Modulation of SR Ca2+ release by the triadin-to-calsequestrin ratio in ventricular myocytes.

Calsequestrin (CSQ) is a Ca(2+) storage protein that interacts with triadin (TRN), the ryanodine receptor (RyR), and junctin (JUN) to form a macromolecular tetrameric Ca(2+) signaling complex in the cardiac junctional sarcoplasmic reticulum (SR). Heart-specific overexpression of CSQ in transgenic mice (TG(CSQ)) was associated with heart failure, attenuation of SR Ca(2+) release, and downregulation of associated junctional SR proteins, e.g., TRN. Hence, we tested whether co-overexpression of CSQ and TRN in mouse hearts (TG(CxT)) could be beneficial for impaired intracellular Ca(2+) signaling and contractile function. Indeed, the depressed intracellular Ca(2+) concentration ([Ca](i)) peak amplitude in TG(CSQ) was normalized by co-overexpression in TG(CxT) myocytes. This effect was associated with changes in the expression of cardiac Ca(2+) regulatory proteins. For example, the protein level of the L-type Ca(2+) channel Ca(v)1.2 was higher in TG(CxT) compared with TG(CSQ). Sarco(endo)plasmic reticulum Ca(2+)-ATPase 2a (SERCA2a) expression was reduced in TG(CxT) compared with TG(CSQ), whereas JUN expression and [(3)H]ryanodine binding were lower in both TG(CxT) and TG(CSQ) compared with wild-type hearts. As a result of these expressional changes, the SR Ca(2+) load was higher in both TG(CxT) and TG(CSQ) myocytes. In contrast to the improved cellular Ca(2+), transient co-overexpression of CSQ and TRN resulted in a reduced survival rate, an increased cardiac fibrosis, and a decreased basal contractility in catheterized mice, working heart preparations, and isolated myocytes. Echocardiographic and hemodynamic measurements revealed a depressed cardiac performance after isoproterenol application in TG(CxT) compared with TG(CSQ). Our results suggest that co-overexpression of CSQ and TRN led to a normalization of the SR Ca(2+) release compared with TG(CSQ) mice but a depressed contractile function and survival rate probably due to cardiac fibrosis, a lower SERCA2a expression, and a blunted response to ß-adrenergic stimulation. Thus the TRN-to-CSQ ratio is a critical modulator of the SR Ca(2+) signaling.

Altered signal transduction in cardiac ventricle overexpressing A(1)-adenosine receptors.

OBJECTIVE: The aim of the present study was to assess the effects of A(1)-adenosine receptor (A1-AR) stimulation in ventricle of A(1)-adenosine receptor overexpressing mice (transgenic mice, TG). METHODS: Effects of the A(1)-adenosine receptor agonist R-PIA ((-)-N(6)-phenylisopropyladenosine) on phosphorylation of phospholamban (PLB), Ca(2+) transients, Ca(2+) currents and cell shortening were studied in isolated ventricular cardiomyocytes. RESULTS: R-PIA alone did not affect contractility in isolated electrically stimulated cardiomyocytes from wild-type mice (WT) or TG. However, after pre-stimulation of beta-adrenoceptors by isoproterenol, R-PIA reduced contractility in cardiomyocytes from WT but increased contractility in TG. Under the same conditions, R-PIA reduced isoproterenol-stimulated currents through L-type Ca(2+) channels, Ca(2+) transients and phosphorylation of PLB in cardiomyocytes from WT. In contrast, R-PIA diminished phospholamban phosphorylation induced by isoproterenol but augmented isoproterenol-elevated currents through L-type Ca(2+) channels, and isoproterenol-heightened Ca(2+) transients in cardiomyocytes from TG. CONCLUSIONS: We suggest that A(1)-adenosine receptor overexpression reverses the interaction of beta-adrenergic and A(1)-adenosine receptor stimulation, at least in part. Hence, the receptor/effector coupling is dependent on receptor density in this model.