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.