Regulation of cardiac muscle Ca2+ release channel by sarcoplasmic reticulum lumenal Ca2+.
The cardiac muscle sarcoplasmic reticulum Ca2+ release channel (ryanodine receptor) is a ligand-gated channel that is activated by micromolar cytoplasmic Ca2+ concentrations and inactivated by millimolar cytoplasmic Ca2+ concentrations. The effects of sarcoplasmic reticulum lumenal Ca2+ 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 Ca2+ concentrations, lumenal-to-cytosolic Ca2+ fluxes >/=0.25 pA activated the channel. At the maximally activating cytosolic Ca2+ concentration of 4 microM, lumenal Ca2+ fluxes of 8 pA and greater caused a decline in channel activity. Lumenal Ca2+ fluxes primarily increased channel activity by increasing the duration of mean open times. Addition of the fast Ca2+-complexing buffer 1,2-bis(2-aminophenoxy)ethanetetraacetic acid (BAPTA) to the cytosolic side of the bilayer increased lumenal Ca2+-activated channel activities, suggesting that it lowered Ca2+ concentrations at cytosolic Ca2+-inactivating sites. Regulation of channel activities by lumenal Ca2+ could be also observed in the absence of caffeine and in the presence of 5 mM MgATP. These results suggest that lumenal Ca2+ can regulate cardiac Ca2+ release channel activity by passing through the open channel and binding to the channel's cytosolic Ca2+ activation and inactivation sites.
Xu
L
L
Department of Biochemistry & Biophysics, University of North Carolina, Chapel Hill, North Carolina 27599-7260, USA.
Meissner
G
G
D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology*; D015220:Calcium Channels / Q000502:physiology; D002135:Calcium-Binding Proteins / Q000502:physiology; D004285:Dogs; D004533:Egtazic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D004594:Electrophysiology; D008081:Liposomes / Q000737:chemistry; D008274:Magnesium / Q000494:pharmacology; D009206:Myocardium / Q000378:metabolism*; D019837:Ryanodine Receptor Calcium Release Channel / Q000502:physiology*; D012519:Sarcoplasmic Reticulum / Q000502:physiology
D015220:Calcium Channels; D002135:Calcium-Binding Proteins; D008081:Liposomes; D019837:Ryanodine Receptor Calcium Release Channel; D002110:Caffeine; D004533:Egtazic Acid; D000255:Adenosine Triphosphate; D008274:Magnesium; C025603:1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; D002118:Calcium