Fig. 1. (  A ) A normal (malignant hyperthermia–nonsusceptible) skeletal muscle fiber was perfused with a mock intracellular solution containing 120 nm Ca²⁺and 1 mm free Mg²⁺. A series of control responses (two shown) was initiated by brief application of a solution containing 40 mm caffeine and 0 Mg²⁺(40 caf/0 Mg²⁺) at 4-min intervals (  left ). During the period indicated by the break in the trace (//), the preparation was perfused for a further 4 min to allow the sarcoplasmic reticulum to reaccumulate Ca²⁺, and perfusion was then stopped. The solution within the bath was then rapidly replaced with one containing 5 mm halothane, and perfusion was stopped again. The perfusion was then restarted, and the protocol was repeated in the same preparation but with 10 mm, 20 mm, and 40 mm halothane. (  B ) Accumulated data summarizing the proportion of normal fibers responding at each halothane concentration. (  C ) Confocal x–y images using the same protocol shown in  A . Representative frames are shown 0, 10, 20, and 60 s after exposure to 40 mm halothane in the presence of 1 mm free Mg²⁺.