Enhanced sensitivity to
caffeine is part of the standard tests for susceptibility to
malignant hyperthermia (MH) in humans and pigs. The
caffeine sensitivity of skeletal muscle contraction and Ca(2+) release from the sarcoplasmic reticulum is enhanced, but surprisingly, the
caffeine sensitivity of purified porcine
ryanodine receptor Ca(2+)-release channels (RyRs) is not affected by the MH mutation (Arg(615)Cys). In contrast, we show here that native malignant hyperthermic pig RyRs (incorporated into
lipid bilayers with RyR-associated
lipids and
proteins) were activated by
caffeine at 100- to 1000-fold lower concentrations than native normal pig RyRs. In addition, the results show that the mutant
ryanodine receptor channels were less sensitive to high-affinity activation by a
peptide (C(S)) that corresponds to a part of the II-III loop of the skeletal
dihydropyridine receptor (DHPR). Furthermore, subactivating concentrations of
peptide C(S) enhanced the response of normal pig and rabbit RyRs to
caffeine. In contrast, the
caffeine sensitivity of MH RyRs was not enhanced by the
peptide. These novel results showed that in MH-susceptible pig muscles 1). the
caffeine sensitivity of native RyRs was enhanced, 2). the sensitivity of RyRs to a skeletal II-III loop
peptide was depressed, and 3). an interaction between the
caffeine and
peptide C(S) activation mechanisms seen in normal RyRs was lost.