REGULATION OF CARDIAC-MUSCLE CA2-RETICULUM LUMENAL CA2+( RELEASE CHANNEL BY SARCOPLASMIC)

The cardiac muscle sarcoplasmic reticulum Ca2+ release channel (ryanod ine receptor) is a ligand-gated channel that is activated by micromola r cytoplasmic Ca2+ concentrations and inactivated by millimolar cytopl asmic Ca2+ concentrations. The effects of sarcoplasmic reticulum lumen al Ca2+ on the purified release channel were examined in single channe l measurements using the planar lipid bilayer method. In the presence of caffeine and nanomolar cytosolic Ca2+ concentrations, lumenal-to-cy tosolic Ca2+ fluxes greater than or equal to 0.25 pA activated the cha nnel. At the maximally activating cytosolic Ca2+ concentration of 4 mu M, lumenal Ca2+ fluxes of 8 pA and greater caused a decline in channe l activity. Lumenal Ca2+ fluxes primarily increased channel activity b y increasing the duration of mean open times. Addition of the fast Ca2 +-complexing buffer 1,2-bis(2-aminophenoxy)ethanetetraacetic acid (BAP TA) to the cytosolic side of the bilayer increased lumenal Ca2+-activa ted channel activities, suggesting that it lowered Ca2+ concentrations at cytosolic Ca2+-inactivating sites. Regulation of channel activitie s by lumenal Ca2+ could be also observed in the absence of caffeine an d in the presence of 5 mM MgATP. These results suggest that lumenal Ca 2+ can regulate cardiac Ca2+ release channel activity by passing throu gh the open channel and binding to the channel's cytosolic Ca2+ activa tion and inactivation sites.