The interaction of a neutral ryanoid with the ryanodine receptor channel provides insights into the mechanisms by which ryanoid binding ls modulated by voltage

In an earlier investigation, we demonstrated that the likelihood of interac tion of a positively charged ryanoid, 21-amino-9 alpha-hydroxyryanodine, wi th the sarcoplasmic reticulum Ca2+-release channel (ryanodine receptor, RyR ) is dependent on holding potential (Tanna, B., W. Welch, L. Ruest, J.L. Su tko, and A.J. Williams. 1998, J. Gen. Physiol. 112:55-69) and suggested tha t voltage dependence could result from either the translocation of the char ged ligand to a site within the voltage drop across the channel or a voltag e-driven alteration in receptor affinity. We now report experiments that al low us to assess the validity of these alternate mechanisms. Ryanodol is a neutral ryanoid that binds to RyR and induces modification of channel funct ion. By determining the influence of transmembrane potential on the probabi lity of channel modification by ryanodol and the rate constants of ryanodol association and dissociation, we demonstrate that the influence of voltage is qualitatively the same for both the neutral and positively charged ryan oids. These experiments establish that most, if not all, of the modificatio n of ryanoid interaction with RyR by transmembrane holding potential result s from a voltage-driven alteration in receptor affinity.