Hypersensitivity of malignant hyperthermia-susceptible swine skeletal muscle to caffeine is mediated by high resting myoplasmic [Ca2+ ].

BACKGROUND Malignant hyperthermia (MH) is an inherited pharmacogenetic syndrome that is triggered by halogenated anesthetics and/or depolarizing muscle relaxants. MH-susceptible (MHS) skeletal muscle has been shown to be more sensitive to caffeine-induced contracture than muscle from nonsusceptible (MHN) subjects and is the basis for the most commonly used clinical diagnostic test to determine MH susceptibility. METHODS We studied the effects of caffeine on myoplasmic free calcium concentration ([Ca2+]i) in MHN and MHS swine muscle fibers by means of Ca2+-selective microelectrodes before and after K+-induced partial depolarization. RESULTS [Ca2+]i in untreated MHN fibers was 123 +/- 8 nm versus 342 +/- 33 nm in MHS fibers. Caffeine (2 mm) caused an increase in [Ca2+]i in both groups (296 +/- 41 nm MHN vs. 1,159 +/- 235 nm MHS) with no change in resting membrane potential. When either MHN or MHS, muscle fibers were incubated in 10 mm K+ [Ca2+]i transiently increased to 272 +/- 22 nm in MHN and 967 +/- 38 nm in MHS for 6-8 min. Exposure of MHN fibers to 2 mm caffeine while resting [Ca2+]i was elevated induced an increment in [Ca2+]i to 940 +/- 37 nm. After 6-8 min of exposure to 10 mm K+, [Ca2+]i returned to control levels in all fibers, and the effect of 2 mm caffeine on resting [Ca2+]i returned to control, despite continued partial membrane depolarization. CONCLUSIONS These results suggest that the increased "sensitivity" to caffeine of MHS swine muscle fibers is a nonspecific response related, at least in part, to the high resting [Ca2+]i and not an increased caffeine sensitivity of the sarcoplasmic reticulum Ca2+ release channel per se.