A modified local control model for Ca2+ transients in cardiomyocytes: junctional flux is accompanied by release from adjacent non-junctional RyRs.

Excitation-contraction coupling in cardiomyocytes requires Ca(2+) influx through dihydropyridine receptors in the sarcolemma, which gates Ca(2+) release through sarcoplasmic ryanodine receptors (RyRs). Ca(2+) influx, release and diffusion produce a cytosolic Ca(2+) transient. Here, we investigated the relationship between Ca(2+) transients and the spatial arrangement of the sarcolemma including the transverse tubular system (t-system). To accomplish this, we studied isolated ventricular myocytes of rabbit, which exhibit a heterogeneously distributed t-system. We developed protocols for fluorescent labeling and triggered two-dimensional confocal microscopic imaging with high spatiotemporal resolution. From sequences of microscopic images, we measured maximal upstroke velocities and onset times of local Ca(2+) transients together with their distance from the sarcolemma. Analyses indicate that not only sarcolemmal release sites, but also those that are within 1 μm of the sarcolemma actively release Ca(2+). Our data also suggest that release does not occur at sites further than 2.5 μm from the sarcolemma. The experimental data are in agreement with results from a mathematical model of Ca(2+) release and diffusion. Our findings can be explained by a modified local control model, which constrains the region of regenerative activation of non-junctional RyR clusters. We believe that this model will be useful for describing excitation-contraction coupling in cardiac myocytes with a sparse t-system, which includes those from diseased heart tissue as well as atrial myocytes of some species.