Malignant hyperthermia: molecular defects in membrane permeability

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.

Calcium ATPase in the sarcoplasmic reticulum of muscle from normal and malignant hyperthermia susceptible pigs

The density of Mg(2+)-dependent Ca2+ ATPase in the terminal cisternae of pig skeletal muscle fibers was investigated to discover whether a reduction in Ca2+ ATPase content impairs Ca2+ sequestration and contributes to the elevated myoplasmic Ca2+ concentration in malignant hyperthermia. Unexpectedly, immunogold electron microscopy showed an increase in Ca2+ ATPase, while densitometry of SDS-polyacrylamide gels suggested that the Ca2+ ATPase content of terminal cisternae vesicles did not change. The affinity of Ca2+ ATPase in vesicles for our monoclonal antibody was not altered. We suggest that the availability of antigenic sites in malignant hyperthermia increases after processing for electron microscopy, perhaps as a consequence of altered sarcoplasmic reticulum membrane properties.

Mechanisms of exercise-induced muscle fibre injury

Exercise for which a skeletal muscle is not adequately conditioned results in focal sites of injury distributed within and among the fibres. Exercise with eccentric contractions is particularly damaging. The injury process can be hypothesised to occur in several stages. First, an initial phase serves to inaugurate the sequence. Hypotheses for the initial event can be categorised as either physical or metabolic in nature. We argue that the initial event is physical, that stresses imposed on sarcolemma by sarcomere length inhomogeneities occurring during eccentric contractions cause disruption of the normal permeability barrier provided by the cell membrane and basal lamina. This structural disturbance allows Ca++ to enter the fibre down its electrochemical gradient, precipitating the Ca++ overload phase. If the breaks in the sarcolemma are relatively minor, the entering Ca++ may be adequately handled by ATPase pumps that sequester and extrude Ca++ from the cytoplasm ('reversible' injury). However, if the Ca++ influx overwhelms the Ca++ pumps and free cytosolic Ca++ concentration rises, the injury becomes 'irreversible'. Elevations in intracellular Ca++ levels activate a number of Ca(++)-dependent proteolytic and phospholipolytic pathways that are indigenous to the muscle fibres, which respectively degrade structural and contractile proteins and membrane phospholipids; for instance, it has been demonstrated that elevation of intracellular Ca++ levels with Ca++ ionophores results in loss of creatine kinase activity from the fibres through activation of phospholipase A2 and subsequent production of leukotrienes. This autogenetic phase occurs prior to arrival of phagocytic cells, and continues during the inflammatory period when macrophages and other phagocytic cells are active at the damage site. The phagocytic phase is in evidence by 2 to 6 hours after the injury, and proceeds for several days. The regenerative phase then restores the muscle fibre to its normal condition. Repair of the muscle fibres appears to be complete; the fibres adapt during this process so that future bouts of exercise of similar type, intensity, and duration cause less injury to the muscle.

Membrane properties of the sarcolemma and sarcoplasmic reticulum of pigs susceptible to malignant hyperthermia. Action of halothane

The role of sarcolemma and sarcoplasmic reticulum in malignant hyperthermia was studied by the esr technique using the trapezius muscle membrane of both normal and genetically susceptible pigs. Normal and malignant hyperthermia membranes from sarcolemma as well as from sarcoplasmic reticulum did not show significant differences near the polar heads of the phospholipidic bilayer. In contrast, the fluidity and activation energy of normal membranes differed from those in malignant hyperthermia; in both sarcolemma and sarcoplasmic reticulum the mobility of the label was greater than the controls. The presence of halothane was examined, by inducing this disease anesthetically. The drug effect confirmed the above results, i.e. the disease affects mainly the hydrophobic core of the lipid bilayer of both sarcolemma and sarcoplasmic reticulum membranes.

Halothane-induced functional and structural modifications in sarcoplasmic reticulum membranes from pig skeletal muscle

We investigated the effect of halothane on lipid and protein components of sarcoplasmic reticulum membranes isolated from pig trapezius muscle. We studied the relationships between the (Ca2(+)-Mg2+)-ATPase activity and the interaction of the anesthetic with lipid and protein moieties by means of EPR and fluorescence spectroscopic techniques. Our results clearly show that below 5 mumol per mg protein, halothane interacts mainly with the lipid components of the membrane. This interaction is shown to be localized in the central core of the phospholipid bilayer and to induce an increase of the membrane calcium permeability. The interaction with protein components only occurs at higher halothane concentrations and affects its conformational and functional states. These results are discussed with respect to new insights into diethylether-SR membrane interaction and to malignant hyperthermia syndrome in the pig.

Calcium accumulation by sarcoplasmic reticulum in whole muscle homogenate preparations of malignant hyperthermia diagnostic patients and pigs

Calcium accumulation by the sarcoplasmic reticulum in whole muscle homogenate preparations of malignant hyperthermia-susceptible (MH+) and non-susceptible (MH-) humans and pigs was investigated using a calcium electrode at 35 degrees C. Sarcoplasmic reticulum of MH+ humans and pigs showed normal Ca2+ accumulation, with no difference being observed in the rate and the time taken to achieve maximal accumulation. However, the capacity for Ca2+ accumulation by the sarcoplasmic reticulum in MH+ humans and pigs is considerably less stable than normal after prolonged ageing of the whole muscle homogenate preparations in ice. In MH+ patients, the capacity for Ca2+ accumulation by the sarcoplasmic reticulum showed a decline of 62% at 22 h ageing and 70% at 48 h ageing, as compared with a reduction of only 23% in MH- patients. In MH+ pigs, the sarcoplasmic reticulum showed a 96% deterioration in the capacity for Ca2+ accumulation as compared with a loss of only 40% in MH- pigs at 7 h ageing in ice. In both humans and pigs, the decline in Ca2+ accumulation was prevented by incubating the whole muscle homogenate preparations for 2 h at 35 degrees C prior to ageing the preparations. The diminished Ca2+ accumulating capacity of the sarcoplasmic reticulum in whole muscle homogenate preparations of MH-susceptible individuals in our experimental protocol provides a potential diagnostic test for malignant hyperthermia susceptibility.

Erythrocyte osmotic fragility in porcine malignant hyperthermia and effects of halothane

Malignant Hyperthermia (MH) has been shown to be due to an inherited disorder in cellular membrane systems, which also seem to be affected by halothane. With the erythrocyte osmotic fragility test we tried to find out if the erythrocyte membrane can serve as a diagnostic tool in malignant hyperthermia and as a model for halothane effects on membranes. We obtained MH susceptible and MH resistant pigs and their crossbreeds. The MH susceptible pigs showed the greatest degree of haemolysis in the erythrocyte osmotic fragility test. Furthermore, halothane increased haemolysis more in the erythrocytes of MH susceptible pigs. The MH resistant pigs were only slightly affected in both and the crossbreeds showed intermediate results. The variance occurring within one halothane group could be used to add a further distinguishing characteristic apart from the discrete halothane challenge test and so enable selection within a pure halothane group.

Membrane abnormality in malignant hyperthermia

In this study we report on the effect of halothane exposure on the skeletal muscle membrane and calcium localization in 9 patients with confirmed susceptibility to malignant hyperthermia and compare them with 11 non-susceptible cases of malignant hyperthermia. Two normal muscle specimens with physiologically induced muscle contractures were also included for comparison. Halothane exposure in malignant hyperthermia-susceptible cases showed severe muscle contractures, definite breaks in the plasma membrane with peroxidase penetration, and large number of calcium granules within the mitochondria and sarcoplasmic reticulum. Malignant hyperthermia-non-susceptible cases and muscle with physiological contractures did not show discontinuity of the plasma membrane or excessive calcium deposits. Based on these findings we conclude that the halothane exposure in malignant hyperthermia-susceptible patients causes breaks in the muscle plasma membrane.

Skeletal muscle mitochondrial respiration of malignant hyperthermia-susceptible patients. Ca2+-induced uncoupling and free fatty acids

1. Skeletal muscle mitochondria of malignant hyperthermia (MH)-susceptible patients showed normal oxidative phosphorylation but were more easily uncoupled than normal by exogenous Ca2+. 2. Fatty acids, in stimulating the mitochondrial ATPase activity, are responsible for the enhanced State 4 respiration in MH-susceptible patients. 3. These results imply that skeletal muscle mitochondria and free fatty acids are associated with the development of MH syndrome.

Temperature-dependent abnormalities of the erythrocyte membrane in porcine malignant hyperthermia

The temperature dependence of ATPase activities and stearic acid spin label motion in red blood cells of normal and MH-susceptible pigs have been examined. Arrhenius plots of red blood cell ghost Ca-ATPase and calmodulin-stimulable Ca-ATPase activities were identical for both normal and MH erythrocyte ghosts. Arrhenius plots of Mg-ATPase activity exhibited a break (defined as a change in slope) at 24 degrees C in both MH and normal erythrocyte ghosts. However, below 24 degrees C the apparent activation energy for this activity was less in MH than normal ghosts. To determine whether breaks in ATPase Arrhenius plots could be correlated with changes in the physical state of the red blood cell membrane, the spin label 16-doxyl-stearate was introduced into the bilayer of both erythrocyte ghosts and red blood cells. With both ghosts and intact cells, at each temperature examined, the mobility of the probe in the lipid bilayer, as measured by electron paramagnetic resonance, was greater in normal than in MH membranes. While there were no breaks in Arrhenius plots for probe motion in the erythrocyte ghosts, the apparent activation energy for probe motion was significantly greater in normal than in MH ghost membranes. While there was no break in the Arrhenius plot of probe motion in normal intact red blood cell membranes, there were breaks in the Arrhenius plot of probe motion at both 24 and 33 degrees C in intact MH red blood cell membranes. Based on the altered temperature dependence of Mg-ATPase activity and spin probe motion in membranes derived from MH red blood cells, we conclude that there may be a generalized membrane defect in MH pigs which is reflected in the red blood cell as an altered membrane composition or organization.

Influence of the Hal locus and standardized stress on antibody response and in vitro reactivity of peripheral blood lymphocytes in pigs

The antibody response to two Escherichia coli antigens (O149 and K88) and the in vitro reactivity of peripheral blood lymphocytes (PBLs) to pokeweed mitogen was studied in growing pigs after exposure to varying degrees of stress. Three experimental groups were used; transportation by lorry, transportation by lorry after a previous injection of a tranquillizing drug (amperozide), no transportation but an amperozide injection. Another group was used as a control group. This group was not transported and received no amperozide injection. The animals were the offspring of boars and sows heterozygous (Nn) for the Hal gene, and all 3 Hal genotypes (NN, Nn, nn) were identified and could thus be compared within litters. Immediately after the experimental treatment, the highest PBL reactivity was found for the amperozide-treated animals and for the non-transported animals, with no differences in reactivity between Hal genotypes. Two weeks later, the treatments caused different effects on pigs of the 3 Hal genotypes, both with regard to the PBL reactivity and the IgG response to O149. The NN pigs had a higher PBL reactivity than the nn pigs for all treatments except the 'drug + transport' class where the reverse rank order was found. The NN pigs also had a higher IgG response to O149 than the nn pigs in the 'drug + no transport' class. The amperozide treatment was followed by a higher PBL reactivity in non-transported NN pigs and in transported nn pigs. The amperozide-treated non-transported NN pigs also had a higher IgG response to O149. The highest PBL reactivity and IgG response to O149 were found mainly in pigs with the lowest cortisol levels. Pronounced differences between litters were found for both the antibody response and the PBL reactivity.