Human heart failure: cAMP stimulation of SR Ca2+-ATPase activity and phosphorylation level of phospholamban

Failing human myocardium has been associated with decreased sarcoplasmic re ticulum (SR) Ca2+-ATPase activity. There remains controversy as to whether the regulation of SR Ca2+-ATPase activity is altered in heart failure or wh ether decreased SR Ca2+-ATPase activity is due to changes in SR Ca2+-ATPase or phospholamban expression. We therefore investigated whether alterations in cAMP,dependent phosphorylation of phospholamban may be responsible for the reduced SR Ca2+-ATPase activity in human heart failure. Protein levels of phospholamban and SR Ca2+-ATPase, detected by Western blot, were unchang ed in failing compared with nonfailing human myocardium. There was decrease d responsiveness to the direct activation of the SR Ca2+-ATPase activity by either cAMP (0.01-100 mu mol/l) or protein kinase A (1-30 mu g) in failing myocardium. Using the backphosphorylation technique, we observed a decreas e of the cAMP-dependent phosphorylation level of phospholamban by 20 +/- 2% . It is concluded that the impaired SR function in human end-stage heart fa ilure may be due, in part, to a reduced cAMP-dependent phosphorylation of p hospholamban.