Transverse tubule calcium regulation in malignant hyperthermia

Enhanced response to caffeine and 4-chloro-m-cresol in malignant hyperthermia-susceptible muscle is related in part to chronically elevated resting [Ca2+]i

Malignant hyperthermia (MH) is a potentially fatal pharmacogenetic syndrome caused by exposure to halogenated volatile anesthetics and/or depolarizing muscle relaxants. We have measured intracellular Ca(2+) concentration ([Ca(2+)](i)) using double-barreled, Ca(2+)-selective microelectrodes in myoballs prepared from skeletal muscle of MH-susceptible (MHS) and MH-nonsusceptible (MHN) swine. Resting [Ca(2+)](i) was approximately twofold in MHS compared with MHN quiescent myoballs (232 +/- 35 vs. 112 +/- 11 nM). Treatment of myoballs with caffeine or 4-chloro-m-cresol (4-CmC) produced an elevation in [Ca(2+)](i) in both groups; however, the concentration required to cause a rise in [Ca(2+)](i) elevation was four times lower in MHS than in MHN skeletal muscle cells. Incubation of MHS cells with the fast-complexing Ca(2+) buffer BAPTA reduced [Ca(2+)](i), raised the concentration of caffeine and 4-CmC required to cause an elevation of [Ca(2+)](i), and reduced the amount of Ca(2+) release associated with exposure to any given concentration of caffeine or 4-CmC to MHN levels. These results suggest that the differences in the response of MHS skeletal myoballs to caffeine and 4-CmC may be mediated at least in part by the chronic high resting [Ca(2+)](i) levels in these cells.

The effects of mutated skeletal ryanodine receptors on calreticulin and calsequestrin expression in the brain and pituitary gland of boars

Mutations in skeletal ryanodine receptors (sRyR) result in malignant hyperthermia in humans and Porcine Stress Syndrome (PSS) in pigs. Whether the sRyR is expressed in neuronal tissue and what impact it has on neuronal function is relatively unexplored. We have hypothesized that the presence of mutated sRyR may be accompanied by compensatory changes in Ca(2+)-binding protein expression. We were interested in whether pigs heterozygous for mutated sRyR would show changes in the expression of Ca(2+)-binding proteins, in specific regions of the brain, and whether changes in this expression would be accompanied by the presence of sRyR within that region. The objectives of the current experiments were to determine (1) whether calreticulin (CR) and calsequestrin (CS) are expressed in the pituitary gland and brain of the pig, (2) if boars heterozygous for mutated sRyR differed from wild-type boars in the expression of CR or CS, and (3) if altered Ca(2+)-binding protein expression would be accompanied by the presence of sRyR mRNA. Boars either heterozygous or wild-type (n=6) for the mutation in sRyR known to cause PSS, were euthanized and the pituitary gland and brains were collected for western blotting for CR and CS. An additional four wild-type boars were sacrificed and brains were collected for in situ hybridization for sRyR mRNA. Immunoreactive CR was expressed in porcine tissues with highest (p<0.0001) expression in the pituitary gland and lower but equivalent expression in the hypothalamus, frontal cortex, and hippocampus. Immunoreactive CS was not detectable in the pituitary gland while low levels were observed in the hypothalamus and frontal cortex. Dramatically higher (p<0.0001) levels of CS were found in the hippocampus. Genotype did not affect CR expression in the pituitary gland or any brain region examined. Immunoreactive CS levels were lower (p<0.002) in the hippocampus of heterozygous compared to wild-type boars. In situ hybridization experiments revealed the presence of sRyR mRNA in the hippocampus equally distributed across all cell subfields. In conclusion, both CR and CS were expressed in the porcine brain with specific patterns of expression across the brain regions examined. Boars heterozygous for mutated sRyR had lower CS in the hippocampus, which was accompanied by the expression of mRNA for sRyR.

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.

The effect of calcium channel antagonists and BAY K 8644 on calcium fluxes of malignant hyperpyrexia-susceptible muscle

1. The calcium channel antagonists verapamil (100 microM) and nifedipine (100 microM) inhibited twitch response and KCl induced hypercontractility in malignant hyperpyrexia (MH)-susceptible porcine skeletal muscle. These calcium channel antagonists did not effect hypercontractility induced by 3% halothane or 2 mM caffeine. 2. The calcium channel agonist BAY K 8644 (50 microM) induced contracture in MH-susceptible muscle but did not potentiate contracture response induced by 2 mM caffeine or 3% halothane. BAY K 8644 did not increase the resting tension of control muscle or increase the sensitivity of control muscle to 4 mM caffeine, 3% halothane or 80 mM KCl. 3. The sarcoplasmic reticulum (SR) from MH-susceptible and control porcine skeletal muscle was separated into vesicular fractions enriched in the membrane elements of the terminal cisternae and longitudinal tubules. 4. Verapamil and diltiazem [which has been previously shown to inhibit the hypercontractility of MH-susceptible porcine muscle to caffeine, halothane and KCl (Foster and Denborough, 1989 Br. J. Anaesth. 62, 566-572)] did not effect Ca2+ uptake or Ca(2+)-dependent ATPase activities of SR longitudinal tubule membranes, from MH-susceptible or control muscle. These calcium channel antagonists did not effect Ca2+ release from terminal cisternae preparations. 5. The skeletal muscle relaxant dantrolene inhibited Ca2+ efflux and equilibrium-Ca2+ exchange associated with the terminal cisternae membrane of MH-susceptible and control skeletal muscle. 6. Calcium channel antagonists modify Ca2+ fluxes in MH-susceptible and control muscle by acting at a site distal to the SR. Calcium channel antagonists may inhibit contractile response by modifying events of excitation-contraction coupling associated with the voltage sensor molecule (dihydropyridine-receptor) of the transverse-tubule membrane, whereas dantrolene directly acts on the terminal cisternae membrane to inhibit Ca2+ efflux and equilibrium Ca2+ exchange. Different calcium channel antagonists seem to modify the voltage-sensor mechanism in different ways in MH-susceptible muscle. 7. An abnormality in the coupling mechanism of the voltage sensor-SR calcium release channel may exist in MH-susceptible muscle. This dysfunction may be an adaptation to the elevated levels of myoplasmic Ca2+ and/or the molecular defect described in the Ca2+ release channel of the SR of MH-susceptible porcine muscle. In view of these results it is unlikely that nifedipine or verapamil would be of therapeutic value for the treatment of MH.

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.

Calcium handling by platelets from normal and malignant hyperthermia-susceptible pigs

Platelets from normal and malignant hyperthermia (MH)-susceptible pigs were evaluated for differences in 45calcium uptake in the absence or presence of caffeine (2-16 mM), halothane (0.05-0.5%), or halothane and caffeine together. There were no statistically significant differences in basal or halothane-inhibited calcium uptake by platelets from either source. There was a small statistically significant difference in calcium uptake between platelets from normal and MH-susceptible pigs in the presence of 16 mM caffeine and 0.5% halothane. Calcium uptake by platelets from one pedigree of MH-susceptible pigs were stimulated in a concentration-dependent manner by caffeine. These data suggest that exposure of platelets to caffeine may have potential for identifying MH-susceptibility.

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.

An electron paramagnetic resonance study of skeletal muscle membrane fluidity in malignant hyperthermia

Skeletal muscle sarcolemma (SL), transverse tubule (TT) and heavy sarcoplasmic reticulum (HSR) membranes were isolated from malignant hyperthermia susceptible (MHS) and normal pigs, and the rotational dynamics of lipid hydrocarbon chain motion was examined by electron paramagnetic resonance (EPR) spectroscopy. The stearic acid spin probe 16-SASL was incorporated into MHS and normal membranes and both the order parameter (S) and effective correlation time (tau r) of probe motion were calculated from spectra recorded over the temperature range of 2 to 40 degrees C. At any given temperature, TT membranes exhibited significantly greater values for both the S and tau r of probe motion than did SL, which exhibited significantly greater values than did HSR membranes. The order of decreasing S and tau r values for 16-SASL mobility correlated with the decreasing cholesterol content of these membranes (TT greater than SL greater than HSR), however there was no difference in the S or tau r values for a given membrane fraction isolated from both MHS and normal muscle. Arrhenius plots of 16-SASL mobility in SL, TT and HSR were linear from 2 to 40 degrees C, indicating no abrupt thermotropic change in the lipid hydrocarbon phase of any of the membrane types studied. Apparent activation energies (Ea), calculated from the Arrhenius plots, were similar for MHS and normal membranes derived from a given cellular location. However, the Ea of probe motion for TT membranes (2.3 +/- 0.1 and 2.4 +/- 0.1 kcal/mol/degree for MHS and normal, respectively) was significantly less than for SL (3.4 +/- 0.4 and 2.9 +/- 0.1 kcal/mol/degree for MHS and normal, respectively) which, in turn, was significantly less than the Ea for HSR (3.7 +/- 0.1 and 3.7 +/- 0.1 kcal/mol/degree for MHS and normal, respectively). Since 16-SASL motion was similar in MHS and normal membranes, we conclude that there is no evidence for a generalized membrane defect affecting lipid mobility in these MHS muscle membranes.