Fatty acids markedly lower the threshold for halothane-induced calcium release from the terminal cisternae in human and porcine normal and malignant hyperthermia susceptible skeletal muscle

Malignant hyperthermia is caused by an abnormal increase in Ca2+ levels in skeletal muscle in response to anesthetics, including halothane. Since fatty acid production is elevated in skeletal muscle from individuals with malignant hyperthermia, the effects of fatty acids on the threshold of halothane-induced Ca2+ release were examined. In the absence of fatty acids halothane caused Ca2+ release from porcine and human heavy sarcoplasmic reticulum fractions, but only at concentrations above the clinically relevant range. Oleic acid (20 microM), an unsaturated fatty acid, reduced the threshold at which halothane induced Ca2+ release to concentrations used for anesthesia. Stearic acid, a saturated fatty acid had considerably less effect on the threshold of halothane action. The greater sensitivity of malignant hyperthermia muscle to halothane can be explained by elevated fatty acid production.     

Lipid analysis of skeletal muscle from pigs susceptible to malignant hyperthermia

Previous studies demonstrated that lipid profiles of humans and pigs susceptible to malignant hyperthermia (MH) differ from those of normal humans and pigs. Lipid extraction techniques retaining in vivo lipid profiles most closely were used in the present study to determine if stimulation of lipolysis by the processes of homogenization or extraction might account for the reported differences in lipid profiles. No differences were observed among three genotypes of British Landrace pigs with respect to cholesterol levels, triglyceride levels, or total lipid phosphorus values of whole muscle (longissimus dorsi). Phospholipid distributions were the same for all three groups. Individual free fatty acids and fatty acids acylated to triglycerides were similar among the genotypes. These results do not support altered lipid profiles in vivo in MH-susceptible swine. Previously used homogenization and extraction procedures most likely affect the lipolytic activity to a different extent in muscle from MH-susceptible pigs and normal pigs.     

Increased sensitivity of the ryanodine receptor to halothane-induced oligomerization in malignant hyperthermia-susceptible human skeletal muscle.

Mutations in the skeletal muscle RyR1 isoform of the ryanodine receptor (RyR) Ca2+-release channel confer susceptibility to malignant hyperthermia, which may be triggered by inhalational anesthetics such as halothane. Using immunoblotting, we show here that the ryanodine receptor, calmodulin, junctin, calsequestrin, sarcalumenin, calreticulin, annexin-VI, sarco(endo)plasmic reticulum Ca2+-ATPase, and the dihydropyridine receptor exhibit no major changes in their expression level between normal human skeletal muscle and biopsies from individuals susceptible to malignant hyperthermia. In contrast, protein gel-shift studies with halothane-treated sarcoplasmic reticulum vesicles from normal and susceptible specimens showed a clear difference. Although the alpha2-dihydropyridine receptor and calsequestrin were not affected, clustering of the Ca2+-ATPase was induced at comparable halothane concentrations. In the concentration range of 0.014-0.35 mM halothane, anesthetic-induced oligomerization of the RyR1 complex was observed at a lower threshold concentration in the sarcoplasmic reticulum from patients with malignant hyperthermia. Thus the previously described decreased Ca2+-loading ability of the sarcoplasmic reticulum from susceptible muscle fibers is probably not due to a modified expression of Ca2+-handling elements, but more likely a feature of altered quaternary receptor structure or modified functional dynamics within the Ca2+-regulatory apparatus. Possibly increased RyR1 complex formation, in conjunction with decreased Ca2+ uptake, is of central importance to the development of a metabolic crisis in malignant hyperthermia.     

Lipid peroxidation, antioxidant concentrations, and fatty acid contents of muscle tissue from malignant hyperthermia-susceptible swine.

Homogenates of semitendinosus muscle from malignant hyperthermia (MH)-susceptible pigs produced threefold more pentane than those from MH-resistant pigs, indicating enhanced free radical-mediated peroxidation of n-6 fatty acids. This did not reflect a deficiency in tissue antioxidants or antioxidant-enzymes but glutathione concentrations and glutathione peroxidase activities were increased in the tissue from MH-susceptible swine, consistent with an adaptive response to a sustained oxidant stress. A lower proportion of linoleic acid (18:2 n-6) in phospholipids and neutral lipids in muscle from MHS pigs indicated increased peroxidation or metabolism (desaturation and elongation). The increased oleic acid (18:1 n-9) in the MHS muscle indicated that desaturase activity was elevated in all lipid classes. The results are consistent with the hypothesis that enhanced free radical activity and lipid peroxidation contributes to the abnormalities in Ca2+ homeostasis and polyunsaturated fatty acid metabolism in MH.     

Microassay for malignant hyperthermia susceptibility: hypersensitive ligand-gating of the Ca channel in muscle sarcoplasmic reticulum causes increased amounts and rates of Ca-release

A crude preparation of heavy sarcoplasmic reticulum (HSR) was isolated using 1 gram of muscle obtained from swine susceptible to malignant hyperthermia (MH) and from control swine. The caffeine and ATP concentration-dependency of Ca-release was determined using suction filtration with radioisotopic ⁴⁵Ca as a tracer. Rates of release were determined using a rapid filtration system. Caffeine and ATP-induced Ca-release from MH-susceptible (MHS) HSR occurred at one-tenth the concentration of agonist that was required for control muscle HSR. No differences in rates and amounts of release were observed when agonist concentrations were used that caused maximum release for controls. However, at the threshold concentration of caffeine causing release for control HSR, the MHS HSR released 4-times as much Ca and at 3-times the rate of controls. These findings indicate that increased rates and amounts of Ca-release are due to the hypersensitivity of the Ca-release channel of HSR and that this abnormality can be detected using 1 gram of muscle.     

Changes in cytoplasmic free calcium caused by halothane. Role of the plasma membrane and intracellular Ca2+ stores

Malignant hyperthermia is a muscle disease characterized by an abnormal response to anaesthetics, stress, and exercise. It is typified by muscle contracture and a dramatic elevation in body temperature. A defect in the regulation of the concentration of cytoplasmic free calcium, [Ca2]i, is thought to underlie this disease, but the actual [Ca2+]i was not measurable until recently. We have shown that the anaesthetic halothane increases [Ca2+]i in isolated lymphocytes from malignant hyperthermia-susceptible humans and pigs but not in the normal counterparts. In this report we extend these observations to a larger number of cases and analyze the molecular mechanisms responsible for the increase. The halothane-mediated rise in [Ca2+]i required external Ca2+ and was prevented by nifedipine, an inhibitor of the voltage-sensitive Ca2+ channels of the cell membrane. In addition, the effect of halothane on the releasable Ca2+ from intracellular stores was determined by measuring the size of the releasable pool before and after addition of the anaesthetic. After addition of halothane, about 73% of this Ca2+ pool was still available for release by the Ca2+ ionophore ionomycin in cells from normal humans and pigs. In contrast, only about 45% of the free Ca2+ in intracellular stores was left after treatment with halothane in cells from malignant hyperthermia-susceptible humans and swine. These results indicate that halothane acts both at the cell membrane and at intracellular organelles, and that this action results in a net increase in [Ca2+]i in malignant hyperthermia, but not in normal cells. The action at the cell membrane appears to be on the voltage-sensitive Ca2+ channels.(ABSTRACT TRUNCATED AT 250 WORDS)     

Dantrolene does not block calcium pulse-induced calcium release from a putative calcium channel in sarcoplasmic reticulum from malignant hyperthermia and normal pig muscle

Calcium pulse additions to isolated SR membranes can cause a reversible efflux of calcium. The threshold level of calcium loading at which calcium efflux occurs is lower for SR membranes isolated from malignant hyperthermia susceptible (MHS) swine. Dantrolene, a unique muscle relaxant, had no effect on threshold calcium load, amounts and rates of calcium release from SR isolated from control and MHS skeletal muscle. It is concluded that the putative calcium channel through which this calcium pulse-induced calcium release mechanism occurs is not affected by dantrolene under these experimental conditions.     

Skeletal muscle mitochondrial respiration of malignant hyperthermia-susceptible patients. Ca2+-induced uncoupling and free fatty acids

1. Skeletal muscle mitochondria of malignant hyperthermia (MH)-susceptible patients showed normal oxidative phosphorylation but were more easily uncoupled than normal by exogenous Ca2+. 2. Fatty acids, in stimulating the mitochondrial ATPase activity, are responsible for the enhanced State 4 respiration in MH-susceptible patients. 3. These results imply that skeletal muscle mitochondria and free fatty acids are associated with the development of MH syndrome.     

Adenine nucleotide stimulation of Ca2+-induced Ca2+ release in sarcoplasmic reticulum

Rabbit skeletal muscle sarcoplasmic reticulum was fractionated into a "Ca2+-release" and "control" fraction by differential and sucrose gradient centrifugation. External Ca2+ (2-20 microM) caused the release of 40 nmol of 45Ca2+/mg of protein/s from Ca2+-release vesicles passively loaded at pH 6.8 with an internal half-saturation Ca2+ concentration of 10-20 mM. Ca2+-induced Ca2+ release had an approximate pK value of 6.6 and was half-maximally inhibited at an external Ca2+ concentration of 2 X 10(-4) M and Mg2+ concentration of 7 X 10(-5) M. 45Ca2+ efflux from control vesicles was slightly inhibited at external Ca2+ concentrations that stimulated the rapid release of Ca2+ from Ca2+-release vesicles. Adenine, adenosine, and derived nucleotides caused stimulation of Ca2+-induced Ca2+ release in media containing a "physiological" free Mg2+ concentration of 0.6 mM. At a concentration of 1 mM, the order of effectiveness was AMP-PCP greater than cAMP approximately AMP approximately ADP greater than adenine greater than adenosine. Other nucleoside triphosphates and caffeine were minimally effective in increasing 45Ca2+ efflux from passively loaded Ca2+-release vesicles. La3+, ruthenium red, and procaine inhibited Ca2+-induced Ca2+ release. Ca2+ flux studies with actively loaded vesicles also indicated that a subpopulation of sarcoplasmic reticulum vesicles contains a Ca2+ permeation system that is activated by adenine nucleotides.     

Dantrolene prevents the malignant hyperthermic syndrome by reducing free intracellular calcium concentration in skeletal muscle of susceptible swine

Malignant hyperthermia (MH) is a pharmacogenetic disorder of skeletal muscle triggered when susceptible subjects are exposed to volatile anesthetic agents and/or depolarizing muscle relaxants. We have used Ca2+ selective microelectrodes to measure in vivo the intracellular free [Ca2+] in skeletal muscle of MH susceptible swine before and after the administration of dantrolene. We have investigated the effectiveness of this muscle relaxant in preventing clinical MH and the relationship between the resting intracellular free [Ca2+] and the probability of inducing the MH syndrome. The resting intracellular free [Ca2+] was 0.41 +/- 0.01 microM (M +/- SEM), which agrees with our previous measurements in susceptible swine. The administration of 0.5, 1, 2, 2.5 and 3 mg/Kg Dantrolene, reduced the intracellular free [Ca2+] to 0.31, 0.21, 0.09, 0.08, 0.08 microM respectively. The 0.5 mg/Kg dose induced a moderate decrease of [Ca2+]i and failed to prevent the MH syndrome after exposure to halothane (2%). The 1 mg/Kg dose produced a further reduction in [Ca2+]i and was sufficient to prevent the clinical syndrome in 2 out of 3 animals. The 2.5 mg/Kg dose was uniformly protective in all animals. These results suggest that the mechanism by which dantrolene protects susceptible animals exposed to triggering agents is by reducing the intracellular free [Ca2+] in skeletal muscle.     

Altered [3H]ryanodine binding is not associated with malignant hyperthermia susceptibility in terminal cisternae preparations from swine

Based on comparisons between Pietrain malignant hyperthermia susceptible swine and Yorkshire control swine, other investigators have reported a 3-fold lower Kd for [3H]ryanodine binding to terminal cisternae in the malignant hyperthermia swine. However, the Kd of [3H]ryanodine binding did not correlate with malignant hyperthermia susceptibility when examined within the same strain (a Yorkshire/Duroc cross) in the present study. The values of Kd for the malignant hyperthermia susceptible and control swine in the present study were similar to those previously reported for the Pietrain strain, suggesting that the control strain chosen, not malignant hyperthermia susceptibility, accounts for what appeared to be a low Kd in Pietrain muscle.     

Defective Mg2+ regulation of RyR1 as a causal factor in malignant hyperthermia

In skeletal muscle, Mg(2+) exerts a dual inhibitory effect on RyR1, by competing with Ca(2+) at the activation site and binding to a low affinity Ca(2+)/Mg(2+) inhibitory site. Pharmacological activators of RyR1 must overcome the inhibitory action of Mg(2+) before Ca(2+) efflux can occur. In normal muscle, where the free [Mg(2+)](i) is approximately 1mM, even prolonged exposure to millimolar levels of volatile anesthetics does not initiate SR Ca(2+) release. However, when the cytosolic [Mg(2+)] is reduced below the physiological range, low levels of volatile anesthetic within the clinically relevant range (1mM) can initiate SR Ca(2+) release, in the form of a propagating Ca(2+) wave. In human muscle fibers from malignant hyperthermia susceptible patients, such Ca(2+) waves occur when 1mM halothane is applied at physiological [Mg(2+)](i). There is increasing evidence to suggest that defective Mg(2+) regulation of RyR1 confers susceptibility to malignant hyperthermia. At the molecular level, interactions between critical RyR1 subdomains may explain the clustering of RyR1 mutations and associated effects on Mg(2+) regulation.     

Inhibition of CD3-induced Ca2+ signals in Jurkat T-cells by myristic acid.

Stimulation of T lymphocytes with antibodies against the T cell receptor/CD3 complex induces within seconds a rise in the concentration of intracellular free Ca2+. Here we show that treatment with 20 microM free myristic acid completely inhibits this Ca2+ signal and the cellular proliferation in Jurkat T cells. Also lauric acid inhibited cell growth while its blocking effect on the Ca2+ signal was weaker than that of myristic acid. Other saturated free fatty acids were inactive. The inhibitory effect of myristic acid could be reversed by the addition of fatty acid free albumin, which will bind the fatty acid. Myristic acid, but not its methyl ester, inhibited both the anti-CD3-induced Ca2+ influx across the cell membrane and Ca2+ release from intracellular stores, but not the formation of inositol phosphates. In contrast, thapsigargin-induced release of Ca2+ from the same intracellular stores was unaffected by myristic acid. Thus, myristic acid specifically blocks T cell antigen receptor-CD3 induced Ca2+ mobilization in T cells.     

InsP3- and Ca2(+)-induced Ca2+ release in single mouse oocytes

To better understand the mechanism of intracellular Ca2+ mobilization, mouse oocytes were micro-injected with 'caged'-inositol-1,4,5 triphosphate caged-InsP3) together with the Ca2+ indicator Fluo-3 to directly induce and monitor Ca2+ redistribution. Photo-released InsP3 elicits [Ca2+]i changes exhibiting several kinetic phases and threshold behaviour. Often Ca2+ oscillations were induced after a single InsP3 pulse. Autoregenerative Ca2+ transients could also be induced by injections of Ca2+ itself, demonstrating unequivocally the presence of a Ca2(+)-induced Ca2(+)-release mechanism in these cells.     

The anti-cancer agent distamycin A displaces essential transcription factors and selectively inhibits myogenic differentiation

A microassay is demonstrated for functional characterization of the Ca2+-release channel (CRC) of sarcoplasmic reticulum (SR) of skeletal muscle using swine with susceptibility to malignant hyperthermia (MH). Diluted muscle homogenates, indo-1 and ratiometric dual-emission spectrofluorometry are used to monitor Ca2+-lowering activity in real-time in the presence and absence of ryanodine at exposures that open and close the CRC. Reactions are initiated with 50 µM CaCl2 to raise ionized Ca2+ concentration near 1 µM and MgATP to activate the Ca2+-ATPase pump. Oxalate is included to precipitate Ca2+ within the SR. The assay requires less than 30 mg muscle, which may be cryopreserved, and is completed within 20 min of thawing the tissue. Maximum SR Ca2+-ATPase pumping and CRC activities, degree of CRC activation, and Ca2+-buffering capacity can be determined. Using this assay we studied muscle from MH-susceptible swine and demonstrated that whereas maximal Ca2+-ATPase pumping and CRC activities are normal, the CRC activity after addition of a bolus of Ca2+ is 50% greater in heterozygotes and 100% greater in homozygotes for the MH mutation. Hypersensitivity to CRC agonists, such as caffeine, and an associated hyposensitivity to CRC antagonists such as Mg2+ is also demonstrated. Genotypes for the MH mutation site can be discriminated from each other by determining Ca2+-lowering activities and the effect of ryanodine on them. (Mol Cell Biochem 167: 61-72, 1997)     

Ca(2+)-oscillations and Ca(2+)-waves in mammalian cardiac and vascular smooth muscle cells

In this article, we review briefly the available theories and data on [Ca2+]i-waves and [Ca2+]i-oscillations in mammalian cardiac and vascular smooth muscles. In addition to our review, we also report: (i) the existence and characterization of rapid agonist-induced [Ca2+]i-waves in cultured vascular smooth muscle cells (A7r5 cells); and (ii a new method for studying rapid [Ca2+]i-waves in mammalian cardiac ventricular cells. In mammalian cardiac muscle several types of Ca(2+)-release from sarcoplasmic reticulum (SR) are known to occur and might be involved in Ca(2+)-waves and Ca(2+)-oscillations: (a) Ca(2+)-induced release of Ca2+, of the type thought to be important in normal excitation-contraction coupling; (b) spontaneous, cyclic release of Ca2+ related to a Ca(2+)-overload of the SR; and (c) Ins(1,4,5)P3-induced Ca(2+)-release. The available data support the idea that [Ca2+]i-waves in heart propagate by a mechanism somewhat different than that involved in normal excitation-contraction coupling (a, above), perhaps involving spontaneous release of Ca2+ from an overloaded SR (b, above). In mammalian vascular smooth muscle, our data support the idea that agonist-receptor interaction (vasopressin, in this case) initiates [Ca2+]i-waves that then propagate via some form of Ca(2+)-induced release of Ca2+, perhaps in a manner similar to that proposed by Berridge and Irvine [1].     

Abnormal ryanodine receptor channels in malignant hyperthermia.

Previous studies have demonstrated a defect associated with the calcium release mechanism of sarcoplasmic reticulum (SR) from individuals susceptible to malignant hyperthermia (MH). To examine whether SR calcium release channels were indeed altered in MH, SR vesicles were purified from normal and MH susceptible (MHS) porcine muscle. The Ca2+ dependence of calcium efflux rates from 45Ca2(+)-filled SR vesicles was then compared with the Ca2+ dependence of single-channel recordings of SR vesicles incorporated into planar lipid bilayers. The rate constants of 45Ca2+ efflux from MHS SR were two to threefold larger than from normal SR over a wide range of myoplasmic Ca2+. Normal and MHS single channels were progressively activated in a similar fashion by cis Ca2+ from pCa 7 to 4. However, below pCa 4, normal channels were inactivated by cis Ca2+, whereas MHS channels remained open for significantly longer times. The altered Ca2+ dependence of channel inactivation in MHS SR was also evident when Ca2+ was increased on the trans side while cis Ca2+ was held constant. We propose that a defect in a low-affinity Ca2+ binding site is responsible for the altered gating of MHS SR channels. Such a defect could logically result from a mutation in the gene encoding the calcium release channel, providing a testable hypothesis for the molecular basis of this inherited disorder.     

Molecular tools to elucidate problems in excitation-contraction coupling.

In this review, constituting the 1990 International Lecture of the Biophysical Society, research is described in two areas in which molecular genetic techniques were used to dissect problems related to sarcoplasmic reticulum proteins: the use of site-directed mutagenesis to gain insight into the mechanism of Ca2+ transport by the Ca2(+)-ATPase; and the use of cloning and genetic linkage analysis to identify the Ca2+ release channel (RYR1) gene as a candidate gene for the predisposition to malignant hyperthermia, a neuromuscular disease of humans and domestic animals.     

Identification of the Ca2+-release activity and ryanodine receptor in sarcoplasmic-reticulum membranes during cardiac myogenesis.

Ca2+-induced Ca2+ release and pH-induced Ca2+ release activities were identified in sarcoplasmic-reticulum (SR) vesicles isolated from adult- and fetal-sheep hearts. Ca2+-induced Ca2+ release and pH-induced Ca2+ release appear to proceed via the same channels, since both phenomena are similarly inhibited by Ruthenium Red. Ca2+ release from fetal SR vesicles is inhibited by higher concentrations of Ruthenium Red than is that from adult membranes. Both fetal and adult SR vesicles bind ryanodine. Fetal SR shows higher ryanodine-binding capacity than adult SR vesicles. Scatchard analysis of ryanodine binding revealed only one high-affinity binding site (Kd 6.7 nM) in fetal SR vesicles compared with two distinct binding sites (Kd 6.6 and 81.5 nM) in the adult SR vesicles. SR vesicles isolated from fetal and adult hearts were separated on discontinuous sucrose gradients into light (free) and heavy (junctional) SR vesicles. Heavy SR vesicles isolated from adult hearts exhibited most of the Ca2+ release activities. In contrast, Ca2+-induced Ca2+ release, pH-induced Ca2+ release and ryanodine receptors were detected in both light and heavy fetal SR. These results suggest that fetal SR may not be morphologically and functionally as well differentiated as that of adult cardiac muscle and that it may contain a greater number of Ca2+-release channels than that present in adult SR membranes.     

Reduced threshold for luminal Ca2+ activation of RyR1 underlies a causal mechanism of porcine malignant hyperthermia

Naturally occurring mutations in the skeletal muscle Ca(2+) release channel/ryanodine receptor RyR1 are linked to malignant hyperthermia (MH), a life-threatening complication of general anesthesia. Although it has long been recognized that MH results from uncontrolled or spontaneous Ca(2+) release from the sarcoplasmic reticulum, how MH RyR1 mutations render the sarcoplasmic reticulum susceptible to volatile anesthetic-induced spontaneous Ca(2+) release is unclear. Here we investigated the impact of the porcine MH mutation, R615C, the human equivalent of which also causes MH, on the intrinsic properties of the RyR1 channel and the propensity for spontaneous Ca(2+) release during store Ca(2+) overload, a process we refer to as store overload-induced Ca(2+) release (SOICR). Single channel analyses revealed that the R615C mutation markedly enhanced the luminal Ca(2+) activation of RyR1. Moreover, HEK293 cells expressing the R615C mutant displayed a reduced threshold for SOICR compared with cells expressing wild type RyR1. Furthermore, the MH-triggering agent, halothane, potentiated the response of RyR1 to luminal Ca(2+) and SOICR. Conversely, dantrolene, an effective treatment for MH, suppressed SOICR in HEK293 cells expressing the R615C mutant, but not in cells expressing an RyR2 mutant. These data suggest that the R615C mutation confers MH susceptibility by reducing the threshold for luminal Ca(2+) activation and SOICR, whereas volatile anesthetics trigger MH by further reducing the threshold, and dantrolene suppresses MH by increasing the SOICR threshold. Together, our data support a view in which altered luminal Ca(2+) regulation of RyR1 represents a primary causal mechanism of MH.