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Each heartbeat is accompanied by the coordinated release of calcium ions into cardiac cells through ryanodine receptors, which span the membrane of the sarcoplasmic reticulum. We show that an intra–sarcoplasmic reticulum protein, junctin, interacts with ryanodine receptor channels and appears to activate them when calcium inside the sarcoplasmic reticulum is low. Conversely, junctin appears to act as an inhibitor of ryanodine receptors when calcium inside the sarcoplasmic reticulum is high. Knowledge of how junctin interacts with ryanodine receptors helps us to understand how calcium within the sarcoplasmic reticulum helps to regulate ryanodine receptor activity in normal hearts and also helps us to understand why junctin is decreased in patients diagnosed with certain forms of heart failure.Junctin, a 26 kDa intra–sarcoplasmic reticulum (SR) protein, forms a quaternary complex with triadin, calsequestrin and the ryanodine receptor (RyR) at the junctional SR membrane. The physiological role for junctin in the luminal regulation of RyR Ca2+ release remains unresolved, but it appears to be essential for proper cardiac function since ablation of junctin results in increased ventricular automaticity. Given that the junctin levels are severely reduced in human failing hearts, we performed an in–depth study of the mechanisms affecting intracellular Ca2+ homeostasis in junctin–deficient cardiomyocytes. In concurrence with sparks, JCN–KO cardiomyocytes display increased Ca2+ transient amplitude, resulting from increased SR [Ca2+] ([Ca2+]SR). Junctin ablation appears to affect how RyRs ‘sense’ SR Ca2+ load, resulting in decreased diastolic SR Ca2+ leak despite an elevated [Ca2+]SR. Surprisingly, the β–adrenergic enhancement of [Ca2+]SR reverses the decrease in RyR activity and leads to spontaneous Ca2+ release, evidenced by the development of spontaneous aftercontractions. Single channel recordings of RyRs from WT and JCN–KO cardiac SR indicate that the absence of junctin produces a dual effect on the normally linear response of RyRs to luminal [Ca2+]: at low luminal [Ca2+] (<1 mmol l−1), junctin–devoid RyR channels are less responsive to luminal [Ca2+]; conversely, high luminal [Ca2+] turns them hypersensitive to this form of channel modulation. Thus, junctin produces complex effects on Ca2+ sparks, transients, and leak, but the luminal [Ca2+]–dependent dual response of junctin–devoid RyRs demonstrates that junctin normally acts as an activator of RyR channels at low luminal [Ca2+], and as an inhibitor at high luminal [Ca2+]. Because the crossover occurs at a [Ca2+]SR that is close to that present in resting cells, it is possible that the activator–inhibitor role of junctin may be exerted under periods of prevalent parasympathetic and sympathetic activity, respectively.