Cardiac myocyte adenosine A2a receptor activation fails to alter cAMP or contractility:: role of receptor localization

Adenosine A(2a) receptors are found in coronary vascular tissue although, their presence in myocardium is subject to investigation. Although there have been numerous studies on adenosine A(2a) receptor agonist effects on contractility and cAMP levels in ventricular myocytes, these have yielded conflicting results. Negative pharmacological studies have even led to the conclusion that A(2a) receptors are not present in cardiac myocytes. The purpose of this study was to determine whether A(2a) receptors are expressed in rat ventricular myocytes and what physiological effects are mediated via activation of these receptors. Western blot analysis with a polyclonal antibody raised against a peptide sequence specific to the carboxy terminus of the A(2a) receptor revealed the presence of a band at similar to45 kDa. However, the immunoreactivity was located in the nonmembrane fraction of the cell lysate. The membrane fraction only exhibited an immunoreactive band greater than or equal to50 kDa. Treatment of isolated myocytes with the adenosine A(2a) agonist 2-[4-[(2-carboxyethyl)-phenyl]ethylamino]-5'-N-ethylcarboxamidoadenosine (CGS-21680) exerted no effects on cAMP levels or myocyte twitch amplitude. These results indicate that although rat ventricular myocytes appear to express adenosine A(2a) receptors, stimulation with an A(2a) agonist exerts no functional effects, possibly because of the subcellular localization of the A(2a) receptor.