Malignant hyperthermia (MH) is a pharmacogenetic disorder of skeletal muscle triggered when susceptible subjects are exposed to volatile anesthetic agents and/or depolarizing muscle relaxants. We have used Ca2+ selective micro-electrodes to measure in, vivo the intracellular free [Ca2+] in skeletal muscle of MH susceptible swine before and after the administration of dantrolene. We have investigated the effectiveness of this muscle relaxant in preventing clinical MH and the relationship between the resting intracellular free [Ca2+] and the probability of inducing the MH syndrome. The resting intracellular free [Ca2+] was 0.41 ± 0.01 μM (M ± SEM), which agrees with our previous measurements in susceptible swine. The administration of 0.5, 1, 2, 2.5 and 3 mg/Kg Dantrolene, reduced the intracellular free [Ca2+] to 0.31, 0.21, 0.09, 0.08, 0.08 μM respectively. The 0.5 mg/Kg dose induced a moderate decrease of [Ca2+]i and failed to prevent the MH syndrome after exposure to halothane (2%). The 1 mg/Kg dose produced a further reduction in [Ca2+]i and was sufficient to prevent the clinical syndrome in 2 out of 3 animals. The 2.5 mg/Kg dose was uniformly protective in all animals. These results suggest that the mechanism by which dantrolene protects susceptible animals exposed to triggering agents is by reducing the intracellular free [Ca2+] in skeletal muscle.
