Calcium Channel Dysfunction in a Mutant Mouse Model of Malignant Hyperthermia(CaV1.1R174W)

Malignant hyperthermia (MH) is a potentially fatal pharmacogenetic disorder of skeletal muscle that is triggered by exposure to volatile anesthetics. MH has been studied extensively in mice and pigs carrying causative mutations in the type 1 ryanodine receptor (RyR1). However, no in vivo information exists regarding how mutations in the skeletal muscle L-type Ca2+ channel (CaV1.1) precipitate MH crises. For this reason, we generated a mouse line carrying the R174W mutation. Homozygous R174W mice ambulated efficiently, reproduced and had normal lifespans. When exposed to isoflurane, homozygous R174W mice entered a hypermetabolic state ending ultimately in death. On the ultrastructural level, R174W muscle displayed limited and variable changes: some variability of the SR calsequestrin content, displacement of mitochondria in some soleus fibers of aged mice and occasional accumulation of SR stacks. On the cellular level, homozygous R174W muscle had elevated resting myoplasmic Ca2+ levels that were greatly increased upon exposure to isoflurane. Flexor digitorum brevis (FDB) fibers dissociated from homozygous R174W mice lacked L-type Ca2+ current even though intramembrane charge movements of were of similar magnitude and voltage-dependence to those recorded from wild-type fibers. Ca2+ released from the SR in response to depolarization was substantially reduced in homozygous R174W fibers suggesting a depleted SR Ca2+ store. Lipid bilayer recordings showed that the Po of RyR1s isolated from homozygous R174W mice was significantly increased at all cis Ca2+ concentrations (Feng et al., this meeting). Taken together, our results support a mechanism for MH susceptibility in which CaV1.1 R174W promotes SR Ca2+ leak without affecting EC coupling per se. This work was supported by grants from the NIH (AR055104 to KGB, AR052534 to PDA, KGB, PMH, CFA and INP) and MDA (MDA277475 to KGB). DB received a stipend from 2T32AG000279-11.