Malignant hyperthermia: effects of halothane on the surface membrane

Voltage modulates halothane-triggered Ca2+ release in malignant hyperthermia-susceptible muscle

Malignant hyperthermia can result from mutations in the ryanodine receptor that favor anesthetic-induced Ca²⁺ release. Zullo et al. find that membrane potential modulates the effect of the volatile anesthetic halothane on skeletal muscle ryanodine receptors possessing the Y524S mutation.

Malignant hyperthermia (MH) is a fatal hypermetabolic state that may occur during general anesthesia in susceptible individuals. It is often caused by mutations in the ryanodine receptor RyR1 that favor drug-induced release of Ca²⁺ from the sarcoplasmic reticulum. Here, knowing that membrane depolarization triggers Ca²⁺ release in normal muscle function, we study the cross-influence of membrane potential and anesthetic drugs on Ca²⁺ release. We used short single muscle fibers of knock-in mice heterozygous for the RyR1 mutation Y524S combined with microfluorimetry to measure intracellular Ca²⁺ signals. Halothane, a volatile anesthetic used in contracture testing for MH susceptibility, was equilibrated with the solution superfusing the cells by means of a vaporizer system. In the range 0.2 to 3%, the drug causes significantly larger elevations of free myoplasmic [Ca²⁺] in mutant (YS) compared with wild-type (WT) fibers. Action potential–induced Ca²⁺ signals exhibit a slowing of their time course of relaxation that can be attributed to a component of delayed Ca²⁺ release turnoff. In further experiments, we applied halothane to single fibers that were voltage-clamped using two intracellular microelectrodes and studied the effect of small (10-mV) deviations from the holding potential (−80 mV). Untreated WT fibers show essentially no changes in [Ca²⁺], whereas the Ca²⁺ level of YS fibers increases and decreases on depolarization and hyperpolarization, respectively. The drug causes a significant enhancement of this response. Depolarizing pulses reveal a substantial negative shift in the voltage dependence of activation of Ca²⁺ release. This behavior likely results from the allosteric coupling between RyR1 and its transverse tubular voltage sensor. We conclude that the binding of halothane to RyR1 alters the voltage dependence of Ca²⁺ release in MH-susceptible muscle fibers such that the resting membrane potential becomes a decisive factor for the efficiency of the drug to trigger Ca²⁺ release.

Death during or following surgical procedure and the allegation of medical negligence: An overview

A great variety of mishaps can occur during or following the administration of anesthesia and operative or investigational procedures that do not necessarily convey an error of judgment or negligence on the part of the surgeon or the anesthetist. However, all deaths occurring during the course of anesthesia and surgery or within a reasonable period thereafter (commonly referred to as peri-operative period) have to be reported to the police as these deaths cannot be regarded as natural. Despite the death occurring due to some preexisting disease or some co-existent condition, there may be a tendency on the part of the relatives of the deceased to impute negligence on the part of the anesthetist and/or the surgeon merely because of the fact that the death was closely associated with the anesthesia and surgical intervention. As such, any death suspected to be caused, or contributed to, by any of these procedures needs to be adequately investigated both from the point of view of the relatives of the deceased as well as instituting future safety measures. This paper examines the medical, ethical and legal aspects of such deaths with reference to the allegations of medical negligence in these cases.

Xenon does not induce contracture in human malignant hyperthermia muscle

Xenon has many characteristics of an ideal anaesthetic agent. It is not known whether xenon is a safe alternative to the potent inhalational anaesthetics in patients susceptible to malignant hyperthermia (MH). We investigated the effect of xenon, halothane and caffeine on muscle specimens of 31 individuals, referred to the MH Unit of the University of Ulm, and performed genetic epidemiology. Thirteen individuals were classified as MH susceptible and 18 as MH negative. Xenon 70% did not cause an increase in baseline tension of any MH-susceptible muscle specimen in contrast to halothane and caffeine. The evoked twitch response increased transiently in MH-susceptible and normal specimens indicating a mechanism independent of MH susceptibility. These results suggest that xenon, in concentrations up to 70% may be a safe anaesthetic for MH-susceptible patients.

Malignant hyperthermia: fatigue characteristics of skeletal muscle

Although the defects in cellular Ca(2+) homeostasis associated with malignant hyperthermia (MH) have been extensively studied, the functional consequences of the MH mutation are not clear. We used continuous and intermittent high-frequency stimulation to determine whether this mutation might alter the fatigue resistance of muscle from MH susceptible (MHS) pigs. Force decline with 10 s continuous stimulation (150 Hz) was significantly less in MHS muscle (58.4 +/- 1.0%) than in normal muscle (50.5 +/- 3.0%). With intermittent stimulation, there was no significant difference in tension decline between muscle types. Post-stimulation twitch and tetanus responses were similar in MHS and normal muscles except: 1) twitch potentiation was significantly greater in normal muscle after continuous stimulation, and 2) recovery of tetanic tension was slowed in MHS muscle. Although the MH defect does not cause major functional abnormalities, subtle differences in MHS muscle response to fatiguing stimulation are apparent. Therefore, it is unlikely the work capacity of MH patients would be limited by any MH associated defect within the muscle.

Altered mechanical responses of malignant hyperthermic skeletal muscle during repetitive stimulation

This investigation examined the mechanical responses of malignant hyperthermic (MH) and normal porcine skeletal muscle to repetitive stimulation. Twitch and maximal tetanic tensions were not significantly different between muscle types. Tensions produced during stimulation at 20-80 Hz were significantly less in MH muscle than in normal muscle. In addition, MH muscle showed significantly greater force decline (tetanic fade) at the end of contractions evoked by 20-80 Hz stimulation. When stimulated to fatigue, both normal and MH muscle exhibited similar rates of tension decline during the initial minutes. Further stimulation caused additional decline in normal muscle, but a tension plateau in MH muscle. In all cases, normal muscle had greater magnitudes of fatigue than did MH muscle. Results show that there are marked differences between MH and normal muscle in the mechanical responses to repetitive stimulation. Due to its inability to properly regulate intracellular Ca2+ exchange, it is possible that MH muscle might be a useful tool for identifying the mechanisms of muscle fatigue in normal muscle.

Free radicals and calcium homeostasis: relevance to malignant hyperthermia?

The regulation of intracellular free calcium ions (Ca2+) in skeletal muscle at rest and during contraction depends on mechanisms such as Na(+)-Ca2+ exchangers, Ca(2+)-ATPases, and the voltage-sensitive ryanodine receptor. The susceptibility of these regulatory mechanisms to free-radical-mediated damage may be increased because of their location within the lipid membranes of sarcolemma, sarcoplasmic reticulum, and mitochondrion with resultant uncontrolled increases in myoplasmic Ca2+ concentration and cell death. The potentially fatal pharmacogenetic disorder, malignant hyperthermia (MH), is characterised by muscle rigidity, arrhythmias, lactic acidosis, and a rapid rise in body temperature. The sequence of events responsible for the MH syndrome remains uncertain, but it has been variously ascribed to faults in many of the Ca2+ regulatory mechanisms. In swine the condition is associated with a specific mutation in the ryanodine receptor, whereas in humans the syndrome is genetically heterogenous. Free-radical-mediated peroxidation of membrane lipids and proteins also results in the rapid efflux of Ca2+ from organelles, and the detection of products of free radical reactions in tissue from MH-susceptible individuals using electron spin resonance spectroscopy provides evidence for the involvement of free radicals in the MH syndrome.

Separate sites for the dantrolene-induced inhibition of contracture of the rat diaphragm preparation due to depolarization or to caffeine

Addition of dantrolene 8.5 x 10(-5) M caused a mono-exponential decay of the depolarization contractures caused by inhibition of the sarcolemmal Na,K-ATPase with propranolol 1 mM or by depolarization of the sarcolemma and T tubular membranes with KCl 100 mM. The half-times of the inhibitory effects were 6 s for the propranolol contracture and 11 s for the KCl contracture. The inhibition of both contractures was complete. Inhibition of the caffeine (10 mM) contracture was bi-exponential with half-times of 45 s and 9.5 min. Inhibition was incomplete; 29.6 +/- 5.0% of the contracture tension could not be inhibited. The inhibition of twitch contractions was similar to that of the caffeine contracture, with half-times of 48 s and 9.1 min, and 20.6 +/- 1.2% of the initial twitch tension could not be inhibited. The contracture tensions induced by release of Ca from the mitochondria with dicumarol, and by actin-myosin binding with the sulfhydryl inhibitor, N-ethyl-maleimide, could not be inhibited by dantrolene. The present results indicate that dantrolene inhibits depolarization signals from the sarcolemma and the T tubular membranes, in addition to inhibition of the coupling between the T tubules and the sarcoplasmic reticulum, and of the release of Ca from the sarcoplasmic reticulum. All these effects of dantrolene may contribute to its therapeutic effect in malignant hyperthermia.

Malignant hyperthermia: an altered phospholipid and fatty acid composition in muscle membranes

There is thought to be a genetic defect within the calcium release channel of the sarcoplasmic reticulum in malignant hyperthermia (MH). This primary alteration is hypothesized to influence the function and/or structure of various muscle membrane systems; e.g., to have a direct effect on the composition of the lipid matrix. Therefore, in striated muscle samples, we determined the quantity and fatty acid composition of the various types of membrane phospholipids. German Landrace pigs were classified as normal or susceptible to MH. Total lipid content from longissimus dorsi, semi-membranosus muscle, and heart left ventricular (HLV) samples were extracted with chloroform/methanol and subsequently separated by high performance liquid chromatography. The single phospholipid fractions were collected and, following derivatization, the quantities of individual fatty acids were determined using a capillary gas chromatographic method. In general, samples from the susceptible pigs contained lower absolute amounts of individual phospholipids. The most notable differences occurred in the HLV, where phosphatidylinositol, phosphatidylserine, phosphatidylethanolamine and sphingomyelin were all significantly less (P less than or equal to 0.05). The muscle from the susceptible animals also contained decreased amounts of the polyunsaturated phospholipid-bound fatty acids (P less than or equal to 0.05). These differences in phospholipid and fatty acid concentrations of membranes isolated from swine susceptible to MH may relate to their apparently increased sensitivities to halothane (e.g., fluidizing effects) or elevated temperature.

Multiple-pulse stimulation and dantrolene in malignant hyperthermia

A potentially fatal condition, yet preventable, malignant hyperthermia (MH) lacks a satisfactory noninvasive diagnostic test. Studying the effects of intravenous dantrolene (3 mg/kg) on electrically stimulated skeletal muscle, we found that this approach does not conclusively distinguish between normal humans and those susceptible to malignant hyperthermia but nonetheless yielded important information about the action of dantrolene in man and in MH. Supramaximal single- and multiple-pulse stimulation of the common peroneal nerve produced stable torque responses of the dorsiflexor muscles (monitored in vivo), which dantrolene suppressed. With the multiple-pulse stimulation (5-6 pulses) this torque suppression was significantly less in MH-susceptible subjects than in control subjects. This distinction, also observed in MH swine, confirms this animal as a good model for human MH. That dantrolene's effect in MH can be more completely reversed with high frequency stimulation is intriguing; presumably, excitation-contraction coupling differs in MH and normal muscle.

Skeletal muscle excitation-contraction coupling. II. Plasmalemma voltage control of intact bundle contractile properties in normal and malignant hyperthermic muscles

Bundles of cells, intact from tendon to tendon, were dissected from muscles of normal and malignant hyperthermia susceptible (MHS) pigs. Intact bundles were stimulated either (1) electrically, to elicit twitches and tetani, or (2) ionically with elevated extracellular K+ (K+0), to elicit K-contractures. Maximal tetanic force was the same for MHS and normal intact bundles. In MHS muscles, when responses were elicited from control resting plasmalemma polarization (4 mM K+0), twitches and K-contractures were significantly larger and the K-contracture activation curve was shifted towards lower [K+]0 with respect to normal bundles. Resting hyperpolarization (2 mM K+0) selectively reduced MHS twitch force to normal and K-contracture force toward normal. For K-contracture and twitches, there was a range of K+0 concentrations (7-10 mM), representing resting depolarization, which enhanced subsequent twitch and K-contracture magnitude in both MHS and normal intact bundles as compared to responses elicited from control (4 mM K+0). These results are consistent with the hypothesis that resting plasmalemma voltage sets the gain of sarcoplasmic reticulum calcium release in MHS and normal intact bundles independent of type of stimulation and suggest that a defect in this mechanism may be responsible for the enhanced twitches and K-contractures of MHS muscles.

The in vitro determination of susceptibility to malignant hyperthermia

To evaluate the reliability of the in vitro contracture test for susceptibility to malignant hyperthermia, we studied muscles from normal pigs and those susceptible to malignant hyperthermia. We performed the contracture test with various muscles from the same animal. Trapezius and intercostal muscles gave similar results, whereas the extensor digiti II muscle had lower sensitivities to both caffeine and halothane. Thus, the muscle chosen to determine susceptibility to malignant hyperthermia is important. In several animals, a false negative diagnosis would have resulted if only the distal muscle had been studied, and this was true even if weak contractures (less than 200 mg) were considered significant. In addition, we compared the response to caffeine or halothane of cut and intact muscle fibers. Although the cut fibers were depolarized, the sensitivity to these drugs was unchanged. Hence, results of the in vitro contracture test are independent of the resting membrane potential.