Elevated resting [Ca2+]iin myotubes expressing malignant hyperthermia RyR1 cDNAs is partially restored by modulation of passive calcium leak from the SR

Abstract

Malignant hyperthermia (MH) is a pharmacogenetic disorder of skeletal muscle triggered in susceptible individuals by inhalation anesthetics and depolarizing skeletal muscle relaxants. This syndrome has been linked to a missense mutation in the type 1 ryanodine receptor (RyR1) in more than 50% of cases studied to date. Using double-barreled Ca2+microelectrodes in myotubes expressing wild-type RyR1 (WTRyR1) or RyR1 with one of four common MH mutations (MHRyR1), we measured resting intracellular Ca2+concentration ([Ca2+]i). Changes in resting [Ca2+]iproduced by several drugs known to modulate the RyR1 channel complex were investigated. We found that myotubes expressing any of theMHRyR1s had a 2.0- to 3.7-fold higher resting [Ca2+]ithan those expressingWTRyR1. Exposure of myotubes expressingMHRyR1s to ryanodine (500 μM) or (2,6-dichloro-4-aminophenyl)isopropylamine (FLA 365; 20 μM) had no effects on their resting [Ca2+]i. However, when myotubes were exposed to bastadin 5 alone or to a combination of ryanodine and bastadin 5, the resting [Ca2+]iwas significantly reduced ( P < 0.01). Interestingly, the percent decrease in resting [Ca2+]iin myotubes expressingMHRyR1s was significantly greater than that forWTRyR1. From these data, we propose that the high resting myoplasmic [Ca2+]iinMHRyR1 expressing myotubes is due in part to a related structural conformation ofMHRyR1s that favors “passive” calcium leak from the sarcoplasmic reticulum.