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

Failing human myocardium has been associated with decreased sarcoplasmic reticulum (SR) Ca2+-ATPase activity. There remains controversy as to whether the regulation of SR Ca2+-ATPase activity is altered in heart failure or whether 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 unchanged in failing compared with nonfailing human myocardium. There was decreased 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 decrease of the cAMP-dependent phosphorylation level of phospholamban by 20 +/- 2%. It is concluded that the impaired SR function in human end-stage heart failure may be due, in part, to a reduced cAMP-dependent phosphorylation of phospholamban.