Sarcoplasmic Reticulum Ca2+ Release Uses a Cascading Network of Intra-SR and Channel Countercurrents.

In muscle, Ca2+ release from the sarcoplasmic reticulum (SR) into the cytosol is mediated through the ryanodine receptors (RyRs) and sustained by countercurrents that keep the SR membrane potential near 0 mV. Likewise, Ca2+ reuptake by the sarco/endoplasmic reticulum Ca2+ ATPase pump requires countercurrent. Although evidence has suggested that TRIC K+ channels and/or RyR K+ influx provide these countercurrents, the exact sources have not yet been determined. We used an equivalent circuit compartment model of a cardiac SR, the surrounding cytosol, and the dyadic cleft to probe the sources of countercurrent during a complete cardiac cycle. By removing and relocating TRIC K+ channels, as well as limiting when they are active, we explored the various possible sources of SR countercurrent under many conditions. Our simulations indicate that no single channel type is essential for countercurrent. Rather, a cascading network of countercurrents is present with anion fluxes within the SR redistributing charges throughout the full SR volume. This allows ion channels in the entire SR membrane, far from the Ca2+ fluxes through the RyRs in the junctional SR and sarco/endoplasmic reticulum Ca2+ ATPase pump in the nonjunctional SR, to mediate countercurrents that support Ca2+ release and reuptake. This multifactorial network of countercurrents allows Ca2+ release to be remarkably robust.