Greater adenosine A(2A) receptor densities in cardiac and skeletal muscle in endurance-trained men: a [11C]TMSX PET study

We examined the densities of adenosine A_2A receptors in cardiac and skeletal muscles between untrained and endurance-trained subjects using positron emission tomography (PET) and 7-methyl-¹¹C]-(E)-8-(3,4,5-trimethoxystyryl)-1,3,7-trimethylxanthine (¹¹C]TMSX), a newly developed radioligand for mapping adenosine A_2A receptors. Five untrained and five endurance-trained subjects participated in this study. The density of adenosine A_2A receptors was evaluated as the distribution volume of ¹¹C]TMSX in cardiac and triceps brachii muscles in the resting state using PET. The distribution volume of ¹¹C]TMSX in the myocardium was significantly greater than in the triceps brachii muscle in both groups. Further, distribution volumes ¹¹C]TMSX in the trained subjects were significantly grater than those in untrained subjects (myocardium, 3.6±0.3 vs. 3.1±0.4 ml g^?1; triceps brachii muscle, 1.7±0.3 vs. 1.2±0.2 ml g^?1, respectively). These results indicate that the densities of adenosine A_2A receptors in the cardiac and skeletal muscles are greater in the endurance-trained men than in the untrained men.