Cyclic ADP-ribose competes with ATP for the adenine nucleotide binding site on the cardiac ryanodine receptor Ca(2+)-release channel.

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Abstract
We have investigated the mechanism of action of the putative second messenger, cyclic ADP-ribose (cADPR), on the cardiac ryanodine-sensitive Ca(2+)-release channel. Current fluctuations through single Ca(2+)-release channels have been monitored after incorporation into planar phospholipid bilayers. We demonstrate that activation of the channel by cADPR is dependent on activating levels of cytosolic Ca2+ and lifetime analysis indicates that the mechanism of action may be sensitization of the channel to Ca2+. In the absence of ATP, cADPR activates the channel in a concentration-dependent manner in the presence of 10 mumol/L cytosolic Ca2+. However, in the presence of ATP, cADPR tends to decrease open probability, indicating that cADPR may be acting at the adenine nucleotide binding site. In addition, we demonstrate that the precursor of cADPR, beta-NAD+, and the breakdown product, ADP-ribose, also activate the channel. As cADPR will have to compete with much higher concentrations of beta-NAD+, ADP-ribose, and ATP, we suggest that cADPR does not act as a direct endogenous trigger for the opening of the cardiac Ca(2+)-release channel.