The selenocysteine incorporation machinery: interactions between the SECIS RNA and the SECIS-binding protein SBP2

The decoding of UGA as a selenocysteine (Sec) codon in mammalian selenoprotein mRNAs requires a selenocysteine insertion sequence (SECIS) element in the 3' untranslated region. The SECIS is a hairpin structure that contains a non-Watson-Crick base-pair quartet with a conserved G.A/A.G tandem in the core of the upper helix. Another essential component of the Sec insertion machinery is SECIS-binding protein 2 (SBP2). In this study, we define the binding site of SBP2 on six different SECIS RNAs using enzymatic and hydroxyl radical footprinting, gel mobility shift analysis, and phosphate-ethylation binding interference. We show that SBP2 binds to a variety of mammalian SECIS elements with similar affinity and that the SBP2 binding site is conserved across species. Based on footprinting studies, SBP2 protects the proximal part of the hairpin and both strands of the lower half of the upper helix that contains the non-Watson-Crick base pair quartet. Gel mobility shift assays showed that the G.A/A.G tandem and internal loop are critical for the binding of SBP2. Modification of phosphates by ethylnitrosourea along both strands of the non-Watson-Crick base pair quartet, on the 5' strand of the lower helix and part of the 5' strand of the internal loop, prevented binding of SBP2. We propose a model in which SBP2 covers the central part of the SECIS RNA, binding to the non-Watson-Crick base pair quartet and to the 5' strands of the lower helix and internal loop. Our results suggest that the affinity of SBP2 for different SECIS elements is not responsible for the hierarchy of selenoprotein expression that is observed in vivo.

North American malignant hyperthermia population: screening of the ryanodine receptor gene and identification of novel mutations

BACKGROUND

Malignant hyperthermia (MH) is a disorder of skeletal muscle manifested as a life-threatening hypermetabolic crisis in susceptible individuals after exposure to inhalational anesthetics and depolarizing muscle relaxants. Mutations in the gene encoding the skeletal muscle ryanodine receptor (RYR1) are considered a common cause of the disorder, and, to date, more than 20 RYR1 mutations have been reported in European and Canadian families. Some studies suggest that differences may exist in the frequencies and distribution of mutations in the RYR1 gene between European and North American MH families the frequency and distribution of mutations in the RYR1 gene.

METHODS

Skeletal muscle samples from 73 unrelated individuals diagnosed as MH susceptible according to the North American MH caffeine-halothane contracture test were studied. Genomic DNA of MH-susceptible patients was investigated by polymerase chain reaction-based restriction fragment length polymorphism, single-strand conformation polymorphism, and sequencing analysis. The majority of known RYR1 mutations were analyzed using the restriction fragment length polymorphism method, whereas new mutations were searched by single-strand conformation polymorphism in exons 12, 15, 39, 40, 44, 45, and 46 of the gene.

RESULTS

Seven known RYR1 mutations (Arg163Cys, Gly248Arg, Arg614Cys, Val2168Met, Thr2206Met, Gly2434Arg, and Arg2454His) were detected at frequencies of 2.7, 1.4, 1.4, 1.4, 1.4, 5.5, and 4.1%, respectively. In addition, three novel amino acid substitutions (Val2214Ile, Ala2367Thr, and Asp2431Asn) were detected at frequency of 1.4% each. These 10 mutations account for 21.9% of the North American MH-susceptible population.

CONCLUSION

Three novel candidate mutations in the RYR1 gene were identified in these MH patients. The frequency and distribution of RYR1 mutations observed in this North American MH population was markedly different from that previously identified in Europe. Larger-scale studies are necessary to clarify the type and frequency of mutations in RYR1 associated with MH in North American families.

Use of a physiologic scoring system during interhospital transport of pediatric patients

OBJECTIVE

To determine the incidence of physiologic deterioration in critically ill and injured pediatric patients during interhospital transport with air and ground ambulance

DESIGN

Prospective, descriptive study

SETTING

All children were treated in regional hospitals and then transported to a pediatric tertiary care center.

PATIENTS

Children (n = 100) with a median age of 1.4 years (range 1 week to 18 years)

MAIN RESULTS

Three sets of physiologic scores were calculated: at the time of referral, on departure from the referring hospital, and arrival at the tertiary care center. The incidence of significant physiologic deterioration based on the calculated physiologic scores was 5.6% (n = 4) during ground and 3.4% (n = 1) during air ambulance transports. Critical events occurred in 15% of ground and 31% of air ambulance transports.

CONCLUSION

No difference existed in the incidence of adverse events or physiologic deterioration when air ambulance transports were compared with ground ambulance transports for critically ill children by our team. The physiologic scoring system we chose is simple and easy to use for quality assurance.

Anaesthetic technique for transoesophageal echocardiography in children

OBJECTIVE

To document the safety and efficacy of an anaesthetic technique in paediatric patients undergoing transoesophageal echocardiography (TOE).

METHODS

Prospective descriptive study performed in a children's hospital with all patients undergoing TOE. Topical analgesia of the pharynx was achieved with lidocaine. Anaesthesia was induced with midazolam (25 microg.kg-1), fentanyl (1 microg.kg-1), and propofol (0.5-1 mg.kg-1), followed by a continuous infusion of propofol (5-10 mg.kg-1.h-1).

RESULTS

Thirty patients are reported. The mean age was 11.4 +/- 5.1 years (range 1-22) and weight 40.5 +/- 22.1 kg (range 10-110). All the patients tolerated the procedure well. Two patients experienced brief oxygen desaturations during induction, 10 patients coughed during the procedure, and six patients had significant muscle activity requiring supplemental doses of propofol. None of the patients experienced nausea or vomiting.

CONCLUSION

We conclude that our anaesthetic technique in spontaneously breathing paediatric patients during TOE is effective and appears to be safe in children with heart disease.

Identification of the Arg1086His mutation in the alpha subunit of the voltage-dependent calcium channel (CACNA1S) in a North American family with malignant hyperthermia

Individuals from a large North American population were screened for the presence of the mutation in the alpha1 subunit of the voltage-dependent calcium channel (CACNA1S) that has recently been associated with malignant hyperthermia (MH). This Arg1086His mutation was screened for in 154 MH normal (MHN) individuals and 112 MH susceptible (MHS) individuals, who were diagnosed by the North American protocol of the in vitro contracture test. PCR and restriction enzyme analysis was used to test for the mutation. The Arg1086His mutation in the CACNA1S was not found in any of the MHN individuals. In contrast, two related individuals (grandfather and grandson, father and son of the MH proband) among the MHS group exhibited this mutation. However, a third MHS individual in the same family (granddaughter, cousin of the grandson) did not exhibit this mutation. These results indicate that this mutation may be associated with MH in this family. Genetic alterations in the CACNA1S associated with MH are present in approximately 1% of this North American MHS population.

Polysome distribution of phospholipid hydroperoxide glutathione peroxidase mRNA: evidence for a block in elongation at the UGA/selenocysteine codon

The translation of mammalian selenoprotein mRNAs requires the 3' untranslated region that contains a selenocysteine insertion sequence (SECIS) element necessary for decoding an in-frame UGA codon as selenocysteine (Sec). Selenoprotein biosynthesis is inefficient, which may be due to competition between Sec insertion and termination at the UGA/Sec codon. We analyzed the polysome distribution of phospholipid hydroperoxide glutathione peroxidase (PHGPx) mRNA, a member of the glutathione peroxidase family of selenoproteins, in rat hepatoma cell and mouse liver extracts. In linear sucrose gradients, the sedimentation velocity of PHGPx mRNA was impeded compared to CuZn superoxide dismutase (SOD) mRNA, which has a coding region of similar size. Selenium supplementation increased the loading of ribosomes onto PHGPx mRNA, but not CuZn SOD mRNA. To determine whether the slow sedimentation velocity of PHGPx mRNA is due to a block in elongation, we analyzed the polysome distribution of wild-type and mutant mRNAs translated in vitro. Mutation of the UGA/Sec codon to UGU/cysteine increased ribosome loading and protein synthesis. When UGA/Sec was replaced with UAA or when the SECIS element core was deleted, the distribution of the mutant mRNAs was similar to the wild-type mRNA. Addition of SECIS-binding protein SBP2, which is essential for Sec insertion, increased ribosome loading and translation of wild-type PHGPx mRNA, but had no effect on the mutant mRNAs. These results suggest that elongation is impeded at UGA/Sec, and that selenium and SBP2 alleviate this block by promoting Sec incorporation instead of termination.

Comparison of sensitivity of sodium currents to tetrodotoxin in equine muscle specimens with that in murine and human muscle specimens

OBJECTIVE

To determine sensitivity of equine skeletal muscle to tetrodotoxin and compare that with sensitivity of murine and human skeletal muscles.

SAMPLE POPULATION

Semimembranosus, vastus lateralis, triceps brachii, and masseter muscle specimens from 22 euthanatized horses, vastus lateralis muscle biopsy specimens from 25 clinically normal humans, and diaphragmatic muscle specimens from 6 mice.

PROCEDURE

Electrically elicited twitch responses were measured in muscle specimens incubated in medium alone and with tetrodotoxin (100 nM, 400 nM, 1.6 microM for equine specimens and 100 nM, 200 nM, 400 nM, 800 nM, 1.6 microM for murine and human specimens). Percentages of tetrodotoxin-sensitive and -resistant sodium channels were determined and compared among muscles and species.

RESULTS

2 sodium channels with different sensitivities to tetrodotoxin were identified in equine muscle. One was blocked with 100 nM tetrodotoxin and the other was unaffected by tetrodotoxin at concentrations up to 1.6 microM. The only difference detected among the 4 equine muscles was that masseter muscle specimens had a higher percentage of tetrodotoxin-sensitive channels than triceps brachii muscle specimens. Tetrodotoxin-resistant sodium channels constituted 31 to 66% of the equine muscle twitch response, which was greater than that determined for normal human and murine muscle (< 5%).

CONCLUSION AND CLINICAL RELEVANCE

Equine skeletal muscle contains a high percentage of tetrodotoxin-resistant sodium channels. The 4 equine muscles evaluated were more similar to each other than to murine and human muscles. Shifts in expression of sodium channel subtypes may play a role in the manifestation of certain myopathies.

A novel RNA binding protein, SBP2, is required for the translation of mammalian selenoprotein mRNAs

In eukaryotes, the decoding of the UGA codon as selenocysteine (Sec) requires a Sec insertion sequence (SECIS) element in the 3' untranslated region of the mRNA. We purified a SECIS binding protein, SBP2, and obtained a cDNA clone that encodes this activity. SBP2 is a novel protein containing a putative RNA binding domain found in ribosomal proteins and a yeast suppressor of translation termination. By UV cross-linking and immunoprecipitation, we show that SBP2 specifically binds selenoprotein mRNAs both in vitro and in vivo. Using (75)Se-labeled Sec-tRNA(Sec), we developed an in vitro system for analyzing Sec incorporation in which the translation of a selenoprotein mRNA was both SBP2 and SECIS element dependent. Immunodepletion of SBP2 from the lysates abolished Sec insertion, which was restored when recombinant SBP2 was added to the reaction. These results establish that SBP2 is essential for the co-translational insertion of Sec into selenoproteins. We hypothesize that the binding activity of SBP2 may be involved in preventing termination at the UGA/Sec codon.

The potency (ED50) and cardiovascular effects of rapacuronium (Org 9487) during narcotic-nitrous oxide-propofol anesthesia in neonates, infants, and children

We studied the neuromuscular blocking effects of rapacuronium (Org 9487) (dose-response curve, onset, and 50% effective dose [ED50] value), and changes in heart rate and blood pressure, as well as evidence of histamine release in neonates, infants, and children in an open-label, randomized, two-center study. Fifteen neonates, 30 infants, and 30 children were studied. Anesthesia was induced and maintained with propofol, nitrous oxide:oxygen (60:40), and fentanyl. Mechanomyographic monitoring of neuromuscular function was performed at the thumb. The potency (ED50) for neonates, infants, and children were 0.32 (95% confidence interval [CI] 0.15-0.61), 0.28 (95% CI 0.11-0.61), and 0.39 (95% CI 0.17-0.85) mg/kg, respectively. Neonates who received 0.3, 0.6, or 0.9 mg/kg Org 9487 developed a maximum T1 twitch depression of 34 +/-28%, 98 +/- 3%, and 99 +/- 2%, respectively. Time-to-peak effect (onset time) for 0.9 mg/kg Org 9487 was 57 +/- 20 s. Maximum percent T1 twitch depression (+/-SD) in infants who received 0.3, 0.6, or 0.9 mg/kg rapacuronium was 41 +/- 34%, 96 +/- 7%, and 100 +/- 1%, respectively. Time-to-peak effect for 0.9 mg/kg Org 9487 was 62 +/- 29 s. In children 0.3, 0.6, and 0.9 mg/kg rapacuronium resulted in an average percent T1 twitch suppression of 29 +/- 23, 83 +/- 11, and 90 +/- 16, respectively. Time-to-peak effect of 0.9 mg/kg Org 9487 was 96 +/- 33 s, respectively. There was no evidence of histamine release or significant changes in heart rate or blood pressure in either group at any dose. Rapacuronium is a low-potency nondepolarizing muscle relaxant with a fast onset of relaxation and minimal cardiovascular effects. Its potency (ED50) is similar in neonates (0.32 mg/kg), infants (0.28 mg/kg), and children (0.39 mg/kg). T1 suppression (90% +/- 16) is less and time to peak effect (96 +/- 33 s) is greater (0.9 mg/kg rapacuronium) in children, compared with the combined group of infants and neonates.

IMPLICATIONS

This study assesses the potency of rapacuronium (Org 9487) in pediatric patients. The potency of rapacuronium is similar in neonates (0.32 mg/kg), infants (0.28 mg/kg), and children (0.39 mg/kg).

Isolation and structural characterization of a cytotoxic L-amino acid oxidase from Agkistrodon contortrix laticinctus snake venom: preliminary crystallographic data

We have purified a cytotoxic L-amino acid oxidase (LAO) from Agkistrodon contortrix laticinctus snake venom by means of Superdex-200 gel filtration, followed by phenyl-Sepharose CL-4B chromatography. The purified enzyme (ACL LAO) is a dimer on gel filtration, with a M(r) of 60,000 for the monomer as estimated by SDS-PAGE. LAO activity was tested against 15 amino acids, but only 9 were oxidized by the enzyme, suggesting that it presents some degree of specificity. ACL LAO has apoptosis-inducing activity in an HL-60 cell culture assay. After 24 h treatment with 25 micrograms/ml of ACL LAO, the typical DNA fragmentation pattern of apoptotic cells was observed on agarose gel electrophoresis. NMR analysis showed the presence of a flavin mononucleotide prosthetic group. To solve its 3-D structure, crystals of the purified protein were grown in 0.1 M Tris-HCl, pH 8.5, and 2 M (NH(4))(2)SO(4). Diffraction data collected to 3.5 A showed that the protein crystallized in the tetragonal system, with unit cell a = b = 103.22 A, c = 183.45 A. This is the first report of preliminary crystallization data for a snake venom L-amino acid oxidase.

ATX II, a sodium channel toxin, sensitizes skeletal muscle to halothane, caffeine, and ryanodine

BACKGROUND

The function or expression of subtypes of the sodium ion (Na+) channel is altered in biopsies or cultures of skeletal muscle from many persons who are susceptible to malignant hyperthermia (MH). ATX II, a specific Na+ channel toxin from a sea anemone, causes delayed inactivation of the channel similar to that seen in cell cultures of MH muscle. ATX II was added to skeletal muscle to determine whether altered Na+ channel function could increase the sensitivity of normal skeletal muscle to agents (halothane, caffeine, ryanodine) to which MH muscle is hypersensitive.

METHODS

Studies were performed of fiber bundles from the vastus lateralis muscle of persons who were deemed not MH susceptible (MH-) or MH susceptible (MH+) according to the MH diagnostic test and of strips of diaphragm muscle from rats. Preparations in a tissue bath containing Krebs solution were connected to a force transducer. ATX II was introduced 5 min before halothane, caffeine, or ryanodine.

RESULTS

ATX II increased the magnitude of contracture to halothane in preparations from most MH-, but not MH+, human participants. After ATX II treatment, preparations from 9 of 24 MH- participants generated contractures to halothane, 3%, that were of the same magnitude as those from MH+ participants. Preparations from four of six ATX II-treated healthy participants also gave responses of the same magnitude as those of MH-susceptible participants to a graded halothane challenge (0.5-3%). The contractures to bolus doses of halothane in specimens from male participants were more than three times larger than the contractures in specimens from female participants. In rat muscle, ATX II increased the magnitude of contracture to caffeine (2 mM) and decreased the time to produce a 1-g contracture to ryanodine (1 microM).

CONCLUSIONS

ATX II, which causes delayed inactivation of the Na+ channel in cell cultures similar to that reported in cultures of MH+ skeletal muscle, increased the sensitivity of normal muscle to three agents to which MH+ muscle is hypersensitive. The increased sensitivity to halothane, 3%, occurred in most (79%), but not all, MH- participants, and this effect was most evident in male participants. Therefore, abnormal function of the Na+ channel, even if it is a secondary event in MH, may contribute to a positive contracture test result for MH.

Comparison of European and North American malignant hyperthermia diagnostic protocol outcomes for use in genetic studies

BACKGROUND

Halothane and caffeine diagnostic protocols and an experimental ryanodine test from the North American Malignant Hyperthermia (MH) Group (NAMHG) and the European MH Group (EMHG) have not been compared in the same persons until now.

METHODS

The outcomes of the NAMHG and EMHG halothane and caffeine contracture tests were compared in 84 persons referred for diagnostic testing. In addition, the authors assessed the experimental ryanodine protocol in 50 of these persons.

RESULTS

Although the NAMHG and EMHG halothane protocols are slightly different methodologically, each yielded outcomes in close (84-100%) agreement with diagnoses made by the other protocol. Excluding 23 persons judged to be equivocal (marginally positive responders) by the EMHG protocol resulted in fewer persons classified as normal and MH susceptible (42 and 19, respectively) than those classified by the NAMHG protocol (48 and 34, respectively). For the 61 persons not excluded as equivocal, the diagnoses were identical by both protocols, with the exception of one person who was diagnosed as MH susceptible by the NAMHG protocol and as "normal" by the EMHG protocol. The NAMHG protocol produced only two equivocal diagnoses. Therefore, a normal or MH diagnosis by the NAMHG protocol was frequently associated with an equivocal diagnosis by the EMHG protocol. The time to 0.2-g contracture after the addition of 1 microM ryanodine completely separated populations, which was in agreement with the EMHG protocol and, except for one person, with the NAMHG protocol.

CONCLUSIONS

Overall, the NAMHG and EMHG protocols and the experimental ryanodine test yielded similar diagnoses. The EMHG protocol reduced the number of marginal responders in the final analysis, which may make the remaining diagnoses slightly more accurate for use in genetic studies.

Lysine 49 phospholipase A2 proteins

The structures of several K49 PLA2 proteins have been determined and these differ as a group in several regions from the closely related D49 PLA2 enzymes. One outstanding difference is the presence of a high number of positively charged residues in the C-terminal region which combined with the overall high number of conserved lysine residues gives the molecule an interfacial adsorption surface which is highly positively charged compared to the opposite surface of the molecule. Although some nucleotide sequences have been reported, progress in obtaining active recombinant proteins has been slow. The K49 proteins exert several toxic activities, including myotoxicity, anticoagulation and edema formation. The most studied of these activities is myotoxicity. The myotoxicity induced by the K49 PLA2 proteins is histologically similar to that caused by the D49 PLA2 myotoxins, with some muscle fiber types possibly more sensitive than others. Whereas it is clear that the K49 PLA2 myotoxins lyse the plasma membrane of the affected muscle cell in vivo, the exact mechanism of this lysis is not known. Also, it is not known whether the toxin is internalized before, during or after the initial lysis or ever. The K49 PLA2 toxins lyse liposomes and cells in culture and in the latter, the PLA2 myotoxins exert at least two distinct mechanisms of action, neither of which is well-characterized. While the K49 PLA2 proteins are enzymatically inactive on artificial substrates, the toxins cause fatty acid production in cell cultures. Whether the fatty acid release is due to the enzymatic activity of the K49 PLA2 or stimulation of tissue lipases, is unknown. While there may be a role for fatty acid production in one mechanism of myotoxicity, a second mechanism appears to be independent of enzymatic activity. Although we are beginning to understand more about the structure of these toxins, we still know little about the precise mechanism by which they interact with the skeletal muscle cell in vivo.

LYS49 phospholipase A2 myotoxins lyse cell cultures by two distinct mechanisms

Three Lys49 phospholipase A2 (PLA2) myotoxins from Agkistrodon contortrix laticinctus (ACLMT), Bothrops jararacussu (bothropstoxin-I) and Bothrops asper (myotoxin II) snake venoms are enzymatically inactive on artificial substrates, yet addition of these toxins to cell cultures causes the release of fatty acids derived from the hydrolysis of membrane phospholipids. Bothropstoxin-I treated with p-bromophenacyl bromide is no longer enzymatically active on cell cultures, suggesting the toxin, not tissue PLA2, may hydrolyze the phospholipids. The NB41A3 cell line is sensitive to lysis by ACLMT by two separate mechanisms. The first mechanism is predominant at lower concentrations of ACLMT (0.1-0.5 microM) and over long incubation periods (24 h) with toxin. This mechanism is antagonized by methylprednisolone (MePDN). The second is predominant at higher concentrations of toxin (1-5 microM) incubated over a short period (1 h) and is not antagonized by MePDN. There is no correlation between enzymatic activity and toxicity at the higher concentrations (5 microM; 1 h) when the enzymatic activity of ACLMT is compared with a noncytolytic PLA2 from Naja naja atra venom (1 microM). However, over a 24 h period, triglyceride formation relative to fatty acids remaining free is about 10-fold greater for ACLMT (ratio about 40:1) than for the PLA2 from Naja naja atra venom (ratio about 4:1), suggesting the two enzymes act on substrates associated with different cellular compartments under this condition. Therefore, two mechanisms of Lys49 PLA2-induced myonecrosis exist and these are dependent on toxin concentration. The MePDN-sensitive mechanism associated with triglyceride accumulation correlates with myotoxicity.

Phenytoin alters transcript levels of hormone-sensitive lipase in muscle from horses with hyperkalemic periodic paralysis

In equine hyperkalemic periodic paralysis (HyperPP), there is evidence suggesting that the primary defect in the sodium channel is associated with a secondary alteration in triacylglycerol-associated fatty acid metabolism (TAFAM) in skeletal muscle. Furthermore, TAFAM may be involved in the therapeutic action of phenytoin. The effects of phenytoin treatment on the transcript levels of three key proteins in TAFAM, hormone sensitive lipase (HSL), carnitine palmitoyltransferase (CPT), and fatty acid binding protein (FABP), were examined. These transcripts were quantitated by competitive reverse transcription polymerase chain reaction in undifferentiated and differentiated primary cultures of equine skeletal muscle from control, heterozygous HyperPP, and homozygous-affected HyperPP horses. There was a 10-fold lower level of HSL transcript in both undifferentiated and differentiated cultures from homozygous-affected horses than from horses of the other genotypes. Phenytoin selectively increased the HSL transcript in homozygous-affected differentiated cultures to levels similar to those of the other genotypes. The levels of CPT and FABP transcripts were unaffected by genotype, differentiation, and phenytoin treatment. These results suggest that the primary defect in HyperPP may secondarily decrease HSL transcript levels and that the therapeutic action of phenytoin may include regulation of mRNA transcripts in skeletal muscle.

Failure to identify the ryanodine receptor G1021A mutation in a large North American population with malignant hyperthermia

Anesthesia-induced malignant hyperthermia (MH) is a rare inherited disorder of skeletal muscle. Several mutations in the ryanodine receptor (RYR1) have been found to be causative of MH. The G1021A mutation in the RYR1 is one of the most frequently occurring mutations in European populations. MH normal (165) and MH susceptible (114) North American patients were screened for the presence of the G1021A mutation. This mutation was not found in any of the patients tested. These studies support the absence of this mutation in the normal population. Furthermore, these findings emphasize the importance of viewing the distribution of MH mutations as variable gene pools with frequencies dependent on the geographical location of the population examined.

A report of the use of the Dynamic Objective Risk Assessment (DORA) score in the changing pediatric intensive care environment

OBJECTIVE

To assess the clinical use of the Dynamic Objective Risk Assessment (DORA) severity of illness score in a site remote from its development.

DESIGN

Prospective chart review.

SETTING

Tertiary referral pediatric intensive care unit (PICU).

PATIENTS

One hundred sixty consecutive admissions involving 621 patient days.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Pediatric Risk of Mortality (PRISM) scores were collected daily for all PICU patient days. Collection of data was performed by a physician not directly involved in the ordering of vital signs or laboratory data. The daily DORA score was calculated from the previous day's PRISM score and the admission PRISM score according to a previously described formula. The DORA score determines the patient's risk of mortality for the next 24 hrs. Also documented were the tests not ordered for each patient day. The sensitivity and specificity of the DORA score in our patient population were very similar to that previously reported using the previously described 1% cutoff for predicted mortality. We also noted that the tests ordered were related to the physician's perception of the patient's degree of sickness, and were themselves predictive of outcome.

CONCLUSION

An outcome scoring system created in one group of PICUs can be applied to patients in another PICU remote from where the scoring system was developed with similar ability to predict outcome.

The G protein beta5 subunit interacts selectively with the Gq alpha subunit

The diversity in the heterotrimeric G protein alpha, beta, and gamma subunits may allow selective protein-protein interactions and provide specificity for signaling pathways. We examined the ability of five alpha subunits (alphai1, alphai2, alphao, alphas, and alphaq) to associate with three beta subunits (beta1, beta2, and beta5) dimerized to a gamma2 subunit containing an amino-terminal hexahistidine-FLAG affinity tag (gamma2HF). Sf9 insect cells were used to overexpress the recombinant proteins. The hexahistidine-FLAG sequence does not hinder the function of the beta1gamma2HF dimer as it can be specifically eluted from an alphai1-agarose column with GDP and AlF4-, and purified beta1gamma2HF dimer stimulates type II adenylyl cyclase. The beta1gamma2HF and beta2gamma2HF dimers immobilized on an anti-FLAG affinity column bound all five alpha subunits tested, whereas the beta5gamma2HF dimer bound only alphaq. The ability of other alpha subunits to compete with the alphaq subunit for binding to the beta5gamma2HF dimer was tested. Addition of increasing amounts of purified, recombinant alphai1 to the alphaq in a Sf9 cell extract did not decrease the amount of alphaq bound to the beta5gamma2HF column. When G proteins in an extract of brain membranes were activated with GDP and AlF4- and deactivated in the presence of equal amounts of the beta1gamma2HF or beta5gamma2HF dimers, only alphaq bound to the beta5gamma2HF dimer. The alphaq-beta5gamma2HF interaction on the column was functional as GDP, and AlF4- specifically eluted alphaq from the column. These results indicate that although the beta1 and beta2 subunits interact with alpha subunits from the alphai, alphas, and alphaq families, the structurally divergent beta5 subunit only interacts with alphaq.

Sodium channel in human malignant hyperthermia

BACKGROUND

The abundance of a specific sodium channel subunit (SkM2) appeared to be altered in vitro in cell cultures from persons susceptible to malignant hyperthermia. This study sought to determine whether these findings are artifacts of cell culture or whether they may be relevant to malignant hyperthermia.

METHODS

Regulation of transcript levels of SkM2, a specific sodium channel alpha-subunit, was determined by mRNA analysis. Functional SkM2 protein was estimated in biopsy sections of vastus lateralis muscle by inhibiting the directly elicited muscle twitch by tetrodotoxin, which can differentiate at least three sodium currents in skeletal muscle.

RESULTS

The transcript levels of SkM2 were depressed by 115-fold in six of seven persons susceptible to malignant hyperthermia; and the functional expression of the SkM2 protein, based on the tetrodotoxin sensitivity of the directly elicited twitch, was decreased by at least fourfold in muscle from persons susceptible to malignant hyperthermia compared with persons who were not susceptible.

CONCLUSIONS

As in previous studies in cell culture, altered mRNA and/or the functional expression of a specific subunit of the sodium channel (SkM2) was found in biopsy sections of muscle from all 12 persons examined who were susceptible to malignant hyperthermia but in none of the 16 nonsusceptible participants. Human malignant hyperthermia is a heterogeneous disorder, and the down-regulation of SkM2 may be involved in the final common pathway through which mutations in any one of several proteins, including the ryanodine receptor, could render a person susceptible.

Melittin and phospholipase A2 from bee (Apis mellifera) venom cause necrosis of murine skeletal muscle in vivo

Melittin and phospholipase A2 (PLA2) from bee (Apis mellifera) venom were rested for their ability to induce necrosis of skeletal muscle cells after intramuscular injection into mice. Light and electron microscopic examination of tissue indicated that both melittin (4 micrograms/g) and bee venom PLA2 (4 micrograms/g) caused necrosis of skeletal muscle cells within 30 min after i.m. injection. Early changes in the cells consisted of delta lesions, indicating a ruptured plasma membrane, and hypercontraction of myofibrils. By 24 hr the affected cells appeared as an amorphous mass of disorganized and disrupted myofibrils contained in an intact basal lamina. To ensure that the myotoxic activity of the melittin preparation was not due to contaminating. PLA2 activity, the preparation was treated with p-bromophenacyl bromide (p-BPB), a known inhibitor of PLA2 activity. The p-BPB-treated melittin was determined to have no detectable PLA2 activity using a sensitive muscle cell culture assay, and it still induced myonecrosis, although to a lesser extent and of a slower onset. Additionally, p-BPB treatment of purified bee venom PLA2 completely inhibited its myotoxic activity. These results indicate that both melittin and bee venom PLA2 are capable of inducing necrosis of skeletal muscle cells upon i.m. injection, and that the catalytic and myotoxic activities of bee venom PLA2 are inihibited by p-BPB. Also, melittin and contaminating PLA2 in the melittin fraction may be acting synergistically to induce a stronger and more rapid myotoxic effect than occurs with either alone.

Similarities and differences in mechanisms of cardiotoxins, melittin and other myotoxins

Myonecrosis induced in vivo by cardiotoxin, melittin, and Asp49 and Lys49 phospholipase A2 (PLA2) myotoxins involves rapid lysis of the sarcolemma, myofibril clumping, and hypercontraction of sarcomeres. In contrast, skeletal muscle necrosis induced by crotamine and myotoxin a is much slower, consisting of mitochondrial and sarcoplasmic reticulum swelling, myofibril degeneration, and lack of sarcolemma or transverse tubule damage. The mechanisms contributing to the myonecrosis induced by these peptides were evaluated. Two cardiotoxins and two Lys49 PLA2 myotoxins lysed primary cultures of human skeletal muscle within 24 hr at a concentration of 0.25 microM, while melittin, crotamine, and myotoxin a, and an Asp49 PLA2 myotoxin were non-cytolytic at concentrations up to 5.0 microM, suggesting that cytolysis is not a good measure of myotoxicity. Crotamine and the Lys49 PLA2 myotoxin altered Ca2+ ion flux in human heavy sarcoplasmic reticulum by opening the ryanocine receptor. Whole-cell patch-clamp studies demonstrated that administrating crotamine intracellularly increased Na+ currents. Free fatty acids, liberated by activation of tissue phospholipase C or by the PLA2 activity of the myotoxins, were monitored for crotamine, myotoxin a and a Lys49 PLA2 myotoxin in cell cultures in which the lipids had been radiolabeled. Only the Lys49 myotoxin produced significant amounts of fatty acid in cell cultures, supporting a potential role for fatty acid production only in the mechanism of sarcolemma-destroying myotoxins. These findings, coupled with those in the literature, support a hypothesis in which the myotoxins and/or products of lipase activity (e.g. fatty acids) are acting at a site existing on both the Na+ channel and a protein involved in Ca2+ release and probably serving a modulatory function for ion regulation. Based on the similarities in mechanisms between the toxins and fatty acids, the most likely site would be a fatty acid binding site on the protein (either similar to that on fatty acid binding proteins, or an acylated cysteine residue) or in the membrane.

Modulation of human muscle sodium channels by intracellular fatty acids is dependent on the channel isoform

Free fatty acids (FFAs), including arachidonic acid (AA), are implicated in the direct and indirect modulation of a spectrum of voltage-gated ion channels. Skeletal muscle sodium channels can be either activated or inhibited by FFA exposure; the response is dependent on both FFA structure and site of exposure. Recombinant human skeletal muscle sodium channels (hSkM1) were transfected into heterologous human renal epithelium HEK293t cells. Cytoplasmic delivery of 5 microM AA augmented the voltage-activated sodium current of hSkM1 channels by 190% (+/-54 S.E., n = 7) over a 20-min period. Similar results were seen with 5 microM oleic acid. Sodium currents in HEK293t cells transfected with human cardiac muscle sodium channels (hH1) were insensitive to AA treatment, and exposure to oleic acid inhibited the hH1 currents over a 20-min period by 29% (+/-13 S.E., n = 5). The increase in hSkM1 current was not accompanied by shifts in voltage dependence of activation, steady-state inactivation, or markedly altered kinetics of inactivation of the macroscopic current. The FFA-induced increase in sodium currents was not dependent on protein kinase C activity. In contrast, both isoforms were reversibly inhibited by external application of unsaturated FFA. Thus, the differential effects of FFA on skeletal muscle sodium channels first noted in cultured muscle cells can be reproduced by expressing recombinant sodium channels in epithelial cells. Although the responses to applied FFAs could be direct or indirect, we suggest that: 1) SkM1 has two classes of response to FFA, one which produces augmentation of macroscopic currents with intracellular FFA, and a second which produces inhibition with extracellular FFA; 2) H1 has only one class of response, which produces inhibition with extracellular FFA. A testable hypothesis is that the presence or absence of each response is due to a specific structure in SkM1 or H1. These specific structures may directly interact with FFA or may interact with intermediate components.

Role of the prenyl group on the G protein gamma subunit in coupling trimeric G proteins to A1 adenosine receptors

The coupling of receptors to heterotrimeric G proteins is determined by interactions between the receptor and the G protein alpha subunits and by the composition of the betagamma dimers. To determine the role of the gamma subunit prenyl modification in this interaction, the CaaX motifs in the gamma1 and gamma2 subunits were altered to direct modification with different prenyl groups, recombinant betagamma dimers expressed in the baculovirus/Sf9 insect cell system, and the dimers purified. The activity of the betagamma dimers was compared in two assays: formation of the high affinity agonist binding conformation of the A1 adenosine receptor and receptor-catalyzed exchange of GDP for GTP on the alpha subunit. The beta1gamma1 dimer (modified with farnesyl) was significantly less effective than beta1gamma2 (modified with geranylgeranyl) in either assay. The beta1gamma1-S74L dimer (modified with geranylgeranyl) was nearly as effective as beta1gamma2 in either assay. The beta1gamma2-L71S dimer (modified with farnesyl) was significantly less active than beta1gamma2. Using 125I-labeled betagamma subunits, it was determined that native and altered betagamma dimers reconstituted equally well into Sf9 membranes containing A1 adenosine receptors. These data suggest that the prenyl group on the gamma subunit is an important determinant of the interaction between receptors and G protein gamma subunits.

G protein gamma subunits with altered prenylation sequences are properly modified when expressed in Sf9 cells

The gamma subunits of heterotrimeric G proteins undergo post-translational prenylation and carboxylmethylation after formation of the betagamma dimer, modifications that are essential for alpha-betagamma, betagamma-receptor, and betagamma-effector interactions. We have determined the specific prenyl group present on the beta1gamma1, beta1gamma2, and beta1gamma3 dimers purified from baculovirus-infected Sf9 cells by specific binding to G protein alpha subunits immobilized on agarose. These recombinant dimers undergo the same post-translational modifications determined for gamma1 and gamma2 isolated from mammalian tissues. Furthermore, infection of Sf9 cells with a recombinant baculovirus encoding an alteration of the gamma1 CaaX sequence (gamma1-S74L) resulted in geranylgeranylation of the resulting gamma1 subunit, and alteration of the gamma2 CaaX sequence to CAIS (gamma2-L71S) resulted in farnesylation. Both of these altered gamma subunits were able to associate stably with beta1, and the resulting betagamma dimer bound tightly to alpha-agarose and eluted specifically with aluminum fluoride. These results indicate that Sf9 insect cells properly process the CaaX motif in G protein gamma subunits and are a useful model system to study the role of prenylation in the protein-protein interactions in which the betagamma subunits participate.

Altered sodium current response to intracellular fatty acids in halothane-hypersensitive skeletal muscle

Biopsies of human skeletal muscle were analyzed by an in vitro contracture test (IVCT) for responsiveness to a halothane challenge: noncontracting (nonresponsive; IVCT-) and contracting (IVCT+). A muscle biopsy that is IVCT+ indicates potential malignant hyperthermia (MH) susceptibility. Primary cultures were grown from portions of the skeletal muscle biopsies, and voltage-activated currents were measured by whole cell recording in the presence or absence of 2-5 microM intracellular arachidonic or oleic acids. In untreated IVCT- cells, Na+ currents were predominantly tetrodotoxin (TTX) insensitive, indicating that most of the current was carried through the embryonic SkM2 isoform of the Na+ channel. Inclusion of fatty acids in the recording pipette of IVCT- cells produced an increase in voltage-activated Na+ currents during 20 min of recording. Approximately 70% of currents in fatty acid-treated cells were TTX sensitive, indicating activation of the adult SkM1 isoform of the Na+ channel. In contrast to IVCT- cells, IVCT+ cells expressed Na+ currents that were predominantly TTX sensitive even in the absence of added fatty acid, thus showing a relatively large baseline functional expression of SkM1 channels. Addition of fatty acids to the recording pipette produced little further change in the magnitude or TTX sensitivity of the whole cell currents in IVCT+ cells, suggesting altered functional regulation of Na+ channels in MH muscle.

Anaesthetic management of the Kenny-Caffey syndrome using the laryngeal mask

We report the anaesthetic management of a nine-year-old, 6.8 kg, 75 cm tall female with the Kenny-Caffey syndrome presenting for strabismus surgery. Dysmorphic features in our patient included a hypoplastic mandible. A neonatal (size 1) laryngeal mask was successfully used for management of the airway whilst providing surgical access. The general features of this rare syndrome are presented and the literature reviewed.

Presynaptically acting snake venom phospholipase A2 enzymes attack unique substrates

Synaptosomes were incubated with bovine serum albumin (BSA) to examine whether the presynaptic action of snake venom phospholipase A2 (PLA2) toxins is due either to the release of fatty acids resistant to extraction by BSA or to the liberation of a specific fatty acid type. In the presence of BSA (0.5% or 1.0%) two PLA2 enzymes from Naja naja atra and Naja naja kaouthia snake venoms that do not have a predominant presynaptic action at the neuromuscular junction (PS-) did not stimulate acetylcholine (ACh) release from synaptosomes. In contrast, two PLA2 enzymes (beta-bungarotoxin, scutoxin) that do have a predominant presynaptic action at the neuromuscular junction (PS+) did stimulate ACh release. BSA did not antagonize PS- enzymes by more efficiently extracting the fatty acids produced by these enzymes relative to PS+ enzymes. While absolute amounts of total and unsaturated fatty acid produced overlapped for the PS- and PS+ enzymes, the two PS+ enzymes produced a significantly greater absolute amount and relative percentage of palmitic acid (16:0) than did either of the PS- enzymes. However, the levels of free palmitic acid remaining in the synaptosomes where they would exert effects on ACh release were similar for the N. n. kaouthia PLA2 (PS-) and beta-bungarotoxin (PS+). Therefore, the total (supernatant plus synaptosomal) amount of palmitic acid produced per se did not account for stimulation of ACh release, since the greater amounts produced by the PS+ enzymes were removed from the synaptosomes by BSA. The production of higher levels of palmitic acid suggests either that PS+ enzymes gain access to sites containing phospholipid substrates unavailable to the PS- enzymes, or that they have a different substrate preference. These findings suggest new possibilities for the mechanism of PS+PLA2 action, including site-directed enzymatic activity and protein acylation.

No role for enzymatic activity or dantrolene-sensitive Ca2+ stores in the muscular effects of bothropstoxin, a Lys49 phospholipase A2 myotoxin

The role of low levels of phospholipase A2 (PLA2) activity and intracellular Ca2+ stores in the pharmacological action of bothropstoxin (BthTX), a myotoxic Lys49 PLA2 homologue isolated from the venom of Bothrops jararacussu, was investigated. We examined the muscular effects of BthTX in the mouse diaphragm and its PLA2 activity in radiolabeled human and rat primary cultures of skeletal muscle. Although it is a Lys49 PLA2 homologue, BthTX had a low, but easily detectable, level of enzymatic activity relative to two Asp49 PLA2 enzymes from Naja naja kaouthia and Naja naja atra venoms, and this activity was reduced by about 85% in the presence of Sr2+ (4.0 mM). However, the replacement of 1.8 mM Ca2+ by 4 mM Sr2+ did not alter the BthTX-induced contracture and blockade of the muscle twitch tension. In addition, Sr2+ decreased by 50% the time required to cause 50% paralysis, and evoked approximately a four-fold increase in the number of spontaneous spikes. In isolated sarcoplasmic reticulum preparations, BthTX opened the intracellular Ca2+ release channel (ryanodine receptor) and lowered the threshold of Ca(2+)-induced Ca2+ release by a second, as yet unidentified, mechanism. However, in intact muscle, dantrolene, an antagonist of some forms of intracellular Ca2+ release, had no effect on the actions of BthTX. These findings do not support any role for the low levels of PLA2 activity, or dantrolene-sensitive intracellular Ca2+ stores, in the action of BthTX. The mechanism whereby Sr2+ stimulates the pharmacological activity of BthTX remains to be clarified.

Genotype and phenotype relationships for mutations in the ryanodine receptor in patients referred for diagnosis of malignant hyperthermia

Anaesthesia-induced malignant hyperthermia (MH) may be caused by specific gene defects in the skeletal muscle ryanodine receptor. We have studied the frequency of occurrence of the C1840T mutation, analogous to the porcine mutation, and three mutations associated both with MH and central core disease (G7301A, C487T and C1209G). We investigated skeletal muscle specimens from up to 137 patients testing negative and 101 patients testing positive for MH susceptibility by the North American MH Group protocol. The presence or absence of the mutations was determined by polymerase chain reaction and restriction enzyme digestion. The frequencies of occurrence of the C1840T and C487T mutations were 2% and 1%, respectively, in MH-positive subjects and were the only two mutations identified. One subject with central core disease did not have any of the three mutations examined associated with this disorder. Therefore, the porcine and central core disease-associated mutations examined in the ryanodine receptor account for a small proportion (approximately 3%) of MH-positive diagnoses. The mutations examined did not occur in any of the MH-negative patients, supporting an association between defects in the ryanodine receptor and a positive diagnosis for MH.(ABSTRACT TRUNCATED AT 250 WORDS)

Bovine serum albumin does not completely block synaptosomal cholinergic activities of presynaptically acting snake venom phospholipase A2 enzymes

Bovine serum albumin (BSA), which binds fatty acids, was used to test the contribution of free fatty acid to the presynaptic toxicity of phospholipase A2 (PLA2) enzymes. The effects of BSA on inhibition of [14C]choline uptake and stimulation of [14C]acetylcholine (ACh) release in synaptosomes by PLA2 enzymes that do not have a predominant presynaptic action at the neuromuscular junction (PS-) were compared with those on the cholinergic actions of PLA2 enzymes that do have a predominant presynaptic action at the neuromuscular junction (PS+). The inhibition of choline uptake by the Naja naja atra PLA2, a PS- PLA2, was completely antagonized by BSA (0.5%); whereas that by beta-bungarotoxin, a PS+ PLA2, was unaffected by BSA. The inhibition of choline uptake by two other PS+ PLA2 toxins (scutoxin and pseudexin) was partially antagonized by BSA. The effects of the PLA2 enzymes were antagonized in the same manner by BSA whether on Na(+)-dependent or on Na(+)-independent choline uptake. Likewise, the stimulation of ACh release by two PS- PLA2 enzymes (from Naja naja atra and Naja naja kaouthia snake venoms) was completely blocked by BSA; whereas that by beta-bungarotoxin was unaffected and that by scutoxin and pseudexin was only partially antagonized by BSA. The results suggest that the PS- PLA2 enzymes are completely dependent on fatty acid production for their cholinergic toxicity and that BSA can be used to investigate further the neurotoxic mechanisms of PS+ PLA2 enzymes in synaptosomes.

Effect of phenytoin on skeletal muscle from quarter horses with hyperkalaemic periodic paralysis

The contractile activity, the threshold for calcium-induced calcium release in fractions of sarcoplasmic reticulum and the potassium concentration were determined in preparations of semimembranosus muscle from normal quarter horses and quarter horses with hyperkalaemic periodic paralysis before and after they were treated with phenytoin. Before the treatment there was no difference in caffeine contracture or electrically elicited twitch response between the two groups. For one week after the treatment, the time to peak tension of caffeine contractures was significantly (P < 0.005) reduced in the horses with hyperkalaemic periodic paralysis but unchanged in the normal horses. The variance but not the mean values for the threshold for Ca(2+)-induced Ca2+ release from the sarcoplasmic reticulum was greater for the horses with hyperkalaemic period paralysis before but not after the treatment with phenytoin.

Masseter muscle rigidity associated with glycine1306-to-alanine mutation in the adult muscle sodium channel alpha-subunit gene

BACKGROUND

Succinylcholine-induced masseter muscle rigidity (MMR) is a potentially life-threatening complication of anesthesia and is closely correlated with the heterogeneous disorder malignant hyperthermia (MH) susceptibility. MMR also is identified with a variety of neuromuscular disorders, including the myotonias, that are associated with abnormal in vitro contracture test (IVCT) results. Recently, mutations in the adult skeletal muscle sodium channel alpha-subunit gene (SCN4A) have been shown to cause generalized nondystrophic myotonias, some of which are associated with mild nonspecific symptoms. The purpose of the current investigation was to begin to evaluate the molecular genetic relationship between known mutations in the SCN4A gene, MMR, and the results of the IVCT used to diagnose MH-susceptibility.

METHODS

A single extended pedigree of 16 individuals was ascertained through a proband who experienced MMR and whole-body rigidity after succinylcholine administration. Subsequently, four individuals were shown to have a mild form of myotonia on clinical and laboratory examination. IVCT was carried out according to standardized protocols. Mutations in the SCN4A gene were sought in exons 22 and 24 using single-strand conformational analyses. Variability in the SCN4A gene sequence was confirmed by direct DNA sequence analyses.

RESULTS

Four individuals with myotonia were shown to carry a guanine-to-cytosine mutation at nucleotide position 3917 of the reported SCN4A sequence. This DNA mutation was coinherited with MMR and an abnormal IVCT result in this family. Previous studies have demonstrated that the glycine1306-to-alanine substitution is associated with a mild clinical syndrome referred to as myotonia fluctuans.

CONCLUSIONS

The current report provides direct evidence that succinylcholine-induced MMR, whole-body rigidity, and an abnormal IVCT result are associated with a mutation in the SCN4A gene.

Antibodies having markedly different effects on enzymatic activity and induction of acetylcholine release by two presynaptically-acting phospholipase A2 neurotoxins

The enzymatic and acetylcholine-releasing activities of two presynaptically-acting phospholipase A2 neurotoxins (pseudexin B and scutoxin) were studied in a synaptosomal fraction. Scutoxin (100 nM) induced greater [14C]acetylcholine release than did pseudexin B (100 nM). Both toxins caused fatty acid production in the synaptosomal fraction, although pseudexin B was more active than scutoxin. One monoclonal antibody raised against pseudexin B (#4) had no effect on the enzymatic activity of either pseudexin B or scutoxin. Two other monoclonal antibodies (#3 and #7), also raised against pseudexin B, antagonized the enzymatic activity of pseudexin B and scutoxin. Monoclonal antibody #3 was more effective than #7 in reducing the amount of acetylcholine released by the toxins, whereas #7 was more effective than #3 in reducing fatty acid production. Although antibody #3 caused complete inhibition of phospholipase A2 activity of pseudexin B on purified substrates, it only reduced phospholipase A2 activity by 35% in synaptosomes. These findings support the hypothesis that gross phospholipase A2 activity does not play a role in stimulation of acetylcholine release by the presynaptically-acting phospholipase A2 neurotoxins.

An update on the malignant hyperthermia syndrome

Malignant hyperthermia (MH) is a pharmacogenetic disorder of skeletal muscle. In humans, MH is inherited in an autosomal dominant fashion; in swine, the principal model for MH, it is in a recessive fashion. Those with MH susceptibility usually are asymptomatic except in the presence of certain "triggering" anaesthetic agents such as isoflurane, enflurane and the muscle relaxant succinylcholine. Upon such exposure hypermetabolism, increased CO2 production, acidosis, muscle rigidity, rhabdomyolysis and hyperthermia occur. Untreated, death may result in 70% of patients. With prompt diagnosis and treatment with dantrolene sodium, the mortality is less than 10%. The overall incidence of MH is low (perhaps 1:50,000 anaesthetics), but it is more common in children. Children also display a paradoxical increase in jaw muscle tone to succinylcholine which often presages MH, but confusing clinically, may also be a normal response to succinylcholine. The pathophysiology of MH centres around a defect in calcium flux in skeletal muscle. A specific base pair change in the gene that codes for the ryanodine receptor calcium channel in muscle has been demonstrated in susceptible swine, but occurs rarely in humans. It is hoped that the understanding of the molecular genetics of MH will lead to a simpler diagnostic test than is currently available, and enhance our understanding of MH and its relation to other myopathies.

Masseter muscle rigidity and malignant hyperthermia susceptibility in pediatric patients. An update on management and diagnosis

BACKGROUND

Controversy exists regarding the definition of masseter muscle rigidity (MMR) and anesthetic management after MMR. This study reports current anesthetic management after MMR, estimates the incidence of clinical malignant hyperthermia (MH) in patients with MMR, and is the first to evaluate the coincidence of MMR with malignant hyperthermia susceptibility (MHS) according to the 1987 North American Malignant Hyperthermia Group protocol.

METHODS

Practicing anesthesiologists referred pediatric patients for biopsy between 1986 and 1991 based on evidence of MMR after succinylcholine (1975-1991). The clinical scenario was described as MMR alone or MMR followed by signs of MH, including arterial CO2 tension > 50 mmHg, arterial pH < or = 7.25, and base deficit > 8. Patients had caffeine-halothane muscle contracture testing to determine MHS.

RESULTS

Seventy patients (50 boys and 20 girls) were evaluated. Eighty-three percent (58 of 70) of anesthetics were halothane-succinylcholine. In 68% (48 of 70) of cases, the anesthetic was discontinued, whereas anesthesia was continued with nontriggering agents in 11% (8 of 70) and with triggering agents in 13% (9 of 70). Fifty-nine percent (41 of 70) of patients were diagnosed as MHS by muscle biopsy. In 7% (5 of 70) of patients, clinical MH developed within 10 min of MMR.

CONCLUSIONS

This study, by using the current North American Malignant Hyperthermia Group protocol, reaffirms the high incidence (59%, 41 of 70) of MHS associated with MMR as confirmed by muscle biopsy. Of the MHS patients, 5 developed signs of clinical MH. Most anesthesiologists in this study, when confronted with MMR, discontinued anesthesia. Because of the potential lethality of MH and the > 50% concordance between MMR and MHS, the most conservative course of action after MMR is to discontinue the anesthetic and observe the patient for clinical evidence of MH. An acceptable alternative, depending on the urgency of the surgery, would be to continue anesthesia with nontriggering agents for MH, with appropriate monitoring.

Pretreatment with alfentanil reduces pain caused by propofol

We have compared two groups (n = 22) of unpremedicated patients to determine if the pain caused by injection of propofol could be modified by alfentanil. In group I, alfentanil 1 mg was given as a bolus i.v. injection 15 s before administration of propofol i.v., while group II received saline. Propofol was given in 20-mg increments every 4 s. All injections were given through the same i.v. cannula on the dorsum of one hand. We found that alfentanil pretreatment reduced pain on injection of propofol (P = 0.001).

Myotoxin a reduces the threshold for calcium-induced calcium release in skeletal muscle

Myotoxin a, isolated from the venom of the prairie rattlesnake Crotalus viridis viridis, induces necrosis in skeletal muscle. In isolated organelles, it has been reported that myotoxin a reduces Ca2+ uptake into the sarcoplasmic reticulum. The effects of the toxin on Ca2+ regulation were examined in heavy sarcoplasmic reticulum fractions from human and equine skeletal muscle. Ca2+ uptake and release (the threshold of Ca(2+)-induced Ca2+ release) were examined by dual wavelength spectrophotometry. The toxin lowered the threshold of Ca(2+)-induced Ca2+ release in a dose-dependent manner (1-10 microM) and this effect was antagonized by ruthenium red, a Ca2+ release channel blocker. Ca2+ uptake into equine heavy sarcoplasmic reticulum was not decreased by myotoxin a (10 microM) when Ca2+ release was blocked by ruthenium red. [3H]Ryanodine binding to equine heavy sarcoplasmic reticulum was converted from a relatively low affinity state to a higher affinity state by myotoxin a. These results suggest that the dominant effect of myotoxin a is to increase the Ca2+ sensitivity for the opening of the calcium release channel (ryanodine receptor). Myotoxin a may prove to be a useful tool to probe the modulation of calcium release in sarcoplasmic reticulum fractions.