Hypokalemic periodic paralysis (HypoPP) is an inherited muscle channelopathy which has been linked to mutations in the L-type (Cav1.1) Ca2+ channel alpha 1 subunit. Remarkably, 6 of 7 HypoPP mutations in CaV1.1 are at arginines in S4 voltage sensors, as are all 8 HypoPP mutations in NaV1.4. Here, we generated a genetically engineered mouse with one amino acid substitution causing HypoPP located in the S4 voltage sensor of the second domain of the Cav1.1 alpha subunit (S4-II R528H). Homozygous mutant mice (S4-II R528H) were viable, and while spontaneous attacks of weakness were not observed, tetanic force was reduced during low K challenge for in vitro contractions. Single isolated flexor digitorum brevis (FDB) muscle fibers from S4-II R528H mice exhibited 80% reduction in the amplitude of fluo4 Ca2+ transients compared to fibers from wild type (WT) mice when subjected to single or repetitive (30 Hz) field stimulation. In voltage-clamped FDB fibers from S4-II R528H mice, charge movement was not significantly altered in amplitude or voltage dependence whereas calcium current density was significantly reduced by 50%. Depolarization-induced fluo4 transients (F/F0) were also reduced in FDB fibers from S4-II R528H mice when compared to WT counterparts, as were the Ca2+ transients and Ca2+ release calculated from F/F0. Fibers form S4-II R528H mice exhibited a reduction in the amplitude of maximum Ca2+ release rate (Rmax) by ca 80 % when compared to WT fibers. It is concluded that the S4-II R528H mutation of CaV1.1 channel significantly reduces the whole-cell Ca2+ channel current amplitude, and is crucial for Cav1.1/RyR coupling during voltage dependent Ca2+ release in skeletal muscle. Supported by NIAMS (AR42703) and NIAMS (R01-AR055099) of the NIH, and the Muscular Dystrophy Association.