Ryanoid Modification of the Cardiac Muscle Ryanodine Receptor Channel Results in Relocation of the Tetraethylammonium Binding Site

The interaction of ryanodine and derivatives of ryanodine with the high affinity binding site on the ryanodine receptor (RyR) channel brings about a characteristic modification of channel function. In all cases, channel open probability increases dramatically and single-channel current amplitude is reduced. The amplitude of the ryanoid-modified conductance state is determined by structural features of the ligand. An investigation of ion handling in the ryanodine-modified conductance state has established that reduced conductance results from changes in both the affinity of the channel for permeant ions and the relative permeability of ions within the channel (Lindsay, A.R.G., A. Tinker, and A.J. Williams. 1994. J. Gen. Physiol. 104:425–447). It has been proposed that these alterations result from a reorganization of channel structure induced by the binding of the ryanoid. The experiments reported here provide direct evidence for ryanoid-induced restructuring of RyR. TEA⁺ is a concentration- and voltage-dependent blocker of RyR in the absence of ryanoids. We have investigated block of K⁺ current by TEA⁺ in the unmodified open state and modified conductance states of RyR induced by 21-amino-9α-hydroxyryanodine, 21-azido-9α-hydroxyryanodine, ryanodol, and 21-p-nitrobenzoylamino-9α-hydroxyryanodine. Analysis of the voltage dependence of block indicates that the interaction of ryanoids with RyR leads to an alteration in this parameter with an apparent relocation of the TEA⁺ blocking site within the voltage drop across the channel and an alteration in the affinity of the channel for the blocker. The degree of change of these parameters correlates broadly with the change in conductance of permeant cations induced by the ryanoids, indicating that modification of RyR channel structure by ryanoids is likely to underlie both phenomena.