Activation and propagation of Ca(2+) release during excitation-contraction coupling in atrial myocytes

Fast two-dimensional confocal microscopy and the Ca(2+) indicator fluo-4 were used to study excitation-contraction (E-C) coupling in cat atrial myocytes which lack transverse tubules and contain both subsarcolemmal junctional (j-SR) and central nonjunctional (nj-SR) sarcoplasmic reticulum. Action potentials elicited by field stimulation induced transient increases of intracellular Ca(2+) concentration ([Ca(2+)](i)) that were highly inhomogeneous. Increases started at distinct subsarcolemmal release sites spaced approximately 2 microm apart. The amplitude and the latency of Ca(2+) release from these sites varied from beat to beat. Subsarcolemmal release fused to build a peripheral ring of elevated [Ca(2+)](i), which actively propagated to the center of the cells via Ca(2+)-induced Ca(2+) release. Resting myocytes exhibited spontaneous Ca(2+) release events, including Ca(2+) sparks and local (microscopic) or global (macroscopic) [Ca(2+)](i) waves. The microscopic [Ca(2+)](i) waves propagated in a saltatory fashion along the sarcolemma ("coupled" Ca(2+) sparks) revealing the sequential activation of Ca(2+) release sites of the j-SR. Moreover, during global [Ca(2+)](i) waves, Ca(2+) release was evident from individual nj-SR sites. Ca(2+) release sites were arranged in a regular three-dimensional grid as deduced from the functional data and shown by immunostaining of ryanodine receptor Ca(2+) release channels. The longitudinal and transverse distances between individual Ca(2+) release sites were both approximately 2 microm. Furthermore, electron microscopy revealed a continuous sarcotubular network and one peripheral coupling of j-SR with the sarcolemma per sarcomere. The results demonstrate directly that, in cat atrial myocytes, the action potential-induced whole-cell [Ca(2+)](i) transient is the spatio-temporal summation of Ca(2+) release from subsarcolemmal and central sites. First, j-SR sites are activated in a stochastic fashion by the opening of voltage-dependent sarcolemmal Ca(2+) channels. Subsequently, nj-SR sites are activated by Ca(2+)-induced Ca(2+) release propagating from the periphery.