Malignant hyperthermia susceptibility arising from altered resting coupling between the skeletal muscle L-type Ca²⁺ channel and the type 1 ryanodine receptor

Malignant hyperthermia (MH) susceptibility is a dominantly inherited disorder in which volatile anesthetics trigger aberrant Ca²⁺ release in skeletal muscle and a potentially fatal rise in perioperative body temperature. Mutations causing MH susceptibility have been identified in two proteins critical for excitation-contraction (EC) coupling, the type 1 ryanodine receptor (RyR1) and Ca_V1.1, the principal subunit of the L-type Ca²⁺ channel. All of the mutations that have been characterized previously augment EC coupling and/or increase the rate of L-type Ca ²⁺ entry. The Ca_V1.1 mutation R174W associated with MH susceptibility occurs at the innermost basic residue of the IS4 voltage-sensing helix, a residue conserved among all Ca_V channels [Carpenter D, et al. (2009) BMC Med Genet 10:104-115.]. To define the functional consequences of this mutation, we expressed it in dysgenic (Ca_V1.1 null) myotubes. Unlike previously described MH-linked mutations in Ca_V1.1, R174W ablated the L-type current and had no effect on EC coupling. Nonetheless, R174W increased sensitivity of Ca²⁺ release to caffeine (used for MH diagnostic in vitro testing) and to volatile anesthetics. Moreover, in Ca _V1.1 R174W-expressing myotubes, resting myoplasmic Ca²⁺ levels were elevated, and sarcoplasmic reticulum (SR) stores were partially depleted, compared with myotubes expressing wild-type Ca_V1.1. Our results indicate that Ca_V1.1 functions not only to activate RyR1 during EC coupling, but also to suppress resting RyR1-mediated Ca²⁺ leak from the SR, and that perturbation of Ca_V1.1 negative regulation of RyR1 leak identifies a unique mechanism that can sensitize muscle cells to MH triggers.