Regulation of Cardiac Muscle Ca2+ Release Channel by Sarcoplasmic Reticulum Lumenal Ca2+

The cardiac muscle sarcoplasmic reticulum Ca 2+ release channel (ryanodine receptor) is a ligand-gated channel that is activated by micromolar cytoplasmic Ca 2+ concentrations and inactivated by millimolar cytoplasmic Ca 2+ concentrations. The effects of sarcoplasmic reticulum lumenal Ca 2+ on the purified release channel were examined in single channel measurements using the planar lipid bilayer method. In the presence of caffeine and nanomolar cytosolic Ca 2+ concentrations, lumenal-to-cytosolic Ca 2+ fluxes ≥0.25 pA activated the channel. At the maximally activating cytosolic Ca 2+ concentration of 4 μ M, lumenal Ca 2+ fluxes of 8 pA and greater caused a decline in channel activity. Lumenal Ca 2+ fluxes primarily increased channel activity by increasing the duration of mean open times. Addition of the fast Ca 2+ -complexing buffer 1,2-bis(2-aminophenoxy)ethanetetraacetic acid (BAPTA) to the cytosolic side of the bilayer increased lumenal Ca 2+ -activated channel activities, suggesting that it lowered Ca 2+ concentrations at cytosolic Ca 2+ -inactivating sites. Regulation of channel activities by lumenal Ca 2+ could be also observed in the absence of caffeine and in the presence of 5mM MgATP. These results suggest that lumenal Ca 2+ can regulate cardiac Ca 2+ release channel activity by passing through the open channel and binding to the channel's cytosolic Ca 2+ activation and inactivation sites.