REDUCED INHIBITORY EFFECT OF MG2+ ON RYANODINE RECEPTOR CA2+ RELEASE CHANNELS IN MALIGNANT HYPERTHERMIA

Malignant hyperthermia (MH) is a potentially fatal, inherited skeletal muscle disorder in humans and pigs that is caused by abnormal regulat ion of Ca2+ release from the sarcoplasmic reticulum (SR). MH in pigs i s associated with a single mutation (Arg(615)Cys) in the SR ryanodine receptor (RyR) Ca2+ release channel. The way in which this mutation le ads to excessive Ca2+ release is not known and is examined here. Singl e RyR channels from normal and MH-susceptible (MHS) pigs were examined in artificial lipid bilayers, High cytoplasmic (cis) concentrations o f either Ca2+ or Mg2+ (>100 mu M) inhibited channel opening less in MH S RyRs than in normal RyRs. This difference was more prominent at lowe r ionic strength (100 mM versus 250 mM). In 100 mM cis Cs+, half-maxim um inhibition of activity occurred at similar to 100 mu M Mg2+ in norm al RyRs and at similar to 300 mu M Mg2+ in MHS RyRs, with an average H ill coefficient of similar to 2 in both cases. The level of Mg2+ inhib ition was not appreciably different in the presence of either 1 or 50 mu M activating Ca2+, showing that it was not substantially influenced by competition between Mg2+ and Ca2+ for the Ca2+ activation site, Ev en though the absolute inhibitory levels varied widely between channel s and conditions, the inhibitory effects of Ca2+ and Mg2+ were virtual ly identical for the same conditions in any given channel, indicating that the two cations act at the same low-affinity inhibitory site. II seems likely that at the cytoplasmic [Mg2+] in vivo (similar to 1 mM), this Ca2+/Mg2+-inhibitory site will be close to fully saturated with Mg2+ in normal RyRs, but less fully saturated in MHS RyRs. Therefore M HS RyRs should be more sensitive to any activating stimulus, which wou ld readily account for the development of an MH episode.