Phospholamban-mediated stimulation of Ca²⁺ uptake in sarcoplasmic reticulum from normal and failing hearts

Studies in animal models have suggested that alterations affecting phospholamban-mediated stimulation of Ca²⁺ uptake by sarcoplasmic reticulum are involved in the pathophysiology of heart disease. A monoclonal antibody that binds to phospholamban and stimulates Ca²⁺ uptake was used to characterize phospholamban-mediated effects in human cardiac sarcoplasmic reticulum and to compare these effects in tissue from normal and failing hearts. Stimulation of Ca²⁺ uptake by antiphospholamban monoclonal antibody simulated the effect of phosphorylation of phospholamban by cAMP-dependent protein kinase. Binding of anti-phospholamban antibody reduced the K_0.5 5 of the Ca²⁺-transporting ATPase from 0.53 μM ([Ca²⁺]) to 0.29 μM([Ca²⁺]), without affecting V_max or n_Hill. At 0.2 μM Ca²⁺, stimulation was 1.93-fold in sarcoplasmic reticulum prepared from normal human left ventricular myocardium and 1.94-fold in sarcoplasmic reticulum prepared from the left ventricular myocardium of patients with heart failure resulting from idiopathic dilated cardiomyopathy. Stimulation of Ca²⁺ uptake in canine cardiac sarcoplasmic reticulum under identical conditions was 1.89-fold. Phospholamban-mediated stimulation of Ca²⁺ uptake in human cardiac sarcoplasmic reticulum is thus comparable in magnitude to that observed in other species and results from an increase in the apparent affinity of the Ca²⁺-transporting ATPase for Ca²⁺. The pathogenesis of heart failure in idiopathic dilated cardiomyopathy does not, however, appear to involve intrinsic alterations of this mechanism.