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

Re-evaluating methods reporting practices to improve reproducibility: an analysis of methodological rigor for the Langendorff whole-heart technique

In recent decades, the scientific community has seen an increased interest in rigor and reproducibility. In 2017, concerns of methodological thoroughness and reporting practices were implicated as significant barriers to reproducibility within the preclinical cardiovascular literature, particularly in studies employing animal research. The Langendorff, whole-heart technique has proven to be an invaluable research tool, being modified in a myriad of ways to probe questions across the spectrum of physio- and pathophysiologic function of the heart. As a result, significant variability in the application of the Langendorff technique exists. This literature review quantifies the different methods employed in the implementation of the Langendorff technique and provides brief examples of how individual parametric differences can impact the outcomes and interpretation of studies. From 2017-2020, significant variability of animal models, anesthesia, cannulation time, and perfusate composition, pH, and temperature demonstrate that the technique has diversified to meet new challenges and answer different scientific questions. The review also reveals which individual methods are most frequently reported, even if there is no explicit agreement upon which parameters should be reported. The analysis of methods related to the Langendorff technique suggests a framework for considering methodological approach when interpreting seemingly contradictory results, rather than concluding that results are irreproducible.

Malignant hyperthermia in the early days of pediatric anesthesia: an interview with anesthesiology pioneer, Dr. John F. Ryan

Dr. John F. Ryan (1935 - ), Associate Professor of Anaesthesia at the Harvard Medical School, influenced the careers of hundreds of residents and fellows-in-training while instilling in them his core values of resilience, hard work, and integrity. His authoritative textbook, A Practice of Anesthesia for Infants and Children, remains as influential today as it did when first published decades ago. Although he had had many accomplishments, he identified his experiences caring for patients with malignant hyperthermia and characterizing the early discovery of this condition as his defining contribution to medicine. Based on a series of interviews with Dr. Ryan, this article reviews a remarkable career that coincides with the dawn of modern pediatric anesthetic practice.

Enzymological investigations on the causes for the PSE-syndrome, II. Comparative studies on glycogen phosphorylase from pig muscles

In order to investigate the cell biological causes for the fast breakdown of glycogen which is observed during the development of the PSE (pale, soft, exudative) syndrome in muscles of stress-susceptible pigs, muscle glycogen phosphorylase (GP) as a key enzyme in two isoforms, a and b, of the energy turnover was isolated from M. longissimus dorsi of normal and PSE-prone pigs of the German Landrace. GP b as well as GP a from normal and PSE-muscles exist in a dimeric form with a molecular weight of 97 000 D per subunit. The tendency for tetramerization of GP b increases in the presence of ATP, whereas the enzyme activity is simultaneously inhibited. The catalytic activities of GP a and GP b from both groups of animals show an optimum at pH 7.0. GP b can be activated to GP a by phosphorylation with the result of a 25% higher optimum specific activity in the case of normal and PSE-muscles. In interaction with glycogen and glucose-1-phosphate GP b follows the characteristics of a Michaelis-Menten kinetic, whereas the binding of AMP and phosphate proves to be allosteric. In comparison of the structural and kinetic characteristics of GP from normal as well as PSE-muscles no significant differences could be determined, indicating that GP does not belong to those factors which are triggering an accelerated energy turnover of ATP in muscles of stress-susceptible pigs.

Calcium-Induced Calcium Release in Skeletal Muscle

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

Novel regulators of RyR Ca2+ release channels: insight into molecular changes in genetically-linked myopathies

There are many mutations in the ryanodine receptor (RyR) Ca2+ release channel that are implicated in skeletal muscle disorders and cardiac arrhythmias. More than 80 mutations in the skeletal RyR1 have been identified and linked to malignant hyperthermia, central core disease or multi-minicore disease, while more than 40 mutations in the cardiac RyR2 lead to ventricular arrhythmias and sudden cardiac death in patients with structurally normal hearts. These RyR mutations cause diverse changes in RyR activity which either excessively activate or block the channel in a manner that disrupts Ca2+ signalling in the muscle fibres. In a different myopathy, myotonic dystrophy (DM), a juvenile isoform of the skeletal RyR is preferentially expressed in adults. There are two regions of RyR1 that are variably spiced and developmentally regulated (ASI and ASII). The juvenile isoform (ASI(-)) is less active than the adult isoform (ASI(+)) and its over-expression in adults with DM may contribute to functional changes. Finally, mutations in an important regulator of the RyR, the Ca2+ binding protein calsequestrin (CSQ), have been linked to a disruption of Ca2+ homeostasis in cardiac myocytes that results in arrhythmias. We discuss evidence supporting the hypothesis that mutations in each of these situations alter protein/protein interactions within the RyR complex or between the RyR and its associated proteins. The disruption of these protein-protein interactions can lead either to excess Ca2+ release or reduced Ca2+ release and thus to abnormal Ca2+ homeostasis. Much of the evidence for disruption of protein-protein interactions has been provided by the actions of a group of novel RyR regulators, domain peptides with sequences that correspond to sequences within the RyR and which compete with the endogenous residues for their interaction sites.

Enhanced response to caffeine and 4-chloro-m-cresol in malignant hyperthermia-susceptible muscle is related in part to chronically elevated resting [Ca2+]i

Malignant hyperthermia (MH) is a potentially fatal pharmacogenetic syndrome caused by exposure to halogenated volatile anesthetics and/or depolarizing muscle relaxants. We have measured intracellular Ca(2+) concentration ([Ca(2+)](i)) using double-barreled, Ca(2+)-selective microelectrodes in myoballs prepared from skeletal muscle of MH-susceptible (MHS) and MH-nonsusceptible (MHN) swine. Resting [Ca(2+)](i) was approximately twofold in MHS compared with MHN quiescent myoballs (232 +/- 35 vs. 112 +/- 11 nM). Treatment of myoballs with caffeine or 4-chloro-m-cresol (4-CmC) produced an elevation in [Ca(2+)](i) in both groups; however, the concentration required to cause a rise in [Ca(2+)](i) elevation was four times lower in MHS than in MHN skeletal muscle cells. Incubation of MHS cells with the fast-complexing Ca(2+) buffer BAPTA reduced [Ca(2+)](i), raised the concentration of caffeine and 4-CmC required to cause an elevation of [Ca(2+)](i), and reduced the amount of Ca(2+) release associated with exposure to any given concentration of caffeine or 4-CmC to MHN levels. These results suggest that the differences in the response of MHS skeletal myoballs to caffeine and 4-CmC may be mediated at least in part by the chronic high resting [Ca(2+)](i) levels in these cells.

Functional analysis of the R1086H malignant hyperthermia mutation in the DHPR reveals an unexpected influence of the III-IV loop on skeletal muscle EC coupling

Malignant hyperthermia (MH) is an inherited pharmacogenetic disorder caused by mutations in the skeletal muscle ryanodine receptor (RyR1) and the dihydropyridine receptor (DHPR) alpha(1S)-subunit. We characterized the effects of an MH mutation in the DHPR cytoplasmic III-IV loop of alpha(1S) (R1086H) on DHPR-RyR1 coupling after reconstitution in dysgenic (alpha(1S) null) myotubes. Compared with wild-type alpha(1S), caffeine-activated Ca(2+) release occurred at approximately fivefold lower concentrations in nonexpressing and R1086H-expressing myotubes. Although maximal voltage-gated Ca(2+) release was similar in alpha(1S)- and R1086H-expressing myotubes, the voltage dependence of Ca(2+) release was shifted approximately 5 mV to more negative potentials in R1086H-expressing myotubes. Our results demonstrate that alpha(1S) functions as a negative allosteric modulator of release channel activation by caffeine/voltage and that the R1086H MH mutation in the intracellular III-IV linker disrupts this negative regulatory influence. Moreover, a low caffeine concentration (2 mM) caused a similar shift in voltage dependence of Ca(2+) release in alpha(1S)- and R1086H-expressing myotubes. Compared with alpha(1S)-expressing myotubes, maximal L channel conductance (G(max)) was reduced in R1086H-expressing myotubes (alpha(1S) 130 +/- 10.2, R1086H 88 +/- 6.8 nS/nF; P < 0.05). The decrease in G(max) did not result from a change in retrograde coupling with RyR1 as maximal conductance-charge movement ratio (G(max)/Q(max)) was similar in alpha(1S)- and R1086H-expressing myotubes and a similar decrease in G(max) was observed for an analogous mutation engineered into the cardiac L channel (R1217H). In addition, both R1086H and R1217H DHPRs targeted normally and colocalized with RyR1 in sarcoplasmic reticulum (SR)-sarcolemmal junctions. These results indicate that the R1086H MH mutation in alpha(1S) enhances RyR1 sensitivity to activation by both endogenous (voltage sensor) and exogenous (caffeine) activators.

N-terminal region of FKBP12 is essential for binding to the skeletal ryanodine receptor

It is known that the two types of FK506-binding proteins FKBP12 and FKBP12.6 are tightly associated with the skeletal (RyR1) and cardiac ryanodine receptors (RyR2), respectively, and their interactions are important for channel functions of the RyR. In the case of cardiac muscle, three amino acid residues (Gln-31, Asn-32, and Phe-59) of FKBP12.6 could be essential for the selective binding to RyR2 (Xin, H. B., Rogers, K., Qi, Y., Kanematsu, T., and Fleischer, S. (1999) J. Biol. Chem. 274, 15315-15319). In this study to identify amino acid residues of FKBP12 that are important for the selective binding to RyR1, we mutated 9 amino acid residues of FKBP12 that differ from the counterparts of FKBP12.6 (Q3E, R18A, E31Q, D32N, M49R, R57A, W59F, H94A, and K105A), and we examined binding properties of these mutants to RyR1 by in vitro binding assay by using glutathione S-transferase-fused proteins of the mutants and Triton X-100-solubilized, FKBP12-depleted rabbit skeletal sarcoplasmic reticulum vesicles. Among the nine mutants tested, only Q3E and R18A lost their selective binding ability to RyR1. Furthermore, co-immunoprecipitation of RyR1 with 33 various mutants for the 9 positions produced by introducing different size, charge, and hydrophobicity revealed that an integration of the hydrogen bonds by the irreplaceable Gln-3 and the hydrophobic interactions by the residues Arg-18 and Met-49 could be a possible mechanism for the binding of FKBP12 to RyR1. Therefore, these results suggest that the N-terminal regions of FKBP12 (Gln-3 and Arg-18) and Met-49 are essential and unique for binding of FKBP12 to RyR1 in skeletal muscle.

Molecular basis of the high-affinity activation of type 1 ryanodine receptors by imperatoxin A

Both imperatoxin A (IpTx(a)), a 33-residue peptide toxin from scorpion venom, and peptide A, derived from the II-III loop of dihydropyridine receptor (DHPR), interact specifically with the skeletal ryanodine receptor (RyR1), which is a Ca(2+)-release channel in the sarcoplasmic reticulum, but with considerably different affinities. IpTx(a) activates RyR1 with nanomolar affinity, whereas peptide A activates RyR1 at micromolar concentrations. To investigate the molecular basis for high-affinity activation of RyR1 by IpTx(a), we have determined the NMR solution structure of IpTx(a), and identified its functional surface by using alanine-scanning analogues. A detailed comparison of the functional surface profiles for two peptide activators revealed that IpTx(a) exhibits a large functional surface area (approx. 1900 A(2), where 1 A=0.1 nm), based on a short double-stranded antiparallel beta-sheet structure, while peptide A bears a much smaller functional surface area (approx. 800 A(2)), with the five consecutive basic residues (Arg(681), Lys(682), Arg(683), Arg(684) and Lys(685)) being clustered at the C-terminal end of the alpha-helix. The functional surface of IpTx(a) is composed of six essential residues (Leu(7), Lys(22), Arg(23), Arg(24), Arg(31) and Arg(33)) and several other important residues (His(6), Lys(8), Arg(9), Lys(11), Lys(19), Lys(20), Gly(25), Thr(26), Asn(27) and Lys(30)), indicating that amino acid residues involved in RyR1 activation make up over the half of the toxin molecule with the exception of cysteine residues. Taken together, these results suggest that the site where peptide A binds to RyR1 belongs to a subset of macrosites capable of being occupied by IpTx(a), resulting in differing the affinity and the mode of activation.

Caffeine sensitivity of native RyR channels from normal and malignant hyperthermic pigs: effects of a DHPR II-III loop peptide

Enhanced sensitivity to caffeine is part of the standard tests for susceptibility to malignant hyperthermia (MH) in humans and pigs. The caffeine sensitivity of skeletal muscle contraction and Ca(2+) release from the sarcoplasmic reticulum is enhanced, but surprisingly, the caffeine sensitivity of purified porcine ryanodine receptor Ca(2+)-release channels (RyRs) is not affected by the MH mutation (Arg(615)Cys). In contrast, we show here that native malignant hyperthermic pig RyRs (incorporated into lipid bilayers with RyR-associated lipids and proteins) were activated by caffeine at 100- to 1000-fold lower concentrations than native normal pig RyRs. In addition, the results show that the mutant ryanodine receptor channels were less sensitive to high-affinity activation by a peptide (C(S)) that corresponds to a part of the II-III loop of the skeletal dihydropyridine receptor (DHPR). Furthermore, subactivating concentrations of peptide C(S) enhanced the response of normal pig and rabbit RyRs to caffeine. In contrast, the caffeine sensitivity of MH RyRs was not enhanced by the peptide. These novel results showed that in MH-susceptible pig muscles 1). the caffeine sensitivity of native RyRs was enhanced, 2). the sensitivity of RyRs to a skeletal II-III loop peptide was depressed, and 3). an interaction between the caffeine and peptide C(S) activation mechanisms seen in normal RyRs was lost.

Negatively charged amino acids within the intraluminal loop of ryanodine receptor are involved in the interaction with triadin

In mammalian striated muscles, ryanodine receptor (RyR), triadin, junctin, and calsequestrin form a quaternary complex in the lumen of sarcoplasmic reticulum. Such intermolecular interactions contribute not only to the passive buffering of sarcoplasmic reticulum luminal Ca2+, but also to the active Ca2+ release process during excitation-contraction coupling. Here we tested the hypothesis that specific charged amino acids within the luminal portion of RyR mediate its direct interaction with triadin. Using in vitro binding assay and site-directed mutagenesis, we found that the second intraluminal loop of the skeletal muscle RyR1 (amino acids 4860-4917), but not the first intraluminal loop of RyR1 (amino acids 4581-4640) could bind triadin. Specifically, three negatively charged residues Asp4878, Asp4907, and Glu4908 appear to be critical for the association with triadin. Using deletional approaches, we showed that a KEKE motif of triadin (amino acids 200-232) is essential for the binding to RyR1. Because the second intraluminal loop of RyR has been previously shown to contain the ion-conducting pore as well as the selectivity filter of the Ca2+ release channel, and Asp4878, Asp4907, and Glu4908 residues are predicted to locate at the periphery of the pore assembly of the channel, our data suggest that a physical interaction between RyR1 and triadin could play an active role in the overall Ca2+ release process of excitation-contraction coupling in muscle cells.

Interaction of HRC (histidine-rich Ca(2+)-binding protein) and triadin in the lumen of sarcoplasmic reticulum

HRC (histidine-rich Ca(2+) binding protein) has been identified from skeletal and cardiac muscle and shown to bind Ca(2+) with high capacity and low affinity. While HRC resides in the lumen of the sarcoplasmic reticulum, the physiological function of HRC is largely unknown. In the present study, we have performed co-immunoprecipitation experiments and show that HRC binds directly to triadin, which is an integral membrane protein of the sarcoplasmic reticulum. Using a fusion protein binding assay, we further identified the histidine-rich acidic repeats of HRC as responsible for the binding of HRC to triadin. These motifs may represent a novel protein-protein interaction domain. The HRC binding domain of triadin was also localized by fusion protein binding assay to the lumenal region containing the KEKE motif that was previously shown to be involved in the binding of triadin to calsequestrin. Notably, the interaction of HRC and triadin is Ca(2+)-sensitive. Our data suggest that HRC may play a role in the regulation of Ca(2+) release from the sarcoplasmic reticulum by interaction with triadin.

Abnormal uptake and release of Ca 2+ ions from human malignant hyperthermia-susceptible sarcoplasmic reticulum 1 1Abbreviations: CICR, Ca2+-induced Ca2+ release; HEK-293, human embryonic kidney; HSR, heavy sarcoplasmic reticulum; IVCT, in vitro caffeine halothane contracture test; MH, malignant hyperthermia; MHS, malignant hyperthermia-susceptible; MHN, malignant hyperthermia normal; MOPS, 3-[N-Morpholino]propanesulphonic acid; RYR1, ryanodine receptor skeletal muscle gene; and TFP, trifluoperazine

Malignant hyperthermia (MH) is a pharmacogenetic myopathy that occurs in humans and several other mammalian species. There has been limited investigation of Ca2+ transport by human heavy sarcoplasmic reticulum (HSR) vesicles despite the fact that mutations of the ryanodine receptor Ca2+ release channel have been linked to inheritance of MH. In this study, the Ca2+ release and uptake mechanisms in human MH-susceptible HSR (MHS) vesicles were investigated and the kinetics and sensitivity compared to normal vesicles. Alterations in Ca2+ regulation were thereby elucidated. HSR vesicles from 6 normal (MHN) and 5 MHS patients were compared using a dual-wavelength continuous Ca2+ flux assay in the presence of pyrophosphate. The loading capacity and loading rate of Ca2+ in MHS vesicles were reduced by almost 50%. These parameters were restored to normal when the Ca2+ channel blocker ruthenium red was added. Calcium-induced calcium release, halothane-induced calcium release, and trifluoperazine-induced calcium release were clearly elevated in MHS HSR vesicles compared to MHN vesicles. The results suggest that MH ryanodine receptors exist in a more open resting state than those in normal muscle.

Arg(615)Cys substitution in pig skeletal ryanodine receptors increases activation of single channels by a segment of the skeletal DHPR II-III loop

The effect of peptides, corresponding to sequences in the skeletal muscle dihydropyridine receptor II-III loop, on Ca(2+) release from sarcoplasmic reticulum (SR) and on ryanodine receptor (RyR) calcium release channels have been compared in preparations from normal and malignant hyperthermia (MH)-susceptible pigs. Peptide A (Thr(671)-Leu(690); 36 microM) enhanced the rate of Ca(2+) release from normal SR (SR(N)) and from SR of MH-susceptible muscle (SR(MH)) by 10 +/- 3.2 nmole/mg/min and 76 +/- 9.7 nmole/mg/min, respectively. Ca (2+) release from SR(N) or SR(MH) was not increased by control peptide NB (Gly(689)-Lys(708)). AS (scrambled A sequence; 36 microM) did not alter Ca (2+) release from SR(N), but increased release from SR(MH) by 29 +/- 4.9 nmoles/mg/min. RyR channels from MH-susceptible muscle (RyR(MH)) were up to about fourfold more strongly activated by peptide A (> or =1 nM) than normal RyR channels (RyR(N)) at -40 mV. Neither NB or AS activated RyR(N). RyR(MH) showed an approximately 1.8-fold increase in mean current with 30 microM AS. Inhibition at +40 mV was stronger in RyR(MH) and seen with peptide A (> or = 0.6 microM) and AS (> or = 0.6 microM), but not NB. These results show that the Arg(615)Cys substitution in RyR(MH) has multiple effects on RyRs. We speculate that enhanced DHPR activation of RyRs may contribute to increased Ca(2+) release from SR in MH-susceptible muscle.

Genetics and pathogenesis of malignant hyperthermia

Malignant hyperthermia (MH) is a potentially life-threatening event in response to anesthetic triggering agents, with symptoms of sustained uncontrolled skeletal muscle calcium homeostasis resulting in organ and systemic failure. Susceptibility to MH, an autosomal dominant trait, may be associated with congenital myopathies, but in the majority of the cases, no clinical signs of disease are visible outside of anesthesia. For diagnosis, a functional test on skeletal muscle biopsy, the in vitro contracture test (IVCT), is performed. Over 50% of the families show linkage of the IVCT phenotype to the gene encoding the skeletal muscle ryanodine receptor and over 20 mutations therein have been described. At least five other loci have been defined implicating greater genetic heterogeneity than previously assumed, but so far only one further gene encoding the main subunit of the voltage-gated dihydropyridine receptor has a confirmed role in MH. As a result of extensive research on the mechanisms of excitation-contraction coupling and recent functional characterization of several disease-causing mutations in heterologous expression systems, much is known today about the molecular etiology of MH.

HRC (histidine-rich Ca2+ binding protein) resides in the lumen of sarcoplasmic reticulum as a multimer

HRC (histidine-rich Ca2+ binding protein) has been identified from skeletal and cardiac muscle and shown to bind Ca2+ with low affinity and high capacity that is reminiscent of calsequestrin. The physiological role of HRC is largely unknown. In this study, we show that HRC exists as a multimeric complex (probably larger than a pentamer) under physiological conditions. At higher Ca2+ concentrations, the complex appeared to dissociate into dimers or trimers that form a more relaxed structure. This is in striking contrast to the characteristics of calsequestrin. An earlier immuno-electron microscopic study showed that HRC resides in the lumen of the sarcoplasmic reticulum (SR), but this conclusion has been challenged by other data. By tryptic digestion and biotinylation of SR vesicles, we provide compelling evidence showing that HRC is indeed present in the lumen of the SR.

Ca(2+)- and pH-dependent halothane stimulation of Ca2+ release in sarcoplasmic reticulum from frog muscle

The effect of halothane on calcium release kinetics was studied in triad-enriched sarcoplasmic reticulum vesicles from frog skeletal muscle. Release from vesicles passively equilibrated with 3 mM 45CaCl2 was measured in the millisecond time range by use of a fast-filtration system. Halothane (400 microM) increased release rate constants at pH 7.1 and 7.4 as a function of extravesicular pCa. In contrast, halothane at pH 6.8 produced the same stimulation of release from pCa 7.0 to 3.0; no release took place in these conditions in the absence of halothane. Halothane shifted the calcium activation curve at pH 7.1, but not at pH 7.4, to the left and increased channel open probability at pH 7.1 in the cis pCa range of 7.0 to 5.0. These results indicate that cytosolic pCa and pH modulate the stimulatory effects of halothane on calcium release. Furthermore, halothane stimulated release in frog skeletal muscle at low pH and resting calcium concentration, indicating that in frog muscle halothane can override the closing of the release channels produced by these conditions, as it does in malignant hyperthermia-susceptible porcine muscle.

Comparative studies of pyruvate kinase from PSE and normal pig muscles

A fast breakdown of glycogen is observed in muscles of stress-susceptible pigs leading to pale, soft and exudative (PSE) meat. We report a comparative study of pyruvate kinase from muscles of normal and PSE-prone pigs. Compared with the enzyme from normal muscle, pyruvate kinase isolated from PSE muscle shows a five times lower Michaelis constant, Km, for phosphoenol pyruvate and a more than ten times higher Kcat/Km value. The pH dependency of the enzymatic activity is shifted to more acidic values for pyruvate kinase from PSE muscles. According to isoelectric focusing, pyruvate kinase from PSE muscle consists of three isoforms, while only two isoforms are detectable in pyruvate kinase preparations from normal pigs. The various isoforms were isolated by preparative isoelectric focusing and their steady-state properties were compared. Isoform 3, which is found only in PSE muscle, shows a 10-fold higher specific activity, a 30-fold lower Km value and a 100-fold increased kcat/Km value for phosphoenol pyruvate as compared to isoform 1. The presence of isoform 3 in PSE muscle appears to be responsible for the high activity of this enzyme under the more acidic conditions prevailing in PSE muscle. In vitro phosphorylation and dephosphorylation experiments using total enzyme and purified isoenzyme 1 suggest that isoforms 2 and 3 arise from isoform 1 by phosphorylation. Thus protein phosphorylation seems to be responsible for the shift in activity of pyruvate kinase, a key enzyme of glycolysis, under the acidic conditions of PSE muscles.

Molecular genetics of ion channel diseases

Many physiological processes depend upon the proper functioning of plasma membrane ion channels. This is most apparent in absorptive and secretory epithelia, and in electrically excitable tissues such as nerve and muscle. Disturbances in the operation of ion channels in these settings can alter normal physiology and cause disease. This review illustrates the use of molecular genetics in identifying hereditary diseases caused by mutations in genes which encode various skeletal muscle ion channels. Recent advances in the discovery of genetic mutations in the skeletal muscle voltage-gated sodium channel in certain forms of periodic paralysis, mutations in the skeletal muscle chloride channel gene in myotonia congenita, and defects in two distinct calcium channels that underlie disorders of excitation-contraction coupling (murine muscular dysgenesis, malignant hyperthermia susceptibility) will be presented. In each case, prior knowledge of abnormal ion channel function prompted the search for mutations in candidate genes. This work is beginning to shed new light on the relationship between ion channel structure and function by studies of naturally occurring channel mutations.

Altered E-C coupling in triads isolated from malignant hyperthermia-susceptible porcine muscle

Triad vesicles were isolated from normal (N) and homozygous malignant hyperthermia-susceptible (MHS) porcine skeletal muscle, and two types of sarcoplasmic reticulum Ca2+ release were investigated: 1) polylysine-induced Ca2+ release (direct stimulation of the junctional foot protein), and 2) depolarization-induced Ca2+ release (stimulation of the junctional foot protein via the dihydropyridine receptor). At submaximal concentrations of polylysine, the rates of induced Ca2+ release from the MHS triads were greater than from normal triads. The T tubules of polarized triads were depolarized by the K(+)-to-Na+ ionic replacement protocol. Higher grades of T-tubule depolarization resulted in higher rates of Ca2+ release from both MHS and normal triads but, when compared at a given grade of T-tubule depolarization, the release rate was always greater from the MHS than from normal triads. Thus the activity of the SR Ca2+ release channel is always higher in MHS than in normal muscle at a given submaximal dose of release trigger. This difference is observed when the channel is stimulated directly by polylysine or indirectly via a depolarization-induced activation of the T-tubule dihydropyridine receptor.

Skeletal muscle junctional membrane protein content in pigs with different ryanodine receptor genotypes

The content of the sarcoplasmic reticulum (SR) Ca(2+)-ATPase, transverse tubule dihydropyridine receptor (DHPR), and SR ryanodine receptor (RyR) was determined in muscle of pigs homozygous for the normal RyR allele and homozygous or heterozygous for the malignant hyperthermia-susceptible (MHS) RyR allele. Total muscle membranes isolated from 1-day-old pigs of the three different genotypes did not differ in the content of any of these proteins. However, at 28 days of age, crude membranes and total muscle homogenates from homozygous MHS pigs exhibited only 61-81% of the [3H]PN 200-110 or [3H]ryanodine binding of identical preparations isolated from normal pigs; these MHS membranes also contained only 50% of the normal content of each of the DHPR subunits. The crude membranes and muscle homogenates from heterozygous pigs were intermediate to both types of homozygotes in terms of [3H]PN 200-110 binding, [3H]ryanodine binding, and the content of the DHPR subunits. However, membrane preparations enriched in triadic junctional proteins isolated from 3- to 4-mo-old pigs of the three different genotypes did not differ in their [3H]PN 200-110 binding, [3H]ryanodine binding, or Ca(2+)-ATPase activities. We conclude that, although the stoichiometry of the RyR to DHPR is not altered, the presence of the MHS RyR allele during muscle development results in a decreased relative content of these two proteins. This is probably due to a lower junctional membrane content and may be an important ultrastructural consequence of the altered sarcoplasmic Ca2+ regulation in MHS muscle.

The genetics of malignant hyperthermia

Malignant hyperthermia susceptibility remains the commonest cause of death owing to general anaesthesia. This is despite the availability of presymptomatic testing, admittedly by a highly invasive method, and a recognised treatment for implementation immediately a patient shows signs of developing a crisis. Recently the finding of linkage to markers from chromosome 19q13.1-13.2 and the identification of mutations in a candidate gene held out hope of genetic diagnosis being available. However, it is likely that only about 50% of families have a mutation of the skeletal muscle calcium release channel gene. With this degree of genetic heterogeneity, presymptomatic testing based on DNA markers can only be offered at present to a limited number of families where linkage to markers from 19q13.1-13.2 has been clearly shown.

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.

Structural and functional correlates of a mutation in the malignant hyperthermia-susceptible pig ryanodine receptor

The skeletal muscle ryanodine receptor of malignant hyperthermia-susceptible (MHS) pigs contains a mutation at residue 615 that is highly correlated with various abnormalities in the regulation of sarcoplasmic reticulum (SR) Ca2+ channel activity. In isolated SR membranes the Arg615 to Cys615 ryanodine receptor mutation is now shown to be directly responsible for an altered tryptic peptide map, due to the elimination of the Arg615 cleavage site. Furthermore, trypsin treatment released 86-99 kDa ryanodine receptor fragments encompassing residue 615 from the SR membranes. We conclude that the 86-99 kDa domain containing residue 615 is near the cytoplasmic surface of the ryanodine receptor and likely near important Ca2+ channel regulatory sites.

Abnormalities in the regulation of blood platelet free cytosolic calcium in malignant hyperthermia. I. Human platelets

The effect of halothane on the regulation of blood platelet free cytosolic calcium was investigated in Quin-2-loaded cells from patients susceptible to Malignant Hyperthermia (MH) and healthy controls, respectively. The resting level of free cytosolic calcium was slightly, but statistically significantly, enhanced in platelets from patients (90 +/- 10 nM vs 110 +/- 35 nM). Halothane induced a dose-dependent, rapid Ca2+ release from intracellular stores both in normal and in MH derived cells, but the resulting increase in cytosolic calcium was significantly higher in the latter (2 mM halothane: [Ca2+]i = 117 +/- 12 nM vs 218 +/- 117 nM; 4 mM halothane: 225 +/- 35 nM vs. 417 +/- 201 nM). Whereas in platelets from healthy donors a complete reversibility of the halothane effect could be observed within 30-45 min, the cytosolic Ca2+ transients in platelets from patients were different from those in normals either in a higher initial peak or in a diminished decline velocity or in both. The basal Ca2+ permeability of the platelet plasma membrane was very low. Generally, halothane caused a dose-dependent increase in Ca2+ permeability. However, the influx of external calcium was significantly higher in platelets from patients than in controls (2 mM halothane: delta [Ca2+]i = 69 +/- 12 nM vs 135 +/- 63 nM; 4 mM halothane: 127 +/- 33 nM vs. 258 +/- 111 nM). Combining the results, the suggestion can be made that susceptibility to MH is characterized by a generalized membrane defect.(ABSTRACT TRUNCATED AT 250 WORDS)

W-7, a calmodulin antagonist, and contracture of malignant hyperthermia susceptible skeletal muscle

1. In malignant hyperthermia susceptible muscle fibers, the calmodulin antagonist, W-7 (10 microM), evoked contractures and potentiated halothene (3%) induced contracture. No effect was seen at 0.1 or 1.0 microM) W-7. 2. Dantrolene sodium (6 microM) prevented and reversed W-7 induced contracture: nifedipine did not. 3. In chemically skinned fibers, 10 microM, 1.0 microM, and 0.1 microM W-7 released 100%, 30%, and 10% of stored calcium respectively, and the effect of 10 microM W-7 was irreversible in that the SR was unable to re-sequestor calcium after exposure to the drug. 4. The release of calcium by W-7 was not prevented by exogenously added calmodulin (3 microM), nor mimicked by mastoparan (10 microM). 5. Calcium release by W-7 appears to be independent of calmodulin inhibition.

Calcium ATPase in the sarcoplasmic reticulum of muscle from normal and malignant hyperthermia susceptible pigs

The density of Mg(2+)-dependent Ca2+ ATPase in the terminal cisternae of pig skeletal muscle fibers was investigated to discover whether a reduction in Ca2+ ATPase content impairs Ca2+ sequestration and contributes to the elevated myoplasmic Ca2+ concentration in malignant hyperthermia. Unexpectedly, immunogold electron microscopy showed an increase in Ca2+ ATPase, while densitometry of SDS-polyacrylamide gels suggested that the Ca2+ ATPase content of terminal cisternae vesicles did not change. The affinity of Ca2+ ATPase in vesicles for our monoclonal antibody was not altered. We suggest that the availability of antigenic sites in malignant hyperthermia increases after processing for electron microscopy, perhaps as a consequence of altered sarcoplasmic reticulum membrane properties.

In vitro correlation between force and energy metabolism in porcine malignant hyperthermic muscle studied by 31P NMR

A comparative study of mechanical and energetic parameters of superfused muscle strips from normal pigs and malignant hyperthermia susceptible (MHS) pigs has been conducted. Phosphorus nuclear magnetic resonance spectroscopy at 80.9 MHz and mechanical measurements were used to assess muscle metabolic state. At rest, biceps femoris biopsies of MHS pigs displayed reduced phosphocreatine level, higher inorganic phosphate, and a more acidic internal pH. In normal stimulated fibers, caffeine infusion (8 or 16 mM) induced twitch potentiation and contracture while twitch tension was reduced and contracture more pronounced in malignant fibers. In normal and malignant fibers, calcium ionophore A23187 produced effects similar to those of caffeine, with the exception of twitch potentiation, which was not observed. With caffeine or A23187, the ATP level remained constant throughout the rest-stimulation-recovery protocol for normal and malignant fibers but phosphocreatine dropped to undetectable levels upon stimulation of malignant fibers. In both treatments some heterogeneity in the resonances of inorganic phosphate was observed in malignant fibers together with a more severe acidosis which might play a role in the impairment of the excitation-contraction process.

Abnormal rapid Ca2+ release from sarcoplasmic reticulum of malignant hyperthermia susceptible pigs

Using the rapid filtration technique to investigate Ca2+ movements across the sarcoplasmic reticulum (SR) membrane, we compare the initial phases of Ca2+ release and Ca2+ uptake in malignant hyperthermia susceptible (MHS) and normal (N) pig SR vesicles. Ca2+ release is measured from passively loaded SR vesicles. MHS SR vesicles present a 2-fold increase in the initial rate of calcium release induced by 0.3 microM Ca2+ (20.1 +/- 2.1 vs. 6.3 +/- 2.6 nmol mg-1 s-1). Maximal Ca2+ release is obtained with 3 microM Ca2+. At this optimal concentration, rate of Ca2+ efflux in absence of ATP is 55 and 25 nmol mg-1 s-1 for MHS and N SR, respectively. Ca(2+)-induced Ca2+ release is inhibited by Mg2+ in a dose-dependent manner for both MHS and N pig SR vesicles (K1/2 = 0.2 mM). Caffeine (5 mM) and halothane (0.01% v/v) increase the Ca2+ sensitivity of Ca(2+)-induced Ca2+ release. ATP (5 mM) strongly enhances the rate of Ca2+ efflux (to about 20-40-fold in both MHS and N pig SR vesicles). Furthermore, both types of vesicles do not differ in their high-affinity site for ryanodine (Kd = 12 nM and Bmax = 6 pmol/mg), lipid content, ATPase activity and initial rate of Ca2+ uptake (0.948 +/- 0.034 vs. 0.835 +/- 0.130 mumol mg-1 min-1 for MHS and N SR, respectively). Our results show that MH syndrome is associated to a higher rate of Ca2+ release in the earliest phase of the calcium efflux.

Abnormal human sarcoplasmic reticulum Ca2+ release channels in malignant hyperthermic skeletal muscle

Single sarcoplasmic reticulum (SR) Ca2+ release channels were reconstituted from normal and malignant hyperthermic (MH) human skeletal muscle biopsies (2-5 g samples). Conduction, gating properties, and myoplasmic Ca2+ dependence of human SR Ca2+ release channels were similar to those in other species (rabbit, pig). The MH diagnostic procedure distinguishes three phenotypes (normal, MH-equivocal, and MH-susceptible) on the basis of muscle contracture sensitivity to caffeine and/or halothane. Single channel studies reveal that human MH muscles (both MH phenotypes) contain SR Ca2+ release channels with abnormally greater caffeine sensitivity. Muscles from MH-equivocal and MH-susceptible patients appear to contain channels with the same abnormality. Further, our data (n = 115, 21 channels, 11 patients) reveals that human MH muscles (both phenotypes) may contain two populations of SR Ca2+ release channels, possibly corresponding to normal and abnormal isoforms. Thus, whole cell phenotypic variation (MH-equivocal vs. MH-susceptible) arises in muscles containing channels with similar caffeine sensitivity suggesting that human MH does not arise from a single defect. These results have important ramifications concerning (a) correlation of functional and genetic MH studies, (b) identification of other, yet to be determined, factors which may influence MH expression, and (c) characterization of normal SR Ca2+ release channel function by exploring genetic channel defects.

Abnormal fluidity state in membranes of malignant hyperthermia pig skeletal muscle

The fluidity state was analyzed on sarcoplasmic reticulum membranes and phospholipid vesicles prepared from normal and malignant hyperthermia susceptible pig muscle. Electron spin resonance studies were performed to determine the fluidity state at the region near the polar headgroups and in the central core of the bilayer using 5-nitroxide (5-NS) and 16-nitroxide stearic acid (16-NS), respectively. With the 5-NS label, no differences were found between normal and malignant hyperthermia sarcoplasmic reticulum (MH SR) membranes whereas with the 16-NS label, a significant increase of the activation energy was shown with MH membranes. Lower values of fluorescence anisotropy observed with DPH-labeled MH membranes as compared with normal ones, confirmed the higher abnormal fluidity state of these membranes. The fluidizing effect of halothane, a triggering agent of malignant hyperthermia syndrome, was also studied in these membranes. We show that a relatively low concentration of the drug destabilized not only the diseased sarcoplasmic reticulum membranes but also the vesicles made of total phospholipids extracted from MH skeletal muscle. Together, these findings strongly suggest that an overall increase in membrane fluidity may be implied in the MH disease, improving the general membrane defect hypothesis for this syndrome.

Halothane-induced functional and structural modifications in sarcoplasmic reticulum membranes from pig skeletal muscle

We investigated the effect of halothane on lipid and protein components of sarcoplasmic reticulum membranes isolated from pig trapezius muscle. We studied the relationships between the (Ca2(+)-Mg2+)-ATPase activity and the interaction of the anesthetic with lipid and protein moieties by means of EPR and fluorescence spectroscopic techniques. Our results clearly show that below 5 mumol per mg protein, halothane interacts mainly with the lipid components of the membrane. This interaction is shown to be localized in the central core of the phospholipid bilayer and to induce an increase of the membrane calcium permeability. The interaction with protein components only occurs at higher halothane concentrations and affects its conformational and functional states. These results are discussed with respect to new insights into diethylether-SR membrane interaction and to malignant hyperthermia syndrome in the pig.

Stimulation and inhibition of [3H]ryanodine binding to sarcoplasmic reticulum from malignant hyperthermia susceptible pigs

When compared to normal pig sarcoplasmic reticulum (SR), SR from malignant hyperthermia susceptible (MHS) porcine skeletal muscle has been shown to exhibit an increased rate of calcium release, as well as alterations in [3H]ryanodine-binding activity in the presence of microM Ca2+ (Mickelson et al., 1988, J. Biol. Chem. 263, 9310). In the present study, various stimulators (adenine nucleotides and caffeine) and inhibitors (ruthenium red and Mg2+) of the SR calcium release channel were examined for effects on MHS and normal SR [3H]ryanodine binding. The apparent affinity of the MHS SR receptor for ryanodine in the presence of 10 mM ATP (Kd = 6.0 nM) or 10 mM caffeine (Kd = 28 nM) was significantly greater than that of the normal SR (Kd = 8.5 and 65 nM in 10 mM ATP or caffeine, respectively), the Bmax (12-16 pmol/mg) was similar in all cases. The Ca2+(0.5) for inhibition of [3H]ryanodine binding in the presence of 5 mM AMPPNP (238 vs 74 microM for MHS and normal SR, respectively) and the Ca2+(0.5) for stimulation of [3H]ryanodine binding in the presence of 5 mM caffeine (0.049 vs 0.070 microM for MHS and normal SR, respectively) were also significantly different. Furthermore, in the presence of optimal Ca2+, MHS SR [3H]ryanodine binding was more sensitive to caffeine stimulation (C0.5 of 1.7 vs 3.4 mM) and was less sensitive to ruthenium red (C0.5 of 1.9 vs 1.2 microM) or Mg2+ inhibition (C0.5 of 0.34 vs 0.21 mM) than was normal SR. These results further support the hypothesis that differences in the ryanodine/receptor calcium release channel regulatory properties are responsible for the abnormal calcium releasing activity of MHS SR.

Inhibitory action of dantrolene on Ca-induced Ca2+ release from sarcoplasmic reticulum in guinea pig skeletal muscle

The effects of dantrolene on the sarcoplasmic reticulum and contractile machinery were examined in skinned skeletal muscles of guinea pigs. Dantrolene inhibited Ca-induced Ca2+ release at 38 degrees C. The inhibitory effect of dantrolene on Ca2+ release was greater in the presence of caffeine or adenine nucleotide than in their absence at low Ca2+ concentrations. Dantrolene suppressed the maximum rate of Ca-induced Ca2+ release but did not affect the sensitivity to Ca2+. At 20 degrees C, however, dantrolene did not inhibit Ca2+ release induced by any Ca2+ concentration, regardless of the presence and absence of caffeine, adenine nucleotide or Mg2+. Ca2+ uptake into the sarcoplasmic reticulum and the Ca2+ sensitivity of the contractile machinery were not affected by dantrolene at 38 degrees C and 20 degrees C. These results suggest that dantrolene is a selective inhibitor of the Ca-induced Ca2+ release mechanism at physiological temperatures without having an effect on the Ca2+ pump of the sarcoplasmic reticulum or on the contractile machinery of skeletal muscle.

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.

Halothane-dependent release of intracellular Ca2+ in blood cells in malignant hyperthermia

The concentration of ionized cytosolic calcium [( Ca2+]i) was determined in peripheral blood mononuclear cells from normal and malignant hyperthermia (MH)-susceptible humans and pigs, using the fluorescent Ca2+ indicator indo-1. [Ca2+]i was slightly but significantly elevated in cells from MH human cells relative to normal cells (198 +/- 18 nM, n = 15, and 146 +/- 14 nM, n = 11, respectively, P less than 0.05). Anesthetic concentrations of halothane in the cell suspension resulted in a rapid increase in [Ca2+]i in cells from both normal and MH humans or pigs. The increases (delta) were more pronounced in cells from MH subjects than from normal individuals (delta at 5.7 mM halothane: 245 +/- 53 vs. 57 +/- 11 nM, respectively) and from MH than from normal pigs (delta of 241 +/- 63 vs. 53 +/- 27 nM, respectively). Removal of extracellular Ca2+ obliterated the delta[Ca2+]i caused by halothane in cells from normal humans or pigs but only decreased by about half the delta[Ca2+]i in cells from MH humans or pigs. In 1,2-bis-(aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA)-loaded cells, in the absence of extracellular Ca2+, halothane failed to increase [Ca2+]i. This suggests that buffering Cai2+ with BAPTA precludes detection of release of Ca2+ from intracellular stores, explaining the previous observations made with quin2, a highly chelating Ca2+ indicator. It is concluded that clinical concentrations of halothane allow influx of Ca2+ in cells from both normal and MH-susceptible individuals but release Ca2+ from intracellular stores selectively in cells from the latter group.(ABSTRACT TRUNCATED AT 250 WORDS)

Ryanodine receptor gene is a candidate for predisposition to malignant hyperthermia

Malignant hyperthermia (MH) is a potentially lethal condition in which sustained muscle contracture, with attendant hypercatabolic reactions and elevation in body temperature, are triggered by commonly used inhalational anaesthetics and skeletal muscle relaxants. In humans, the trait is usually inherited in an autosomal dominant fashion, but in halothane-sensitive pigs with a similar phenotype, inheritance of the disease is autosomal recessive or co-dominant. A simple and accurate non-invasive test for the gene is not available and predisposition to the disease is currently determined through a halothane- and/or caffeine-induced contracture test on a skeletal muscle biopsy. Because Ca2+ is the chief regulator of muscle contraction and metabolism, the primary defect in MH is believed to lie in Ca2+ regulation. Indeed, several studies indicate a defect in the Ca2+ release channel of the sarcoplasmic reticulum, making it a prime candidate for the altered gene product in predisposed individuals. We have recently cloned complementary DNA and genomic DNA encoding the human ryanodine receptor (the Ca2(+)-release channel of the sarcoplasmic reticulum) and mapped the ryanodine receptor gene (RYR) to region q13.1 of human chromosome 19 (ref. 14), in close proximity to genetic markers that have been shown to map near the MH susceptibility locus in humans and the halothane-sensitive gene in pigs. As a more definitive test of whether the RYR gene is a candidate gene for the human MH phenotype, we have carried out a linkage study with MH families to determine whether the MH phenotype segregates with chromosome 19q markers, including markers in the RYR gene. Co-segregation of MH with RYR markers, resulting in a lod score of 4.20 at a linkage distance of zero centimorgans, indicates that MH is likely to be caused by mutations in the RYR gene.

Effects of halothane on calcium release from sarcoplasmic reticulum of rabbit psoas and semitendinosus skinned muscle fibers

Calcium release from sarcoplasmic reticulum was investigated using skinned fibers isolated from rabbit semitendinosus and psoas muscles, representative of slow and fast fibers, respectively. In both types of fibers, halothane at the concentration of 0.03% (v/v) enhanced the Ca2(+)-induced calcium release. In the absence of cytoplasmic free Ca2+, halothane induced calcium release in a dose-dependent manner, with a similar sensitivity for both semitendinosus and psoas fibers. These results are discussed in connection with muscular diseases such as malignant hyperthermia in which the crisis is triggered during anesthesia by halothane.

Uptake and release of calcium ions by heavy sarcoplasmic reticulum fraction of normal and malignant hyperthermia-susceptible human skeletal muscle

1. Ca2+ uptake, Ca2(+)-dependent ATPase activity and halothane-induced Ca2+ release from the heavy sarcoplasmic reticulum fraction of muscle from malignant hyperthermia susceptible individuals are similar to those of normal human muscle. 2. Ca2(+)-induced Ca2+ release from the diseased muscle was increased by 13%.

An electron paramagnetic resonance study of skeletal muscle membrane fluidity in malignant hyperthermia

Skeletal muscle sarcolemma (SL), transverse tubule (TT) and heavy sarcoplasmic reticulum (HSR) membranes were isolated from malignant hyperthermia susceptible (MHS) and normal pigs, and the rotational dynamics of lipid hydrocarbon chain motion was examined by electron paramagnetic resonance (EPR) spectroscopy. The stearic acid spin probe 16-SASL was incorporated into MHS and normal membranes and both the order parameter (S) and effective correlation time (tau r) of probe motion were calculated from spectra recorded over the temperature range of 2 to 40 degrees C. At any given temperature, TT membranes exhibited significantly greater values for both the S and tau r of probe motion than did SL, which exhibited significantly greater values than did HSR membranes. The order of decreasing S and tau r values for 16-SASL mobility correlated with the decreasing cholesterol content of these membranes (TT greater than SL greater than HSR), however there was no difference in the S or tau r values for a given membrane fraction isolated from both MHS and normal muscle. Arrhenius plots of 16-SASL mobility in SL, TT and HSR were linear from 2 to 40 degrees C, indicating no abrupt thermotropic change in the lipid hydrocarbon phase of any of the membrane types studied. Apparent activation energies (Ea), calculated from the Arrhenius plots, were similar for MHS and normal membranes derived from a given cellular location. However, the Ea of probe motion for TT membranes (2.3 +/- 0.1 and 2.4 +/- 0.1 kcal/mol/degree for MHS and normal, respectively) was significantly less than for SL (3.4 +/- 0.4 and 2.9 +/- 0.1 kcal/mol/degree for MHS and normal, respectively) which, in turn, was significantly less than the Ea for HSR (3.7 +/- 0.1 and 3.7 +/- 0.1 kcal/mol/degree for MHS and normal, respectively). Since 16-SASL motion was similar in MHS and normal membranes, we conclude that there is no evidence for a generalized membrane defect affecting lipid mobility in these MHS muscle membranes.

Effects of the halothane-sensitivity gene on sarcoplasmic reticulum function

Pigs heterozygous for the halothane-sensitivity gene exhibit a distinct phenotype with regard to both in vivo and in vitro muscle responses to halothane (E. M. Gallant, J. R. Mickelson, B. D. Roggow, S. K. Donaldson, C. F. Louis, and W. E. Rempel. Am. J. Physiol. 257 (Cell Physiol. 26): C781-C786, 1989). In this paper heavy sarcoplasmic reticulum (SR) preparations were isolated from the muscles of pigs of all three genotypes. The rate of calcium release from SR of pigs homozygous for the halothane-sensitivity gene was approximately twice that of SR from pigs homozygous for the normal allele. Furthermore, in the presence of 6 microM Ca2+, the binding of [3H]ryanodine to SR isolated from the homozygous halothane-sensitive pigs was of a higher affinity than was the binding to SR isolated from the homozygous normal pigs (Kd = 70-90 vs. 265 nM, respectively). The SR from pigs heterozygous for the halothane-sensitivity gene, however, demonstrated intermediate values for the rate of calcium release and the affinity for [3H]ryanodine (Kd = 192 nM). Thus the alterations in heavy SR calcium release and [3H]ryanodine binding in the pigs containing one copy of the halothane-sensitivity gene demonstrate a distinct heterozygote phenotype. These data also suggest that the protein product of this gene is closely associated with, and perhaps identical to, the SR calcium release channel-ryanodine receptor protein.

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.