Afferent arteriolar vasodilator effect of adenosine predominantly involves adenosine A2B receptor activation

Abstract

Adenosine is an important paracrine agent regulating renal vascular tone via adenosine A1 and A2 receptors. While A2B receptor message and protein have been localized to preglomerular vessels, functional evidence on the role of A2B receptors in mediating the vasodilator action of adenosine on afferent arterioles is not available. The present study determined the role of A2B receptors in mediating the afferent arteriolar dilation and compared the effects of A2B and A2A receptor blockade on afferent arterioles. We used the rat in vitro blood-perfused juxtamedullary nephron technique combined with videomicroscopy. Single afferent arterioles of Sprague-Dawley rats were visualized and superfused with solutions containing adenosine or adenosine A2 receptor agonist (CV-1808) along with adenosine A2B and A2A receptor blockers. Adenosine (10 μmol/l) caused modest constriction and subsequent superfusion with SCH-58261 (SCH), an A2A receptor blocker, at concentrations up 10 μmol/l elicited only slight additional decreases in afferent arteriolar diameter with maximum effect at a concentration of 1 μmol/l (−11.0 ± 2.5%, n = 6, P < 0.05). However, superfusion of adenosine-treated vessels with MRS-1754 (MRS), an A2B receptor blocker, elicited greater decreases in afferent arteriolar diameter (−26.0 ± 4.7%, n = 5, P < 0.01). SCH did not significantly augment the adenosine-mediated afferent constriction elicited by MRS; however, adding MRS after SCH caused further significant vasoconstriction. Superfusion with CV-1808 dilated afferent arterioles (17.2 ± 2.4%, n = 6, P < 0.01). This effect was markedly attenuated by MRS (−22.6 ± 2.0%, n = 5, P < 0.01) but only slightly reduced by SCH (−9.0 ± 1.1%, n = 5, P < 0.05) and completely prevented by adding MRS after SCH (−24.7 ± 1.8%, n = 5, P < 0.01). These results indicate that, while both A2A and A2B receptors are functionally expressed in juxtamedullary afferent arterioles, the powerful vasodilating action of adenosine predominantly involves A2B receptor activation, which counteracts A1 receptor-mediated vasoconstriction.