AMP is a partial agonist at the sheep cardiac ryanodine receptor

1 We have investigated the ability of AMP to modulate the native sheep card iac ryanodine receptor (RyR) channel at various cytosolic [Ca2+]. Channels were incorporated into planar phospholipid bilayers and current fluctuation s through the bilayer were monitored under voltage damp conditions. 2 We demonstrate that AMP only exhibits agonist activity if the cytosolic [ Ca2+] is sufficiently high. Even in the presence of a high cytosolic [Ca2+] (65 mu M), AMP cannot fully open the channel and the maximum open probabil ity (Po) observed is approximately 0.3 at 2 mM AMP. 3 Concentrations of AMP above the maximally activating level cause inactiva tion of the channel. 4 Our experiments indicate that AMP is an agonist with such low efficacy at the ATP sites on the cardiac RyR that it is effectively an antagonist of A TP-induced increases in Po. Our study demonstrates that the number of phosp hates attached to the 5'-carbon of the ribose ring of adenine-based compoun ds determines the efficacy of the ligand to increase the Po of the cardiac RyR. Substitution of groups at this position may lead to the identification of potent antagonists at ATP sites on RyR.