Increased phospholamban phosphorylation limits the force-frequency response in the MLP-/- mouse with heart failure

Reduced Ca2+ release from the sarcoplasmic reticulum (SR) and a negative force-frequency relation characterize end-stage human heart failure. The MLP-/- mouse with dilated cardiomyopathy is used as a model to explore novel therapeutic interventions but the alterations in Ca2+ handling in MLP-/- remain incompletely understood. We Studied [Ca2+](i) in left ventricular myocytes from MLP-/- and WT mice (3-4 months old; whole-cell voltage clamp, 30 degrees C. At 1 Hz stimulation, the amplitude of [Ca2+](i) transients was similar. However, in contrast to WT, at higher frequencies the [Ca2+](i) transient amplitude declined in MLP-/- and there was no increase in SR Ca2+ content. Unexpectedly, the decline of [Ca2+](i) was faster in M LP-/- than in WT (at 1 Hz, tau of 80 +/- 9 vs. 174 +/- 29 ms, P < 0.001) and the frequency-dependent acceleration of the decline was abolished suggesting an enhanced basal SERCA activity. Indeed, the Ca2+ affinity of SR Ca2+ uptake in homogenates was higher in MLP-/-, with the maximal uptake rate similar to WT. Phosphorylation of phospholamban in MLP-/- was increased (2.3-fold at Ser(16) and 2.9-fold at the Thr(17) site, P < 0.001) with similar SERCA and total phospholamban protein levels. On increasing stimulation frequency to 4 Hz, WT, but not MLP myocytes had a net gain of Ca2+, suggesting inadequate Ca2+ sequestration in MLP In conclusion, increased baseline phosphorylation of phospholamban in MLP-/- leads to a reduced reserve for frequency-dependent increase of Ca2+ release. This represents a novel paradigm for altered Ca2+ handling in heart failure, underscoring the importance of phosphorylation pathways. (c) 2006 Elsevier Ltd. All rights reserved.