title: Using large organic cations to probe the nature of ryanodine modification in the sheep cardiac sarcoplasmic reticulum calcium release channel.
abstract: We have reported that the large impermeant organic cations tetrabutyl ammonium (TBA+), tetrapentyl ammonium, and the charged local anesthetic QX314 produce unique reduced conductance states in the purified sheep cardiac sarcoplasmic reticulum Ca2+ release channel when present at the cytoplasmic face of the channel. We have interpreted this as a form of partial occlusion by the blocking cation in wide vestibules of the conduction pathway. Following modification with ryanodine, which causes the channel to enter a reduced conductance state with long open dwell time, these cations block the receptor channel to a level that is indistinguishable from the closed state. The voltage dependence of TBA+'s interaction with the Ca2+ release channel is the same before and after ryanodine modification. The concentration dependence is different, in that the ryanodine-modified channel has one-third the affinity for TBA+, which is accounted for predominantly by changes in the TBA+ on rate. The data are compatible with a structural change in the vestibule of the conduction pathway consequent upon ryanodine binding that reduces the capture radius for blocking ion entry.
authors.affiliation: Department of Cardiac Medicine, National Heart and Lung Institute, University of London, England.
authors.forename: A
authors.initials: A
authors.lastname: Tinker
authors.forename: A J
authors.initials: AJ
authors.lastname: Williams
mesh_terms: D000818:Animals; D055592:Biophysical Phenomena; D001703:Biophysics; D002121:Calcium Channel Blockers; D015220:Calcium Channels / Q000378:metabolism*; D002412:Cations / Q000378:metabolism; D004553:Electric Conductivity; D066298:In Vitro Techniques; D007700:Kinetics; D015335:Molecular Probes; D009124:Muscle Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism*; D009206:Myocardium / Q000378:metabolism*; D000644:Quaternary Ammonium Compounds / Q000378:metabolism / Q000494:pharmacology; D019837:Ryanodine Receptor Calcium Release Channel; D012519:Sarcoplasmic Reticulum / Q000378:metabolism; D012756:Sheep
chemical_list: D002121:Calcium Channel Blockers; D015220:Calcium Channels; D002412:Cations; D015335:Molecular Probes; D009124:Muscle Proteins; D000644:Quaternary Ammonium Compounds; D019837:Ryanodine Receptor Calcium Release Channel; C009405:tetrabutylammonium