Functional and Molecular Characterization of Receptor Subtypes Mediating Coronary Microvascular Dilation to Adenosine

Adenosine is a potent vasodilator of the coronary microvessels and is implicated in the regulation of coronary blood flow during metabolic stress. However, the receptor subtypes and the vasodilatory mechanism responsible for the dilation of coronary microvessels to adenosine remain unclear. In the present study, using an isolated-vessel preparation we demonstrated that porcine coronary arterioles (50–100 μ m) dilated concentration-dependently to adenosine, CPA (adenosine A1receptor agonist) and CGS21680 (adenosine A2Areceptor agonist). These vasodilations were not altered by the A1receptor antagonist CPX, but were abolished by the selective A2Areceptor antagonist ZM241385, indicating that activation of A2Areceptors mediates these vasodilatory responses. The protein kinase A inhibitor Rp-8-Br-cAMPS abolished coronary arteriolar dilations to adenylyl cyclase activator forskolin and cAMP analog 8-Br-cAMP, but failed to inhibit adenosine- and CGS21680-induced dilations. The calcium-activated potassium channel inhibitor iberiotoxin also did not affect vasodilations to adenosine and CGS21680. In contrast, the ATP-sensitive potassium (KATP) channel inhibitor glibenclamide abolished vasodilations to adenosine and CGS21680 but did not affect vasodilations to forskolin and 8-Br-cAMP. In addition, the cAMP level in coronary microvessels was not increased by adenosine or CGS21680. The results from RT/PCR and in situ hybridization indicated that adenosine A2Areceptor mRNA was encoded in coronary arterioles and the left anterior descending (LAD) artery but not in cardiomyocytes, whereas the A1receptor transcript was detected in the LAD artery and cardiomyocytes but not in arterioles. Similarly, adenosine A1and A2Aproteins were expressed in the LAD artery, but only A2Areceptors were expressed in coronary arterioles. Collectively, these functional data suggest that coronary arteriolar dilation to adenosine is primarily mediated by the opening of KATPchannels through activation of A2Areceptors. This conclusion is corroborated by the molecular data showing that coronary arterioles only express adenosine A2Areceptors. Furthermore, the dilation of coronary microvessels to adenosine A2Areceptor activation appears to be independent of cAMP signaling.