RYANODINE RECEPTOR CALCIUM-RELEASE CHANNEL CONFORMATIONS AS REFLECTEDIN THE DIFFERENT EFFECTS OF PROPRANOLOL ON ITS RYANODINE BINDING AND CHANNEL ACTIVITY

1. Propranolol, a beta-blocker, inhibited or stimulated ryanodine bind ing to both the membrane-bound and purified ryanodine receptor (RyR) d epending on the assay conditions. At high NaCl concentrations, propran olol increased the number of ryanodine-binding sites (B-max) with no e ffect on the binding affinity. In the presence of 0.2 M NaCl, ryanodin e binding was inhibited by propranolol. Half-maximal inhibition was ob tained at 1.2 mM and complete inhibition at 2 mM propranolol. The inhi bitory effect of propranolol obtained at low NaCl concentration was no t restored by increasing the NaCl concentration to 1 M. 2. Modulators of the RyR that are known to alter its conformational slates, such as adenine nucleotides, Ca2+ concentration and pH, modified the effect of propranolol on ryanodine binding. In the presence of propranolol and at low NaCl concentrations, ryanodine binding was inhibited and showed no Ca2+-, pH- or time-dependence. 3. Propranolol immediately and comp letely blocked the channel opening of RyR reconstituted into a planar lipid bilayer. Propranolol-modified non-active channel was reactivated to a subconductive state (about 40% of the control conductance) by AT P. 4. Competition experiments between lidocaine (a stimulatory drug) o r tetracaine (an inhibitory drug) and propranolol at 0.2 or 1.0 M NaCl , respectively, suggest the existence of different interaction sites f or local anaesthetics and propranolol. 5. These results suggest that p ropranolol interacts directly with the RyR and modifies its ryanodine binding and single-channel activities. Propranolol effects are altered by the RyR conformational state, suggesting its possible use as a con formational probe for RyR.