Subcellular properties of [Ca2+]itransients in phospholamban-deficient mouse ventricular cells.

The regulatory protein phospholamban exerts a physiological inhibitory effect on the sarcoplasmic reticulum (SR) Ca2+ pump that is relieved with phosphorylation. We have studied the subcellular properties of intracellular Ca2+([Ca2+]i) transients in ventricular myocytes isolated from wild-type (WT) and phospholamban-deficient (PLB-KO) mice. In PLB-KO myocytes, steady-state twitch [Ca2+]itransients revealed an accelerated relaxation and the occurrence of highly localized failures of Ca2+release. The acceleration of SR Ca2+ uptake caused an increase in SR Ca2+ load with the frequent occurrence of spontaneous [Ca2+]iwaves and Ca2+ sparks. [Ca2+]iwaves in PLB-KO cells showed a marked decrease in spatial width and more frequently appeared to abort. Local Ca2+ release events (Ca2+ sparks) were larger and more variable in amplitude and [Ca2+]ideclined faster in PLB-KO myocytes. Increased local buffering and reduction in the refractoriness of SR Ca2+ release caused by the increased SR pump rate led to an overall enhancement of local [Ca2+]igradients and inhomogeneities in the [Ca2+]idistribution during spontaneous Ca2+ release, [Ca2+]iwaves, and excitation-contraction coupling.