Structural factors that determine the ability of adenosine and related compounds to activate the cardiac ryanodine receptor

1 The effects of adenosine and adenine on the gating of native sheep cardia c ryanodine receptor (RyR) channels were investigated. By examining the mec hanisms underlying channel activation and by using comparative molecular fi eld analysis (CoMFA) we have investigated the structural features of adenin e-based ligands involved in channel activation. 2 In the presence of 10 mu M cytosolic Ca2+, adenosine and adenine both act ivate the channel but only to a level approximately 10 and 20% respectively of that of ATP indicating that both are partial agonists of low efficacy. 3 Adenosine was able to antagonize the ATP-induced increase in open probabi lity (Po) as expected for a partial agonist of low efficacy at the ATP site s on the cardiac RyR. 4 GTP (100 mu M-10 mM) had no effect on channel gating indicating that the adenine ring structure is important for agonist activity at the ATP-sites o n RyR. 5 CoMFA revealed an extremely strong correlation between the structural fea tures of the five ATP analogues and the ability to increase (Po). Our model indicates that the high efficacy of ATP results primarily from the large e lectrostatic field established by the ionized phosphate groups. Reducing th e number of phosphate groups lowers the strength of this field, leading to ligands with lower efficacy. In addition, steric interactions between the a lpha-phosphate and ribose moieties and the RyR are correlated with low Po.