Subcellular properties of [Ca²⁺](i) transients 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⁺](i) transients 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⁺](i) waves and Ca²⁺ sparks. [Ca²⁺](i) waves 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⁺](i) declined 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⁺](i) gradients and inhomogeneities in the [Ca²⁺](i) distribution during spontaneous Ca²⁺ release, [Ca²⁺](i) waves, and excitation-contraction coupling.