnormal regulation of muscle contraction similar to that associated with malignant hyperthemia (MH) was evident in intact external intercostal muscle cells from Thoroughbreds with recurrent exertional rhabdomyolysis (RER). Animals 5 adult Thoroughbred horses with RER and 7 clinically normal adult Thoroughbred or mixed-breed horses. Procedures Twitch time course variables and contracture responses to various concentrations of potassium, caffeine, and halothane were measured in small bundles of intact external intercostal muscle cells from clinically normal horses and horses with RER. Results Threshold for significant contracture induced by potassium depolarization was lower for RER-affected muscles, compared with normal muscles, although the relationship between potassium concentration and membrane potential were not different. Thresholds for contracture induced by caffeine and halothane were also lower for RER-affected muscles, compared with normal muscles. Lower thresholds for caffeine- and halothane-induced contractures, as well as depolarization-elicited contractures, in RER-affected muscles suggest a defect in myoplasmic calcium regulation. Conclusions and Clinical Relevance Regulation of muscle contraction is abnormal in Thoroughbreds with RER. The specific defect may be attributable to abnormal intracellular calcium regulation. Knowledge of the specific defect involved in RER may lead to improved prevention and treatment of RER-affected horses. (Am J Vet Res 1999;60:992-999)

Treatment of normal skeletal muscle with FK506 or rapamycin results in halothane-induced muscle contracture.

FKBP12 is a protein that is closely associated with the ryanodine receptor type 1 of skeletal muscle and modulates Ca2+ release by the channel. The immunosuppressants FK506 and rapamycin both bind to FKBP12 and in turn dissociate the protein from the ryanodine receptor. By treating healthy human skeletal muscle strips with FK506 or rapamycin and then subjecting the strips to the caffeine-halothane contracture test, this study determined that FK506 and rapamycin alter the sensitivity of the muscle strip to halothane, caffeine, or both. Skeletal muscle strips from 10 healthy persons were incubated in Krebs medium equilibrated with a 95% oxygen and 5% carbon dioxide mixture, which contained either 12 microM FK506 (n = 8) or 12 microM rapamycin (n = 6), for 15 min at 37 degrees C. The strips were subjected to the caffeine-halothane contracture test for malignant hyperthermia according to the European Malignant Hyperthermia Group protocol. Treatment of normal skeletal muscle strips with FK506 and rapamycin resulted in halothane-induced contractures of 0.44+/-0.16 g and 0.6+/-0.49 g, respectively, at 2% halothane. The results obtained show that pre-exposure of healthy skeletal muscle strips to either FK506 or rapamycin is sufficient to give rise to halothane-induced contractures. This is most likely caused by destabilization of Ca2+ release by the ryanodine receptor as a result of the dissociation of FKBP12. This finding suggests that a mutation in FKBP12 or changes in its capacity to bind to the ryanodine receptor could alter the halothane sensitivity of the skeletal muscle ryanodine receptor and thereby predispose the person to malignant hyperthermia.

Effects of veratridine on mechanical responses of human malignant hyperthermic muscle fibers.

To determine if alteration in the function of the sodium channel may in turn modify halothane-induced changes in mechanical responses of muscle bundles from patients susceptible to malignant hyperthermia (MH). Mechanical responses of muscle bundles from 12 MH-susceptible and 20 MH non-susceptible patients were measured prior to and during administration of halothane alone and in the presence of 10 microM veratridine, an inhibitor of sodium channel inactivation. Peak tension (PT), time to peak tension (TPT), positive peak of isometric tension derivative (+dP/dtmax) were used to characterize the inotropic state. Analysis of relaxation process was performed using half relaxation time (RT 1/2) and the negative peak of isometric tension derivative (-dP/dtmax). The ratio (R) = (+dP/dtmax)/(-dP/dtmax) was used to measure the coupling between contraction and relaxation under isometric condition. Veratridine significantly enhanced the 0.5, 1, 2 and 3 vol% halothane-induced contracture and induced a negative inotropic effect in MH-susceptible muscle bundles. R increased by nearly 90% indicating that the combined effects were more pronounced in the relaxation phase. In MH non-susceptible muscle, veratridine did not significantly enhance the effects of halothane. These results on cut MH-susceptible human muscle bundles support the hypothesis that halothane-induced contracture in MH can be modified by the binding of an inhibitor of sodium channel inactivation.

THE EFFECTS OF THE ATYPICAL ANTIPSYCHOTICS CLOZAPINE, REMOXIPRIDE AND RISPERIDONE ON THE CONTRACTILE CHARACTERISTICS OF RAT SKELETAL MUSCLEIN VITRO

The therapeutic use of the atypical antipsychotics clozapine, remoxipride and risperidone has been reported to be associated with a reduced occurrence of the extrapyramidal side-effects seen during therapy with classical (or typical) antipsychotics such as trifluoperazine and haloperidol. The aim of this study was to determine the effects of the atypical antipsychotics clozapine, remoxipride and risperidone on rat skeletal musclein vitro. Remoxipride and risperidone did not produce contracture in muscle. Clozapine induced a small muscle contracture at high concentrations. Pre-treatment of muscle with trifluoperazine or haloperidolin vitrocaused the muscle to display contracture responses to halothane, and haloperidol also potentiated a contracture response to caffeine. Pre-treatment of muscle with remoxipride and risperidone did not induce contracture in response to halothane and did not potentiate caffeine contracture. Clozapine pre-treatment caused muscle fibre bundles to display a small halothane-induced contracture and caused significant potentiation of caffeine-induced contracture.

Effects of the serotonin2 receptor agonist DOI on skeletal muscle specimens from malignant hyperthermia-susceptible patients.

Administration of serotonin2 (5-HT2) receptor agonists in pigs triggers malignant hyperthermia (MH) and psychotic-like behavior. Both can be reduced by 5-HT2 receptor antagonists. Furthermore, an increase in the plasma concentration of 5-HT has been found during onset of halothane-induced MH in pigs. Therefore, in this study, the in vitro effects of the 5-HT2 receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) were investigated in muscle specimens from MH-susceptible (MHS) and -negative (MHN) patients. After MH classification using the caffeine-halothane contracture test (CHCT), surplus muscle specimens from 23 MHS and 17 MHN patients were used to examine the effects of DOI. In the first study, DOI was added to the bath in a concentration of 0.02 mM. In a second experiment, muscles were preincubated for 60 min with 0.02 mM DOI, and subsequently, halothane was added incrementally to the organ bath (0.11-0.22-0.44 mM) for 15 min according to the CHCT protocol. The in vitro effects of DOI on contracture development and muscle twitch were measured for 120 min in both investigations. Muscle specimens form all patients developed contractures after administration of DOI, characterized by a significantly earlier development of contracture in MHS (16.8 +/- 1.7 min) than in MHN (66.3 +/- 5.8 min) muscles (P < 0.05). There was no overlap between the groups in the range of times. The onset of contracture development after DOI was prolonged by halothane in specimens from MHN patients (89.7 +/- 5.6 min) but not MHS patients. Preincubation with DOI increased the halothane-induced contractures in specimens from MHS patients compared to the results of the CHCT. The contracture development in specimens from MHS patients was larger than from MHN patients. At the end of the experiment, contractures had reached a maximum of 12.9 +/- 1.1 mN in specimens from MHS and 5.3 +/- 0.6 mN in MHN patients (P < 0.05). The additional administration of halothane led to significantly increased contractures in specimens from MHS individuals (15.9 +/- 0.9 mN) at 120 min. However, the contracture development decreased significantly to 3.1 +/- 0.4 mN in MHN muscles. Muscle twitch after DOI administration was reduced significantly in specimens from MHS and MHN patients. A functional or structural altered serotonin system might be involved in the development of MH in humans.

IN VITRO EFFECT OF EPHEDRINE, ADRENALINE, NORADRENALINE AND ISOPRENALINE ON HALOTHANE-INDUCED CONTRACTURES IN SKELETAL MUSCLE FROM PATIENTS POTENTIALLY SUSCEPTIBLE TO MALIGNANT HYPERTHERMIA

We have measured the effects of ephedrine, adrenaline, noradrenaline and isoprenaline on halothane-induced contractures in muscle biopsies from patients potentially susceptible to malignant hyperthermia (MH). At concentrations of 4-24 mmol litre-1, ephedrine induced in vitro contractures in halothane 0.44 mmol litre-1-prechallenged muscle, whilst adrenaline, noradrenaline and isoprenaline had no effect. There was a shift of the ephedrine concentration-response curve to the left and an increased maximum muscle contracture in the MH susceptible group compared with the MH negative group (P < 0.001). We conclude that ephedrine increased halothane-induced muscle contractures in vitro either by an unknown pharmacological mechanism or by an adrenergic stimulation which was different from those of the other investigated adrenoceptor agonists.

Effects of Calcium-free Solution, Calcium Antagonists, and the Calcium Agonist BAY K 8644 on Mechanical Responses of Skeletal Muscle from Patients Susceptible to Malignant Hyperthermia

The purpose of this investigation was to determine if alteration in the function of the dihydropyridine receptor may in turn modify halothane-induced contractures in muscle bundles from patients susceptible to malignant hyperthermia (MH). The effects of Ca(2+)-free Krebs Ringer (KR) solution, 5 microM verapamil, 5 microM nifedipine, and 10 microM of the Ca2+ agonist BAY K 8644 on halothane-induced contracture were therefore investigated. The halothane-induced contracture was prevented in the absence of extracellular Ca2+ and significantly reduced in the presence of verapamil or nifedipine. BAY K 8644 significantly enhanced the 0.5-, 1.0-, and 1.5-vol % halothane-induced contracture in MH-susceptible muscle bundles. When BAY K 8644 was dissolved in Ca(2+)-free KR solution, no contracture was observed in MH-susceptible muscle bundles. These results on cut MH-susceptible human muscle bundles support the hypothesis that halothane-induced contracture in MH can be modified by the binding of Ca2+ agonists or antagonists to the dihydropyridine receptor. The role of Ca2+ entry phenomena remains unclear, but the results suggest that extracellular Ca2+ is required to reprime or to bind to some sites of the dihydropyridine receptors.

USE OF THE CALCIUM AGONIST BAY K 8644 FOR IN VITRO DIAGNOSIS OF SUSCEPTIBILITY TO MALIGNANT HYPERTHERMIA

We have studied the effects of the calcium agonist BAY K 8644 on the in vitro halothane test in 10 malignant hyperthermia-susceptible (MHS), 12 MH "equivocal" to halothane (MHEh), 30 MH non-susceptible (MHN) and 10 control patients. BAY K 8644 potentiated the halothane-induced contracture in muscle strips from both MHS and MHEh patients. The drug produced a more obvious difference in contracture responses between the MHEh group compared with the MHN and control groups.

Alterations in the functional properties of skinned fibers from denervated rabbit skeletal muscle

Isometric tension was recorded in vitro from chemically skinned fibers obtained from normal and 14-day-denervated extensor digitorum longus muscles of the rabbit. Denervation potentiated the tensions elicited by pCa 6.0 but did not modify the pCa value (5.6) required for maximum tension. Ca2+ transport across the membranes of the sarcoplasmic reticulum (SR) was markedly affected by denervation. Thus the rate of ATP-dependent net Ca2+ uptake increased significantly, and the spontaneous release ("leakage") of the Ca2+ stored in the SR was significantly reduced in denervated fibers. These effects lead to increased accumulation of Ca2+ in the lumen of the SR. The dose-response curve for the halothane-induced contractures of Ca2(+)-loaded skinned fibers was displaced to the left after denervation. Thus 0.7 mM halothane, a concentration that elicited no tension in 10 control fibers, induced contractures in the 10 denervated fibers tested. The potentiation of the halothane-induced tensions is attributed mainly to the larger stores of Ca2+ in the SR of denervated fibers. The possibility that denervation may also affect the interaction of halothane with the SR membranes is discussed.

Halothane sensitivity of young pigs in vivo and in vitro

Piglets less than 8 wk of age that are known by genotype to be malignant hyperthermia-susceptible (MHS) do not usually develop characteristic hyperthermia and limb muscle rigidity in response to a brief halothane exposure (5 min of 3%). To determine whether a malignant hyperthermia (MH) episode could nevertheless be provoked by a more rigorous challenge, both genetically MHS (Pietrain) and normal (Yorkshire) 5-wk-old piglets were exposed to a combined halothane-succinylcholine challenge. Only two of eight MHS piglets developed limb rigidity; however, all MHS piglets (and no normal piglets) developed clinical signs of MH episode initiation during the 30-min challenge. Temperatures rose from 37.4 to 38.6 degrees C in MHS piglets while falling slightly in normal piglets. In MHS piglets, venous pH fell from 7.46 +/- 0.02 to 6.88 +/- 0.07, PVCO2 rose from 36 +/- 2 to 126 +/- 17 mmHg, and plasma concentration of K+ rose from 4.0 +/- 0.1 to 7.1 +/- 0.6 mM, whereas all values remained stable in normal piglets. Muscles removed from the same piglets before the halothane-succinylcholine challenge were exposed to halothane in vitro. The muscles from genetically MHS piglets responded to halothane with characteristic depression of tetanic tension and prolonged tetanus relaxation time but did not develop halothane-induced contractures. We conclude that, in the absence of either halothane-induced limb rigidity or in vitro contractures, these young animals were still susceptible to potentially fatal MH episodes on exposure to appropriate triggering agents. The MH defect is apparently partially masked in piglets and expressed fully only in older pigs.

Malignant hyperthermia: slow sodium current in cultured human muscle cells

Voltage-activated ion currents were measured in cultured skeletal muscle myoballs. Cultures were generated from biopsies from patients referred for diagnosis of susceptibility to malignant hyperthermia (MH); diagnosis of susceptibility (MH+) or nonsusceptibility (MH-) was made on the basis of in vitro halothane-induced contracture of a separate piece of biopsy. Measurements of ion currents were made at room temperature in the absence of anesthetic agents, using tight-seal whole-cell recording. Fast transient Na+ currents and delayed outward K+ currents were similar in magnitude and kinetics in cells from MH+ and MH- patients. An additional slowly inactivating inward current component was commonly observed in cells from MH+ patients. This current was blockable by tetrodotoxin and was carried by Na+ but not by Ba2+. The component was less frequently observed and was of a lower magnitude in cells from MH- patients. The increased magnitude of the slow inward current observed in cultured muscle cells from MH+ patients may be a manifestation of the lesion that causes MH.

Halothane-sensitivity gene and muscle contractile properties in malignant hyperthermia.

Malignant hyperthermia (MH) results from the presence of the halothane-sensitivity gene and is characterized by abnormalities in muscle function. Populations of genetically defined pigs were used to determine the in vivo and in vitro expression of this gene in both the homozygous and the heterozygous condition. On exposure to halothane, isolated muscle bundles from the homozygous halothane-sensitive pigs exhibited decreased tetanus tension and increased tetanus half-relaxation time and contracture and were clearly distinguished from homozygous normal muscles. The heterozygous and homozygous normal muscles were similar in contractile responses except for the occurrence of halothane-induced contractures in the heterozygotes. The heterozygous halothane-negative pigs did not exhibit the characteristic signs of an MH episode in response to halothane succinylcholine, although some metabolic responses were significantly altered (e.g., increased venous partial pressure of CO2 and arterial and venous K+ concentration). Thus the heterozygous pigs were not MH susceptible but did represent a phenotype distinct from the homozygous normal pigs both in vitro and in vivo. These data provide the first convincing evidence for the expression of the halothane-sensitivity gene in heterozygotes.

Comparison of the suppressing action of three calcium entry blockers (CEB) on the in vitro halothane-induced contracture in Malignant Hyperthermia (MH) Susceptible subjects

Comparison of the suppressing action of three calcium entry blockers (CEB) on the in vitro halothane-induced contracture in Malignant Hyperthermia (MH) Susceptible subjects

Malignant hyperthermia: effects of halothane on the surface membrane.

We studied the electrical properties of intact muscle fibers from normal and malignant hyperthermia-susceptible (MHS) pigs. Resting membrane potentials, action potentials, and current-voltage relationships were measured with and without the presence of halothane. There were no changes in the resting potentials or the specific membrane conductances at any concentration of halothane in either the normal or MHS fibers. The current-voltage relationships of normal and MHS fibers did not differ. Contractures were observed in MHS muscle when the concentration of halothane was greater than or equal to 0.8%. These halothane-induced contractures were not associated with depolarization of the surface membrane. Contractures were not observed in normal muscle even at concentrations of 6.0% halothane. In contrast, halothane altered the shape of the action potentials of both MHS and normal fibers. However, these changes were significantly greater in MHS fibers, occurred at much lower concentrations, and were partially prevented by preincubation in 10 microM dantrolene.

IN VITRO MUSCLE CONTRACTURES INDUCED BY HALOTHANE AND SUXAMETHONIUM: II: Human Skeletal Muscle from Normal and Malignane Hyperthermia Susceptible Patients

The role of lipolysis in halothane-induced contractures of human skeletal muscle was examined using three inhibitors of phospholipase A2 (PLA2) activity (quinacrine, spermine and indomethacin) and exogenously administered PLA2. In addition, we examined the effects of the same PLA2 inhibitors on contractures induced by halothane in combination with suxamethonium. The studies were conducted on directly stimulated muscle strips isolated from biopsies from patients diagnosed as MH susceptible, or not susceptible, by the halothane contracture test. The liberation of fatty acids from skeletal muscle homogenates was examined to determine whether PLA2 activity might be increased in MH susceptible patients. The three PLA2 inhibitors antagonized halothane-induced contractures of muscle from MH susceptible patients as well as induction of contractures by suxamethonium and halothane. Pre-exposing preparations that were previously unresponsive to halothane to bee venom PLA2 caused the muscle strips to respond with contractures upon subsequent challenge with halothane, but not with suxamethonium. Free fatty acid production during an in vitro incubation of skeletal muscle homogenates was greater in preparations from MH susceptible patients than in those from patients who were not susceptible. These results suggest that excess liberation of fatty acids may be involved in halothane-induced contractures of muscle from MH susceptible individuals, in the synergism observed between halothane and suxamethonium, and in human MH.