Malignant hyperthermia of Poland China swine: studies of a myogenic etiology

Ablation of Calsequestrin-1, Ca2+ unbalance, and susceptibility to heat stroke

Introduction: Ca2+ levels in adult skeletal muscle fibers are mainly controlled by excitation-contraction (EC) coupling, a mechanism that translates action potentials in release of Ca2+ from the sarcoplasmic reticulum (SR) release channels, i.e. the ryanodine receptors type-1 (RyR1). Calsequestrin (Casq) is a protein that binds large amounts of Ca2+ in the lumen of the SR terminal cisternae, near sites of Ca2+ release. There is general agreement that Casq is not only important for the SR ability to store Ca2+, but also for modulating the opening probability of the RyR Ca2+ release channels. The initial studies: About 20 years ago we generated a mouse model lacking Casq1 (Casq1-null mice), the isoform predominantly expressed in adult fast twitch skeletal muscle. While the knockout was not lethal as expected, lack of Casq1 caused a striking remodeling of membranes of SR and of transverse tubules (TTs), and mitochondrial damage. Functionally, CASQ1-knockout resulted in reduced SR Ca2+ content, smaller Ca2+ transients, and severe SR depletion during repetitive stimulation. The myopathic phenotype of Casq1-null mice: After the initial studies, we discovered that Casq1-null mice were prone to sudden death when exposed to halogenated anaesthetics, heat and even strenuous exercise. These syndromes are similar to human malignant hyperthermia susceptibility (MHS) and environmental-exertional heat stroke (HS). We learned that mechanisms underlying these syndromes involved excessive SR Ca2+ leak and excessive production of oxidative species: indeed, mortality and mitochondrial damage were significantly prevented by administration of antioxidants and reduction of oxidative stress. Though, how Casq1-null mice could survive without the most important SR Ca2+ binding protein was a puzzling issue that was not solved. Unravelling the mystery: The mystery was finally solved in 2020, when we discovered that in Casq1-null mice the SR undergoes adaptations that result in constitutively active store-operated Ca2+ entry (SOCE). SOCE is a mechanism that allows skeletal fibers to use external Ca2+ when SR stores are depleted. The post-natal compensatory mechanism that allows Casq1-null mice to survive involves the assembly of new SR-TT junctions (named Ca2+ entry units) containing Stim1 and Orai1, the two proteins that mediate SOCE.

High-Fat Diet Impairs Muscle Function and Increases the Risk of Environmental Heatstroke in Mice

Environmental heat-stroke (HS) is a life-threatening response often triggered by hot and humid weather. Several lines of evidence indicate that HS is caused by excessive heat production in skeletal muscle, which in turn is the result of abnormal Ca2+ leak from the sarcoplasmic reticulum (SR) and excessive production of oxidative species of oxygen and nitrogen. As a high fat diet is known to increase oxidative stress, the objective of the present study was to investigate the effects of 3 months of high-fat diet (HFD) on the HS susceptibility of wild type (WT) mice. HS susceptibility was tested in an environmental chamber where 4 months old WT mice were exposed to heat stress (41 °C for 1 h). In comparison with mice fed with a regular diet, mice fed with HFD showed: (a) increased body weight and accumulation of adipose tissue; (b) elevated oxidative stress in skeletal muscles; (c) increased heat generation and oxygen consumption during exposure to heat stress; and finally, (d) enhanced sensitivity to both temperature and caffeine of isolated muscles during in-vitro contracture test. These data (a) suggest that HFD predisposes WT mice to heat stress and (b) could have implications for guidelines regarding food intake during periods of intense environmental heat.

Aerobic Training Prevents Heatstrokes in Calsequestrin-1 Knockout Mice by Reducing Oxidative Stress

Calsequestrin-1 knockout (CASQ1-null) mice suffer lethal episodes when exposed to strenuous exercise and environmental heat, crises known as exertional/environmental heatstroke (EHS). We previously demonstrated that administration of exogenous antioxidants (N-acetylcysteine and trolox) reduces CASQ1-null mortality during exposure to heat. As aerobic training is known to boost endogenous antioxidant protection, we subjected CASQ1-null mice to treadmill running for 2 months at 60% of their maximal speed for 1 h, 5 times/week. When exposed to heat stress protocol (41°C/1 h), the mortality rate of CASQ1-null mice was significantly reduced compared to untrained animals (86% versus 16%). Protection from heatstrokes was accompanied by a reduced increase in core temperature during the stress protocol and by an increased threshold of response to caffeine of isolated extensor digitorum longus muscles during in vitro contracture test. At cellular and molecular levels, aerobic training (i) improved mitochondrial function while reducing their damage and (ii) lowered calpain activity and lipid peroxidation in membranes isolated from sarcoplasmic reticulum and mitochondria. Based on this evidence, we hypothesize that the protective effect of aerobic training is essentially mediated by a reduction in oxidative stress during exposure of CASQ1-null mice to adverse environmental conditions.

Estrogens Protect Calsequestrin-1 Knockout Mice from Lethal Hyperthermic Episodes by Reducing Oxidative Stress in Muscle

Oxidative stress has been proposed to play a key role in malignant hyperthermia (MH), a syndrome caused by excessive Ca²⁺ release in skeletal muscle. Incidence of mortality in male calsequestrin-1 knockout (CASQ1-null) mice during exposure to halothane and heat (a syndrome closely resembling human MH) is far greater than that in females. To investigate the possible role of sex hormones in this still unexplained gender difference, we treated male and female CASQ1-null mice for 1 month, respectively, with Premarin (conjugated estrogens) and leuprolide (GnRH analog) and discovered that during exposure to halothane and heat Premarin reduced the mortality rate in males (79-27% and 86-20%), while leuprolide increased the incidence of mortality in females (18-73% and 24-82%). We then evaluated the (a) responsiveness of isolated muscles to temperature and caffeine, (b) sarcoplasmic reticulum (SR) Ca²⁺ release in single fibers, and (c) oxidative stress and the expression levels of main enzymes involved in the regulation of the redox balance in muscle. Premarin treatment reduced the temperature and caffeine sensitivity of EDL muscles, normalized SR Ca²⁺ release, and reduced oxidative stress in males, suggesting that female sex hormones may protect mice from lethal hyperthermic episodes by reducing both the SR Ca²⁺ leak and oxidative stress.

Strenuous exercise triggers a life-threatening response in mice susceptible to malignant hyperthermia

In humans, hyperthermic episodes can be triggered by halogenated anesthetics [malignant hyperthermia (MH) susceptibility] and by high temperature [environmental heat stroke (HS)]. Correlation between MH susceptibility and HS is supported by extensive work in mouse models that carry a mutation in ryanodine receptor type-1 (RYR1^Y522S/WT) and calsequestrin-1 knockout (CASQ1-null), 2 proteins that control Ca²⁺ release in skeletal muscle. As overheating episodes in humans have also been described during exertion, here we subjected RYR1^Y522S/WT and CASQ1-null mice to an exertional-stress protocol (incremental running on a treadmill at 34°C and 40% humidity). The mortality rate was 80 and 78.6% in RYR1^Y522S/WT and CASQ1-null mice, respectively, vs. 0% in wild-type mice. Lethal crises were characterized by hyperthermia and rhabdomyolysis, classic features of MH episodes. Of importance, pretreatment with azumolene, an analog of the drug used in humans to treat MH crises, reduced mortality to 0 and 12.5% in RYR1^Y522S/WT and CASQ1-null mice, respectively, thanks to a striking reduction of hyperthermia and rhabdomyolysis. At the molecular level, azumolene strongly prevented Ca²⁺-dependent activation of calpains and NF-κB by lowering myoplasmic Ca²⁺ concentration and nitro-oxidative stress, parameters that were elevated in RYR1^Y522S/WT and CASQ1-null mice. These results suggest that common molecular mechanisms underlie MH crises and exertional HS in mice.-Michelucci, A., Paolini, C., Boncompagni, S., Canato, M., Reggiani, C., Protasi, F. Strenuous exercise triggers a life-threatening response in mice susceptible to malignant hyperthermia.

Antioxidants protect calsequestrin-1 knockout mice from halothane- and heat-induced sudden death

BACKGROUND

Mice lacking calsequestrin-1 (CASQ1-null), a Ca-binding protein that modulates the activity of Ca release in the skeletal muscle, exhibit lethal hypermetabolic episodes that resemble malignant hyperthermia in humans when exposed to halothane or heat stress.

METHODS

Because oxidative species may play a critical role in malignant hyperthermia crises, we treated CASQ1-null mice with two antioxidants, N-acetylcysteine (NAC, Sigma-Aldrich, Italy; provided ad libitum in drinking water) and (±)-6-hydroxy-2,5,7,8-tetramethylchromane-2-carboxylic acid (Trolox, Sigma-Aldrich; administered by intraperitoneal injection), before exposure to halothane (2%, 1 h) or heat (41°C, 1 h).

RESULTS

NAC and Trolox significantly protected CASQ1-null mice from lethal episodes, with mortality being 79% (n = 14), 25% (n = 16), and 20% (n = 5) during halothane exposure and 86% (n = 21), 29% (n = 21), and 33% (n = 6) during heat stress in untreated, NAC-treated, and Trolox-treated mice, respectively. During heat challenge, an increase in core temperature in CASQ1-null mice (42.3° ± 0.1°C, n=10) was significantly reduced by both NAC and Trolox (40.6° ± 0.3°C, n = 6 and 40.5° ± 0.2°C, n = 6). NAC treatment of CASQ1-null muscles/mice normalized caffeine sensitivity during in vitro contracture tests, Ca transients in single fibers, and significantly reduced the percentage of fibers undergoing rhabdomyolysis (37.6 ± 2.5%, 38/101 fibers in 3 mice; 11.6 ± 1.1%, 21/186 fibers in 5 mice). The protective effect of antioxidant treatment likely resulted from mitigation of oxidative stress, because NAC reduced mitochondrial superoxide production, superoxide dismutase type-1 expression, and 3-nitrotyrosine expression, and increased both reduced glutathione and reduced glutathione/oxidized glutathione ratio.

CONCLUSION

These studies provide a deeper understanding of the mechanisms that underlie hyperthermic crises in CASQ1-deficient muscle and demonstrate that antioxidant pretreatment may prevent them.

Hexanol and lidocaine affect the oligomeric state of the Ca-ATPase of sarcoplasmic reticulum

Hexanol at 7 degrees C stimulates the activity of the Ca-ATPase of sarcoplasmic reticulum (SR). Time-resolved phosphorescence spectroscopy studies of SR whose Ca-ATPase is covalently labeled with erythrosin isothiocyanate (ERITC) indicate that at 7 degrees C hexanol (1) cause a concentration-dependent increase in the rate of decay of phosphorescence anisotropy, (2) causes larger oligomers of Ca-ATPase to dissociate into smaller oligomers, and (3) increases the rotational mobility of Ca-ATPase in all its oligomeric states. Electron paramagnetic resonance (EPR) spectroscopy of spin-labeled stearic acid (SASL) in SR suggests that at 7 degrees C hexanol diminishes the fraction of SR lipids in the boundary lipid domain and disorders and fluidizes both the boundary lipid and the unrestricted lipid domain. In protein-free liposomes of extracted SR lipids hexanol increases fluidity and decreases order to a greater extent near the center of the lipid bilayer than near the polar head groups. At 25 degrees C hexanol has biphasic effects on Ca-ATPase activity: at 10 and 20 mM hexanol increases activity, but at 30 mM and especially at 40 mM there is inhibition of Ca-ATPase activity. The influence of hexanol at 25 degrees C on the oligomeric state of Ca-ATPase is also biphasic. At 10 and 20 mM, hexanol promotes the dissociation of larger oligomers into smaller ones, whereas at higher concentrations, 30 and 40 mM, hexanol causes larger oligomers to be formed from smaller ones. Lidocaine at 25 degrees C inhibits Ca-ATPase activity and causes dramatic slowing of the decay of phosphorescence anisotropy of ERITC-labeled SR by causing the formation of larger oligomers of Ca-ATPase from smaller ones. In protein-free liposomes of SR lipids at 25 degrees C, lidocaine disorders and fluidizes the acyl chains near the center of the bilayer (as did hexanol), but has opposite effects near the polar head groups. The opposite effects of hexanol and lidocaine on the oligomeric state of the SR Ca-ATPase provide a new molecular explanation for the opposite effects of hexanol and lidocaine on the activity of the Ca-ATPase. We conclude that the biphasic effects of hexanol on the activity of Ca-ATPase can be accounted for by biphasic effects of hexanol on the oligomeric state of the Ca-ATPase. This study supports the view that anesthetics can alter interactions between membrane proteins.

Calcium-dependent halothane activation of sarcoplasmic reticulum calcium channels from frog skeletal muscle

The effect of halothane on calcium channels present in sarcoplasmic reticulum membranes isolated from frog skeletal muscle was studied at the single channel level after fusing the isolated vesicles into planar lipid bilayers. Addition of 91 microM halothane to the cytosolic compartment containing 1 microM free calcium activated the channel by increasing fractional open time from 0.11 to 0.59, without changing the channel conductance. The activation of the channels by halothane was calcium dependent. At resting calcium concentrations in the cytosolic compartment, halothane failed to activate the channel, whereas maximal activation was found at 10 microM calcium. The free energy of halothane binding to the channel decreased from -5.8 kcal/mol at 1 microM calcium to -6.6 kcal/mol at 10 microM calcium. Halothane increased the open time constants and decreased the closed time constants, indicating that it binds to both the open and the closed configurations of the channel.

31P-NMR spectroscopy: the metabolic profile of malignant hyperpyrexic porcine skeletal muscle

31Phosphorus-NMR spectroscopy may have the potential to help in the noninvasive diagnosis of malignant hyperpyrexia (MH). Changes in the phosphate-metabolite profile of MH-susceptible (MHS) skeletal muscle occur more readily under conditions of anoxia than in control muscle. Induction of anoxia caused a rapid fall in intracellular phosphocreatine, an elevation of inorganic phosphate, and finally a diminution of ATP in MHS muscle. The onset of metabolic change was slower in control tissue. Increased oxygen consumption may occur in anoxic MHS muscle, which leads to accelerated glycolysis and a rapid fall in the intracellular high-energy phosphates. In MHS muscle an abnormality may exist in carbohydrate metabolism linked with poor resynthesis of the high-energy phosphates, which may be precipitated under anaerobic conditions. Accelerated muscle metabolism is also observed in the presence of 2 mM caffeine and 3% halothane in MHS muscle. Changes in the concentrations of metabolites could be mapped noninvasively under anoxic conditions using topical 31P-NMR.

Erythrocyte osmotic fragility in porcine malignant hyperthermia and effects of halothane

Malignant Hyperthermia (MH) has been shown to be due to an inherited disorder in cellular membrane systems, which also seem to be affected by halothane. With the erythrocyte osmotic fragility test we tried to find out if the erythrocyte membrane can serve as a diagnostic tool in malignant hyperthermia and as a model for halothane effects on membranes. We obtained MH susceptible and MH resistant pigs and their crossbreeds. The MH susceptible pigs showed the greatest degree of haemolysis in the erythrocyte osmotic fragility test. Furthermore, halothane increased haemolysis more in the erythrocytes of MH susceptible pigs. The MH resistant pigs were only slightly affected in both and the crossbreeds showed intermediate results. The variance occurring within one halothane group could be used to add a further distinguishing characteristic apart from the discrete halothane challenge test and so enable selection within a pure halothane group.

Pharmacology of calcium release from sarcoplasmic reticulum

Calcium release from sarcoplasmic reticulum (SR) has been elicited in response to additions of many different agents. Activators of Ca2+ release are here tentatively classified as activators of a Ca2+-induced Ca2+ release channel preferentially localized in SR terminal or as likely activators of other Ca2+ efflux pathways. Some of these pathways may be associated with several different mechanisms for SR Ca2+ release that have been postulated previously. Studies of various inhibitors of excitation-contraction coupling and of certain forms of SR Ca2+ release are summarized. The sensitivity of isolated SR to certain agents is unusually affected by experimental conditions. These effects can seriously undermine attempts to anticipate effects of the same pharmacological agents in situ. Finally, mention is made of a new preparation ("sarcoballs") designed to make the pharmacological study of SR Ca2+ release more accessible to electrophysiologists, and some concluding speculations on the future of SR pharmacology are offered.

SR function in malignant hyperthermia

Malignant hyperthermia (MH) is a genetic disease in man and other animal species that predisposes to a catastrophic hypermetabolic syndrome that is triggered by certain anesthetic agents. A working hypothesis is that a defect in regulation of muscle cell calcium is the primary mechanism that initiates the MH syndrome. This paper reviews the evidence for a defect in muscle cell calcium as regulated by the sarcoplasmic reticulum membrane system. Skeletal muscle biopsied from MH man, pigs and dogs has abnormal in vitro contracture response to halothane and caffeine and these responses can be altered by lowering calcium content of the bathing solution and/or the muscle. Measurements of MH muscle cell Ca2+ by Ca2+-specific microelectrodes in vivo and fura-2 in vitro have demonstrated abnormal Ca2+ levels in resting and in caffeine-stimulated states. The SR membrane system is the primary calcium regulating organelle in skeletal muscle and a likely site for the defect in MH muscle. Two Ca2+ regulating functions of the SR have been explored in SR isolated from MH muscle. An abnormality of the 100K Ca2+-ATPase protein that functions to transport Ca2+ from myoplasm to inside the SR does not appear to be responsible for MH. The most probable defective site in the SR appears to be Ca2+ release channels and a Ca2+-induced Ca2+ release pathway has been shown to be abnormal in SR from MH human and pig muscle.

Involvement of the 60 kDa phosphoprotein in the regulation of Ca2+ release from sarcoplasmic reticulum of normal and malignant hyperthermia susceptible pig muscles

Junctional sarcoplasmic reticulum (SR) vesicles isolated from back muscles of normal and malignant hyperthermia susceptible (MHS) pigs were phosphorylated by addition of MgATP in the presence of 5 mM Ca2+ and 1 microM calmodulin (CaM). The major site of phosphorylation was a 60 kDa protein both in normal and MHS SR. The maximal amount of phosphorylation in MHS SR (5 pmol P/mg SR) was significantly lower than that in the normal SR (12 pmol P/mg SR). The phosphorylated 60 kDa protein was spontaneously dephosphorylated both in normal and MHS SR. Ca2+ release from the passively loaded SR was induced by a Ca2+-jump, and monitored by stopped-flow fluorometry using chlorotetracycline. In the absence of preincubation with MgATP, no significant difference was found in any of the kinetic parameters of Ca2+ release between normal and MHS SR. Upon addition of 20 microM MgATP to the passively loaded SR to phosphorylate the 60 kDa protein, the initial rate of Ca2+ release in normal SR significantly decreased from 659 +/- 102 to 361 +/- 105 nmol Ca2+/mg SR per s, whereas in MHS SR the rate decreased from 749 +/- 124 to 652 +/- 179 nmol Ca2+/mg SR per s. Addition of 20 microM adenosine 5'-[beta, gamma-imido]triphosphate (p[NH]ppA) did not significantly alter the initial rate of Ca2+ release both in normal and MHS SR. These results suggest that the previously reported higher Ca2+ release rate in MHS SR (Kim et al. (1984) Biochim. Biophys. Acta 775, 320-327) is at least partly due to the reduced extent of the Ca2+/CaM-dependent phosphorylation of the 60 kDa protein. Two-dimensional gel electrophoresis study showed that amount of a protein with Mr = 55,000 was significantly lower in MHS SR than in normal SR suggesting that the abnormally lower amount of 55 kDa protein would cause the lower amount of phosphorylation of the 60 kDa protein in MHS SR.

The effects of calmodulin antagonist drugs on isolated sarcoplasmic reticulum from malignant hyperpyrexia susceptible swine

1. Because calcium antagonist drugs increase contracture in both control and malignant hyperpyrexia susceptible (MHS) skeletal muscle, the effect of these drugs on the sarcoplasmic reticulum (SR) was investigated. 2. The calmodulin antagonist drugs inhibited the Ca2+ dependent ATPase activity and the ATP-dependent Ca2+ uptake, and accelerated the efflux of Ca2+ from isolated SR preparations from both control and MHS skeletal muscle. These effects of calmodulin antagonist drugs on SR Ca2+ transport functions were consistent with their in vitro pharmacological effects on control and MHS muscle.

Enhanced Ca2+-induced calcium release by isolated sarcoplasmic reticulum vesicles from malignant hyperthermia susceptible pig muscle

To further define the possible involvement of sarcoplasmic reticulum calcium accumulation and release in the skeletal muscle disorder malignant hyperthermia (MH), we have examined various properties of sarcoplasmic reticulum fractions isolated from normal and MH-susceptible pig muscle. A sarcoplasmic reticulum preparation enriched in vesicles derived from the terminal cisternae, was further fractionated on discontinuous sucrose density gradients (Meissner, G. (1984) J. Biol. Chem. 259, 2365-2374). The resultant MH-susceptible and normal sarcoplasmic reticulum fractions, designated F0-F4, did not differ in yield, cholesterol and phospholipid content, or nitrendipine binding capacity. Calcium accumulation (0.27 mumol Ca/mg per min at 22 degrees C), Ca2+-ATPase activity (0.98 mumol Pi/mg per min at 22 degrees C), and calsequestrin content were also similar for MH-susceptible and normal sarcoplasmic reticulum fraction F3. To examine sarcoplasmic reticulum calcium release, fraction F3 vesicles were passively loaded with 45Ca (approx. 40 nmol Ca/mg), and rapidly diluted into a medium of defined Ca2+ concentration. Upon dilution into 1 microM Ca2+, the extent of Ca2+-dependent calcium release measured after 5 s was significantly greater for MH-susceptible than for normal sarcoplasmic reticulum, 65.9 +/- 2.8% vs. 47.7 +/- 3.9% of the loaded calcium, respectively. The C1/2 for Ca2+ stimulation of this calcium release (5 s value) from MH-susceptible sarcoplasmic reticulum also appeared to be shifted towards a higher Ca2+-sensitivity when compared to normal sarcoplasmic reticulum. Dantrolene had no effect on calcium release from fraction F3, however, halothane (0.1-0.5 mM) increased the extent of calcium release (5 s) similarly in both MH-susceptible and normal sarcoplasmic reticulum. Furthermore, Mg2+ was less effective at inhibiting, while ATP and caffeine were more effective in stimulating, this Ca2+-dependent release of calcium from MH-susceptible, when compared to normal sarcoplasmic reticulum. Our results demonstrate that while sarcoplasmic reticulum calcium-accumulation appears unaffected in MH, aspect(s) of the sarcoplasmic reticulum Ca2+-induced calcium release mechanism are altered. Although the role of the Ca2+-induced calcium release mechanism of sarcoplasmic reticulum in situ is not yet clear, our results suggest that an abnormality in the regulation of sarcoplasmic reticulum calcium release may play an important role in the MH syndrome.

Effect of halothane on Ca2+-induced Ca2+ release from sarcoplasmic reticulum vesicles isolated from rat skeletal muscle

Halothane induces the release of Ca2+ from a subpopulation of sarcoplasmic reticulum vesicles that are derived from the terminal cisternae of rat skeletal muscle. Halothane-induced Ca2+ release appears to be an enhancement of Ca2+-induced Ca2+ release. The low-density sarcoplasmic reticulum vesicles which are believed to be derived from nonjunctional sarcoplasmic reticulum lack the capability of both Ca2+-induced and halothane-induced Ca2+ release. Ca2+ release from terminal cisternae vesicles induced by halothane is inhibited by Ruthenium red and Mg2+, and require ATP (or an ATP analogue), KCl (or similar salt) and extravesicular Ca2+. Ca2+-induced Ca2+ release has similar characteristics.

Malignant hyperthermia: molecular defects in membrane permeability

Malignant hyperthermia (MH), a genetically inherited disorder of skeletal muscle, is due to molecular defect in membrane permeability. The alteration in membrane permeability is suggested to be due to enhanced phospholipase A2 activity which is responsible for the increased level in sarcoplasmic Ca2+. The excess Ca2+ is responsible for muscle hyper-rigidity and enhanced rate of glycolysis, resulting in a rapid rate of lactic acid production and a low pH in MH muscle.

Kinetic studies of Ca2+ release from sarcoplasmic reticulum of normal and malignant hyperthermia susceptible pig muscles

The time-course of Ca2+ release from sarcoplasmic reticulum isolated from muscles of normal pigs and those of pigs susceptible to malignant hyperthermia were investigated using stopped-flow spectrophotometry and arsenazo III as a Ca2+ indicator. Several methods were used to trigger Ca2+ release: (a) addition of halothane (e.g., 0.2 mM); (b) an increase of extravesicular Ca2+ concentration ([Ca2+0]); (c) a combination of (a) and (b), and (d) replacement of ions (potassium gluconate with choline chloride) to produce membrane depolarization. The initial rates of Ca2+ release induced by either halothane or Ca2+ alone, or both, are at least 70% higher in malignant hyperthermic sarcoplasmic reticulum than in normal. The amount of Ca2+ released by halothane at low [Ca2+0] in malignant hyperthermic sarcoplasmic reticulum is about twice as large as in normal sarcoplasmic reticulum. Membrane depolarization led to biphasic Ca2+ release in both malignant hyperthermic and normal sarcoplasmic reticulum, the rate constant of the rapid phase of Ca2+ release induced by membrane depolarization being significantly higher in malignant hyperthermic sarcoplasmic reticulum (k = 83 s-1) than in normal (k = 37 s-1). Thus, all types of Ca2+ release investigated (a, b, c and d) have higher rates in malignant hyperthermic sarcoplasmic reticulum than normal sarcoplasmic reticulum. These results suggest that the putative Ca2+ release channels located in the sarcoplasmic reticulum are altered in malignant hyperthermic sarcoplasmic reticulum.

Abnormality in calcium release from skeletal sarcoplasmic reticulum of pigs susceptible to malignant hyperthermia

Two fractions of sarcoplasmic reticulum, one light (LSR) and one heavy (HSR), were isolated from gracilis muscle of control and malignant hyperthermia (MH)-susceptible pigs. Part of the gracilis muscle biopsy was used to compare the contracture sensitivity of the muscle to the calcium-releasing effects of caffeine on isolated SR membranes. Gracilis muscle of MH pigs was more sensitive to the contracture-producing effects of caffeine than control pig muscle. The caffeine dose-cumulative contracture response curve for MH muscle was shifted left of that for controls. The amount of caffeine-induced calcium released from SR is a function of the amount of calcium preload and this did not differ between LSR of MH and control muscle. When LSR fractions were optimally loaded with calcium for caffeine-induced calcium release, no difference in calcium-releasing effects of varying caffeine doses was observed between MH and control LSR. At calcium preloads below optimal, the MH-LSR appeared to be more sensitive to caffeine-induced calcium release. The HSR fractions could not be loaded with calcium in a manner similar to the LSR fractions because of an apparent calcium-induced calcium release phenomenon. Therefore, calcium threshold for calcium-induced calcium release was compared between MH and control HSR fraction. The effect of caffeine on the calcium-induced calcium release was also studied. The average calcium concentration threshold for calcium-induced calcium release was markedly lower for MH vs. control HSR; 20 vs. 63 nmol Ca2+/mg, respectively. Caffeine decreased the threshold for calcium-induced calcium release more in the MH than in control HSR. Under all conditions studied, the amount of calcium released did not differ between the two groups. Ruthenium red increased the threshold calcium concentration for calcium-induced calcium release while it reduced the amount of calcium released. Increasing concentrations of Mg2+ increased the Ca2+ threshold for release and the amount of Ca2+ released but did not significantly affect rate of Ca2+ release. Results of the study suggest a defect in the mechanisms causing calcium release from SR in MH-affected muscle.

Prolonged electromechanical coupling time intervals in skeletal muscle of pigs susceptible to malignant hyperthermia

Electromechanical properties of skeletal muscle were compared between malignant hyperthermia (MH)-susceptible and control pigs. These muscle properties were studied both in vivo and in vitro. The in vivo time interval from nerve stimulation to onset of mechanical tension was longer in muscle of pigs susceptible to MH. Kinetics of formation of the in vivo compound muscle action potential did not differ between pig groups, whereas the time from peak of the muscle action potential to tension onset was longer for MH muscle. In vitro the time from direct stimulation to tension onset was also longer for MH muscle. These results suggest that the mechanism causing longer electromechanical time intervals in MH skeletal muscle is distal to the myoneural junction. Data comparing contraction kinetics between muscle from MH and control pigs indicates that the mechanism contributing to the longer time interval involves the transfer of signal from the transverse tubule to the sarcoplasmic reticulum in the pigs studied.

Malignant hyperthermia: a disease of specific myofiber type?

The present study attempts to determine if malignant hyperthermia susceptibility (MHS) is limited to muscles composed predominantly of a single major fiber type. The MHS trapezius muscle (49 per cent type 1 and 51 per cent type 2 fibers) has greater abnormal contracture response to halothane or to caffeine than does MHS gracilis (11 per cent type 1 and 80 per cent type 2 fibers). The dynamic properties (contraction and relaxation times) of MHS trapezius and MHS gracilis are similar, suggesting that the greater contracture response of MHS trapezius to halothane or to caffeine is more biochemical than physiological in origin. The results of this study are discussed in relevance to the diagnosis and to aetiologic investigation of malignant hyperthermia.

Skeletal muscle sarcoplasmic reticulum in porcine malignant hyperthermia

To examine the function of sarcoplasmic reticulum (SR) in malignant hyperthermia, SR was isolated from semitendinosus muscle of normal and genetically susceptible Poland China swine. Determinations included rate of calcium binding (oxalate absent), rate and capacity of calcium uptake (oxalate present), and spontaneous calcium release (in the absence of ionic depolarization or calcium) with and without halothane, using the millipore filtration technique. Rate of calcium binding, and rate and capacity of calcium uptake were decreased, and spontaneous calcium release was greater in SR fragments from susceptible swine as compared to those from normal swine. Halothane 0.5% slightly increased the rate of calcium binding in susceptible and normal SR. Above 1%, halothane decreased calcium binding rate, and uptake rate and capacity, and increased calcium release similarly in susceptible and normal SR. These differences in SR function were insufficient to explain the etiology of malignant hyperthemia, nor did the effect of halothane account for its triggering action.

Mitochondrial calcium efflux and porcine stress-susceptibility

Mitchondrial Ca2+ efflux rates of M. longissimus dorsi correlate very closely with parameters associated with porcine stress-susceptibility. Experimental data support the measurement of mitochondrial Ca+2 efflux to be a very sensitive and reliable method for differentiating porcine stress-susceptibility.

Studies on normal human skeletal muscle in relation to the pathopharmacology of malignant hyperpyrexia

1. The effects of dantrolene on pharmacologically-induced contractures and potentiated isometric twitches in normal human skeletal muscle have been studied in vitro. 2. Dantrolene sodium, at concentrations of 3 mumol/l or less, attenuates basal twitch, inhibits halothane potentiation of basal twitch and inhibits halothane-potentiated potassium contractures, but has less effect on twitch potentiation by 2 mmol/l caffeine. 3. Caffeine contractures are attenuated by dantrolene concentrations of 12 mumol/l or greater. The effect of dantrolene on caffeine contracture is characterized by decreased contracture tension and by prolonged time to peak contracture. 4. The results indicate that halothane and 2 mmol/l caffeine have agonistic effects on the excitation-contraction (E-C) coupling mechanism, and suggest that they may act at separate E-C coupling sites. The relationships of these findings to the pathopharmacology of malignant hyperpyrexia are discussed.

The adenosine triphosphate (ATP) depletion test: comparison with the caffeine contracture test as a method of diagnosing malignant hyperthermia susceptibility

The adenosine triphosphate (ATP depletion ratio, which is the ratio [ATP] in skeletal muscle equilibrated with carbogen and 4% halothane for 30 minutes divided by [ATP] in skeletal muscle equilibrated with carbogen alone for 30 minutes is less than normal in most but not in all rigid MHS patients. The ratio is normal in non-rigid MHS patients. This diagnostic tool is, therefore, useful in the diagnosis of rigid MH. It is not, however, such a sensitive diagnostic parameter as the caffeine contracture test.