Putative malignant hyperthermia mutation CaV1.1-R174W is insufficient to trigger a fulminant response to halothane or confer heat stress intolerance

Malignant hyperthermia susceptibility (MHS) is an autosomal dominant pharmacogenetic disorder that manifests as a hypermetabolic state when carriers are exposed to halogenated volatile anesthetics or depolarizing muscle relaxants. In animals, heat stress intolerance is also observed. MHS is linked to over 40 variants in RYR1 that are classified as pathogenic for diagnostic purposes. More recently, a few rare variants linked to the MHS phenotype have been reported in CACNA1S, which encodes the voltage-activated Ca²⁺ channel Ca_V1.1 that conformationally couples to RYR1 in skeletal muscle. Here we describe a knock-in mouse line that expresses one of these putative variants, Ca_V1.1-R174W. Heterozygous (HET) and homozygous (HOM) Ca_V1.1-R174W mice survive to adulthood without overt phenotype but fail to trigger with fulminant MH when exposed to halothane or moderate heat stress. All three genotypes (WT, HET, HOM) express similar levels of Ca_V1.1 by qPCR, western blot, [³H]PN-200 receptor binding and immobilization-resistant charge movement densities in flexor digitorum brevis fibers. Although HOM fibers have negligible Ca_V1.1 current amplitudes, HET fibers have similar amplitudes to WT, suggesting a preferential accumulation of the Ca_V1.1-WT protein at triad junctions in HET animals. Never-the-less both HET and HOM have slightly elevated resting free Ca²⁺ and Na⁺ measured with double barreled microelectrode in vastus lateralis that is disproportional to upregulation of TRPC3 and TRPC6 in skeletal muscle. Ca_V1.1-R174W and upregulation of TRPC3/6 alone are insufficient to trigger fulminant MH response to halothane and/or heat stress in HET and HOM mice.