Porcine malignant hyperthermia: genotype and contractile threshold of immature muscles

Relationships between resting conductances, excitability, and t-system ionic homeostasis in skeletal muscle

Activation of skeletal muscle fibers requires rapid sarcolemmal action potential (AP) conduction to ensure uniform excitation along the fiber length, as well as successful tubular excitation to initiate excitation–contraction coupling. In our companion paper in this issue, Pedersen et al. (2011. J. Gen. Physiol. doi:10.1085/jgp.201010510) quantify, for subthreshold stimuli, the influence upon both surface conduction velocity and tubular (t)-system excitation of the large changes in resting membrane conductance (G_M) that occur during repetitive AP firing. The present work extends the analysis by developing a multi-compartment modification of the charge–difference model of Fraser and Huang to provide a quantitative description of the conduction velocity of actively propagated APs; the influence of voltage-gated ion channels within the t-system; the influence of t-system APs on ionic homeostasis within the t-system; the influence of t-system ion concentration changes on membrane potentials; and the influence of Phase I and Phase II G_M changes on these relationships. Passive conduction properties of the novel model agreed with established linear circuit analysis and previous experimental results, while key simulations of AP firing were tested against focused experimental microelectrode measurements of membrane potential. This study thereby first quantified the effects of the t-system luminal resistance and voltage-gated Na⁺ channel density on surface AP propagation and the resultant electrical response of the t-system. Second, it demonstrated the influence of G_M changes during repetitive AP firing upon surface and t-system excitability. Third, it showed that significant K⁺ accumulation occurs within the t-system during repetitive AP firing and produces a baseline depolarization of the surface membrane potential. Finally, it indicated that G_M changes during repetitive AP firing significantly influence both t-system K⁺ accumulation and its influence on the resting membrane potential. Thus, the present study emerges with a quantitative description of the changes in membrane potential, excitability, and t-system ionic homeostasis that occur during repetitive AP firing in skeletal muscle.

A retrograde signal from RyR1 alters DHP receptor inactivation and limits window Ca2+ release in muscle fibers of Y522S RyR1 knock-in mice

Malignant hyperthermia (MH) is a life-threatening hypermetabolic condition caused by dysfunctional Ca(2+) homeostasis in skeletal muscle, which primarily originates from genetic alterations in the Ca(2+) release channel (ryanodine receptor, RyR1) of the sarcoplasmic reticulum (SR). Owing to its physical interaction with the dihydropyridine receptor (DHPR), RyR1 is controlled by the electrical potential across the transverse tubular (TT) membrane. The DHPR exhibits both voltage-dependent activation and inactivation. Here we determined the impact of an MH mutation in RyR1 (Y522S) on these processes in adult muscle fibers isolated from heterozygous RyR1(Y522S)-knock-in mice. The voltage dependence of DHPR-triggered Ca(2+) release flux was left-shifted by approximately 8 mV. As a consequence, the voltage window for steady-state Ca(2+) release extended to more negative holding potentials in muscle fibers of the RyR1(Y522S)-mice. A rise in temperature from 20 degrees to 30 degrees C caused a further shift to more negative potentials of this window (by approximately 20 mV). The activation of the DHPR-mediated Ca(2+) current was minimally changed by the mutation. However, surprisingly, the voltage dependence of steady-state inactivation of DHPR-mediated calcium conductance and release were also shifted by approximately 10 mV to more negative potentials, indicating a retrograde action of the RyR1 mutation on DHPR inactivation that limits window Ca(2+) release. This effect serves as a compensatory response to the lowered voltage threshold for Ca(2+) release caused by the Y522S mutation and represents a novel mechanism to counteract excessive Ca(2+) leak and store depletion in MH-susceptible muscle.

Enhanced excitation-coupled calcium entry in myotubes is associated with expression of RyR1 malignant hyperthermia mutations

Myotubes expressing wild type RyR1 (WT) or RyR1 with one of three malignant hyperthermia mutations R615C, R2163C, and T4826I (MH) were exposed sequentially to 60 mm KCl in Ca(2+)-replete and Ca(2+)-free external buffers (Ca+ and Ca-, respectively) with 3 min of rest between exposures. Although the maximal peak amplitude of the Ca(2+) transients during K(+) depolarization was similar for WT and MH in both external buffers, the rate of decay of the sustained phase of the transient during K(+) depolarization (decay rate) in Ca+ was 50% slower for MH. This difference was eliminated in Ca-, and the relative decay rates were faster for both genotypes than in Ca+. The integrated Ca(2+) transient in Ca-compared with Ca+ was reduced by 50-60% for MH and 20% for WT. The decay rate was not affected by [K(+)] x [Cl(-)] product or NiCl(2) (2 mm) supplementation of Ca-. The addition of La(2+) (0.1 mm), or SKF 96365 (20 microm) to Ca+ significantly accelerated decay rates for both WT and MH, but their effect was significantly greater in MH. Nifedipine (1 microm) had no effect, suggesting that the mechanism for this difference was not a reduction in L-type Ca(2+) channel Ca(2+) current. These data strongly suggest: 1) the decay rate in skeletal myotubes is related in part to Ca(2+) entry through the ECCE channel; 2) the MH mutations enhance ECCE compared with wild type; and 3) the increased Ca(2+) entry might play a significant role in the pathophysiology of MH.

Intracellular Ca2+ dynamics in malignant hyperthermia and central core disease: established concepts, new cellular mechanisms involved

Malignant hyperthermia (MH) and central core disease (CCD) are inherited human disorders of skeletal muscle Ca2+ homeostasis. Both MH and CCD are linked to mutations and/or deletions in the gene encoding the skeletal muscle ryanodine receptor (RyR1), the intracellular Ca2+ release channel, which is essential to excitation-contraction (EC) coupling. Our knowledge on how mutations in RyR1 disrupt intracellular Ca2+ homeostasis and EC coupling, eventually leading to MH and CCD has been recently improved, thanks to multidisciplinary studies ranging from clinical, single channel recordings, patch-clamp experiments, and molecular biology. This review presents a brief historical perspective, on how pioneer studies resulted in associating MH and CCD to RyR1. The review is also focused on discussing novel results in regard to pathophysiological consequences of specific MH/CCD RyR1 mutant proteins, which are representative of the different cellular mechanisms that are linked to either phenotype.

Dynamic alterations in myoplasmic Ca2+ in malignant hyperthermia and central core disease

Ca2+ ions play a pivotal role in a wide array of cellular processes ranging from fertilization to cell death. In skeletal muscle, a mechanical interaction between plasma membrane dihydropyridine receptors (DHPRs, L-type Ca2+ channels) and Ca2+ release channels (ryanodine receptors, RyR1s) of the sarcoplasmic reticulum orchestrates a complex, bi-directional Ca2+ signaling process that converts electrical impulses in the sarcolemma into myoplasmic Ca2+ transients during excitation-contraction coupling. Mutations in the genes that encode the two proteins that coordinate this electrochemical conversion process (the DHPR and RyR1) result in a variety of skeletal muscle disorders including malignant hyperthermia (MH), central core disease (CCD), multiminicore disease, nemaline rod myopathy, and hypokalemic periodic paralysis. Although RyR1 and DHPR disease mutations are thought to alter excitability and Ca2+ homeostasis in skeletal muscle, only recently has research begun to probe the molecular mechanisms by which these genetic defects lead to distinct clinical and histopathological manifestations. This review focuses on recent advances in determining the impact of MH and CCD mutations in RyR1 on muscle Ca2+ signaling and how these effects contribute to disease-specific aspects of these disorders.

Distinct effects on Ca2+ handling caused by malignant hyperthermia and central core disease mutations in RyR1

Malignant hyperthermia (MH) and central core disease (CCD) are disorders of skeletal muscle Ca2+ homeostasis that are linked to mutations in the type 1 ryanodine receptor (RyR1). Certain RyR1 mutations result in an MH-selective phenotype (MH-only), whereas others result in a mixed phenotype (MH + CCD). We characterized effects on Ca2+ handling and excitation-contraction (EC) coupling of MH-only and MH + CCD mutations in RyR1 after expression in skeletal myotubes derived from RyR1-null (dyspedic) mice. Compared to wild-type RyR1-expressing myotubes, MH + CCD- and MH-only-expressing myotubes exhibited voltage-gated Ca2+ release (VGCR) that activated at more negative potentials and displayed a significantly higher incidence of spontaneous Ca2+ oscillations. However, maximal VGCR was reduced only for MH + CCD mutants (Y4795C, R2435L, and R2163H) in which spontaneous Ca2+ oscillations occurred with significantly longer duration (Y4795C and R2435L) or higher frequency (R2163H). Notably, myotubes expressing these MH + CCD mutations in RyR1 exhibited both increased [Ca2+]i and reduced sarcoplasmic reticulum (SR) Ca2+ content. We conclude that MH-only mutations modestly increase basal release-channel activity in a manner insufficient to alter net SR Ca2+ content ("compensated leak"), whereas the mixed MH + CCD phenotype arises from mutations that enhance basal activity to a level sufficient to promote SR Ca2+ depletion, elevate [Ca2+]i, and reduce maximal VGCR ("decompensated leak").

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.

Functional defects in six ryanodine receptor isoform-1 (RyR1) mutations associated with malignant hyperthermia and their impact on skeletal excitation-contraction coupling

Malignant hyperthermia (MH) is a potentially fatal pharmacogenetic disorder of skeletal muscle that segregates with >60 mutations within the MHS-1 locus on chromosome 19 coding for ryanodine receptor type 1 (RyR1). Although some MHRyR1s have been shown to enhance sensitivity to caffeine and halothane when expressed in non-muscle cells, their influence on EC coupling can only be studied in skeletal myotubes. We therefore expressed WTRyR1, six of the most common human MHRyR1s (R163C, G341R, R614C, R2163C, V2168M, and R2458H), and a newly identified C-terminal mutation (T4826I) in dyspedic myotubes to study their functional defects and how they influence EC coupling. Myotubes expressing any MHRyR1 were significantly more sensitive to stimulation by caffeine and 4-CmC than those expressing WTRyR1. The hypersensitivity of MH myotubes extended to K+ depolarization. MH myotubes responded to direct channel activators with maximum Ca2+ amplitudes consistently smaller than WT myotubes, whereas the amplitude of their responses to depolarization were consistently larger than WT myotubes. The magnitudes of responses attainable from myotubes expressing MHRyR1s are therefore related to the nature of the stimulus rather than size of the Ca2+ store. The functional changes of MHRyR1s were directly analyzed using [3H]ryanodine binding analysis of isolated myotube membranes. Although none of the MHRyR1s examined significantly altered EC50 for Ca2+ activation, many failed to be completely inhibited by a low Ca2+ (<or=100 nm), and all were significantly more responsive to caffeine than WTRyR1 at Ca2+ concentrations that approximate those in resting myotubes. All seven mutations had diminished sensitivity to inhibition by Ca2+ and Mg2+. Using a homologous expression system, our study demonstrates for the first time that these 7 MH mutations are all both necessary and sufficient to induce MH-related phenotypes. Decreased sensitivity to Ca2+ and Mg2+ inhibition and inability of MHRyR1s to be fully inactivated at [Ca2+]i typical of normal myotubes at rest are key defects that contribute to the initiation of MH episodes.

Increased sensitivity to 4-chloro-m-cresol and caffeine in primary myotubes from malignant hyperthermia susceptible individuals carrying the ryanodine receptor 1 Thr2206Met (C6617T) mutation

Malignant hyperthermia (MH) is an autosomal-dominant disorder of skeletal muscle, triggered by volatile anaesthetics and depolarizing muscle relaxants. The causative defect lies in the control of Ca(2+) release from the sarcoplasmic reticulum in skeletal muscle. Numerous mutations have been detected in the ryanodine receptor 1 (RyR1) gene, but so far an MH-causative role has only been confirmed for 16 human RyR1 mutations. In this report we show that myotubes derived from individuals carrying the RyR1 Thr2206Met (C6617T) mutation have an abnormal response of the intracellular calcium concentration to 4-chloro-m-cresol and to caffeine. Satellite cells were obtained from muscle biopsies of patients referred for diagnosing MH. The intracellular calcium concentration in response to 4-chloro-m-cresol and to caffeine was investigated by fluorescence calcium imaging. In myotubes the half-maximal activation concentration (EC(50)) for 4-chloro-m-cresol was reduced from 203 micro m (wild type) to 98 micro m (Thr2206Met), and for caffeine from 3.8 mm to 1.8 mm. From the reduction of EC(50) we conclude that the RyR1 Thr2206Met mutation is pathogenic for MH.

Functional effects of central core disease mutations in the cytoplasmic region of the skeletal muscle ryanodine receptor

Central core disease (CCD) is a human myopathy that involves a dysregulation in muscle Ca(2)+ homeostasis caused by mutations in the gene encoding the skeletal muscle ryanodine receptor (RyR1), the protein that comprises the calcium release channel of the SR. Although genetic studies have clearly demonstrated linkage between mutations in RyR1 and CCD, the impact of these mutations on release channel function and excitation-contraction coupling in skeletal muscle is unknown. Toward this goal, we have engineered the different CCD mutations found in the NH(2)-terminal region of RyR1 into a rabbit RyR1 cDNA (R164C, I404M, Y523S, R2163H, and R2435H) and characterized the functional effects of these mutations after expression in myotubes derived from RyR1-knockout (dyspedic) mice. Resting Ca(2)+ levels were elevated in dyspedic myotubes expressing four of these mutants (Y523S > R2163H > R2435H R164C > I404M RyR1). A similar rank order was also found for the degree of SR Ca(2)+ depletion assessed using maximal concentrations of caffeine (10 mM) or cyclopiazonic acid (CPA, 30 microM). Although all of the CCD mutants fully restored L-current density, voltage-gated SR Ca(2)+ release was smaller and activated at more negative potentials for myotubes expressing the NH(2)-terminal CCD mutations. The shift in the voltage dependence of SR Ca(2)+ release correlated strongly with changes in resting Ca(2)+, SR Ca(2)+ store depletion, and peak voltage-gated release, indicating that increased release channel activity at negative membrane potentials promotes SR Ca(2)+ leak. Coexpression of wild-type and Y523S RyR1 proteins in dyspedic myotubes resulted in release channels that exhibited an intermediate degree of SR Ca(2)+ leak. These results demonstrate that the NH(2)-terminal CCD mutants enhance release channel sensitivity to activation by voltage in a manner that leads to increased SR Ca(2)+ leak, store depletion, and a reduction in voltage-gated Ca(2)+ release. Two fundamentally distinct cellular mechanisms (leaky channels and EC uncoupling) are proposed to explain how altered release channel function caused by different mutations in RyR1 could result in muscle weakness in CCD.

Malignant hyperthermia and excitation-contraction coupling

Malignant hyperthermia (MH) is a state of elevated skeletal muscle metabolism that may occur during general anaesthesia in genetically pre-disposed individuals. Malignant hyperthermia results from altered control of sarcoplasmic reticulum (SR) Ca2+ release. Mutations have been identified in MH-susceptible (MHS) individuals in two key proteins of excitation-contraction (EC) coupling, the Ca2+ release channel of the SR, ryanodine receptor type 1 (RyR1) and the alpha1-subunit of the dihydropyridine receptor (DHPR, L-type Ca2+ channel). During EC coupling, the DHPR senses the plasma membrane depolarization and transmits the information to the ryanodine receptor (RyR). As a consequence, Ca2+ is released from the terminal cisternae of the SR. One of the human MH-mutations of RyR1 (Arg614Cys) is also found at the homologous location in the RyR of swine (Arg615Cys). This animal model permits the investigation of physiological consequences of the homozygously expressed mutant release channel. Of particular interest is the question of whether voltage-controlled release of Ca2+ is altered by MH-mutations in the absence of MH-triggering substances. This question has recently been addressed in this laboratory by studying Ca2+ release under voltage clamp conditions in both isolated human skeletal muscle fibres and porcine myotubes.

Malignant hyperthermia mutation Arg615Cys in the porcine ryanodine receptor alters voltage dependence of Ca2+ release

Ca2+ inward current and fura-2 Ca2+ transients were simultaneously recorded in porcine myotubes. Myotubes from normal pigs and cells from specimens homozygous for the Arg615Cys (malignant hyperthermia) mutation of the skeletal muscle ryanodine receptor RyR1 were investigated. We addressed the question whether this mutation alters the voltage dependence of Ca2+ release from the sarcoplasmic reticulum. The time course of the total flux of Ca2+ into the myoplasm was estimated. Analysis showed that the largest input Ca2+ flux occurred immediately after depolarization. Amplitude and time course of the Ca2+ flux at large depolarizations were not significantly different in the Arg615Cys myotubes. Ca2+ release from the sarcoplasmic reticulum was activated at more negative potentials than the L-type Ca2+ conductance. In the controls, the potentials for half-maximal activation V 1/2 were -9.0mV and 16.5 mV, respectively. In myotubes expressing the Arg615Cys mutation, Ca2+ release was activated at significantly lower depolarizing potentials (V = -23.5 mV) than in control myotubes. In contrast, V of conductance activation (13.5 mV) was not significantly different from controls. The specific shift in the voltage dependence of Ca2+ release caused by this mutation can be well described by altering a voltage-independent reaction of the ryanodine receptor that is coupled to the voltage-dependent transitions of the L-type Ca2+ channel.

Calcium ion in skeletal muscle: its crucial role for muscle function, plasticity, and disease

Mammalian skeletal muscle shows an enormous variability in its functional features such as rate of force production, resistance to fatigue, and energy metabolism, with a wide spectrum from slow aerobic to fast anaerobic physiology. In addition, skeletal muscle exhibits high plasticity that is based on the potential of the muscle fibers to undergo changes of their cytoarchitecture and composition of specific muscle protein isoforms. Adaptive changes of the muscle fibers occur in response to a variety of stimuli such as, e.g., growth and differentition factors, hormones, nerve signals, or exercise. Additionally, the muscle fibers are arranged in compartments that often function as largely independent muscular subunits. All muscle fibers use Ca(2+) as their main regulatory and signaling molecule. Therefore, contractile properties of muscle fibers are dependent on the variable expression of proteins involved in Ca(2+) signaling and handling. Molecular diversity of the main proteins in the Ca(2+) signaling apparatus (the calcium cycle) largely determines the contraction and relaxation properties of a muscle fiber. The Ca(2+) signaling apparatus includes 1) the ryanodine receptor that is the sarcoplasmic reticulum Ca(2+) release channel, 2) the troponin protein complex that mediates the Ca(2+) effect to the myofibrillar structures leading to contraction, 3) the Ca(2+) pump responsible for Ca(2+) reuptake into the sarcoplasmic reticulum, and 4) calsequestrin, the Ca(2+) storage protein in the sarcoplasmic reticulum. In addition, a multitude of Ca(2+)-binding proteins is present in muscle tissue including parvalbumin, calmodulin, S100 proteins, annexins, sorcin, myosin light chains, beta-actinin, calcineurin, and calpain. These Ca(2+)-binding proteins may either exert an important role in Ca(2+)-triggered muscle contraction under certain conditions or modulate other muscle activities such as protein metabolism, differentiation, and growth. Recently, several Ca(2+) signaling and handling molecules have been shown to be altered in muscle diseases. Functional alterations of Ca(2+) handling seem to be responsible for the pathophysiological conditions seen in dystrophinopathies, Brody's disease, and malignant hyperthermia. These also underline the importance of the affected molecules for correct muscle performance.

Effect of beta-adrenoceptor activation on [Ca2+]i regulation in murine skeletal myotubes

The present study used real-time confocal microscopy to examine the effects of the beta2-adrenoceptor agonist salbutamol on regulation of intracellular Ca2+ concentration ([Ca2+]i) in myotubes derived from neonatal mouse limb muscles. Immunocytochemical staining for ryanodine receptors and skeletal muscle myosin confirmed the presence of sarcomeres. The myotubes displayed both spontaneous and ACh-induced rapid (<2-ms rise time) [Ca2+]i transients. The [Ca2+]i transients were frequency modulated by both low and high concentrations of salbutamol. Exposure to alpha-bungarotoxin and tetrodotoxin inhibited ACh-induced [Ca2+]i transients and the response to low concentrations of salbutamol but not the response to higher concentrations. Preexposure to caffeine inhibited the subsequent [Ca2+]i response to lower concentrations of salbutamol and significantly blunted the response to higher concentrations. Preexposure to salbutamol diminished the [Ca2+]i response to caffeine. Inhibition of dihydropyridine-sensitive Ca2+ channels with nifedipine or PN-200-110 did not prevent [Ca2+]i elevations induced by higher concentrations of salbutamol. The effects of salbutamol were mimicked by the membrane-permeant analog dibutyryl adenosine 3', 5'-cyclic monophosphate. These data indicate that salbutamol effects in skeletal muscle predominantly involve enhanced sarcoplasmic reticulum Ca2+ release.

Voltage-dependent calcium release in human malignant hyperthermia muscle fibers

Malignant hyperthermia (MH) results from a defect of calcium release control in skeletal muscle that is often caused by point mutations in the ryanodine receptor gene (RYR1). In malignant hyperthermia-susceptible (MHS) muscle, calcium release responds more sensitively to drugs such as halothane and caffeine. In addition, experiments on the porcine homolog of malignant hyperthermia (mutation Arg615Cys in RYR1) indicated a higher sensitivity to membrane depolarization. Here, we investigated depolarization-dependent calcium release under voltage clamp conditions in human MHS muscle. Segments of muscle fibers dissected from biopsies of the vastus lateralis muscle of MHN (malignant hyperthermia negative) and MHS subjects were voltage-clamped in a double vaseline gap system. Free calcium was determined with the fluorescent indicator fura-2 and converted to an estimate of the rate of SR calcium release. Both MHN and MHS fibers showed an initial peak of the release rate, a subsequent decline, and rapid turn-off after repolarization. Neither the kinetics nor the voltage dependence of calcium release showed significant deviations from controls, but the average maximal peak rate of release was about threefold larger in MHS fibers.

Malignant hyperthermia: effects of sarcoplasmic reticulum Ca2+ pump inhibition

In porcine malignant hyperthermia-susceptible (MHS) skeletal muscles, calcium release is abnormal and resting calcium may be elevated. Thus MHS muscles may have prolonged twitch relaxation and lower fusion frequencies, which would be augmented by inhibition of sarcoplasmic reticulum (SR) Ca2+ adenosine triphosphatase (ATPase) activity; bundles of intact muscle cells from MHS and normal pigs were used to investigate this possibility. Cooling and low-frequency stimulation, in combination, enhanced twitch fusion and prolonged twitch relaxation significantly more in MHS than in normal muscles (e.g., 34 +/- 4% versus 16 +/- 4% fusion, and 82.4 +/- 9.4 ms versus 43.2 +/- 7.8 ms half-relaxation time, for MHS and normal muscles, respectively). Similarly, inhibition of the SR Ca2+ ATPase by cyclopiazonic acid resulted in significantly greater twitch fusion in MHS muscles. These results were consistent with predicted effects of enhanced SR Ca2+ release and/or elevated resting calcium in MHS muscles and indicate that cooling during a malignant hyperthermia crisis could actually increase the force of muscle contractures.

Myoglobinuria, malignant hyperthermia, neuroleptic malignant syndrome and serotonin syndrome

This article presents an overview of the causes and manifestations of myoglobinuria and provides criteria for its diagnosis and management. The article also reviews neuroleptic malignant syndrome, malignant hyperthermia, and serotonin syndrome, all of which could cause rhabdomyolysis and myoglobinuria.

Slow calcium current is not reduced in malignant hyperthermic porcine myotubes

Malignant hyperthermia-susceptible (MHS) pigs express a sarcoplasmic reticulum (SR) Ca(2)+-release channel mutation that results in lower than normal contractile thresholds in skeletal muscles. In adult MHS pig muscles the L-type calcium current (ls) is also reduced. We tested the hypothesis that there is a causal relationship between ls and the lower contractile threshold by recording ls from MHS and normal porcine myotubes using the whole cell patch-clamp technique. Current voltage relationships for both MHS and normal myotubes were similar, with peak ls between +20 and +30 mV. Maximum ls amplitudes were not different from (normal: 4976 +/- 566 pA; MHS:6516 +/- 1088 pA) nor was ls specific density (normal: 9.0 +/- 0.8; MHS: 8.8 +/- 1.1 pA/pF). In both MHS and normal myotubes, both the dihydropyridine antagonist PN200-110 (200 nmol/L) and holding the membrane potential at -10mV for 5 min decreased ls significantly (by more than 50%). There was no apparent direct relationship between the mutation in the SR Ca(2)+ -release channel mutation on muscle development.