In cardiac muscle, junctin forms a quaternary protein complex with the ryanodine 
receptor (RyR), calsequestrin, and triadin 1 at the luminal face of the 
junctional sarcoplasmic reticulum (jSR). By binding directly the RyR and 
calsequestrin, junctin may mediate the Ca(2+)-dependent regulatory interactions 
between both proteins. To gain more insight into the underlying mechanisms of 
impaired contractile relaxation in transgenic mice with cardiac-specific 
overexpression of junctin (TG), we studied cellular Ca(2+) handling in these 
mice. We found that the SR Ca(2+) load was reduced by 22% in cardiomyocytes from 
TG mice. Consistent with this, the frequency of Ca(2+) sparks was diminished by 
32%. The decay of spontaneous Ca(2+) sparks was prolonged by 117% in TG. This 
finding was associated with a lower Na(+)-Ca(2+) exchanger (NCX) protein 
expression (by 67%) and a higher basal RyR phosphorylation at Ser(2809) (by 64%) 
in TG. The shortening- and Delta[Ca](i)-frequency relationships (0.5-4 Hz) were 
flat in TG compared to wild-type (WT) which exhibited a positive staircase for 
both parameters. Furthermore, increasing stimulation frequencies hastened the 
time of relaxation and the decay of [Ca](i) by a higher percentage in TG. We 
conclude that the impaired relaxation in TG may result from a reduced NCX 
expression and/or a higher SR Ca(2+) leak. The altered shortening-frequency 
relationship in TG seems to be a consequence of an impaired 
excitation-contraction coupling with depressed SR Ca(2+) release at higher rates 
of stimulation. Our data suggest that the more prominent frequency-dependent 
hastening of relaxation in TG results from a stimulation of SR Ca(2+) transport 
reflected by corresponding changes of [Ca](i).