Diketopyridylryanodine Has Three Concentration-dependent Effects on the Cardiac Calcium-release Channel/Ryanodine Receptor

By interacting with more than one site, ryanoids induce multiple effects on calcium-release channels. To date, the kinetics of interaction of only one of these sites has been characterized. Using C(4),C(12)-diketopyridylryanodine in both [(3)H]ryanodine binding and single channel experiments we characterized another site on the cardiac ryanodine receptor (RyR2) with which ryanoids interact. Competitive binding of this ryanoid to RyR2 implied a minimal two-site binding model. At the single channel level, C(4),C(12)-diketopyridylryanodine induced three distinct effects. At nanomolar concentrations, it increased channel open probability severalfold without inducing a subconductance. This effect was independent of membrane holding potential. As for other ryanoids, low micromolar concentrations of C(4),C(12)-diketopyridylryanodine readily induced a subconductance state. The major subconductance had a current amplitude of 52% of fully open, it was reversible, and its time to induction and duration were voltage- and concentration-dependent, affording Hill slopes of >2. At higher micromolar concentrations C(4),C(12)-diketopyridylryanodine induced long lasting, yet reversible shut states. Using a pharmacological strategy we have discerned an additional ryanoid-binding site on RyR2 that triggers an increase in channel activity. This site likely resides outside the strict confines of the transmembrane conducting pathway.