Abstract 2230: Increased Expression of Adenosine A2A Receptors in Patients with Spontaneous and Head-Up Tilt Induced Syncope
Background - Recent studies have suggested that adenosine may be an endogenous mediator in susceptible patients with vasovagal syncope. Adenosine receptors expression is known to play an important role in heart rate and blood pressure control. But its role in patients with vasovagal syncope has not been investigated. We investigated in patients with vasovagal syncope whether adenosine A2A receptor expression differs in those with a positive head-up tilt test as compared to those with a negative test.
Methods and Results - Adenosine plasma levels (APLs), adenosine A2A receptor number (Bmax), affinity (KD) and mRNA relative amount (mean ± SEM) were measured in 27 patients with unexplained syncope. Expression of A2A receptors was assessed on peripheral mononuclear cells using a selective A2A receptor ligand. In the14 patients with a negative head-up tilt test, Bmax was 39 ± 4 fmol/mg of protein, APLs at baseline was 0.45 ± 0.07 μmol/L, and 0.51 ± 0.07 μmol/L after 45 minutes. In the 13 patients with a positive test, Bmax was 129 ± 11 fmol/mg of protein, APLs at baseline 2.13 ± 0.37 μmol/L, and 2.71 ± 0.29 μmol/L during syncope, significantly higher (p < 0.0001) than in patients with a negative test. No difference was found in KD or in mRNA relative amount between the 2 groups.
Conclusions - This study showed in patients with spontaneous syncope and a positive head-up tilt as compared to those with a negative test, an increased expression of A2A receptors with an up-regulation of the amount of adenosine receptors but without any change in receptor affinity for adenosine, or in receptor synthesis.