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

BACKGROUND Adenosine may play a rote in the triggering of neurocardiogenic syncope, but no information on adenosine receptors is available at the present time. OBJECTIVE The purpose of this study was to investigate whether adenosine A2A receptors expression is altered in patients with neurocardiogenic syncope. METHODS Adenosine 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. RESULTS At 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 mu M, 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. CONCLUSION This 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 rote in the recurrence of syncopal episodes.