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

Distinct pathophysiological characteristics in developing muscle from patients susceptible to malignant hyperthermia

BACKGROUND

Most patients with malignant hyperthermia susceptibility diagnosed by the in vitro caffeine-halothane contracture test (CHCT) develop excessive force in response to halothane but not caffeine (halothane-hypersensitive). Hallmarks of halothane-hypersensitive patients include high incidence of musculoskeletal symptoms at rest and abnormal calcium events in muscle. By measuring sensitivity to halothane of myotubes and extending clinical observations and cell-level studies to a large group of patients, we reach new insights into the pathological mechanism of malignant hyperthermia susceptibility.

METHODS

Patients with malignant hyperthermia susceptibility were classified into subgroups HH and HS (positive to halothane only and positive to both caffeine and halothane). The effects on [Ca2+]cyto of halothane concentrations between 0.5 and 3 % were measured in myotubes and compared with CHCT responses of muscle. A clinical index that summarises patient symptoms was determined for 67 patients, together with a calcium index summarising resting [Ca2+]cyto and spontaneous and electrically evoked Ca2+ events in their primary myotubes.

RESULTS

Halothane-hypersensitive myotubes showed a higher response to halothane 0.5% than the caffeine-halothane hypersensitive myotubes (P<0.001), but a lower response to higher concentrations, comparable with that used in the CHCT (P=0.055). The HH group had a higher calcium index (P<0.001), but their clinical index was not significantly elevated vs the HS. Principal component analysis identified electrically evoked Ca2+ spikes and resting [Ca2+]cyto as the strongest variables for separation of subgroups.

CONCLUSIONS

Enhanced sensitivity to depolarisation and to halothane appear to be the primary, mutually reinforcing and phenotype-defining defects of halothane-hypersensitive patients with malignant hyperthermia susceptibility.

Molecular Aspects Implicated in Dantrolene Selectivity with Respect to Ryanodine Receptor Isoforms

Dantrolene is an intra-cellularly acting skeletal muscle relaxant used for the treatment of the rare genetic disorder, malignant hyperthermia (MH). In most cases, MH susceptibility is caused by dysfunction of the skeletal ryanodine receptor (RyR1) harboring one of nearly 230 single-point MH mutations. The therapeutic effect of dantrolene is the result of a direct inhibitory action on the RyR1 channel, thus suppressing aberrant Ca2+ release from the sarcoplasmic reticulum. Despite the almost identical dantrolene-binding sequence exits in all three mammalian RyR isoforms, dantrolene appears to be an isoform-selective inhibitor. Whereas RyR1 and RyR3 channels are competent to bind dantrolene, the RyR2 channel, predominantly expressed in the heart, is unresponsive. However, a large body of evidence suggests that the RyR2 channel becomes sensitive to dantrolene-mediated inhibition under certain pathological conditions. Although a consistent picture of the dantrolene effect emerges from in vivo studies, in vitro results are often contradictory. Hence, our goal in this perspective is to provide the best possible clues to the molecular mechanism of dantrolene’s action on RyR isoforms by identifying and discussing potential sources of conflicting results, mainly coming from cell-free experiments. Moreover, we propose that, specifically in the case of the RyR2 channel, its phosphorylation could be implicated in acquiring the channel responsiveness to dantrolene inhibition, interpreting functional findings in the structural context.

Ultrasound Elastography for Rapid, Real-time Detection of Localized Muscular Reaction in Malignant Hyperthermia-susceptible Pigs

WHAT WE ALREADY KNOW ABOUT THIS TOPIC

WHAT THIS ARTICLE TELLS US THAT IS NEW: BACKGROUND:: Halothane and caffeine induce excessive sarcoplasmic calcium liberation and skeletal muscle contracture in patients susceptible to malignant hyperthermia (MH) and are utilized for diagnosis in the in vitro contracture test. Intramuscular injection previously caused a marked local lactate increase in MH-susceptible but not in MH-nonsusceptible individuals in vivo. Using shear-wave elastography, this study evaluated localized changes in muscle stiffness after intramuscular injection of halothane and caffeine.

METHODS

Microdialysis probes were placed into the gracilis muscle of 16 pigs (9 MH-susceptible and 7 MH-nonsusceptible). After local injection of either halothane or caffeine in different concentrations, changes of tissue elasticity surrounding the probe were examined by quantitative shear-wave elastography. Local lactate concentrations were analyzed spectrophotometrically.

RESULTS

Ultrasound elastography detected a temporary increase in local muscle rigidity in MH-susceptible but not in MH-nonsusceptible pigs after 2.5 and 5 vol% halothane and after 10, 40, and 80 mM caffeine, whereas there were no differences in the control groups (median [interquartile range] for maximum effect after 5 vol% halothane: MH-susceptible: 97 [31 to 148] vs. MH-nonsusceptible: 5 [-6 to 18] kPa; P = 0.0006; maximum effect after 80 mM caffeine: 112 [64 to 174] vs. -3 [-6 to 35] kPa; P = 0.0002). These effects were seen rapidly within 5 min. Local lactate concentrations were higher in MH-susceptible versus nonsusceptible pigs after 1 and 2.5 vol% halothane and 10, 40, and 80 mM caffeine (2.5 vol% halothane: MH-susceptible: 2.8 [1.9 to 4.4] vs. MH-nonsusceptible: 0.6 [0.6 to 0.7] mmol/l; P < 0.0001; 80 mM caffeine: 5.2 [4.1 to 6.3] vs. 1.6 [1.2 to 2.4] mmol/l; P < 0.0001). After 10 vol% halothane, rigidity and lactate levels were increased in both MH-susceptible and MH-nonsusceptible animals.

CONCLUSIONS

This pilot study revealed shear-wave elastography as a suitable technique for real-time detection of altered tissue elasticity in response to pharmacologic stimulation. By considering the variability of these results, further test protocol optimization is required before elastography could serve as a minimally invasive MH diagnostic test.

Dantrolene Requires Mg2+ and ATP To Inhibit the Ryanodine Receptor

Dantrolene is a ryanodine receptor (RyR) inhibitor, which is used to relax muscles in malignant hyperthermia syndrome. Although dantrolene binds to the RyR protein, its mechanism of action is unknown, mainly because of the controversial results showing that dantrolene inhibited Ca²⁺ release from intact fibers and sarcoplasmic reticulum (SR) vesicles, but failed to inhibit single RyR channel currents in bilayers. Accordingly, it was concluded that an important factor for dantrolene's action was lost during the purification procedure of RyR. Recently, Mg²⁺ was demonstrated to be the essential factor for dantrolene to inhibit Ca²⁺ release in skinned muscle fibers. The aim of the present study was to confirm these results in Ca²⁺ release and bilayer experiments, using SR vesicles and solubilized channels, respectively. Our Ca²⁺ release experiments demonstrated that the effect of dantrolene and Mg²⁺ was cooperative and that ATP enhanced the inhibiting effect of dantrolene. Namely, 10 µM dantrolene reduced RyR channel open probability by ∼50% in the presence of 3 mM free Mg²⁺ and 1 mM ATP, whereas channel activity further decreased to ∼20% of control when [ATP] was increased to 2 mM. Our data provide important complementary information that supports the direct, Mg²⁺-dependent mechanism of dantrolene's action and suggests that dantrolene also requires ATP to inhibit RyR.

Organelle membrane derived patches: reshaping classical methods for new targets

Intracellular ion channels are involved in multiple signaling processes, including such crucial ones as regulation of cellular motility and fate. With 95% of the cellular membrane belonging to intracellular organelles, it is hard to overestimate the importance of intracellular ion channels. Multiple studies have been performed on these channels over the years, however, a unified approach allowing not only to characterize their activity but also to study their regulation by partner proteins, analogous to the patch clamp “golden standard”, is lacking. Here, we present a universal approach that combines the extraction of intracellular membrane fractions with the preparation of patchable substrates that allows to characterize these channels in endogenous protein environment and to study their regulation by partner proteins. We validate this method by characterizing activity of multiple intracellular ion channels localized to different organelles and by providing detailed electrophysiological characterization of the regulation of IP₃R activity by endogenous Bcl-2. Thus, after synthesis and reshaping of the well-established approaches, organelle membrane derived patch clamp provides the means to assess ion channels from arbitrary cellular membranes at the single channel level.

The emerging role of calmodulin regulation of RyR2 in controlling heart rhythm, the progression of heart failure and the antiarrhythmic action of dantrolene

Cardiac output and rhythm depend on the release and the take-up of calcium from the sarcoplasmic reticulum (SR). Excessive diastolic calcium leak from the SR due to dysfunctional calcium release channels (RyR2) contributes to the formation of delayed after-depolarizations, which underlie the fatal arrhythmias that occur in heart failure and inherited syndromes. Calmodulin (CaM) is a calcium-binding protein that regulates target proteins and acts as a calcium sensor. CaM is comprised of two calcium-binding EF-hand domains and a flexible linker. CaM is an accessory protein that partially inhibits RyR2 channel activity. CaM is critical for normal cardiac function, and altered CaM binding and efficacy may contribute to defects in SR calcium release. The present paper reviews CaM binding to RyR2 and how it regulates RyR2 channel activity. It then goes on to review how mutations in the CaM amino acid sequence give rise to inherited syndromes such as Catecholaminergic Polymorphic Ventricular Tachychardia (CPVT) and long QT syndrome (LQTS). In addition, the role of reduced CaM binding to RyR2 that results from RyR2 phosphorylation or from oxidation of either RyR2 or CaM contributes to the progression of heart failure is reviewed. Finally, this manuscript reviews recent evidence that CaM binding to RyR2 is required for the inhibitory action of a pharmaceutical agent (dantrolene) on RyR2. Dantrolene is a clinically used muscle relaxant that has recently been found to exert antiarrhythmic effects against SR Ca²⁺ overload arrhythmias.

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

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

Dantrolene requires Mg2+ to arrest malignant hyperthermia

Malignant hyperthermia (MH) is a clinical syndrome of skeletal muscle that presents as a hypermetabolic response to volatile anesthetic gases, where susceptible persons may develop lethally high body temperatures. Genetic predisposition mainly arises from mutations on the skeletal muscle ryanodine receptor (RyR). Dantrolene is administered to alleviate MH symptoms, but its mechanism of action and its influence on the Ca²⁺ transients elicited by MH triggers are unknown. Here, we show that Ca²⁺ release in the absence of Mg²⁺ is unaffected by the presence of dantrolene but that dantrolene becomes increasingly effective as cytoplasmic-free [Mg²⁺] (free [Mg²⁺]_cyto) passes mM levels. Furthermore, we found in human muscle susceptible to MH that dantrolene was ineffective at reducing halothane-induced repetitive Ca²⁺ waves in the presence of resting levels of free [Mg²⁺]_cyto (1 mM). However, an increase of free [Mg²⁺]_cyto to 1.5 mM could increase the period between Ca²⁺ waves. These results reconcile previous contradictory reports in muscle fibers and isolated RyRs, where Mg²⁺ is present or absent, respectively, and define the mechanism of action of dantrolene is to increase the Mg²⁺ affinity of the RyR (or "stabilize" the resting state of the channel) and suggest that the accumulation of the metabolite Mg²⁺ from MgATP hydrolysis is required to make dantrolene administration effective in arresting an MH episode.

Cardiac Calcium Release Channel (Ryanodine Receptor 2) Regulation by Halogenated Anesthetics

Background

Halogenated anesthetics activate cardiac ryanodine receptor 2–mediated sarcoplasmic reticulum Ca2+ release, leading to sarcoplasmic reticulum Ca2+ depletion, reduced cardiac function, and providing cell protection against ischemia-reperfusion injury. Anesthetic activation of ryanodine receptor 2 is poorly defined, leaving aspects of the protective mechanism uncertain.

Methods

Ryanodine receptor 2 from the sheep heart was incorporated into artificial lipid bilayers, and their gating properties were measured in response to five halogenated anesthetics.

Results

Each anesthetic rapidly and reversibly activated ryanodine receptor 2, but only from the cytoplasmic side. Relative activation levels were as follows: halothane (approximately 4-fold; n = 8), desflurane and enflurane (approximately 3-fold,n = 9), and isoflurane and sevoflurane (approximately 1.5-fold, n = 7, 10). Half-activating concentrations (Ka) were in the range 1.3 to 2.1 mM (1.4 to 2.6 minimum alveolar concentration [MAC]) with the exception of isoflurane (5.3 mM, 6.6 minimum alveolar concentration). Dantrolene (10 μM with 100 nM calmodulin) inhibited ryanodine receptor 2 by 40% but did not alter the Ka for halothane activation. Halothane potentiated luminal and cytoplasmic Ca2+ activation of ryanodine receptor 2 but had no effect on Mg2+ inhibition. Halothane activated ryanodine receptor 2 in the absence and presence (2 mM) of adenosine triphosphate (ATP). Adenosine, a competitive antagonist to ATP activation of ryanodine receptor 2, did not antagonize halothane activation in the absence of ATP.

Conclusions

At clinical concentrations (1 MAC), halothane desflurane and enflurane activated ryanodine receptor 2, whereas isoflurane and sevoflurane were ineffective. Dantrolene inhibition of ryanodine receptor 2 substantially negated the activating effects of anesthetics. Halothane acted independently of the adenine nucleotide–binding site on ryanodine receptor 2. The previously observed adenosine antagonism of halothane activation of sarcoplasmic reticulum Ca2+ release was due to competition between adenosine and ATP, rather than between halothane and ATP.

The couplonopathies: A comparative approach to a class of diseases of skeletal and cardiac muscle

A novel category of diseases of striated muscle is proposed, the couplonopathies, as those that affect components of the couplon and thereby alter its operation. Couplons are the functional units of intracellular calcium release in excitation–contraction coupling. They comprise dihydropyridine receptors, ryanodine receptors (Ca²⁺ release channels), and a growing list of ancillary proteins whose alteration may lead to disease. Within a generally similar plan, the couplons of skeletal and cardiac muscle show, in a few places, marked structural divergence associated with critical differences in the mechanisms whereby they fulfill their signaling role. Most important among these are the presence of a mechanical or allosteric communication between voltage sensors and Ca²⁺ release channels, exclusive to the skeletal couplon, and the smaller capacity of the Ca stores in cardiac muscle, which results in greater swings of store concentration during physiological function. Consideration of these structural and functional differences affords insights into the pathogenesis of several couplonopathies. The exclusive mechanical connection of the skeletal couplon explains differences in pathogenesis between malignant hyperthermia (MH) and catecholaminergic polymorphic ventricular tachycardia (CPVT), conditions most commonly caused by mutations in homologous regions of the skeletal and cardiac Ca²⁺ release channels. Based on mechanistic considerations applicable to both couplons, we identify the plasmalemma as a site of secondary modifications, typically an increase in store-operated calcium entry, that are relevant in MH pathogenesis. Similar considerations help explain the different consequences that mutations in triadin and calsequestrin have in these two tissues. As more information is gathered on the composition of cardiac and skeletal couplons, this comparative and mechanistic approach to couplonopathies should be useful to understand pathogenesis, clarify diagnosis, and propose tissue-specific drug development.

Oxidation of ryanodine receptor (RyR) and calmodulin enhance Ca release and pathologically alter, RyR structure and calmodulin affinity

Oxidative stress may contribute to cardiac ryanodine receptor (RyR2) dysfunction in heart failure (HF) and arrhythmias. Altered RyR2 domain-domain interaction (domain unzipping) and calmodulin (CaM) binding affinity are allosterically coupled indices of RyR2 conformation. In HF RyR2 exhibits reduced CaM binding, increased domain unzipping and greater SR Ca leak, and dantrolene can reverse these changes. However, effects of oxidative stress on RyR2 conformation and leak in myocytes are poorly understood. We used fluorescent CaM, FKBP12.6, and domain-peptide biosensor (F-DPc10) to measure, directly in cardiac myocytes, (1) RyR2 activation by hydrogen peroxide (H2O2)-induced oxidation, (2) RyR2 conformation change caused by oxidation, (3) CaM-RyR2 and FK506-binding protein (FKBP12.6)-RyR2 interaction upon oxidation, and (4) whether dantrolene affects 1-3. H2O2 was used to mimic oxidative stress. H2O2 significantly increased the frequency of Ca(2+) sparks and spontaneous Ca(2+) waves, and dantrolene almost completely blocked these effects. H2O2 pretreatment significantly reduced CaM-RyR2 binding, but had no effect on FKBP12.6-RyR2 binding. Dantrolene restored CaM-RyR2 binding but had no effect on intracellular and RyR2 oxidation levels. H2O2 also accelerated F-DPc10-RyR2 association while dantrolene slowed it. Thus, H2O2 causes conformational changes (sensed by CaM and DPc10 binding) associated with Ca leak, and dantrolene reverses these RyR2 effects. In conclusion, in cardiomyocytes, H2O2 treatment markedly reduces the CaM-RyR2 affinity, has no effect on FKBP12.6-RyR2 affinity, and causes domain unzipping. Dantrolene can correct domain unzipping, restore CaM-RyR2 affinity, and quiet pathological RyR2 channel gating. F-DPc10 and CaM are useful biosensors of a pathophysiological RyR2 state.

Essential Role of Calmodulin in RyR Inhibition by Dantrolene

Dantrolene is the first line therapy of malignant hyperthermia. Animal studies suggest that dantrolene also protects against heart failure and arrhythmias caused by spontaneous Ca(2+) release. Although dantrolene inhibits Ca(2+) release from the sarcoplasmic reticulum of skeletal and cardiac muscle preparations, its mechanism of action has remained controversial, because dantrolene does not inhibit single ryanodine receptor (RyR) Ca(2+) release channels in lipid bilayers. Here we test the hypothesis that calmodulin (CaM), a physiologic RyR binding partner that is lost during incorporation into lipid bilayers, is required for dantrolene inhibition of RyR channels. In single channel recordings (100 nM cytoplasmic [Ca(2+)] + 2 mM ATP), dantrolene caused inhibition of RyR1 (rabbit skeletal muscle) and RyR2 (sheep) with a maximal inhibition of Po (Emax) to 52 ± 4% of control only after adding physiologic [CaM] = 100 nM. Dantrolene inhibited RyR2 with an IC50 of 0.16 ± 0.03 µM. Mutant N98S-CaM facilitated dantrolene inhibition with an IC50 = 5.9 ± 0.3 nM. In mouse cardiomyocytes, dantrolene had no effect on cardiac Ca(2+) release in the absence of CaM, but reduced Ca(2+) wave frequency (IC50 = 0.42 ± 0.18 µM, Emax = 47 ± 4%) and amplitude (IC50 = 0.19 ± 0.04 µM, Emax = 66 ± 4%) in the presence of 100 nM CaM. We conclude that CaM is essential for dantrolene inhibition of RyR1 and RyR2. Its absence explains why dantrolene inhibition of single RyR channels has not been previously observed.

The effect of succinylcholine on malignant hyperthermia events in susceptible swine

BACKGROUND

While the impact of volatile anaesthetics to induce malignant hyperthermia (MH) is abundantly clear, the role of succinylcholine still remains controversial. To evaluate the influence of succinylcholine on porcine MH events, the authors investigated the hemodynamic and metabolic responses in MH susceptible (MHS) and non-susceptible (MHN) swine following either succinylcholine or halothane application alone or a combination of both substances.

METHODS

With approval of the local animal care committee 27 MHS and 30 MHN pigs were anaesthetized and mechanically ventilated. Fiberoptic probes for continuous PCO2 measurement were inserted into the femoral vein and the triceps muscle. Group A received succinylcholine 4 mg/kg, group B incremental doses of halothane (0.5, 1.0 vol%) and group C succinylcholine and halothane simultaneously. Vital signs were recorded continuously.

RESULTS

Prior to drug application measured values did not differ between MHS and MHN. While MHN pigs did not show relevant alterations, succinylcholine, halothane and the combination of both lead to significant hemodynamic and metabolic changes in MHS swine.

CONCLUSIONS

Hemodynamic and metabolic alterations following succinylcholine were similar to halothane in MHS pigs. The combination of both pharmacological agents potentiated the observed effects. According to these results succinylcholine acted as an independent and supportive factor during onset of an MH episode.

Drosophila ryanodine receptors mediate general anesthesia by halothane

BACKGROUND

Although in vitro studies have identified numerous possible targets, the molecules that mediate the in vivo effects of volatile anesthetics remain largely unknown. The mammalian ryanodine receptor (Ryr) is a known halothane target, and the authors hypothesized that it has a central role in anesthesia.

METHODS

Gene function of the Drosophila Ryr (dRyr) was manipulated in the whole body or in specific tissues using a collection of mutants and transgenes, and responses to halothane were measured with a reactive climbing assay. Cellular responses to halothane were studied using Ca imaging and patch clamp electrophysiology.

RESULTS

Halothane potency strongly correlates with dRyr gene copy number, and missense mutations in regions known to be functionally important in the mammalian Ryrs gene cause dominant hypersensitivity. Tissue-specific manipulation of dRyr shows that expression in neurons and glia, but not muscle, mediates halothane sensitivity. In cultured cells, halothane-induced Ca efflux is strictly dRyr-dependent, suggesting a close interaction between halothane and dRyr. Ca imaging and electrophysiology of Drosophila central neurons reveal halothane-induced Ca flux that is altered in dRyr mutants and correlates with strong hyperpolarization.

CONCLUSIONS

In Drosophila, neurally expressed dRyr mediates a substantial proportion of the anesthetic effects of halothane in vivo, is potently activated by halothane in vitro, and activates an inhibitory conductance. The authors' results provide support for Ryr as an important mediator of immobilization by volatile anesthetics.

Magnesium in health and disease

Mammalian cells tightly regulate cellular Mg(2+) content through a variety of transport and buffering mechanisms under the control of various hormones and cellular second messengers. The effect of these hormones and agents results in dynamic changes in the total content of Mg(2+) being transported across the cell membrane and redistributed within cellular compartments. The importance of maintaining proper cellular Mg(2+) content optimal for the activity of various cellular enzymes and metabolic cycles is underscored by the evidence that several diseases are characterized by a loss of Mg(2+) within specific tissues as a result of defective transport, hormonal stimulation, or metabolic impairment. This chapter will review the key mechanisms regulating cellular Mg(2+) homeostasis and their impairments under the most common diseases associated with Mg(2+) loss or deficiency.

Effects of physical training with different intensities of effort on lipid metabolism in rats submitted to the neonatal application of alloxan

BACKGROUND

Type 2 diabetes mellitus (T2DM) is a chronic disease that is characterized by insulin resistance. Its development is directly connected with the inability of insulin to exert its action, not just on carbohydrate metabolism but also on primarily on lipid metabolism. The present study aimed to compare the effects of continuous, intermittent, and strength training on serum and tissue variables on the lipid metabolism of alloxan rats.

METHODS

Wistar rats were divided into eight groups: sedentary alloxan (SA), sedentary control (SC), continuous training alloxan (CA), intermittent training alloxan (IA), strength training alloxan (StA), continuous training control (CC), intermittent training control (IC) and strength training control (StC). Alloxan (250 mg/kg bw) was injected into neonatal rats at 6 days of age. The continuous training protocol consisted of 12 weeks of swimming training for 1 uninterrupted hour/day, five days/week, supporting a load that was 5% bw. The intermittent training protocol consisted of 12 weeks of swimming training with 30 s of activity interrupted by 30 s of rest, for a total of 20 min/day, five days/week, supporting a load that was 15% bw. The strength-training protocol consisted of 12 weeks of training, five days/week with 4 sets of 10 jumps in water with 1 min rest between sets, supporting a load that was a 50% bw.

RESULTS

At 28 days, the alloxan animals exhibited higher insulin resistance as measured by the disappearance of glucose serum (% Kitt/min) during the ITT. At 120 days, the sedentary alloxan animals showed higher FFA values than continuous and intermittent training alloxan. In addition, the alloxan animals that underwent intermittent and strength training showed lower FFA values compared to the corresponding controls. The continuous training protocol was less effective than the strength training protocol for reducing the levels of total cholesterol in the alloxan animals. Serum total lipid values revealed that intermittent training increased serum levels in alloxan animals

CONCLUSION

Thus, it was concluded that physical training at different intensities of effort is of great importance in attenuation and control of changes in the lipid metabolism in alloxan animals.

Coupled gating of skeletal muscle ryanodine receptors is modulated by Ca2+, Mg2+, and ATP

Coupled gating (synchronous openings and closures) of groups of skeletal muscle ryanodine receptors (RyR1), which mimics RyR1-mediated Ca(2+) release underlying Ca(2+) sparks, was first described by Marx et al. (Marx SO, Ondrias K, Marks AR. Science 281: 818-821, 1998). The nature of the RyR1-RyR1 interactions for coupled gating still needs to be characterized. Consequently, we defined planar lipid bilayer conditions where ∼25% of multichannel reconstitutions contain mixtures of coupled and independently gating RyR1. In ∼10% of the cases, all RyRs (2-10 channels; most frequently 3-4) gated in coupled fashion, allowing for quantification. Our results indicated that coupling required cytosolic solutions containing ATP/Mg(2+) and high (50 mM) luminal Ca(2+) (Ca(lum)) or Sr(2+) solutions. Bursts of coupled activity (events) started and ended abruptly, with all channels activating/deactivating within ∼300 μs. Coupled RyR1 were heterogeneous, where highly active RyR1 ("drivers") seemed open during the entire coupled event (P(o) = 1), while other RyR1s ("followers") displayed abundant flickering and smaller amplitude. Drivers mean open time increased with cytosolic Ca(2+) (Ca(cyt)) or caffeine, whereas followers flicker frequency was Ca(cyt) independent and more sensitive to inhibition by cytosolic Mg(2+). Coupled events were insensitive to varying lumen-to-cytosol Ca(2+) fluxes from ∼1 to 8 pA, which does not corroborate coupling of neighboring RyR1 by local Ca(2+)-induced Ca(2+) release. However, coupling requires specific Ca(lum) sites, as it was lost when Ca(lum) was replaced by luminal Ba(2+) or Mg(2+). In summary, coupled events reveal complex interactions among heterogeneous RyR1, differentially modulated by cytosolic ATP/Mg(2+), Ca(cyt), and Ca(lum,) which under cell-like ionic conditions may parallel synchronous RyR1 gating during Ca(2+) sparks.

Dantrolene prevents arrhythmogenic Ca2+ release in heart failure

In heart failure (HF), arrhythmogenic Ca(2+) release and chronic Ca(2+) depletion of the sarcoplasmic reticulum (SR) arise due to altered function of the ryanodine receptor (RyR) SR Ca(2+)-release channel. Dantrolene, a therapeutic agent used to treat malignant hyperthermia associated with mutations of the skeletal muscle type 1 RyR (RyR1), has recently been suggested to have effects on the cardiac type 2 RyR (RyR2). In this investigation, we tested the hypothesis that dantrolene exerts antiarrhythmic and inotropic effects on HF ventricular myocytes by examining multiple aspects of intracellular Ca(2+) handling. In normal rabbit myocytes, dantrolene (1 μM) had no effect on SR Ca(2+) load, postrest decay of SR Ca(2+) content, the threshold for spontaneous Ca(2+) wave initiation (i.e., the SR Ca(2+) content at which spontaneous waves initiate) and Ca(2+) spark frequency. In cardiomyocytes from failing rabbit hearts, SR Ca(2+) load and the wave initiation threshold were decreased compared with normal myocytes, Ca(2+) spark frequency was increased, and the postrest decay was potentiated. Using a novel approach of measuring cytosolic and intra-SR Ca(2+) concentration (using the low-affinity Ca(2+) indicator fluo-5N entrapped within the SR), we showed that treatment of HF cardiomyocytes with dantrolene rescued postrest decay and increased the wave initiation threshold. Additionally, dantrolene decreased Ca(2+) spark frequency while increasing the SR Ca(2+) content in HF myocytes. These data suggest that dantrolene exerts antiarrhythmic effects and preserves inotropy in HF cardiomyocytes by decreasing the incidence of diastolic Ca(2+) sparks, increasing the intra-SR Ca(2+) threshold at which spontaneous Ca(2+) waves occur, and decreasing the loss of Ca(2+) from the SR. Furthermore, the observation that dantrolene reduces arrhythmogenicity while at the same time preserves inotropy suggests that dantrolene is a potentially useful drug in the treatment of arrhythmia associated with HF.

Magnesium does not influence the clinical course of succinylcholine-induced malignant hyperthermia

BACKGROUND

Malignant hyperthermia (MH) is a potentially lethal hypermetabolic syndrome. Volatile anesthetics and/or succinylcholine lead to an increase of the intracellular calcium concentration resulting in activation of various intracellular processes. A production of carbon dioxide, and later lactate, are early signs of increased cellular energy consumption. On a cellular level, magnesium acts as a physiological calcium inhibitor resulting in less-intense calcium liberation from the sarcoplasmic reticulum. In this study, we examined the effects of IV magnesium administration on the clinical course of an MH crisis.

METHODS

Sixteen Pietrain pigs (10 MH-susceptible [MHS] and 6 MH-nonsusceptible [MHN]) were anesthetized without an MH trigger substance. Invasive hemodynamic monitoring was established before 4 mg/kg succinylcholine was administered. Four of the MHS pigs received 10 mg/kg magnesium sulfate 10 minutes later. Hemodynamic changes (heart rate, mean arterial blood pressure, and oxygen saturation as measured by pulse oximetry) were continuously monitored. Venous and arterial blood gases (pH, Pco(2), Po(2), base excess, and lactate) were taken at 15-minute intervals. The H test and U test were used with P < 0.05 for significant differences among the groups.

RESULTS

No differences among the groups were seen for weight, hemodynamic, and metabolic variables before administration of succinylcholine. In all MHS animals, succinylcholine led to a marked decrease of mean arterial blood pressure and increase of heart rate. Animals in both MHS groups developed combined metabolic and respiratory acidosis. Succinylcholine had no effect on animals in the MHN group. Hemodynamic and metabolic values were not different between the 2 MHS groups but were between groups MHS and MHN.

CONCLUSION

Succinylcholine led to a hemodynamic and metabolic reaction in only MHS pigs. Treatment with magnesium did not influence the clinical course. The intervention had no beneficial effect in the acute phase of an MH crisis.

TRPM channels in the vasculature

Recent studies show that mammalian melastatin TRPM nonselective cation channels (TRPM1-8), members of the largest and most diverse TRP subfamily, are widely expressed in the endothelium and vascular smooth muscles. When activated, these channels similarly to other TRPs permit the entry of sodium, calcium and magnesium, thus causing membrane depolarisation. Although membrane depolarisation reduces the driving force for calcium entry via TRPMs as well as other pathways for calcium entry, in smooth muscle myocytes expressing voltage-gated Ca(2+) channels the predominant functional effect is an increase in intracellular Ca(2+) concentration and myocyte contraction. This review focuses on several best documented aspects of vascular functions of TRPMs, including the role of TRPM2 in oxidant stress, regulation of endothelial permeability and cell death, the connection between TRPM4 and myogenic response, significance of TRPM7 for magnesium homeostasis, vessel injury and hypertension, and emerging evidence that the cold and menthol receptor TRPM8 is involved in the regulation of vascular tone.

CGP-37157 inhibits the sarcoplasmic reticulum Ca²+ ATPase and activates ryanodine receptor channels in striated muscle

7-Chloro-5-(2-chlorophenyl)-1,5-dihydro-4,1-benzothiazepin-2(3H)-one [CGP-37157 (CGP)], a benzothiazepine derivative of clonazepam, is commonly used as a blocker of the mitochondrial Na+/Ca²+ exchanger. However, evidence suggests that CGP could also affect other targets, such as L-type Ca²+ channels and plasmalemma Na+/Ca²+ exchanger. Here, we tested the possibility of a direct modulation of ryanodine receptor channels (RyRs) and/or sarco/endoplasmic reticulum Ca²+-stimulated ATPase (SERCA) by CGP. In the presence of ruthenium red (inhibitor of RyRs), CGP decreased SERCA-mediated Ca²+ uptake of cardiac and skeletal sarcoplasmic reticulum (SR) microsomes (IC₅₀ values of 6.6 and 9.9 μM, respectively). The CGP effects on SERCA activity correlated with a decreased V(max) of ATPase activity of SERCA-enriched skeletal SR fractions. CGP (≥ 5 μM) also increased RyR-mediated Ca²+ leak from skeletal SR microsomes. Planar bilayer studies confirmed that both cardiac and skeletal RyRs are directly activated by CGP (EC(50) values of 9.4 and 12.0 μM, respectively). In summary, we found that CGP inhibits SERCA and activates RyR channels. Hence, the action of CGP on cellular Ca²+ homeostasis reported in the literature of cardiac, skeletal muscle, and other nonmuscle systems requires further analysis to take into account the contribution of all CGP-sensitive Ca²+ transporters.

Drugs and pharmaceuticals: management of intoxication and antidotes

The treatment of patients poisoned with drugs and pharmaceuticals can be quite challenging. Diverse exposure circumstances, varied clinical presentations, unique patient-specific factors, and inconsistent diagnostic and therapeutic infrastructure support, coupled with relatively few definitive antidotes, may complicate evaluation and management. The historical approach to poisoned patients (patient arousal, toxin elimination, and toxin identification) has given way to rigorous attention to the fundamental aspects of basic life support--airway management, oxygenation and ventilation, circulatory competence, thermoregulation, and substrate availability. Selected patients may benefit from methods to alter toxin pharmacokinetics to minimize systemic, target organ, or tissue compartment exposure (either by decreasing absorption or increasing elimination). These may include syrup of ipecac, orogastric lavage, activated single- or multi-dose charcoal, whole bowel irrigation, endoscopy and surgery, urinary alkalinization, saline diuresis, or extracorporeal methods (hemodialysis, charcoal hemoperfusion, continuous venovenous hemofiltration, and exchange transfusion). Pharmaceutical adjuncts and antidotes may be useful in toxicant-induced hyperthermias. In the context of analgesic, anti-inflammatory, anticholinergic, anticonvulsant, antihyperglycemic, antimicrobial, antineoplastic, cardiovascular, opioid, or sedative-hypnotic agents overdose, N-acetylcysteine, physostigmine, L-carnitine, dextrose, octreotide, pyridoxine, dexrazoxane, leucovorin, glucarpidase, atropine, calcium, digoxin-specific antibody fragments, glucagon, high-dose insulin euglycemia therapy, lipid emulsion, magnesium, sodium bicarbonate, naloxone, and flumazenil are specifically reviewed. In summary, patients generally benefit from aggressive support of vital functions, careful history and physical examination, specific laboratory analyses, a thoughtful consideration of the risks and benefits of decontamination and enhanced elimination, and the use of specific antidotes where warranted. Data supporting antidotes effectiveness vary considerably. Clinicians are encouraged to utilize consultation with regional poison centers or those with toxicology training to assist with diagnosis, management, and administration of antidotes, particularly in unfamiliar cases.

Importance of melastatin-like transient receptor potential 7 and magnesium in the stimulation of osteoblast proliferation and migration by platelet-derived growth factor

Bone is a dynamic tissue that is continuously being remodeled throughout life. Specialized cells called osteoclasts transiently break down old bone (resorption process) at multiple sites as other cells known as osteoblasts are replacing it with new tissue (bone formation). Usually, both resorption and formation processes are in balance and thereby maintain skeletal strength and integrity. This equilibrium is assured by the coordination of proliferation, migration, differentiation, and secretory functions of the osteoblasts, which are essential for adequate formation and resorption processes. Disturbances of this equilibrium may lead to decreased bone mass (osteoporosis), increased bone fragility, and susceptibility to fractures. Epidemiological studies have linked insufficient dietary magnesium (Mg2+) intake in humans with low bone mass and osteoporosis. Here, we investigated the roles of Mg2+ and melastatin-like transient receptor potential 7 (TRPM7), known as Mg2+ channels, in human osteoblast cell proliferation and migration induced by platelet-derived growth factor (PDGF), which has been involved in the bone remodeling process. PDGF promoted an influx of Mg2+, enhanced cell migration, and stimulated the gene expression of TRPM7 channels in human osteoblast MG-63 cells. The stimulation of osteoblast proliferation and migration by PDGF was significantly reduced under culture conditions of low extracellular Mg2+ concentrations. Silencing TRPM7 expression in osteoblasts by specific small interfering RNA prevented the induction by PDGF of Mg2+ influx, proliferation, and migration. Our results indicate that extracellular Mg2+ and TRPM7 are important for PDGF-induced proliferation and migration of human osteoblasts. Thus Mg2+ deficiency, a common condition among the general population, may be associated with altered osteoblast functions leading to inadequate bone formation and the development of osteoporosis.

Malignant hyperthermia: a pharmacogenetic disorder

Malignant hyperthermia (MH) is a pharmacogenetic disorder triggered by volatile anesthetics or depolarizing muscle relaxants in predisposed individuals. Exercise or stress-induced MH episodes, in the absence of any obvious pharmacological trigger, have been reported, but these are rare. A considerable effort has taken place over the last two decades to identify mutations associated with MH and characterize their functional effects. A number of different, but complementary systems, have been developed and implemented to this end. The results of such studies have identified commonalities in functional affects of mutations, and also uncovered unexpected complexities in both the structure and function of the skeletal muscle calcium-release channel. The following review is an attempt to provide a summary of the background to current MH research, and highlight some recent advances in our knowledge of the molecular basis of the phenotypic expression of this disorder.

Bradykinin regulates calpain and proinflammatory signaling through TRPM7-sensitive pathways in vascular smooth muscle cells

Transient receptor potential melastatin-7 (TRPM7) channels have recently been identified to be regulated by vasoactive agents acting through G protein-coupled receptors in vascular smooth muscle cells (VSMC). However, downstream targets and functional responses remain unclear. We investigated the subcellular localization of TRPM7 in VSMCs and questioned the role of TRPM7 in proinflammatory signaling by bradykinin. VSMCs from Wistar-Kyoto rats were studied. Cell fractionation by sucrose gradient and differential centrifugation demonstrated that in bradykinin-stimulated cells, TRPM7 localized in fractions corresponding to caveolae. Immunofluorescence confocal microscopy revealed that TRPM7 distributes along the cell membrane, that it has a reticular-type intracellular distribution, and that it colocalizes with flotillin-2, a marker of lipid rafts. Bradykinin increased expression of calpain, a TRPM7 target, and stimulated its cytosol/membrane translocation, an effect blocked by 2-APB (TRPM7 inhibitor) and U-73122 (phospholipase C inhibitor), but not by chelerythrine (PKC inhibitor). Expression of proinflammatory mediators VCAM-1 and cyclooxygenase-2 (COX-2) was time-dependently increased by bradykinin. This effect was blocked by Hoe-140 (B2 receptor blocker) and 2-APB. Our data demonstrate that in bradykinin-stimulated VSMCs: 1) TRPM7 is upregulated, 2) TRPM7 associates with cholesterol-rich microdomains, and 3) calpain and proinflammatory mediators VCAM-1 and COX2 are regulated, in part, via TRPM7- and phospholipase C-dependent pathways through B2 receptors. These findings identify a novel signaling pathway for bradykinin, which involves TRPM7. Such phenomena may play a role in bradykinin/B2 receptor-mediated inflammatory responses in vascular cells.