Calcium sparks and excitation-contraction coupling in phospholamban-deficient mouse ventricular myocytes

1. We examined [Ca2+](i) and L-type Ca2+ channel current (I-Ca) in single cardiac myocytes to determine how the intracellular protein phospholamban (PLB) influences excitation-contraction (E-C) coupling in heart. Wild type (WT) and PLB-deficient (KO) mice were used. Cells were patch clamped in whole-cell mode while [Ca2+](i) was imaged simultaneously using the Ca2+ indicator fluo-3 and a confocal microscope. 2. Although I-Ca was similar in magnitude, the decay; of I-Ca was faster in KO than in WT cells and the [Ca2+](i) transient was larger and decayed faster. Furthermore, the E-C coupling 'gain' (measured as Delta[Ca2+](i)/I-Ca) was larger in KO cells than in WT cells. 3. Spontaneous Ca2+ sparks were three times more frequent and larger in KO cells than in WT myocytes but, surprisingly, the time constants of decay were similar. 4. SR Ca2+ content was significantly greater in KO than in WT cells. When the SR Ca2+ content in KO cells was reduced to that in WT cells, Ca2+ sparks in these 'modified' (KO') cells decayed faster. E-C coupling gain, [Ca2+](i) transient amplitude and the kinetics of decay of I-Ca were similar in KO' and WT cells. 5. We conclude that SR Ca2+ content influences (1) I-Ca, (2) the amplitude and kinetics of Ca2+ sparks and [Ca2+](i) transients, (3) the sensitivity of the RyRs to triggering by: [Ca2+](i), (4) the amount of Ca2+ released, (5) the magnitude of the E-C coupling 'gain' function, and (6) the rate of Ca2+ re-uptake by the SR Ca2+-ATPase. In KO cells, the larger [Ca2+](i) transients and Ca2+ sparks speed up I-Ca inactivation. Finally, we conclude that PLB plays an important regulatory role in E-C coupling by: modulating SR Ca2+-ATPase activity, which establishes the SR Ca2+ content and consequently influences the characteristics of local and global Ca2+ signalling.