Ca2+ current-gated focal and local Ca2+ release in rat atrial myocytes: evidence from rapid 2-D confocal imaging

In atrial myocytes immunocytochemistry has shown two groups of ryanodine receptors (RyRs): those at the periphery colocalized with dihydropyridine receptors (DHPRs) and those at the cell interior not associated with DHPRs. The extent to which the two sets of RyRs are controlled by Ca2+ current (I(Ca)) or Ca2+ diffusion remains to be determined. Here, using rapid (240 Hz) two-dimensional confocal Ca2+ imaging in rat atrial myocytes, we examine directly the role of I(Ca) on the two-dimensional patterns of local and focal Ca2+ releases. I(Ca) evoked peripheral Ca2+ release within 1-4 ms, causing a rapid monophasic local rise of Ca2+, which then propagated into the cell interior along sarcomeric lines (approximately 2 microm) with a velocity of approximately 230 microm s(-1), even though we found no evidence for organized t-tubules using di-8-ANEPPS staining. I(Ca)-triggered Ca2+ release in the cell centre, on the other hand, had both a rapid (12 ms) and slower delayed components (12-50 ms). The voltage dependence of peripheral Ca2+ release and the two components of central release was bell shaped, and the magnitude of each release component was linearly related to I(Ca). Premature termination (2-10 ms) of I(Ca) was equally effective in abbreviating both the peripheral and slow central Ca2+ release. High concentration of Ca2+ buffers (2-5 mM EGTA plus 1 mM fluo-3) completely abolished the I(Ca)-gated propagation wave and the slow delayed component of Ca2+ release, but had little or no effect on the rapid component of central release. The efficacy of I(Ca) to trigger Ca2+ release in periphery of the myocyte was approximately 5 times higher than in the centre, consistent with the smaller measured central Ca2+ release. The quantification of central Ca2+ release as a function of peripheral release suggests a cooperative gating mechanism(s) for central release. These findings indicate that both I(Ca) and diffusion of Ca2+ from the peripheral sites contribute to the gating of Ca2+ release from central SR. How in fact the I(Ca)-dependent fast component of central release is activated remains to be determined.