Increased expression of adenosine A2A receptors in patients with spontaneous and head-up-tilt-induced syncope

BackgroundAdenosine may play a role in the triggering of neurocardiogenic syncope, but no information on adenosine receptors is available at the present time.ObjectiveThe purpose of this study was to investigate whether adenosine A2A receptors expression is altered in patients with neurocardiogenic syncope.MethodsAdenosine plasma levels (APLs), the expression of A2A receptors, were measured (mean ± standard error of the mean) during tilt testing. Expression of receptors was assessed on mononuclear cells using a selective receptor ligand.ResultsAt baseline, the APLs of 16 patients with a positive test were higher than those of 17 patients with a negative test and of those of a control group (2.10 ± 0.30 vs. 0.40 ± 0.05 and 0.41 ± 0.06 μM, respectively; P <.0001). The number of receptors was higher in patients tested positive than in patients tested negative or in the control group (122 ± 10 vs. 38 ± 4 and 44 ± 4 fmol/g of proteins, respectively; P <.0001). No difference was found in the affinity or synthesis among the three groups.ConclusionThis study showed an increased number and an up-regulation of adenosine A2A receptors in patients with spontaneous syncope and a positive head-up tilt, which in the context of high APLs may play a role in the recurrence of syncopal episodes.