USING LARGE ORGANIC CATIONS TO PROBE THE NATURE OF RYANODINE MODIFICATION IN THE SHEEP CARDIAC SARCOPLASMIC-RETICULUM CALCIUM-RELEASE CHANNEL

We have reported that the large impermeant organic cations tetrabutyl ammonium (TBA+), tetrapentyl ammonium, and the charged local anestheti c QX314 produce unique reduced conductance states in the purified shee p cardiac sarcoplasmic reticulum Ca2+ release channel when present at the cytoplasmic face of the channel. We have interpreted this as a for m of partial occlusion by the blocking cation in wide vestibules of th e conduction pathway. Following modification with ryanodine, which cau ses the channel to enter a reduced conductance state with long open dw ell 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 a fter ryanodine modification. The concentration dependence is different , in that the ryanodine-modified channel has one-third the affinity fo r TBA+, which is accounted for predominantly by changes in the TBA+ on rate. The data are compatible with a structural change in the vestibu le of the conduction pathway consequent upon ryanodine binding that re duces the capture radius for blocking ion entry.