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 Ca(2+) 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 Ca(2+) homeostasis in junctin-deficient 
cardiomyocytes. In concurrence with sparks, JCN-KO cardiomyocytes display 
increased Ca(2+) transient amplitude, resulting from increased SR [Ca(2+)] 
([Ca(2+)](SR)). Junctin ablation appears to affect how RyRs 'sense' SR Ca(2+) 
load, resulting in decreased diastolic SR Ca(2+) leak despite an elevated 
[Ca(2+)](SR). Surprisingly, the β-adrenergic enhancement of [Ca(2+)](SR) 
reverses the decrease in RyR activity and leads to spontaneous Ca(2+) 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 [Ca(2+)]: at low luminal [Ca(2+)] (<1 mmol l(-1)), junctin-devoid RyR 
channels are less responsive to luminal [Ca(2+)]; conversely, high luminal 
[Ca(2+)] turns them hypersensitive to this form of channel modulation. Thus, 
junctin produces complex effects on Ca(2+) sparks, transients, and leak, but the 
luminal [Ca(2+)]-dependent dual response of junctin-devoid RyRs demonstrates 
that junctin normally acts as an activator of RyR channels at low luminal 
[Ca(2+)], and as an inhibitor at high luminal [Ca(2+)]. Because the crossover 
occurs at a [Ca(2+)](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.