A, when the SR Ca2+ levels are low, maximal interaction may occur between HRC and SERCA thereby mediating the rate of SR Ca2+ uptake, while interaction between JCN and CSQ reduces Ca2+ leak through the RyR. B, when the SR Ca2+ content is high, HRC dissociates from SERCA and promotes interaction with triadin, which may simultaneously affect Ca2+ uptake and RyR activity. Junctin interaction with CSQ is reduced, possibly facilitating Ca2+ leak through the RyR.