9199552: positive IVCT; 11260227: positive IVCT; 12411788: positive IVCT; 16163667: positive IVCT; 15201141: Similar to that observed for MH mutations in RYR1, the R1086H MH mutation in the 1S-subunit of the skeletal muscle DHPR increases the sensitivity of the calcium release mechanism to activation by both pharmacological (caffeine) and endogenous (voltage sensor) activators. Threshold for caffeine-induced calcium release is approximately fivefold lower in R1086H- expressing myotubes. Increased "sensitivity" of RYR1 to activation by caffeine may be a consequence of an elevated resting calcium level in R1086H- expressing myotubes. The data suggest that the intracellular III-IV linker of the DHPR acts as a negative regulatory module for release channel activation and that the R1086H MH mutation in 1S disrupts this critical negative allosteric regulatory mechanism.