Afferent arteriolar adenosine A(2a) receptors are coupled to K(ATP) in in vitro perfused hydronephrotic rat kidney

Adenosine is known to exert dual actions on the afferent arteriole, eliciting vasoconstriction, by activating A1 receptors, and vasodilation at higher concentrations, by activating lower-affinity A2 receptors. We could demonstrate both of these known adenosine responses in the in vitro perfused hydronephrotic rat kidney. Thus, 1.0 μM adenosine elicited a transient vasoconstriction blocked by 8-cyclopentyl-1,3- dipropylxanthine (DPCPX), and 10-30 μM adenosine reversed KCl- induced vasoconstriction. However, when we examined the effects of adenosine on pressure-induced afferent arteriolar vasoconstriction, we observed a third action. In this setting, a high-affinity adenosine vasodilatory response was observed at concentrations of 10-300 nM. This response was blocked by both 4- (2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-yl- amino]ethyl)phenol (ZM-241385) and glibenclamide and was mimicked by 2-phenylaminoadenosine (CV-1808) (IC50 of 100 nM), implicating adenosine A(2a) receptors coupled to ATP-sensitive K channels (K(ATP)). Like adenosine, 5'-N-ethylcarboxamidoadenosine (NECA) elicited both glibenclamide-sensitive and glibenclamide- insensitive vasodilatory responses. The order of potency for the glibenclamide-sensitive component was NECA > adenosine = CV-1808. Our findings suggest that, in addition to the previously described adenosine A1 and low-affinity A(2b) receptors, the renal microvasculature is also capable of expressing high- affinity adenosine A(2a) receptors. This renal adenosine receptor elicits afferent arteriolar vasodilation at submicromolar adenosine levels by activating K(ATP).