The interactions of ATP, ADP, and inorganic phosphate with the sheep cardiac ryanodine receptor

The effects of ATP, ADP, and inorganic phosphate (P-i) on the gating of native sheep cardiac ryanodine receptor channels incorporated into planar phospholipid bilayers were investigated. We demonstrate that ATP and ADP can activate the channel by Ca2+-dependent and Ca2+-independent mechanisms. ATP and ADP appear to compete for the same site/s on the cardiac ryanodine receptor, and in the presence of cytosolic Ca2+ both agents tend to inactivate the channel at supramaximal concentrations. Our results reveal that ATP not only has a greater affinity for the adenine nucleotide site/s than ADP, but also has a greater efficacy. The EC50 value for channel activation is similar to 0.2 mM for ATP compared to 1.2 mM for ADP. Most interesting is the fact that, even in the presence of cytosolic Ca2+, ADP cannot activate the channel much above an open probability (P-o) of 0.5, and therefore acts as a partial agonist at the adenine nucleotide binding site on the channel. We demonstrate that P-i also increases P-o in a concentration and Ca2+-dependent manner, but unlike ATP and ADP, has no effect in the absence of activating cytosolic [Ca2+]. We demonstrate that P-i does not interact with the adenine nucleotide site/s but binds to a distinct domain on the channel to produce an increase in P-o.