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Spatiotemporal Characteristics of Junctional and Nonjunctional Focal Ca²⁺ Release in Rat Atrial Myocytes

From the Department of Pharmacology, Georgetown University Medical Center, Washington, DC.

Correspondence to Prof Martin Morad, Dept of Pharmacology, Georgetown University Medical Center, 3900 Reservoir Rd NW, Washington, DC 20057. E-mail moradm{at}georgetown.edu

Atrial myocytes have two functionally separate Ca²⁺ release sites: those in peripheral sarcoplasmic reticulum (SR) adjacent to the Ca²⁺ channels of surface membrane and those in central SR not associated with Ca²⁺ channels. Recently, we have reported on the gating of these two different Ca²⁺ release sites by Ca²⁺ current. In the present study, we report on the spatiotemporal properties of focal Ca²⁺ releases (sparks) occurring spontaneously in central and peripheral sites of voltage-clamped rat atrial myocytes, using rapid 2-dimensional (2-D) confocal Ca²⁺ imaging. Peripheral and central sparks were similar in size and release time (300 000 Ca²⁺ ions for 12 ms), but significantly larger and longer than ventricular sparks. Both sites were resistant to Cd²⁺ and inhibited by ryanodine. Peripheral sparks were brighter and flattened against surface membrane, had 5-fold higher frequency, 2 times faster diffusion coefficient, and dissipated abruptly. Central sparks, in contrast, occurred less frequently, were elongated along the cellular longitudinal axis, and dissipated slowly. Compound sparks (composed of 2 to 5 unitary focal releases) aligned longitudinally and occurred more frequently at the center. The diversity of peripheral and central sparks with respect to shape, frequency, and speed of spatial development and decay is consistent with regional ultrastructural heterogeneity of SR. The retarded dissipation of central atrial sparks, and high prevalence of compound sparks in cell center may be critical in facilitating the propagation of Ca²⁺ waves in atrial myocytes lacking t-tubular system and provide the atrial myocytes with functional Ca²⁺ signaling diversity. The full text of this article is available at http://www.circresaha.org.

Key Words: atrial myocyte • Ca²⁺ spark • two-dimensional confocal imaging

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