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

Carbohydrate and lipid metabolism at rest and under physical and metabolic load in normal pigs and those with the genetic mutation for malignant hyperthermia

The plasma concentrations of glucose, insulin, lactate, cortisol and free fatty acids were investigated in normal pigs and those with the homozygous and heterozygous genetic mutation for malignant hyperthermia (MH). Blood samples were obtained during a 24-h resting period, a glucose tolerance test and during a treadmill test with submaximal physical load conditions. The homozygous MH positive animals elicited the highest glucose concentrations and the homozygous MH negative animals the lowest at each sampling time during the 24-h period (P < 0·01). During the glucose tolerance test, the most pronounced post-prandial increase and the highest plasma concentrations of glucose were found for the homozygous MH positive animals (P < 0·01), which was associated with a late release of insulin. Increasing work loads during the treadmill test run resulted in an increase of plasma cortisol concentrations in all animals (P < 0·01), with the more pronounced increase for the homozygous and heterozygous MH positive animals. Plasma free fatty acid concentrations during the treadmill test differed between the genotypes (P < 0·05). A pronounced increase was found for the homozygous and heterozygous MH positive animals, whereas the MH negative animals provided very constant free fatty acid values, at a low level, independent of the work load. These results indicate that the genetic mutation for MH in pigs affects the metabolic pathways of the energy metabolism, resulting in higher plasma glucose concentrations at rest and higher lipolytic activities under physical load conditions.

Pharmacological distinction between dantrolene and ryanodine binding sites: evidence from normal and malignant hyperthermia-susceptible porcine skeletal muscle

Dantrolene inhibits and ryanodine stimulates calcium release from skeletal-muscle sarcoplasmic reticulum (SR), the former by an unknown mechanism, and the latter by activating the ryanodine receptor (RyR), the primary Ca2+-release channel of SR. Dantrolene is used to treat malignant hyperthermia (MH), a genetic predisposition to excessive intracellular Ca2+ release upon exposure to volatile anaesthetics. Porcine MH results from a point mutation in the SR RyR that alters the open probability of the channel, and is reflected in altered [3H]ryanodine binding parameters. Specific binding sites for [3H]dantrolene and [3H]ryanodine co-distribute on SR that has been isolated by discontinuous sucrose gradient centrifugation. If the two drug-binding sites are functionally linked, [3H]dantrolene binding might be affected both by pharmacological and by genetic modulators of the functional state of the RyR. Accordingly, we compared the characteristics of [3H]dantrolene binding to porcine malignant-hyperthermia-susceptible and normal-skeletal-muscle SR, and examined the effects of RyR modulators on [3H]dantrolene binding to these membranes. Additionally, the feasibility of separating the SR binding sites for [3H]dantrolene and [3H]ryanodine was investigated. No significant differences in [3H]dantrolene binding characteristics to SR membranes from the two muscle types were detected, and the Bmax ratio for [3H]dantrolene/[3H]ryanodine was 1.4(+/-0.1):1 in both muscle types. [3H]Dantrolene binding is unaffected by the RyR modulators caffeine, ryanodine, Ruthenium Red and calmodulin, and neither dantrolene nor azumolene have any effect on [3H]ryanodine binding. Additionally, distinct peaks of [3H]dantrolene and [3H]ryanodine binding are detected in SR membranes fractionated by linear sucrose centrifugation, although no differences in protein patterns are detected by SDS/PAGE or Western-blot analysis. We suggest that the binding sites for these two drugs are pharmacologically distinct, and may exist on separate molecules.

Thermal response in acute porcine malignant hyperthermia

This study was designed to evaluate how vital organ and skin-surface temperatures correlate with other clinical signs of a malignant hyperthermia (MH) episode. Six susceptible swine were anesthetized with thiopental and nitrous oxide and kept normothermic (approximately equal to 38 degrees C). After a 30-min control period, halothane (1 minimum alveolar anesthetic concentration) was administered, followed in 5 min by a bolus of succinylcholine (2 mg/kg intravenously). Monitoring included: 1) ETCO2; 2)PaO2, PaCO2, pHa; 3) cardiovascular function; 4) core temperatures (esophagus, pulmonary artery, and rectum); 5) organ temperatures (brain, kidney, liver, and four skeletal muscles); and 6) skin temperatures (forehead, neck, and axilla). Within 10 min after exposure to halothane and succinylcholine, all animals developed fulminant MH. Kidney, liver, and brain temperatures increased more rapidly than pulmonary artery temperature with the onset of MH. Temperatures significantly increased in the visceral organs prior to the detection of contractures within skeletal muscles. The masseter, longissimus dorsi, quadriceps, deltoid, and extensor digiti II intramuscular temperatures were 1-2 degrees C less than pulmonary artery and esophageal temperatures during the episodes, whereas those of the kidney, liver, and brain were the same or slightly greater. When it occurs, core hyperthermia during acute MH results largely from heat produced in central organs, not in skeletal muscle per se. In these swine, changes in axilla skin surface temperatures correlated well with core temperature trends, whereas those of the neck and forehead did not. Unless a skin-surface probe can be placed in close proximity to a major vessel, cutaneous temperatures should not be substituted for measurements at an appropriate core site.

Specific modifications of the membrane fatty acid composition of human myotubes and their effects on the muscular sodium channels

The fatty acid (FA) composition of human myotube primary cultures was varied by modifications of the contents of FA in the culture medium. An incubation time of 18 h with a defined FA mixture resulted in the most effective alteration of the original FA pattern of the cells. The increases reached for the relative amounts of palmitic acid (16:0), linoleic acid (18:2) or arachidonic acid (20:4) were 3-5-fold. More than 50% of the extra FA were incorporated in the phospholipid fraction, the remaining share in the triglyceride fraction. Shorter incubation times resulted in less FA incorporation, longer incubation times raised the uptake of FA into the triacylglycerol fraction. For a study of the influence of the membrane modification on the function of the sodium channels, the myotubes were converted into myoballs. The sodium channel properties were then determined using the whole-cell clamp technique. The modified cultures showed no significant alterations in the time constants of activation and inactivation, in the voltage dependence of inactivation (h infinity curves) or in the average amplitudes of the sodium currents.