Phospholamban is a low molecular weight phosphoprotein in cardiac sarcoplasmic reticulum. The regulatory role of phospholamban in vivo has recently been elucidated by targeting the gene of this protein in embryonic stem cells and generating phospholamban-deficient mice. The phospholamban knockout hearts exhibited significantly enhanced contractile parameters and attenuated responses to beta-agonists. The hyperdynamic cardiac function of the phospholamban knockout mice was not accompanied by any cytoarchitectural abnormalities or alterations in the expression levels of the cardiac sarcoplasmic reticulum Ca(2+)-ATPase, calsequestrin, Na(+)-Ca2+ exchanger, or the contractile proteins. Furthermore, the attenuation of the cardiac responses to beta-agonists was not due to alterations in the phosphorylation levels of the other key cardiac phosphoproteins in the phospholamban knockout hearts. However, ablation of phospholamban was associated with down-regulation of the ryanodine receptor, which suggests that a cross-talk between cardiac sarcoplasmic reticulum Ca2+ uptake and Ca2+ release occurred in an attempt to maintain Ca2+ homeostasis in these hyperdynamic phospholamban knockout hearts.