Abstract
Changes in heart rate affect Ca2+ signalling and contractility in ventricular muscle, but the effects on atrial Ca2+ signalling are poorly understood. Here, we explored how increased stimulation frequency affects right atrial (RA) and left atrial (LA) local Ca2+ signalling and underlying cellular mechanisms. We used two-dimensional confocal Ca2+ imaging, patch clamping, immunocytochemistry and western blotting in isolated rat atrial myocytes. Centripetal Ca2+ waves were common in both RA and LA myocytes. Increasing the stimulation frequency from 1 to 3 Hz reduced local Ca2+ transients in LA but not in RA myocytes. LA myocytes consistently exhibited threefold faster centripetal Ca2+ propagation than RA myocytes. RA myocytes had a faster Ca2+ decay rate at higher frequencies. Most LA myocytes displayed fast release sites in the interior upon depolarization and significant transverse-axial tubules (TATs) that were partly co-localized with junctophilin-2, unlike RA myocytes. Increased frequency similarly reduced the Ca2+ current (ICa) in both cell types, but ICa was larger in RA cells. At increased frequencies, sarcoplasmic reticulum (SR) Ca2+ loading and fractional release (FR) remained stable in RA cells, while peripheral SR content and FR decreased in LA cells. RA cells had higher levels of peripheral SERCA2 and protein expressions of phospholamban (PLB) and phosphorylated PLB. Our data and integrative modelling suggest that LA myocytes may contract faster than RA myocytes due to TAT-associated faster central Ca2+ release. However, LA Ca2+ signalling is more prone to maladaptation to frequency increases due to less effective SR Ca2+ uptake and a smaller trigger ICa. KEY POINTS: Changes in heart rate affect Ca2+ signalling and contractility in ventricular muscle, but the effects on atrial Ca2+ signalling are less well understood. Here we determine how increased electrical stimulation frequency affects right (RA) and left atrial (LA) local Ca2+ signalling and underlying cellular mechanisms. We demonstrate that, during depolarization, centripetal Ca2+ propagation occurs approximately threefold faster in LA myocytes compared to RA myocytes possibly due to fast releases in the interior, associated with transverse-axial tubules, and less peripheral sarcoplasmic reticulum Ca2+ pumps. Increasing stimulation frequency more readily compromises peripheral sarcoplasmic reticulum Ca2+ loading in LA myocytes, thereby impairing local Ca2+ releases, unlike in RA myocytes. The lower peripheral density of sarcoplasmic reticulum Ca2+ pumps, along with reduced levels of phospholamban monomer and phosphorylated phospholamban, in LA myocytes compared to RA myocytes underlies the defective Ca2+ signalling adaptation to increased frequency in the left atrium.