Ser16-, but not Thr17-phosphorylation of phospholamban influences frequency-dependent force generation in human myocardium

Beta-adrenoceptor/cAMP-dependent Ser16-phosphorylation as well as Ca(2+)-dependent Thr17-phosphorylation of phospholamban (PLB) influences SERCA 2a activity and thus myocardial contractility. To determine the cross-signaling between Ca2+ and cAMP pathways, the phosphorylation of Ser16-PLB and Thr17-PLB was studied at increasing stimulation frequencies as well as in the presence of beta-adrenergic stimulation in isolated ventricular trabeculae from failing (dilative cardiomyopathy, DCM, heart transplants, n=9) and non-failing human myocardium (donor hearts, NF, n=9). In addition, we measured the intracellular Ca(2+)-transient (fura-2) at increasing stimulation frequencies (0.5-3.0 Hz). Protein expression of SERCA 2a and phospholamban was similar in DCM and NF. In DCM, diastolic [Ca2+]i was increased and systolic [Ca2+]i as well as Ser16 PLB-phosphorylation were decreased as compared to NF at 0.5 Hz. The positive force-frequency relationship in human non-failing myocardium was accompanied by a frequency-dependent increase in Ser16-PLB, but not Thr17-PLB phosphorylation. In DCM, Ser16-PLB as well as Thr17-PLB phosphorylation were not altered at higher stimulation frequencies. After application of isoprenaline (1 microM), a profound increase in Ser16-PLB phosphorylation was accompanied by a small increase in Thr17-PLB phosphorylation, only in NF. The frequency-dependent phosphorylation of Ser16-PLB may favor an increase in Ca2+ transient and force generation in humans. Cross talk signaling of Ser16/Thr17-PLB phosphorylation after beta-adrenergic stimulation exists in non-failing, but not in failing human myocardium. The Ca(2+)-dependent CaM-kinase activity may be altered in human heart failure.