J. Kockskamper et al., Activation and propagation of Ca2+ release during excitation-contraction coupling in atrial myocytes, BIOPHYS J, 81(5), 2001, pp. 2590-2605
Fast two-dimensional confocal microscopy and the Ca2+ 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 (
I-SR) and central nonjunctional (nj-SR) sarcoplasmic reticulum. Action pote
ntials elicited by field stimulation induced transient increases of intrace
llular Ca2+ concentration ([Ca2+](i)) that were highly inhomogeneous. Incre
ases started at distinct subsarcolemmal release sites spaced similar to2 mu
m apart. The amplitude and the latency of Ca2+ release from these sites var
ied from beat to beat. Subsarcolemmal release fused to build a peripheral r
ing of elevated [Ca2+](i), which actively propagated to the center of the c
ells via Ca2+-induced Ca2+ release. Resting myocytes exhibited spontaneous
Ca2+ release events, including Ca2+ sparks and local (microscopic) or globa
l (macroscopic) [Ca2+](i) waves. The microscopic [Ca2+]l waves propagated i
n a saltatory fashion along the sarcolemma ("coupled" Ca2+ sparks) revealin
g the sequential activation of Ca2+ release sites of the j-SR. Moreover, du
ring global [Ca2+](i) waves, Ca2+ release was evident from individual nj-SR
sites. Ca2+ release sites were arranged in a regular three-dimensional gri
d as deduced from the functional data and shown by immunostaining of ryanod
ine receptor Ca2+ release channels. The longitudinal and transverse distanc
es between individual Ca2+ release sites were both similar to2 tm. Furtherm
ore, 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-in
duced whole-cell [Ca2+](i) transient is the spatio-temporal summation of Ca
2+ release from subsarcolemmal and central sites. First, j-SR sites are act
ivated in a stochastic fashion by the opening of voltage-dependent sarcolem
mal Ca2+ channels. Subsequently, nj-SR sites are activated by Ca2+-induced
Ca2+ release propagating from the periphery.