Calcium Sparks and Excitation–Contraction Coupling in Phospholamban‐Deficient Mouse Ventricular Myocytes

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Abstract
1 We examined [Ca2+]i and L‐type Ca2+ channel current (ICa) 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 ICa was similar in magnitude, the decay of ICa 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 Δ[Ca2+]i/ICa) 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 ICa were similar in KO' and WT cells. 5 We conclude that SR Ca2+ content influences (1) ICa, (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 ICa 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.