Fig. 2. (  A ) A normal (MHN) skeletal muscle fiber was perfused with a solution containing 120 nm Ca²⁺and 1 mm Mg²⁺. A series of control responses (two shown) was initiated by brief application of a solution containing 40 mm caffeine and 0 Mg²⁺. 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 reload with Ca²⁺, and perfusion was then stopped. The [Mg²⁺] was then changed to either 1.5, 1, 0.4, 0.2, or 0.1 mm for a further minute (not shown) before rapidly introducing 1 mm halothane (at the same [Mg²⁺]). (  B ) The same protocol applied to a malignant hyperthermia–susceptible (MHS) fiber. (  C ) Cumulative data for MHN (  filled ), MHS (  open ), and malignant hyperthermia–equivocal (MHE) fibers (  hatched ). All values are expressed as a percentage of the total number of fibers: MHN, 71; MHS, 35; and MHE, 9 (1 fiber/patient).