Relaxation by adenosine and its analogs of potassium-contracted human coronary arteries

The present study was an attempt to characterize the adenosine receptor in human coronary arteries, and to establish the dependence of the relaxations mediated by this receptor on a functional endothelium. Human coronary arteries were obtained from organ donors. Adenosine and its analogs (5'-N-ethyl-carboxamido-adenosine, NECA; N6-L-phenylisopropyladenosine, L-PIA; 2-chloroadenosine, CAD), all inhibited the contraction induced by 25 mmol/l KCl in a concentration-dependent manner and the order of potency was found to be: NECA greater than CAD greater than L-PIA greater than adenosine. These relaxations were antagonized by 8-phenyltheophylline (8PT). At higher concentrations of KCl, the relaxations were attenuated. In rings which relaxed in response to endothelium-dependent relaxing agents (bradykinin and A23187), NECA and CAD produced relaxations similar to those produced in rings which did not show endothelium-dependent responses. The results suggest that the coronary adenosine receptor (probably A2) mediates relaxations which may not be dependent on the relaxing function of the endothelium.