Malignant hyperthermia: effects of sarcoplasmic reticulum Ca2+ pump inhibition.

In porcine malignant hyperthermia-susceptible (MHS) skeletal muscles, calcium release is abnormal and resting calcium may be elevated. Thus MHS muscles may have prolonged twitch relaxation and lower fusion frequencies, which would be augmented by inhibition of sarcoplasmic reticulum (SR) Ca2+ adenosine triphosphatase (ATPase) activity; bundles of intact muscle cells from MHS and normal pigs were used to investigate this possibility. Cooling and low-frequency stimulation, in combination, enhanced twitch fusion and prolonged twitch relaxation significantly more in MHS than in normal muscles (e.g., 34 +/- 4% versus 16 +/- 4% fusion, and 82.4 +/- 9.4 ms versus 43.2 +/- 7.8 ms half-relaxation time, for MHS and normal muscles, respectively). Similarly, inhibition of the SR Ca2+ ATPase by cyclopiazonic acid resulted in significantly greater twitch fusion in MHS muscles. These results were consistent with predicted effects of enhanced SR Ca2+ release and/or elevated resting calcium in MHS muscles and indicate that cooling during a malignant hyperthermia crisis could actually increase the force of muscle contractures.