Reduced Mg2+ inhibition of Ca2+ release in muscle fibers of pigs susceptible to malignant hyperthermia

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.

MG53 suppresses interferon-β and inflammation via regulation of ryanodine receptor-mediated intracellular calcium signaling

TRIM family proteins play integral roles in the innate immune response to virus infection. MG53 (TRIM72) is essential for cell membrane repair and is believed to be a muscle-specific TRIM protein. Here we show human macrophages express MG53, and MG53 protein expression is reduced following virus infection. Knockdown of MG53 in macrophages leads to increases in type I interferon (IFN) upon infection. MG53 knockout mice infected with influenza virus show comparable influenza virus titres to wild type mice, but display increased morbidity accompanied by more accumulation of CD45+ cells and elevation of IFNβ in the lung. We find that MG53 knockdown results in activation of NFκB signalling, which is linked to an increase in intracellular calcium oscillation mediated by ryanodine receptor (RyR). MG53 inhibits IFNβ induction in an RyR-dependent manner. This study establishes MG53 as a new target for control of virus-induced morbidity and tissue injury.

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.

In vivo Ca2+ dynamics induced by Ca2+ injection in individual rat skeletal muscle fibers

In contrast to cardiomyocytes, store overload‐induced calcium ion (Ca²⁺) release (SOICR) is not considered to constitute a primary Ca²⁺ releasing system from the sarcoplasmic reticulum (SR) in skeletal muscle myocytes. In the latter, voltage‐induced Ca²⁺ release (VICR) is regarded as the dominant mechanism facilitating contractions. Any role of the SOICR in the regulation of cytoplasmic Ca²⁺ concentration ([Ca²⁺]_i) and its dynamics in skeletal muscle in vivo remains poorly understood. By means of in vivo single fiber Ca²⁺ microinjections combined with bioimaging techniques, we tested the hypothesis that the [Ca²⁺]_i dynamics following Ca²⁺ injection would be amplified and fiber contraction facilitated by SOICR. The circulation‐intact spinotrapezius muscle of adult male Wistar rats (n = 34) was exteriorized and loaded with Fura‐2 AM to monitor [Ca²⁺]_i dynamics. Groups of rats underwent the following treatments: (1) 0.02, 0.2, and 2.0 mmol/L Ca²⁺ injections, (2) 2.0 mmol/L Ca²⁺ with inhibition of ryanodine receptors (RyR) by dantrolene sodium (DAN), and (3) 2.0 mmol/L Ca²⁺ with inhibition of SR Ca²⁺ ATPase (SERCA) by cyclopiazonic acid (CPA). A quantity of 0.02 mmol/L Ca²⁺ injection yielded no detectable response, whereas peak evoked [Ca²⁺]_i increased 9.9 ± 1.8% above baseline for 0.2 mmol/L and 23.8 ± 4.3% (P < 0.05) for 2.0 mmol/L Ca²⁺ injections. The peak [Ca²⁺]_i in response to 2.0 mmol/L Ca²⁺ injection was largely abolished by DAN and CPA (−85.8%, −71.0%, respectively, both P < 0.05 vs. unblocked) supporting dependence of the [Ca²⁺]_i dynamics on Ca²⁺ released by SOICR rather than injected Ca²⁺ itself. Thus, this investigation demonstrates the presence of a robust SR‐evoked SOICR operant in skeletal muscle in vivo.

Critical Role of Intracellular RyR1 Calcium Release Channels in Skeletal Muscle Function and Disease

The skeletal muscle Ca²⁺ release channel, also known as ryanodine receptor type 1 (RyR1), is the largest ion channel protein known and is crucial for effective skeletal muscle contractile activation. RyR1 function is controlled by Ca_v1.1, a voltage gated Ca²⁺ channel that works mainly as a voltage sensor for RyR1 activity during skeletal muscle contraction and is also fine-tuned by Ca²⁺, several intracellular compounds (e.g., ATP), and modulatory proteins (e.g., calmodulin). Dominant and recessive mutations in RyR1, as well as acquired channel alterations, are the underlying cause of various skeletal muscle diseases. The aim of this mini review is to summarize several current aspects of RyR1 function, structure, regulation, and to describe the most common diseases caused by hereditary or acquired RyR1 malfunction.

Anatomy and bronchoscopy of the porcine lung. A model for translational respiratory medicine

The porcine model has contributed significantly to biomedical research over many decades. The similar size and anatomy of pig and human organs make this model particularly beneficial for translational research in areas such as medical device development, therapeutics and xenotransplantation. In recent years, a major limitation with the porcine model was overcome with the successful generation of gene-targeted pigs and the publication of the pig genome. As a result, the role of this model is likely to become even more important. For the respiratory medicine field, the similarities between pig and human lungs give the porcine model particular potential for advancing translational medicine. An increasing number of lung conditions are being studied and modeled in the pig. Genetically modified porcine models of cystic fibrosis have been generated that, unlike mouse models, develop lung disease similar to human cystic fibrosis. However, the scientific literature relating specifically to porcine lung anatomy and airway histology is limited and is largely restricted to veterinary literature and textbooks. Furthermore, methods for in vivo lung procedures in the pig are rarely described. The aims of this review are to collate the disparate literature on porcine lung anatomy, histology, and microbiology; to provide a comparison with the human lung; and to describe appropriate bronchoscopy procedures for the pig lungs to aid clinical researchers working in the area of translational respiratory medicine using the porcine model.

Activation and propagation of Ca2+ release from inside the sarcoplasmic reticulum network of mammalian skeletal muscle

Skeletal muscle fibres are large and highly elongated cells specialized for producing the force required for posture and movement. The process of controlling the production of force within the muscle, known as excitation-contraction coupling, requires virtually simultaneous release of large amounts of Ca(2+) from the sarcoplasmic reticulum (SR) at the level of every sarcomere within the muscle fibre. Here we imaged Ca(2+) movements within the SR, tubular (t-) system and in the cytoplasm to observe that the SR of skeletal muscle is a connected network capable of allowing diffusion of Ca(2+) within its lumen to promote the propagation of Ca(2+) release throughout the fibre under conditions where inhibition of SR ryanodine receptors (RyRs) was reduced. Reduction of cytoplasmic [Mg(2+)] ([Mg(2+)]cyto) induced a leak of Ca(2+) through RyRs, causing a reduction in SR Ca(2+) buffering power argued to be due to a breakdown of SR calsequestrin polymers, leading to a local elevation of [Ca(2+)]SR. The local rise in [Ca(2+)]SR, an intra-SR Ca(2+) transient, induced a local diffusely rising [Ca(2+)]cyto. A prolonged Ca(2+) wave lasting tens of seconds or more was generated from these events. Ca(2+) waves were dependent on the diffusion of Ca(2+) within the lumen of the SR and ended as [Ca(2+)]SR dropped to low levels to inactivate RyRs. Inactivation of RyRs allowed re-accumulation of [Ca(2+)]SR and the activation of secondary Ca(2+) waves in the persistent presence of low [Mg(2+)]cyto if the threshold [Ca(2+)]SR for RyR opening could be reached. Secondary Ca(2+) waves occurred without an abrupt reduction in SR Ca(2+) buffering power. Ca(2+) release and wave propagation occurred in the absence of Ca(2+)-induced Ca(2+) release. These observations are consistent with the activation of Ca(2+) release through RyRs of lowered cytoplasmic inhibition by [Ca(2+)]SR or store overload-induced Ca(2+) release. Restitution of SR Ca(2+) buffering power to its initially high value required imposing normal resting ionic conditions in the cytoplasm, which re-imposed the normal resting inhibition on the RyRs, allowing [Ca(2+)]SR to return to endogenous levels without activation of store overload-induced Ca(2+) release. These results are discussed in the context of how pathophysiological Ca(2+) release such as that occurring in malignant hyperthermia can be generated.

Nonspecific sarcolemmal cation channels are critical for the pathogenesis of malignant hyperthermia

Malignant hyperthermia (MH) susceptibility has been attributed to a leaky sarcoplasmic reticulum (SR) caused by missense mutations in RYR1 or CACNA1S, and the MH crisis has been attributed solely to massive self-sustaining release of Ca(2+) from SR stores elicited by triggering agents. Here, we show in muscle cells from MH-RyR1(R163C) knock-in mice that increased passive SR Ca(2+) leak causes an enlarged basal influx of sarcolemmal Ca(2+) that results in chronically elevated myoplasmic free Ca(2+) concentration ([Ca(2+)]i) at rest. We discovered that Gd(+3) and GsMTx-4 were more effective than BTP2 or expression of the dominant-negative Orai1(E190Q) in reducing both Ca(2+) entry and [Ca(2+)]i, implicating a non-STIM1/Orai1 SOCE pathway in resetting resting [Ca(2+)]i. Indeed, two nonselective cationic channels, TRPC3 and TRPC6, are overexpressed, and [Na]i is chronically elevated in MH-RyR1(R163C) muscle cells. [Ca(2+)]i and [Na(+)]i are persistently elevated in vivo and further increased by halothane in MH-RyR1(R163C/WT) muscle. These increases are markedly attenuated by local perfusion of Gd(+3) or GsMTx-4 and completely suppressed by dantrolene. These results contribute a new paradigm for understanding MH pathophysiology by demonstrating that nonselective sarcolemmal cation channel activity plays a critical role in causing myoplasmic Ca(2+) and Na(+) overload both at rest and during the MH crisis.-Eltit, J. M., Ding, X., Pessah, I. N., Allen, P. D., Lopez, J. R. Nonspecific sarcolemmal cation channels are critical for the pathogenesis of malignant hyperthermia.

Gene dose influences cellular and calcium channel dysregulation in heterozygous and homozygous T4826I-RYR1 malignant hyperthermia-susceptible muscle

Malignant hyperthermia susceptibility (MHS) is primarily conferred by mutations within ryanodine receptor type 1 (RYR1). Here we address how the MHS mutation T4826I within the S4-S5 linker influences excitation-contraction coupling and resting myoplasmic Ca(2+) concentration ([Ca(2+)](rest)) in flexor digitorum brevis (FDB) and vastus lateralis prepared from heterozygous (Het) and homozygous (Hom) T4826I-RYR1 knock-in mice (Yuen, B. T., Boncompagni, S., Feng, W., Yang, T., Lopez, J. R., Matthaei, K. I., Goth, S. R., Protasi, F., Franzini-Armstrong, C., Allen, P. D., and Pessah, I. N. (2011) FASEB J. doi:22131268). FDB responses to electrical stimuli and acute halothane (0.1%, v/v) exposure showed a rank order of Hom ≫ Het ≫ WT. Release of Ca(2+) from the sarcoplasmic reticulum and Ca(2+) entry contributed to halothane-triggered increases in [Ca(2+)](rest) in Hom FDBs and elicited pronounced Ca(2+) oscillations in ∼30% of FDBs tested. Genotype contributed significantly elevated [Ca(2+)](rest) (Hom > Het > WT) measured in vivo using ion-selective microelectrodes. Het and Hom oxygen consumption rates measured in intact myotubes using the Seahorse Bioscience (Billerica, MA) flux analyzer and mitochondrial content measured with MitoTracker were lower than WT, whereas total cellular calpain activity was higher than WT. Muscle membranes did not differ in RYR1 expression nor in Ser(2844) phosphorylation among the genotypes. Single channel analysis showed highly divergent gating behavior with Hom and WT favoring open and closed states, respectively, whereas Het exhibited heterogeneous gating behaviors. [(3)H]Ryanodine binding analysis revealed a gene dose influence on binding density and regulation by Ca(2+), Mg(2+), and temperature. Pronounced abnormalities inherent in T4826I-RYR1 channels confer MHS and promote basal disturbances of excitation-contraction coupling, [Ca(2+)](rest), and oxygen consumption rates. Considering that both Het and Hom T4826I-RYR1 mice are viable, the remarkable isolated single channel dysfunction mediated through this mutation in S4-S5 cytoplasmic linker must be highly regulated in vivo.

Effects of conformational peptide probe DP4 on bidirectional signaling between DHPR and RyR1 calcium channels in voltage-clamped skeletal muscle fibers

In skeletal muscle, excitation-contraction coupling involves the activation of dihydropyridine receptors (DHPR) and type-1 ryanodine receptors (RyR1) to produce depolarization-dependent sarcoplasmic reticulum Ca²⁺ release via orthograde signaling. Another form of DHPR-RyR1 communication is retrograde signaling, in which RyRs modulate the gating of DHPR. DP4 (domain peptide 4), is a peptide corresponding to residues Leu²⁴⁴²-Pro²⁴⁷⁷ of the central domain of the RyR1 that produces RyR1 channel destabilization. Here we explore the effects of DP4 on orthograde excitation-contraction coupling and retrograde RyR1-DHPR signaling in isolated murine muscle fibers. Intracellular dialysis of DP4 increased the peak amplitude of Ca²⁺ release during step depolarizations by 64% without affecting its voltage-dependence or kinetics, and also caused a similar increase in Ca²⁺ release during an action potential waveform. DP4 did not modify either the amplitude or the voltage-dependence of the intramembrane charge movement. However, DP4 augmented DHPR Ca²⁺ current density without affecting its voltage-dependence. Our results demonstrate that the conformational changes induced by DP4 regulate both orthograde E-C coupling and retrograde RyR1-DHPR signaling.

Measurement of fluid secretion from intact airway submucosal glands

Human airways are kept sterile by a mucosal innate defense system that includes mucus secretion. Mucus is secreted in healthy upper airways primarily by submucosal glands and consists of defense molecules mixed with mucins, electrolytes, and water and is also a major component of sputum. Mucus traps pathogens and mechanically removes them via mucociliary clearance while inhibiting their growth via molecular (e.g., lysozyme) and cellular (e.g., neutrophils, macrophages) defenses. Fluid secretion rates of single glands in response to various mediators can be measured by trapping the primary gland mucus secretions in an oil layer, where they form spherical bubbles that can be optically measured at any desired interval to provide detailed temporal analysis of secretion rates. The composition and properties of the mucus (e.g., solids, viscosity, pH) can also be determined. These methods have now been applied to mice, ferrets, cats, pigs, sheep, and humans, with a main goal of comparing gland secretion in control and CFTR-deficient humans and animals.

HCO3(-) secretion by murine nasal submucosal gland serous acinar cells during Ca2+-stimulated fluid secretion

Airway submucosal glands contribute to airway surface liquid (ASL) composition and volume, both important for lung mucociliary clearance. Serous acini generate most of the fluid secreted by glands, but the molecular mechanisms remain poorly characterized. We previously described cholinergic-regulated fluid secretion driven by Ca²⁺-activated Cl⁻ secretion in primary murine serous acinar cells revealed by simultaneous differential interference contrast (DIC) and fluorescence microscopy. Here, we evaluated whether Ca²⁺-activated Cl⁻ secretion was accompanied by secretion of HCO₃⁻, possibly a critical ASL component, by simultaneous measurements of intracellular pH (pH_i) and cell volume. Resting pH_i was 7.17 ± 0.01 in physiological medium (5% CO₂–25 mM HCO₃⁻). During carbachol (CCh) stimulation, pH_i fell transiently by 0.08 ± 0.01 U concomitantly with a fall in Cl⁻ content revealed by cell shrinkage, reflecting Cl⁻ secretion. A subsequent alkalinization elevated pH_i to above resting levels until agonist removal, whereupon it returned to prestimulation values. In nominally CO₂–HCO₃⁻-free media, the CCh-induced acidification was reduced, whereas the alkalinization remained intact. Elimination of driving forces for conductive HCO₃⁻ efflux by ion substitution or exposure to the Cl⁻ channel inhibitor niflumic acid (100 μM) strongly inhibited agonist-induced acidification by >80% and >70%, respectively. The Na⁺/H⁺ exchanger (NHE) inhibitor dimethylamiloride (DMA) increased the magnitude (greater than twofold) and duration of the CCh-induced acidification. Gene expression profiling suggested that serous cells express NHE isoforms 1–4 and 6–9, but pharmacological sensitivities demonstrated that alkalinization observed during both CCh stimulation and pH_i recovery from agonist-induced acidification was primarily due to NHE1, localized to the basolateral membrane. These results suggest that serous acinar cells secrete HCO₃⁻ during Ca²⁺-evoked fluid secretion by a mechanism that involves the apical membrane secretory Cl⁻ channel, with HCO₃⁻ secretion sustained by activation of NHE1 in the basolateral membrane. In addition, other Na⁺-dependent pH_i regulatory mechanisms exist, as evidenced by stronger inhibition of alkalinization in Na⁺-free media.

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

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

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

Malignant hyperthermia (MH) susceptibility is a genetic disorder of skeletal muscle associated with mutations in the ryanodine receptor isoform 1 (RyR1) of sarcoplasmic reticulum (SR). In MH-susceptible skeletal fibers, RyR1-mediated Ca(2+) release is highly sensitive to activation by the volatile anesthetic halothane. Indeed, studies with isolated RyR1 channels (using simple Cs(+) solutions) found that halothane selectively affects mutated but not wild-type RyR1 function. However, studies in skeletal fibers indicate that halothane can also activate wild-type RyR1-mediated Ca(2+) release. We hypothesized that endogenous RyR1 agonists (ATP, lumenal Ca(2+)) may increase RyR1 sensitivity to halothane. Consequently, we studied how these agonists affect halothane action on rabbit skeletal RyR1 reconstituted into planar lipid bilayers. We found that cytosolic ATP is required for halothane-induced activation of the skeletal RyR1. Unlike RyR1, cardiac RyR2 (much less sensitive to ATP) responded to halothane even in the absence of this agonist. ATP-dependent halothane activation of RyR1 was enhanced by cytosolic Ca(2+) (channel agonist) and counteracted by Mg(2+) (channel inhibitor). Dantrolene, a muscle relaxant used to treat MH episodes, did not affect RyR1 or RyR2 basal activity and did not interfere with halothane-induced activation. Studies with skeletal SR microsomes confirmed that halothane-induced RyR1-mediated SR Ca(2+) release is enhanced by high ATP-low Mg(2+) in the cytosol and by increased SR Ca(2+) load. Thus, physiological or pathological processes that induce changes in cellular levels of these modulators could affect RyR1 sensitivity to halothane in skeletal fibers, including the outcome of halothane-induced contracture tests used to diagnose MH susceptibility.

Postulated role of interdomain interaction between regions 1 and 2 within type 1 ryanodine receptor in the pathogenesis of porcine malignant hyperthermia

We have demonstrated recently that CICR (Ca2+-induced Ca2+ release) activity of RyR1 (ryanodine receptor 1) is held to a low level in mammalian skeletal muscle ('suppression' of the channel) and that this is largely caused by the interdomain interaction within RyR1 [Murayama, Oba, Kobayashi, Ikemoto and Ogawa (2005) Am. J. Physiol. Cell Physiol. 288, C1222-C1230]. To test the hypothesis that aberration of this suppression mechanism is involved in the development of channel dysfunctions in MH (malignant hyperthermia), we investigated properties of the RyR1 channels from normal and MHS (MH-susceptible) pig skeletal muscles with an Arg615-->Cys mutation using [3H]ryanodine binding, single-channel recordings and SR (sarcoplasmic reticulum) Ca2+ release. The RyR1 channels from MHS muscle (RyR1MHS) showed enhanced CICR activity compared with those from the normal muscle (RyR1N), although there was little or no difference in the sensitivity to several ligands tested (Ca2+, Mg2+ and adenine nucleotide), nor in the FKBP12 (FK506-binding protein 12) regulation. DP4, a domain peptide matching the Leu2442-Pro2477 region of RyR1 which was reported to activate the Ca2+ channel by weakening the interdomain interaction, activated the RyR1N channel in a concentration-dependent manner, and the highest activity of the affected channel reached a level comparable with that of the RyR1MHS channel with no added peptide. The addition of DP4 to the RyR1MHS channel produced virtually no further effect on the channel activity. These results suggest that stimulation of the RyR1MHS channel caused by affected inter-domain interaction between regions 1 and 2 is an underlying mechanism for dysfunction of Ca2+ homoeostasis seen in the MH phenotype.

Regulation of ryanodine receptors from skeletal and cardiac muscle during rest and excitation

1. In muscle, intracellular calcium concentration, hence skeletal muscle force and cardiac output, is regulated by uptake and release of calcium from the sarcoplasmic reticulum (SR). The ryanodine receptor (RyR) forms the calcium release channel in the SR. 2. Calcium release through RyRs is modulated by a wide variety of endogenous molecules, including small diffusible ligands such as ATP, Ca2+ and Mg2+. The regulation of RyR channels by ATP, Ca2+ and Mg2+ is a complex interplay of several regulatory mechanisms, which are still being unravelled. Consequently, it is not clearly known how RyRs are regulated in resting muscle and during contraction. 3. The present paper reviews factors controlling the activity of RyRs in skeletal and cardiac muscle with an emphasis on mechanistic insights derived from single channel recording methods. 4. In addition, the nature of dihydropyridine receptor (DHPR) control of RyRs in skeletal muscle derived from experiments with peptide fragments of the DHPR II-III loop is reviewed. 5. Finally, recent experiments on coupled RyRs in lipid bilayers and their potential for resolving the elusive mechanisms controlling calcium release during cardiac contraction are discussed.

Mg2+ dependence of Ca2+ release from the sarcoplasmic reticulum induced by sevoflurane or halothane in skeletal muscle from humans susceptible to malignant hyperthermia

BACKGROUND

In normal resting muscle, cytosolic Mg(2+) exerts a potent inhibitory influence on the sarcoplasmic reticulum (SR) Ca(2+) release channel (ryanodine receptor, RyR1). Impaired Mg(2+)-regulation of RyR1 has been proposed as a causal factor in malignant hyperthermia (MH). The aim of this study was to compare the effects of cytosolic Mg(2+) on SR Ca(2+) release induced by halothane or sevoflurane in normal (MHN) and MH susceptible (MHS) human skeletal muscle fibres.

METHODS

Samples of vastus medialis muscle were obtained from patients under investigation for MH susceptibility. Single fibres were mechanically skinned and perfused with solutions mimicking the intracellular milieu. Changes in [Ca(2+)](i) were detected using fura-2 fluorescence after application of equimolar halothane or sevoflurane.

RESULTS

In MHN fibres, concentrations of sevoflurane or halothane as high as 10 mM typically failed to induce SR Ca(2+) release at physiological free [Mg(2+)] (1 mM). However, when [Mg(2+)] was decreased to 0.4 mM, SR Ca(2+) release occurred in 51% (16/33) and 6% (2/33) of MHN fibres after the addition of 1 mM halothane or 1 mM sevoflurane, respectively. Further decreases in [Mg(2+)] increased the proportion of responsive fibres. In the presence of 0.1 mM [Mg(2+)], Ca(2+) release occurred in all fibres (33/33) after the introduction of 1 mM halothane or 1 mM sevoflurane. In MHS fibres, 1 mM halothane or 1 mM sevoflurane-induced Ca(2+) release in 54% (7/13) or 15% (2/13) of fibres, respectively, at 1 mM Mg(2+). A decrease in [Mg(2+)] to 0.2 mM Mg(2+) was sufficient to render 100% of MHS fibres (13/13) responsive to 1 mM halothane or 1 mM sevoflurane.

CONCLUSIONS

In both MHS and MHN fibres (i) halothane is a more potent activator of SR Ca(2+) release than sevoflurane and (ii) as with halothane, the efficacy of sevoflurane-induced SR Ca(2+) release exhibits a marked dependence on cytosolic [Mg(2+)]. The marked potentiation of SR Ca(2+) release after a moderate reduction in cytosolic [Mg(2+)] suggests that conditions which cause hypomagnesaemia will increase the probability and possibly severity of an MH event. Conversely, maintenance of a normal or slightly increased cytosolic [Mg(2+)] may reduce the probability of MH.

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

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

Cystic fibrosis and airway submucosal glands

The chronic pulmonary infections and inflammation associated with cystic fibrosis (CF) are responsible for almost all the morbidity and mortality of this disease. Our understanding of the mechanisms that underlie the very early stages of CF lung disease, that result directly from mutations in the CF gene, is relatively poor. However, the demonstration that the predominant sites of expression of the CF gene in normal lungs are the submucosal glands, together with the histological observations showing that hyperplasia of these glands and mucin occlusion of the gland ducts are the earliest signs of disease in the CF lung, suggest that malfunction of the submucosal glands may be an important factor contributing to the early pathophysiology of CF lung disease. This review describes the function of submucosal glands in normal lungs, and the way in which their function may be disrupted in CF and may thus contribute to the early stages of CF lung disease.

Coupled calcium release channels and their regulation by luminal and cytosolic ions

Contraction in skeletal and cardiac muscle occurs when Ca(2+) is released from the sarcoplasmic reticulum (SR) through ryanodine receptor (RyR) Ca(2+) release channels. Several isoforms of the RyR exist throughout the animal kingdom, which are modulated by ATP, Ca(2+) and Mg(2+) in the cytoplasm and by Ca(2+) in the lumen of the SR. This review brings to light recent findings on their mechanisms of action in the mammalian isoforms RyR-1 and RyR-2 with an emphasis on RyR-1 from skeletal muscle. Cytoplasmic Mg(2+) is a potent RyR antagonist that binds to two classes of cytoplasmic site, identified as low-affinity, non-specific inhibition sites and high-affinity Ca(2+) activation sites (A-sites). Mg(2+) inhibition at the A-sites is very sensitive to the cytoplasmic and luminal milieu. Cytoplasmic Ca(2+), Mg(2+) and monovalent cations compete for the A-sites. In isolated RyRs, luminal Ca(2+) alters the Mg(2+) affinity of the A-site by an allosteric mechanism mediated by luminal sites. However, in close-packed RyR arrays luminal Ca(2+) can also compete with cytoplasmic ions for the A-site. Activation of RyRs by luminal Ca(2+) has been attributed to either Ca(2+) feedthrough to A-sites or to Ca(2+) regulatory sites on the luminal side of the RyR. As yet there is no consensus on just how luminal Ca(2+) alters RyR activation. Recent evidence indicates that both mechanisms operate and are likely to be important. Allosteric regulation of A-site Mg(2+) affinity could trigger Ca(2+) release, which is reinforced by Ca(2+) feedthrough.

Effects of elastase and cathepsin G on the levels of membrane and soluble TNFα

Neutrophil elastase (NE) and cathepsin G (CG), the proteolytic enzymes localized in azurophil granules of neutrophils (PMN), are involved in PMN responses to various stimuli. When released at sites of inflammation, they participate in the degradation of numerous proteins involved in the regulation of the immune response. In this study, we employed ADAM17(-/-) fibroblasts stably transfected with cDNA of human pro-tumor necrosis factor alpha (proTNFalpha) (ADAM17(-/-)TNF(+)) to investigate the effects of NE and CG on shedding and degradation of TNFalpha. Both NE and CG were found to diminish the level of membrane TNFalpha (mTNFalpha) as measured by flow cytometry. This process was accompanied by the accumulation of biologically active soluble TNFalpha (sTNFalpha) in the culture medium, as determined by an increase in both the cytotoxic activity of TNFalpha and its ability to serve as a co-stimulator in the induction of inducible nitric oxide synthase (iNOS). However, in contrast to CG, NE at high concentrations was able to degrade sTNFalpha released from the cell surface. Using soluble recombinant human TNFalpha, we identified Val(93)-Ala(94) and Val(117)-Glu(118) as the NE cleavage sites within the sTNFalpha molecule. Taken together, the ability of NE and CG to modulate levels of membrane and soluble forms of TNFalpha may contribute to the proinflammatory activity of neutrophils.

Dantrolene stabilizes domain interactions within the ryanodine receptor

Interdomain interactions between N-terminal and central domains serving as a "domain switch" are believed to be essential to the functional regulation of the skeletal muscle ryanodine receptor-1 Ca(2+) channel. Mutational destabilization of the domain switch in malignant hyperthermia (MH), a genetic sensitivity to volatile anesthetics, causes functional instability of the channel. Dantrolene, a drug used to treat MH, binds to a region within this proposed domain switch. To explore its mechanism of action, the effect of dantrolene on MH-like channel activation by the synthetic domain peptide DP4 or anti-DP4 antibody was examined. A fluorescence probe, methylcoumarin acetate, was covalently attached to the domain switch using DP4 as a delivery vehicle. The magnitude of domain unzipping was determined from the accessibility of methylcoumarin acetate to a macromolecular fluorescence quencher. The Stern-Volmer quenching constant (K(Q)) increased with the addition of DP4 or anti-DP4 antibody. This increase was reversed by dantrolene at both 37 and 22 degrees C and was unaffected by calmodulin. [(3)H]Ryanodine binding to the sarcoplasmic reticulum and activation of sarcoplasmic reticulum Ca(2+) release, both measures of channel activation, were enhanced by DP4. These activities were inhibited by dantrolene at 37 degrees C, yet required the presence of calmodulin at 22 degrees C. These results suggest that the mechanism of action of dantrolene involves stabilization of domain-domain interactions within the domain switch, preventing domain unzipping-induced channel dysfunction. We suggest that temperature and calmodulin primarily affect the coupling between the domain switch and the downstream mechanism of regulation of Ca(2+) channel opening rather than the domain switch itself.

Neutrophil Serine Proteinases Inactivate Surfactant Protein D by Cleaving within a Conserved Subregion of the Carbohydrate Recognition Domain

Surfactant protein D (SP-D) plays important roles in innate immunity including the defense against bacteria, fungi, and respiratory viruses. Because SP-D specifically interacts with neutrophils that infiltrate the lung in response to acute inflammation and infection, we examined the hypothesis that the neutrophil-derived serine proteinases (NSPs): neutrophil elastase, proteinase-3, and cathepsin G degrade SP-D. All three human NSPs specifically cleaved recombinant rat and natural human SP-D dodecamers in a time- and dose-dependent manner, which was reciprocally dependent on calcium concentration. The NSPs generated similar, relatively stable, disulfide cross-linked immunoreactive fragments of approximately 35 kDa (reduced), and sequencing of a major catheptic fragment definitively localized the major sites of cleavage to a highly conserved subregion of the carbohydrate recognition domain. Cleavage markedly reduced the ability of SP-D to promote bacterial aggregation and to bind to yeast mannan in vitro. Incubation of SP-D with isolated murine neutrophils led to the generation of similar fragments, and cleavage was inhibited with synthetic and natural serine proteinase inhibitors. In addition, neutrophils genetically deficient in neutrophil elastase and/or cathepsin G were impaired in their ability to degrade SP-D. Using a mouse model of acute bacterial pneumonia, we observed the accumulation of SP-D at sites of neutrophil infiltration coinciding with the appearance of approximately 35-kDa SP-D fragments in bronchoalveolar lavage fluids. Together, our data suggest that neutrophil-derived serine proteinases cleave SP-D at sites of inflammation with potential deleterious effects on its biological functions.

Association of a mutation in the ryanodine receptor 1 gene with equine malignant hyperthermia

Equine malignant hyperthermia MH has been suspected but never genetically confirmed. In this study, we investigated whether mutations in a candidate gene, RyR1, were associated with MH in two clinically affected horses. RyR1 gene sequences revealed polymorphisms in exons 15, 17, and 46 in WTRyR1 and MHRyR1 horses with one derived amino acid change in MHRyR1 exon 46, R2454G. The MHRyR1 horses were genetically heterozygous for this mutation, but presented an MH phenotype with halothane challenge. Skeletal sarcoplasmic reticulum from a R2454G heterozygote collected during a fulminant MH episode showed significantly higher affinity and density of [3H]ryanodine-binding sites compared to WTRyR1, but no differences in Ca2+, Mg2+, and caffeine modulation. In conclusion, an autosomal missense mutation in RyR1 is associated with MH in the horse, providing a screening test for susceptible individuals. [3H]ryanodine-binding analysis suggests that long-lasting changes in RyR1 conformation persists in vitro after the triggering event.

Evidence that elastase is the TNF-R75 shedding enzyme in resting human polymorphonuclear leukocytes

We previously showed that a metalloprotease and a serine protease mediate shedding of the TNF-R75 (75-kDa tumor necrosis factor receptor) in neutrophils. Here we show that elastase is the TNF-R75 solubilizing serine protease. Release of the TNF-R75 by resting cells was almost totally inhibited by the serine protease inhibitor diisopropylfluorophosphate (DFP), by two synthetic, chemically unrelated, elastase-specific inhibitors and by alpha1-protease inhibitor. Release after TNF or FMLP (N-formyl-L-methionyl-L-leucyl-L-phenylalanine) stimulation was blocked by DFP and a metalloprotease inhibitor used in combination. Supernatants from resting neutrophils contained a 28-kDa fragment of the receptor, compatible with that generated by elastase, whose appearance was inhibited by DFP. Upon FMLP stimulation, the release of 28-kDa and 40-kDa fragments was observed, which was inhibited by DFP and a metalloprotease inhibitor, respectively. We conclude that elastase is the TNF-R75 sheddase of resting neutrophils and that it contributes to shedding of this receptor in stimulated cells.

Mg2+ dependence of halothane-induced Ca2+ release from the sarcoplasmic reticulum in rat skeletal muscle

The effect of cytosolic Mg2+ on halothane-induced Ca2+ release from the sarcoplasmic reticulum (SR) was investigated in mechanically skinned fibres from the rat extensor digitorum longus (EDL) muscle. Preparations were perfused with solutions mimicking the intracellular milieu and changes in [Ca2+] were detected using Fura-2 fluorescence. In the presence of 1 mM Mg2+, brief (500 ms) applications of 40 mM halothane failed to induce Ca2+ release from the SR. However, Ca2+ release became detectable when [Mg2+] was reduced to 0.4 mM, and the amplitude of the response increased progressively as [Mg2+] was further reduced to 0.2 and 0.1 mM. Lower halothane concentrations within the range found during anaesthesia or induction (0.1-1.2 mM) failed to induce SR Ca2+ release at 0.2 or 0.4 mM Mg2+. However, in further experiments, preparations were exposed to 1 mM halothane for 2-3 min under conditions where the volume of solution surrounding the preparation was restricted by stopping the flow. In the absence of perfusion, 1 mM halothane induced Ca2+ release from the SR at 0.4 mM Mg2+ in two out of six preparations, and release was observed consistently at 0.2 and 0.1 mM Mg2+. Responses to 1 mM halothane induced in the presence of 0.4 and 0.2 mM Mg2+ were typically delayed in onset and involved a localised release of Ca2+ that propagated along the fibre. These results suggest that halothane-induced Ca2+ release is strongly inhibited at normal physiological levels of Mg2+. However, when Mg2+-induced inhibition of the ryanodine receptor (RYR) is reduced, levels of halothane within the range found during anaesthesia can induce a marked efflux of Ca2+ from the SR. This may be of relevance to the condition of malignant hyperthermia, where the inhibition of RYRs by Mg2+ is reportedly reduced.

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.

Identification of proteases involved in the proteolysis of vascular endothelium cadherin during neutrophil transmigration

Transmigration of neutrophils across the endothelium occurs at the cell-cell junctions where the vascular endothelium cadherin (VE cadherin) is expressed. This adhesive receptor was previously demonstrated to be involved in the maintenance of endothelium integrity. We propose that neutrophil transmigration across the vascular endothelium goes in parallel with cleavage of VE cadherin by elastase and cathepsin G present on the surface of neutrophils. This hypothesis is supported by the following lines of evidence. 1) Proteolytic fragments of VE cadherin are released into the culture medium upon adhesion of neutrophils to endothelial cell monolayers; 2) conditioned culture medium, obtained after neutrophil adhesion to endothelial monolayers, cleaves the recombinantly expressed VE cadherin extracellular domain; 3) these cleavages are inhibited by inhibitors of elastase; 4) VE cadherin fragments produced by conditioned culture medium or by exogenously added elastase are identical as shown by N-terminal sequencing and mass spectrometry analysis; 5) both elastase- and cathepsin G-specific VE cadherin cleavage patterns are produced upon incubation with tumor necrosis factor alpha-stimulated and fixed neutrophils; 6) transendothelial permeability increases in vitro upon addition of either elastase or cathepsin G; and 7) neutrophil transmigration is reduced in vitro in the presence of elastase and cathepsin G inhibitors. Our results suggest that cleavage of VE cadherin by neutrophil surface-bound proteases induces formation of gaps through which neutrophils transmigrate.

Effects of Mg(2+) and SR luminal Ca(2+) on caffeine-induced Ca(2+) release in skeletal muscle from humans susceptible to malignant hyperthermia

Regulation of the ryanodine receptor (RYR) by Mg(2+) and SR luminal Ca(2+) was studied in mechanically skinned malignant hyperthermia susceptible (MHS) and non-susceptible (MHN) fibres from human vastus medialis. Preparations were perfused with solutions mimicking the intracellular milieu and changes in [Ca(2+)] were detected using fura-2 fluorescence. At 1 mM cytosolic Mg(2+), MHS fibres had a higher sensitivity to caffeine (2-40 mM) than MHN fibres. The inhibitory effect of Mg(2+) on caffeine-induced Ca(2+) release was studied by increasing [Mg(2+)] of the solution containing 40 mM caffeine. Increasing [Mg(2+)] from 1 to 3 mM reduced the amplitude of the caffeine-induced Ca(2+) transient by 77 +/- 7.4 % (n = 8) in MHN fibres. However, the caffeine-induced Ca(2+) transient decreased by only 24 +/- 8.1 % (n = 9) in MHS fibres. In MHN fibres, reducing the Ca(2+) loading period from 4 to 1 min (at 1 mM Mg(2+)) decreased the fraction of the total sarcoplasmic reticulum (SR) Ca(2+) content released in response to 40 mM caffeine by 90.4 +/- 6.2 % (n = 6). However, in MHS fibres the response was reduced by only 31.2 +/- 17.4 % (n = 6) under similar conditions. These results suggest that human malignant hyperthermia (MH) is associated with reduced inhibition of the RYR by (i) cytosolic Mg(2+) and (ii) SR Ca(2+) depletion. Both of these effects may contribute to increased sensitivity of the RYR to caffeine and volatile anaesthetics.

Mucus secretion from single submucosal glands of pig. Stimulation by carbachol and vasoactive intestinal peptide

Secretion rates of >700 individual glands in isolated tracheal mucosa from 56 adult pigs were monitored optically. "Basal" secretion of 0.7 +/- 0.1 nl x min(-1) gland(-1) was observed 1-9 h post-harvest but was near zero on day 2. Secretion to carbachol (10 microm) peaked at 2-3 min and then declined to a sustained phase. Peak secretion was 12.4 +/- 1.1 nl x min(-1) gland(-1); sustained secretion was approximately one-third of peak secretion. Thapsigargin (1 microm) increased secretion from 0.1 +/- 0.05 to 0.7 +/- 0.2 nl x min(-1) gland(-1); thapsigargin did not cause contraction of the trachealis muscles. Isoproterenol and phenylephrine (10 microm each) were ineffective, but vasoactive intestinal peptide (1 microm) and forskolin (10 microm) each produced sustained secretion of 1.0 +/- 0.5 and 1.7 +/- 0.2 nl x min(-1) gland(-1), respectively. The density of actively secreting glands was 1.3/mm(2). Secretion to either carbachol or forskolin was inhibited (approximately 50%) by either bumetanide or HCO(3)(-) removal and inhibited approximately 90% by the combined treatments. Mucus secreted in response to carbachol or forskolin was acidic by approximately 0.2 pH units relative to the bath and remained acidic by approximately 0.1 pH units after bumetanide. The strong secretory response to vasoactive intestinal peptide, the acidity of [cAMP](i)-stimulated mucus, and its inhibition by bumetanide were unexpected.

Interdomain interactions within ryanodine receptors regulate Ca2+ spark frequency in skeletal muscle

DP4 is a 36-residue synthetic peptide that corresponds to the Leu(2442)-Pro(2477) region of RyR1 that contains the reported malignant hyperthermia (MH) mutation site. It has been proposed that DP4 disrupts the normal interdomain interactions that stabilize the closed state of the Ca(2)+ release channel (Yamamoto, T., R. El-Hayek, and N. Ikemoto. 2000. J. Biol. Chem. 275:11618-11625). We have investigated the effects of DP4 on local SR Ca(2)+ release events (Ca(2)+ sparks) in saponin-permeabilized frog skeletal muscle fibers using laser scanning confocal microscopy (line-scan mode, 2 ms/line), as well as the effects of DP4 on frog SR vesicles and frog single RyR Ca(2)+ release channels reconstituted in planar lipid bilayers. DP4 caused a significant increase in Ca(2)+ spark frequency in muscle fibers. However, the mean values of the amplitude, rise time, spatial half width, and temporal half duration of the Ca(2)+ sparks, as well as the distribution of these parameters, remained essentially unchanged in the presence of DP4. Thus, DP4 increased the opening rate, but not the open time of the RyR Ca(2)+ release channel(s) generating the sparks. DP4 also increased [(3)H]ryanodine binding to SR vesicles isolated from frog and mammalian skeletal muscle, and increased the open probability of frog RyR Ca(2)+ release channels reconstituted in bilayers, without changing the amplitude of the current through those channels. However, unlike in Ca(2)+ spark experiments, DP4 produced a pronounced increase in the open time of channels in bilayers. The same peptide with an Arg(17) to Cys(17) replacement (DP4mut), which corresponds to the Arg(2458)-to-Cys(2458) mutation in MH, did not produce a significant effect on RyR activation in muscle fibers, bilayers, or SR vesicles. Mg(2)+ dependence experiments conducted with permeabilized muscle fibers indicate that DP4 preferentially binds to partially Mg(2)+-free RyR(s), thus promoting channel opening and production of Ca(2)+ sparks.

Sepsis research in the next millennium: concentrate on the software rather than the hardware

Today the basic principles of septic conditions are understood. Nevertheless, sepsis research has reached a critical point. To integrate our knowledge towards a consistent theory of the disease process and to derive effective therapies, new perspectives for future research that fit the complexity of the problem have to be found. We conducted a review of the literature concerning systemic inflammatory response syndrome (SIRS) and sepsis with particular reference to liver pathophysiology. And compared our findings with characteristic features of complex systems. The complexity of sepsis is broadly recognized. A review of the different aspects of liver inflammation during SIRS and sepsis, i.e. endotoxin challenge, cytokine induced dysfunction, the mechanisms of leukocyte transmigration, and hormonal and neuroendocrine regulatory mechanisms is given. Key aspects of complex systems, including parallelism, locality, emergence, and cross-scale interactions are introduced. We conclude that sepsis research needs new perspectives that allow us to handle the complex interactions occurring during the disease process. We propose to focus research on the interactions between the constituents of the system rather than only describing isolated aspects of the disease process. We also conclude that the ideas and techniques of non-linear systems theory are suitable tools for the analysis of complex and dynamic diseases like SIRS and sepsis.

Characteristics of irreversible ATP activation suggest that native skeletal ryanodine receptors can be phosphorylated via an endogenous CaMKII

Phosphorylation of skeletal muscle ryanodine receptor (RyR) calcium release channels by endogenous kinases incorporated into lipid bilayers with native sarcoplasmic reticulum vesicles was investigated during exposure to 2 mM cytoplasmic ATP. Activation of RyRs after 1-min exposure to ATP was reversible upon ATP washout. In contrast, activation after 5 to 8 min was largely irreversible: the small fall in activity with washout was significantly less than that after brief ATP exposure. The irreversible activation was reduced by acid phosphatase and was not seen after exposure to nonhydrolyzable ATP analogs. The data suggested that the channel complex was phosphorylated after addition of ATP and that phosphorylation reduced the RyR's sensitivity to ATP, adenosine, and Ca(2+). The endogenous kinase was likely to be a calcium calmodulin kinase II (CaMKII) because the CaMKII inhibitor KN-93 and an inhibitory peptide for CaMKII prevented the phosphorylation-induced irreversible activation. In contrast, phosphorylation effects remained unchanged with inhibitory peptides for protein kinase C and A. The presence of CaMKIIbeta in the SR vesicles was confirmed by immunoblotting. The results suggest that CaMKII is anchored to skeletal muscle RyRs and that phosphorylation by this kinase alters the enhancement of channel activity by ATP and Ca(2+).

Aquaporin-5 dependent fluid secretion in airway submucosal glands

Fluid and macromolecule secretion by submucosal glands in mammalian airways is believed to play an important role in airway defense and surface liquid homeostasis and in the pathogenesis of cystic fibrosis. Immunocytochemistry revealed strong expression of aquaporin water channel AQP5 at the luminal membrane of serous epithelial cells in submucosal glands throughout the mouse nasopharynx and upper airways and AQP4 at the contralateral basolateral membrane in some glands. Novel methods were applied to measure secretion rates and composition of gland fluid in wild type mice and knockout mice lacking AQP4 or AQP5. In mice breathing through a tracheotomy, total gland fluid output was measured from the dilution of a volume marker present in the fluid-filled nasopharynx and upper trachea. Pilocarpine-stimulated fluid secretion was 4.3 +/- 0.4 microl/min in wild type mice, 4.9 +/- 0.9 microl/min in AQP4 null mice, and 1.9 +/- 0.3 microl/min in AQP5 null mice (p < 0.001). Similar results were obtained when secreted fluid was collected in the oil-filled nasopharyngeal cavity. Real-time video imaging of fluid droplets secreted from individual submucosal glands near the larynx in living mice showed a 57 +/- 4% reduced fluid secretion rate in AQP5 null mice. Analysis of secreted fluid showed a 2.3 +/- 0.2-fold increase in total protein in AQP5 null mice and a smaller increase in [Cl(-)], suggesting intact protein and salt secretion across a relatively water impermeable epithelial barrier. Submucosal gland morphology and density did not differ significantly in wild type versus AQP5 null mice. These results indicate that AQP5 facilitates fluid secretion in submucosal glands and that the luminal membrane of gland epithelial cells is the rate-limiting barrier to water movement. Modulation of gland AQP5 expression or function might provide a novel approach to treat hyperviscous gland secretions in cystic fibrosis and excessive fluid secretions in infectious or allergic bronchitis/rhinitis.

Divergent effects of the malignant hyperthermia-susceptible Arg(615)-->Cys mutation on the Ca(2+) and Mg(2+) dependence of the RyR1

The sarcoplasmic reticulum (SR) Ca(2+) release channel (RyR1) from malignant hyperthermia-susceptible (MHS) porcine skeletal muscle has a decreased sensitivity to inhibition by Mg(2+). This diminished Mg(2+) inhibition has been attributed to a lower Mg(2+) affinity of the inhibition (I) site. To determine whether alterations in the Ca(2+) and Mg(2+) affinity of the activation (A) site contribute to the altered Mg(2+) inhibition, we estimated the Ca(2+) and Mg(2+) affinities of the A- and I-sites of normal and MHS RyR1. Compared with normal SR, MHS SR required less Ca(2+) to half-maximally activate [(3)H]ryanodine binding (K(A,Ca): MHS = 0.17 +/- 0.01 microM; normal = 0.29 +/- 0.02 microM) and more Ca(2+) to half-maximally inhibit ryanodine binding (K(I,Ca): MHS = 519.3 +/- 48.7 microM; normal = 293.3 +/- 24.2 microM). The apparent Mg(2+) affinity constants of the MHS RyR1 A- and I-sites were approximately twice those of the A- and I-sites of the normal RyR1 (K(A,Mg): MHS = 44.36 +/- 4.54 microM; normal = 21.59 +/- 1.66 microM; K(I,Mg): MHS = 660.8 +/- 53.0 microM; normal = 299.2 +/- 24.5 microM). Thus, the reduced Mg(2+) inhibition of the MHS RyR1 compared with the normal RyR1 is due to both an enhanced selectivity of the MHS RyR1 A-site for Ca(2+) over Mg(2+) and a reduced Mg(2+) affinity of the I-site.

The power of single channel recording and analysis: its application to ryanodine receptors in lipid bilayers

1. Since the inception of the patch-clamp technique, single-channel recording has made an enormous impact on our understanding of ion channel function and its role in membrane transport and cell physiology. 2. However, the impact of single-channel recording methods on our understanding of intracellular Ca2+ regulation by internal stores is not as broadly recognized. There are several possible reasons for this. 3. First, ion channels in the membranes of intracellular organelles are not directly accessible to patch pipettes, requiring other methods that are not as widely known as the patch-clamp techniques. 4. Second, bulk assays for channel activity have proved successful in advancing our knowledge of Ca2+ handling by intracellular stores. These assays include Ca2+ imaging, ryanodine binding assays and measurements of muscle tension and Ca2+ release and uptake by vesicles that have been isolated from internal stores. 5. The present review describes methods used for single- channel recording and analysis, as applied to the calcium release channels in striated muscle, and details some of the unique contributions that single-channel recording and analysis have made to our current understanding of the release of Ca2+ from the internal stores of muscle. 6. With this in mind, the review focuses on three aspects of channel function and shows how single-channel investigations have led to an improved understanding of physiological processes in muscle. 7. Finally, the review describes some of the latest improvements in membrane technology that will underpin future advances in single-channel recording.

Submucosal gland secretions in airways from cystic fibrosis patients have normal [Na(+)] and pH but elevated viscosity

Fluid and macromolecule secretion by submucosal glands in mammalian airways is believed to be important in normal airway physiology and in the pathophysiology of cystic fibrosis (CF). An in situ fluorescence method was applied to measure the ionic composition and viscosity of freshly secreted fluid from airway glands. Fragments of human large airways obtained at the time of lung transplantation were mounted in a humidified perfusion chamber and the mucosal surface was covered by a thin layer of oil. Individual droplets of secreted fluid were microinjected with fluorescent indicators for measurement of [Na(+)], [Cl(-)], and pH by ratio imaging fluorescence microscopy and viscosity by fluorescence recovery after photobleaching. After carbachol stimulation, 0.1--0.5 microl of fluid accumulated in spherical droplets at gland orifices in approximately 3--5 min. In gland fluid from normal human airways, [Na(+)] was 94 +/- 8 mM, [Cl(-)] was 92 +/- 12 mM, and pH was 6.97 +/- 0.06 (SE, n = 7 humans, more than five glands studied per sample). Apparent fluid viscosity was 2.7 +/- 0.3-fold greater than that of saline. Neither [Na(+)] nor pH differed in gland fluid from CF airways, but viscosity was significantly elevated by approximately 2-fold compared to normal airways. These results represent the first direct measurements of ionic composition and viscosity in uncontaminated human gland secretions and indicate similar [Na(+)], [Cl(-)], and pH to that in the airway surface liquid. The elevated gland fluid viscosity in CF may be an important factor promoting bacterial colonization and airway disease.

Effects of a domain peptide of the ryanodine receptor on Ca2+ release in skinned skeletal muscle fibers

Mutations in the central domain of the skeletal muscle ryanodine receptor (RyR) cause malignant hyperthermia (MH). A synthetic peptide (DP4) in this domain (Leu-2442-Pro-2477) produces enhanced ryanodine binding and sensitized Ca2+ release in isolated sarcoplasmic reticulum, similar to the properties in MH, possibly because the peptide disrupts the normal interdomain interactions that stabilize the closed state of the RyR (Yamamoto T, El-Hayek R, and Ikemoto N. J Biol Chem 275: 11618-11625, 2000). Here, DP4 was applied to mechanically skinned fibers of rat muscle that had the normal excitation-contraction coupling mechanism still functional to determine whether muscle fiber responsiveness was enhanced. DP4 (100 microM) substantially potentiated the Ca2+ release and force response to caffeine (8 mM) and to low [Mg2+] (0.2 mM) in every fiber examined, with no significant effect on the properties of the contractile apparatus. DP4 also potentiated the response to submaximal depolarization of the transverse tubular system by ionic substitution. Importantly, DP4 did not significantly alter the size of the twitch response elicited by action potential stimulation. These results support the proposal that DP4 causes an MH-like aberration in RyR function and are consistent with the voltage sensor triggering Ca2+ release by destabilizing the closed state of the RyRs.

Protein kinase C regulates the flow rate-dependent decline in human nasal ciliary beat frequency in vitro

Cilia provide the driving force for mucociliary clearance, the process that removes mucus from the airways. Protein kinase C (PKC) plays a poorly understood regulatory role in phosphorylation-based signal transduction cascades, including the control of human mucociliary clearance, especially with respect to ciliary beat frequency (CBF). Ciliary studies minimize the importance of fluid flow, because it is generally accepted that flow increases CBF. Here, we studied postflow events by measuring CBF in vitro in volunteers. Rose chamber-loaded cells were pulsed for 5 minutes at 30 mL/h in medium-199 +/- PKC modulators at 20 degrees C. The 5-minute pulse precipitated a fall in CBF noted within 1 minute after flow (acute dip response [ADR] to 84 +/- 2% of preflow baseline). Thereafter, CBF rose to 8% below baseline for 30 minutes [postrecovery plateau at 92 +/- 3%]. Preincubation with 1 microM of phorbol 12-myristate 13-acetate (PMA), a PKC-activating phorbol ester attenuated the ADR (c. 95%) and restored the postrecovery plateau almost to baseline levels (98 +/- 0.7%; p > 0.10 compared with baseline CBF). With respect to the ADR, the PMA protective effect was lost in the presence of the selective PKC inhibitor myristoylated epidermal growth factor peptide 651d-658 (Myr-PKCI; 10 microM). Myr-PKCI alone changed the ADR pattern such that the CBF remained at 15% below preflow baseline. We conclude that CBF fall and recovery after a fluid pulse is regulated by PKC activity either directly or indirectly.

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.

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.

A bovine stress syndrome associated with exercise-induced hyperthermia

OBJECTIVE

To investigate an exercise-induced bovine stress syndrome under field and controlled experimental conditions.

DESIGN AND PROCEDURE

In the field study, cattle affected with the stress syndrome were observed while they were grazing and during normal mustering using horses. This study served to define the clinical nature of the syndrome. The experimental study utilised three affected and five normal unaffected cattle. These animals were compared on the basis of their response to a defined exercise program, which consisted of walking 3.6 km in 2 h. Blood samples and measurements of respiratory rate, ambient temperature and rectal temperature were taken immediately before exercise, and at 0.5, 1.0, 1.5 and 2.0 h during the exercise and 24 h later. Clinical and blood constituent data were subjected to standard analysis of variance and repeated measures analysis.

RESULTS

In the field study, affected cattle were observed to show abnormally anxious and hyperactive behaviour. This behaviour was exhibited by affected cattle during the experimental exercise program where it was shown to be accompanied by hyperthermia and hyperventilation. The experimental study showed that affected cattle developed metabolic acidosis and became hyperglycaemic. Their plasma creatine kinase activity remained markedly increased at 24 h after exercise but other clinical and blood constituent variables had returned to normal values.

CONCLUSION

The clinical and biochemical changes detected in affected cattle were consistent with exercise-induced malignant hyperthermia.

Oxidation and reduction of pig skeletal muscle ryanodine receptors

Time-dependent effects of cysteine modification were compared in skeletal ryanodine receptors (RyRs) from normal pigs and RyR(MH) (Arg(615) to Cys(615)) from pigs susceptible to malignant hyperthermia, using the oxidizing reagents 4,4'-dithiodipyridine (4, 4'-DTDP) and 5,5'-dithio-bis(2-nitrobenzoic acid) (DTNB) or the reducing agent dithiothreitol (DTT). Normal and RyR(MH) channels responded similarly to all reagents. DTNB (1 mM), either cytoplasmic (cis) or luminal (trans), or 1 mM 4,4'-DTDP (cis) activated RyRs, introducing an additional long open time constant. 4,4'-DTDP (cis), but not DTNB, inhibited channels after >5 min. Activation and inhibition were relieved by DTT (1-10 mM). DTT (10 mM, cytoplasmic or luminal), without oxidants, activated RyRs, and activation reversed with 1 mM DTNB. Control RyR activity was maintained with 1 mM DTNB and 10 mM DTT present on the same or opposite sides of the bilayer. We suggest that 1) 4,4'-DTDP and DTNB covalently modify RyRs by oxidizing activating or inhibiting thiol groups; 2) a modified thiol depresses mammalian skeletal RyR activity under control conditions; 3) both the activating thiols and the modified thiols, accessible from either cytoplasm or lumen, reside in the transmembrane region; 4) some cardiac sulfhydryls are unavailable in skeletal RyRs; and 5) Cys(615) in RyR(MH) is functionally unimportant in redox cycling.

Elastase and metalloproteinase activities regulate soluble complement receptor 1 release

Complement receptor 1 (CR1) is cleaved from the surface of polymorphonuclear cells (PMN) in the membrane-proximal region to yield a soluble fragment (sCR1) that contains the functional domains. The enzymes involved in this cleavage are produced by the PMN itself, since in vitro stimulation of purified PMN is followed by sCR1 release. Purified human neutrophil elastase (HNE) cleaved CR1 from erythrocytes and urinary vesicles originating from podocytes and enhanced tenfold the cleavage of CR1 from activated PMN. The largest fragment released from PMN by HNE was identical in size to CR1 shed spontaneously. The CR1 fragments cleaved from erythrocytes were functional. The shedding of sCR1 by activated PMN was inhibited by phenylmethylsulfonyl fluoride (80 +/- 10%), alpha1-antiprotease (50 +/- 5%) and elafin (60 +/- 5%). Furthermore the cleavage was blocked by the metalloprotease inhibitor 1,10-phenanthroline (70 +/- 6 %) as well as by a monoclonal antibody against human neutrophil collagenase MMP8 (40 +/- 10%). Maximal inhibition of sCR1 shedding was obtained by a combination of 1,10-phenanthroline with elafin (86 +/- 6%). These inhibitors had no effect on L-selectin shedding, indicating that the cleavage of CR1 was specific. In conclusion, elastase or elastase-like activity may be responsible for the shedding of functional sCR1 in vivo, and this activity is controlled by the local release of PMN metalloproteases and alpha1antiprotease.

Measurement of resting cytosolic Ca2+ concentrations and Ca2+ store size in HEK-293 cells transfected with malignant hyperthermia or central core disease mutant Ca2+ release channels

Malignant hyperthermia (MH) and central core disease (CCD) mutations were introduced into full-length rabbit Ca2+ release channel (RYR1) cDNA, which was then expressed transiently in HEK-293 cells. Resting Ca2+ concentrations were higher in HEK-293 cells expressing homotetrameric CCD mutant RyR1 than in cells expressing homotetrameric MH mutant RyR1. Cells expressing homotetrameric CCD or MH mutant RyR1 exhibited lower maximal peak amplitudes of caffeine-induced Ca2+ release than cells expressing wild type RyR1, suggesting that MH and CCD mutants might be "leaky." In cells expressing homotetrameric wild type or mutant RyR1, the amplitude of 10 mM caffeine-induced Ca2+ release was correlated significantly with the amplitude of carbachol- or thapsigargin-induced Ca2+ release, indicating that maximal drug-induced Ca2+ release depends on the size of the endoplasmic reticulum Ca2+ store. The content of endogenous sarco(endo)plasmic reticulum Ca2+-ATPase isoform 2b (SERCA2b), measured by enzyme-linked immunosorbent assay, 45Ca2+ uptake, and confocal microscopy, was increased in HEK-293 cells expressing wild type or mutant RyR1, supporting the view that endoplasmic reticulum Ca2+ storage capacity is increased as a compensatory response to an enhanced Ca2+ leak. When heterotetrameric (1:1) combinations of MH/CCD mutant and wild type RyR1 were expressed together with SERCA1 to enhance Ca2+ reuptake, the amplitude of Ca2+ release in response to low concentrations of caffeine and halothane was higher than that observed in cells expressing wild type RyR1 and SERCA1. In Ca2+-free medium, MH/CCD mutants were more sensitive to caffeine than wild type RyR1, indicating that caffeine hypersensitivity observed with a variety of MH/CCD mutant RyR1 proteins is not dependent on extracellular Ca2+ concentration.