Molecular mechanisms of reduced sarcoplasmic reticulum Ca(2+) uptake in human failing left ventricular myocardium

Background: Human failing heart due to idiopathic dilated cardiomyopathy is associated with decreased sarcoplasmic reticulum Ca 2+ uptake. However, it is unknown as to which mechanism leads to this abnormality. Methods: Immunodetectable sarcoplasmic reticulum proteins (phospholamban [PLB], phosphorylated PLB at serine-16 or threonine-17, calsequestrin and Ca 2+ -ATPase levels), the activities of Ca 2+ -calmodulin-dependent protein kinase and protein phosphatase and Ca 2+ uptake at varying Ca 2+ concentrations were determined in left ventricular specimens from the same 7 failing hearts (ejection fraction 20 ± 2%) due to idiopathic dilated cardiomyopathy and 5 non-failing explanted control donor hearts. Results: In failing hearts, compared with control donors, decreased maximal velocity and affinity of Ca 2+ uptake for Ca 2+ were found to be associated with reduced expression levels of Ca 2+ -adenosine triphosphatase (ATPase), PLB and phosphorylated PLB at serine-16, but not of calsequestrin and phosphorylated PLB at threonine-17. In contrast, protein phosphatase activity increased significantly and the activity and protein expression level of the δ isoform of Ca 2+ -calmodulin-dependent protein kinase remained unchanged in failing hearts compared with control donors. Conclusions: The impaired maximal velocity of sarcoplasmic reticulum Ca 2+ uptake may be due in part to reduced protein expression level of Ca 2+ -ATPase, whereas the reduced affinity may be due in part to the reduced ratio of Ca 2+ -ATPase to PLB and reduced PLB phosphorylation at serine-16 in failing hearts. The latter abnormality may be due in part to increased protein phosphatase activity. These results suggest that selective enhancement of Ca 2+ uptake into the sarcoplasmic reticulum by pharmaceutical agents, or by molecular tools that inhibit phosphatase activity, would be a valuable therapeutic approach for treating, or at least retarding, the process of heart failure.