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.