Mechanisms underlying local Ca signalling differences between right and left atrial myocytes at normal and increased frequencies.

Changes in heart rate affect Ca signalling and contractility in ventricular muscle, but the effects on atrial Ca signalling are poorly understood. Here, we explored how increased stimulation frequency affects right atrial (RA) and left atrial (LA) local Ca signalling and underlying cellular mechanisms. We used two-dimensional confocal Ca imaging, patch clamping, immunocytochemistry and western blotting in isolated rat atrial myocytes. Centripetal Ca waves were common in both RA and LA myocytes. Increasing the stimulation frequency from 1 to 3 Hz reduced local Ca transients in LA but not in RA myocytes. LA myocytes consistently exhibited threefold faster centripetal Ca propagation than RA myocytes. RA myocytes had a faster Ca 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 Ca current (I) in both cell types, but I was larger in RA cells. At increased frequencies, sarcoplasmic reticulum (SR) Ca 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 Ca release. However, LA Ca signalling is more prone to maladaptation to frequency increases due to less effective SR Ca uptake and a smaller trigger I. KEY POINTS: Changes in heart rate affect Ca signalling and contractility in ventricular muscle, but the effects on atrial Ca signalling are less well understood. Here we determine how increased electrical stimulation frequency affects right (RA) and left atrial (LA) local Ca signalling and underlying cellular mechanisms. We demonstrate that, during depolarization, centripetal Ca 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 Ca pumps. Increasing stimulation frequency more readily compromises peripheral sarcoplasmic reticulum Ca loading in LA myocytes, thereby impairing local Ca releases, unlike in RA myocytes. The lower peripheral density of sarcoplasmic reticulum Ca pumps, along with reduced levels of phospholamban monomer and phosphorylated phospholamban, in LA myocytes compared to RA myocytes underlies the defective Ca signalling adaptation to increased frequency in the left atrium.