Table 1.

Effect of adenosine on the ability of PPPi to activate RyR2.

Adenosine concentration	Mean RyR2 open probability (Po) ± SEM
	Adenosine alone	Adenosine + 10 mM PPPi
100 μM	0.068 ± 0.014 (n = 6)	0.374 ± 0.056 (n = 6)**
1 mM	0.080 ± 0.032 (n = 5)	0.331 ± 0.099 (n = 5)
10 mM	0.155 ± 0.081 (n = 5)	0.572 ± 0.107 (n = 5)**

To avoid inaccuracies in the [adenosine] due to the limits of adenosine solubility, 1 and 10 mM adenosine were perfused into the cytosolic chamber following fusions of the SR vesicles and therefore it is not possible to determine a ‘control’ value in the absence of adenosine (although comparison with other channels at 10 µM cytosolic Ca²⁺ could be made). 100 μM adenosine was obtained by addition of a 10 mM stock solution of adenosine, and in this case the control Po in the presence of 10 μM Ca²⁺ alone was 0.038 ± 0.009 (SEM, n = 6). Inactivation was not observed in any experiment but since adenosine binds reversibly to RyR2, one might expect to see occasional channel inactivation that occurred less frequently as the concentration of adenosine increased. However, this is difficult to show at the single-channel level and hence the [³H]ryanodine binding data in Fig. 4A is likely to provide a more complete picture of overall RyR2 activities for populations of channel. Note that adenosine is a partial agonist at the RyR2 ATP site²⁹. Adenosine alone does not increase Po above approx. 0.15, thus in the presence of an agonist with higher efficacy such as ATP or PPPi, it can appear to act as an antagonist. We have previously demonstrated that 100 μM and 1 mM adenosine inhibit channel activation by 1 mM ATP by 57% and 74% respectively²⁹. Therefore, unless PPPi and adenosine are both simultaneously aligned together in the ATP binding site (and the probability of this occurring will increase with increasing concentrations of adenosine), adenosine will appear to act as an antagonist. This is in line with the results since 10 mM adenosine and 10 mM PPPi gives the highest Po. (Mean values ± SEM are shown. **p < 0.01. p values relative to adenosine alone).