Functional and molecular characterization of receptor subtypes mediating coronary microvascular dilation to adenosine

Adenosine is a potent vasodilator of the coronary microvessels and is impli cated in the regulation of coronary blood flow during metabolic stress. How ever, the receptor subtypes and the vasodilatory mechanism responsible For the dilation of coronary microvessels to adenosine remain unclear. In the p resent study, using an isolated-vessel preparation we demonstrated that por cine coronary arterioles (50-100 mum) dilated concentration-dependently to adenosine, CPA (adenosine A(1) receptor agonist) and CGS21680 (adenosine A( 2A) receptor agonist). These vasodilations were not altered by the A(1) rec eptor antagonist CPX, but were abolished by the selective A(2A) receptor an tagonist ZM241385, indicating that activation of A(2A) receptors mediates t hese vasodilatory responses. The protein kinase A inhibitor Rp-8-Br-cAMPS a bolished coronary arteriolar dilations to adenylyl cyclase activator forsko lin and cAMP analog 8-Br-cAMP but failed to inhibit adenosine-and CGS21680- induced dilations. The calcium-activated potassium channel inhibitor iberio toxin also did not affect vasodilations to adenosine and CGS21680. In contr ast, the ATP-sensitive potassium (K-ATP) channel inhibitor glibenclamide ab olished vasodilations to adenosine and CGS21680 but did not affect vasodila tions to forskolin and 8-Br-cAMP. In addition, the cAMP level in coronary m icrovessels was not increased by adenosine or CGS21680, The results from RT /PCR and in situ hybridization indicated that adenosine A(2A) receptor mRNA was encoded in coronary arterioles al-td the left anterior descending (LAD ) artery but not in cardiomyocytes, whereas the A(1) receptor transcript wa s detected in the LAD artery and cardiomyocytes but not in arterioles. Simi larly, adenosine A(1) and A(2A) proteins were expressed in the LAD artery b ut only A(2A) receptors were expressed in coronary arterioles. Collectively , these functional data suggest that coronary arteriolar dilation to adenos ine is primarily mediated by the opening of K-ATP channels through activati on of A(2A) receptor. This conclusion is corroborated by the molecular data showing that coronary arterioles only express adenosine A(2A) receptors. F urthermore, the dilation of coronary microvessels to adenosine A(2A) recept or activation appears to be independent of cAMP signaling. (C) 2001 Academi c Press.