Structure-function relationships among ryanodine derivatives. Pyridyl ryanodine definitively separates activation potency from high affinity.

Ryanodine derivatives are differentially effective on the two limbs of the ryanodine concentration-effect curve. This study comparing ryanodine, ryanodol, and pyridyl ryanodine and nine C10Oeq esters of them focuses on structure-function relations underlying their differential effectiveness. Ryanodol and pyridyl ryanodine had significantly lower affinities than ryanodine, but their EC50act values (concentration of ryanoid that induces one-half of full efficacy), potencies, and efficacies were not diminished in like fashion. Ryanodine and ryanodol were partial agonists, whereas pyridyl ryanodine was a full agonist, having a diminished deactivation potency. C10Oeq esterifications enhanced affinities and efficacies of the base ryanoids. The C10-Oeq ester derivatives of ryanodine and pyridyl ryanodine, but not those of ryanodol, lost their capacity to deactivate RyR1s. Thus, affinity differences among ryanoids clearly do not predicate functional differences as regards activation of Ca2+ release channels. The pyrrole carboxylate on the C3 of ryanodine is dispensable to ryanoid activation of Ca2+ release channels. Ryanodol lacks this ring, but it nevertheless effects substantial activation. Moreover, its C10-Oeq esters display full efficacy. The increased ability of all the C10-Oeq derivatives to release Ca2+ from the vesicles strengthens their role in directly impeding deactivation of RyR1, perhaps by interaction with some component within the transmembrane ionic flux pathway.