Functional identification of adenylate cyclase-coupled adenosine receptors in rat brain microvessels.

It was investigated whether adenosine affects the adenylate cyclase system of microvessels, isolated from the rat cerebral cortex, via an external receptor recently classified as R-site receptor. Several adenosine analogs caused GTP-dependent stimulation of adenylate cyclase. The rank order of potency: NECA (5'-(N-ethylcarboxamido)-adenosine, EC50 0.2 microM) greater than adenosine (0.71 microM) = 2-chloroadenosine (0.72 microM) greater than L-PIA (N6-(L-phenylisopropyl)-adenosine, 1.03 microM) greater than D-PIA (N6-(D-phenylisopropyl)-adenosine, 5.27 microM) is consistent with that for agonism at activatory (Ra-site) adenosine receptors in other tissues. Adenylate cyclase stimulation by PIA displayed stereoselectivity. The action of NECA was competitively antagonized by 8-phenyltheophylline. These findings provide functional evidence for Ra-site adenosine receptors in rat brain microvessels. However, direct identification of these receptors by binding studies was not possible. Binding of [3H]NECA to rat brain microvessels displayed rapid on-off kinetics and saturability, but equivocal specificity.