A role for the A3 adenosine receptor in determining tissue levels of cAMP and blood pressure: studies in knock-out mice.

Adenosine administration has been reported to lower blood pressure by activating specific membrane receptors. The rat and human heart and aorta have been previously found to express both A2-type adenosine receptors, which activate adenylyl cyclase, and A3 adenosine receptors (A3AR), which inhibit adenylyl cyclase. In the current study, we used A3 adenosine receptor (A3AR) knock-out mice to examine the hypothesis that the relative levels of the A2-type adenosine receptors and A3AR determine the steady-state levels of cAMP in the cells and may affect blood pressure. We found that the A3AR knock-out mice express normal levels of the A1- and A2-type adenosine receptors. In situ hybridization demonstrated that the level of A3AR is high in the vascular smooth muscle layer of aortas derived from wild-type mice, but is not detectable in the knock-out mice. The steady-state level of cAMP is elevated in the aorta and heart of knock-out mice, as compared to wild-type mice, but is not altered in platelets, where A3AR is not expressed naturally. A3AR knock-out mice possess a blood pressure comparable to this in wild-type mice. However, when challenged with adenosine, the knock-out mice display a further increase in cAMP levels in the heart and vascular smooth muscle and a significant decrease in blood pressure, as compared to wild-type mice. In contrast, the effect of adenosine on ADP-induced platelet aggregation is similar in both types of mice. These studies indicate that the A3AR affects the steady-state level of cAMP in the tissues where it is naturally expressed, and that it influences the blood pressure in response to adenosine.