Malignant hyperthermia: effects of sarcoplasmic reticulum Ca2+ pump inhibition

Inhibition of sarcoplasmic Ca2+-ATPase increases caffeine- and halothane-induced contractures in muscle bundles of malignant hyperthermia susceptible and healthy individuals

Background

Malignant hyperthermia (MH) is triggered by halogenated anaesthetics and depolarising muscle relaxants, leading to an uncontrolled hypermetabolic state of skeletal muscle. An uncontrolled sarcoplasmic Ca²⁺ release is mediated via the ryanodine receptor. A compensatory mechanism of increased sarcoplasmic Ca²⁺-ATPase activity was described in pigs and in transfected cell lines. We hypothesized that inhibition of Ca²⁺ reuptake via the sarcoplasmic Ca²⁺-ATPase (SERCA) enhances halothane- and caffeine-induced muscle contractures in MH susceptible more than in non-susceptible skeletal muscle.

Methods

With informed consent, surplus muscle bundles of 7 MHS (susceptible), 7 MHE (equivocal) and 16 MHN (non-susceptible) classified patients were mounted to an isometric force transducer, electrically stimulated, preloaded and equilibrated. Following 15 min incubation with cyclopiazonic acid (CPA) 25 μM, the European MH standard in-vitro-contracture test protocol with caffeine (0.5; 1; 1.5; 2; 3; 4 mM) and halothane (0.11; 0.22; 0.44; 0.66 mM) was performed. Data as median and quartiles; Friedman- and Wilcoxon-test for differences with and without CPA; p < 0.05.

Results

Initial length, weight, maximum twitch height, predrug resting tension and predrug twitch height of muscle bundles did not differ between groups. CPA increased halothane- and caffeine-induced contractures significantly. This increase was more pronounced in MHS and MHE than in MHN muscle bundles.

Conclusion

Inhibition of the SERCA activity by CPA enhances halothane- and caffeine-induced contractures especially in MHS and MHE skeletal muscle and may help for the diagnostic assignment of MH susceptibility. The status of SERCA activity may play a significant but so far unknown role in the genesis of malignant hyperthermia.

Quadriceps muscle function and fatigue in women with Addison's disease

In nine patients with Addison's disease (mean +/- SE: 51 +/- 2 years) receiving conventional steroid treatment, and nine age-matched healthy controls (56 +/- 2 years), we investigated maximum voluntary quadriceps force (MVC) and contractile properties evoked with stimulation and central activation both at rest and during a submaximal intermittent fatigue task. The MVC was similar (-3%), but twitch tension (-27%) and central activation were significantly less (-7%), and tetanic half-relaxation time was approximately 40% slower in the patients. Twitch amplitudes were potentiated by 6% in the patients, but unchanged in the control group. The patients self-terminated a submaximal intermittent fatigue protocol (0.6 duty cycle) at approximately 5 +/- 1 min, whereas the controls stopped when they lost 50% of MVC force ( approximately 10 +/- 1 min). Force loss was similar between groups over the first 5 min of the fatigue task. In the patient group, maximal and submaximal relative integrated electromyogram (IEMG) increased significantly in the first minute of fatigue and remained elevated, whereas the controls exhibited a gradual increase in submaximal IEMG with little change in maximal IEMG. These results indicate that conventionally treated Addison's patients have similar MVC strength, but altered contractile properties and decreased endurance compared with controls.

The effects of toxins on muscle

Substances known to be myotoxic are reviewed, including descriptions of the resultant clinical syndromes and the mechanisms important in their development. Certain host characteristics such as altered immune and metabolic regulation are known to modulate the effects of individual myotoxins, producing varied clinical syndromes. Evaluation procedures important in identifying a toxin-induced myopathy are described.

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.