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).