Diversity of atrial local Ca2+ signalling: evidence from 2-D confocal imaging in Ca2+-buffered rat atrial myocytes

Atrial myocytes, lacking t-tubules, have two functionally separate groups of ryanodine receptors (RyRs): those at the periphery colocalized with dihydropyridine receptors (DHPRs), and those at the cell interior not associated with DHPRs. We have previously shown that the Ca 2+ current ( I Ca )-gated central Ca 2+ release has a fast component that is followed by a slower and delayed rising phase. The mechanisms that regulate the central Ca 2+ releases remain poorly understood. The fast central release component is highly resistant to dialysed Ca 2+ buffers, while the slower, delayed component is completely suppressed by such exogenous buffers. Here we used dialysis of Ca 2+ buffers (EGTA) into voltage-clamped rat atrial myocytes to isolate the fast component of central Ca 2+ release and examine its properties using rapid (240 Hz) two-dimensional confocal Ca 2+ imaging. We found two populations of rat atrial myocytes with respect to the ratio of central to peripheral Ca 2+ release ( R c/p ). In one population (‘group 1’, ∼60% of cells), R c/p converged on 0.2, while in another population (‘group 2’, ∼40%), R c/p had a Gaussian distribution with a mean value of 0.625. The fast central release component of group 2 cells appeared to result from in-focus Ca 2+ sparks on activation of I Ca . In group 1 cells intracellular membranes associated with t-tubular structures were never seen using short exposures to membrane dyes. In most of the group 2 cells, a faint intracellular membrane staining was observed. Quantification of caffeine-releasable Ca 2+ pools consistently showed larger central Ca 2+ stores in group 2 and larger peripheral stores in group 1 cells. The R c/p was larger at more positive and negative voltages in group 1 cells. In contrast, in group 2 cells, the R c/p was constant at all voltages. In group 1 cells the gain of peripheral Ca 2+ release sites (Δ[Ca 2+ ]/ I Ca ) was larger at −30 than at +20 mV, but significantly dampened at the central sites. On the other hand, the gains of peripheral and central Ca 2+ releases in group 2 cells showed no voltage dependence. Surprisingly, the voltage dependence of the fast central release component was bell-shaped and similar to that of I Ca in both cell groups. Removal of extracellular Ca 2+ or application of Ni 2+ (5 m m ) suppressed equally I Ca and Ca 2+ release from the central release sites at +60 mV. Depolarization to +100 mV, where I Ca is absent and the Na + –Ca 2+ exchanger (NCX) act